Browsing by Author "Khandekar, Sameer"

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Open Access
Thermo-hydrodynamics of closed loop pulsating heat pipes
(2004) Khandekar, Sameer; Groll, Manfred (Prof. Dr.-Ing. habil.)
Thermal management of electronics is a contemporary issue which is increasingly gaining importance in line with the advances in packaging technology. Immediate and consistent multi-disciplinary research is needed to cater to the prevailing trends of net power and flux levels of upcoming microelectronics products. Material science, packaging concepts, fabrication technology and novel cooling strategies are some of the key areas requiring synchronal research for successful thermal management. Focusing on the latter area, this thesis attempts to describe the complex thermo-hydrodynamics of Closed Loop Pulsating Heat Pipes (CLPHPs) which are new entrants in the family of closed passive two-phase heat transfer systems. These apparently simple looking devices are extremely intriguing for theoretical and experimental investigations alike. It is rare to find a combination of such events and mechanisms, like bubble nucleation/collapse and agglomeration, bubble pumping action, pressure/temperature perturbations, flow regime changes, dynamic instabilities, metastable non-equilibrium conditions, flooding/bridging etc., all together contributing towards the thermal performance of a device. The thermal performance objective function is multi-dimensional and embodies major multi-disciplinary two-phase flow physics. To achieve the goal, five different experimental set-ups have been envisioned, fabricated and tested. The set-ups are designed for flow visualization (including videography and infra-red thermography) coupled with standard thermometry. Six major parameters have emerged as the primary influence parameters which affect the system dynamics. These include:  Internal diameter of the CLPHP tube,  Volumetric filling ratio of the working fluid,  Input heat flux,  Total number of turns,  Operational orientation and,  Thermo-physical properties of the working fluid. The thesis provides detailed discussion on the various design parameters. Apart from the multitude of geometrical, physical and operational variables, the performance is also strongly linked with the flow patterns existing inside the device. Subtle aspects of this two-phase flow dynamics and their interactions with the heat transfer characteristics have been highlighted leading to the formulation of primary design rules. Mathematical modeling of the device operation has also been successfully accomplished by applying two approaches which are quite diverse in nature, viz. (a) Semi-empirical modeling with non-dimensional groups, and (b) Modeling by artificial neural networks. The handicaps and problems of conventional modeling by ‘first principles’, e.g Navier-Stokes equation, are also scrutinized. At the end of this study program, although some nuances of the device operation still remain unexplored, it is believed that major advancement in the understanding of the thermo-hydrodynamics of CLPHPs has been accomplished. With the progress achieved so far, the prospects for this exemplary and unprecedented technology seems quite promising.