Investigation of the effects of miscible and immiscible binary fluids on thermal performance of pulsating heat pipes

Abstract

Present paper reports and discusses experimental results related to thermal and flow characteristics of asymmetrical designed flat plate closed loop pulsating heat pipe charged with miscible and immiscible binary fluids. Extensive experimental range is provided for a comprehensive analysis; which covers different binary mixing ratios (H:P, M:H and P:M = 1:1, 1:4 and 4:1) and inclination angles (0 degrees and 90 degrees). The filling ratio is kept constant as 50 %. H, P and M represent hexane, pentane and methanol, respectively. Results of pure fluids are also compared to binary ones, and all the processes are qualitatively analyzed via high speed visualization. It is concluded that methanol is the best choice among pure fluids; however, results completely changed when two fluids are mixed. Strongly depending on mixing ratio, immiscible binary mixtures show significantly better thermal performance compared to both the miscible binary one (H:P) and pure fluids. In this regard, optimum thermal characteristics are obtained with immiscible mixture of P:M = 1:1. On the other hand, increasing volume fraction of methanol in pentane-methanol mixture adversely affects thermal characteristics; and even, the poorest thermal performance is presented by the mixture of P:M = 1:4. Bubble dynamics and relevant flow patterns play critical role on heat transport phenomenon.