Experimental Investigation of the Coupled Thermal–Hydraulic Performance of ZnO Nanofluids in a Heat Pipe System: Effects of Nanoparticle Size, Concentration, and Flow Behavior

This study presents an experimental investigation of the coupled thermal–hydraulic performance of a heat pipe operating with ZnO nanofluids as the working fluid. ZnO nanoparticles with average diameters of ۳۵ and ۵۵ nm were dispersed in distilled water at concentrations of ۱, ۱۰, ۵۰, and ۱۰۰ ppm. Experiments were conducted under heat inputs of ۳۵, ۴۵, ۵۵, and ۶۵ W to evaluate the effects of nanoparticle size, concentration, and operating conditions on axial wall temperature distribution, thermal resistance, pressure drop, and Reynolds number. The results demonstrated that the incorporation of ZnO nanoparticles significantly reduced wall temperature and thermal resistance compared to distilled water under all operating conditions. The optimum nanoparticle concentration was found to be ۵۰ ppm for both particle sizes. At this concentration, thermal resistance decreased by up to ۷۲% and ۶۴% for the ۳۵ and ۵۵ nm nanoparticles, respectively. Nanofluids containing ۳۵ nm particles exhibited superior thermal performance owing to their higher effective thermal conductivity and improved colloidal stability. Pressure drops increased with nanoparticle concentration and Reynolds number due to enhanced viscosity and flow resistance. At ۵۰ ppm, the pressure drops increased by approximately ۲۵% for the ۳۵ nm nanoparticles and ۳۰% for the ۵۵ nm nanoparticles. Despite this hydraulic penalty, the thermal enhancement achieved under optimum conditions outweighed the associated increase in flow resistance. Uncertainty analysis based on the root-sum-square method yielded maximum uncertainties of ±۴.۵%, ±۳.۸%, and ±۳.۲% for thermal resistance, pressure drop, and Reynolds number, respectively. Repeatability tests showed measurement variations below ۵%, confirming acceptable experimental reliability. The findings demonstrate that nanoparticle size and concentration strongly influence the thermal and hydraulic behavior of heat pipe systems and confirm the potential of ZnO nanofluids for enhancing heat transfer performance under optimized operating conditions.

کلیدواژه ها:

Heat pipe ، Zinc oxide nanofluid ، Concentration ، Flow behavior ، Nanoparticle size