TRANSIENT-BEHAVIOR OF FINNED-TUBE CROSS-FLOW HEAT-EXCHANGERS

Three different approaches are developed for the prediction of transie nt performance of cross-flow finned-tube, liquid/gas heat exchangers f or the step change in the inlet temperature of the hot fluid. These ar e called the zero solid capacity, one solid capacity and two solid cap acity approaches (ZCA, OCA and TCA respectively). In the analysis usin g ZCA, the heat capacities of the wall and tins are added to the capac ities of the hot and cold fluids. The temperature variation of both fl uids between inlet and exit is assumed to be linear, and the effective rate of heat transfer is assumed to be proportional to the difference of the average temperatures on both sides. In the analysis using OCA the fins and tube wall are considered as one thermal capacity and the thermal resistance between them is neglected. However, in the analysis using TCA the capacities of the fins and tube wall are considered sep arately. Energy equations for the hot and cold fluid, tube wall and th e fins, if they are to be considered, are derived for each of these ap proaches and solved numerically using a finite-difference method. The variation of the dimensionless exit temperature of the hot and cold fl uid with time is obtained for a step change in the inlet temperature o f the hot fluid. To show the appreciability of these approaches, an ex perimental study is performed and the numerical results are compared w ith the experimental results.