Response of parallel-flow and counterflow heat exchangers to sinusoidal flow rate changes of large amplitude

A computational procedure for obtaining both the steady-state responses and the describing functions of parallel flow and counterflow heat exchangers subject to sinusoidal flow rate changes of large amplitude is presented. The describing functions are defined in terms of both amplitude and circular frequency and represent the relationship between input and output fundamental components. The describing functions are depicted in a complex plane in terms of both amplitude and circular frequency, and the effects of the amplitude on the input-output relationship of fundamental components are shown. The computed steady-state periodic responses are in good agreement with the digital simulation results obtained with the Runge-Kutta-Gill method. The method discussed allows the input-output relationships of fundamental components to be obtained and the nonlinear behavior of the steady-state periodic responses to be computed without experiments.