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2.2 Air 145 With p points, a system of p non-linear equations ensues. With known boundary conditions, the p unknown air pressures follow from solving that system. Insertion into the flow equations gives the airflows between points. Non-linearity makes iteration necessary. Assume values for the p air pressures, calculate the related air permeances and solve the system. Recalculate the permances with the p air pressures found, solve the system again and continue until the standard deviation between preceding and new air pressures equals a preset value. In the exceptional case that the flow in a flat assembly is nonetheless one-dimensional, a series circuit of linear and non-linear air permeances emerges, with the same air flux ga migrating across all: Layer 1 ga ˆ Ka1ΔPa1 ˆ a1ΔPb1 a1 or 1 ga a1 b1 ˆ ΔPa1 Layer 2 ga ˆ Ka2ΔPa2 ˆ a2ΔPb2 a2 or 1 ga a2 b2 ˆ ΔPa2 Layern ga ˆ KanΔPan ˆ anΔPbn an or 1 ga an bn ˆ ΔPan Sum : ga Pg 1 b1 i a a 1 bi i 2 64 3 75 ˆ ΔPa (2.29) ΔPa is the air pressure differential. The solution again requires iteration. Once the ‘correct’ air flux is known, the air pressure distribution in the assembly follows from the layer equations. 2.2.6 Air transfer at the building level 2.2.6.1 Definitions Air in buildings may flow inter- and intra-zone. ‘Inter’ stands for air exchanges among spaces and the outdoors. Modelling starts from a grid of zone points linked by flow paths. ‘Intra’ relates to air movement within spaces and relates to questions such as ‘What about looping? How does the zone and ventilation air mix? Which corners lack air washing?’. Solving requires computational fluid dynamics (CFD). The discussion here deals with interzone airflow only. 2.2.6.2 Thermal stack Thermal stack acts vertically. With the outdoors as a reference, the equation is: Pa gρaz ˆ Pa z gPa Ra 1 Te 1 Ti Pa 3460z 1 Te 1 Ti (2.30)


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