Conference Paper

NREL/CP-540-48055

August 2010

Enhancement of Heat Transfer 

with Pool and Spray Impingement 

Boiling on Microporous and 

Nanowire Surface Coatings 

S.J. Thiagarajan, W. Wang, and R. Yang 

University of Colorado

S. Narumanchi and C. King

National Renewable Energy Laboratory 

Presented at the 14th International Heat Transfer Conference 

(IHTC-14)

Washington, DC

August 8-13, 2010 
NOTICE

The submitted manuscript has been offered by an employee of the Alliance for Sustainable Energy, LLC 

(Alliance), a contractor of the US Government under Contract No. DE-AC36-08GO28308. Accordingly, the US 

Government and Alliance retain a nonexclusive royalty-free license to publish or reproduce the published form 

of this contribution, or allow others to do so, for US Government purposes.

This report was prepared as an account of work sponsored by an agency of the United States government. 

Neither the United States government nor any agency thereof, nor any of their employees, makes any 

warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or 

usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not 

infringe privately owned rights. Reference herein to any specific commercial product, process, or service by 

trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, 

recommendation, or favoring by the United States government or any agency thereof. The views and 

opinions of authors expressed herein do not necessarily state or reflect those of the United States 

government or any agency thereof.

Available electronically at http://www.osti.gov/bridge

Available for a processing fee to U.S. Department of Energy

and its contractors, in paper, from:

U.S. Department of Energy

Office of Scientific and Technical Information

P.O. Box 62

Oak Ridge, TN 37831-0062

phone: 865.576.8401

fax: 865.576.5728

email: mailto:reports@adonis.osti.gov

Available for sale to the public, in paper, from:

U.S. Department of Commerce

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

phone: 800.553.6847

fax: 703.605.6900

email: orders@ntis.fedworld.gov

online ordering: http://www.ntis.gov/ordering.htm

Printed on paper containing at least 50% wastepaper, including 20% postconsumer waste
 1 

Proceedings of the 14th International Heat Transfer Conference 

IHTC-14 

August 8-13, 2010, Washington, D.C., USA 

IHTC14-23284 

ENHANCEMENT OF HEAT TRANSFER WITH POOL AND SPRAY IMPINGEMENT 

BOILING ON MICROPOROUS AND NANOWIRE SURFACE COATINGS 

Suraj Joottu Thiagarajan 

Department of Mechanical Engineering 

University of Colorado 

Boulder, Colorado, USA 

Sreekant Narumanchi*†

 and Charles King†

National Renewable Energy Laboratory 

Golden, Colorado, USA 

*sreekant.narumanchi@nrel.gov

 303-275-4062 

Wei Wang and Ronggui Yang 

University of Colorado 

Department of Mechanical Engineering 

Boulder, Colorado, USA 

ABSTRACT 

The National Renewable Energy Laboratory (NREL) is 

leading a national effort to develop next-generation cooling 

technologies for hybrid vehicle electronics, as part of the 

Advanced Power Electronics and Electrical Machines program 

area in the U.S. Department of Energy’s (DOE’s) Vehicle 

Technologies Program. The overarching goal is to reduce the 

size, weight, and cost of power electronic modules that convert 

direct current from the batteries to alternating current for the 

motor, and vice versa. Aggressive thermal management 

techniques help in achieving the goals of increased power 

density and reduced weight and volume, while keeping the chip 

temperatures within acceptable limits. The viability of 

aggressive cooling schemes such as spray and jet impingement 

in conjunction with enhanced surfaces is being explored as part 

of the program. In this work, we present results from a series of 

experiments with pool and spray boiling on enhanced surfaces, 

such as a microporous layer of copper and copper nanowires, 

using HFE-7100 as the working fluid. Spray impingement on 

the microporous coated surface showed an enhancement of 

100%−300% in the heat transfer coefficient at a given wall 

superheat with respect to spray impingement on a plain surface 

under similar operating conditions. The critical heat flux also 

increased by 7%−20%, depending on the flow rates. Heat 

transfer coefficients obtained on the nanowire-grown surface 

are considerably better than those obtained on the plain surface, 

although the enhancement is lower than those obtained on the 

microporous surface. The critical heat flux is also considerably 

lower for the nanowire surface than for the plain surface. 

KEY WORDS 

 Electronics cooling, pool boiling, spray cooling, surface 

enhancements, microporous surface, nanowire, HFE-7100 

INTRODUCTION 

The unprecedented increase in interest in the development 

of a new electric propulsion system to enable the transition 

from conventional engines to economical hybrid vehicles and 

fully electric vehicles in the last few years has led to a major 

collaborative effort through the DOE FreedomCAR and Fuel 

Partnership [1]. The goal in developing new electric propulsion 

systems is to decrease their cost to a level that renders hybrid 

and fuel cell vehicles economically justifiable for the consumer. 

The state-of-the-art cooling system for power electronics 

modules using liquid heat exchangers adds a significant amount 

of cost and weight to the modules. To achieve FreedomCAR 

goals, significant advances must be achieved in the thermal 

control of both the power electronics and electrical machines. 

By optimizing existing technologies and extending them to new 

cooling methods, NREL aims to achieve higher power 

densities, smaller volumes and weights, and increased 

reliability for the drivetrain components. 

Achieving high heat flux dissipation is dependent on the 

chip packaging. Direct cooling of the chips using dielectric 

fluids will eliminate much of the resistances due to packaging. 

However, the thermophysical properties of dielectric coolants 

are considerably worse than those of common coolants such as 

water-ethylene glycol mixtures. Hence, it is necessary to 

achieve very high convective heat transfer coefficients to 

reduce the chip-to-fluid resistance below that of the total 

resistance in an indirectly cooled package to obtain an 

advantage with direct cooling. Given the inferior 

†

Employees of the Alliance for Sustainable Energy, LLC (ASE), a contractor of 

the U.S. Government under Contract No. DE-AC36-08GO28308. The U.S. 

Govt. and ASE retain a nonexclusive royalty-free license to publish or reproduce 

this contribution, or allow others to do so, for U.S. Govt. purposes.
 2 

thermophysical properties of the dielectric coolants, high heat 

transfer coefficients can be obtained by using phase change in 

conjunction with aggressive flow configurations such as spray 

or jet impingement. In addition, even higher heat transfer 

coefficients can be obtained by engineering the surface to 

provide the maximum nucleation site density with minimum 

superheat. 

Several studies in the past explored the effect of enhanced 

surfaces on pool boiling. A summary of the different surface 

enhancements and their effect on pool boiling performance is 

provided in reference [2], in which several forms of 

microstructure enhancement are reviewed, including laser 

drilled cavities, reentrant cavities, micro-fins, porous coatings 

and sputtered surfaces. It has been found that, in general, 

micro-fins are more effective at increasing the critical heat flux 

(CHF), while microporous structures are more effective at 

increasing nucleate boiling heat transfer. Chang and You 

performed pool boiling experiments [3] with FC-72 on an 

optimized microporous organic coating made with thermally 

conductive particles in an organic binder (a “DOA” coating). 

Despite the low thermal conductivity (~0.95 W/mK), these 

coatings led to a significantly lower nucleate boiling incipience 

superheat of <10 K, in comparison to ~30 K for the plain 

surface, and resulted in a significant increase in the heat 

transfer coefficient and critical heat flux. Kim et al. [4] studied 

the mechanism of pool boiling with FC-72 on cylindrical 

platinum wire using the consecutive-photo method [5]. They 

determined the differences (between the microporous coated 

surface and plain surface) in the latent heat contribution and the 

bubble size and frequency of departure. They found that the 

microporous coating enhances the boiling heat transfer by 

increasing the nucleation site density, and not by increasing the 

surface area. El-Genk and Parker [6] performed pool boiling 

tests on commercially available porous graphite with HFE7100. They observed the onset of nucleate boiling in the porous 

surface at a surface superheat of only 0.5−0.8 K. For 

comparison, onset occurred on plain copper at a superheat of 

~40K. This resulted in a large increase (1600% at 10 K 

superheat, and 60% near CHF) in the heat transfer coefficient. 

The CHF also increased by 60%. They attributed the 

performance enhancement on the porous graphite layer to the 

presence of trapped air in the surface pores and re-entrant 

cavities that form nucleation sites at very small superheat 

temperatures. 

Porous coated surfaces lead to boiling enhancement as a 

result of a combination of the following factors: an increase in 

the effective surface area, an increase in the nucleation site 

density, the presence of capillaries that facilitate liquid flow, 

and the dependence of the vapor escape path on the pore 

distribution in the layer adjacent to the liquid. The 

interconnected pore assists the flow of fluid toward the heated 

surface by capillary action. By proper design of the porous 

layer, the CHF could also be enhanced by the capillary-assisted 

liquid flow toward the phase-change interface. This reduces 

liquid vapor counter flow resistance and impedes the 

development of localized dry-out conditions leading to a higher 

CHF [7]. 

More recently, copper and silicon nanowires grown on the 

heater surfaces under pool boiling with water have been shown 

to enhance the nucleation site density at a given superheat and 

bubble release frequency [8] and to increase both the heat 

transfer and CHF [9]. Contrary to the traditional belief that 

nanoscale structures will not enhance nucleate boiling heat 

transfer because the size scale is too small to induce bubble 

nucleation, these works have shown enhanced performance [8]. 

This is attributed to the increased wetting of the surface due to 

the presence of the nanowires and to micron-sized defects or 

voids formed between the nanowires, which form favorable 

sites for bubble nucleation. However, in the case of highly 

wetting dielectric fluids like HFE-7100, the effect of wetting 

enhancement due to the nanowires may be small, and the 

presence of the voids of the optimum shape and size may be the 

more important factor in the boiling enhancement. 

The use of enhanced surfaces on spray cooling has 

received much less attention in comparison to their application 

to pool boiling. Comparison of spray cooling with other phasechange cooling techniques such as pool boiling and 

microchannel cooling and jet impingement [10, 11] shows that 

significantly higher quantities of heat can be removed at a low 

superheat with spray cooling. A uniform dispersal of the liquid 

droplets impinging on the heater surface in a spray gives rise to 

a more uniform spatial surface temperature distribution over the 

entire spray impact area. Further, boiling incipient superheat, 

which may cause a severe thermal shock to electronic 

components and make the heat transfer performance highly 

unpredictable, is much less pronounced in spray cooling 

systems than in pool or flow boiling systems. Spray cooling can 

also provide significantly higher CHF at low values of 

subcooling in comparison to jet cooling [10]. A review of the 

spray cooling phenomena as of 2007 is provided in reference 

[12]. Horacek et al. [13] found that heat flux dissipated from a 

spray-cooled surface can be correlated to the three-phase 

contact line length density, and not to the total wetted area. The 

CHF was reached at the highest contact line density. Kim et al. 

[14] investigated evaporative spray cooling on a microporous 

coated surface using water at very low flow rates up to 0.025 

cm3

/cm2/s. The low thermal conductivity (estimated to be ~0.95 

W/mK) porous layer was fabricated using a mixture of 

methylethylketone, epoxy, and aluminum powder, and its 

maximum thickness was 500 μm. They found that the CHF 

increased by 50% relative to the uncoated surface. Bostanci et 

al. [15] performed spray cooling experiments with ammonia on 

microstructured surfaces with indentations and protrusions with 

heat fluxes up to 500 W/cm2

. They observed an enhancement of 

49%−112% in the heat transfer coefficient with respect to a 

smooth surface. The increase is believed to be due to the 

increased surface area, availability of a range of the cavity 

sizes, and increased contact line length density over the heater 

surface. 

Because of the desirable properties of the fluid HFE-7100 

(such as low global warming potential, low ozone depletion 
 3 

potential, and high dielectric strength, in addition to good 

thermophysical properties), it has been identified as a potential 

coolant for the cooling of power electronic modules in 

automobiles. However, little research has been carried out with 

HFE-7100 with aggressive cooling schemes such as sprays and 

jets. In particular, there exists no study in the literature on the 

performance of HFE-7100 coolant with spray impingement, in 

conjunction with enhanced surfaces. The main objective of the 

present study is to investigate the performance enhancement 

that can be obtained by novel surface coatings on copper, under 

pool boiling and spray impingement configurations with the 

HFE-7100 coolant. Two types of surface coatings were tested: 

microporous copper coating and electrolytically grown copper 

nanowire array. The effect of the spray flow rate and liquid 

subcooling were also studied. In the following sections, the 

experimental setup and procedure and target surface 

enhancements are described, followed by a discussion of the 

results. 

EXPERIMENTS 

Setup 

Figure 1 shows the construction of the test heater that is 

used to simulate the chip. The test heater is a single cylindrical 

block of oxygen-free copper with a cubical projection whose 

top face, which has a surface area of 1 cm x 1 cm, is exposed to 

the coolant. The cubical projection from the cylindrical block is 

surrounded by an insulating fiberglass (G7) block, which is 

fabricated to fit tightly along the sides of the cube. Three 

cartridge heaters, together capable of delivering over 200 W, 

are placed in holes that are bored in the back of the copper 

block. The entire heater assembly is mounted on a stainless 

steel flange, which is then clamped to the chamber. Two type K 

thermocouple probes are inserted into the copper block, at a 

distance of 1.25 mm and 7.25 mm from the test surface. The 

heat flux through the surface (=qcalc) is determined by the 

temperature gradient across the two thermocouples. This 

information is also used to determine the surface temperature 

(assuming one-dimensional heat flow), and hence the surface 

heat transfer coefficient. Dow Corning TC-5022 thermal grease 

is used to provide good thermal contact between the 

thermocouples/heaters and the copper block. 

Heat loss is estimated to range from 6% to 15% for a 

variety of heat transfer coefficients (h) and input power levels 

(qin) in the nucleate boiling regime. The highest losses occur 

with the lowest h and the lowest qin. Higher losses (up to 50%) 

occur in the single-phase regime in the pool boiling 

configuration. To ensure that the temperatures read by the 

thermocouples are the true values (so that the heat flux could be 

reliably measured by the gradient), steady-state thermal 

analysis was performed with a 3D ANSYS Workbench model. 

The analysis was performed at different surface heat fluxes, 

assuming a uniform heat transfer coefficient at the target 

surface and a set of heat transfer coefficients for the different 

modes of heat loss. The set of the coefficients for heat loss used 

in the analysis was arrived at by iteratively trying different 

values until temperatures obtained in the ANSYS model (and 

heat flux from the surface) match the corresponding 

experimental values. 

(a) 

(b) 

Figure 1 (a) Construction of test heater assembly, and (b) photo of the 

assembly, showing the square target surface that is exposed to the 

coolant (photo by S. Thiagarajan, co-author). 

In the model, the input values were the power input (qin = 

VI through the cartridge heaters) and the loss heat transfer 

coefficients. The heat loss through the different surfaces 

(including the target surface) and the temperature at different 

points within the copper block were the outputs. Figure 2 shows 

the results for an input heat flux of 100 W/cm2

 with h = 2.06 

W/cm2

K on the target surface. The temperature in the planes 

where the thermocouples are placed in the copper block is 

shown. The temperature is uniform within 0.15°C in the plane 

closer to the surface, and within 0.5°C in the plane farther from 

the surface. This confirms that the surface temperature in the 

experiments could be taken as the extrapolation of the two 

thermocouples. 

The schematic of the experimental test loop is shown in Fig. 

3. It is designed to deliver the test fluid at the desired pressure, 

temperature, and flow rate to the spray nozzle located inside the 

test chamber. The coolant is partially evaporated upon impact 

with the test heater. The remaining liquid accumulates in the 

bottom of the test vessel, while the vapor is separated from it by 

buoyancy into the vessel’s top region. The liquid from the test 

vessel drains directly to a pump and then passes through a valve 
 4 

and subsequently through a plate-type heat exchanger, where 

the fluid is subcooled to the required temperature. The 

subcooled liquid then passes through the flow meter and then 

through a filter before reaching the spray nozzle. The 

temperature of the incoming fluid is read by a type K 

thermocouple approximately 2.5 cm from the nozzle exit. The 

pressure ahead of the nozzle entrance and inside the chamber is 

measured using pressure transducers. The spray nozzle is a 

Unijet full cone pressure nozzle, with an orifice diameter of 1.7 

mm and spray angle of 48.5°, made by Spraying Systems Inc. 

(a) (b) 

Figure 2 Temperature (°C) in the plane of each target thermocouple: 

(a) closer to the surface; (b) farther from the surface. 

A separate air-cooled finned-tube heat exchanger is placed 

above the test vessel for the removal of noncondensable gases 

from the liquid before each experiment. The test vessel 

temperature is maintained at a fixed point, within 0.5°C, using 

band heaters that heat the liquid through the wall of the vessel. 

The spray nozzle exit in all experiments is located at a distance 

of 11.1 mm, so that the square surface is just covered by the 

spray for maximizing the CHF [16]. The same setup (Fig. 3) is 

also used for pool boiling. For this purpose, the liquid is filled 

until it completely submerges the target surface. Note that the 

heater surface is vertical to the ground in the pool boiling 

experiments. The input from the different sensors feed in to a 

data acquisition system and are read using LabVIEW. All the 

controls are manually operated. 

Uncertainties 

The flow meter was calibrated by measuring the total 

liquid flowing through the circuit in a given time. The 

uncertainty in the flow rate is estimated to be ±0.17 cm3

/s in the 

range of 1.25 to 17 cm3

/s. The pressure transducers were 

calibrated to an accuracy of ±350 Pa using a Fluke pressure 

calibrator. A Hart Scientific thermal bath was used for the 

calibration of all the thermocouples. The uncertainty in the 

temperature measured is within ±0.03°C. The uncertainties in 

voltage and current are negligibly small. Because of the 

uncertainty in the positions of the two target thermocouples 

(estimated to be ±0.125 mm from the centerline of the holes), 

and hence the temperature measurements in the target block, it 

is estimated that the measured heat flux, based on the gradient 

in the two thermocouple readings, has an uncertainty of ±6 %. 

Procedure 

In all the experiments reported in this study, the chamber 

vapor temperature was maintained at ~61°C, corresponding to a 

pressure of 1 atmosphere at sea level (~101 kPa). Before each 

experiment, the fluid was deaerated for 20−45 minutes to expel 

any dissolved noncondensable gases using the deaeration 

condenser, while the pump is on and set at a constant 

predetermined flow rate. After this procedure, the system is 

allowed to settle on to a steady state with no input power. All 

the sensor readings are continuously recorded and monitored 

through LabVIEW. 

(a) 

(b) 

Figure 3 (a) Schematic of experimental test loop; (b) photo of the test 

facility (photo by S. Thiagarajan, co-author). 

For the spray experiments, two levels of liquid 

temperatures (in the nozzle) are considered—31°C (subcooling 
 5 

ΔTsub = 30°C, henceforth referred to as “subcooled”), and 60°C 

(subcooling ΔTsub~1°C, henceforth referred to as “nearsaturated”). Boiling curves are generated by raising the voltage 

through the cartridge heaters in small increments and allowing 

the temperatures to achieve steady state, which was determined 

by less than 0.1°C change in 5−10 minutes (depending on the 

surface). To accurately determine the CHF, the flux increment 

is kept very small near CHF. The highest heater power with 

stable target surface temperature is considered to be the CHF, 

beyond which a sudden, large surge in the temperature 

measurement occurs with only a small increment in the heat 

flux. Once the CHF is detected, the heater voltage is cut off and 

the target is allowed to cool down. 

(a) 

(b) 

Figure 4 SEM images showing the microstructure of the (a) 

microporous coating (image by Bobby To, NREL) and (b) nanowire 

array (image by Wei Wang, co-author).

Target Surface Enhancements 

Three types of surfaces were studied: the baseline case of a 

plain copper surface with no coating, a copper microporous 

coating, and copper nanowire arrays. To ensure reproducible 

results with the plain copper surface, it was deoxidized before a 

series of tests with NOXON solution (composition: ammonium 

hydroxide, limestone, ethanedioic acid, ethanol and 

isopropanol) and polished using #600 and #2000 grit paper in a 

specific pattern. The surfaces with the enhancements could not 

be cleaned throughout the experiments because of concerns 

about damaging the surface. 

The target with the copper microporous coating was 

obtained from 3M Corporation. The procedure for fabricating 

the microporous layer on the copper target surface involves 

fusing copper-rich microparticles on the copper surface at a 

temperature of ~850°C. The coating is about 150 μm thick and 

is made with copper particles ≤ 20 μm in diameter [17,18]. 

The copper nanowires were grown on the copper surface 

by means of electrodeposition using a porous anodic alumina as 

the template. The procedure for the fabrication followed 

reference [19]. The nanowires have a diameter of 250-300 nm, 

an interwire gap of ~50-100 nm, and a length of 2 μm. Figure 4 

shows the SEM image of (a) the microporous coating, and (b) 

the nanowire array (top view). 

RESULTS AND DISCUSSIONS 

In the present study, we focused on the determination of 

the effect of the microporous and nanowire surface structures 

on the pool/spray boiling performance in comparison to the 

baseline case of a plain surface. Furthermore, for the 

microporous coating, we investigated the effect of the liquid 

flow rate and the subcooling level, but only the effect of the 

subcooling level for the nanowire sample (in the spray tests). 

The samples with the coatings are measured in the order of 

increasing flow rate (after the pool boiling test), so as to 

minimize the effect of possible surface degradation due to the 

impact of the high-velocity spray. However, the surface with 

the nanowire coating showed degraded performance due to the 

spray even at the lower flow rate. So, spray cooling results for 

the nanowire surface are provided only for the lowest flow rate. 

Pool Boiling Experiments 

The boiling curves for the three surfaces are shown in Fig. 

5(a). The heat transfer coefficient, ݄ ൌ ݍ௖௟௔௖"⁄൫ܶ௪ െ ܶ௟௜௤൯, 

calculated with liquid temperature as the reference temperature, 

at various levels of the heat flux is shown in Fig. 5(b). The 

onset of nucleate boiling occurs on the microporous surface at a 

superheat of only ~3°C (Fig. 5(a)), in comparison to ~8°C in 

the nanowire surface and ~12°C in the plain surface. The 

microporous surface augmented the heat transfer coefficient by 

more than 500%, with respect to the plain surface, throughout 

the nucleate boiling regime at each power level. The CHF is 

also enhanced by 10% for the microporous surface with respect 

to the plain surface. Thus, while there is a considerable 

enhancement of the h, the CHF is only slightly increased. This 

result is different from that of Rainey and You [20], who 

obtained almost 60% enhancement in the CHF with the 

microporous surface over plain surface using FC-72. Based on 

the visualization studies performed on platinum wire with 

microporous coating, Kim et al. (4) explained the enhancement 

in the CHF as being due to the increase in the microconvection 

caused by the bubbles. The presence of many active nucleation 

sites prevents the bubbles from becoming too large before 

departure and increases the frequency of bubble departure, 

leading to a better convection heat transfer. It appears that the 
 6 

same effect may also be at play in the current experiments, 

though its effect on CHF seems minimal. The difference in the 

enhancements of the CHF could also be due to the difference in 

the properties of the two liquids, in addition to the structure and 

composition of the microporous layer itself. 

(a) 

(b) 

Figure 5 Plots showing (a) boiling curve and (b) heat transfer 

coefficient for the pool boiling tests with the three different surfaces, 

with the surfaces oriented vertically in the pool.

The nanowire surface, while showing an improvement in the 

h of about 60% for the same heat flux near CHF, resulted in a 

lower CHF than the plain surface. Chen et al. [9] studied pool 

boiling on surfaces with copper nanowire arrays in saturated 

water. They observed large enhancements (100%) in both the h

and CHF. The length of the nanowires used in that work was 

40−50 μm, which is long enough to allow for the agglomeration 

of the top layer of the nanowires, resulting in the formation of 

many microscale cavities leading to a higher nucleate site 

density and hence a higher heat transfer coefficient. 

Furthermore, they also calculated that the CHF is enhanced as a 

result of the large capillary action provided by the presence of 

the nanowires. By reducing the contact angle, the nanowires 

also increase the surface area wetted by water. The sample used 

in the current study has nanowires that are only about 2 μm 

long, and hence much too short to agglomerate and form 

microscale cavities. Moreover, the highly wetting nature of the 

HFE-7100 could potentially render any additional wetting 

provided by the nanowires insignificant. Hence, it appears that 

the presence of the short nanowires does not seem to influence 

the heat transfer process significantly, while it also reduces the 

CHF. Thus, coating with nanowires of optimum length could 

increase the heat transfer coefficient in the nucleate boiling 

regime through its effect on the site density, and that is what we 

are pursuing currently. 

In Fig. 5(a), after the onset of nucleation, the slope of the 

boiling curve for the plain surface is seen to increase at first and 

decrease as it nears the CHF. This is indicative of the gradual 

activation of the nucleation sites as the superheat increases. The 

subsequent drop in the slope is possibly due to the 

agglomeration and coalescence of the bubbles on the surface 

leading to local dryouts. In contrast, the boiling curves 

corresponding to both the microporous surface and the 

nanowire surface show a sharp increase in the slope, indicating 

the simultaneous activation of a large number of nucleation 

sites with the wall temperature/superheat remaining almost 

constant. 

Spray Impingement Experiments 

Experiments with spray impingement were performed on 

the different surfaces for the combination of parameters shown 

in Table 1. The mean velocity shown in the table is simply the 

ratio of the volumetric flow rate of the liquid to the area of the 

nozzle orifice (1.7 mm in diameter). 

Table 1 Matrix of the experiments performed. 

Surface Flow rate 

(cm3

/s) 

Mean velocity 

(m/s) 

Subcooling 

level 

Plain 

4.7 2.1 

10.2 4.5 30°C, 1°C 

15.8 7.0 

Microporous 

4.7 2.1 

10.2 4.5 30°C, 1°C 

15.8 7.0 

Nanowire 4.7 2.1 30°C, 1°C 

Effect of liquid subcooling 

Figures 6, 7, and 8 show the boiling curves for the 

subcooled and near-saturated conditions for the different 

surfaces at flow rates of 4.7 cm3

/s, 10.2 cm3/s, and 15.8 cm3/s. 

In each plot, the solid symbols correspond to the “subcooled” 

case (Tnozzle = 31°C), and the open symbols correspond to the 

“near-saturated” case (Tnozzle = 60°C). In the discussion that 

follows, the heat transfer coefficient is calculated with vapor 

temperature (chamber Tsat) as the reference 

(݄ ൌ ݍ௖௟௔௖"⁄ሺܶ௪ െ ܶ௦௔௧ሻ), rather than liquid temperature in the 

nozzle. In this way, the effect on the heat transfer coefficients of 

subcooling the liquid is more evident. The heat transfer 

coefficients corresponding to the data shown in Figs. 6−8 are 

shown in Fig. 9(b) and Fig. 10(b). In those figures, the h for the 

single-phase regime is not shown as it is not meaningful, as 

defined here. 
 7 

Figure 6 Effect of subcooling on the different surfaces at a spray flow 

rate = 4.7 cm3

/s. 

Figure 7 Effect of subcooling on the different surfaces at a spray flow 

rate = 10.2 cm3

/s. 

The enhancement of the h and CHF seen in pool boiling is 

reproduced with the spray at the low flow rate (4.7 cm3

/s, Fig. 

6): the microporous surface shows an increase of 100%−300% 

in the h in the nucleate boiling regime and ~7% increment in 

the CHF, while the nanowire surface shows a marginal increase 

in the h while also showing a considerably lower CHF. Evident 

from the plots is also the fact that the subcooling of the liquid 

considerably increases the h and also the CHF in the case of the 

low flow rate throughout the nucleate boiling regime. While the 

nanowire surface performs better than the plain surface in the 

subcooled case, the shape of the boiling curve in the nearsaturated test is possibly indicative of the degradation of the 

surface, resulting in a performance close to that of the plain 

surface, including the CHF. 

Figure 8 Effect of subcooling on the different surfaces at a spray flow 

rate = 15.8 cm3

/s. 

As expected, the heat transfer coefficient obtained with the 

subcooled spray is higher than that of the spray with nozzle 

temperature close to the saturation temperature. This is due to 

the additional sensible heat removed by the liquid from the 

surface to heat the liquid up to the saturation temperature. The 

plots in Figs. 7 and 8 show the performance of the microporous 

surface, with respect to the plain surface, for the higher flow 

rates (10.2 cm3

/s and 15.8 cm3/s). The large enhancement in the 

h is seen to continue throughout the range of the flow rate. The 

CHF also shows a greater enhancement of about 15%−20% for 

the higher flow rates. 

The favorable effect of the liquid subcooling is clearly seen 

in the lower power levels for both the surfaces (Fig. 6, 7, and 

8). However, as the heat flux approaches the CHF, the 

subcooling has little effect on the h (for the higher flow rates, 

Figs. 7 and 8), in contrast to the low-flow-rate case (Fig. 6) in 

which the subcooling is helpful throughout the range. The 

convergence of the near-saturated and the subcooled curves at 

the higher heat fluxes shows that the phase-change heat transfer 

is the dominant heat transfer mechanism. The effect of 

subcooling is undiminished in the low-flow-rate case (Fig. 6), 

probably because the CHF occurs before any convergence 

could be seen, while in the case of the higher flow rates, the 

CHF is sufficiently delayed for the curves to converge (Fig. 7 

and 8). Thus, although the effect of the subcooling is small on 

the heat transfer coefficient at high heat fluxes for the higher 

flow rates, it helps in increasing the CHF (very significantly in 

the case of the spray with the low flow rate, Fig. 6). 

Effect of spray flow rate 

Figures 9 and 10 show the boiling curves and heat transfer 

coefficients for the subcooled spray and the near-saturated 

spray, respectively. The liquid flow rate has been observed to 

have a relatively minor effect on the nucleate boiling heat 

transfer coefficient [16]. However, the CHF can be enhanced by 

higher flow rates, because, for a given spray nozzle, increasing 

the liquid flow rate is accompanied by an increased number of 
 8 

droplets, which causes more frequent bubble puncturing and 

increases the number of secondary nuclei, enhancing 

heatfluxes and CHF [21]. 

(a) 

(b) 

Figure 9 Effect of the spray flow rate, with subcooled spray: (a) 

boiling curves and (b) heat transfer coefficients. 

For the subcooled spray (Fig. 9), the effect of flow rate on 

h is quite small for both surfaces. However, in the case of the 

near-saturated spray (Fig. 10), the increasing flow rate leads to 

a concomitant increase in the boiling heat transfer, especially 

for the plain surface. While there is a clear trend of increasing 

heat transfer performance with increasing flow rate, from pool 

boiling to 15.8 cm3

/s on the plain surface (Fig. 10), the same 

cannot be said about the microporous surface. At low heat 

fluxes on the microporous surface, the liquid spray seems to 

have little effect on the heat transfer performance, and even 

pool boiling performs as well as the spray. This is possibly 

because, on the microporous surface, phase change heat 

transfer is the dominant part of the heat transferred from the 

surface because of the large nucleation site density, and the 

convective heat transferred is smaller because of the higher 

resistance to fluid flow on the surface. In the case of the plain 

surface, because of the lower bubble density and smoother 

surface, the resistance to the liquid flow on the surface is 

considerably smaller, and the convective heat transferred forms 

a larger part of the total heat flux removed. Consequently, the 

flow rate has a considerably greater effect on the h on the plain 

surface. The CHF, however, increases monotonically with the 

spray flow rate. 

(a) 

(b) 

Figure 10 Effect of flow rate with the near-saturated spray: (a) boiling 

curves and (b) heat transfer coefficients. The pool boiling data are also 

shown for comparison. 

CONCLUSIONS 

Pool boiling and spray impingement boiling experiments 

were performed on copper surfaces coated with a thermally 

conductive copper microporous coating and copper nanowire 

coating, for comparisons to a plain surface. Promising heat 

transfer performances were obtained with the enhanced 
 9 

surfaces in comparison to the plain surface. Under all 

conditions tested, both enhanced surfaces showed lower 

incipience temperatures than those of the plain surface. In pool 

boiling with the surfaces oriented vertically in the pool, the 

microporous surface showed ~500% increase in the heat 

transfer coefficient at the same heater power and ~10% increase 

in the CHF in comparison to the plain surface. The nanowire 

surface showed approximately a 60% increase in the h near the 

CHF, while the CHF itself was considerably lower. 

Spray tests were performed at flow rates of 4.7 cm3

/s, 10.2 

cm3

/s, and 15.8 cm3/s on the microporous surface, with liquid 

subcooling of 30°C and 1°C. Only the lowest flow rate was 

tested on the nanowire surface. The performance of the spray 

cooling at low flow rate on the nanowire surface, in comparison 

to the plain surface, was similar to that observed in the pool 

boiling experiment (enhanced h and lower CHF with respect to 

the plain surface). However, the nanowire surface showed 

degradation in further tests. The short nanowire array used in 

the current study is not optimized for heat transfer. We expect 

that a further increase in the heat transfer, along with a higher 

CHF, may be obtained once the nanowire surface is optimized. 

The microporous surface showed a 100%−300% increase in the 

h at all subcooling levels and flow rate levels over the plain 

surface. The CHF increased by 7%−20% for the microporous 

surface over the plain surface for identical conditions, 

depending on the flow rate. We observed that subcooling of the 

liquid had a strong positive effect at low flow rates throughout 

the nucleate boiling regime and at the lower heat fluxes in the 

sprays with higher flow rates. However, at the highest heat 

fluxes with high flow rate sprays, the subcooling had an 

insignificant effect. The effect of the flow rate on h was found 

to be small for the subcooled spray, while in the saturated case, 

h increased with increasing flow rate. The CHF increased with 

increasing flow rate irrespective of the level of liquid 

subcooling. 

A full understanding of the mechanisms leading to the 

performance improvements observed here is necessary in order 

to further optimize the surface modifications. We are currently 

developing methods to obtain quantitative information 

regarding the relations between the surface structure and the 

flow field so as to develop a mechanistic model to explain the 

observations made here. 

With suitable modifications of the processing conditions, 

some of these enhanced surfaces could potentially be employed 

in power electronics package cooling, thereby resulting in more 

efficient and greater heat dissipation from power electronics 

devices. This will help in achieving the broader goals of 

increased power density and more compact, lightweight power 

electronics modules. 

ACKNOWLEDGMENTS 

The authors would like to acknowledge the support 

provided by Susan Rogers, Technology Development Manager 

for Power Electronics and Electric Machines, Vehicle 

Technologies Program, DOE Office of Energy Efficiency and 

Renewable Energy. They are also grateful to Phillip Tuma (3M 

Company) for providing the microporous coated surface, and to 

members of the Power Electronics team at NREL for fruitful 

interactions. 

NOMENCLATURE 

CHF = critical heat flux, W/cm2 

h = heat transfer coefficient ൌ ݍ"/ሺ∆ܶሻ, W/cm2/K 

I = current, A 

 q = power, W 

q” = heat flux, W/cm2

T = temperature, °C 

ΔT = surface superheat ൌ ܶ௪ െ ܶ௦௔௧, °C 

V = voltage, V 

Subscripts 

calc = calculated from the two thermocouples 

in = input 

liq = liquid 

sat = saturation 

sub = subcooling 

w = wall (surface) 

REFERENCES 

 

1 Bennion, K., 2007, Plug-in Hybrid Electric Vehicle Impacts 

on Power Electronics and Electric Machines, National 

Renewable Energy Laboratory report NREL/MP-540-36085, 

Golden, CO. 

2 Honda, H., and Wei, J.J., 2004, “Enhanced Boiling Heat 

Transfer from Electronic Components by Use of Surface 

Microstructures,” Experimental Thermal and Fluid Science, 28, 

159. 

3 Chang, J.Y., and You, S.M., 1997, “Boiling Heat Transfer 

Phenomena from Microporous and Porous Surfaces in 

Saturated FC-72,” International Journal of Heat and Mass 

Transfer, 40, 4437. 

4 Kim, J.H., Rainey, K.N., You, S.M., and Pak, J.Y., 2002, 

“Mechanism of Nucleate Boiling Heat Transfer Enhancement 

from Microporous Surfaces in Saturated FC-72,” ASME 

Journal of Heat Transfer, 124, 500. 

5 Ammerman, C.N., and You, S.M., 1998, “Consecutive-Photo 

Method to Measure Volume Flow Rate During Boiling From a 

Wire Immersed in Saturated Liquid,” ASME Journal of Heat 

Transfer, 120, 561. 

6 El-Genk, M.S., and Parker, J.S., 2005, “Enhanced Boiling of 

HFE-7100 Dielectric Liquid on Porous Graphite,” Energy 

Conversion and Management, 46, 2455. 

7 Liter. S.G., and Kaviany, M., 2001, “Pool Boiling CHF 

Enhancement by Modulated Porous Layer Coating: Theory and 

Experiment,” International Journal of Heat and Mass Transfer, 

44, 4287. 
 10 

 

8 Li, C., Wang, Z., Wang, P., Peles, Y., Koratkar, N., and 

Peterson, G.P., 2008, “Nanostructured Copper Interfaces for 

Enhanced Boiling,” Small, 4, 1084. 

9 Chen, R., Lu, M.C., Srinivasan, V., Wang, Z., Cho, H.H., and 

Majumdar, A., 2009, “Nanowires for Enhanced Boiling Heat 

Transfer,” Nano Letters, 9, 548. 

10 Estes, K.A., and Mudawar, I., 1995, “Comparison of TwoPhase Electronic Cooling Using Free Jets and Sprays,” Journal 

of Electronic Packaging, 117, 323. 

11Mudawar, I., 2001, “Assessment of High-Heat-Flux Thermal 

Management Schemes,” IEEE Transactions in Components and 

Packaging Technologies, 24,122. 

12 Kim, J., 2007, “Spray Cooling Heat Transfer: The State of 

the Art,” International Journal of Heat and Fluid Flow, 28, 753. 

13 Horacek, B., Kiger, K.T., and Kim, J., 2005, “Single Nozzle 

Spray Cooling Heat Transfer Mechanisms,” International 

Journal of Heat and Mass Transfer, 48, 1425. 

14 Kim, J.H., You, S. M., and Choi, S.U.S., 2004, “Evaporative 

Spray Cooling of Plain and Microporous Coated Surfaces,” 

International Journal of Heat and Mass Transfer, 47, 3307. 

15 Bostanci, H., Rini, D.P., Kizito, J.P., and Chow, L.C., 2009, 

“Spray Cooling With Ammonia on Microstructured Surfaces: 

Performance Enhancement and Hysteresis Effect,” ASME 

Journal of Heat Transfer, 131, 071401. 

16 Mudawar, I., and Estes, K.A., 1996, “Optimizing and 

Predicting CHF in Spray Cooling of a Square Surface,” ASME 

Journal of Heat Transfer, 118, 672. 

17 3M Product Brochure, 2008, 3M Microporous Metallic 

Boiling Enhancement Coating (BEC) L-20227. 

18 Tuma, P.E., 2006, “Evaporator/Boiler Design for 

Thermosyphons Utilizing Segregated Hydrofluoroether 

Working Fluids,” IEEE Semiconductor Thermal Measurement 

and Management Symposium, 69−77. 

19 Chen, X., Duan, H., Zhou, Z., Liang, J., and Gnanaraj, J., 

2008, “Fabrication of Free-Standing Cu Nanorod Arrays on Cu 

Disc by Template-Assisted Electrodeposition,” Nanotechnology, 19, 

365306. 

20 Rainey, K.N., and You, S.M., 2001, “Effects of Heater Size 

and Orientation on Pool Boiling Heat Transfer From 

Microporous Coated Surfaces,” International Journal of Heat 

and Mass Transfer, 44, 2589. 

21 Rini, D. P., Chen, R.H., and Chow, L.C., 2002, “Bubble 

Behavior and Nucleate Boiling Heat Transfer in Saturated FC72 Spray Cooling,” ASME Journal of Heat Transfer, 124, 63. 
F1147-E(10/2008) 

REPORT DOCUMENTATION PAGE Form Approved

OMB No. 0704-0188

The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, 

gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this 

collection of information, including suggestions for reducing the burden, to Department of Defense, Executive Services and Communications Directorate (0704-0188). Respondents 

should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a 

currently valid OMB control number.

PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ORGANIZATION.

1. REPORT DATE (DD-MM-YYYY)

August 2010

2. REPORT TYPE

Conference Paper

3. DATES COVERED (From - To)

 

4. TITLE AND SUBTITLE

Enhancement of Heat Transfer with Pool and Spray Impingement 

Boiling on Microporous and Nanowire Surface Coatings

5a. CONTRACT NUMBER

DE-AC36-08-GO28308

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

S. Thiagarajan, W. Wang, R. Yang, S. Narumanchi, and C. King 

5d. PROJECT NUMBER

NREL/CP-540-48055

5e. TASK NUMBER

FC08.7000

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Renewable Energy Laboratory

1617 Cole Blvd.

Golden, CO 80401-3393

8. PERFORMING ORGANIZATION

REPORT NUMBER

NREL/CP-540-48055

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR'S ACRONYM(S)

NREL

11. SPONSORING/MONITORING

AGENCY REPORT NUMBER

12. DISTRIBUTION AVAILABILITY STATEMENT

National Technical Information Service

U.S. Department of Commerce

5285 Port Royal Road

Springfield, VA 22161

13. SUPPLEMENTARY NOTES

14. ABSTRACT (Maximum 200 Words)

The DOE National Renewable Energy Laboratory (NREL) is leading a national effort to develop next-generation cooling 

technologies for hybrid vehicle electronics. The goal is to reduce the size, weight, and cost of power electronic modules that 

convert direct current from batteries to alternating current for the motor, and vice versa. Aggressive thermal management 

techniques help to increase power density and reduce weight and volume, while keeping chip temperatures within acceptable 

limits. The viability of aggressive cooling schemes such as spray and jet impingement in conjunction with enhanced surfaces is 

being explored. Here, we present results from a series of experiments with pool and spray boiling on enhanced surfaces, such as 

a microporous layer of copper and copper nanowires, using HFE-7100 as the working fluid. Spray impingement on the 

microporous coated surface showed an enhancement of 100%-300% in the heat transfer coefficient at a given wall superheat with 

respect to spray impingement on a plain surface under similar operating conditions. Critical heat flux also increased by 7%-20%, 

depending on flow rates.

15. SUBJECT TERMS

vehicle power electronics; power electronics thermal management; spray cooling; jet impingement cooling; 

immersion boiling; pool boiling

16. SECURITY CLASSIFICATION OF: 17. LIMITATION 

OF ABSTRACT

UL

18. NUMBER 

OF PAGES

19a. NAME OF RESPONSIBLE PERSON

a. REPORT

Unclassified

b. ABSTRACT

Unclassified

c. THIS PAGE

Unclassified 19b. TELEPHONE NUMBER (Include area code)

Standard Form 298 (Rev. 8/98) Prescribed by ANSI Std. Z39.18