Convective heat transfer within mechanically-ventilated building spaces

摘要

A hierarchy of interacting and interdependent approaches havebeen developed for calculating internal surface convective heattransfer coefficients within mechanically-ventilated rooms. A'high-level' computer code is developed for non-bucyant andbuoyant flow based on the "elliptic' code of Pun and Spalding(1977), in which 'upwind' finite-difference approximations to thegoverning partial-differential equations for continuity, momentumand thermal energy are formulated in terms of 'primitive'pressure-velocity variables. Closure of these time-averaged,elliptic equations is obtained via transport equations for boththe turbulence kinetic energy and its dissipation rate. Thehigh-level code solves the difference equations for apredetermined size, staggered grid in an iterative 'line-by-line'manner using a guess-and-correct procedure.An 'intermediate-level' computer code (the ROOM-CHT program)has also been developed for the above purpose, which employs'informed" estimates of the flow and thermal field based on theknown mean flow properties of wall-jets. The corresponding heattransfer distribution across the room surface is calculated usingwall-jet profile analysis or improved data correlations forbucyancy-driven convection as appropriate.Caqputations are presented for a room into which air isinjected through a low or high side wall register. The supply ofair governed by both cyclic and modulating control was examined.The intermediate-level code is advocated as being the mostappropriate for meeting the requirements of dynamic buildingthermal models. This code was verified by comparison with thehigh-level code and with experimental measurements. The oomputedheat transfer coefficients from the intermediate-level code werefound to be in good agreement with that of the high-level code.Both indicate significantly higher values than those which wouldbe obtained from established design guides. These high valuessuggest errors in building thermal models based on guide data,including substantial under-estimation of preheat times.