TRACTION DRIVE INVERTER COOLING WITH 

SUBMERGED LIQUID JET IMPINGEMENT ON 

MICROFINNED ENHANCED SURFACES

Scot Waye, Sreekant Narumanchi, Gilbert Moreno

National Renewable Energy Laboratory

SAE 2014 Thermal Management Systems Symposium

September 22–24, 2014

Denver, Colorado

14TMSS-0020

NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

NREL/PR-5400-62921
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Thermal Management of Power Electronics

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Prius 2004 Camry 2007

Prius 2010 Camry 2012

Images courtesy: ORNL

Impacts: Lower cost, volume, and weight

Enabling technology : Enhanced heat transfer/thermal management

Leaf 2011

State of the Art
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Research Objectives

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Objectives

• Design and develop a light-weight, 

single-phase, liquid-cooled, 

automotive inverter-scale heat 

exchanger based on impinging jets 

and enhanced surfaces

• Enable use of high-temperature 

water-ethylene glycol (WEG) coolant 

for power electronics cooling

Advanced thermal management technologies are critical to enabling higher power 

densities and meeting DOE targets for specific power, power density, and cost
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Jets on Enhanced Surfaces

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0

25,000

50,000

75,000

100,000

125,000

150,000

0 3 6 9 12 15

h (W/m2-K)

U (m/s)

Submerged Jets

Baseline (Ra=0.3um)

Sandpaper (Ra=4.1um)

Sandblasted (Ra=4.16um)

Finned (140% area increase)

Microcool (Wolverine)

Microporous (3M)

Spray Pyrolysis (NREL)

Nanowire (CU)

0%

50%

100%

150%

0 5 10 15

Heat Transfer Enhancement

Nozzle Velocity (m/s)

Wolverine MicroCool Surface

Why use a microfinned enhanced surface? 

Large increase in heat transfer coefficient
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Three Configurations Were Tested

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Baseline heat exchanger

Aluminum channel flow cold plate

First version heat exchanger prototype

Plastic manifold (lower weight)

Same flow path as baseline

Submerged jet impingement on plain surfaces

Submerged jet impingement on microfinned enhanced surfaces

Second version heat exchanger prototype

Plastic manifold (lower weight)

Simplified flow path to reduce pressure loss

Submerged jet impingement on plain surfaces

Submerged jet impingement on microfinned enhanced surfaces
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Experimentation and Modeling Approach

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• 50%–50% WEG at 70oC; 5, 8, 10 L/min

• Power 4 diodes (105 W heat)

• Metrics: Rth,j-l using T3ster, ∆P

Experiments 

(low power)

• Validate models at 105-W heat 

(50%–50% WEG at 70oC; 5, 8, 10 L/min)

• Model 2.5 kW heat (24 IGBTs, 24 diodes)

• Metrics: Rth,j-l

, ∆P

Modeling 

(CFD)

• 40%–60% WEG at 30oC, 10 L/min

• 40, 60, 80, 100 kW Electrical Power

• Metrics: ∆T between probes and coolant

Experiments 

(inverter level)
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Reliability Characterization

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By-pass

Pump

Test 

samples

T

Flow 

meters

Valves

Flow to eight 

samples within 

the test section

Strainer

Cartridge 

heaters

12-month testing of free WEG jet 

impinging on microfinned surface

• 35oC WEG

• 2 m/s jet, 12 m/s jet

Long-term testing of submerged

WEG jet on 2 microfinned

surfaces (nickel-plated), 3 DBC 

substrates, and 3 DBA substrates

• 65oC automotive-grade WEG

• 5 m/s jet (1.3-mm nozzle, 3-

mm jet distance)

W1

W2

t = 0 (initial) t = 12 months

previous work

DBA = direct-bond-aluminum

DBC = direct-bond-copper
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Thermal Resistance Map from T3ster (105-W experiment)

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10.3%

5.1%

Reduction in thermal resistance:
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Performance at 2.5 kW Heat Dissipation (model)

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• 9% (plain) to 32% (microfinned) reduction in thermal resistance 

• 5% (plain) to 40% (microfinned) increase in COP (1/Rth[∆P⋅V])

• 29% (plain) to 55% (microfinned) increase in specific power (kW/kg)

• 6% (plain) to 28% (microfinned) increase in power density (kW/L)

• Values from modeling represent idealized limit due to external adiabatic 

boundaries driving heat into the coolant

Temperature reduction (jets to baseline):

• 5oC – 6oC (plain)

• 15oC – 16oC (microfinned)

Jets provide localized 

cooling on devices

Heat Transfer Coefficient (W/m2-K)
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Passive Stack Resistance Dependence

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Decreased Convective 

Cooling Resistance

Decreased 

Rth,j-l

Increased Heat Flux 

and Distribution

Channel Flow 

Cold Plate

Jets on Plain 

Surface

Jets on Enhanced 

Surface

4 diodes 

(105 W)

24 IGBTs / 

24 diodes 

(2520 W)
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Inverter Testing on Dynamometer

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40, 60, 80, 100 kW Electrical Power

0.9, 1.3, 1.6, 2.2 kW heat dissipation (model estimate)

0.9, 1.1, 1.6, 1.9 kW heat dissipation (simple energy balance)

Thermocouple locations 

(middle of copper baseplate)

Channel Flow Jet Impingement

∆T = TTC,avg – Tcoolant, avg
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Thermocouple Temperatures

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Power [kW] 40 60 80 100

Channel, Plain 46.6 52.5 60.2 68.7

Jet, Plain N/A 53.5 60.0 67.6

Jet, Microfinned 44.8 50.2 56.1 62.3

Average thermocouple temperature [oC]

Lower temperatures

Less variability
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Temperature Difference (Baseplate to Coolant)

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∆T proportional to thermal resistance (Rth = ∆T/Q)
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Full Inverter Thermal Performance

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Power [kW] 40 60 80 100

Coefficient of Performance

Jet, Plain -28.5% -0.8% 1.7% -0.5%

Jet, Microfinned 4.8% 14.2% 17.0% 17.4%

Specific Power

Jet, Plain 15.8% 20.1% 22.5% 21.1%

Jet, Microfinned 26.4% 33.6% 35.6% 35.9%

Power Density

Jet, Plain -4.5% -0.9% 1.1% -0.1%

Jet, Microfinned 4.3% 10.2% 11.9% 12.1%

• Jets with plain surfaces: little 

improvement

• Jets with microfinned enhanced 

surfaces: considerable 

improvement

Improvement over baseline
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Inverter-Scale Experimental Thermal Performance Summary

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• Full inverter testing (40 – 100 kW) with jets and microfinned surfaces:

Reduction in thermal resistance

Increase in Coefficient of Performance

Increase in power density

Increase in specific power (reduction in heat exchanger weight by 

~ 3 kg) compared to channel flow

• Jets with microfinned surfaces provide localized cooling and improved 

temperature uniformity (50C less spread) than channel flow case

• Jet reliability is good 

17%

17%

12%

36%
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Jet and Surface Reliability (First Round Testing)

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• Negligible change in jet nozzle diameter after 12 months 

of nearly continuous impingement

• Degradation in thermal performance of surfaces due to 

oxidation (no coating, no corrosion inhibitors)

G. Moreno, S. Narumanchi and T. Venson, "Micro-structured surfaces for single-phase jet impingement heat transfer enhancement," ASME Journal of Thermal Science and Engineering Applications, vol. 5, pp. 1-9, 2013. 
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Surface Reliability (Second Round Testing)

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• Initial break-in period

• Some degradation on surfaces

• Corrosion inhibitors appear to coat surfaces
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Thermal Performance Summary (Compared to Baseline Channel 

Flow)

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Jet, Plain Thermal 

Resistance

COP Specific 

Power

Power 

Density

105 W Experiment 

(70oC 50%-50% WEG)

2.5 kW Model 

(70oC 50%-50% WEG)

~2 kW Experiment 

(30oC 40%-60% WEG)

Jet, Microfinned

105 W Experiment

(70oC 50%-50% WEG)

2.5 kW Model 

(70oC 50%-50% WEG)

~2 kW Experiment 

(30oC 40%-60% WEG)

5%

9%

2%

10%

32%

17%

0%

5%

0%

9%

40%

17%

29%

21%

55%

36%

6%

0%

28%

12%

•Jets provide localized 

cooling on devices

• Enhanced surfaces 

increase jet effectiveness
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Potential for Further Jet Optimization

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Current 

Convective 

Cooling 

Resistance

Baseline (Channel Flow)

Jet Impingement

More aggressive jet cooling strategies can further lower resistance 

(need to be balanced to not overly increase pressure drop/fluid power)

Previous work illustrating thermal resistances
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Conclusions

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• Light-weighting with plastics possible with jet impingement 

because thermal path does not require high conductivity

• Jets provide localized increase in heat transfer (where you need it)

• Enhanced surfaces increase jet heat transfer effectiveness (greater 

surface area and flow dynamics)