Force-Fed Microchannels for High Flux Cooling Applications

In this chapter, the working principles of force-fed microchannel (micro-grooved) heat exchangers (FFMHX) in single-phase and two-phase heat transfer modes are defined and the associated benefits of using these systems are highlighted. The literature on FFMHX is reviewed, and important conclusions are summarized. The main benefits of optimally designed next generation force-fed heat transfer (FFHT) configurations are the short, parallel microchannel network system and equal flow distribution among the channels through careful manifold design. The collective benefits include substantially higher available heat/mass transfer surface area, precise flow distribution among the channels, creation of thin film cooling in microchannels such that the thin film component dominates the flow regime, and low to moderate pressure drops while achieving record-high heat/mass transfer coefficients. The information presented in this chapter includes experimental evaluation of the thermal performance of FFMHXs in two-phase heat transfer mode using refrigerant R-245fa. Two distinct heat transfer trends were observed. At high hydraulic diameter, high mass flux, and high heat flux, the heat transfer coefficients had a slowly increasing trend with increase in heat flux and outlet quality. At low hydraulic diameters, low mass flux, and low heat flux, the heat transfer coefficients experience a bell-shaped behavior with a sharp increase at low vapor qualities until they reach the maximum peak point. Most recent research has focused on increasing the vapor quality at the exit of the evaporator for energy efficiency and optimum system performance. It was demonstrated that the tested FFMHX heat sink configuration can cool a heat flux of q″base = 1.23kW/cm2 with a superheat of Δ T sat = 56.2°C and pressure drop of only Δ P = 60.3 kPa. The results clearly suggest that under optimum design conditions, FFMHXs have the proven potential to achieve substantially higher heat transfer coefficients with low to moderate pressure drops, and substantially less working fluid in circulation, and thus substantial compaction (weight/volume reductions), improved energy efficiency, and reduced capital and operational cost. When compared to the state-of-the-art thermal management/conventional systems.