Airflow Dominated by Thermal Plumes
In rooms where air and contaminant movement is dominated by thermal energy of heat sources (e. g., in rooms with natural or displacement ventilation), temperature and contaminant stratification along the room height is created. Air supply and exhaust in such rooms are designed not to disturb the natural pattern of air movement created by heat sources: cooled air enters the room in
The lower zone close to the floor level and is exhausted from the upper zone. Under the influence of buoyancy, cold air spreads along the floor and floods the lower zone of the room. The air close to the heat source is heated and rises upward as a convective airstream (Fig. 7.8). In the upper zone this stream spreads along the ceiling. The lower part of the convective stream induces the colder air of the lower zone of the room, and the upper part of the convective airstream induces the heated air of the upper zone of the room. The height of the lower zone depends on the air volume discharged into the occupied zone and on the amounts of convective heat discharged by the sources (Fig. 7.9). In the presence of the temperature gradient, the convective plume may reach the height where the temperature difference in the plume and in the ambient air at the corresponding height disappears. This can happen w’ith convective plumes above weak heat sources (e. g., above cigarette, point welding, person’s body) in the presence of stronger heat sources (Fig. 7.10).
Temperature and contaminant gradients along the room height and separation stability between the upper and lower zones are influenced by turbulent exchange between these zones. The heat flux density due to turbulent exchange can be determined as7
50 i —
^7lurb — ^turbCpgp, ( ‘-.l5)
Where 80/82 is the air temperature gradient in the separation zone. To calculate the turbulent exchange coefficient, Aturi„ for the separation zone,
Shilkrot7 used the V. H. Munk and E. R. Anderson relationship,9 w’hich is in good agreement with empirical data:
Aturb = ^ex(1 + 3.3 Ri)-3/1 (7.36)
The exchange coefficient, j4cx, has been evaluated experimentally.
Z, m 0.6
FIGURE 7.9 Influence of exhausted airflow on airflow pattern in the naturally ventilated room: (a) airflow in the convective plume smaller than exhausted airflow; (fa) airflow In the convective pluime equal to the exhausted airflow; (c) airflow In the thermal plume at the stratification level equal to the exhausted airflow.8 (t, — air temperature along the room height, tr = average room temperature)
Z, m 0.6 0.5
0.3 0.2 0.1 0
FIGURE 7.10 Stratification in rooms with several heat sources of different strength.9
To characterize the airflow in the stratified space, Elterman2 proposed K, which is a ratio of kinetic energy dissipating in the ventilated space to the energy used to suppress the buoyancy forces:
SHAPE \* MERGEFORMAT
The criteria K is similar to the Archimedes number introduced in 1930 by Baturin and Shepelev7 to characterize air jets influenced by buoyancy, or to the Richardson criteria used in meteorology to characterize the ratio of the turbulence suppression by the buoyancy forces over the turbulence generation by the Reynolds tension.10’11 In the case of displacement ventilation, the Richardson criteria can be defined by the relationship’2
Ri = !■-
7.3 S’ I
Where Si>/&z is the room’s velocity gradient in the separation zone. Analysis conducted by Shilkrot7 for the parameter ranges
• Convective component of heat sources up to 11.6 x 103 kW
• Heights of the temperature separation level up to 15 m
• Room air exchange rates up to 50 h’1,
• Room heat intensity up to 116 W/m2
• Velocity of air supply up to 2 m/s
• Richardson number below 5
Showed that the heat flux density value due to the turbulent exchange between the upper and lower zones does not exceed 10% of the total heat released into the occupied zone, and thus can be neglected.
Energy generated by physical activity in the room (i. e., movement of people, transport, conveyor, operation of machines) increases turbulent exchange between the upper and the lower zones and may even disrupt temperature and contaminant stratification along the room height.
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