EFFECT OF POWER DENSITY DISTRIBUTION ON PERFORMANCE OF FLAT PLATE OSCILLATING HEAT PIPES

Abstract

Oscillating Heat Pipes (OHPs) are passive, wickless thermal transport devices that utilize the self-sustained oscillation of a two-phase working fluid within a serpen- tine capillary channel to achieve efficient heat transfer. Their simple structure, absence of wicking material, and high effective thermal conductivity make them attractive for compact and high-performance thermal management applications. While OHPs require surface tension effects to form slugs and bubbles, their performance is generally not otherwise limited by surface tension effects. These characteristics make OHPs partic- ularly well-suited for microgravity environments, where conventional capillary-driven heat pipes experience functional limitations. The ASETS-II (Advanced Space Experi- ment on Two-Phase Flow and Thermal Control Systems-II) program demonstrated a major milestone in this field by successfully operating OHPs aboard the X-37B Orbital Test Vehicle, validating their long-term performance and stability in microgravity. As part of the ASETS-II program, they reported that smaller heaters with concentrated power density had higher thermal resistance than a competing design where the heat was more spread out. However it was unclear whether the higher thermal resistance was due to the concentration of the heat, or the fact that the smaller heater did not span the full width of the channels. In this thesis, the primary objective is to distinguish the effects heater power density and geometry on the performance of OHPs. The layout of the thesis is as follows: Chapter 1 will describe the necessary background of conventional and oscillating heat pipes, along with a more detailed description of the ASETS-II experiment, a key benchmark for our experiments and simulations. In Chapter 2, we describe a set of experiments that we conducted whereby an acetone-charged copper flat-plate OHP design was designed, nearly identical in channel geometry to the ASETS-II experiment and tested. Its transient and thermal response was investigated under terrestrial conditions using a novel rearrangable heater design, which allowed independent control over the heater area and span of the fluid- charged channels. Systematic experiments were conducted over a range of heat inputs and geometrical configurations to observe changes in startup temperature evolution, thermal conductance providing insight into the coupling between geometry and thermal transport efficiency. In Chapter 3, we describe the results of those experiments. In all heater con- figurations and powers we tested, we found that acetone-charged OHPs have higher conductance than a control experiment consisting of identical empty copper OHPs, in some cases by 100% - despite the fact that the OHPs fluid channels only consti- tute ≈ 11% of the cross sectional area. Overall, the results demonstrate that thermal conductance in the oscillating heat pipe is predominantly controlled by the spatial dis- tribution, coverage, and orientation of the heat input rather than by total power alone, as these factors directly regulate the initiation, coherence, and intensity of two-phase oscillatory flow, thereby dictating the effective thermal resistance of the system. In Chapter 4, we describe, modify, and use a recent open-source simulation tool (developed at UCLA) to model the behavior of our OHPs under different heater configurations. The simulations, in principle, can provide much more insight into the detailed evolution of the OHP, tracking the slug, film, and vapor dynamics as well as the temperature distribution in the solid regions of the OHP. The simulation results are compared against our experimental observations, and used to interpret experimental trends where possible. Interestingly, we found that while the simulation results do a fanastic job of replicating the ASETS-II experimental observations, they do a poor job of predicting observations in our experiments: in every case, the simulations apparently incorrectly predict that OHPs in our configurations will dry out or be non-functional. Direct experimental comparison of empty cells to fluid-charged cells proves otherwise. We discuss possible shortcomings of the model.