Quang-Viet Nguyen and David A. Jacqmin

Glenn Research Center, Cleveland, Ohio

A Study of Cavitation-Ignition

Bubble Combustion

NASA/TM—2005-213599

August 2005
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Quang-Viet Nguyen and David A. Jacqmin

Glenn Research Center, Cleveland, Ohio

A Study of Cavitation-Ignition

Bubble Combustion

NASA/TM—2005-213599

August 2005

National Aeronautics and

Space Administration

Glenn Research Center
Acknowledgments

This work was supported by the U.S. Department of Energy under a partially-reimbursable Space Act Agreement

SAA–3–547 (Department of Energy Interagency Agreement number DE–A105–02OR22929), and in part, by

the Strategic Research Fund (SRF) at NASA Glenn Research Center. The authors thank Gregg Calhoun,

QSS Group, Inc., for the facilities operations engineering services, gas analysis, and trouble-shooting;

John Zilka, University of Toledo, for his assistance in conducting the experiments; Ray Lotenero and

James Sexton, Akima Corporation, for building the flow reactor apparatus; Rod Berriker, ZIN

Technologies, for the flow systems design; Jackie Corrigan, University of Dayton, for help

with the parametric studies using the computer model; Martin Rabinowitz

and James Conklin, ORNL, for helpful discussions; and Paul Penko

for his suggestions in the preparation of the manuscript.

Available from

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22100

Trade names or manufacturers’ names are used in this report for

identification only. This usage does not constitute an official

endorsement, either expressed or implied, by the National

Aeronautics and Space Administration.

Available electronically at http://gltrs.grc.nasa.gov
NASA/TM—2005-213599 1

A Study of Cavitation-Ignition Bubble Combustion 

Quang-Viet Nguyen and David A. Jacqmin 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

We present the results of an experimental and computational study of the physics and chemistry of 

cavitation-ignition bubble combustion (CIBC), a process that occurs when combustible gaseous mixtures 

are ignited by the high temperatures found inside a rapidly collapsing bubble. The CIBC process is 

similar to that in a diesel engine. However, the length scales are very small (μm to nm) and the time 

scales are very brief (μs to fs). Furthermore, the process takes place inside a micro-spherical chamber 

with variable dimensions rather than in a macro-cylindrical metal chamber with fixed dimensions. We 

computationally model the CIBC process using a 1- and 0-dimensional time-dependent compressible 

fluid-dynamics code that includes finite-rate chemistry. The computational model of the CIBC process 

indicates that gas-phase reactions within the bubble occur and produce CO and other by-products of 

combustion, heat and mechanical energy release through a bubble volume-expansion phase. The model 

shows that the CIBC process is sensitive to the fuel-air mixture ratio, the initial bubble diameter, and the 

acoustic pressure forcing amplitude. We experimentally demonstrate the CIBC process using an 

ultrasonically excited cavitation flow reactor. In the flow reactor, we subject gaseous mixtures of C3H8-air 

and CH4-air bubbles in liquid water, and methanol vapor-air bubbles in liquid methanol, to circa 100 W of 

acoustic power at a frequency of 20 kHz. We measure small amounts (up to 160 ppm) of carbon 

monoxide (CO) emitted as a byproduct of the gas-phase chemical reactions or combustion within the 

collapsing bubbles. We find that the CO production is proportional to the acoustic power level delivered 

to the CIBC flow reactor. The results of the model were found to be consistent with the measured 

experimental results. Based on the experimental data, and supported by the results of the computational 

model and previous reports of the “micro-diesel effect” in industrial hydraulic systems, we determined 

that gas-phase chemical reactions that are initiated by the high temperatures within a collapsing bubble 

are indeed possible and exist in ultrasonically- and hydrodynamically-induced cavitation. Using the 

results from the computer model of CIBC of a CH4-air bubble in water, we find that, in theory, it may be 

possible to develop a hydraulic-CIBC engine that produces net power. The results of the model indicate 

that such an engine would require at minimum the following conditions to operate at a break-even point: a 

liquid flow rate of about 114 liter ⋅ min–1 (31 gal ⋅ min–1) with a 10 percent bubble void fraction of 

approximately 10 μm diameter CH4-air bubbles in water, with an equivalence ratio of about 1.25, and a 

pressure drop of about 21 kPa (3 psid) provided by a venturi with a pressure recovery factor of at least 

85 percent. A significant result of this finding is that a hydraulic-CIBC engine requires a venturi or other 

pressure recovering device to operate in a self-sustaining mode.

Introduction 

In recent years sonoluminescence (SL), the production of light from sound resulting from the 

cavitational collapse of a bubble, has been an active and intense area of study because of the uniquely 

high temperatures and pressures obtainable using SL. Useful reviews of this phenomenon have been 

written by Barber (1997) and Brenner et al. (2000). This report deals with a related but new field of 

research called cavitation-ignition bubble combustion (CIBC), or the ignition and subsequent burning of a 

gaseous mixture inside a fuel-air bubble caused by adiabatic compression heating that results from 
NASA/TM—2005-213599 2

Bubble Radius

Time

Ignition

Compression

Expansion

R0

Bubble Radius

Time

Ignition

Compression

Expansion

R0

Figure 1.—Graphical depiction of the cavitation-ignition bubble 

combustion (CIBC) process. After an initial expansion phase 

resulting from cavitation, a bubble collapses rapidly due to high 

inertial forces that act on the bubble wall; the collapse is so sudden 

that an adiabatic compression heating of the contents occurs. This 

heating ignites the contents much like a diesel engine. In fact, this 

process has long been known to cause degradation of hydraulic 

oils and is termed the “micro-diesel” effect. The heat release and 

change in product moles resulting from combustion causes the 

bubble radius to grow to a larger radius than before, thereby 

producing volumetric expansion work. 

cavitational collapse as depicted in figure 1. The occurrence of combustion in a fuel-air mixture inside a 

hydraulic-cavitation-induced collapsing bubble has been identified and known for quite some time 

(Lohrentz, 1968). However, it has always been considered a problem and many studies have been aimed 

at eliminating it as an undesirable occurrence in hydraulic fluid systems. Here however, we look at CIBC 

as a possible means of generating mechanical power in a way similar to a diesel engine. Furthermore, we 

do not limit our study to hydraulic cavitation but also consider sound-induced cavitation (sonocavitation). 

There have been many previous studies of effects in or caused by cavitation. In many of these studies, 

cavitation has long been regarded as a problem in hydraulic systems as it causes the mechanical 

degradation of hydraulic components such as valves and orifices (Hobbs and McCloy, 1972). 

Additionally, cavitation can be responsible for the chemical degradation of the working hydraulic fluid 

(Totten et al., 1998; Koivula and Ellman, 1998). In particular, it has long been known that the entrainment 

of air into hydrocarbon-based hydraulic fluids (oils), causes a rapid degradation in the fluid properties 

(Staeck, 1987) from the so-called “micro-diesel” effect (Backe and Lipphardht, 1976; Svedberg et al., 

1999) in which the cavitational collapse of the oil heats air inside the bubbles to the point of oxidizing or 

burning the oil vapor. The early investigations of Lohrentz (1968) followed by the experiments of Backe 

and Lipphardt (1976) showed that flashes of light resulting from the combustion of the oil vapor and air, 

are emitted from these bubbles as they collapse. Various techniques have been advanced to separate air 

bubbles from hydraulic oils to minimize this degradation and to reduce the oil temperature (see for 

example, Suzuki et al. 1997). 

Separate from the above studies of cavitation and its effect on the properties of the working hydraulic 

fluid are studies of bubble combustion in the context of two-phase flow reactors in which bubbles provide 

the interface and surface area for gas-liquid reactions. In these studies, cavitation is not used for ignition 

but the contents of the gaseous bubble are ignited and reaction takes place through bulk heating. Studies 

include the theory of bubble displacement reactors (Likachev et al., 1991), bubble-column reactors 

(Carbonell and Guirardello, 1997), studies of oscillating combustible gas bubbles (Gol’dshtein et al., 

1998), and premixed combustion by blowing air bubbles into liquid fuels (Kitano and Shiba, 1996). 
NASA/TM—2005-213599 3

Cavitation to purposefully effect chemical and morphological changes was pioneered by Suslick who 

showed that sonocavitation could be used to produce new nano-particles, synthesize new compounds, and 

even process bio-medical materials (Suslick et al., 1999). Hydrodynamic cavitation has also been 

experimentally shown to effect chemical reactions (Suslick et al., 1997; Pandit et al., 1999) in aqueous 

compounds. More recently, efforts have been made by several groups to model both the hydrodynamics 

and chemistry of cavitating flows (Gong and Hart, 1998 and 1999; Colussi et al., 1998). These studies 

indicate that the rapid heating and cooling of cavitational collapse provides several benefits not found 

using other chemical processing techniques, the most notable of which is the extremely rapid cooling rate 

of 1010 K sec–1 (Suslick et al., 1999) that enables the “freezing” of certain compounds after they are 

generated under the high and intense pressures encountered within the collapsing bubbles. Other effects 

such as mixture segregation from differential mass transport within collapsing bubbles (Storey and Szeri, 

1999) and the role of shock waves inside collapsing bubbles (Vuong et al., 1999) have also been 

addressed. However, there is still debate as to the importance of bubble non-uniformity in affecting 

chemical interactions and the overall cavitation process. 

Approach 

Bubble cavitation can be initiated by any number of means including acoustic waves (sonocavitation), 

hydraulic pressure drops (hydraulic-cavitation), and laser-induced plasmas. For this report, the emphasis 

was principally centered on acoustically generated cavitation as this has been shown to produce the most 

intense temperatures and pressures in collapsing bubbles. We generated multi-bubble mixtures of 

premixed fuel/oxidizer in a small chamber containing (1) liquid water, or (2) a single-component liquid 

hydrocarbon fuel (such as methanol) that were acoustically forced to produce cavitation. The liquid water 

was seeded with a stream of premixed hydrocarbon fuel and oxidizer bubbles consisting of propane-air or 

methane-air. In the case of liquid methanol, the flow was seeded with air bubbles. The gas bubble seeds 

were then subjected to a high intensity acoustic field generated with a 200 W (max) acoustic driver/horn 

submerged in the liquid chamber. The horn produces intense localized pressure fields in excess of several 

atmospheres to drive the cavitation. Some of the collapsing bubbles (depending on how well the acoustic 

power couples into the bubble as a resonator) heat up to a high temperature sufficient to ignite the bubble 

contents. 

To study this process, we applied a suite of diagnostics including the measurement of light emission 

resulting from chemical reactions, acoustic signatures, and time-averaged bulk gaseous emissions. 

However, we concentrated our measurements on the continuous gaseous emissions measurements of 

carbon dioxide (CO2), oxides of nitrogen (NOX) which include NO and NO2, and carbon monoxide (CO) 

given off by the combusting bubble, as these are the main indicators of chemical reactions resulting from 

combustion. Tests conducted in liquid water, however, have the problem of gas solubility, as both CO2

and NOX are highly soluble in water. Thus, the major gaseous emissions were expected to be CO as it is 

much less soluble in water. 

In parallel, we theoretically modeled the bubble-collapse event using the Gilmore equation describing 

the 1-dimensional (1-D) time-dependent collapse of a single bubble. A design-of-experiments approach 

was used for modeling and was aimed at determining the optimal conditions for cavitation-driven 

chemical reactions. The internal flow of the bubble was assumed to be a 1-D, time-dependent problem 

involving nano-scales of both length and time, and was not modeled in detail. Instead, the assumption of 

uniform properties within the bubble was used to simplify the modeling efforts. 

The objectives of this study were as follows: 

a.) Determine whether or not sonocavitation can be used to ignite mixtures of gaseous hydrocarbon 

fuels and oxidizers inside a bubble to release heat as evidenced by measurements of combustion 

by-products, heat-release, light emission, and/or other means of energy production. 
NASA/TM—2005-213599 4

b.) Develop a computational model of the CIBC process using fundamental principles of fluid 

dynamics, physics and chemistry. Perform an analysis of the CIBC process using the model to 

determine critical parameters for CIBC. 

c.) From the experimental data and model, perform a final determination of the possibility or 

existence of CIBC. 

Theory 

We modeled the time-dependent behavior of the collapsing bubble with the assumption of radial 

symmetry to simplify the governing equations (Storey and Szeri, 1998). The flow in the cavitating bubble 

is then governed by the compressible Navier-Stokes equations for radial momentum, the continuity 

equation and the energy equation. The continuity equation is given by: 

( ) 2

2

1

r 0

t r r

∂ρ ∂ + ρυ = ∂ ∂ , (1) 

where r is the radial direction, t is time, ρ is the gas density and υ is the radial velocity. The momentum 

equation is given by: 

( ) ( ) 22 2

2 2

1 1 rrp r r

t r rr r r r

θθ φφ ∂ρυ ∂ ∂ ∂ τ + τ

+ ρυ = − − τ + ∂ ∂ ∂∂ , (2) 

where p is the pressure. The τ terms represent the normal stresses in the fluid. Because of radial 

symmetry, these reduce to 

1 2

2 3 rr r r θθ φφ

⎛ ⎞ ∂υ υ

τ =τ =− τ = μ − ⎜ ⎟ ⎝ ⎠ ∂ . (3) 

The viscosity μ is in general a function of temperature and composition. The evolution equation for the 

temperature T is given by: 

2 2

2 2

1 1

rr C

DT r q p r CT Q Dt r T r r r r r

θθ φφ

υ

ρ

∂ ∂ ∂ υ ∂υ τ +τ ⎛ ⎞ ρ =− − −τ −υ + ⎜ ⎟ ∂∂ ∂ ∂ ⎝ ⎠ . (4) 

Here, the term DT/Dt is the material derivative given by ∂/∂t + υ∂/∂r. The term QC is the heat per unit 

volume that is generated due to chemical reactions. The term q is the heat transfer due to conduction, and 

is given by: 

T q k

r

∂ = − ∂ , (5) 

where k is the thermal conductivity. The term Cυ is the specific heat at constant volume per unit mass of 

bubble gas. Hence, k and Cυ may both be a function of temperature, bubble gas composition and pressure. 

The conservation equations for the chemical species are 

( ) 2

2

i 1

i i r M

t r r

∂ρ ∂

+ ρυ = ∂ ∂ , (6) 
NASA/TM—2005-213599 5

where Mi is the rate of change in the density of the chemical species i from chemical reactions. Diffusion 

currents are neglected in the calculations. The chemical heat release term, QC, is given in terms of the Mi

by 

C ii

i

Q MU =∑ , (7) 

where Ui is the internal energy per unit mass of material i. 

The radius of the bubble changes through the mechanical interactions between its gaseous contents 

and liquid surroundings. The flow in the liquid is nearly incompressible and can be expressed by the 

Gilmore equation (Brenner et al., 2002). This is: 

3 1 2 (1 ) (1 3 ) (1 ) (1 ) 2

d

t tt t t t d t

dH R R cR R cR R cH R cR

c dt

− + − =+ + − , (8) 

where R is the radius of the bubble, c the sound speed at the bubble wall, and Hd the liquid enthalpy at the 

bubble wall minus the enthalpy at infinity. The t subscripts indicate time derivatives. We use a liquid 

equation of state of the modified Tait form (Prosperetti and Lezzi, 1986) 

0 0

n P B

P B

+ ρ ⎛ ⎞ = ⎜ ⎟

+ ρ⎝ ⎠

, (9) 

where B = 3049.13 bar and n = 7.15 gives an excellent fit for water vapor up to 105 bar. The enthalpy 

difference between the bubble wall and the far-field can then be written as 

1

B

d

B

n P BP B H

n

∞

∞

⎛ ⎞ + + = − ⎜ ⎟

−ρ ρ ⎝ ⎠

 (10) 

and 

d 1 1 B

B

dH dP dP

dt dt dt

∞

∞

= − ρ ρ. (11) 

The B subscript indicates liquid quantities at the bubble wall. P∞ is the far-field acoustic pressure, and is 

what forces the bubble motion. 

Multi-Shell Numerical Model 

Prior to our adoption of the assumption of uniform internal bubble flow properties, we performed a 

series of computational tests using a multi-shell model to incorporate the effects of internal-flow spatial 

non-uniformity. The multi-shell model divided the internal flow-field into a series (up to 20) of concentric 

shells with heat, mass, and momentum transfer. To keep the chemical kinetics simple, the effects of 

combustion were tested in the multi-shell model using some very simple chemical reaction models: 1) 

instantaneous chemical conversion and heat release where the gas temperature exceeds a given 

temperature; 2) heat release and chemical conversion that depends exponentially on temperature and 

linearly on the remaining amount of combustible material. The first model was chosen to maximize the 

likelihood of shock formation. We found that this occurred only under extreme conditions. In most 

situations, conditions within the bubble stayed fairly smooth. This was especially true for pressure, and 

mostly true for temperature, even near the bubble wall. This finding of little formation of spatial structure 
NASA/TM—2005-213599 6

is consistent with the review of Brenner et al., (2002) that show sonoluminescing bubbles are usually 

shock-free, though sometimes can be generated (Yuan et al., 1998) 

We concluded that a single-shell (or homogeneous properties) model is adequate for further 

computational investigations of CIBC. The single-shell model has the advantage of fast computation and 

therefore offers the opportunity to fully compute complex chemical mechanisms. 

Single-Shell Numerical Model 

The single-shell model is equivalent to approximating the conditions within the cavitating bubble as 

uniform. The mass M in the bubble is constant, as evaporation effects are neglected. The amount of each 

chemical species and the total number of moles of gaseous chemical species vary as a function of time 

from the effects of chemical reactions (combustion). The chemistry was calculated using the CHEMKIN 

subroutine package (Kee et al., 1989) in conjunction with a methane-air chemical kinetic mechanism 

(Bowman et al.). As with the continuum model, the changes in the radius and volume of the bubble are 

determined by the Gilmore equation. 

The perfect gas law is assumed, 

 / P NR T V = g , (12) 

where N is the time-varying number of moles, V is the bubble volume, and Rg is the universal gas 

constant. The temperature of the gas varies with the internal energy according to 

1 1

C C

m n

dT dU dU Q Q dt Mc dt Nc dt υ υ

⎛ ⎞⎛ ⎞ = += + ⎜ ⎟⎜ ⎟ ⎝ ⎠⎝ ⎠, (13) 

where cυm is the constant-volume specific heat per mixture unit mass and cυm is the constant-volume 

specific heat of mixture per unit mole. These are functions of temperature and bubble composition as 

calculated by the CHEMKIN subroutine package (Kee et al., 1989). The time variation of U is given by

g

k

dU dV NR T

Q dt V dt = − , (14) 

where QC is the chemical heat release, and Qk is the amount of heat conducted into the bubble is given by 

( )/ Q kT T A k LIQ GAS =α − , (15) 

Where α is a proportionality constant (adjustment factor) and A is the bubble wall area. Results were 

found to be insensitive to the value of α (Brenner et al., 2002). This is because the bubble-collapse period, 

which is the period of greatest interest, is so fast that it is essentially adiabatic. The conductivity k was 

simplified to a constant, chosen to be typical of the gases at the given liquid temperature. This is the 

temperature range where the bubble exists most of the time. 

With regards to the numerical time-steps chosen for the simulations: all calculations were explicit, 

chemical reactions therefore required short-time steps at high temperatures. This apparent inefficiency is 

acceptable because at high temperatures the bubble radius evolution proceeds extremely rapidly, which, 

to capture this change, also requires small time-steps in the picosecond to femtosecond range. The timestep size was calculated at each time-step using simple formulas involving the powers of bubble radius 

and temperature. The formulations for calculating the variable time-steps turned out well: when we 

included the full chemical kinetic mechanism with 279 elementary reaction-steps, it was still possible to 

calculate hundreds of thousands of bubble radius time-steps per CPU second. As a result, the 
NASA/TM—2005-213599 7

computational time for a few bubble oscillations is typically on the order of several minutes to an hour on 

a standard desktop workstation. 

Experimental Apparatus 

We used an acoustically excited sonocavitation flow reactor setup to demonstrate the existence of 

CIBC. The experimental apparatus is given schematically in figure 2. This shows the recirculating liquid 

flow system which provides the bulk flow of the working fluid, the gaseous flow system which provides a 

controllable mixture of gaseous reactants, the gaseous emissions analyzer, and an automated facility data 

acquisition and control system. 

At the center of the system is an optically accessible flow reactor that is constructed using standard 

stainless steel vacuum hardware with fused-silica windows on 3 sides of the two 6-way crosses. The 

working liquid (distilled water or spectroscopic grade methanol) enters from the bottom of the reactor 

where a gaseous mixture of fuel and oxidizer is introduced into the liquid using a sintered stainless steel 

filter element (7 μm). The resulting two-phase flow of liquid and bubbles is then introduced to the upper 

section of the flow reactor through a 2 mm diameter orifice located at the center of a 20 mm diameter disk 

made of titanium. This is referred to as the “anvil” after a configuration used by Hobbs et al. (1969) and 

Hobbs and Rachman (1970). The two-phase flow issuing from the orifice forms a jet that impinges on the 

face of a 12.7 mm diameter titanium ultrasonic transducer horn that is spaced between 2 to 10 mm from 

the anvil. Titanium is used for wear resistance as it has one of the lowest cavitational erosion rates of any 

metal (Hobbs and McCloy, 1972). Figure 3 shows a photograph of the hammer and anvil configura-tion 

with the acoustic horn in the off state. Figure 3 shows a toroid-like collection of bubbles trapped in 

Gas Sampling

Line

* Gas Analyzer

CO2, CO, 

O2 , NOx

Fuel Flow

Controller

Air Flow

Controller

Sump Tank

Turbine

Flowmeter

Chiller/

Heater

Pump

Exhaust

Anvil

Ultrasonic Transducer

Gas Mixer 7 micron sintered

S.S. Bubbler

Fused Silica

Windows (4)

Fused Silica

Windows (4)

Facility Control and

Data Acquisition

System (PLC/W.W.)

Gas Sampling

Line

* Gas Analyzer

CO2, CO, 

O2 , NOx

Fuel Flow

Controller

Air Flow

Controller

Sump Tank

Turbine

Flowmeter

Chiller/

Heater

Pump

Exhaust

Anvil

Ultrasonic Transducer

Gas Mixer 7 micron sintered

S.S. Bubbler

Fused Silica

Windows (4)

Fused Silica

Windows (4)

Facility Control and

Data Acquisition

System (PLC/W.W.)

Figure 2.—Schematic of the sonocavitation flow reactor system. There are two separate flow systems: 

recirculating liquid and gaseous reactants. The recirculating liquid flow system uses a gear pump to 

drive the bulk liquid through the flow reactor. The gaseous flow control system meters and mixes the 

fuel/oxidizer mixture before delivering it into the reactor using a sintered filter bubbler. The two-phase 

mixture then impinges onto an ultrasonic transducer horn in a “hammer and anvil” geometry. The 

gaseous emissions that result from the CIBC process are then separated in the sump and are analyzed. 

All aspects of the apparatus are controlled and recorded from a programmable logic controller (PLC) 

in conjunction with software and a PC. 
NASA/TM—2005-213599 8

the recirculating flow field set up by the stagnation point flow resulting from the impingement of the jet 

on the face of the acoustic transducer horn. We chose this type of flow geometry because: (1) it provides 

an intense and uniform acoustic pressure field that all portions of the flow must pass through; (2) it also 

provides excellent optical access to view the process from the outside; (3) the residence time that the 

fluids are in contact with the high intensity sound field can be varied by adjusting the gap between the 

hammer and anvil or the bulk flow rate of the fluid; and (4) the stagnation point flow geometry is wellunderstood and provides clear boundary conditions that are useful from a computational-fluid dynamics 

(CFD) modeling standpoint. 

After the two-phase flow of liquid and bubbles undergoes acoustically induced cavitation, and in 

some cases, cavitation-ignition bubble combustion, the flow and its resulting gaseous combustion 

byproducts are then deposited into a stainless steel sump tank (8 liter capacity, typically filled to 4 liters) 

to permit the liquid and gases to settle and separate. The gaseous emissions that are separated from the 

liquid exit through a vent tube directed to an exhaust ventilation hood. A small gaseous-sampling probe 

inside the vent tube extracts a portion of the vented gases. This is then dried using an anhydrous calcium 

sulfate desiccant pack, then drawn into a gas analyzer for the determination of oxygen (O2), carbon 

dioxide (CO2), carbon monoxide (CO), oxides of nitrogen (NOX), and hydrocarbons (HC) or sulfur 

dioxide (SO2). The gas analyzer (Horiba, PG-250) uses non-dispersive infrared (NDIR) techniques to 

measure the CO, CO2 and HC/SO2 concentrations, chemiluminescence to measure the NOX concentration, 

and an electro-galvanic cell to measure the O2 concentration. All gases are calibrated using NIST 

traceable calibration gas standards. The liquid flow in the sump is then recirculated through the system 

after going through a temperature controlled heat exchanger that stabilizes the temperature of the liquid 

(20-22 C). A filter (10 μm) then removes larger particulates resulting from the cavitational erosion. A 

magnetically-coupled ceramic gear pump (up to 1.5 liter ⋅ min–1) provides the positive pressure to drive 

the fluid through the system. A turbine flow meter measures the fluid flow rate. Throughout the flow path, 

the temperatures and pressures of the bulk fluid at critical points are monitored and recorded using type-K 

0.500 inch Anvil

Ultrasonic Horn

Bubble/Fluid

Flow 

(2 mm orifice)

Figure 3.—Photograph of the hammer and anvil flow reactor 

geometry. The two-phase flow enters from the bottom 

through a 2 mm diameter orifice and impinges on the 

12.7 mm (0.50 inch) diameter ultrasonic transducer horn. 

Both the ultrasonic transducer horn (the hammer) and the 

anvil are made of titanium for cavitational erosion resistance. 

In this photograph, the ultrasonic transducer is turned off, 

but the liquid flow is turned on. The stagnation point flow 

produces a small recirculation zone that traps bubbles in a 

toroidal pattern above the anvil. 
NASA/TM—2005-213599 9

thermocouples and high precision electronic pressure transducers (0.5 kPa accuracy). The flow leaving 

the turbine flow meter is then directed to the bottom of the flow reactor where the premixed gases are 

injected as bubbles. 

The gases are provided from pressure regulated cylinders and metered and controlled using precision 

thermal conductivity mass flow controllers with a specified 2 percent accuracy over the flow range 

(350 sccm max). The flow meters are calibrated using an electronic soap-bubble type volumetric flow 

calibrator. The gases used are dry air, argon (Ar), nitrogen (N2), propane (C3H8), and methane (CH4). The 

metered gas flows from the fuel and oxidizer side of the flow systems are then mixed using a custom coannular tube mixer. In this type of flow mixer the fuel flows down the inner central tube, while the 

oxidizer is fed in through the annular space. The oxidizer is injected through radially directed holes along 

the inner tube wall and mixed with the fuel. In this way, a fully premixed gas flow is established quickly 

and efficiently through the resulting high-shear mixing action of the oxidizer jets upon the central core 

flow. The gases exiting the mixer then pass through a check valve, and finally to the sintered filter 

element that serves as the gas bubbler. All gas flow pressures, temperatures and flow rates are measured 

and recorded with 16-bit precision at a rate of 10 Hz using the automated data acquisition and control 

system. Typical gas flow rates range from 10 to 100 sccm for the fuel and 100 to 300 sccm for the air. 

Typical liquid flow rates ranged from 1.0 to 1.5 liter ⋅ min–1(1 cm3 ⋅ min–1 accuracy). 

The automated data acquisition and control system uses a programmable logic controller (PLC) 

system with various analog and digital input/output interfaces. The PLC, in conjunction with custom 

developed software and two rack-mounted PC’s, control all aspects of the apparatus including safety 

related interlocks and shutdown processes, remote valve actuation, pump speed, gas flow rate control, the 

ultrasonic power setting, all measured instrumentation data such as pressures and temperatures, and 

gaseous emissions analyzer readings. Figure 4 shows a photograph of the apparatus and the laboratory 

that was dedicated to this study. In figure 4, the PLC instrumentation rack is in the background along with 

the computer monitors used to control the software. The gas analyzer is on the left, the power supply and 

driver for the ultrasonic system is behind the computer monitor in the foreground, and a stereomicroscope in the foreground is focused on the optically accessible hammer and anvil flow reactor. Figure 

5 shows a closer view of the flow reactor setup where the optical access windows for the flow reactor can 

be seen; at the top of the flow reactor is the ultrasonic transducer mounted in a vertical position. 

Figure 4.—Photograph of the CIBC flow reactor system and laboratory. 
NASA/TM—2005-213599 10

Figure 5.—Photograph (detail) of the optically 

accessible flow reactor system. 

Results of Computer Model of CIBC Process 

We begin with a discussion of the results of the computational model of cavitation-ignition bubble 

combustion. We chose to model the process for a gaseous bubble in liquid water. The working fluid used 

in many hydraulic devices is similar to automatic transmission fluid (ATF). However, ATF is a widely 

varying complex mixture of mineral oils and many chemical additives designed to stabilize the oil, 

prevent oxidation, enhance lubricity, etc., and these additives complicate the interpretation of 

experimental results. Thus, for the purposes of demonstrating CIBC, we chose two single component 

liquids: water and methanol. Water has other advantages such as being clean and easy to work with, has a 
NASA/TM—2005-213599 11

high surface tension force, and is relatively inert from a combustion standpoint. We used methanol 

(CH3OH) as a hydrocarbon based working fluid because it too, is relatively clean and easy too work with. 

The acoustic or sonocavitation pressure wave that is used to drive the CIBC process is modeled as a 

far-field sine wave at 20 kHz, with an amplitude up to 1.2 atm. We further assume that the bubble rest 

radius is initially 4.5 μm (baseline case). The 4.5 μm radius is used as the baseline case so that the noncombustion results of the simulations can be compared with other studies (Storey and Szeri, 1999). 

Comparisons of the results of our baseline simulation (no chemistry) with these other studies showed 

excellent agreement. In order to model the chemical kinetics properly, we use methane-air as the reactant 

gases inside the bubble as there are well characterized and validated chemical kinetic mechanisms for 

methane-air chemistry (Bowman et al.). Basically, we are trying to remove as many unknowns and 

ambiguities in the problem as possible in order to distill the computational problem to a case in which the 

properties of both the liquid phase and gas phase are well known, or readily calculated or estimated. 

Effect of Pressure Amplitude on Peak Temperature 

Figure 6 shows the results of the CIBC simulation for a methane-air bubble with a stoichiometric 

equivalence ratio (φ = 1.0) and a 1.110 atm pressure amplitude forcing at 20 kHz for a 4.5 μm radius 

bubble in 350 K water. The upper plot shows the bubble radius and temperature as a function of time (out 

to 150 microseconds). In this plot we can see that the bubble oscillations are steady and repeatable from 

cycle to cycle. We can also see that there is the characteristic bubble “bounce” that decays within 10 

small oscillations at the end of the collapse due to the acoustic impedance mismatch between the bubble 

and the water. The lower plot shows the chemical species concentrations for two of the reactants: methane 

and oxygen. In figure 6 we can see that although the peak temperatures reach as high as 1800 K, there is 

little or no combustion as evidenced by the lack of consumption of either methane or oxygen. This is due 

to the short times (< 1 ns) that high temperatures persist, and this condition is insufficient to result in 

enough 

0

0.05

0.10

0.15

0.20

0.25

0 0.00005 0.00010 0.00015

O2

CH4

Time (sec)

Mole Fraction

0

0.0005

0.0010

0.0015

0.0020

0 0.00005 0.00010 0.00015

0

500

1000

1500

2000

Radius (cm)

Temp (K)

1.11 atm, Phi-1.0, R=4.5 microns

Figure 6.—Results of CIBC simulation for methane-air bubble in water 

where combustion does not occur (1.11 atm forcing amplitude). 
NASA/TM—2005-213599 12

molecular collisions to result in significant chain-branching chemical reactions that are required of 

combustion. Figure 7 shows the simulation for the same bubble, except that the pressure amplitude has 

been slightly raised to 1.115 atm, and at this condition, the peak temperatures are just above 2000 K. At 

this temperature, signs of chemical reactions are apparent as evidenced by the slight consumption of CH4, 

and the production of H2O, OH and CO at the peak temperature points. In figure 8, we now increase the 

pressure amplitude a little more to 1.14 atm, and we can see the resulting dramatic increase in the 

chemistry and what appears to be combustion resulting from the cavitational heating. From figure 8, we 

can also see that the peak temperatures are still continuing to rise with each major bubble collapse as 

more energy is released from the methane oxidation with each cycle. This can clearly be seen in the lower 

plot of figure 8 which shows the CH4 concentration dropping in a step-like function with each 

temperature peak. Similarly, the O2 is being consumed with each oscillation and H2O and CO are both 

being generated in a step-like manner. Note that because this condition is marginally enough to produce 

combustion, approximately 3 major oscillation cycles are required to completely consume the CH4. 

Similar step-like changes in chemical concentrations following each peak temperature spike were also 

reported by Gong and Hart (1998) in their simulation of a collapsing bubble with chemistry. 

0

0.05

0.10

0.15

0.20

0.25

0 0.00005 0.00010 0.00015

0

1x10-5

2x10-5

3x10-5

4x10-5

5x10-5

OH CO H2O

O2

CH4

Time (sec)

Mole FractionMole Fraction

0

0.0005

0.0010

0.0015

0.0020

0 0.00005 0.00010 0.00015

0

500

1000

1500

2000

2500

Radius (cm)

Temp (K)

1.115 atm, Phi-1.0, R=4.5 microns

Figure 7.—Results of CIBC simulation for methane-air 

bubble in water where combustion caused by bubble 

collapse has just started to occur (1.115 atm forcing 

amplitude). Note the small amount of H2O and OH 

being generated after each collapse. 
NASA/TM—2005-213599 13

0

0.05

0.10

0.15

0.20

0 0.00005 0.00010 0.00015

H2O

O2

CO CO2

CH4

Time (sec)

Mole Fraction

0

0.0005

0.0010

0.0015

0.0020

0.0025

0 0.00005 0.00010 0.00015

0

1000

2000

3000

4000

Radius (cm)

Temp (K)

1.14 atm, Phi=1.0, R=4.5 micron

Figure 8.—Results of CIBC simulation for methane-air 

bubble in water where combustion is just starting to occur 

(1.14 atm forcing amplitude). The upper plot shows the 

radius and temperature of the bubble, the lower plot 

shows the chemical species mole fraction for the major 

combustion products. Note that the methane is essentially 

completely consumed by the third cycle. 

Figure 9 shows the results of the CIBC simulation for the same bubble with a more powerful pressure 

amplitude of 1.19 atm. At this amplitude, the initial peak temperature is in excess of 4000 K, but notice 

that the subsequent peak temperatures are much lower (about 3700 K). This indicates that the majority of 

the heat or energy release occurred in the first collapse which had sufficient energy to oxidize all the CH4. 

However, due to the rapid cooling rates within the bubble, intermediate by-products of combustion such 

as CO are frozen and are slowly oxidized in subsequent bubble collapse cycles. From the change in the 

bubble radius resulting in the heat release after combustion (highlighted by the arrows), volumetric 

change in the bubble may provide a way of extracting useful ∫pdV work. Figure 9 also highlights another 

aspect of CIBC: the production of nitric oxide (NO) as a result of the thermal NOX mechanism. From the 

lower plot in figure 9, we can see that there is a significant amount of NOX generated with each collapse. 

Note that there does not even have to be combustion to have NOX as a plain air bubble subjected to the 

effects of cavitation will also produce it, as observed by (Wakeford et al., 1998). 

Figure 10 summarizes the effect of the acoustic pressure amplitude on the peak bubble temperature. 

From figure 10, we can see that acoustic amplitudes of less than 1.00 atm really have no effect on the 

cavitational heating of a bubble; above 1.05 atm the bubble peak temperature starts to rise rapidly, and 

above 1.12 atm chemical reactions that can be described as combustion start to take place. Our 

simulations show that bubble combustion as evidenced by the actual chemical production of combustion 

by-products does not happen in stoichiometric methane-air bubbles in water until a temperature of 2000 K 

is reached. The behavior of the peak temperature curve does not show an abrupt change at 2000 K, as the 
NASA/TM—2005-213599 14

effect is gradual. The result shown in figure 9 indicates that the ignition of the contents of a bubble 

depends on the peak temperature in a subtle way, and that the temperature change due to heat release is 

less than the temperature rise of the bubble resulting from cavitational collapse alone. The significance of 

this finding is that the flows used by hydraulic-CIBC devices must undergo a significant flow restriction 

in order to generate the necessary pressure drop to induce hydrodynamic cavitation with a high pressure 

amplitude differential to produce cavitation ignition. With hydrodynamic cavitation, a significant pressure 

drop, if not recuperated using devices such as a venturi, will cause a significant energy loss penalty. Thus 

it is highly unlikely that a hydrodynamic cavitation based CIBC engine will produce net power without 

the implementation of some form of pressure loss recovery system. 

0

0.05

0.10

0.15

0.20

0.25

0 0.00005 0.00010 0.00015 0.00020

0

0.02

0.04

0.06

0.08

0.10

NO

CO2 OH CO

H2O O2

CH4

Time (sec)

Mole FractionMole Fraction

0

0.001

0.002

0.003

0 0.00005 0.00010 0.00015 0.00020

0

2000

4000

6000

Temp Radius

Radius (cm)

Temp (K)

1.19 atm, Phi=1.0, R=4.5 microns

Figure 9.—Results of the computer model of CIBC with full 

chemistry. A stoichiometric (φ = 1.0) methane-air bubble in 

water with an initial radius of 4.5 μm is excited with 20 kHz 

sound waves at an amplitude of 1.19 atm. The lower plot 

shows the radius and temperature as a function of time; the 

upper plot shows the chemical species concentrations of 

several key molecules. Note how the methane fuel is 

completely consumed after the initial collapse which then 

generates CO2, H2O, CO, OH, and NO. The subsequent 

collapses serve to consume the CO that was left behind from 

the first cycle as a result of incomplete combustion. 

Chemical reactions only occur when the temperature is high 

(where OH is visible), which corresponds to the peak of the 

bubble collapse. 
NASA/TM—2005-213599 15

0

1000

2000

3000

4000

1.00 1.05 1.10 1.15 1.20

NO IGNITION

IGNITION

Pressure (atm)

Temp (K)

Figure 10.—Results of CIBC simulation for methane-air 

bubble in water showing the dependence of the temperature vs. far-field acoustic pressure. Ignition occurs at approximately 2000 K. 

Figure 10 summarizes the effect of the acoustic pressure amplitude on the peak bubble temperature. 

From figure 10, we can see that acoustic amplitudes of less than 1.00 atm really have no effect on the 

cavitational heating of a bubble; above 1.05 atm the bubble peak temperature starts to rise rapidly, and 

above 1.12 atm chemical reactions that can be described as combustion start to take place. Our 

simulations show that bubble combustion as evidenced by the actual chemical production of combustion 

by-products does not happen in stoichiometric methane-air bubbles in water until a temperature of 2000 K 

is reached. The behavior of the peak temperature curve does not show an abrupt change at 2000 K, as the 

effect is gradual. The result shown in figure 9 indicates that the ignition of the contents of a bubble 

depends on the peak temperature in a subtle way, and that the temperature change due to heat release is 

less than the temperature rise of the bubble resulting from cavitational collapse alone. The significance of 

this finding is that the flows used by hydraulic-CIBC devices must undergo a significant flow restriction 

in order to generate the necessary pressure drop to induce hydrodynamic cavitation with a high pressure 

amplitude differential to produce cavitation ignition. With hydrodynamic cavitation, a significant pressure 

drop, if not recuperated using devices such as a venturi, will cause a significant energy loss penalty. Thus 

it is highly unlikely that a hydrodynamic cavitation based CIBC engine will produce net power without 

the implementation of some form of pressure loss recovery system. 

Effect of Bubble Radius on Peak Temperatures 

We now examine the effects of variations in the initial bubble radius on the peak temperature, and 

hence, its effect on the ignition of the bubble contents. We start with the baseline case using a 

stoichiometric methane-air bubble being forced at 20 kHz with a 1.19 atm pressure amplitude. Figures 11 

and 12 show how the dynamic behavior of the bubble changes completely for increasingly larger bubble 

radii. In figure 11, the initial bubble radius is set at 20 μm, and this causes both the peak temperature and 

the maximum radius to vary significantly from cycle-to-cycle. Figure 12 shows the same simulation but 

with an initial bubble radius of 65 μm, here we can see that in addition to the peak temperatures and 

maximum radii being completely chaotic, the number of smaller bounce cycles is reduced and is also 
NASA/TM—2005-213599 16

0

0.002

0.004

0.006

0.008

0.010

0 0.00005 0.00010 0.00015 0.00020

0

1000

2000

3000

4000

5000

Time (sec)

Radius (cm)

Temp (K)

Figure 11.—Calculated variation of radius and temperature 

for a 20 μm initial radius methane-air bubble with 

φ = 1.0, at 1.19 atm and 20 kHz. Note the non-uniform, 

almost random behavior of the bubble. 

0

0.005

0.010

0.015

0.020

0.025

0 0.0002 0.0004 0.0006 0.0008

0

1000

2000

3000

4000

5000

Time (sec)

Radius (cm)

Temp (K)

Figure 12.—Calculated variation of radius and temperature 

for a 65 μm initial radius methane-air bubble with φ = 1.0, 

1.19 atm, 20 kHz. Note the non-uniform, chaotic behavior 

of the bubble. 
NASA/TM—2005-213599 17

chaotic, in that sometimes there are no after bounces. These figures show that in general, the dynamics of 

collapsing bubbles, coupled with the chemical interactions which release heat and affect the bubble 

dynamics, yields a highly non-linear system (see figs. 11 and 12). If we carry out these simulations for an 

extended time period (10 million points) or about 1 ms of actual bubble time, and record the peak bubble 

temperature, we can see that the variation in the peak temperature as a function of bubble radius as shown 

in figure 13 is highly non-linear. Figure 13 shows that for bubble radii ranging from about 2.5 to about 

150 μm, the peak bubble temperature is greater than 2000 K, which is sufficient for igniting the contents 

of the bubble. Figure 13 also indicates that for bubbles larger than about 20 μm in radius, multiple bubble 

oscillations are required to produce an ignition event (solid line). The dashed line represents the peak 

temperature from the first collapse, and this curve falls off significantly after about 

50 μm radius. In a hydraulic-CIBC process, the cavitation collapse occurs only once for each bubble as it 

passes through a flow restriction device that produces the cavitation (unless the bubble is forced past 

multiple flow restrictions to drop the pressure repeatedly). Therefore, the lesson of this finding is that for 

a hydrodynamic based CIBC process it is critical to have a precise and narrow distribution of bubble sizes 

that are within the size range that will lead to ignition events. For sonocavitation devices, where the 

bubble can be subjected to repeated cycles, the sizing and distribution of the bubble radius is less critical 

as shown in figure 13. 

0

2000

4000

6000

1 10 100 1000

Initial Radius (microns)

Temperature (K)

Figure 13.—Variation of maximum bubble temperature 

with initial bubble radius (solid line) for a φ = 1.0 

methane-air bubble in water with 1.19 atm forcing at 

20 kHz. Note that there for bubble radii ranging from 

about 2.5 to 150 μm temperatures will reach above 

1500 K, which should be sufficient for ignition. The 

maximum temperature of the first collapse is shown 

with the dashed line. For bubbles larger than 20 μm, 

more than one bubble oscillation is required to attain 

the maximum temperature. Thus, for the case of 

hydrodynamic cavitation where the bubble has only 

one cycle to ignite, bubbles have to be smaller than 

20 μm to achieve ignition. 
NASA/TM—2005-213599 18

Work Produced by CIBC Process and Effect of Equivalence Ratio 

The problem of extracting useful work or energy from the CIBC process, although it is not one of the 

primary questions to be answered in this study, is nonetheless important to examine from the standpoint 

of practical applications that could utilize CIBC. We use the computational model of CIBC to calculate 

the net-energy release (work output) per bubble and examine the effect of methane-air equivalence ratio 

to determine the optimum stoichiometry for methane-air bubbles in water. From this number we can then 

estimate the maximum volumetric power density of a CIBC device. As the temperature change in the bulk 

liquid resulting from the combustion heat-release in CIBC is negligible because of the small (1:1000) 

ratio of bubble density compared to liquid density, we have to assume that the energy released by a 

bubble resulting from CIBC must be harnessed through some other means than a net bulk temperature 

increase in the working fluid. As there is a change in the volume of the bubble after a CIBC event, and 

this change is impressed on the surrounding fluid and is sensible from a two-phase bulk fluid standpoint 

given enough bubbles (void fraction of say 10 percent), this volumetric change can potentially be 

harnessed and converted to useful work. This volumetric-change or boundary expansion work is given by: 

0

( ) () ()V t

V W t p t dV = ∫ , (16) 

where, p(t) is the time-varying pressure and the dV is the infinitesimal change in the volume of the bubble 

from an initial bubble volume V0 to the volume at time t given by V(t). We define a positive value as net 

energy out of the bubble, and negative values as energy into the bubble (required to expand or compress 

the walls of the bubble for instance). We calculate W(t) using a discrete summation of equation (16), and 

plot the results for different equivalence ratios as shown in figure 14. From figure 14, we can see that 

peak energy resulting from the bubble expansion (after the initial collapse and bubble oscillations) occurs 

-0.02

-0.01

0

0.01

0 0.2x10-4 0.4x10-4 0.6x10-4 0.8x10-4 1.0x10-4

Phi=4.0

Phi=1.25

Phi=1.05

Phi=0.5

Phi=0.25

Phi=0.0

Phi-1.05 - no combustion

Peak is 0.00322 erg energy release/bubble at Phi=1.25

Time (sec)

Energy (erg)

Bubble Combustion Integral PdV Work

1.19 atm, 20 kHz, 4.5 micron radius

Figure 14.—Results of the computer model showing the effect 

of equivalence ratio on the production of ∫PdV work. 
NASA/TM—2005-213599 19

at about 400 microseconds. Again, the simulations assume the baseline conditions of 4.5 μm bubble 

radius, 20 kHz acoustic frequency, and 1.19 atm pressure amplitude for a methane-air bubble in 350 K 

water. We can see that for fuel-lean values, the amount of energy required to induce the cavitation 

(pressure forcing) is greater than energy released as a result of combustion. In fact there is a break-even 

point at about φ = 0.5. Thus, this indicates that the CIBC process cannot produce net work for very fuellean fuel-air mixtures. 

If we plot the maximum energy release as a function of equivalence ratio, we see that the maximum 

energy release produced after the bubble bounce-oscillations subsides, is 3.22×10-3 erg ⋅ bubble–1 

(1 erg = 107 J), and occurs at φ = 1.25, as shown in figure 15. In conventional internal combustion 

engines, the peak energy release is typically very close to a slightly fuel-rich, but near stoichiometric 

(φ = 1.0) point. We now calculate the number of bubbles required to produce 7.46×109

 erg ⋅ sec–1 (746 W 

or 1 hp) given that the maximum energy release per bubble is 3.22×10-3 erg ⋅ bubble–1. These values, 

when combined, result in a value of 2.32×1012 bubble ⋅ sec–1, which translates to a requirement of a flow 

rate of 53 liter ⋅ min–1 of gaseous flow per 746 W (1 hp) assuming uniform 4.5 μm radius bubbles. If we 

further assume a bubble void fraction of 10 percent, then the liquid flow rate requirement per 746 W (hp) 

is 530 liter ⋅ min–1 ⋅ hp–1 or 140 gal ⋅ min–1 ⋅ hp–1. 

For a pump to produce a flow with a sufficient pressure differential of 1.2 atm (121 kPa or 17.6 psid) 

to produce ignition-capable cavitation, the pump will require at least 783 W (1.05 hp) of power input 

(Lee Co., 1997) assuming that the pressure drop is obtained through a conventional flow orifice with a 

27 percent pressure recovery factor (Baker, 2000). Although 783 W (1.05 hp) is only slightly higher than 

the 746 W (1.00 hp) input required to drive the CIBC process, we have not accounted for other losses 

such as friction, and inefficiencies. However, if a pressure-recovering device such as a cavitating venturi 

with an 85 percent pressure recovery is used (Hu et al., 1998), then the pressure drop experienced by the 

-0.004

-0.003

-0.002

-0.001

0

0.001

0.002

0.003

0.004

0 1 2 3 4

Equivalence Ratio

Peak Work Output (erg/bubble)

Peak Work Output vs. Equivalence Ratio

CH4-Air, 4.5 micron radius, 20 kHz, 1.19 atm

Figure 15.—Results of the computer model showing the effect 

of equivalence ratio on the peak energy output per bubble. 
NASA/TM—2005-213599 20

pump will be less than 21 kPa (3 psid), which translates to a pump power requirement of only 164 W 

(0.22 hp). Thus, if the volumetric bubble expansion power extraction from the CIBC process was perfect, 

a minimum liquid pump flow rate of about 114 liter ⋅ min–1 (31 gal ⋅ min–1) with a pressure drop of about 

21 kPa (3 psid) would be required for the engine to operate in a break-even or self-sustaining mode. The 

estimated specific volumetric flow rate from the above values is approximately 0.7 liter ⋅ min–1 ⋅ W–1. 

More importantly though, this analysis reveals a very important finding: a hydraulic-CIBC engine cannot 

run in a self-sustaining mode unless venturi-type pressure restrictors are used to recover the excess 

pressure drop. The flow values above represent the minimum conditions required to develop a selfsustaining hydraulic CIBC device. 

For a system operating on a hydrocarbon-based oil such as ATF, the value would be significantly 

different as the vapor pressure of oil based liquids is significantly lower, and this would mean that higher 

liquid flow rates (and higher pump input power) would be required to produce a net power output. 

However, the values calculated above using a methane-air bubble in water are good starting point for 

future estimates. 

Experimental Evidence of Bubble Combustion 

We performed several experiments using the previously described apparatus in an attempt to obtain 

data that would indicate the existence of CIBC. Perhaps the most conclusive evidence of bubble 

combustion would be the release of a gaseous emission consistent with the products of combustion such 

as CO2, CO, or NOX. Other indicators of combustion would include the emission of light, the generation 

of heat, and even the generation of a sound. However, the solubility of both CO2 and NOX in liquid water 

and methanol is high (England, 1986), and the most likely molecule for an observable gaseous emissions 

measurement is CO. We attempted to measure the emission of visible light (400 to 700 nm) from the 

CIBC process using a lens-fiber-coupled spectrograph and high sensitivity CCD camera, similar to the 

one used by Kojima and Nguyen (2003) for spontaneous Raman scattering studies. Although this 

spectrometer/CCD camera combination has single-photon sensitivity, the optical emissions in the visible 

spectrum were nonetheless, very faint. Thus the results were inconclusive so we do not present them here. 

We noticed an intermittent but audible change in the sound that the apparatus made when the system 

was operated with combustion-capable gas mixtures. We then mounted a small piezoresistive pressure 

transducer (200 kHz bandwidth, 0.07 kPa sensitivity) to a stainless steel window blank affixed to the 

acoustic reaction chamber portion of the flow reactor. The transducer or microphone was mounted so that 

the sensor tip protruded 3 mm past the blank plate surface. The output from the microphone was then 

sent to an oscilloscope for storage and analysis. The sound traces showed distinct high amplitude (up to 

1.5 atm) pressure peaks. The analysis of the acoustic signature data from the microphone however, proved 

inconclusive due to the intermittent nature of the signal and its irreproducibility. The intermittent nature 

of the sounds generated by the sonication process probably resulted from an inefficient coupling of the 

acoustic energy into the bubbles. Ideally, the bubble diameters should fall within the distribution shown in 

figure 13 for efficient acoustic energy transfer. We speculate that an acoustic impedance mismatch 

between the wavelength of the sonication waves and the bubble sizes resulted from the rather wide 

distribution of the bubbles. This was confirmed by our measurements of the bubble sizes discussed in the 

following paragraph. 

The 7 μm pore bubbler element when used with distilled water produced rather large bubbles that 

ranged from approximately 200 μm to 2 mm in diameter (measured using microphotographs taken with a 

stereomicroscope and digital camera combination). This was due to two causes: small bubbles in pure 

water coalesce readily to form larger bubbles, and the co-flowing liquid water velocity past the bubbler 

pores was too slow to quickly detach the small bubbles from the surface as they were being filled by the 

exiting gas flow. According to the study performed by Chuang and Goldschmidt (1970) co-flowing water 

velocities of about 2 m ⋅ sec–1 are required to permit the generation of the smallest bubble sizes, whereas 
NASA/TM—2005-213599 21

the co-flow velocities in our study were much lower (0.01 m ⋅ sec–1). Thus, several things can be done in 

future studies to improve the bubble generation process: a salt such as MgSO4 can be added in small 

quantities to increase the short range hydration repulsion forces between two gas-liquid interfaces to 

prevent bubble coalescence (Zenit et al., 2001), and the velocity can be increased substantially by 

providing a direct impingement jet of water onto the bubble generator. Due to the rather large bubbles 

created by the bubble generator used in this apparatus, the creation of the smaller bubbles conducive to 

ignition were most likely generated as a result of the acoustic cavitation process which breaks larger 

bubbles into successively smaller ones until they are small enough to undergo CIBC. 

Figure 16 shows the measured CO emissions data for propane-air bubbles in water. Here, propane 

was used as the fuel because it is inherently easier to burn than methane. A mixture of propane air with an 

equivalence ratio of φ = 0.95 and φ = 1.27 was injected into the system with a water flow rate of 1.0 liter ⋅

min–1 through a 9.5 mm anvil gap. Approximately 50 W of acoustic power was delivered to the probe tip 

to produce cavitation. From figure 16 we can see that upon the introduction of the propane gas, the 

generation of CO is immediately visible and steadily climbs and then exponentially levels off. This effect 

is a result of the time required to fill the void space (approx. 4 liter) above the liquid sump tank. At about 

90 minutes, the acoustic power was increased to about 100 W, and we immediately see that this makes the 

rate of CO production increase. The CO levels continue to rise, and at about 110 minutes, we increase the 

equivalence ratio to φ = 1.27. Again, we see the CO emissions rise as a result of more CO being generated 

at fuel-rich equivalence ratios compared to fuel-lean, and reach a peak value of about 160 ppm (2.5 ppm 

accuracy). At about 120 minutes, the acoustic horn suffers overheating and the power output drops 

resulting in a reduced CO emissions. Clearly, there is a direct correlation between the acoustic cavitation 

and the production of CO. As shown in figure 17, the measurement of CO production and how it tracks 

acoustic power or acoustic pressure amplitude is consistent with the computer model of CIBC. 

0

40

80

120

160

200

0 15 30 45 60 75 90 105 120 135

Acoustic Horn

Overheating

Air Flow Increase 200 to 300 sccm

Propane Flow Increase 8 to 16 sccm

Propane Flow Turned On

50% to 75% Acoustic

Power Increase

Time (minutes)

CO (ppm)

Figure 16.—Experimental data showing the production of 

CO as a function of time for propane-air bubbles in 

water with acoustic cavitation. The water flow rate 

was 1.0 liter/min through a 9.5 mm wide anvil gap. 

Note that a 75 percent acoustic power setting 

corresponds to approximately 100 W of acoustic 

power delivered at the probe tip. The equivalence ratio 

was set at φ = 0.95 initially and increased to φ = 1.27. 
NASA/TM—2005-213599 22

10-8

10-6

10-4

10-2

1.10 1.12 1.14 1.16 1.18 1.20

Pressure (atm)

Peak CO Mole Fraction

Figure 17.—Results from the computer model of CIBC 

showing the production of CO as a function of 

acoustic pressure (20 kHz) for a 4.5 μm radius 

methane-air (φ = 1.0) bubble in water. 

We finally examine the effect of sonication on air bubbles in a hydrocarbon liquid. We use methanol 

as the liquid for the previously stated reasons, and flow it through the apparatus with conditions similar to 

water. However, we decreased the temperature of the methanol by using the chiller in order to reduce the 

effect of methanol vapor evaporation and condensation on the gaseous sampling line. At a temperature of 

15 °C, the saturation properties of methanol in air result in a bubble with an equivalence ratio of 0.67. 

Although this is a fuel-lean case at the liquid temperature, it is expected that the higher temperatures 

inside the bubble will produce a more fuel-rich bubble prior to the time of ignition. We were not able to 

simulate the methanol-air bubble using the computer model due to the lack of a chemical kinetic 

mechanism for methanol-air chemistry. However, the experiment yields a result similar to the data 

acquired for propane-air bubbles in water. The important distinction here is that the fuel vapor is now 

provided by the working liquid and is not injected as a gas bubble. Figure 18 shows the experimental 

results of CIBC for air bubbles in methanol flowing at 1.0 liter ⋅ min–1 through a 9.5 mm anvil gap. 

Between 100 W and 140 W of acoustic power was delivered to the probe tip with a mixture of either air 

or a combination of argon-air. The use of argon generally provides higher adiabatic compression 

temperatures as indicated by earlier studies (Barber et al., 1997). From figure 18 we can see that as there 

is a definite correlation between the generation of CO and the acoustic power as the CO emissions drops 

towards zero when the acoustic power is cut off at about 130 minutes. The peak value of the CO (35 ppm) 

was less than the case using CH4-air bubbles in water, and this can be partially attributed to the fuel-lean 

equivalence ratio (φ = 0.67) of the methanol-air mixture. 
NASA/TM—2005-213599 23

0

10

20

30

40

0 50 100 150 200 250

250 SCCM Air

150 SCCM Argon

100% Acoustic Power

250 SCCM Air

150 SCCM Argon

0% Acoustic Power

250 SCCM Air

150 SCCM Argon

75% Acoustic Power

250 SCCM Air

75% Acoustic Power

350 SCCM Air

75% Acosustic Power

Time (minutes)

CO (ppm)

Air Bubbles in Methanol (22 C), 1 Liter/min

Figure 18.—Experimental data showing the production of 

CO as a function of time for air bubbles in methanol 

with acoustic cavitation. Methanol flow rate was 

1.0 liter ⋅ min–1 through a 9.5 mm anvil gap. Methanol 

temperature was maintained at 15 C which provides a 

saturation equivalence ratio of φ = 0.67. 

Conclusions 

The principle of using a collapsing bubble to effect ignition and combustion has been studied by both 

experiments and computational simulations of the cavitation-ignition bubble combustion (CIBC) process. 

A review of the literature related to hydraulic cavitation revealed that the micro-diesel effect (or the 

ignition of oil vapor mixtures within cavitating bubbles) has been known since the late 1960s. 

For this study, we utilized ultrasonically induced cavitation (sonication) to initiate combustion in 

gaseous hydrocarbon-air bubbles in liquid water, and methanol vapor-air bubbles in liquid methanol. An 

ultrasonic cavitation flow reactor developed for this study provided a means of studying CIBC in a 

controlled manner with instrumentation and optical diagnostics. The apparatus provided a concentrated 

and uniform pressure field for acoustically exciting the two-phase flow that underwent cavitation. Visible 

light emission from CIBC was not found to provide any conclusive information regarding the existence of 

CIBC. The most conclusive proof of combustion came from the gaseous emission measurements: up to 

160 ppm of carbon monoxide (CO) was found to be emitted as a byproduct of the gas-phase chemical 

reactions or combustion within the collapsing bubbles. The CO production occurred for both propane-air 

bubbles in water and for air bubbles in methanol, and was proportional to the acoustic power level 

delivered to the CIBC flow reactor. 

In parallel with the experiments, we computationally modeled the CIBC process using a 1-D timedependent compressible flow fluid dynamics code that included a comprehensive finite-rate chemical 

kinetics model. The computational model of the CIBC process indicated that gas-phase reactions within 

the bubble occurred and produced CO, CO2, NOX, and other gases. In addition, heat and mechanical 
NASA/TM—2005-213599 24

energy were released through a bubble volume expansion phase that occurs in the combusting bubble. 

The model showed that the CIBC process is sensitive to the fuel-air mixture ratio, the initial bubble 

diameter, the acoustic pressure forcing amplitude, and fluid properties. The results of the model were 

found to be consistent with the measured experimental results. In particular, the increase in the acoustic 

pressure (or sound intensity) resulted in a higher production of CO emissions. 

For the simulations, we used a baseline case of 4.5 μm radius methane-air (φ = 1.0) bubbles in liquid 

water undergoing a far-field acoustic pressure excitation of 1.19 atm at a frequency of 20 kHz. From this 

case we varied parameters such as far-field pressure, bubble radius, and equivalence ratio to examine their 

effects on bubble ignition and power output. Specifically, the model predicts that bubbles do not undergo 

ignition unless the internal bubble temperature reaches 2000 K, which corresponds to a far-field pressure 

of about 1.12 atm. The model also predicts that for a 20 kHz excitation frequency, the bubble diameters 

have to be within a certain size range (5 to 300 μm diameter for multiple acoustic cycle oscillations) in 

order for ignition to occur efficiently. This result is further restricted to bubbles smaller than 40 μm in 

diameter in the case of hydrodynamic cavitation as it occurs only once as a bubble passes through a 

pressure-dropping device such as an orifice. We also discovered that in order for net volume expansion 

work to be generated, the equivalence ratio must be greater than 0.5. Thus very fuel-lean mixtures cannot 

be used. 

We found that in theory, it may be possible to develop a hydraulic-CIBC engine that produces net 

power using methane-air bubbles in water. The results of the model indicate that such a device, would 

require the following conditions to operate at a breakeven (self-sustaining mode): approximately 10 μm 

diameter methane-air bubbles in water, approximate φ = 1.25, a liquid flow rate of at least 114 liter ⋅ min–1

(31 gal ⋅ min–1) with a 10 percent bubble void fraction, a total pressure drop of less than 21 kPa (3 psid) 

provided by a venturi or other device with a pressure recovery factor of at least 85 percent, and no other 

losses in the device such as friction. Without the pressure recovery provided by the venturi, the hydraulic 

pump power required to provide conditions suitable for cavitation ignition is greater than the power that 

can be extracted from the resulting CIBC. 

The above conclusions can be used as a basis to estimate the potential performance of a hydraulicCIBC engine prototype that uses ATF as the working fluid. It is the opinion of the authors that such an 

engine prototype, in principle, may be able to produce net mechanical power if optimal flow conditions 

(low pressure drop), and optimal bubble ignition conditions (correct equivalence ratio control and bubble 

diameters) are satisfied, and an efficient power extraction process capable of harnessing the volumetric 

expansion work generated by the combusting bubble is developed. 

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NASA/TM—2005-213599 25

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NASA/TM—2005-213599 27
NASA/TM—2005-213599 28
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WBS–22R–722–96–0224–03

34

A Study of Cavitation-Ignition Bubble Combustion

Quang-Viet Nguyen and David A. Jacqmin

Cavitation flow; Combustion chemistry; Bubbles; Fuels; Kinetics; Mathematical models

Unclassified -Unlimited

Subject Categories: 23, 25, 28, 64, 70, 71, and 77

Responsible person, Quang-Viet Nguyen, organization code RTB, 216–433–3574.

We present the results of an experimental and computational study of the physics and chemistry of cavitation-ignition

bubble combustion (CIBC), a process that occurs when combustible gaseous mixtures are ignited by the high temperatures found inside a rapidly collapsing bubble. The CIBC process was modeled using a time-dependent compressible

fluid-dynamics code that includes finite-rate chemistry. The model predicts that gas-phase reactions within the bubble

produce CO and other gaseous by-products of combustion. In addition, heat and mechanical energy release through a

bubble volume-expansion phase are also predicted by the model. We experimentally demonstrate the CIBC process

using an ultrasonically excited cavitation flow reactor with various hydrocarbon-air mixtures in liquid water. Low

concentrations (< 160 ppm) of carbon monoxide (CO) emissions from the ultrasonic reactor were measured, and

found to be proportional to the acoustic excitation power. The results of the model were consistent with the measured

experimental results. Based on the experimental findings, the computational model, and previous reports of the

“micro-diesel effect” in industrial hydraulic systems, we conclude that CIBC is indeed possible and exists in

ultrasonically- and hydrodynamically-induced cavitation. Finally, estimates of the utility of CIBC process as a

means of powering an idealized heat engine are also presented.