AN EXPERIMENTAL INVESTIGATION OF THE AUGMENTATION OF FLOW BOILING INSIDE A HORIZONTAL TUBE

Abstract

In the present work an experimental investigation of flow boiling heat transfer augmentation of R-134a inside a horizontal tube using a `flexible start-up insert' has been carried out and significant enhancement observed. The plain tube data were also obtained to establish the integrity of the test set-up and to judge the insert performance. An - experimental set-up was designed and fabricated in the laboratory to study the flow boiling augmentation. Tests were conducted using a single tube evaporator test facility. The test section used was 1000 mm (1 m) long, 12.5 mm inside diameter, horizontal, hard drawn Copper tube with `flexible start-up insert' made from Copper . A `flexible start-up insert' in the form of an `L' shaped flag, hinged on a diametric rod, was installed at the in let of the tube. The present study covered the range of two important parameters viz, mass flux, G, 80.39 kg/m2-s to 401.99 kg/m2-s and heat flux 10.8kW/m2 to 33.54kW/r2. The refrigerant vapor quality, x, varied from 0.08 to 0.83 An augmentation, in heat transfer coefficient, h, up to 75% was observed over a tube length equivalent to 50 diameters downstream of the insert for mass .flux of 401.99 kg/m2-s and heat flux of 33.54 kW/m2. The destabilization of the hydrodynamic boundary later, which reduces the thermal resistance, and core mixing of the fluid, due to oscillating motion of the insert, may be the reasons for heat transfer enhancement. Augmentation ofheat transfer and the augmented length are found to be dependant on mass flux, G and heat flux, q. A flow map was developed to identify the flow pattern for different mass flux and heat flux conditions, for plain and augmented tube. For lower mass flux transition of flow pattern from stratified to stratified and intermittent to annular is observed. For higher mass fluxes intermittent flow is found to be the dominant flow regime.