Visualization of bubble mechanism of pulsating heat pipe with conventional working fluids and surfactant solution

Visualization experiment is a must in realizing functional characteristics of an operational pulsating heat pipe (PHP). So far there is no general formulation which can foretell the complex and chaotic flow nature for every working fluid. Different response of working fluids and their distinct flow nature as well as their behavior can be visualized which thereby helps to understand the operational mechanism of the PHP. In this experiment, tests were conducted in a transparent PHP with 3 conventional working fluids, viz. de-ionized (DI) water, methanol, ethanol, and 300 ppm cetyltrimethyl ammonium chloride (CTAC) solution, each with fill ratio (FR) of 50%. With the help of high speed camera, flow characteristics at different operational stages for each working fluid are captured. Difference in the generation, growth, movement, and flow direction of bubbles are observed and the consequence of combined effects of various thermal properties of the fluid rather than a dominating single property. Start-up characteristics and dominating flow pattern for each fluid are reported in this paper. Moreover, peculiar flow characteristics with 300 ppm CTAC like bubble detachment, movement of cluster of micro-bubbles, and swirling are also presented.