Performance Simulation And Experimental Study Of The Vacuum Condensation Heat Transfer

Due to its advantages of safety, reliability, high efficiency and the flexibility of temperature, the vacuum boiler receives more attention in the field of heat ventilation and air conditioning (HVAC) and boiler. The topic relies on a sample vacuum boiler supplied by Ningbo Fulton Thermal Equipment Corporation Limited, whose rated power is0.35MW. According to the monitoring of the operational thermal efficiency and the feedback of the user’s experience on the sample vacuum boiler, there are unstable performances happening to vacuum boiler, such as unstable heat exchange, resulting to cooling water’s temperature fluctuation. This research aims to solve the problems presented during the process of the vacuum boiler. However, the actual operation of the heat transfer process is unstable and fluctuant, which does not fulfill the actual situation seriously. From a theoretical point, the paper describes the cause of the instability of the heat transfer, respectively from the two aspects of the coexistence of boiling and condensation in the vacuum environment and the bundle effect exists in the shell and tube condensers. Conclusions can be drawn that the fluctuation of heat transfer in the vacuum boiler is caused by the vapor-liquid two-phase countercurrent, geyser effect, bundle effect, and so on. This research can be divided into the following areas:1. Heat transfer process in the vacuum shell is analyzed by means of theoretical analysis. The paper identifies the factors affecting the heat transfer process in vacuum boiler in various stages and overall process. By comparing the existing actual operating parameters and the theoretical results on the vacuum boiler. It can be concluded that the main factors affecting the instability of the vacuum boiler heat transfer are steam humidity ratio, the steam flow field, the flow of condensate, the pressure and temperature in the vacuum space, the size of the heat exchange, etc.2. The simulation software, fluent, was used to simulate the heat transfer tubes. The simulation is based on orthogonal regression experimental design method. The heat transfer tubes are modeled, respectively, with film and without film. A group of simulation result is selected to analyze the fluid temperature and the tube wall temperature changing along the axial direction. In addition, heat transfer tubes are simulated with different liquid film thickness from the perspective of different steps of the bundle. The effects on the tubes heat transfer ability by the bundle effect are analyzed from the theoretical simulation point of view.3. Boiling-condensation coexistence condensing heat transfer of the single tube is studied in the vacuum shell. The test tubes are designed in the EES, the heat transfer capability is largely affected by length-diameter ratio. By comprehensive consideration, three kinds of tubes with the same length and wall thickness. The overall heat transfer coefficient is influenced by three factors which are turbulent heat transfer coefficient, fluid flow velocity, inlet temperature. Therefore, there are three factors and three levels in the experiment. In order to reduce the number of experiments, orthogonal regression experimental design method is applied to the simulation. It can be concluded by the comparison of experimental results and theoretical results that:tube wall temperature is gradually increased along the direction of fluid flow, rather than theoretically the same temperature. The results can be seen from Heat transfer tubes of different film thickness, condensing heat transfer thermal resistance is the most obvious changing aspects of resistance during the whole process. The gap between the heat transfer coefficient of the tubes in the vacuum circumstance with the theoretical results is about20%.4. The monitoring data on the inlet and outlet temperature of the Sample vacuum boiler tubes at different locations is analyzed. In the heat exchange bundle, crossing line and column were selected to measure the outlet temperature. The heat transfer coefficient of the measured heat transfer tubes are calculated with the measurement results. Then the simulation results were compared with the experimental results. The result is that they almost have the same trend. The heat transfer coefficient of the tubes arranged near the center was significantly lower than that arranged in the edge of the bundle. Thus, the heat transfer efficiency of the vacuum boiler can be better improved by changing the steam flow field in the vacuum shell to make it sufficient contact to each of the tubes. 5. On the basis of research two kinds of new structural vacuum boiler are the proposed:Separate vacuum boiler and vacuum boiler with embedded boiler barrel. Two kinds of the boilers separate steam and condensate channels, increasing the steam dryness, eliminating the counter-flow of the condensate and steam, weakening the bundle effects.The main reason for instability of the heat transfer in the vacuum boiler is the low condensation heat transfer coefficient outside the tube, whether it is from the perspective of experiment or simulation. Tube condensation heat transfer coefficient should be optimized in two aspects which are improving the external heat transfer coefficient of the tube and the bundle. To Improve the outer tube condensation heat transfer coefficient of the single tube, kind of the tube can be switched to drop-wise condensation tubes to prevent the tubes and other effects caused by condensate, reducing the thermal resistance caused by film condensation. The heat transfer coefficient of the Bundles can be improved by the installation of baffle plate, clear the steam channels and so on. All of these provide a clear direction to research and develop the vacuum boiler in future.