Wehmeyer, Geoff2024-08-302024-08-302024-082024-08-05August 202Shimokusu, Trevor J.. Measurements and modeling of passive nonlinear thermal devices: diodes and regulators. (2024). PhD diss., Rice University. https://hdl.handle.net/1911/117818https://hdl.handle.net/1911/117818This thesis investigates the thermal science of three nonlinear thermal devices: the jumping droplet thermal diode (JDTD), the heterojunction diode, and the oscillating heat pipe (OHP). I use experiments and modeling to gain insight into physical mechanisms and characteristics that underpin thermal nonlinearities required for thermal rectification and regulation. Furthermore, in contrast to existing research that has primarily focused on understanding and improving the steady-state performance of thermal diodes and regulators, my work also considers the durability and transient performance of passive nonlinear thermal devices. I begin by discussing my work developing superhydrophobic aluminum surfaces with improved robustness against steam degradation. Using these surfaces in a JDTD, I parametrically study steady-state thermal rectification and demonstrate half-wave thermal rectification in response to an alternating current (ac) thermal input. Then, building on insight gained from half-wave thermal rectification experiments, I present our work studying the time-periodic thermal response of a heterojunction diode. Using analytical perturbation methods supported by finite element modeling, I show that a direct current (dc) heat flux emerges in response to an ac thermal input, and that this dc response may differ depending on which side the ac input is applied. The emergence of additional harmonics (e.g., dc heat flow) from a single harmonic input (i.e., sinusoidal temperature input) for nonlinear heat conduction in heterojunction diodes agrees with our half-wave thermal rectification measurements with the JDTD and could be useful for transient applications such as thermal energy storage. Lastly, I discuss my strain gauge and temperature measurements of an oscillating heat pipe (OHP). By correlating the strain frequency response with thermal resistance measurements as function of heat flow, I delineate the transition between operating regimes characterized by intermittent and stable fluid oscillations, which is responsible for strong nonlinearity and thermal regulatory behavior in OHPs. Overall, my research motivates to the need to conduct more nonlinear thermal device- and circuit-level research that complements steady-state demonstrations, such as transient thermal measurements and modeling and durability characterization.application/pdfengCopyright is held by the author, unless otherwise indicated. Permission to reuse, publish, or reproduce the work beyond the bounds of fair use or other exemptions to copyright law must be obtained from the copyright holder.heat transfernonlinear transportThesis2024-08-30