Loop heat pipes (LHPs) transport energy from an evaporator to a condenser in the form of latent heat. In conventional LHPs, the vapor pressure is significantly higher than the liquid pressure across the liquid-vapor interface due to the small pores and the corresponding capillary forces in the wick. This large pressure difference transports the single phase vapor after evaporation from the evaporator to the condenser and once the vapor is condensed, a single phase liquid from the condenser back to the evaporator. This current work involves the development of a steady state design model of the LHP system consisting of a planar evaporator package and a finned copper tube loop, which serves as an air-cooled condenser. Although evaporation due to the heat transfer creates the pressure in the vapor which drives the flow, contrasting to the conventional loop heat pipes, the pressure drop across the liquid-vapor interface is much smaller. A positive hydrostatic head is applied to the liquid above the wick and there is entrainment of liquid from the wick in the evaporator. Therefore, the fluid flow leaving the evaporator package is a two-phase flow, assumed to be saturated liquid and saturated vapor in equilibrium. The primary objective of this non-conventional LHP device is to remove the thermal energy dissipated from a Light Emitting Diode (LED) array. A major portion of this energy causes evaporation of the working fluid within the wick. The remaining energy reheats the liquid in both the liquid return line and within the evaporator package. The evaporator package is modeled as a one-dimensional thermal resistance network and these resistances are empirically determined from experiments. It is found that the convective heat transfer co-efficient of air plays a pivotal role in the heat dissipation and hence, is empirically determined in this work. This value is fairly agreeable with the Nusselt number correlation on the air side developed by Hahne et al. . A mass balance relates the fill volume with the length of the condenser. The temperatures within the LHP are predicted by applying the principle of conservation of energy over the evaporator, the condenser and the sub-cooler sections of the heat exchanger loop. Finally, this LHP model predicts an approximate fill volume necessary for the LHP to operate properly.
Steady State Numerical Modeling of Non-Conventional Loop Heat Pipes (LHPs)
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Remella, KS, Gerner, FM, Shuja, A, & Medis, P. "Steady State Numerical Modeling of Non-Conventional Loop Heat Pipes (LHPs)." Proceedings of the ASME 2012 International Mechanical Engineering Congress and Exposition. Volume 7: Fluids and Heat Transfer, Parts A, B, C, and D. Houston, Texas, USA. November 9–15, 2012. pp. 1467-1478. ASME. https://doi.org/10.1115/IMECE2012-88217
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