Daniel P. Rini

Bubble Behavior and Nucleate Boiling Heat Transfer in Saturated FC-72 Spray Cooling

Bubble behavior during saturated FC-72 spray cooling was experimentally investigated. A heater previously used for pool boiling was used to allow direct comparison. The results are analyzed to reveal the interaction between bubbles and impinging droplets. The following are presented: (1) the importance of secondary nuclei entrained by impingement droplets, (2) the role of impinging droplets on bubble parameters such as growth, diameter at puncture, lifetime, life cycle and bubble number density, and (3) the relative contribution of nucleation, especially that of secondary nuclei, to the heat transfer. It is concluded that increasing the droplet flux increases the number of secondary nuclei helps to lower surface temperature for a given heat flux, increases the overall heat transfer coefficient, and increases heat transfer due to both nucleate boiling and enhanced convection. Increasing the droplet flux also shortens the bubble growth time (i.e., resulting in earlier bubble removal) and life cycle. However, increasing the droplet flux (and, therefore, secondary nucleation) for each of the three heat flux values does not affect the percentage of either nucleate or convection heat transfer. This suggests that both the nucleate and convection heat transfer are enhanced, as a result of increased secondary nuclei and turbulent mixing due to the impinging droplets.

BUBBLE BEHAVIOR AND HEAT TRANSFER MECHANISM IN FC-72 POOL BOILING

A transparent heater made of a thin synthetic diamond substrate along with a high-speed camera was used to investigate bubble behavior during pool boiling. The heater design, combined with the selected FC-72 liquid, overcame the difficulty of previous thin-film heater experiments where transparency and adequate heat flux could not be simultaneously achieved. It also resulted in an essentially uniform temperature field over the heater surface. The growth and merging of bubbles were visualized and quantitatively documented. The relative contribution from phase change to the overall heat flux was determined at several heat flux levels. At a heat flux level half of the critical heat flux (CHF), surface bubble nucleation was found to contribute to more than 70% of the heat transfer from the heater surface. At a similar heat flux level, the ratio of dry to wetted area was determined to exceed 1/3, significantly higher than that predicted by a recent hydrodynamic model for CHF (approximately 1/16). This result s...A transparent heater made of a thin synthetic diamond substrate along with a high-speed camera was used to investigate bubble behavior during pool boiling. The heater design, combined with the selected FC-72 liquid, overcame the difficulty of previous thin-film heater experiments where transparency and adequate heat flux could not be simultaneously achieved. It also resulted in an essentially uniform temperature field over the heater surface. The growth and merging of bubbles were visualized and quantitatively documented. The relative contribution from phase change to the overall heat flux was determined at several heat flux levels. At a heat flux level half of the critical heat flux (CHF), surface bubble nucleation was found to contribute to more than 70% of the heat transfer from the heater surface. At a similar heat flux level, the ratio of dry to wetted area was determined to exceed 1/3, significantly higher than that predicted by a recent hydrodynamic model for CHF (approximately 1/16). This result suggests that modifications are needed for the hydrodynamic model when applied to highly wetting fluid on nearly isothermal surfaces. The merging of bubbles to form vapor blankets over the heater surface was observed, as has been assumed in recent hydrodynamic models.

Spray Cooling With Ammonia on Microstructured Surfaces: Performance Enhancement and Hysteresis Effect

Experiments were performed to investigate spray cooling on microstructured surfaces. Surface modification techniques were utilized to obtain microscale indentations and protrusions on the heater surfaces. A smooth surface was also tested to have baseline data for comparison. Tests were conducted in a closed loop system with ammonia using RTIs vapor atomized spray nozzles. Thick film resistors, simulating heat source, were mounted onto 1 ×2 cm 2 heaters, and heat fluxes up to 500 W/cm 2 (well below critical heat flux limit) were removed. Two nozzles each spraying 1 cm 2 of the heater area used 96 ml/cm 2 min (9.7 gal/in. 2 h) liquid and 13.8 ml/cm 2 s (11.3 ft 3 /in. 2 h) vapor flow rate with only 48 kPa (7 psi) pressure drop. Comparison of cooling curves in the form of surface superheat (ΔT sat =T surf ―T sat ) versus heat flux in the heating-up and cooling-down modes (for increasing and decreasing heat flux conditions) demonstrated substantial performance enhancement for both microstructured surfaces over smooth surface. At 500 W/cm 2 , the increases in the heat transfer coefficient for microstructured surfaces with protrusions and indentations were 112% and 49% over smooth surface, respectively. Moreover, results showed that smooth surface gives nearly identical cooling curves in the heating-up and cooling-down modes, while microstructured surfaces experience a hysteresis phenomenon depending on the surface roughness level and yields lower surface superheat in the cooling-down mode, compared with the heating-up mode, at a given heat flux. Microstructured surface with protrusions was further tested using two approaches to gain better understanding on hysteresis. Data indicated that microstructured surface helps retain the established three-phase contact lines, the regions where solid, liquid, and vapor phases meet, resulting in consistent cooling curve and hysteresis effect at varying heat flux conditions (as low as 25 W/cm 2 for the present work). Data also confirmed a direct connection between hysteresis and thermal history of the heater.

Spray Cooling of IGBT Devices

The popularity and increased usage of insulated gate bipolar transistors (IGBTs) in power control systems have made the problem of cooling them a subject of considerable interest in recent years. In this investigation, a heat flux of 825 W/cm 2 at the die was achieved when air-water spray cooling was used to cool IGBTs at high current levels. The junction temperature of the device was measured accurately through voltage-to-temperature characterization. Results from other cooling technologies and other spray cooling experiments were reviewed. A discussion of electrical power losses in IGBTs, due to switching and conduction, is included in this paper. Experiments were conducted on 19 IGBTs, using data collection and software control of the test set. Three types of cooling were explored in this investigation: single-phase convection with water, spray cording with air-water and spray cooling with steam-water. The results of these experiments show clear advantages of air-water spray cooling IGBTs over other cooling technologies. The applications of spray cooling IGBTs are discussed in open (fixed) and closed (mobile) systems. Current and heat flux levels achieved during this investigation could not have been done using ordinary cooling methods. The techniques used in this investigation clearly demonstrate trio superior cooling performance of air-water spray cooling over traditional cooling methods.

Spray cooling of power electronics using high temperature coolant and enhanced surface

A spray cooling system was developed and tested for thermal management of power inverter modules utilized in automotive applications. System featured an array of 1×2 pressure atomized nozzles that used 90 °C antifreeze coolant with 0.15 L/min-cm2 liquid flow rate and 145 kPa pressure drop. Two cm2 simulated device, having an enhanced spray surface with micro scale structures, reached up to 400 W/cm2 heat flux with only 14 °C surface superheat. These experimental results demonstrated the capability of greatly reducing the overall thermal resistance of the inverter modules that are commonly cooled with single phase convective systems. Performance of the presented system proved the spray cooling of power electronics as an attractive option that enables high power densities while maintaining satisfactory and uniform device temperatures. In addition, due to the use of high temperature coolant at low flow rates, spray cooling offers compact and efficient system design.

Thermal Management of Power Inverter Modules at High Fluxes via Two-Phase Spray Cooling

A spray cooling system was developed and tested for thermal management of power inverter modules utilized in automotive applications. The system featured an array of 1×2 pressure atomized nozzles that used 88°C boiling point antifreeze coolant with 0.15-l/min.cm2 liquid flow rate and 145-kPa pressure drop. A 2-cm2 simulated device, having two kinds of enhanced spray surface with microscale structures, reached up to 400-W/cm2 heat flux with as low as 14 °C surface superheat. These experimental results demonstrated the capability of greatly reducing the overall thermal resistance of the inverter modules that are commonly cooled with single-phase convective systems. The long-term reliability of the spray cooling was assessed with 2000 h of testing time. Performance of the presented system proved the spray cooling of power electronics as an attractive option that enables high power densities while maintaining acceptable and uniform device temperatures. In addition, due to the use of high temperature coolant at low flow rates, the spray cooling offers a compact and efficient system design.

Evaporative spray cooling of power electronics using high temperature coolant

A pressure atomized evaporative spray cooling nozzle array was used to thermally manage the power electronics of a 3 phase inverter module. The module tested was a COTS module manufactured by Semikron, Inc., and has a maximum DC power input of 180 kW (450 VDC and 400 A) with 25degC coolant. However, the standard heat sink that the module uses is a single phase liquid heat sink and when 100degC coolant is used (as in automotive applications), the maximum module power is de-rated to 45 kW so that the IGBT chips will not overheat. The module tested here incorporated a custom heat sink that allowed for the use of spray cooling nozzles, which were designed and developed by RTI. The spray liquid was a 50/50 mixture of water and propylene glycol (WPG) at a temperature of 100degC. The sprays impinged directly onto the bottom surface of the DBC boards to which the power electronics were mounted. This arrangement, combined with the high heat transfer coefficient of evaporative spray cooling, greatly reduced the thermal resistance of the power electronics material stack up, but did so without directly wetting the electronics. The results of this work were that the unique evaporative spray cooling nozzle design and patented electronics interface design allowed the module to be run to full power while keeping the IGBT junction temperatures acceptable, despite the high coolant temperature. The junction temperatures of the IGBTs were measured by electrically insulated type T thermocouples placed on top of the devices, and the thermocouple readings at the full load were within several degrees of one another. Consistent and uniform junction temperatures are an important factor in long term device reliability. For the standard heat sink, which uses single phase liquid cooling, the pressure drop and flow rate required for maximum heat removal would be 17 psi and 5.3 GPM. For the pressure atomizer spray nozzles, the module would require a pressure drop and flow rate of 40 psi and only 2.7 GPM.

Droplet and Bubble Dynamics in Saturated FC-72 Spray Cooling on a Smooth Surface

Droplet and bubble dynamics and nucleate heat transfer in saturated FC-72 spray cooling were studied using a simulation model. The spray cooling system simulated consists of three droplet fluxes impinging on a smooth heater, where secondary nuclei outnumber the surface nuclei. Using the experimentally observed bubble growth rate on a smooth diamond heater, submodels were assumed based on physical reasoning for the number of secondary nuclei entrained by the impinging droplets, bubble puncturing by the impinging droplets, bubble merging, and the spatial distribution of secondary nuclei. The predicted nucleate heat transfer was in agreement with experimental findings. Dynamic aspects of the droplets and bubbles, which had been difficult to observe experimentally and their ability in enhancing nucleate heat transfer were then discussed based on the results of the simulation. These aspects include bubble merging, bubble puncturing by impinging droplets, secondary nucleation, bubble size distribution, and bubble diameter at puncture. Simply increasing the number of secondary nuclei is not as effective in enhancing nucleate heat transfer as when it is also combined with increased bubble puncturing frequency by the impinging droplets. For heat transfer enhancement, it is desirable to have as many small bubbles and as high a bubble density as possible.

Evaluation of Compact and Effective Air-Cooled Carbon Foam Heat Sink

This study investigates a V-shaped corrugated carbon foam heat sink for thermal management of electronics with forced air convection. Experiments were conducted to determine the heat sink performance in terms of heat transfer coefficient and pressure drop. The test section, with overall dimensions of 51 mm L X51 mm W ×19 mm H, enabled up to 166 W of heat dissipation, and 3280 W/m 2 K and 2210 W/m 2 K heat transfer coefficients, based on log mean and air inlet temperatures, respectively, at 7.8 m/s air flow speed, and 1320 Pa pressure loss. Compared to a solid carbon foam, the V-shaped corrugated structure enhances the heat transfer, and at the same time reduces the flow resistance. Physical mechanisms underlying the observed phenomena are briefly explained. With benefits that potentially can reduce overall weight, volume, and cost of the air-cooled electronics, the present V-shaped corrugated carbon foam emerges as an alternative heat sink.

Microscale surface modifications for heat transfer enhancement.

In this experimental study, two surface modification techniques were investigated for their effect on heat transfer enhancement. One of the methods employed the particle (grit) blasting to create microscale indentations, while the other used plasma spray coating to create microscale protrusions on Al 6061 (aluminum alloy 6061) samples. The test surfaces were characterized using scanning electron microscopy (SEM) and confocal scanning laser microscopy. Because of the surface modifications, the actual surface area was increased up to 2.8× compared to the projected base area, and the arithmetic mean roughness value (Ra) was determined to vary from 0.3 μm for the reference smooth surface to 19.5 μm for the modified surfaces. Selected samples with modified surfaces along with the reference smooth surface were then evaluated for their heat transfer performance in spray cooling tests. The cooling system had vapor-atomizing nozzles and used anhydrous ammonia as the coolant in order to achieve heat fluxes up to 500 W/cm(2) representing a thermal management setting for high power systems. Experimental results showed that the microscale surface modifications enhanced heat transfer coefficients up to 76% at 500 W/cm(2) compared to the smooth surface and demonstrated the benefits of these practical surface modification techniques to enhance two-phase heat transfer process.