Effect of Sintered Powder Size on the Convective Boiling Performance in a Larger Diameter Vertical Pipe – An Experimental Investigation

The study investigates the effect of powder size on the performance of flow boiling inside a sintered-coated pipe using R134a coolant in a vertical orientation. The pipe is uniformly heated with a diameter and a length of 27 mm and 100 mm, respectively. The powder size used for sintering on the tube are 45 μm, 75 μm, and 150 μm, respectively. The heat flux varies between 32 kW/m2 - 160 kW/m2. The outlet quality ranges from 0.1 to 0.9. The mass flow rate varies between 0.015 kg/s and 0.030 kg/s, and operating pressure varies between 500 kPa and 600 kPa. Tests are conducted on a vertical tube with a 27 mm inner diameter. The performance of two-phase flow boiling inside the smooth tube was compared with Chen’s correlation of convective boiling, and results revealed that the difference of heat transfer coefficient of ∼7% - 18% at high heat flux region and ∼36% - 40% at low heat flux region was observed. In addition, results also showed that the heat transfer coefficient increases with the rise of powder size, and an improvement of heat transfer coefficient of ∼33% to 81% was reported subject to different operating conditions for powder size of 150 μm. This is because the sintering powder will increase the roughness to initiate more nucleation sites, considerably enhancing bubble generation.