Indoor airflow characteristics

Indoor Airflow Characteristics: A Study on the Building Air Flow Induced by Environmental Control Systems and Characteristics of Air Diffusion Devices

(This is my PhD project work under the supervision of Prof. W.K. Chow)

Brief:

Air diffusion system is the key component of the air-conditioning and mechanical ventilating systems which maintain desired indoor thermal environmental conditions through controling local air speeds and temperatures in the occupied zone. Issues firstly considered after review of existing ventilation theories and the parameters related to the thermal comfort and air diffusion system are, (1) Characteristics of the air diffusion terminal devices and the induced air flow patterns; (2) Thermal comfort and acceptability of occupants.

Air diffusers

A survey study in air conditioned spaces was conducted to identify popular designs of air diffusion system [1].

Example diffusers (Source: Energy Engineering 95;6:50-79,1998 [1])

Chamber tests

Air flow characteristics induced by common air diffusion terminal devices including ceiling-mounted linear diffuser, high sidewall grill and high side wall opening were then studied experimentally in an environmental chamber [2-4]. The included the flow visualization inside and near a square diffuser. Thermal comfort indices related to local air speeds were assessed. Empirical correlation relationships between thermal comfort indices and macroscopic flow parameters were derived.

Turbulence intensity (Source: Building and Environment 29;4:523-530,1994 [4])

Test Chamber (Source: Indoor Built Environment 5;2:82-98,1996 [2])

Field measurements

For understanding the actual building thermal environments in typical air conditioned and mechanically ventilated spaces, field measurements on a sport stadium [2], a large underground carpark [5] and 7 large mechanically ventilated waiting halls [6-7] were studied. Median values of mean air speeds and percentage of thermal discomfort were proposed to specify thermal environments. Additional subjective rating of building occupants for the car park were studied to justifying the thermal environment assessment results. It is because thermal comfort environment was not on the first priority. Both of the air speeds and turbulence intensity were found to be good thermal comfort parameters and they show some correlation to the carbon monoxide concentration in the car park.

From the measurements, macroscopic flow numbers including number of air changes rate, ventilation rate, diffuser inlet velocity, 2 jet momentum numbers, 2 new flow numbers X1 and X2, 3 jet momentum ratios and 2 jet-volume numbers were evaluated for deriving correlation relationships among local environmental conditions (specified by air speeds and percentage of thermal discomfort) in the occupied zone of the buildings concerned [7-8]. 2 new quantities, the jet momentum ratio RM2 and the jet-volume number Y1 were found to be good parameters for describing environmental conditions due to air flow in the occupied zone.

Computational fluid dynamics (CFD)

The correlation relationships were further investigated using Computational Fluid Dynamics [9]. Correlation relationships among the mean air speeds U50, Umean and the percentage of dissatisfied PD50 with macroscopic numbers were derived for spaces installed with wall-mounted air supply inlets. The correlation relationships deduced from CFD were compared with the experimental data. Good agreements were found for the jet momentum ratio RM2 with RM2 between 0.0481 and 0.2992 m³×s^-1. The jet momentum ratio RM2 is thus recommended to predict environmental conditions in the occupied zone of indoor spaces installed with wall-mounted air supply inlets.

CFD simulations (Source: Indoor and Built Environment 7;5/6:276-288,1998)

Applications (illustration purposes)

Potential application of the results is on large spaces. This study take atria as example for discussion. Atrium environments were classified systematically for further studies on thermal environmental control systems [10]. A total number of 138 atria were surveyed. Air diffusion systems design in atrium using result of this study was discussed. The chamber test results were also applied to design cabin air diffusion for congressed passengers such as in train or similar environment [11].

Age of air measurement for large spaces

Conventional methods of evaluating the ventilation system in large spaces using the age of air measured by tracer gas might not be practical. It is diffcult to achieve ‘well-mixed’ condition throughout the huge space; and the cost of discharging large volume of tracer gas is too expensive. A new method for evaluating the ventilation in large enclosed spaces is proposed [12]. This is based on the conventional step-down or decay method. Instead of flling up the entire hall with tracer gas, only a small control volume is considered. Tracer gas is discharged into this control volume with the ‘local’ mean age of air measured by studying the transient decay curve of the tracer gas concentration. Several positions are considered and the ‘ages’ measured can be used to evaluate the ventilation in the big hall. Field measurements were made in two big halls for illustrating the method. Further, the age measured in the control volume is compared with the age measured by thorough mixing of tracer gas in a smaller room.

Trace gas (SF6) concentrations in a large space (Source: Building and Environment 37;2:145-152,2002)

References:

[1] Chow WK, Wong LT, 1998. Survey on the air diffusion devices for air-conditioning systems in Hong Kong, Energy Engineering 95(6) 50-79.

[2] Chow WK, Wong LT, 1996. Experimental studies on the air flow characteristics induced by a high sidewall grill in a climate chamber, Indoor Built Environment 5(2) 82-98.

[3] Chow WK, Wong LT, Chan KT, Yiu JMK, 1994. Experimental studies on the airflow characteristics of air-conditioned spaces, ASHRAE Transactions 100(1) 256-263.

[4] Chow WK, Wong LT, 1994. Experimental studies on air diffusion of a linear diffuser and associated thermal comfort indices in an air-conditioned space, Building and Environment 29(4) 523-530.

[5] Chow WK, Wong LT, Fung WY, 1996. Field study on the indoor thermal environment and carbon monoxide levels in a large underground car park, Tunnelling and Underground Space Technology 11(3) 333-343.

[6] Chow WK, Wong LT, Fung WY, Field measurement of the air flow characteristics of big mechanically ventilated spaces, Building and Environment 31(6) 541-550.

[7] Chow WK, Wong LT, 1999. Local air speeds measurement in mechanically ventilated spaces, Building and Environment 34(5) 553-563.

[8] Chow WK, Wong LT, 1998. Air diffusion terminal devices: macroscopic numbers describing jet momentum, Building Services Engineering Research and Technology 19(1) 49-54.

[9] Chow WK, Wong LT, 1998. Equations for a ventilation design derived from computational fluid dynamics, Indoor and Built Environment 7(5/6) 276-288.

[10] Chow WK, Wong LT, 1999. Thermal environment design of atria in the Hong Kong Special Administrative Region: A survey study, Architectural ScienceReview 42(4) 235-252.

[11] Chow WK, Wong LT, 1997. Design of air diffusion terminal devices in passenger train vehicle, Journal of Environmental Engineering (ASCE) 123(12) 1203-1207.

[12] Chow WK, Fung WY, Wong LT, 2002. Preliminary studies on a new method for assessing ventilation in large spaces, Building and Environment 37(2) 145-152.