Analysis of the thermal response of a serpentine finned-tube, liquid-to-gas, cross-flow heat exchanger.

摘要

An analytical and numerical study of the transient response of a single-row, serpentine, finned-tube, liquid-to-gas crossflow heat exchanger is presented. Transients are due to step or ramp disturbances in liquid inlet flow rate or step changes in liquid inlet temperature. Three models are used in the investigation; a model that accounts for two-dimensional temperature gradients in the fins, a model that accounts for longitudinal tube conduction, and a model in which the fins are lumped with the tube and longitudinal tube conduction is neglected.;Tube conduction effects were found to be negligible for a typical multi-pass serpentine heat exchanger. However, effects are found to be important for short, single pass heat exchangers.;Using a constant and uniform gas-side heat transfer coefficient for all fins in the two-dimensional fin conduction model, response times of available experimental data were overpredicted. The effects of position-dependent heat transfer coefficient values were examined and found to have a negligible effect on the prediction of overall heat exchanger response times. However, the effect of a time-dependent heat transfer coefficient was shown to be significant through the use of calculated time-dependent fin efficiency.;Closed form analytical solutions are obtained for the lumped fin model for both step changes in liquid inlet flow rate and temperature. A perturbation solution is additionally obtained for the lumped fin model for the case of a ramp change in inlet liquid flow rate. Each of the three models were solved numerically. Limited experimental results available in the literature were used for comparison purposes. For the available experimental data, the lumped fin model was found to over predicts response times, but in general performed satisfactorily. The effect of tube bends was found to have a significant influence on predicted response times. The first order perturbation solution was found to favorably predict the response time of step and ramp changes in liquid inlet temperature, when compared with numerical solutions.