Experimental investigation of non-uniform pulsating heat pipes charged with mixtures

In this thesis, heat transfer characteristics and flow phenomena of non-uniform (variable cross-sectional area in a turn) flat plate closed loop pulsating heat pipes (FP-CLPHPs) charged with pure fluids and binary and ternary mixtures (and/or fluid pairs) of fluids at different volumetric mixing ratios were experimentally investigated. Extensive experiments have been carried out for different volumetric mixing ratios (MR), filling ratios (FR), inclination angles (IA), heat loads and fluid types. In addition, during the experiments, flow images were taken with a high-speed camera simultaneously with the measurements, and the flow patterns were examined, and the relevant images were used to examine the physical mechanism. Two FP-CLPHPs were used in the experiments. In each heat pipe, the cross-sectional values of the parallel channels in a turn are different from each other. This thesis was divided into three main parts. In the first part, experimental investigation of pulsating heat pipe with variable parallel channels (2mm x 2mm and 1mm x 2mm) under different conditions of filling ratio, mixing ratio and inclination angle was conducted. Binary mixtures of ethanol and pentane were used as the working fluid. It was determined that the change in the mixing ratio had a significant effect on the results. The heat pipe charged with working fluid in the form of E:P=1:3 presented the best thermal characteristics for both low (FR=30%) and high (FR=60%) fill rates. In the second part, an experimental investigation of the non-uniform (2mm x 2mm and 1.5mm x 2mm) FP-CLPHP charged with miscible or immiscible binary fluids at a single filling ratio was conducted. Single forms and binary mixtures of hexane, pentane and methanol were used as working fluids. Measurements were taken at different positions (horizontal and vertical). Among the pure fluids, hexane and pentane presented similar thermal performance. However, methanol performed better than these two pure fluids. For MR=1:1 and MR=4:1, the thermal performance of the immiscible fluid pairs (methanol-hexane and pentane-methanol) was higher than the thermal performance of the miscible fluid pair (hexane-pentane mixture). The best thermal performance was observed in the heat pipe charged with the working fluid of P:M=1:1. In the third part, the performance of an asymmetrical designed (2mm x 2 mm and 1.5 mm x 2 mm) flat plate closed-loop pulsating heat pipe charged with ternary mixtures was experimentally investigated at different filling ratios, mixing ratios and inclination angles. Three fluids, water, methanol and pentane, were considered. The pure state of each fluid was also studied as a working fluid. At IA=90°, in general, ternary mixtures showed better thermal performance than the pure fluids. The ternary mixture of W:M:P=1:2:3 presented the highest thermal performance; while the lowest thermal performance was obtained for W:M:P=1:0:0 (pure water). Within the scope of each study, four important forces acting on the working mechanism in non-uniform channels were defined: (1) evaporative momentum force, (2) surface tension force, (3) shear force, and (4) gravity force.