Evaluation of thermo-fluidic performance of micro pulsating heat pipe with and without evaporator side surface structures

Interfacial behaviour and thermal performance of a 4-loop micro pulsating heat pipe are examined through experiments performed at different filling ratio (30-60 %), working fluid (acetone, ethanol, and methanol), and heat input (0-40 W). During the heat pipe operation, nucleation, growth and merging of bubbles, elongation, and movement of vapor plug, and dry out conditions are noticed upon ramp heating (0-40 W) inside smooth surface evaporator pulsating heat pipe. Experiments show that preparation and boiling inception lead to steady operation of heat pipe. Pulsating heat pipe showed minimum thermal resistance when operated with 40 % filling ratio and acetone as working fluid, which has low viscosity, surface tension and boiling point. Additionally, effect of surface structures (rectangular pillar protrusion and hemispherical dimple cavity) on the evaporator section is tested against base smooth surface micro pulsating heat pipe to understand augmentation of heat transfer. Hemispherical dimple cavities show better nucleation, bubble growth, and vapor plug elongation and thereby reduction in thermal resistance. On the other hand, rectangular pillar protrusions showed clogged bubble, least bubble detachment and vapor plug length in the adiabatic zone which results in higher thermal resistance than even smooth surface.