The Study Of The Prediction Method Of Airflow Rate And The Enhancement Ventilation For Solar Chimney

Natural ventialtion is an important sustainable building design strategy and has attracted a strong growing interest because of its potential advantages in terms of economic, energy requirement and environmental benefits. However, duo to the weak driven force of natural ventilation, i.e., wind pressure or thermal pressure, the natural ventilation rate is usually smaller and is difficult to control than that of mechanical ventilation. Solar chimney is an excellent passive ventilation strategy for enhancing natural ventilation through the buildings and reducing buildings consumption. Solar chimney exploits the solar radiation to heat the air inside chimney channel. A sufficient temperature difference can be created by maximizing the slolar gain and the thermal buoyancy can be increased. The combined radiation and convection inside a solar chimney results in appreciable air movement and in turn enhances natural ventilation.Theoretical analysis, experimental study and numerical simulation were used to study the overpredictions of the existing airflow rate prediction method available in the literature and the law of the occurrence of reverse flow at the chimney outlet, the main research contents are shown as follows.Two reasons of overpredictions of the existing airflow rate prediction method available in the literature are reported: firstly, the existing airflow rate prediction method available in the literature is based on the assumption of uniform temperature distribution at the same height inside the chimney. The assumption results in the incorrect calculation of stack pressure inside the chimney. Secondly, the normal forced flow pressure loss coefficients are used to the existing airflow rate prediction method available in the literature. The normal forced flow pressure loss coefficients can result in the incorrect evaluations of the pressure loss along the chimney. At extreme situations, the reverse flow can occur at the solar chimney outlet. The prediction method available in the literature based on the nomal forced flow pressure loss coefficients can significantly overpredict the airflow rate.For a vertical solar chimney with both open-ended, the chimney outlet connects the hot air inside chimney and cold air outside the chimney. The airflow at the chimney outlet resembles a horizontal vent problem. Based on the theoretical analysis of horizontal vent problem, a new pressure loss coefficient correlation taken into account the reverse flow at the chimney outlet can be obtained. Further, a improved prediction method for airflow rate in solar chimney, which takes into account the variation of pressure loss coefficient for different flow conditions at the chimney out, is presented and compared with the experimental results obtained in this work and available in the literature. It is shown that the improved prediction method is better agreement with the experimental results than the existing prediction method available in the lieterature. Although the improved prediction method for airflow rate in solar chimney is not perfect, the success in this study suggests that the correct calculation of the pressure loss coefficient at the solar chimney outlet is the right direction for improving the airflow rate predictions in the solar chimney.The experimental investigations are carried out for the temperature and velocity field inside a solar chimney. Flow visualization experirments are focused on revealing further insight in the flow behavior, especially the reverse flow occurring at the solar chimney outlet.A CFD numerical simulation study on turbulent flow for solar chimney with a wide range of chimney heights, chimney gaps and heat fluxes has been performed. The reverse flow at the chimney outlet can be influenced mainly by the solar chimney height and gap. The occurence of reverse flow can influenced negatively the airflow rate inside the chimney. For the solar chimney with 0.5m, 1m and 2m heights, the gap-to-height ratio, which is the beginning occurrence of the reverse flow at the chimney outlet, are all 0.2. The optimum gap-to-height ratios that maximize the airflow rate in the solar chimney are reported. For diffirent solar chimney height, the optimum gap-top-height ratio change. The optimum gap-to-height ratio is 0.2 for the solar chimney with 0.5m height, and 0.1 for the solar chimney with 1m and 2m height.Finally, on the basis of the CFD numerical simulation results two empirical formulate are found, giving Reynolds number(Re) and Nusselt number(Nu) as functions of Rayleigh number(Ra*), the gap-to-height ratio and the height.