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

Three different approaches are developed for the prediction of transient performance of cross-flow finned-tube, liquid/gas heat exchangers for the step change in the inlet temperature of the hot fluid. These are called the zero solid capacity, one solid capacity and two solid capacity approaches (ZCA, OCA and TCA respectively). In the analysis using ZCA, the heat capacities of the wall and tins are added to the capacities of the hot and cold fluids. The temperature variation of both fluids 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 separately. Energy equations for the hot and cold fluid, tube wall and the fins, if they are to be considered, are derived for each of these approaches and solved numerically using a finite-difference method. The variation of the dimensionless exit temperature of the hot and cold fluid with time is obtained for a step change in the inlet temperature of the hot fluid. To show the appreciability of these approaches, an experimental study is performed and the numerical results are compared with the experimental results.