Marit Meyer

Glenn Research Center, Cleveland, Ohio

Design of a Thermal Precipitator for the

Characterization of Smoke Particles From

Common Spacecraft Materials

NASA/TM—2015-218746

June 2015
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Marit Meyer

Glenn Research Center, Cleveland, Ohio

Design of a Thermal Precipitator for the

Characterization of Smoke Particles From

Common Spacecraft Materials

NASA/TM—2015-218746

June 2015

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
Acknowledgments

Marit Meyer: Research of previous devices, aerosol physics calculations, general design and geometry parameters, fi nite element 

modeling resulting in fi nal design parameters, initial testing and fi nal use in smoke experiments. Daniel Gotti (National Center 

for Space Exploration Research, NSCER): CAD design and modeling in ProE, thermal analysis and thermal design, packaging, 

hardware selection and assembly of TP, instrumentation with thermocouples, assistance with initial testing. Victoria Bryg (National 

Center for Space Exploration Research, NSCER): Choice & preparation of microscopy substrates, microscopy (SEM and TEM).

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NASA/TM—2015-218746 1 

Design of a Thermal Precipitator for the Characterization of 

Smoke Particles From Common Spacecraft Materials 

Marit Meyer 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

A thermal precipitator (TP) was designed to collect smoke aerosol particles for microscopic analysis 

in fire characterization research. Information on particle morphology, size and agglomerate structure 

obtained from these tests supplements additional aerosol data collected. Modeling of the thermal 

precipitator throughout the design process was performed with the COMSOL Multiphysics finite element 

software package, including the Eulerian flow field and thermal gradients in the fluid. The COMSOL 

Particle Tracing Module was subsequently used to determine particle deposition. Modeling provided 

optimized design parameters such as geometry, flow rate and temperatures. The thermal precipitator was 

built and testing verified the performance of the first iteration of the device. The thermal precipitator was 

successfully operated and provided quality particle samples for microscopic analysis, which furthered the 

body of knowledge on smoke particulates. This information is a key element of smoke characterization 

and will be useful for future spacecraft fire detection research. 

Introduction 

Characterization of the smoke from pyrolysis of common spacecraft materials provides insight for the 

design of future smoke detectors and post-fire clean-up equipment on the International Space Station. 

Real-time measurements of smoke particles in terms of number and mass concentration are useful, 

however, a wealth of additional information can be obtained by microscopy techniques. Magnified 

images show morphology, degree of particle agglomeration and also provide insight into particle 

formation mechanisms. Further techniques such as energy dispersive X-ray spectroscopy (EDS) give 

elemental composition of individual particles. An appropriate sampling method must be used to obtain 

quality microscopy results, and thermophoretic sampling directly on microscopy substrates is preferred to 

minimize handling and potential contamination. Thus, an appropriate thermal precipitator (TP) was 

created to augment existing spacecraft smoke data. 

The thermal precipitator design was targeted towards smoke particles from typical oxidative pyrolysis 

tests at NASA’s White Sands Test Facility (WSTF) conducted to challenge and verify performance of 

post-fire cleanup equipment under development for spacecraft (Meyer et al. 2013). These tests were 

characterized by aerosol mass and number concentrations ranging from 40 to 70 mg/m3

 and 1105 to 

1106

 particles/cm3 and particle diameters ranging from 100 to 1000 nm. The goal of characterizing the 

smoke particles by microscopy was twofold: to verify that a repeatable fire challenge was produced in the 

facility and to determine the particle morphology and elemental composition resulting from various fuels 

and temperatures. The intention was not to perform a size distribution analysis, so numerical modeling 

was restricted to the 100 to 1000 nm particle size range. Sizes outside of this size range were captured, as 

seen in the microscopy results, however, there was no attempt to calculate deposition efficiency of the 

device beyond the design range. There was also no requirement to make the TP compact, low power or 

portable, in the first iteration effort. 
NASA/TM—2015-218746 2 

Existing Thermal Precipitator Designs 

At the time of the early TP design phase, there was no thermal precipitator commercially available, 

and it was determined that an original device should be created and tailored to WSTF smoke testing. A 

number of references were consulted in order to gain insight from successful instruments, and some 

design rules-of-thumb emerged based on the thermal precipitators listed in Table 1. Note that the devices 

are listed in order of increasing particle size ranges which were either documented as design goals or the 

range of particle sizes tested in experiments reported in the references. 

The basic design trade is between the magnitude of the thermal gradient and the flow rate—which 

determines residence time and thus deposition efficiency of the desired size ranges. Notional geometry of 

the phenomenon of thermophoretic deposition is shown in Figure 1, which shows particles traveling 

axially through a thermal gradient directed perpendicularly to the flow path between two parallel flat 

surfaces. In a thermal precipitator, the thermophoretic force acting on the particle must drive it to the cold 

collection substrate before it reaches the end of the thermal gradient region in the device. An added 

complication for the smallest particles is the effect of diffusion, which causes particle losses that increase 

with residence time in the TP, and simultaneously, residence time is also increased by slip. This is evident 

from the first three devices listed in the table, which have extremely high flow rates for short residence 

times, coupled with very small gaps and the largest thermal gradients, for maximum thermophoretic force 

leading to deposition of smaller particles. 

Several designs in Table 1 use a transmission electron microscope (TEM) grid as the collection 

substrate to facilitate microscopic analysis. A larger collection substrate allows more latitude in the design 

parameters for thermal gradients and flow rates required for high collection efficiencies. The TP designs 

of Azong-Wara and Tsai and Lu use larger collection substrates (4 and 74 times larger area than a TEM 

grid, respectively) and thus can tolerate lower flow rates and a smaller thermal gradient and still achieve a 

sufficient residence time for high deposition rates. Collecting particles directly on a microscopy substrate 

is preferable, so the goal of this TP design was to use a scanning electron microscopy (SEM) specimen 

holder, called a stub, integrated into the cold surface. 

The TP design for smoke particles has a much larger desired size range than the other devices in 

Table 1. Since the aerosol physics influencing thermal precipitator design is determined based on the 

Knudsen number, the dynamics of larger particles in a thermal precipitator can be modeled using the 

TABLE 1.—DESIGN PARAMETERS OF OTHER THERMAL PRECIPITATORS IN THE LITERATURE 

Reference Thermal gradient, 

K/mm 

Gap, 

mm 

Flow rate, 

cc/min 

Intended particle 

size range, 

nm 

Collection substrate 

Wen & Wexler 2007 500 0.1 1500 less than 10 TEM grid 

Maynard 1995 1000 0.15 1010 less than 100 TEM grid 

Lorenzo et al. 2007 400 0.3 2000 15 to 300 TEM grid 

Azong-Wara et al. 2009 15 1.25 2 up to 300 206 mm plate 

Tsai & Lu 1995 83.7 and 50.3 0.38 400 40 to 500 37.1 cm plate 

Current TP design 56 1.25 110 100 to 1000 SEM stub with TEM grids 
NASA/TM—2015-218746 3 

following formulas. When the particle size is larger than the mean free path of the surrounding air 

(dp  λ, or Kn is small), then the thermophoretic force on a particle, Fth, is given by (Hinds, 1999) 

g p

p

th T

d H T

F



     29 2

 (1) 

where μ is the viscosity of air, ρg is the density of the surrounding gas (air), Tp is the particle absolute 

temperature (the temperature of the aerosol, typically considered to be ambient temperature) and T is the 

thermal gradient in the gas. The coefficient H accounts for a temperature gradient within the particle and 

thus depends on particle diameter and is given by 

























  

 





















  

p p

a

p p

a

p k d

k

k d

k

d

H

1 2 8.8

4.4

6 1

1 (2) 

where ka is the thermal conductivity of the air and kp is the thermal conductivity of the particle. 

The thermophoretic velocity for particle diameters larger than the mean free path of air is given by 

(Hinds, 1999) 

g p

c th T

C H T V



    23 (3) 

where the Cunningham slip correction factor, Cc is included. Ambient pressure is an influencing variable 

by way of the gas density in the force and velocity equations. Thus, the thermophoretic force was 

enhanced by approximately 20 percent at WSTF, considering the effect of lower pressure, which is 

85150 Pa (0.84 atm) owing to the altitude in Las Cruces, New Mexico. To estimate the thermal 

conductivity of pyrolysis smoke particles, they were assumed to consist of re-condensed polymer vapors 

on small primary particles. An average of nine representative WSTF fuel polymer materials ranging from 

0.12 to 0.3 W/m-K was used to obtain the value kp = 0.19 W/m-K used in calculations and modeling. At 

larger sizes, the effect of the particle material is more pronounced. Figure 2 shows calculations of the 

thermophoretic force (Eq. (1)) versus particle size, and it is evident that the thermal conductivity of salt 
NASA/TM—2015-218746 4 

particles, which has approximately 25 times the thermal conductivity of air, results in a smaller force on 

particle sizes larger than 500 nm, when compared to the polymer smoke. Particle temperature is assumed 

to be equilibrated with the typical WSTF smoke chamber temperature of 40 C (313 K). 

Finite Element Modeling to Narrow Design Space 

A design space consists of all possible configuration options to achieve an engineering outcome, 

which is narrowed or optimized based on constraints of the laws of physics and available hardware to 

generate conditions required to achieve the outcome. Finite element modeling is a rapid and cost-effective 

method of exploring design spaces, so that interconnected design parameters can be optimized virtually, 

and fully functioning hardware can be built on the first iteration. This approach was used in the TP 

design, with the COMSOL Multiphysics software package, version 4.2. This product is a finite element 

analysis (FEA) solver which provides the ability to couple various physical phenomena described by the 

term “multiphysics.” 

First, an Eulerian numerical model was created to represent laminar fluid dynamics and the heat 

transfer in the fluid of the device. Subsequently, the COMSOL Multiphysics Particle Tracing Module was 

used to apply user-defined forces on particles for Lagrangian simulations based on previous Eulerian 

solutions. Thus, the iterative design approach was to solve the Eulerian model, then calculate 

thermophoretic force as a function of particle size, parametrically modify the particle forces in the model 

(including slip-corrected Stokes drag) and simulate monodisperse particle deposition in the TP geometry 

through multiple simulations over the particle size range of interest. After a number of modeling 

iterations, the design space was suitably narrowed and one of many possible optimized solutions was 

chosen for the final TP configuration. 

The COMSOL fluid flow interfaces uses the Navier-Stokes equations, for incompressible flow 

consisting of conservation of mass 

 u  0 (4) 

(where u is the three-dimensional fluid velocity vector) and conservation of momentum 

  p t

      





 u u u

u

f f

2 (5) 

where p is pressure. 

The COMSOL heat transfer in fluids model uses the simplified heat equation 

C T   k T

t

T

gCp  g p    a





 (6) 

where Cp is the fluid heat capacity at constant pressure and T is the absolute temperature of the air. 

The particle tracing module uses Newton’s second law to define the motion of a particle, which 

includes the two important user-defined forces for the TP 

th d dt

d

m F F x   2

2

 (7) 

where the thermophoretic force is in the direction of the thermal gradient, given by Equation (1). Fd is the 

Stokes drag given by 

 

c

p p

d C

d u u

F    3

 (8) 
NASA/TM—2015-218746 5 

where up is the three-dimensional particle velocity vector, defined as the fluid velocity in the initial 

conditions (at time = 0 of the particle trajectory). No additional body forces (such as gravity) were 

imposed on the particles. 

The TP geometry had no requirements, other than starting and ending with connectors for 1/4-in.-

tubing diameter. This was approximated by a square in the model, as it had no bearing on the 

computational fluid dynamics at the scale of interest for this project. Achieving laminar flow through a 

very narrow gap was the goal that dictated the shape of the TP, and the solution was to use a cubic spline 

function in the COMSOL geometry tools to gradually widen the body as the height was reduced to a 

uniform gap on the order of a millimeter. Typical WSTF test duration of approximately 20 min was a 

design requirement for sufficient sampling, so other variables such as flow rate, temperature boundary 

conditions, gap height and overall geometry could be manipulated to achieve the desired particle 

residence time in the gradient to provide adequate deposition concentration on the SEM stubs in that 

length of time. 

Figure 3 shows the flow field inside the long and narrow TP body indicating that the velocity is 

reduced from a fully developed parabolic flow inlet condition to a uniform parabolic profile in the gap 

(color bar has units of m/s). The initial estimate of a suitable gap height was based on spreadsheet 

calculations of the thermophoretic velocity for particles from 100 to 1000 nm and the time required for a 

particle to travel the entire gap at that speed. This provided insight on the range of residence times 

required for deposition of different particle sizes and narrowed the choice of gap heights. The Reynolds 

number 

















  h uD Re was calculated to determine whether the internal TP flow is in the laminar regime. 
NASA/TM—2015-218746 6 

The hydraulic diameter, Dh, is twice the height of the TP body, as the fluid can be considered to be 

flowing between two plane parallel surfaces where the gap height is much less than the width. For all 

design iterations, Re was well below the critical value of 2300 for the full range of each design parameter 

considered, typically below 50. 

Simple temperature boundary conditions were applied to simulate various cold and hot plate areas in 

the TP model, creating a gradient on the order of 50 to 100 C/m, an initial estimate based on the design 

of Tsai and Lu (Table 1). Although a more detailed thermal model can easily be made in COMSOL, there 

was no benefit for the TP geometry design iterations. Subsequent thermal design was perfected through 

experiments and thermocouple measurements, as contact conductances between parts of the final 

assembly were unknown. The temperature gradient is the only Eulerian model input needed for the 

thermophoretic force which was applied in the z-direction (vertical axis in figures) using Equation (1). 

Stokes drag (a function of the three-dimensional velocity field solved in the Eulerian simulation) was 

applied on particles in the x- and y- and z-directions according to Equation (8). 

Flow field iterations were used as inputs for particle trajectory simulations corresponding to 100 nm, 

500 and 1000 nm spherical particles subjected to Stokes drag and material-specific thermophoretic forces. 

Figure 4 shows COMSOL Lagrangian particle trajectory simulation results with moving particles 

(left) and deposition pattern (right) of 1000 particles with 100 nm diameter, accounting for particle 

thermal conductivity in the vertical thermophoretic force and three-dimensional Stokes drag. The 

deposition pattern does not reach the SEM stubs (circles), so this intermediate set of design parameters 

was not accepted. Subsequent to this simulation, the thermal gradient was reduced in the model so that 

particle deposition would be delayed and cover the circular SEM stubs further downstream. 

Overall, COMSOL modeling narrowed the design space significantly. Simulation results showed that 

an elongated TP body and a correspondingly longer section of constant thermal gradient would allow for 

longer residence time and deposition of the size range of interest. Additionally, placing two stubs in series 

provided the ability to collect a larger size range of particles. Final variables in the TP design are given in 

the last entry of Table 1. The COMSOL model was considered a design tool, not an end in itself, and once 

the parameters were defined within the realm of available heaters, coolers and pumps, focus was placed 

on the physical TP design and testing to complete the project. 
NASA/TM—2015-218746 7 

Mechanical and Thermal Design 

The thermophoretic force on a particle subjected to a temperature gradient is directed towards the 

cold surface, which in this TP design is the collection substrate for microscopy, a 15 mm diameter 

aluminum SEM specimen mount (Hitachi M4) with threaded hole. By making two stubs in series, the 

collection area is increased and thus a larger range of particle sizes are deposited. A portion of each stub 

surface was covered with a strip of carbon two-sided sticky tape in the flow direction, in order to attach 

3.01 mm TEM grids enabling multiple types of electron microscopy. The carbon tape provided an 

alternate surface for particle deposition in addition to the aluminum stub surface. Carbon tape backing 

was peeled back slightly to adhere the TEM grids and then pressed back down to maintain cleanliness of 

the tape surface until testing took place. The tape backing was removed prior to each sampling test after 

the new clean stub was installed. 

Large thermal gradients in small devices can be achieved using a thermoelectric cooler, also known 

as a Peltier cooler or heater. A layer of semiconductor pellets between two outer plates create a heat flux 

between the junction of two different types of materials (N and P-type semiconductor pellets), creating a 

hot side and a cold side of the Peltier device, which in this thermal precipitator, was used for the cold 

particle collection surface. Therefore, in this work it is referred to as a thermoelectric cooler. In the TP, 

there are three circular thermoelectric coolers in parallel, one for each of the two SEM stubs and one on 

the cold plate which surrounds the stubs (upstream of the stubs). Based on numerical modeling, a 70 C 

thermal gradient was required to deposit particles in the desired size range, so a variable autotransformer 

was used to adjust the current from a voltage-limited power supply until the desired thermal gradient was 

achieved, with best results achieved after a warm-up period of 30 min or less. Opposite the cold surface of 

the thermoelectric cooler was the hot side where the waste heat removed from the cold side was pumped. 

The efficiency of heat removal from this area is directly proportional to the cold temperatures that can be 

achieved on the cold side. Therefore a heat sink with a large fin assembly was designed to remove the 

heat, and ultimately required a muffin fan to improve the rate of removal by forced convection (Fig. 5). 

The cold plate was chosen as the upper surface to take advantage of natural convection in removing heat 

from the back of the thermoelectric coolers, so the thermophoretic force on particles is not assisted by 

gravity in this design. Spacers were added above the SEM stubs, according to recommendations by 

thermoelectric cooler manufacturer, to create more distance between the intended cold surface and the hot 

surface attached to the fins which eject heat. Spacers are shown in grey in the CAD model image in 

Figure 6. Contact conductances between the segments were improved by using thermal grease, and 

insulation was added to both sides of the cold plate to reduce heat transfer in the plane of the TP body. 
NASA/TM—2015-218746 8 

A Kapton (DuPont, Wilmington, DE) heater was connected to the hot plate with thermal grease 

opposite of the cold aluminum plate and stubs. There was no direct control of temperature as the required 

thermal gradient was consistently achieved once the heat removal was improved by forced convection. 

Permanent type K thermocouples were installed at the hot plate and the cold plate to provide real-time 

information on the gradient and to indicate when the thermal precipitator had fully warmed up (reached 

equilibrium). Preliminary tests were performed with 1000, 670, and 105 nm polystyrene latex (PSL) test 

aerosol spheres and with polydisperse NaCl aerosol and microscopy verified that good deposition was 

obtained for all sizes, which validated the thermal design of the precipitator. Based on Figure 2, the 

thermophoretic force on NaCl particles is expected to be weaker than that of polymer pyrolysis smoke 

particles, so improved collection efficiency was expected in the actual smoke testing. Additional extended 

durations of testing were performed with a zero filter (small HEPA filter) at the inlet of the thermal 

precipitator and a condensation particle counter (CPC) at the outlet in order to verify that particles were 

not generated from within the device itself. In this test, particle-free air entered the TP and the aerosol 

number concentration measured by the CPC was verified to be zero. Subsequent microscopy also showed 

no particles on the SEM stubs. 

Figure 6 shows a cut-away view of the thermal precipitator CAD model. The aerosol outlet is 

connected to a zero filter followed by a small pump. This prevents particles from entering the pump, 

however, acid gases which are products of the WSTF pyrolysis tests will still enter the pump, so it must 

be monitored for performance and replaced when internal parts are compromised by the harsh 

environment. 
NASA/TM—2015-218746 9 

The SEM stubs were numbered 1 and 2, and there were slight differences in the temperatures of the 

stubs, which may have influenced the sizes of particles deposited. A stub instrumented with a 

thermocouple was used to verify the surface temperatures at both stub positions. The downstream stub 

was slightly warmer as the upstream SEM stub was closer to the thermoelectric cooler for the cold plate 

with fewer warm adjacent surfaces. Between sampling tests, stubs were removed and replaced with clean 

stubs by using an allen wrench on set screws that allowed the entire fin/spacer/stub assemblies to be 

removed (Fig. 7, left photo). The threaded stubs were removed and placed in a protective box (Fig. 7, 

right photo), and a new stub was screwed in securely, with care to compress the o-ring that seals off a 

potential leak path for aerosol flow. 

The highest temperature difference achieved between plates during initial testing was 79.5 C, 

however, it was determined by modeling and verified with microscopy that a 70 C difference is 

sufficient to successfully deposit adequate numbers of particles. This reduced the warm-up time to 30 min 

or less. During actual smoke particle sampling, the cold plate temperature ranged from –3.4 to 1.3 C and 

the hot plate temperature ranged from 66.9 to 77.5 C. Condensation on the SEM stubs was problem with 

zero and negative cold plate temperatures during preliminary experiments with PSL and NaCl, test 

aerosols however, this was not a problem in the WSTF smoke tests, which have negative dew point 

temperatures, between –9 and –18 C. At end of each test, the thermoelectric coolers were immediately 

turned off by placing the power supply on standby to warm them and thus prevent condensation of room 

air moisture while removing and inserting stubs. 

The TP body was made by stereolithography (SLA), which is an additive manufacturing technique 

that creates parts directly from three-dimensional CAD data by converting liquid materials into solid 

cross-sections, layer by layer, using an ultraviolet laser. This technique can save significantly on cost of 

manufacturing unique shapes such as the cubic spline transition from the 1⁄4 in. round tubing connector to 

the wide thermal gradient section of the TP. Figure 5 shows the translucent SLA body (grey) which has a 

rectangular outer cross-section, with the internal flow path transitioning from narrow to wide and back to 

narrow. The 1.25 mm gap is well within the tolerances of SLA capability, and this geometry would not 

have been possible to machine from metal in one piece. The potential for inexpensive re-design is a 

benefit of this free-form fabrication technique, as additional TP bodies can be updated and created 

inexpensively, however, the first iteration of the design was successful. Caution must be observed as there 
NASA/TM—2015-218746 10 

is a tendency of SLA parts to warp, shrink or sag over time. Therefore, before using the TP, a leak-check 

is performed to assure that the seals are intact. 

An aluminum housing was constructed to contain and protect all wires and parts. Figure 8 shows the 

outer housing with the fan and the hinged lid opened revealing the heatsinking fins. The aerosol inlet is 

below the fan, and the outlet is at the opposite end of the housing. 

Conclusion 

The thermal precipitator was successfully built after finite element modeling iteratively determined 

key design parameters. Testing verified the performance of the first iteration of the device and no 

subsequent design modifications were necessary. The thermal precipitator was successfully operated and 

provided quality particle samples for microscopic analysis, which furthered the body of knowledge on 

smoke particulates. Morphology and chemical composition information are important aspects of smoke 

characterization and will be useful for future spacecraft fire detection research. 

History 

 TP design conceived, modeled and iterated starting June 2011 

 Constructed November 2011 

 Tested December 2011 and January 2012 

 Successfully used February 2012 

References 

Azong-Wara, N., et al.: Optimisation of a Thermophoretic Personal Sampler for Nanoparticle Exposure 

Studies. J. Nanoparticle Res., vol. 11, 2009, pp. 1611–1624. 

COMSOL 4.2a Multiphysics User’s Guide: COMSOL, Inc., Burlington, MA, 2011. 

Hinds, W.C.: Aerosol Technology. Second ed., Wiley Interscience, New York, 1999. 

Lorenzo, R., et al.: A Thermophoretic Precipitator for the Representative Collection of Atmospheric 

Ultrafine Particles for Microscopic Analysis. Aerosol Sci. Technol., vol. 41, no. 10, 2007, 

pp. 934−943. 
NASA/TM—2015-218746 11 

Maynard, A.D.: The Development of a New Thermophoretic Precipitator for Scanning-Transmission 

Electron-Microscope Analysis of Ultrafine Aerosol-Particles. Aerosol Sci. Technol., vol. 23, 1995, 

pp. 521–533. 

Meyer, M.E., et al.: Materials Combustion Testing and Combustion Product Sensor Evaluations in FY12. 

AIAA−2013−3432, 2013. 

Tsai, C.J.; and Lu, H.C.: Design and Evaluation of a Plate-To-Plate Thermophoretic Precipitator. Aerosol 

Sci. Technol., vol. 22, 1995, pp. 172–180. 

Wen, J.; and Wexler, A.S.: Thermophoretic Sampler and Its Application in Ultrafine Particle Collection. 

Aerosol Sci. Technol., vol. 41, no. 6, 2007, pp. 624−629.