STUDY OF FLOW BOILING HEAT TRANSFER ENHANCEMENT 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 `spring 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 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, .91.3-471:6 kg/m?-sec, and Heat flux, q, 4.0-12.7 kW/m2. The refrigerant vapor quality, X, was varied from the range of 0.0-0.30. It was found that a spring insert could enhance the heat transfer by 1.2 to 2.0 based on the mass flux and heat flux. The highest heat transfer was observed at mass flux of 471.6 kg/m2-sec and heat flux of.6.455kW/m2. The wire coil insert provides enhancement by flow separation at the wire, causing fluid mixing in the down stream boundary layer. Augmentation of heat transfer and augmented length are found to be dependent on mass flux, G, and heat flux, q. A flow map was developed by using the 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. 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.