Two-phase modeling for porous micro-channel evaporators

Abstract

Two-phase (liquid/vapor) micro-channel cold plate evaporators are leading candidates for use in high-power electronics thermal management due to their superior convection heat transfer capabilities in comparison to single-phase (liquid) cold plates. In this context, novel micro-channel geometries and surface structuring are being pursued with the goal of obtaining thermofluidic performance enhancements and improved flow stability. Understanding the complex phenomena occurring within evaporators that govern the overall behavior, performance, and limitations of two-phase systems such as gravity-driven thermosyphon loops (TSL) is critical to the application of this thermal technology. Thus, developing comprehensive models that can provide insights into evaporator design, performance, and transient behavior is a key objective.We develop and validate a detailed numerical model for an evaporator design incorporating porous domains, where capillary pumping at the evaporator length scale augments the overall flow circulation within a gravity-driven TSL using either R245fa or R1233zd(E) as the working fluid. The modeling ambition includes capturing the physics at the interface between the porous and vapor domains, where a mix of direct evaporation, dry-out and liquid leakage can occur. The challenges addressed for such modeling include the implementation of the local variable phase interface conditions and a computationally efficient method of tracking the free liquid-vapor interface. We present results that suggest the design can realize significant improvements in thermofluidic performance, in terms of evaporator performance index defined as the inverse product of the pressure drop and thermal resistance, over a range of heat loads relevant to cooling high-power hardware components. The implications of the proposed evaporator design is the potential for extending the operating envelope of a TSL by reducing the overall pressure drops and modulating the passive refrigerant flow rate to achieve stable cooling and higher critical heat fluxes.