1 Copyright @1999 by ASME

Proceeding of the

5

th ASME/JSME Joint Thermal Engineering Conference

1999, San Diego, California

AJTE99-6367

WATER MIST IMPINGEMENT ONTO A HEATED SURFACE

Chunming Fu and Paul E. Sojka

Maurice J. Zucrow Laboratories 

School of Mechanical Engineering

Purdue University

West Lafayette, IN 47907-1003 USA

sojka@ecn.purdue.edu

and

Yudaya R. Sivathanu

En’Urga, Inc.

West Lafayette, IN 47906 USA

Keywords: mist impingement, evaporation, size distribution

ABSTRACT

An experimental study on the interaction of a water mist 

with a heated surface is described. The long term objective is to 

produce experimental data that can be used to validate

submodels for four key physical phenomena involved in the

interaction of sprays with burning surfaces: (1) the effect of

buoyancy (caused by the hot combustion products) on the

trajectory of a single droplet, (2) the effect of evaporation on

the trajectory of a single droplet, (3) the cessation of reaction

and reduction in flame spread caused by the droplets on

flaming surface combustion, and (4) the reduction in surface

temperature caused by the effect of drop impingement,

spreading, and evaporation, on surface combustion.

The short term objective was to complete experiments that 

mapped the size history of water droplets as they approached a 

hot surface using a Malvern Spray Analyzer to study the effect 

of both buoyancy and evaporation on droplet trajectory. Surface 

temperature profiles were recorded using fine-wire 

thermocouples. The influence of water mass flow rate and drop 

size distribution on the hot surface temperature profile is

presented and discussed. 
 2 Copyright @1999 by ASME

BACKGROUND

Interaction of water mists with fires have been studied 

for almost four decades (Braidech et al., 1995; Rasbash et al., 

1960; Lugar, 1979; Mawhinney et al., 1994). However, there

are no basic models that describe all interactions at this time. 

Braidech et al. (1955) have concluded correctly that, in most

cases, the efficacy of a water spray is predominantly due to the 

oxygen displacement effect of the water evaporating, rather

than thermal cooling. However, there are no detailed models

describing the size reduction and evaporation of droplets in

flames, or their effect on surface reaction kinetics. Indeed, even 

models that predict what size droplets will reach the surface are 

not available. The present work is aimed at providing

experimental data that describe the interactions of water sprays 

with flaming and non-flaming surface combustion. 

A recent study by Hicks and Senser (1995) shows that 

spray-target interactions can be described by dividing the

process into a near-nozzle flow-development (NNFD) region

and a target-interaction (TI) region. Key features of the NNFD 

region include spray formation, atomization, and entrainment of 

surrounding air. The TI region is characterized by two-way 

coupling between gas and liquid phase momenta where the

spray can no longer be considered dilute, i.e. in the boundary 

layer near the target itself. Key features are deposition of some 

drops, depending on their sizes (big drops are more likely to

impact the surface due to their larger momentum-toaerodynamic drag ratio), and the formation and growth of a

boundary layer which makes it difficult for drops to deposit as 

one moves further from the spray centerline. However, the flow 

physics studied are different for flames/heated surfaces than

they are for unheated surfaces accurate, spatially resolved

measurements of droplet sizes and velocities are necessary for 

submodel development.

Many studies have been performed that quantify the

vaporization process of both single and multiple droplet arrays 

impacting on hot surfaces (DiMarzo and Evans, 1986, DiMarzo 

et al., 1989). These studies have also identified the need for

spatial and temporally resolved data of the droplet trajectory as 

it approaches a hot surface (including buoyancy effects) as well 

as the temperature distribution of the surface material. The

different components that describe both the droplet trajectory

and the temperature distribution over the surface as a function 

of time will be studied in detail in this project by separating the 

various physical processes. This paper concentrates on one

aspect.

EXPERIMENTAL APPARATUS

The experimental apparatus consists of the spray nozzle 

and its associated feed systems, the heated surface and its

associated power supply, a Malvern Spray Analyzer to measure 

near-plate water mist droplet size distributions, and fine-wire 

 Water

 Air

 Hot plate

 1 2 

Figure 1 The schematic diagram of experimental 

apparatus.

 Signals go to

 Computer 1

 5 K-type equally-spaced

 thermocouples

 20mm

 D=200mm

Figure 2 The structure of the heated surface with the 

thermocouples.

type-K thermocouples connected to a computer to record

transient temperature distribution on the heated surface.

Thermocouple voltage signals are transferred to the computer

using an amplifier. All items are summarized below. Figure 1 

and 2 are the detailed schematics. 

The spray nozzle is an effervescent design, and was 

originally used by Lund et al. (1993). The operation principle is 

that of a metered flowrate of air injected into a known flowrate 

of liquid such that bubbles are formed inside the nozzle body. 

The resulting two-phase flow exits the final orifice in an

annular configuration. The rapidly moving air core causes the 

slower moving liquid annulus to break up into ligaments, with 

the ligaments then breaking up into drops. 

The heated surface is a 20cm diameter, 5mm thick

copper plate located 25cm beneath the nozzle. It is placed on a 

spiral wound resistance heater whose power is supplied by a

pair of Variacs and a transformer.

Nozzle

Transmitter Receiver

Laser beam

Droplet size 

data collection

Thermocouple

data collection
 3 Copyright @1999 by ASME

 A Malvern model 2600 Spray analyzer was used to obtain 

all drop size data. The Malvern instrument operates on the

principle of Fraunhofer diffraction. It operates by collecting

light diffracted by particles in the medium, through use of a

Fourier transform lens, and focusing it on a planar detector

array. The detector array consists of thirty concentric

semicircular elements of varying area. Each element is a

photodiode that converts incident light energy into a

proportional current. The currents are transformed to voltages 

that are sampled and stored by a dedicated microcomputer.

After storage, the microcomputer iteratively determines a

particle size distribution that would yield the diffracted light

profile as measured by the detector array. Optical effects,

which results in sampling errors must be considered when

extending the Malvern sizer to media with time varying index 

of refraction gradient. In our experiments, some beam steering 

was expected because the index of refraction gradient of air in 

the measuring volume was influenced by the heated surface

below. However, spurious signals were removed by “killing” 

data on the innermost detector elements, as suggested by Dodge 

and Cerwin (1984) and the manufacturer. The Malvern Spray 

Analyzer calibration was checked using the procedure specified 

by Dodge (1984). 

RESULTS AND DISCUSSION

The baseline temperature distribution on the copper 

plate in the absence of a spray was quite uniform, as Table 1 

shows. By changing the spray mass flow rate and initial drop 

size distribution, their influence was investigated as presented 

below.

Table 1: Average temperature distribution of the 

copper plate before spray

Temperature (oC)

 r=0 mm 596.5

r=20 mm 601.3

r=40 mm 601.8

r=60 mm 605.8

r=80mm 602.4

Average Temp.(oC) 601.6

Standard deviation(oC) 3.0

Note: The expected thermocouple system accuracy is ? 2.2oC;

 32 samples are collected at each radial location at 

frequency f=1000Hz. 

 (a) Influence of water mass flowrate

By applying sprays of D32=100 ? m (obtained though

adjusting the flow rates of water and air simultaneously), the

influence of water mass flow rate was investigated. The mass 

flow rates used in this experiment were 0.214, 0.323, 0.480,

0.609, 0.742 and 0.923 g/s, respectively. Table 2 and Figure 3 

shows the temperature distributions at t=8 s (this instant was

chosen since for the case of m=0.923 g/s, the temperature of the 

center point was nearly at the boiling point; and after this

instant, there was some water deposition on the plate, which is 

beyond our interest of study). 

Figure 3 shows that flow rate has a significant influence on 

the temperature profiles, especially in the center. This is 

because of the cone spray structure, which concentrates a large 

part of the mass flow along the spray axis. The larger the flow 

rate, the more rapidly the temperature decreases, since more 

Figure 3 Temperature distributions at t=8s for 

different flow rate. D32= 100 ? m sprays.

Table 2. Temperature Distribution at t=8 s

Radial distance (mm)

Flow rate (g/s)

0 20 40 60 80

0 596.5 601.3 601.8 605.8 602.4

0.214 575.9 596.1 598.3 598.8 600.6

0.323 540.2 573.3 579.6 584.1 589.7

0.480 500.7 560.6 564.8 589.7 573.2

0.609 449.7 543.3 560.9 566.8 571.7

0.742 306.3 463.0 540.0 555.0 568.0

0.923 102.5 430.0 522.0 538.0 553.0

0

100

200

300

400

500

600

700

0 20 40 60 80 100

Radial distance (mm)

Temperature Distribution (

oC)

T0(C)

m=0.214g/s

m=0.323g/s

m=0.480g/s

m=0.609g/s

m=0.742g/s

m=0.923g/s
 4 Copyright @1999 by ASME

energy is required to vaporize the droplets. For the largest flow 

rate, m=0.923 g/s, when t=8 s, the center temperature was very 

close to the boiling point of water. A small area of water

deposition on the plate was observed quite soon after this. For 

m=0.742g/s, this situation occurred much later at about t=120 s. 

For other cases, droplets were visually observed bouncing from 

the plate and vanished rapidly, with the final equilibrium states 

reached after different periods of time.

Figure 4 shows the time history of the temperature

distribution for the case of m=0.742 g/s. It should be pointed

out that because there were radiation, convection and

conduction heat losses at the same time, it is difficult to tell

accurately what percent of energy was consumed by

evaporation at this time.

Figure 4. The time history of temperature profile for 

m=0.742 g/s.

(b) Influence of droplet size distribution

Three mean drop sizes were used to investigate the

influence of droplet size distribution on the temperature profile, 

all at the flow rate of 0.923g/s. Sizes used here are D32=70, 100 

and 200 ? m. Detailed information is provided in Figure 5.

From this figure, we see that for the same flow rate, the

smaller drop size spray exhibits temperature difference between 

the plate center line and edge. When small droplets approach 

the heated surface, they evaporate rapidly and consume more

energy, while for large droplets, energy is mainly consumed

after they impact the plate, disintegrate into small droplets, 

bounce (because of their high speed), and finally evaporate. 

(c) Influence of evaporation and buoyancy on nearplate spray size distribution

 Measurements of spray size distribution when they

approach the hot plate were obtained at a height of 40 mm 

Figure 5. Temperature distribution at m=0.923 g/s, 

t=8 s for sprays having D32=70, 100 and 200 ? m.

above the plate. Measurements could not be made closer to the 

plate because the temperature of the air just above the surface 

was very high, thus the index of the reflection would be

seriously influenced and too much beam steering would be

present. Table 3 gives a preliminary indication of the size

distribution near the plate:

Table 3: Near-plate spray size distribution at t=1 s

Original size: D32=100 ? m

Mass flow rate (g/s) Average size (? m)

0.214 158

0.323 175

0.480 180

0.609 188

0.742 184

0.923 176

From the table we see that , the average size measured

increases as the flow rate increases, reaches its maximum at

m=0.609 g/s, then decreases. One possible reason for the

observed behavior is that the proportions of small and large

drops are nearly the same for all sprays. Here we have constant 

heat supply. In the low flow rate case, small drops vanish

rapidly, and large drops have partly evaporated. As the flow

rates increases, the number of small droplets increases, the

portion of energy used to evaporate the small drops becomes

greater and greater, thus D32 of the spray increases. When the 

flow rate is quite large, there is not enough energy to evaporate 

all the small drops during the short time between the droplets 

0

100

200

300

400

500

600

700

0 20 40 60 80 100

Radial distance (mm)

Temperature distribution (C)

t=0 s

t=4 s

t=8 s

t=16 s

t=32 s

t=64 s

t=120 s

0

100

200

300

400

500

600

700

0 50 100

Radial distance (mm)

Temperature distribution ( C)

T0(C)

D32=200 um

D32=100 um

D32=70 um
 5 Copyright @1999 by ASME

exit the nozzle and impact on the plate, thus the D32 decreases 

again. Further modeling should help resolve this issue. 

The influence of buoyancy on droplet size distribution may 

be considered as follows: (1) Buoyancy increases the time

between when the droplets exit the nozzle and reach the heated 

plate, hence increases the evaporation. (2) The rising air stream 

enhanced the interaction and heat transfer between the droplets 

and the ambient air, thus indirectly enhancing evaporation rate. 

Therefore, for small flow rates, since buoyancy has more

significant influence on small droplets (decelerating their

velocities much faster), the small droplets’ evaporation rate and 

the evaporated portion both increase. This contributes to

increasing D32. For the larger flow rates, buoyancy decreases 

D32 for the same reason. In both cases, buoyancy narrows the 

size distribution. However, the influence of buoyancy on

droplet size distribution is much less than evaporation. 

SUMMARY

The effect of both evaporation and buoyancy on droplet

size history and the corresponding temperature distribution on a 

hot surface at 600oC are described. The influence of water mass 

flow rate and drop size distribution on the hot surface

temperature profiles is presented and discussed. Experiments

investigating the separate effects of buoyancy and evaporation 

using monodisperse spray will be carried out in the near future. 

ACKNOWLEDGEMENT

This work was sponsored by the Department of Defense

Next-Generation Fire Suppression Technology Program,

funded by the DOD Strategic Environmental Research and

Development Program (SERDP) under Contract number

DASW01-98-C-0029, with Dr. Richard Gann serving as the

NGP Technical Program Manager. 

REFERENCES

Braidech, M.M., Neale, J.A., Matson, A.F., And Dufour,

R.R. (1955), “The mechanism of extinguishment of fire by

finely divided water”, Underwirters Laboratories Inc. for the

National Board of Fire Underwirters, NY, p.73

Di Marzo, M., Kavoosi, and Klassen, M. (1989),

“Transient cooling of a hot surface by droplets evaporation”, 

Report prepared for the National Institute of Standards and

Technology, NIST-GCR-89-559, University of Maryland,

College Park, MD.

DiMarzo, M., and Evans, D. D., (1986) “Evaporation of

water droplet deposited on a hot high thermal conductivity solid 

surface”, National Bureau of Standards Interagency/Internal

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Dodge, L. G. (1984), “Calibration of the Malvern Particle 

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vol. 117, pp. 713-719.

Lugar, J.R., (1979), “Preliminary test results of fine water 

mist fire protection systems study”, David W. Taylor Navel

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Mawhinney, J.R., Dlugogorski, B.Z., and Kim, A. K.

(1994), Proceedings of the Fourth International Symposium on 

Fire Safety Science, (Ed. T. Kashiwagi), Ottawa, CA, pp47-60.

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Combust. Flame, vol. 4, pp223-234.