Exploration of Condensation Heat Transfer Coefficient and Pressure Drop on Serrated Fin Surfaces for R134a

Abstract

The compact plate fin heat exchanger (CPFHE) plays a major role in aerospace industry and various process plants because of their compactness, low weight and high effectiveness. The performance of a plate fin heat exchanger is determined, among other things, by the geometry and type of fins. The most popularly used fin surfaces in compact heat exchangers are the serrated (offset strip) fins, wavy fins, louvered fins and plain fins. Amongst these fin types the serrated fins assume lot of importance due to its enhanced thermo-hydraulic performance. Thermo-hydraulic design of CPFHEs is strongly dependent upon the predicted/measured dimensionless performance (Colburn j factor and Fanning friction f vs. Reynolds number) of heat transfer surfaces. Two phase flow analysis for the condensation of refrigerants within the CPFHEs is an area of ongoing research. The condensation average refrigerant heat transfer coefficient ( ) and two phase frictional pressure drop ( of the finned surface constitute the most important parameters for CPFHEs design. These parameters are functions of fin geometry, fluid properties (thermal conductivity and Prandtl number) and mass flow rate (Reynolds number). However, the complexity of the flow phenomena through the passages of a compact heat exchanger makes any theoretical analysis formidable. Experimental investigation becomes necessary for determining the thermo-hydraulic performance of such devices. In aircraft and automobiles, the demand on performance that is the volume and weight of the heat exchangers should be kept minimum. An experimental apparatus, based on steady state method has been developed for conducting condensation experiments on four different serrated fin surfaces. Based on theoretical design, four test condensers are developed using vacuum brazing technology. The effects of saturation temperature (pressure), mass flux, heat flux, effect of fin surface characteristics and fluid properties are investigated inside a small brazed plate fin heat exchanger with serrated fin surface for R134a. The average condensation heat transfer coefficients and frictional pressure drops were determined experimentally for refrigerant R134a at different saturated temperatures (ranging fromiv 32 0C to 44 0C). A transition point between gravity controlled and forced convection condensation has been found for a refrigerant mass flux around 20 kg/m2s. Correlations are provided for the measured heat transfer coefficients and frictional pressure drops separately for four different types of serrated fins. CFD (Computational fluid dynamics) methodology has been used to develop the single phase water heat transfer coefficient and friction factor correlations for serrated fins using ANSYS Fluent 14.5. The results are compared with previous aircooled models and experimental results of water. The water cooled CFD analysis results shows that the Prandtl number has a large effect on the Nusselt number of the serrated fin geometry. Finally, the generalized correlations are developed for serrated fins taking all geometrical parameters into account. The influence of hydraulic diameter on experimental heat transfer coefficient and pressure drop measured during R134a saturated vapour condensation inside a brazed compact plate fin heat exchanger with serrated fins was studied. Test condensers are selected with four different hydraulic diameter (1.1894 mm , 1.345 mm, 1.7461 mm and 1.8667 mm) serrated fins to analyse the affect of heat transfer and frictional pressure drop characteristics. The environmental control system (ECS) for a typical fighter aircraft is used for cabin cooling & pressurization, demisting operations and for avionics cooling. Most of the passenger and fighter aircrafts use bleed-air cycle for ECS. A new ECS called all electric environmental control system (AEECS) which works on vapour compression refrigeration system using ram air as medium. The AEECS is found to reduce the power requirement of the system to 80 kW compared to the bleed air cycle ECS which requires a power of 0.8 MW to run the system. In AEECS one of the components is condenser.