Continuous vs. pulsating flow boiling. Part 1: Experimental comparison and visualization

High heat exchanger performance is crucial to meet efficiency standards with low cost and environmental impact. Heat transfer performance may be improved by passive and active enhancement techniques applicable to air, liquid and phase change heat exchangers. This experimental study investigates an active method for flow boiling heat transfer enhancement by means of fluid flow pulsation. The pulsations are believed to increase the flow boiling heat transfer by means of better bulk fluid mixing, wall wetting and flow-regime destabilization. The pulsations are introduced by a flow modulating expansion device and compared with continuous flow from a stepper-motor expansion valve. The cycle time range from 1 to 9 seconds for the pulsations. Transient heat transfer coefficients are measured downstream from the expansion valve at low vapor qualities and the time-averaged heat transfer coefficients are compared to those with continuous fluid flow. The results show that the pulsations improve the time-averaged heat transfer coefficient by up to 5 % at low cycle time, whereas the pulsations reduce the time-averaged heat transfer coefficient by up to 8 % at high cycle time and heat flux. The latter reduction is adhered to significant dry-out when the expansion valve is closed. The test section consists of the two exchangeable expansion valves and a tube-in-tube evaporator with four subsections for measuring the heat transfer coefficient. Refrigerant R134a flows internally and water flows externally. The inner tube internal diameter is 8 mm and each subsection is 0.25 m long. Temperatures in and out of each subsection are measured as well as the water flow rate. Wall temperatures are measured outside on the internal tube in the top and the bottom of each subsection as well as refrigerant temperature, pressure and flow rate. The evaporation temperature is kept at 5°C during the experiments and the fluid state before the expansion valve is kept at a condensing temperature and subcooling of 32.5°C and 2 K, respectively. The time-averaged refrigerant mass flux varies from 41 to 167 kg/m2s, vapor qualities from 0.18 to 0.59 and heat fluxes from 0.8 to 45 kW/m2. The paper includes a description of the test rig, the data reduction method, uncertainty propagation and the results and comparison.