Avijit Bhunia

Flow around a bubble on a heated wall in a cross-flowing liquid under microgravity condition

Abstract Fluid motion around a bubble placed on a heated wall of a flowing liquid channel is studied under microgravity condition, using a spectral element based two-dimensional numerical model. It is shown that the flow and temperature fields around the bubble are governed by an interaction between thermocapillary and forced convection. An opposing interaction between the two convection mechanisms creates a recirculation cell at the downstream side of the bubble. Channel flow velocity, length of the heated wall before the bubble, and temperature difference between the heated wall and the bulk liquid are shown to be the most important variables. Their effects on the bubble surface temperature, surface velocity, stagnation point on the bubble surface, length of the recirculation cell along the heated wall, and wall heat transfer near the bubble are investigated.

Liquid Jet Impingement Cooling of a Silicon Carbide Power Conversion Module for Vehicle Applications

Thermal management of power electronics is an extremely challenging problem in the harsh environment of military hybrid vehicles, where the local air and liquid coolants temperature exceed 100 °C under regular operating conditions. This paper presents the development work of a high heat flux, jet impingement-cooled heat exchanger for a 600-V/50-A silicon carbide (SiC) power module (rated at 175 °C device junction temperature), used for bidirectional power conversion between a 28-V battery and a 300-V dc bus. A total of 50 volume% mixture of water-ethylene glycol (WEG) coolant at 100 °C inlet temperature is the only available coolant. An array of WEG coolant microjets impinges on the base plate of the SiC module. The jet impingement cooling system has been optimized by experimental studies on a surrogate module, along with a high-fidelity computational model, to accurately estimate the SiC device junction temperature in relevant operating conditions. Results indicate that at the design heat load of 151 W (worst-case scenario), the SiC device junction temperature is reduced from 290 °C with commercial-off-the-shelf (COTS) cold plate cooling and 215 °C with COTS microchannel heat exchanger cooling, to 169 °C with a jet impingement-cooled heat exchanger, using the same flow rate.

High temperature semiconductor diode laser pumps for high energy laser applications

Existing thermal management technologies for diode laser pumps place a significant load on the size, weight and power consumption of High Power Solid State and Fiber Laser systems, thus making current laser systems very large, heavy, and inefficient in many important practical applications. To mitigate this thermal management burden, it is desirable for diode pumps to operate efficiently at high heat sink temperatures. In this work, we have developed a scalable cooling architecture, based on jet-impingement technology with industrial coolant, for efficient cooling of diode laser bars. We have demonstrated 60% electrical-to-optical efficiency from a 9xx nm two-bar laser stack operating with propylene-glycolwater coolant, at 50 °C coolant temperature. To our knowledge, this is the highest efficiency achieved from a diode stack using 50 °C industrial fluid coolant. The output power is greater than 100 W per bar. Stacks with additional laser bars are currently in development, as this cooler architecture is scalable to a 1 kW system. This work will enable compact and robust fiber-coupled diode pump modules for high energy laser applications.

Transitions of Heat Transfer Modes on Microfabricated Copper Wick Structures

Electroplating has been implemented in the fabrication of radio frequency circuits for many years. Combined with a photolithography process, this technology can be employed to precisely define both the particle size and distributions of copper wick structures in a phase-change heat transfer system. In this paper, two electroplated copper wick structures are developed to investigate characteristics of phase-change heat transfer at the early phase-change stage when applied heat flux is below ∼250  W/cm2. Using subcooled operating fluid, both visualization and heat transfer characterization indicate that the appearance of the onset of nucleation boiling plays a critical role in the transitions of the heat transfer model. Before the onset of nucleation boiling is triggered, heat transfer is dictated by the heat conduction of the wick, as well as evaporation on the meniscus interface. After the onset of nucleation boiling, the heat transfer coefficient is rapidly enhanced by boiling and evaporation within the ...

A Phase Change Study of Electroplated Copper Wick Structures

Electroplating has been implemented in the fabrication of RF circuits for many years. Combined with a photolithography process, this technology can be employed to precisely define both the particle size and geometry of copper wick structures in a phase change heat transfer system. In this article, two electroplated copper wick structures are developed to investigate characteristics of phase change heat transfer at the early stage when applied heat flux is below 250W/cm. Using subcooled operating fluid, both visualization and heat transfer characterization indicate that the appearance of the onset of nucleation boiling (ONB) plays a critical role in determining the heat transfer model. Before the ONB is triggered, heat transfer is dictated by the heat conduction of the wick, as well as evaporation on the meniscus interface. After the ONB, the heat transfer coefficient is rapidly enhanced by phase change within the wick structures. On the characterization curves of heat flux versus the substrate temperature, the wick properties, such as porosity, directly affect the position of the ONB. Submicron porous structures on electroplated copper pillars accelerate the ONB and enhance the heat transfer coefficients of the phase change.