transient behavior simulation of fin-and-tube heat exchangers for the variation of the inlet temperatures of both fluids

in this article, a transient behavior simulation of fin-and-tube heat exchangers has been studied. energy equation for fluid flow and tube wall is derived and solved numerically. the variation of the temperatures of both fluids with time and position are obtained for a step-change in the inlet temperatures of the water and air fluids. the results show that in step-change of inlet water-side temperature, outlet water-side temperature will get steady faster than air-side, while in step-change of inlet air-side temperature, outlet air-side temperature will become steady state faster than water-side. also, the time constant (the time interval that the flow will reach steady state) of the system is not influenced by the step-change amplitude of inlet air and water temperatures. the inlet water temperature expands along the tube and after a time interval, it reaches the outlet section of the tube. but, the inlet air temperature reaches the outlet section without time delay.