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State of the art in oscillating heat pipes in practical application and in theoretical modeling
Keywords: Oscillating Heat Pipe, Two phase heat transfer, Modeling and simulation
Oscillating heat pipes (OHPs) have been the subject of significant research since 1990, but their entry into broad real-world applications is just now occurring. Continued research and development of OHPs has been driven by the perpetual advancements in semiconductor devices and the ever-rising power levels and power densities associated with modern optical and electronic components. The resultant heat flux increases pose equally unending challenges to acquire, spread, and reject the device-generated waste heat in order to maintain temperature control of such components. [slide 1 will restate the recurring thermal challenge of rising heat fluxes in modern technology systems] Like wick-based heat pipes, OHPs are passive heat transfer devices made from an envelope material with internal, saturated two-phase working fluid; and OHPs utilize such working fluids two-phase flow to transfer heat from relatively hot zones to relatively cool ones. Unlike conventional heat pipes, OHPs do not use wicking structures to return the liquid-phase fluid in a counter-current flow relative to the vapor-phase fluid. Instead, OHPs are thermally pumped two-phase flow devices where the liquid and vapor phase fluid move in a co-current regime. OHPs are made by forming by a meandering capillary channel that wraps between hot and cold zones, typically forming distinct evaporator and condenser sections with an adiabatic zone in between. The channel volume is evacuated and then partially filled with a working fluid. Because of the capillary dimension of the tubing, the fluid forms distinct liquid plugs and vapor bubbles within the channel. Filling ratios are typically between 20 and 80% liquid/ total volume. As heat is applied, evaporation occurs and local pressure rises, driving the liquid plug away from the evaporator. As the liquid plug moves away from the evaporator, it pushes liquid plugs and vapor bubbles from the condenser back into the evaporator, replenishing the fluid displaced. With a sufficient number of connected channels, this process repeats continuously so long as heat is applied and no dryout conditions or operating limits are met. [slides approx. 2-3 will be illustrate of OHPs operating mechanism and construction] Due to their unique heat transfer mechanism and relatively simple production processes, OHPs have proven capable of handling high heat fluxes and to be manufactured in a variety of form factors enabling multi-functional thermal-mechanical structures. ThermAvant Technologies with considerable Small Business Innovation and Research (SBIR) support from the Air Force Research Laboratory, Space Vehicles Directorate (AFRL/RV) has made advancements in producing OHP- based heat spreaders and heat sinks for high power density, densely- packaged electronics systems. OHPs have been integrated into structural heat sinks with dimensions from less than 1mm thick to greater than 1m long. Further, OHPs have been proven operable with chip-level heat flux inputs >1200W/cm2. [slides approx. 4-7 will be photographs / outcomes of OHPs designed, built and tested under Air Force Research Laboratory SBIRs awarded to ThermAvant Technologies pending approval from AFRL/RV] In addition to demonstrating OHPs producibility, considerable research and development is being conducted on better understanding their operational limits. Initially presented as Paper 2016-3097 at the 46th AIAA Thermophysics Conference, Washington, D.C., 1317 June 2016, the second half of this presentation presentation will be an updated description of the theoretical and empirical performance limits of oscillating heat pipe (OHP) and how they can be designed for integration into high power density electronic packages and systems. This presentation presents analytical and experimental investigations of the various performance limits of oscillating heat pipes. The Bond number limit, vapor inertia limit, heat flux limit, viscous limit, sonic limit, start-up limit, and limitations on heated length are discussed in detail and developed with analytical tools for prediction. These limits are then tested against experimental studies. It will be shown that the Bond number coefficient can be as high as 2.4 2.74, rather than the typically reported 1.82.0. The established vapor inertia, heat flux, viscous, and sonic limits show good agreement with published experimental data sets, as well as controlled experiments run specifically to test these limits. Finally, under certain operating conditions it can be determined that longer evaporator lengths result in a reduction of overall heat transport capability. An analytical approach is presented to predict this performance loss, showing good agreement with experimental results. [slides approx. 8-15 will discuss the theoretical approach to predicting OHPs operating limits and the comparisons of predicted vs experimental results] [slide 16 will be references]
Dr Bruce L Drolen & Mr. Joe Boswell, Drolen = Consultant; Boswell = CEO
ThermAvant Technologies
Columbia, Missouri
United States

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