Working fluid study for loop heat pipes

Loop heat pipes (LHPs) provide unique applications for high heat flux and long distance heat transfer. Working fluid aspects, including liquid charging ratio, type and fluid dynamics, in loop heat pipe is quite different as compared to conventional heat pipe. This paper will particularly endeavor to explain these differences, and provide optimal design conditions achieved via experimental study. Design considerations and operational aspects of LHP with respect to working fluid is evaluation and discussed in detail. Important thermal-fluid properties with respect to pressure-temperature relation and merit number are outlined for various heat transfer fluids to choose best option for given conditions. LHP with capillary evaporator, 25 mm diameter and 155 mm long, and heat transfer length ∼250 mm was designed to study and optimize different working fluid aspects including optimum working fluid range for different orientations, effect of fluid type on performance and operational dynamics of loop related to fluid (start-up, oscillations, hysteresis and buoyancy effects). Loop utilized copper containment, nickel wick and mainly water as working fluid. As an outcomes of this investigation, it has been concluded that liquid ratio in range of 40 to 60 % of loop volume provide optimum charge, for different orientations, with 50 % charge showing best thermal performance. In summary, considered working fluid type and optimized overall liquid amount, calculated by giving due consideration to volumes of different loop components, can help to get best performance from the loop systems.