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This paper reviews recent advances in pool and flow boiling of dilute emulsions, including initial theories on the boiling mechanisms. The current consensus is that pool boiling is initiated on nucleation sites on the surface and that increased heat transfer coefficients are a result of mixing produced by vapor bubbles in the thermal boundary layer. Visualization of the boiling process substantiates this conclusion. Current understanding of the influence of various quantities on the convective heat transfer coefficient is enumerated with respect to the underlying thermodynamics and energy transport. Prior research on flow boiling is limited in comparison, and much more needs to be done with respect to the boiling and enhancement mechanism. Various models for boiling of emulsions are discussed, including a RANS model that incorporates droplet contact with the heated surface, collisions between droplets and bubbles, and chain boiling due to collision during the boiling process. Key modeling elements are how interphase transport and droplet interactions affect nucleation rates and droplet behavior within the viscous and thermal boundary layers at and near a heated surface. It is clear now that boiling of a suspended phase with a lower saturation temperature than the continuous phase can produce marked increases in overall heat transfer coefficients. A re-conceptualization of two-phase heat exchange equipment is a future possibility, and various applications are identified. Boiling of dilute emulsions may be an attractive cooling strategy when restrictions on size, weight and pumping power must be addressed, such as in thermal management of avionics systems.

Flow boiling in an onboard variable microgap is demonstrated as a viable cooling method for multidevice electronics. The microgap is created by a bonded conformal encapsulation that delivers uniform subcooled inlet coolant flow across a multidevice layout comprising a processor and two in-line, symmetrically placed memory devices. Each device is simulated with a ceramic resistance heater on a 1:1 scale, and the heights of the devices create the variable microgap under the roof line of the encapsulation. The gap height for the processor is 0.5 mm and 1 mm for the memory devices. Parameters investigated are pressure drop, average device temperature, processor power, and coefficient of performance (COP). For inlet coolant flow first over the memory devices, the average device temperature exceeds the 95 °C limit when processor power is ∼50 W or less. For inlet flow over the processor, memory device temperatures are approximately the same over all the levels of processor and memory chip power. For processor power 40 W, two-phase heat transfer dominates, and processor power of 120 W is reached within the 95 °C threshold. Volumetric power density across the data set is 134 to 1209 W/cm3.

Onboard liquid cooling of electronic devices is demonstrated with liquid delivered externally to the point of heat removal through a conformal encapsulation. The encapsulation creates a flat microgap above the integrated circuit (IC) and delivers a uniform inlet coolant flow over the device. The coolant is Novec™ 7200, and the electronics are simulated with a resistance heater on a 1:1 scale. Thermal performance is demonstrated at power densities of ∼1 kW/cm3 in the microgap. Parameters investigated are pressure drop, average device temperature, heat transfer coefficient, and coefficient of performance (COP). Nusselt numbers for gap sizes of 0.25, 0.5, and 0.75 mm are reduced to a dimensionless correlation. With low coolant inlet subcooling, two-phase heat transfer is seen at all mass flows. Device temperatures reach 95 °C for power dissipation of 50–80 W (0.67–1.08 kW/cm3) depending on coolant flow for a gap of 0.5 mm. Coefficients of performance of ∼100 to 70,000 are determined via measured pressure drop and demonstrate a low pumping penalty at the device level within the range of power and coolant flow considered. The encapsulation with microgap flow boiling provides a means for use of higher power central processing unit and graphics processing unit devices and thereby enables higher computing performance, for example, in embedded airborne computers.

A new technology for onboard liquid cooling of high power density electronic devices is introduced via conformal encapsulation of the devices and direct contact liquid cooling. This research effort addresses size, weight and power constraints of onboard application with a CFD-enabled design that delivers a uniform coolant flow over single- and multi-device layouts through a microgap channel. The paradigm shift is the replacement of inefficient remote air cooling and associated high resistance conduction paths with the use of microgap flow boiling with direct coolant contact at the device level. The coolant used in all measurements is Novec™ 7200, and the electronics are emulated with resistance heaters on a 1∶1 scale. Thermal performance is demonstrated at power densities on the order of 1 KW/cm3. Parameters investigated include average device temperature, pressure drop, flow field characterization, and overall heat transfer coefficients. For single chip encapsulation, thermal-fluid performance with microgaps of 0.25, 0.5 and 0.75 mm is determined. With low coolant inlet subcooling, two-phase heat transfer is seen at all coolant mass flows. Device temperatures reach 95 °C for power dissipation of 50 – 80 W depending on coolant flow for a gap of 0.5 mm. Inlet subcooling of 25 and 51 °C permits higher power dissipation with nucleate flow boiling on the device surface. For multi-device encapsulation comprising two memory chips arranged symmetrically in line with a larger processor, the best thermal performance is obtained for inlet flow over the processor. For all measurements, the gap between the processor and encapsulation is 0.5 mm, and the gap above the memory chips is 1.0 mm. For inlet coolant flow first over the memory chips, the small chips exceed the 95°C limit when processor power is ∼50 W or less. Processor temperature reaches 95 °C at ∼80 W over the range of coolant flows tested. For inlet flow first over the processor, memory device temperatures are approximately the same over all levels of processor and memory chip powers. For processor power 40 W, two-phase heat transfer dominates, and a processor power of 120 W is reached within the 95 °C threshold.