| dc.description.abstract | Embargo up to 15/1/2027
A Pulsating Heat Pipe (PHP) is a wickless meandering capillary tube bent back and forth parallel to itself. PHPs are classified into single and multiturn, closed-loop and open-loop. A single loop PHP can be thought of as a building block for multiturn, single plane or multiplane PHPs. The term “capillary” means that the diameter of the tube is small enough for surface tension effect to become prominent. When the capillary tubes are initially evacuated and then partially filled with the working fluid, a train of liquid slugs and vapour plugs is formed inside the PHP. A PHP contains evaporator, adiabatic and condenser zones. In the present work, the possibility of employing a multiturn PHP as a pin-finned heat sink attached to a base plate is considered. Hence all the regions, other than those belonging to evaporators, function as condenser zones. Differential pressure created in a PHP due to differential heating and cooling between the evaporator and condenser acts as the driving force for producing momentum changes and oscillations in the two-phase fluid. This two-phase oscillatory flow causes efficient heat transmission from evaporator to condenser with very low temperature difference.
Experimental investigations are carried on PHPs with multiturn, single plane and multiplane configurations. The condenser (or fin portions) of the PHPs is cooled by forced air flow and radiation to the surroundings. Experimental parametric runs are conducted by varying the number of turns, fill ratio, orientations, and heat loads. Three working fluids, namely, water, methanol and FC-72 are tested. PHPs with 9, 12 and 18 turns are tested for fill ratios of 30%, 50% and 70% and five orientations (+90°, +45º, 0°, -45º and -90º measured with respect to horizontal). Heat loads are increased in steps of 50 W from 50 W to 400 W for testing the PHPs. Based on the experimental data obtained, a heat transfer correlation is constructed with a deviation of less than ± 20% for the Kutateladze number in terms of the relevant dimensionless numbers for each of the five orientations. Each correlation is based on 274 data points across the parametric space. Startup characteristics of the multiturn PHPs are also obtained for both single plane and multiplane configurations.
For different PHP physical geometries, the flow regimes change with respect to heat loads. The optimum angle for best thermal resistance is found to lie between 60° and 70°. Gravity independent operation observed in all the multiplane models and generally, the bottom heating mode (+90°) produce the highest heat transfer.
During the startup, for lower heat loads, a temperature overshoot is observed before the establishment of pulsations. At higher heat loads, pulsations are set up without appreciable temperature overshoot, following a smooth startup. The results have shown enhancement in fin thermal conductivity compared to solid copper. The average thermal resistances ( ºC ⁄W) for 9 single plane (0.3857), 9 multiplane (0.2221), 12 multiplane (0.2863) and 18 multiplane (0.3408) compare to dry fin thermal resistance ( ºC ⁄W).
The thermo-hydraulics of a PHP are very complex as they involve moving liquid slugs and vapour plugs of varying size, phase change mechanisms of evaporation and condensation and wall heat conduction effects. Hence most of the work reported in the literature has been experimental. In the recent times some efforts were made to formulate mathematical models for PHPs and to numerically solve the same. In the present work also an attempt is made to formulate a reasonably sophisticated mathematical model of a PHP. The numerical formulation consists of discretized governing equations and the required auxiliary equations. A computer program is written to carry out the numerical procedures and some numerical results with the working fluids methanol and water are obtained. | en_US |
