An insight into thermo-hydrodynamic coupling in closed loop pulsating heat pipes

A Closed Loop Pulsating Heat Pipe (CLPHP) is a complex heat transfer device with a strong thermo-hydrodynamic coupling governing its thermal performance. To better understand its operational characteristics, a two-phase loop is constructed with a capillary tube (ID = 2.0 mm) having no internal wick structure. The loop is heated at one end and cooled at the other and partially made up of glass to assist visualization. The working fluid employed is ethanol. It is concluded from the study that a two-phase loop does represent the thermo-fluidic characteristics of a multi-turn CLPHP. Dynamic two-phase instabilities are present in a two-phase loop also; although the number of turns in a CLPHP increases the level of internal perturbations. The existence of an optimum number of turns for a given heat throughput requirement is explained. Also, it is shown that classical thermodynamics based on quasi-equilibrium theory seems not to be sufficient for complete system analysis. The performance (i.e., overall thermal resistance) is strongly dependent on the flow pattern existing inside the tubes. The role of gravity in the operation characteristics is clarified. (C) 2003 Elsevier SAS. All rights reserved.