HEAT TRANSFER STUDIES TO BINARY MIXTURES IN A VERTICAL TUBE THERMOSYPHON REBOILER

Abstract

In the present experimental investigation, studies of heat transfer to boiling pure liquids, viz., water, acetone and methanol; and boiling binary mixtures, viz. acetone-water and methanol-water have been carried out under natural convective flow through a vertical tube of thermosiphon reboiler at 1 atmospheric pressure. The experimental apparatus used for this study consisted of an electrically heated stainless steel tube of 21 mm inside diameter and 1850 mm heated length. Experimental data were generated for both pure liquids and binary mixtures by imposing various parameters like heat flux, submergence and inlet liquid temperature. The effect of composition, which is an another controlling parameter, in case of boiling binary mixtures has also been included in the study. The uniform heat fluxes in the range of about 0.49 x 104 Wirn2 to 2.6 x 104 W/m2 were used. The liquid submergence levels were maintained approximately at • 100, 75. 50 and 307.The inlet liquid temperatures for all the test liquids were kept far below compared to their respective saturation temperatures in most of the cases, The two binary liquid systems viz. acetone-water and methanol-water were used for a variety of concentrations. The effect of heat flux, liquid submergence, inlet liquid temperature and mixture composition on heat transfer coefficients as well as wall temperatures along the heated length has been presented graphically and discussed. The values of heat transfer coefficient are observed to enhance as the heat flux is raised. This may be attributed to the higher levels of temperature set-up and increased induced flow rates of the liquids. The variation of heat transfer coefficient, along the heated length of the tube clearly indicates the existence of natural convective non-boiling and boiling regions. The value of heat transfer coefficient increases at a very low rate in the non-boiling region while it increases sharply in the boiling region. With the lowering of liquid level in the reboiler tube, the length of boiling region in the tube gets extended. The heat transfer coefficient of the binary mixtures are found to be strongly influenced by the mixture composition. The coefficient profile shifts to lower value with increase in concentration of more volatile component. The reason of this may he due to the change in properties on mixing and the effect of mass diffusion iii which comes to play because of concentration gradient between the liquid and the vapor phase. The values of heat transfer coefficient over the non-boiling region are found to be of lower magnitude compared to those in saturated boiling region. The heat transfer for both pure and binary liquids. in the non-boiling region, is mainly influenced by the induced flow of liquid through the tube. On the other hand, the heat transfer coefficients, in the boiling region, have been found to be strongly influenced by both micro-convective (due to bubble growth and detachment from the heating surface) and macro-convective (due to the bulk flow of the two-phase mixture through the tube) mechanisms. In case of binary mixtures, an additional mechanism, i.e., mass diffusion plays an important role in the heat transfer. Due to presence of two inherently different mechanisms of heat transfer in the two regions along the tube, two separate correlations have been proposed in literature for predicting the heat transfer coefficient. But as the purpose of the present study is to investigate the heat transfer phenomenon in two phase pure component and binary mixture systems, particularly in the boiling region, efforts have been made to develop correlations for pure liquids as well as binary mixtures in boiling region only..........