INVESTIGATIONS ON THREE-FLUID COMPACT PLATE-FIN HEAT EXCHANGER

Abstract

Cryogenic industries are well served by the compact plate-fin heat exchanger due to its ability of commendably very high heat transfer rate per unit volume. The purification of air, liquefaction of various gases such as nitrogen, hydrogen, helium, argon, etc., synthesis of ammonia are the cryogenic processes that involve successive compression and cooling process, due to the limitation in the maximum allowable pressure drop. During compression, the gases or the fluids are pushed towards the saturation liquid state for condensation and the temperature rises immensely. Hence, here comes the role of the compact plate-fin heat exchangers which serve the system by cooling the gases at a very high heat transfer rate. Moreover, compact plate-fin heat exchangers are also the prime component of aerodynamic industries due to its compactness. Practically, the above mentioned cryogenic process involves the three or multi fluids in stacking. The boiling point of hydrogen, nitrogen, oxygen, and helium are -252.9 °C, -195.8 °C, -183 °C and -268.9 °C respectively. Hence, oxygen is liquefied first followed by nitrogen, hydrogen, and helium. Three-fluid plate-fin (compact) heat exchanger is utilized for liquefaction of helium by utilizing liquid nitrogen and liquid hydrogen or oxygen; or both fluids as liquid nitrogen (-195.8 °C) used as a refrigerant. In aircraft, the atmospheric air is preheated with heated coolant fluid (circulated around the engine for cooling) and exhaust gas from the engine using a three-fluid plate-fin heat exchanger. Due to the various complexities involved in the modelling of three-fluid plate-fin heat exchangers as all three fluids are at different temperature levels, the research conducted by the analyst is limited. Accordingly, the objective of the thesis is to investigate in detail the three-fluid plate-fin heat exchanger considering various aspects such as ambient heat ingression, the perturbation in flow, temperature, and flow non –uniformity that dominantly affects the efficacy. The investigation is conducted in three phases that is a numerical investigation, experimental investigation, and design optimisation. A transient mathematical model for a cross-flow arrangement considering longitudinal heat conduction within the parting walls, axial fluid dispersion, and abovementioned aspects is proposed and the numerical solution is attained by using the Implicit Finite Difference Method. The extensive experimental investigation of thermo-hydraulic efficacy of both parallel and crossflow arrangement is conducted and ANN modelling is proposed for the same. The experimental investigation is extended by artificially inducing the flow and temperature non-uniformity using a specially designed tri-entry header at the inlet of the core. Further, the design optimisation of both parallel and cross-flow arrangement is done by considering the minimum number of entropy generation as an objective. The experimental validation is delineated for both the numerical and optimisation model and a very decent agreement is achieved. For parallel flow arrangement, the optimisation model is developed using the ε-NTU relationship for four different types of platefins (plain rectangular, offset strip, corrugated louvered, and wavy fin) for co-current and counter-current flow arrangements. Next, for cross-flow three-fluid heat exchanger with offset strip fin, a novel approximate equivalence model is proposed for all four feasible flow arrangements.