Performance-Governing Transport Mechanisms for Heat Pipes at Ultrathin Form Factors

Abstract

Abstract-Heat pipes and vapor chamber heat spreaders offer a potential solution to the increasing thermal management challenges in thin-form-factor mobile computing platforms, where efficient spreading is required to simultaneously prevent overheating of internal components and formation of hot regions on the device exterior surfaces. Heat pipe performance limitations unique to such ultra-thin form factors, and the key heat transfer mechanisms governing the performance, must be characterized. A thermal resistance network model and a detailed two-dimensional numerical model are used to analyze the performance of heat pipes under these conditions. A broad parametric study of geometries and heat inputs using the reduced-order model helps delineate the performance thresholds within which the effectiveness of a heat pipe is greater than a comparable solid heat spreader. A vapor-phase threshold unique to ultra-thin heat pipes operating at low power inputs is observed. At this threshold, the vapor-phase thermal resistance imposed by the saturation pressure/temperature gradient in the heat pipe causes a crossover in the thermal resistance relative to a solid heat spreader. The higher-fidelity numerical model is used to assess the accuracy of the resistance network model and to verify the validity and applicability of each assumption made regarding the transport mechanisms. Key heat transfer mechanisms not captured by the reduced-order thermal network models are identified. These include the effects of boundary conditions on the interface mass flux profile, convective effects on the vapor core temperature drop, and two-dimensional conduction on smearing of evaporation/condensation mass flux into the adiabatic section.

Electronics Thermal Management in Information and Communications Technologies: Challenges and Future Directions

“…The requirements for mobile thermal management are starkly different, and call for heat spreading efficiency to meet ergonomic constraints at ultra-thin form factors. The key heat transfer mechanisms governing the performance of vapor chambers with shrinking thickness have been recently identified in order to delineate the performance thresholds within which the effectiveness of a vapor chamber, as a function of geometry and heat input, is greater than that of a comparable solid heat spreader [74]. For ultra-thin vapor chambers operating at a low power, the heat spreading resistance is dictated by the vapor-phase behavior; design approaches and working fluid selection criteria must be revised with these considerations.…”

A validated time-stepping analytical model for 3D transient vapor chamber transport

“…For steady-state behavior, models are readily available for all levels of dimensionality. Prasher [9] and Yadavalli et al [10] …”

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

“…Patankar et al [11] experimentally observed this variation in temperature along the condenser surface when characterizing the performance of ultra-thin vapor chambers; the vapor chamber resistance changed with operating temperature due to changes in the thermophysical properties with temperature. Yadavalli et al [12] analyzed the performance limitations of a thin heat pipe using a resistance-network-based model. In the limit of low power (where the capillary limit is not of concern), the authors developed a figure of merit based on the thermophysical fluid properties that affect the vapor core thermal resistance, given as…”