Technical Background: Micro-Channel Cooling

Physics of Fluid Flow and Heat Transfer in Micro-channel Heat SinksMicro-channel cooling is commonly achieved with the aid of a heat sink consisting of a high conductivity substrate that contains a large number of parallel, small diameter channels. These heat sinks are very compact and lightweight, and by allowing the coolant to undergo phase change (boiling) along the channels, they provide heat transfer coefficients far greater than those possible with single-phase liquid counterparts.  This greatly reduces the coolant flow rate required to dissipate the same amount of heat compared to a single-phase heat sink, and also helps reduce coolant inventory for the entire system.  Two-phase heat sinks also provide better temperature uniformity by maintaining surface temperatures close to the coolant’s saturation temperature. These attributes, coupled with design simplicity, are key reasons behind the unprecedented popularity of micro-channel heat sinks for high-heat-flux cooling applications. 

Flow Boiling in Micro-channelsBut implementation of these heat sinks requires use of highly accurate predictive tools to capture the detailed two-phase behavior prevalent inside the channels, including two-phase flow and heat transfer regimes, and spatial boundaries between these regimes.  And two-phase micro-channel heat sinks are not without shortcomings.  One of the key complications to two-phase flow in micro-channels is the influence of flow acceleration caused by axial reduction in two-phase mixture density.  High heat fluxes can greatly increase pressure drop, and result in significant variations in properties of vapor and liquid.  These variations can produce appreciable compressibility (specific volume variations of vapor and liquid with pressure) and/or flashing (vapor and liquid enthalpy variations with pressure), as well as increased likelihood of two-phase choking.  Complications may also arise from instabilities commonly encountered in micro-channel heat sinks.  These instabilities take form of either severe pressure drop oscillations or mild parallel channel oscillations. Severe pressure drop oscillations, the more serious of the two instabilities, can be eliminated by mounting a throttling valve upstream of the heat sink.  But perhaps the most important performance limit for a micro-channel heat sink is critical heat flux (CHF), which is the upper possible heat flux limit before the heat sink incurs a sharp, unsteady temperature rise.  Micro-channel heat sinks are also prone to “premature CHF”, which is the result of flow instabilities, especially at low flow rates and inlet temperatures close to saturation.