An OHP consists of a capillary sized tube and a refrigerant. Most OHPs are formed in closed serpentine loops with multiple turns, but OHPs can operate in other configurations –Closed End (serpentine but not looped) –Open End The tube is evacuated then partially filled with the working fluid Capillary forces create a natural separation of liquid slugs and vapor plugs. Oscillating (Pulsating) Heat Pipes Khandekar ‘04

Operating Regimes Critical Heat Flux Static Slug/Plug distribution, generation of small bubbles at high temps. similar to nucleate boiling Oscillatory slug/plug flow develops in individual loops ‘Stable’ oscillations develop, amplitudes increase with increasing heat flux, and oscillations in multiple tubes come into phase with one another -direction is arbitrary and periodically changes (more so with lower heat flux) -local oscillations superimposed on system oscillation Annular flow develops in fluid traveling from the evaporator to condenser References 1.Khandekar, S., 2004, “Thermo-hydrodynamics of Closed Loop Pulsating Heat Pipes,” Institut fur Kernenergetik und Energiesysteme der Universitat Stuttgart. 2.Ma., H. B., Borgmeyer, B., Cheng, P., Zhang, Y., 2008, “Heat Transport Capability in an Oscillating Heat Pipe,” Journal of Heat Transfer, 130(8), pp Borgmeyer, B., 2005, “Theoretical Analysis and Experimental Investigation of A Pulsating Heat Pipe for Electronics Cooling,” Master’s Thesis, University of Missouri-Columbia. Heat flux

Modeling OHP modeling has focused on the “simpler” slug/plug flow regime Multiple modeling approaches have been tried –Chaos –Continuity, Momentum & Energy –Spring-Mass-Damper –Non-Dimensional Analysis –Artificial Neural Networks Our model has its roots in the spring-mass-damper models presented by Ma, Borgmeyer, et al. 2,3 EES/MATLAB based thermo-hydrodynamic model of an OHP operating in slug-plug flow. Q. TcTc Vapor Plug CondenserEvaporatorAdiabatic Region Liquid Slug

Project Objective: Further the development of the technology/understanding required for successful implementation of an OHP in spacecraft thermal control applications. Approach: theoretical and experimental Deliverable: OHP design guidelines and a physics-based, but semi-empirical, model suitable for design The OHP Advantage and Project Goals Advantages –OHPs are simpler/lighter than other two-phase heat transfer devices No wick No additional fluid reservoir –OHPs may be more robust? Dry-out problems, while not extensively studied, do not appear to be a major problem…OHPs may be able to handle higher heat fluxes (convective heat transfer vs. phase change) –May be used as a thermal switch Disadvantages Lower effective conductance than Capillary Pumped Loops and Loop Heat Pipes