TRANSIENT BEHAVIOUR OF TWO FLUIDS CROSS FLOW COMPACT HEAT EXCHANGER

Abstract

Heat exchangers are generally designed to meet certain performance requirements under steady operating conditions. However, transient response of heat exchangers needs to be known for designing the control strategy of different HVAC systems, cryogenic and chemical process plants. Problems such as start-up, shutdown, failure, and accidents have motivated investigations of transient thermal response in crossflow heat exchangers. In present work, transient temperature response of crossflow heat exchangers with finite wall capacitance and both fluids unmixed is investigated numerically for step, ramp and exponential perturbations provided in hot fluid inlet temperature. Effect of variable heat transfer coefficient due to variation of temperature over constant heat transfer coefficient on the transient response has been investigated. All of these studies report a significant change in the heat exchanger performance if the effect of variable heat transfer is included. It has also been shown that the presence of variable heat transfer coefficient has more effect on the exit temperature response of cold fluid and it has no significant effect on the same of hot fluid. Effect of two-dimensional longitudinal conduction in separating sheet and axial dispersion in fluids on the transient response has been investigated assuming both constant heat transfer and variable heat transfer coefficient.