13 Modelling Considerations - 3 Boundary ConditionsAir supply ductsprescribed sources of mass, momentum, turbulence and residence timeAir Extract ductsspecified fixed-pressure outlet sinksLFU’sinlets and outlets specified as above
14 Representation of LFU’s Air flow in individual units not solvedtreated as internal blockages in domainair discharged from base at prescribed ratematching intake from front faceinteractive updating of discharge residence timeconstant internal residence time assumed
15 Room 30Small room x 5.94 x 2.92 m.Used for processing, filling and packaging of productsContains central plinth with filling-machine enclosure aboveConveyor-linked trays outboard of enclosure, for containers & finished goods
41 Room 30 Ventilation Summary (after refurbishment) Complex flow paths between inlet and outletPattern complicated by LFU intake extraction flowsair has to pass around central plinth and filling-machine enclosureair in near-side passage has to “run gauntlet” of LFU’s
42 Room 30 Ventilation Summary (after refurbishment) Outflow-weighted residence time of 121 seconds meets client criterion“near” extract air considerably “older” than “far” extract airsome “dead zones” still apparentevidence of entrainment into more than one unit
47 LFU Unit intake timesUnits D1, D2, B2, C2, A1 receive considerably fresher air than their neighbours A2-A4, B2 and C1Air entering A4 is 2.2 times older than that entering D1Air leaving A4 exceeds air-change criterion en route to outlet
48 Conclusions“Residence time” analysis concept adds considerable value to vector and contour plots in assessment of complex ventilation flows.