Flow and diffusion fields in rooms are studied by means of numerical simulation. The rooms used are models of conventional flow-type clean rooms. In the rooms, the exhaust as well as the supply openings are set in the ceiling. The exhaust openings are installed so that the supply and exhaust air flow rates are balanced locally in that space. This ventilation system is intended to avoid formation of a large recirculating flow in the whole room. The flow field is composed of a series of "flow units" in which supply-exhaust flows are locally "confined" and "closed off" from other units. Each flow unit consists of a supply jet and the rising recirculating streams around it. This locally confined flow field is effective in limiting diffusion of contaminants to within a local area, compared with conventional methods. Numerical simulation is performed for the following four cases: (1) supply and exhaust air flow rates are locally balanced for each flow unit, (2) supply and exhaust air flow rates are set to agree with the results of a model experiment, (3) a flow obstacle is arranged, and (4) a balance between the supply and exhaust air flow rates for one flow unit is not maintained. Supply and exhaust air flow rates can be set more exactly in numerical simulation than in experiments or actual trials. Therefore, precise examination of the effects of an imbalance between supply and exhaust rates can be conducted quantitatively only by means of numerical simulation. The ventilation efficiency of the room was quantitatively examined using the scale of ventilation efficiency (SVE 1- 3) proposed earlier, based on the results of numerical study.

KEYWORDS: air diffusion, air change rate, exhaust air, rooms, calculating, clean rooms, ventilation, recirculating, exhaust air ventilation, air flow rate, balanced ventilation, extract and input ventilation, efficiency