Date of Award

Fall 2014

Degree Type


Degree Name

Master of Science (MS)


Mechanical Engineering

First Advisor

Suresh V. Garimella

Second Advisor

Justin A. Weibel

Committee Member 1

Xiulin Ruan

Committee Member 2

Michael T. Weibel


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. The operating conditions for such applications are also characterized by low input heat fluxes, which in combination with the geometric constraints, give rise to unique performance limitations that require examination. This thesis aims to characterize the steady-state and transient heat pipe performance limitations unique to such ultra-thin form factors, and characterizes the key heat transfer mechanisms governing the performance.

A thermal resistance network model and a detailed two-dimensional model are used to analyze the steady-state 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 that of 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, where performance becomes worse than a solid heat spreader. The higher-fidelity numerical model is used to assess the accuracy of the thermal 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 effect 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. Lastly, the numerical model was used to compare the transient performance between ultra-thin heat pipes and heat spreaders during the initial start-up period was conducted to demonstrate an initial crossover period under which the performance of the heat pipe was lower than that of a heat spreader.

This thesis establishes the performance thresholds of ultra-thin form factor heat pipes operating at low input heat fluxes under steady-state operation, and identifies key performance traits that must be considered under transient operation.