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A typical three-fluid cross-flow heat exchanger with nonuniform inlet temperature in the central (hot) fluid is considered for the present analysis. Steady and transient state behavior of the heat exchanger is observed for four different temperature nonuniformity models along with step excitation in inlet temperature of the central fluid. Longitudinal heat conduction in the separating walls and the effect of fluid back-mixing along with axial dispersion effect are considered within the fluids with constant thermophysical fluid properties. The solution of governing equations has been obtained using implicit finite difference scheme. Temperature distribution over the separating walls has been depicted providing a clear view of the thermal stresses generated in separating walls. The performance for all the four cross-flow arrangements has been analyzed by comparing that with and without nonuniform conditions. It is found that the nonuniformity in inlet temperature has an adverse effect on the performance of heat exchanger.

Cross-flow three-fluid plate-fin heat exchanger is analyzed under both steady-state and transient conditions with a nonuniform inlet temperature of all the three fluids. The influence of the longitudinal heat conduction and axial dispersion in the separating sheets and three fluids, respectively, is also considered. Five different combinations (modes) of temperature nonuniformity in the three fluids have been considered and compared for the performance. An important phenomenon of temperature cross between/among the fluids has been observed and presented for certain modes of temperature nonuniformity and operating conditions. The effect in the performance has been presented on the basis of mean exit temperature and deterioration factor. Implicit finite difference technique has been used for the numerical solution. The heat exchanger's performance is found to be dependent on the mode of temperature nonuniformity, number of transfer units, and the operating parameters.

Three-fluid compact heat exchanger of plate-fin type with parallel-flow configuration is optimised for the entropy generation. Four different types of plate fins (plain rectangular, offset strip, corrugated louvered and wavy fin) are embodied within heat exchanger for both co-current and counter-current flow arrangements have been selected for the study. Genetic algorithm is selected as an optimisation tool having apt in handling various continuous variables and discrete variables and the problems with complexities in the objective function as well as in constraints. Validation of the optimization model is carried out by comparing the results with that from experimental results, Particle swarm optimization (without heat duty constraint) and from graphical method (with heat duty constraint). It is observed that for a specified heat duty and given operating conditions corrugated louvered fin with counter-current flow arrangement offers the minimum entropy generation amongst all.