FLOW BOILING HEAT TRANSFER OF OZONE SAFE REFRIGERANTS

Abstract

An investigation of flow boiling heat transfer augmentation of R-134a inside a horizontal tube using a `flexible start-up insert' on the basis of an experimentation work in the laboratory, has been carried out and its effective enhancement observed. To. establish the integrity of the experimental set-up and to analyze the insert performance, the plain tube data were also recorded. 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 kept horizontally of 1000 mm long with 12.2 mm inside diameter of hard drawn Copper tube with 'spring insert' also be made of Copper. A full length spring of 12.2mm outside diameter is inserted inside horizontal tube with the help of a rod and a wire. The present study covers the range of two parameters likes, Mass flux G, 100-475 kg/m2sec, and Heat flux q, 6.5-19.0 kW/m2 The refrigerant vapor quality X was varied from the range of 0.01-0.30. From the experimentation it was seen that a spring insert could enhance the heat transfer by 1.2 to 2.0 based on the mass flux and heat flux. The wire coil insert creates the turbulence disturbance due to fluctuation of velocity which causes the lamina sub-layer breaks and at also depends upon fluid mixing. Augmentation of heat transfer and augmented length are found to be dependent on mass flux G and heat flux q. According to Kattan's and Steiner's correlation[25,26,27] for the identification of the flow pattern for different mass flux and heat flux conditions, for plain and augmented tube, flow maps were also developed. From the experimentation it was found that for lower mass flux transition of flow pattern from stratified to stratified-wavy and intermittent to annular is observed and for higher mass flux intermittent flow is found to be the dominant flow regime.