CFD study of natural ventilation in a room using a solar chimney

Abstract

The aim of this dissertation was to investigate the potential of incorporating natural ventilation in buildings in a Maltese context. Published literature indicated the solar chimney as a feasible method of naturally ventilating a dwelling. It was identified that limited research was conducted with regards to the feasibility of side inlets into the SC, as well as methods of minimising reversed flow at the SC outlet. ANSYS DesignModeler was used to create 2D and 3D geometries which were then transferred to the ANSYS mesher to generate a mesh. The 2D model was then used as a foundation for the 3D model as well as a comparison tool to validate the final 3D results. Several limitations of the ANSYS mesher were identified, which prompted a switch to the FLUENT mesher. The FLUENT Mesher was capable of overcoming the limitations in mesh generation and was able to generate a good quality mesh. Convergence issues were encountered in 3D, potentially due to the more complex nature of the flow in three planes. To overcome these issues, a transient method was adopted with the aim of obtaining a steady state response. The transient model was initialised with a steady state model followed by a transient model with a very low time step size for the initial 15 iterations. This process resulted in a robust and accurate simulation. Multiple configurations were tested, these being a smaller room inlet, a smaller solar chimney cavity gap, a louver at the solar chimney outlet and closing of the solar chimney side inlets. The outlet mass flow rate ranged from 0.120 – 0.147 kgs-1, corresponding to an air changes per hour of between 13.38 to 16.28 for a volume of 21.15m3. It was found that a decrease of 25% in the room inlet area increased turbulence and subsequently decreased the outlet mass flow rate by 15.65%. Reversed flow at the solar chimney outlet was reported in all simulations but was minimised through the louver. The issue of hot air from the solar chimney reentering back into the room from the side inlets was noted in all cases with open side inlets. This resulted in an increase of up to 4.16K in the room average temperature. Closing the side inlets remedied this issue, eliminating the bled air and improving the outlet mass flow rate by 3.23%. This indicates that side inlets are detrimental to the thermal comfort and solar chimney performance. Further research should focus on reducing room air velocities to a more comfortable level as well as optimising the room inlet and solar chimney side inlets to minimise reversed flow and heat bleed.