Effects of Sub-atmospheric Pressure and Ethylene Glycol on Heat Transfer Performance of Counter Current Flow Diverging Microchannel Heat Sink: Thermal Management Solutions For High-power Electronics

Abstract

Driven by advancements such as artificial intelligence, effective thermal management has become a critical issue for high-power electronic devices. An innovative counter current flow diverging microchannel (CFDM) heat sink was previously developed in our group. This design leverages channel-to-channel heat transfer to deliver ultrahigh cooling performance. This study focuses on the application of the CFDM heat sink for the cooling of high-power chips and insulated gate bipolar transistor (IGBT) modules.

To achieve a lower saturation temperature when using water as the working fluid, the system must be operated at a sub-atmospheric pressure. Therefore, investigating the heat transfer performance of CFDM heat sinks under such pressures is essential for evaluating their suitability in electronics cooling applications with stringent low-temperature requirements. The bubble dynamics of water at inlet pressures of 40, 60, 80, and 120 kPa in a CFDM heat sink with an area of 20 mm×40 mm is studied to better understand the effect of sub-atmospheric pressure on convective boiling heat transfer, and the effects of bubble dynamics are investigated. Across all pressure conditions, bubble growth and shrinkage were examined at or near the onset of nucleate boiling (ONB) heat flux in the heat sink. Bubble shrinkage reflects the cooling effect provided by neighboring channels through the counterflow design. Furthermore, with decreasing pressure, the average peak bubble length and shrinking time both showed a general increasing trend, which is attributed to the rising liquid-to-vapor density ratio. Concurrently, bubble frequency decreases significantly, while the bubble departure length increases as inlet pressure is reduced. Under an inlet pressure of 40 kPa, the saturation temperature corresponding to the system pressure of 31.4 kPa is approximately 70.0 °C and density ratio of liquid to vapor is 4898, which is significantly different from that of about 1000 at 1 atm. Such a large density ratio makes the bubble very long and deteriorates the boiling heat transfer.

The current heat fluxes in the power IGBT modules of an electric vehicle range from 100–150 W/cm². With increasing current density and switching frequency, the upper limit is projected to reach 500 W/cm²; thus, exploring thermal management for IGBT modules is of significant interest and importance. With the development of high-temperature packaging technologies, the junction temperature of silicon carbide (SiC) IGBT modules reaches 200 °C. This indicates that a two-phase heat transfer process of water or ethylene glycol/water (EGW) mixtures (boiling point is equal to or higher than 100 °C at 1 atm) can be employed for IGBT cooling. EGW mixtures are antifreeze liquids that have been widely used in electric vehicle cooling systems. In this study, the heat transfer performance levels of deionized (DI) water and EGW mixtures (10%, 30%, and 50% volumetric concentrations) in the CFDM heat sink are compared experimentally and analyzed. On the basis of the dimensions of the electric vehicle IGBT modules, the contact heating area is designed to be 30 mm × 40 mm. The results of this study reveal that with increasing concentration of ethylene glycol, the heat transfer performance deteriorates because of the lower thermal conductivity and mass diffusion of water to the vapor-liquid interface. At the onset of nucleate boiling (ONB) in deionized water, discrete and very small bubbles emerge in the middle region of the channel. In contrast, the bubble size is considerably increased in the EGW mixture. In addition, it is more difficult for bubble shrinkage to occur in the EGW mixture because of the lower channel-to-channel heat transfer performance due to lower thermal conductivity of the liquid film with rich ethylene glycol. For the 50% EGW mixture, the present CFDM heat sink with cavity arrays demonstrates a heat flux of about 250 W/cm² with wall temperature being lower than 170 °C. For the similar wall temperature, the heat flux may reach 407 W/cm² if microgrooves are further laser etched on the bottom wall of the microchannels in the heat sink. Such a high heat dissipation flux is about 100% higher and the corresponding HTC is about 140% higher than the highest values reported in the literature. This is much higher than the thermal management requirements of current IGBT modules and has great potential meeting the cooling requirement of next generation electric vehicles.

Date of Award	5 Feb 2026
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Chin PAN (Supervisor) & Jiyun ZHAO (Co-supervisor)