Thermal analysis of a loop heat pipe evaporator and compensation chamber

The Loop Heat Pipe (LHP) is a two-phase, capillary-force driven, thermal control device that utilizes the latent heat of vaporization of a working fluid to transfer heat. An understanding of the underlying heat transfer and fluid flow processes within LHPs is important in designing the devices and in the development of models to accurately predict its performance. Although the evaporator and compensation chamber (CC) are key components of an LHP, detailed investigations of the processes within these components are very limited. In addition, the small number of studies on these components do not address the effect of including a bayonet tube that delivers cooled liquid into the evaporator.; The performance of an ammonia-filled, nickel-wick LHP containing a bayonet tube was investigated in the present study. Experimental and analytical methods were used in this investigation. Experimental tests revealed temperature stratification in the CC. A three-dimensional computational fluid dynamics model of the heat transfer and fluid flow in the evaporator and CC of the LHP was used to study the influences of natural convection, liquid subcooling, and the effective thermal conductivity of the wick. Temperatures in the evaporator core, farthest from the CC, decreased when natural convection effects are excluded. Small values of liquid subcooling and mass flowrates produced fairly uniform temperatures and temperature gradients in the components. Increasing the values of these parameters decreased temperatures in the evaporator and increased temperature gradients. The temperature profile at the inner surface of the evaporator wick was shown to vary with circumferential and axial location. An assumption of constant temperature over this surface was not always appropriate. Temperatures at the inner wick surface were also shown to be lower than the corresponding CC saturation temperature except for cases with very little liquid subcooling or very small mass flowrates.; It was demonstrated that the conventional assumption used in analyses of LHP performance was not always valid for designs that did or did not include a bayonet tube. The development and use of models for predicting LHP performance that treat the temperatures in the CC and the evaporator core as separate entities are required.