Nanoscale Wicking Structures

Abstract

Heat pipes are ubiquitous in various heat transfer applications due to their low maintenance and lack of moving parts. Their simplicity makes them compact and ideally suited for microelectronics use. Recirculation of the coolant in a heat pipe is done passively by means of a wicking structure that induces capillary-driven flow from the condenser to the evaporator. This fluidic scheme is highly desirable but requires precise optimization of the wicking structure geometry to provide the required coolant flow rates under different heat loads. In this paper we present an ab initio model that simulates the capillary flow within a wicking structure of regular and periodic geometry. An energy formulation incorporating capillary equations for pressure gradient and the Stokes flow equation for frictional dissipation were used in the analysis. The feasibility of using nanostructures for capillary-driven flow was assessed using this theoretical analysis. This model is specifically designed to simulate a nanopillar array wick (or nanowick) but was also extended to incorporate commercially available homogenous wicks through the use of a general Darcy’s flow approach. A Darcy’s flow analysis requires knowledge of the porous structure permeability (κ), which must be empirically determined. However, our first principles approach can be used to estimate the effective permeability of various commercial wicks. Only the characteristic structural dimensions of a wick are needed in our model for an accurate estimate of the permeability and the maximum flow rate the wick can sustain without the necessity for an empirical correlation. The results of the theoretical model were corroborated through experimental measurements of baseline mesh wicks and nanowicks. Since the thermal performance of most heat pipes is usually capped by the capillary limit, this threshold was examined for each wick by measuring the mass flow over time at different heat fluxes. At high heat fluxes, the wick cannot sustain the fluid flow necessary for heat removal and burnout occurs. This phenomenon occurs at the thermal capillary limit. The mass flow ceases to increase in the case of burnout and may actually decrease if a disruptive vapor film is created. Experiments show that the baseline wicks were found to have higher mass flow rates when compared to a nanowick due to the difference in thickness of the wicks. However, when the data were normalized to produce velocity values, the nanowick was found to have a higher velocity than most of the baseline wicks. These experimental results were weighed against the theoretical model results showing very good agreement of the two.