Mathematical Model And Numerical Simulation Of Natural Ventilation Enhanced By Solar Chimney

Natural ventilation is an energy-saving ventilation technology. However, due to the weak driven force of natural ventilation, i.e., wind pressure or thermal pressure, the natural ventilation rate is usually smaller than that of mechnical ventilation. Therefore, it's necessary to study the basic mechanism of natural ventilation and its enhancement methods. Solar chimney is an effective technology to improve natural ventilation performance, which ustilizes the solar radiation to heat the air inside the chimney channel and increase the thermal buoyancy to enhance the natural ventilation. It is important to study the natural ventilation performance of solar chimney due to its high academic value and engineering guidance before its commercial application.In this thesis, the heat transfer in a vertical soalr chimney was modeled mathematically and numerically. The effect of solar chimney sizes and solar radiation on the temperature of air, absorb wall and glass cover, the ventilation rate was calculated and discussed.First, based on the principle of energy conservation, a revised one-dimensional steady mathematical model was proposed to improve the prediction of the natural ventilation performance in a solar chimney. The thermal resistance of the glass cover and absorber wall was taken into account to calculate the heat transfer in the solar chimney. Results of the present model were compared to related experimental data and reasonable agreement was validated. According to the results of the present model, the best ratio of the solar chimney height, solar chimney depth and air inlet height is closed to 10:1:1.Second, a one-dimensional unsteady mathematical model for predicting the airflow rate in a solar chimney was proposed on the basis of the steady model. Crank-Nicolson finite difference scheme was used to solve the differential equations. The transient temperature variation in the glass cover, the absorber wall, the air in the channel and the airflow rate were calculated with a variable soalr radiation. Results show that the temperature in the glass cover, the absorber wall, the air in the channel increase when solar radiation increases, leading to more air flow during daytime condition. The thermal resistance of the glass cover and absorber wall could not be ignored during the nighttime due to the heat loss and storage effect of the absorber wall.Finally, computational fluid dynamic (CFD) method was used to numerically study the natural ventilation in the solar chimney. Realizableκ-εturbulent model was used to simulate the airflow and DO radiation model was used to calculate the heat transfer between the air and absorber wall. SIMPLEC algorithm was used for the calculation. The performance of ventilation in the solar chimney with different structural parameters of the solar chimney was studied. The local heat transfer coefficient and Grashof number were also calculated. The following main conclusions can be drawn from the results.1.The performance of ventilation in the solar chimney is improved as the height and depth of the solar chimney increases, but the air inlet height of solar chimney has a smaller influence on the performance of ventilation than the other structure parameters.2. The temperature and velocity distribution of the air in the soalr chimney is not uniform at the same level along the depth direction. Larger temperature and velocity gradients are found within the boundary layer near the heated wall.3. The internal airflow is laminar when the solar chimney is no higher than 2 meters, while the airflow in the boundary layer changed from laminar flow into transitional flow at about 1.5m vertically measured from the air inlet when the solar chimney is as high as 3 meters. When the height of solar chimney is 4 meters, the airflow in the boundary layer becomes turbulent after the vertical height is more than 3.3 meters.The results demonstrated that reasonable design of SC would improve the natural ventilation performance and theoretically provided the important reference to the optimized design of solar chimney for natural ventilation application.