On the thermofluidic behaviour of deformed multi-source heat pipes

The context for this thesis is the thermo-fluidic behaviour of deformed heat pipes for the thermal management of multiple, high flux (10−102 W/cm2 ) heat sources for space-constrained 5G wireless communication hardware. There are four principal aspects to this thesis: the development of a precision apparatus for the thermal characterization of a concentric tube heat pipe; an experimental investigation into the thermal performance of deformed, multiple heat source (MHS) heat pipes in comparison to their straight single heat source (SHS) counterpart; fluidic modeling and experimental analysis of graded wicks for the potential enhancement of liquid flux in MHS heat pipe applications; and the demonstration of a non-destructive methodology for visualization and dimensional characterization of the microstructures in straight and deformed heat pipes. A calorimeter-based method is developed to characterize the thermal performance of concentric tube sintered copper heat pipes. High precision thermistors positioned at fixed radial locations within two cylindrical calorimeters are used to measure the heat supplied to the evaporator and the heat extracted from the condenser. One-dimensional radial conduction is assumed to occur within each calorimeter, which enables the accurate quantification of heat flows from the temperature readings with uncertainties in thermal characterization of <7.5% for a range of thermal loads (5−25W). This study concludes that previous characterization techniques for heat pipes can greatly overestimate thermal performance (15−32%) due to inaccuracies in quantifying heat flow. The thermal performance of a series of bent MHS heat pipes is tested for a range of bend angles (0−90°) and bend locations. The analysis reveals that: adding MHS configurations to a heat pipe can increase thermal resistance by up to ~65% in comparison to its SHS counterpart; bending an MHS heat pipe up to 90° between the condenser and evaporator section can result in as much as a ~65% rise in thermal resistance; and changing the location of the bend to somewhere between heat sources can add up to ~18% to the thermal resistance of the bent MHS heat pipe. It is speculated that the increase in thermal resistance and evaporator temperature in bent MHS configurations is due to higher radial heat fluxes caused by the MHS configuration, where local liquid cooling demands are not met because of dry-out, and deformations to the wick and vapor channels caused by the bending process. Discretely graded porous media have shown potential in applications where evaporator dry-out, or insufficient liquid fluxes, limit the operation of a heat pipe. Accurate prediction models for imbibition within these wicks are essential for design optimization, but are not yet fully developed. Here, an analytical model for use with discretely graded porous media is developed and validated against benchmark capillary rate-of-rise experiments. A general observation is that graded particulate wicks should always transition from larger- to smaller-sized particles; also, the relative lengths of the wick layers strongly influence the potential for enhancement. The utility of the model as a design tool is illustrated by formulating it to predict wick flow rates. The model demonstrates that the optimized discretely graded wicks analyzed in this thesis can more than double wicking velocities. The reduction in the thermal performance of deformed heat pipes is well-established in the literature. Until recently, root-cause analysis for this degradation in performance has generally been carried out using destructive observational techniques, resulting in a limited understanding of the influence of deformation. X-ray tomography is demonstrated here as a novel non-destructive methodology that can be used to visualize the microstructure of a heat pipe, thereby quantifying the local vapor and liquid pressure drops caused by deformations to the wick and the vapor channel, and their influence on the capillary limit. The method reveals that bending a heat pipe can increase the gaseous and liquid phase pressure drops by up to ~14% and 130%, respectively. It is concluded that the additional pressure drop in the liquid phase is shown to exert the dominant influence (~28 times greater than the gaseous phase) on the capillary limit of a bent heat pipe. Finally, the findings of this thesis enhance the understanding of the thermo-fluidic behavior of deformed MHS heat pipes for space-constrained applications, forming the foundation for future developments in the thermal management of 5G wireless hardware.