Analytical and experimental response time to flow rate step along a counter flow double pipe heat exchanger

Temperatures transient response along a tubular counter flow heat exchanger is investigated when mass flow rate is subjected to sudden change. The dyn amic behaviour is approximated by a first order response with a time consta nt. The hot fluid subjected to the flow rate step presents a decreasing lin ear time constant according to flow axis. The cold fluid, which is not subm itted to the flow rate step, shows two types of transient response. The fir st one corresponds to a linear spatial increasing function of time constant while the second one presents a uniform time constant along the heat excha nger. Considering the conditions of the transient response on the boundarie s of the heat exchanger, analytical expressions of time constants are obtai ned for each case, The comparison of the theoretical results with experimen tal data allows to validate the analytical expressions, which depend on ini tial and final steady-states. The influence of the flow rate step magnitude on transient behaviour is studied by maintaining the initial steady-state. (C) 2001 Elsevier Science Ltd. All rights reserved.