NUMERICAL AND EXPERIMENTAL INVESTIGATIONS ON PARALLEL - FLOW THREE-FLUID HEAT EXCHANGER

Abstract

Three-fluid heat exchangers are used in different chemical processes, cryogenics and aerospace applications. Three-fluid heat exchangers consist of three-fluid passages and thus heat transfer improves due to the third passage giving larger heat transfer area per unit exchanger length and hence leading to a more compact and economical design. The present thesis concentrates on numerical investigations on the transient behaviour of parallel-flow three-fluid heat exchanger with two thermal communications. Experimental investigations on thermo-hydraulic behaviour of triple concentric-tube heat exchanger have also been carried out for steady as well as for transient conditions. Heat exchangers generally operate under steady state conditions however, transient thermal behavior is also important in case of start-up, shutdown, failure and accidents. Therefore, knowledge of transient behaviour of heat exchangers is required for designing control and regulation strategies of various industrial processes and operations like HVAC systems, cryogenics, nuclear reactors and petrochemical process plants. Transient response of parallel-flow three-fluid heat exchanger has been obtained for number of transfer units (NTU), heat capacity rate ratio (E), conductance ratio (R), dimensionless inlet temperature, core longitudinal conduction and fluid axial dispersion using a finite difference scheme. The performance has also been observed for the perturbations in temperature as well as flow and for the nonuniformities present in temperature and flow. Four possible arrangements of three-fluid parallel-flow heat exchanger have also been compared under transient conditions It has been found that the exit temperature response of the heat exchanger is decided by the inlet temperatures, geometrical and process parameters. Temperature distribution and flow distribution at the entry and position of the fluid moving devices also influences. The thermo-hydraulic behaviour for triple concentric-tube heat exchanger with two thermal communications has been experimentally carried out at steady state and transient conditions. Experiments were performed using water as the working fluid at all possible flow arrangements for Reynolds number ranging from 2800 to 11000 of hot water in inner annulus. Comparison of the experimental and the numerical results has also been carried out. Distribution of temperature along the heat exchanger length, variation of friction factor (f) and Nusselt number (Nu) with Reynolds number for the hot water in inner annulus has been obtained and the empirical correlations have been proposed for Nu and f. Transient behaviour of triple concentric-tube heat exchanger has also been experimentally found for a sudden change in hot water flow rate and the time constants have been determined for all possible flow arrangements. The results obtained will be useful in obtaining better designs and in deciding control strategies of parallel-flow three-fluid heat exchangers. Keywords: axial dispersion, flow perturbation, heat exchanger, longitudinal conduction, temperature nonuniformities, thermo-hydraulic behavior, three-fluid, transient behavior, two thermal communication