The Experiment And Numerical Simulation Study On Thermal Cycle Mechanism Of Compact All-glass Evacuated Tube Water Heaters

Currently, all-glass evacuated tube solar water heater is the most widely usedsolar water heaters in the domestic market. in this paper, the influence of evacuatedtube structural parameters on the flow field and temperature field inside the evacuatedtube water heater do the following several aspects research on the evacuated tubewater heater by the way of experimental and numerical methods.1. Taking the domestic evacuated tube water heater which is from the samemanufacturer, as the research object to carry out thermal performance test onstructural parameters of tube length. The tube length structural parameter of all-glassevacuated solar water heater of the same manufacturers were producing thermalperformance test. The influence of tube length and the daily irradiation on dailyenergy and efficiency of evacuated tube water heaters has been analyzed. The resultsshowed that the daily irradiation has comparatively great impact on the system dailyheat gain than daily average efficiency for the same set of evacuated tube waterheater. Under the same test conditions, the daily average efficiency and daily heatgain of evacuated tube water heater system, which consists of1800mm tube, isgreater than the one with2100mm tube0.5%. which shows that tube length ofevacuated tube has a little influence on the daily heat gain and the averageefficiency.2. On experiment data and the use condition of evacuated tube water heater inthe domestic, the three-dimensional numerical model of evacuated tubes with vacuumand water two domains in different tube length, diameter and tilt-angle have been established by CFD packages in view of the experimental data and the domesticactual use of evacuated tube water heater. In addition, meshing, numerical simulationand data analysis have also been discussed. In the model, the tube length that insertsthe tank has been taken into consideration, and also the model reliability been verifiedby test. Take solar load model in fluent to study flow and heat transfer changes insidethe system with heating time in the model.3. The results showed that, the temperature above the opening was slightlychanged with the tilt-angle increased, and the volume of low-temperature region weredecreased in the bottom of tank. The average Nusselt number is increasing within thesystem with tilt-angle of the vacuum water heater increased from20°to60°. Whentilt-angle is greater than60°, the average Nusselt number is unchanged. The velocitydistribution is the same at the evacuated tube and the tank cross-sectional, the flowrate increases with the tilt-angle increases. The velocity at the center of the tube ismuch smaller the velocity on both sides of the center.4. The velocity on the tube center cross-sectional position showed symmetricaldistribution. The Nusselt number of system increases with the tube length increase.It indicated that convective heat transfer became stronger with the tube lengthincrease. With the tube length from1500mm to2400mm, the flow rate inside the tubebuttom is smaller than1~2orders compared with the middle, showed that the bottomof the tube does not sufficient.5. To the same set of evacuated tube water heater system, convective heattransfer is most intense at the beginning, with the heating time increase, the internalconvection heat transfer is weaken, For evacuated heater system with diameter of47mm and58mm, Nuseelt number for58mm system slightly smaller than the systemfor47mm. The temperature difference on evacuated tube center and the tank is smallfor both diameters. two tube diameter has no effect on the flow rate of the system.The temperature field and flow field distribution of two tube diameters showed thatthe influence of tube diameter on the thermal properties of the water heater is verysmall.The study in this paper aims to have a certain understanding for heat and masstransfer and internal flow distribution in the evacuated tube water heater, provide some scientific basis for improving the daily average efficiency of evacuated tubewater heater system and production applications at the same time.