Characterization, Modeling and Design Of Ultra-Thin Vapor Chamber Heat Spreaders Under Steady-State and Transient Conditions

Gaurav Patankar, Purdue University

Abstract

This dissertation is focused on studying transport behavior in vapor chambers at ultra-thin form factors so that their use as heat spreaders can be extended to applications with extreme space constraints. Both the steady-state and transient thermal transport behaviors of vapor chambers are studied. The steady-state section presents an experimental characterization technique, methodologies for the design of the vapor chamber wick structure, and a working fluid selection procedure. The transient section develops a low-cost, 3D, transient semi-analytical transport model, which is used to explore the transient thermal behavior of thin vapor chambers: 1) The key mechanisms governing the transient behavior are identified and experimentally validated; 2) the transient performance of a vapor chamber relative to a copper heat spreader of the same external dimensions is explored and key performance thresholds are identified; and 3) practices are developed for the design of vapor chambers under transient conditions. These analyses have been tailored to ultra-thin vapor chamber geometries, focusing on the application of heat spreading in mobile electronic devices. Compared to the conventional scenarios of use for vapor chambers, this application is uniquely characterized by compact spaces, low and transient heat input, and heat rejection via natural convection. Under steady-state conditions and at ultra-thin form factors, the performance-governing transport mechanisms of vapor chambers differ compared to a conventional scenario. Performance is primarily limited by the vapor core thermal resistance, rather than the thermal resistance across the evaporator wick. Additionally, thermal management requirements of mobile electronic devices are increasingly governed by user comfort, and hence vapor chamber performance needs to be characterized by the temperature at its condenser surface. This new paradigm creates a need for new experimental characterization techniques and design methodologies. Also, the heat load in mobile electronic devices is inherently transient in nature. However, no knowledge exists for the transient thermal behavior of vapor chambers. Hence, a new model is developed to solve for transient transport in vapor chambers. The model is used to identify mechanisms governing the transient thermal behavior of vapor chambers, to compare the transient performance with metal spreaders and to develop design practices for improving transient performance. In the experimental characterization approach developed in the steady-state section, the evaporator-side and ambient temperatures are measured directly; the condenser-side surface temperature distribution is measured using an infrared camera. The high thermal resistance imposed by natural convection in the vapor chamber heat dissipation pathway requires accurate prediction of the parasitic heat losses from the test facility using a combined experimental and numerical calibration procedure. Performance metrics are developed to characterize heat spreader performance in terms of the effective thermal resistance and the condenser-side temperature uniformity. The performance characterization technique offers a rigorous approach for testing and analysis of new designs of vapor chamber as heat spreaders, with accurate characterization of their performance relative to other heat spreaders. To design vapor chambers under steady-state conditions, unlike previous approaches that have focused on designing evaporator-side wicks for reduced thermal resistance and delayed dryout at higher operating powers, this work focuses on manipulating the condenser-side wick to improve lateral heat spreading. The proposed condenser-side wick designs are evaluated using a 3D numerical vapor chamber transport model that accurately captures conjugate heat transport, phase change at the liquid–vapor interface, and pressurization of the vapor core due to evaporation.

Degree

Ph.D.

Advisors

Weibel, Purdue University.

Subject Area

Design|Mathematics|Statistics

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