MODELING OF RADIAL HEAT-TRANSPORT IN WALL-COOLED PACKED-BEDS - CONFIDENCE-INTERVALS OF ESTIMATED PARAMETERS AND CHOICE OF BOUNDARY-CONDITIONS

The heat transport in a wall-cooled packed tube in which a hot gas is cooled down is often described with a pseudo-homogeneous one-dimension al or two-dimensional model. Assuming a radially flat inlet temperatur e profile at the bed entrance can lead to erroneous results, if the ac tual profile at the entrance is curved. It can cause an apparent lengt h dependence of the effective heat transport coefficients, the so call ed ''length effect''. The reason being that the amount of heat enterin g the packed bed is overestimated, which is compensated for by higher values for the heat transport coefficients. Using a parabolic inlet te mperature profile, as measured in the packed bed at a certain minimal bed length, eliminates the length dependence of the heat transport coe fficients. An experimental investigation showed that for the gas flow rates applied, Pe(p)s = 52 to 785, a wall heat transfer coefficient al pha(w) has to be used for modelling the heat transport with a two-dime nsional model. Confidence intervals are given for the effective radial heat conductivity lambda(e,r,) the wall heat transfer coefficient alp ha(w) and the overall heat transfer coefficient U(ov). It is shown tha t lambda(e,r) and alpha(w) are strongly cross-correlated and have larg e confidence intervals. Especially at low gas flow rates alpha(w) is d ifficult to determine accurately. The confidence intervals for U(ov) a re much smaller. It is shown that although values for lambda(e,r) and alpha(w) can scatter much for different measurements due to the cross- correlation of these coefficients, the scatter in U(ov) is reduced sig nificantly if this coefficient is calculated with the so called ''lump equation''.