Natural ventilation induced by combined wind and thermal forces

Analytical solutions are derived for calculating natural ventilation flow rates and air temperatures in a single-zone building with two openings when no thermal mass is present. In these solutions, the independent variables are the heat source strength and wind speed, rather than given indoor air temperatures. Three air change rate parameters α, β and γ are introduced to characterise, respectively, the effects of the thermal buoyancy force, the envelope heat loss and the wind force. Non-dimensional graphs are presented for calculating ventilation flow rates and air temperatures, and for sizing ventilation openings. The wind can either assist the buoyancy force or oppose the airflow. For assisting winds, the flow is always upwards and the solutions are straightforward. For opposing winds, the flow can be either upwards or downwards depending on the relative strengths of the two forces. In this case, the solution for the flow rate as a function of the heat source strength presents some complex features. A simple dynamical analysis is carried out to identify the stable solutions.