Effect of filling ratio on heat transfer characteristics and performance of a closed loop pulsating heat pipe

Thermal control is a generic need for any heat dissipation system. Heat pipes emerge as the most appropriate technology and most thermal effective solution due to their excellent heat transfer capability, heat transfer efficiency and structural simplicity. In many engineering applications higher heat flux is important. Two phase passive devices are proven solutions for modern microelectronics thermal management. This paper attempts to describe the heat transfer characteristics of closed loop pulsating heat pipe (CLPHP) which are new entrants in the family of closed passive two phase heat transfer system. It also shows the comparison of thermal efficiency of CLPHP for different filling ratios with two different working fluids, water and ethanol. This device is a combination of lot of events and mechanisms like bubble nucleation, collapse and agglomeration, bubble pumping action, pressure and temperature perturbations, flow regime changes, dynamic instabilities, meta-stable non equilibrium conditions, flooding, bridging etc. All contribute towards the thermal performance of a device. But, such a complex operating mechanism is not understood well yet and the present state of the art cannot predict the required design parameters for a given task. The aim of research work presented in this paper is to better understand the heat transfer characteristics of these mechanisms through experimental investigations. Experiments are conducted on a CLPHP made of capillary tube of 2.2mm inner diameter. The heat transfer characteristics and the performance of the CLPHP are investigated for filling ratios of 100%, 82.5%, 63%, 41.3% and 28%. The results indicate that the performance of this device changes with the changing of working fluid, filling ratios and heat input. (C) 2012 The authors, Published by Elsevier Ltd.