Working-fluid selection for minimized thermal resistance in ultra-thin vapor chambers

Abstract

The behavior of a vapor chamber is strongly coupled to the thermophysical properties of the working fluid within. It is well known that these properties limit the maximum power (heat load) at which a vapor chamber can operate, due to incidence of the capillary limit. At this limit, the available capillary pressure generated within the wick structure balances the total pressure drop incurred along the path of fluid flow within the wick. A common figure of merit prioritizes working fluids that maximize this capillary-limited operating power. The current work explores working fluid selection for ultra-thin vapor chambers based on a thermal performance objective, rather than for maximized power dissipation capability. A working fluid is sought in this case that provides the minimal thermal resistance while ensuring a capillary limit is not reached at the target operating power. A resistance-network-based model is used to develop a simple analytical relationship for the vapor chamber thermal resistance as a function of the working fluid properties, operating power, and geometry. At small thicknesses, the thermal resistance of vapor chambers becomes governed by the saturation temperature gradient in the vapor core, which is dependent on the thermophysical properties of the working fluid. To satisfy the performance objective, it is shown that the choice of working fluid cannot be based on a single figure of merit containing only fluid properties. Instead, the functional relationship for thermal resistance must be analyzed taking into account all operating and geometric parameters, in addition to the thermophysical fluid properties. Such an approach for choosing the working fluid is developed and demonstrated.Keywords: mobile device thermal management, ultra-thin vapor chamber, heat pipe, working fluid, figure of merit

Simultaneous wick and fluid selection for the design of minimized-thermal-resistance vapor chambers under different operating conditions

“…The premise of the current design approach is to minimize the thermal resistance of the vapor chamber while operating within the capillary limit. This can be achieved by designing for the minimum wick thickness that utilizes the complete capillary pressure head available [11]; at this minimum thickness, both the wick and vapor core thermal resistances will be minimized. Introducing a factor of safety, and equating equations (7) and 8…”

“…Hence, for a given operating condition (working thickness t and heat load Q), a working fluid can be selected from a given set of fluids that would provide the minimum thermal resistance. The fluid choice can thus be mapped on a t-Q map, where each point on the map represents the working fluid with properties that minimizes the total thermal resistance [11].…”

“…A detailed discussion of the mechanisms governing fluid choice based on the vapor core resistance alone is available in Ref. [11].…”

“…Ml or Mv for fluid selection. Patankar et al[11] developed a coupled fluid selection and wick thickness design approach to achieve optimized thermal performance for thin vapor chambers. In this approach, the vapor chamber was designed to have the minimum wick thickness required to sustain the imposed heat load without suffering a capillary limit, so as to maximize the space made available to the vapor core and thus minimizing the vapor pressure drop and thermal resistance.…”

“…It was shown that working fluid selection using this design approach should be based on a combination of both Ml and Mv in addition to the given heat load. However, this approach[11] is valid only for thin form factors at which the vapor core thermal resistance dominates, and does not consider the impact of the wick thermal resistance or wick properties that become critical at thick form factors and high heat loads.…”