Conjugate Heat Transfer in Single-Phase and Two-Phase Flow Boiling in Mini/ Microchannels

Abstract

Miniaturization of modern electronic packing led to very high power density, and cooling of such high power density is now a critical neck task. Cooling of such electronic devices is necessary to ensure the safe and reliable functioning. Owing to its ability of high heat dissipation and compact in size, makes microchannel based heat exchanger very attractive choice to satisfy cooling requirements. However, in many microchannel heat sink, hot spots are found due to uneven distribution of heat flux. Also, thickness of microchannel substrate is usually higher as compared to hydraulic diameter of microchannel. This causes flow of large amount of heat towards the inlet region, in a direction opposite to that of fluid flow. This tendency of heat transfer is called “axial wall conduction” or “axial back conduction”. This has motivated the present study to develop effective design of heat sink that can be directly embedded on the back of heat source for uniform heat flux removal.

Numerical study of conjugate heat transfer effect in different microchannel, subjected to constant wall heat flux at the bottom surface is presented. Single and two-phase flow analysis of different microchannel heat sinks are carried out separately with different underlying assumptions. Small hydraulic diameters in the range of 0.4 mm ≤ Dh ≤ 0.72 mm are considered which is engraved on solid rectangular substrate. For comparison purpose substrate width (W = 1.8 mm) and length (L = 30 mm) are kept constant throughout the study. To explore conjugate effect on overall heat transfer, substrate thickness to channel height ratio (δsf) and wall to fluid conductivity ratio (ksf) are varied.
Study of single-phase flow problem is considereded in four parts where the objective in first three part is to highlight conjugate heat transfer effect in single-phase laminar developing flow in microchannel is studied theoretically and numerically to find basic difference in conjugate heat transfer process experienced in a microchannel subjected to constant heat flux imposed on its bottom surface. Water and nanofluids such as water-Al2O3, water-Cuo, Sio2 used as coolant and their individual effect on thermal performance of different microchannels is directly compared. Besides that, other factors such as convective heat transfer, thermal resistance, pressure drop, and uniform heat flux distribution affecting thermal performance of microchannel are studied in detail. In the remaining part is to compute effect of axial wall conduction in microtube. For this purpose, a new Nusselt number equation is proposed which include the axial wall conduction. Additionally, in an actual microprocessor, overall size of microchannel based heat sink is a bit larger due to which the ends of microchannel heat sink are not in direct contact with the processor unit need to be cooled. This corresponds to heat transfer from microprocessor to microchannel heat sink only in the central portion while ends of the heat sink remain adiabatic. To understand conjugate effect in such scenario, partial heated microtube is analysed which will serve as the reference for other geometries. This unique assumption helps to observe impact of axial wall conduction at entrance and exit region of microtube.

Flow boiling is a promising solution to cool down high heat flux electronic devices to operate within safe operating temperature range. Thus, two-phase flow boiling also investigated to analyze effect of axial wall conduction in different microchannel heat sink. Effect of conjugate heat transfer on behavior of bubble growth rate and flow pattern observed for wide range of parametric variation in different microchannel heat sink.
Validation of present study is done by comparson with existing study available in literature. Results are presented in terms of dimensionless heat flux, local wall and bulk fluid temperature, local and average Nusselt number, friction factor, thermal resistance, and performance factor in single and two-phase laminar flow. General conclusion drawn from results is that wall to fluid conductivity ratio (ksf), substrate thickness to channel height ratio (δsf) and geometrical configurations are important parameters which are affecting thermal performance of microchannel. Geometrical modification of raccoon microchannel performed to reduce overall pressure drop while enhancing overall heat transfer along with temperature uniformity. The author believes that the observation obtained in the present thesis will be helpful to fulfill demands of further research works in this area.