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

Horizontal 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 .

KEYWORDS:

• Natural ventilation
• smoke exhaust
• ceiling vent
• envelope heat transfer
• tracer-gas method

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

The authors acknowledge the support from the National Natural Science Foundation of China (NSFC) [grant number 51578087], [grant number 51106189], Project No. 106112016CDJXY210004 supported by the Fundamental Research Funds for the Central Universities, and the 111 Project (No. B13041).

Notes on contributors

Tao Du

Tao Du is a PhD student at Department of Urban Construction and Environmental Engineering, Chongqing University. His study focuses on natural ventilation and smoke control.

Dong Yang

Dong Yang is an associate professor at Department of Urban Construction and Environmental Engineering, Chongqing University. His research is mainly about fluid mechanics, building environment, ventilation and smoke control strategy.

Shini Peng

Shini Peng is a professor of gas engineering in Chongqing University. He has research interests in fluid mechanics, gas transmission and distribution, gas burner.

Baizhan Li

Baizhan Li is a professor of Chongqing University. He is interested in green buildings, building physics and thermal comfort.