Measurement of buoyancy-driven transient exchange flow rate across a thin horizontal ceiling vent of a non-adiabatic enclosure using a modified tracer-gas decay method

Abstract

ABSTRACTHorizontal openings are important pathways for mass and energy transfer between different zones in buildings or between a building and the outdoor environment. The temperature difference between indoor air and envelopes and its resulting heat transfer is common in buildings, especially in ones with transient ventilation. The transient, buoyancy-driven ventilation flow rates across a thin horizontal ceiling vent of a non-adiabatic enclosure are experimentally studied. Rather than using brine/water to imitate the buoyancy difference between indoor and outdoor fluids as been done in most previous studies, this study employs air as the working fluid and thus the effects of heat transfer between indoor air and envelopes can be taken into account. To measure the buoyancy-driven exchange flow rate that is dependant of the difference between indoor and outdoor temperature, a modified tracer-gas concentration decay method is proposed. In this method, the ventilation rate is determined by the combination of decay curve of tracer-gas concentration and that of temperature-rise. It is shown that the heat transfers at the enclosure envelopes have an influence on the exchange flow rate of a ceiling vent. Both a case with big rate of heat transfer from envelopes to indoor air and the one from indoor air to envelopes could produce dimensionless flow rates as twice as the model predictions that ignore the heat transfers at the envelopes (e.g. the Epstein's formula derived from salt-water experiments). It is also shown that is independent of the dimensionless temperature rise of indoor air .