| In view of the disadvantages of the conventional packed cooling tower (PCT), the SCT (Shower Cooling Tower) broke the designing way of the conventional cooling tower by doing away with the fills of the original countercurrent type of cooling tower and ameliorating the spray distribution of water. Eliminating the fill makes the tower fully empty which is of far-reaching significance in circulating water with high temperature, high turbidity. The research included the moving principles of hot water droplet in the cool air in the SCT and the heat and mass transfer between the droplet and the air based on the numerical model, the exergy analysis based on the second thermodynamic law, the experimental research guided by the theoretical research and through combining the head-on spray method, salt water cooling in the SCT is analysed, and the upward-spray SCT is designed. The research provided a theoretical basis for the application of the SCT. The research contents and conclusions are as follows:(1) Based on the force and the moving research of the droplet, the moving equations of the droplet in the SCT was derived and solved in detail. So the height of an SCT, the fitting location of the nozzles and the distance between the nozzles and the wall of the tower can be obtained in the designing of an SCT, which brings theoretical basis for the SCT designing. According to the different way of the water droplet and the air current, the water droplet flying process in the SCT was analyzed in detail, and the maximum and minimum radii of the possible existent water droplet under the different wind velocity were obtained. The influence of the equivalent diameter and the initial velocity of the water droplets on the moving up height was analysed. In order to investigate the coupled heat and mass transfer process between the droplet and the air, the evaporative model of water droplet was studied; and the conclusion that the evaporative mass of a droplet will not exceed 4%, so the decrease of its diameter after evaporation will not exceed 1%. So that the diameter of water droplet can be assumed to be constant, which will decrease calculated quantity greatly. Through the investigation of horizontal moving of the droplet, we found that as the diameter of the droplet is smaller, the horizontal moving distance is smaller. So in view of preventing the water sprayed on the wall, the smaller droplet is better.(2) Theoretical and experimental research on the heat and mass transfer of the SCT was conducted. A one-dimensional heat and masss transfer (HMT) numerical model based on Merkel assumptions and an artificial neural network (ANN) model were built. And experimental study was conducted. The results of the HMT model and ANN model were compared with the experimental data. It is shown that the ANN model is more close to the cooling process in the SCT. So the nonlinear model based on backpropagate ANN can improve the calculated precision of the outlet water temperature of the SCT. But the ANN model can only obtain the result, it is impossible to analyse and optimize each parameter. So it does not work for the ANN model to comprehend the heat and mass transfer status. However, the BP neural neural networks is of great significance in evaluating the operating characteristics of the SCT. Based on the investigation of the characteristics of the tower, experimental research on the tower is conducted, and then the influencing factors are analysed and the numerical and experimental results are compared.(3) In order to investigate the cooling characteristics of the impinging SCT, the impinging status of droplets was analysed through new breakup model. It is shown that the impinging phenomena of water droplets rely greatly on the attribution of the droplets. Through the experimental data published before (Ashgriz&Poo, 1990), dissipation factors are relavent with the weber number. By combining the impinging status of the water and salt water, the salt water number caused by stretching breakup and reflexive breakup is smaller than the water, because the surface tension of salt water is greater than the water. Based on impinging breakup model, salt water cooling in an SCT was analysed, which brought theoretical basis for salt water shower cooling tower.(4) Without applying the three of Merkel assumptions in model 1, SCT model 2 was built, the heat and mass transfer between the supersaturated air and the water was simulated, and the computed results and the experimental results were compared. It is shown that the results applying model 2 is more close to the experimental results. When the temperature of the ambient air is higher and the humidity is smaller, the outlet water temperature evaluated by model 2 is lower than model 1. This is because there is great difference between the outlet air temperature evaluated by model 1 and model 2. As the temperature of the ambient air is lower, the difference of the outlet air temperature between the two methods is smaller. As the ambient air temperature increases, if the air humidity is small, the difference of the evaluated outlet air temperature between the two methods will increase. Because the curve of the constant enthalpy in the supersaturated area is nearly upright, for constant air enthalpy, the air temperature is nearly constant and the moist air and mist included in the saturated air has no influence. But in the unsaturated area, the constant enthalpy curve is not upright, so the diffrence between the two points has big difference. So when the outlet air temperature is unsaturated, the results precision by applying model 1 is worse than the status that the outlet air temperature is supersaturated.(5) Based on the heat and mass model, the characteristics of the SCT by applying exergy is analyzed, which bring a method to study the SCT deeply. It is shown that the water exergy is defined as the usable energy included in the water that is decreased from the top to the bottom. The exergy quantity that the water provides is greater than obsorbs by the air, the system produes entropy. In order to dipict the usable exergy between the air and the water, the exergy of each fluid through the tower is given. It is shown that the air exergy via evaporative heat transfer is prominent. The exergy destruction increases from the top to the bottom, the distribution characteristics of exergy destruction can be used to optimize the operating characteristics of the SCT. Combining this conclusion, an important factor restricting SCT's characteristics is that as the droplet falling to the bottom, the water droplet velocity increases greatly, which means the heat and mass transfer process between the droplet and the air is short, so the usable energy from the water to the air decreases and the exergy destruction increases. Moreover, in SCTs the air velocity in upper tower is greater, which causes the exergy destruction on the top is less than the bottom.
|
PDF Full Text Request