Investigation of subcooled boiling in micro-channel heat sink for indirect refrigeration cooling applications
Abstract
A new cooling scheme is proposed where the primary working fluid is pre-cooled to low temperature using an indirect refrigeration cooling system. The cooling performance was explored using HFE 7100 as working fluid and four different microchannel sizes. High-speed video imaging was employed to help explain the complex interrelated influences of channel geometries and flow conditions on cooling performance. Unlike most prior two-phase flow studies, which involved annular film evaporation due to high void fraction, the low coolant temperatures used in this study produced subcooled flow boiling conditions. Increasing liquid subcooling decreased two-phase pressure drop because of decreased void fraction, caused by strong condensation at bubble interfaces, as well as decreased likelihood of bubble coalescence. It is shown macro-channel subcooled boiling pressure drop and heat transfer correlations are unsuitable for micro-channel flows. A new model is proposed to predict the pressure drop characteristics of subcooled two-phase micro-channel heat sinks. This model depicts the subcooled flow as consisting of a homogeneous two-phase flow layer near the heated walls of the microchannel and a second subcooled bulk liquid layer. Mass, momentum and energy control volume conservation equations are combined to predict flow characteristics for thermodynamic equilibrium qualities below zero. The model shows good predictions of pressure drop data for different mass velocities and subcoolings for four different micro-channel sizes. The high subcooling greatly reduced both bubble departure diameter and void fraction, and precluded flow pattern transitions beyond the bubbly regime. CHF was triggered by vapor blanket formation along the micro-channel walls despite the presence of abundant core liquid. CHF increased with increasing mass velocity and/or subcooling and decreasing hydraulic diameter for a given total mass flow rate. A pre-mature type of CHF was caused by vapor backflow into the heat sink's inlet plenum at low mass velocities and small inlet subcoolings, and was associated with significant fluctuations in inlet and outlet pressure, as well as wall temperature. A systematic technique is developed to modify existing CHF correlations to more accurately account for features unique to micro-channel heat sinks, including rectangular cross-section, three-sided heating, and flow interaction between micro-channels.
Degree
Ph.D.
Advisors
Mudawar, Purdue University.
Subject Area
Mechanical engineering
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