Experimental technique to determine unsteady flow in natural ventilation stacks at model scale

A major uncertainty in the design of naturally ventilated buildings with stacks (and with chimneys attached to fuel-burning appliances) is the effect of the unsteady wind on the stack flow. If the stack outlets are not sited correctly, flow reversal can be induced by the wind and this reversal may become established even in the presence of buoyancy. The paper describes two approaches which are being used to investigate the unsteady flow in ventilation stacks, namely: a wind tunnel modelling technique; and theoretical envelope flow modelling. The wind tunnel approach has required the development of a technique for determining the instantaneous magnitude and direction of the flow in a stack. The paper concentrates on this development and examples of measurements are presented. Following a brief description of the theoretical model, some preliminary comparisons between prediction and measurement are given.