The thermal resistance of a vapor chamber is primarily governed by conduction across the evaporator wick and the saturation temperature gradient in the vapor core. The relative contributions of these two predominant resistances can vary dramatically with vapor chamber operating conditions and geometry. In the limit of very thin form factors, the contribution from the vapor core thermal resistance dominates the overall thermal resistance of the vapor chamber; recent work has focused on working fluid selection to minimize overall thermal resistance in this limit. However, the wick thermal resistance becomes increasingly significant as its thickness increases to support higher heat inputs while avoiding the capillary limit. It therefore becomes critical to simultaneously consider the contributions of the wick and vapor core thermal resistances in the development of a generalized methodology for vapor chamber working fluid selection. The current work uses a simplified thermal-resistance-network-based vapor chamber model to explore selection of working fluids and wick structures that offer the minimum overall thermal resistance as a function of the vapor chamber thickness and heat input. An illustrative example of working fluid selection, for cases with and without the contribution of wick thermal resistance, is first used to demonstrate the potential significance of the wick thermal resistance on fluid choice. This influence of the wick on working fluid selection is further explained based on the wick properties (effective pore radius, permeability, and effective thermal conductivity). The ratio of effective pore radius to wick permeability is found to be the most critical wick parameter governing the overall vapor chamber resistance at thin form factors where minimizing the wick thickness is paramount; the wick conductivity becomes an equally important parameter only at thicker form factors. Based on this insight, a new approach for vapor chamber design is demonstrated, which allows simultaneous selection of the working fluid and wick that provides minimum overall thermal resistance for a given geometry and operating condition.
Vapor chamber, Heat pipe, Wick, Working fluid, Thermal resistance
Date of this Version
K. Baraya, J. A. Weibel, and S. V. Garimella, “Simultaneous Wick and Fluid Selection for the Design of Minimized-Thermal-Resistance Vapor Chambers under Different Operating Conditions,” International Journal of Heat and Mass Transfer, Vol. 136, pp. 842-850, 2019.