SOLAR INDUCED VENTILATION

Abstract

The present work deals with a numerical study on the airflow and heat transfer characteristics due to natural convection in a solar chimney system. The primary objective is to evaluate the effects of the air velocity on the thermal comfort of occupants due to buoyancy driven flows. The solar chimney consists of two vertical walls and a roof wall which were subjected to heating at various isothermal conditions. Based on the conservation laws and steady state conditions, governing equations were formulated using the ?-? turbulence model which included the effects of buoyancy. The three dimensional equations were solved by the SIMPLE finite difference scheme with the appropriate pressure boundary conditions at the air inlet and outlet of the system. At the air inlet, the velocity was not imposed but was an outcome of the numerical solution using an elliptic scheme. The numerical predictions were verified with data from experimental results conducted on a scale model. The key parameters controlling the behaviour of the system were identified and analysed on a two-dimensional basis. Numerical simulations covering the practical range of Grashof numbers and Reynolds numbers for a solar chimney system with different normalised air inlet heights and chimney aspect ratios were performed. The airflow and heat transfer developments in the system as well as the turbulence structure were investigated. The separate effect of each system parameter was also studied. Using the numerical results obtained, the airflow and heat transfer characteristics were correlated to the critical system parameters. Results indicate that the Nusselt number of the solar chimney is very much affected by the chimney aspect ratio and the heating effects of the chimney walls as embodied in the Grashof number. However, the heat transfer characteristics are less sensitive to the normalised air inlet height to the room although there is a sizable recirculation zone present at the upper edge of the air inlet. The correlations also indicate that the Reynolds number is very much affected by the chimney aspect ration, B/D. This is expected as more air is induced into the system due to the chimney effect. The Grashof number also has a strong effect on the Reynolds number because of the higher buoyancy induced at higher temperature differentials. It was found that the airflow rates induced by the solar chimney though the room amply meet the criteria of air exchange rates according to the local code of practice even at low heating temperatures of the solar chimney. But on a performance basis, it was found that the airflows in the occupied zone are rather weak. For all the cases studied, the maximum air velocities were located in the suction downstream of the air inlet. In the analysis of the air velocity distribution for the occupied zone, the air velocity that was exceeded for a certain percentage of the interior floor area was calculated. None of the air velocity calculated for each node in the occupied zone exceeded the limiting velocity which was required for improving the predicted percentage dissatisfied, PPD under still air conditions. Thus, it can be concluded that for pure buoyancy flows, the magnitudes of the air velocities are too low to have any influence on the predicted percentage dissatisfied PPD and hence the thermal comfort of occupants in the room. This implies that for the air velocity to have any impact on the thermal comfort of occupants, a combination of airflow clue to buoyancy natural convection and wind effects have to be considered.