EVALUATING MODELS FOR SUPERPOSITION OF WIND AND STACK EFFECT IN AIR INFILTRATION

Models designed for routine calculations of air infiltration rates int o buildings often use a semi-empirical function to combine the separat ely calculated wind-effect and stack-effect flow rates. The actual sup erposition of wind and buoyancy-generated pressure fields is a complic ated non-linear process that is strongly influenced by the distributio n of leakage sites on the building envelope. In the present study larg e sets of hourly-averaged air infiltration measurements using a consta nt concentration tracer gas injection system were sorted to separate s tack-driven, wind-driven and wind-direction shelter effects. These dat a sets were then used to test superposition errors for linear, quadrat ure and flow coefficient methods for superposition of wind and stack e ffects. By using measured values of wind and stack dominated extremes, tests of the superposition methods were made independent of theoretic al models for the wind-effect and stack-effect flows. The best superpo sition model was one using simple pressure addition. Results show that simple non-linear superposition models are an acceptable approximatio n to estimate average infiltration rates for combined wind and stack e ffect.