A NUMERICAL STUDY OF TUBE-IN-TUBE HELICAL COIL HEAT EXCHANGER

Abstract

In the present work a numerical study is carried out to predict heat transfer and friction factor characteristics for both Newtonian and non-Newtonian fluid flow in tube-in-tube helical coil (TTHC) heat exchanger for both laminar and turbulent power law fluids. The various TTHC heat exchanger configurations has been considered: (i) parallel flow TTHC heat exchanger with baffles, (ii) parallel flow TTHC heat exchanger without baffles, (iii) counter flow TTHC heat exchanger with baffles, and (iv) counter flow heat exchanger without baffles. In case of TTHC heat exchanger with baffles, the baffles are 50% cut (i.e. semi-circular) plates fitted in the annular section. The baffles provided in the annular section not only enhanced the heat transfer coefficient in the annular section but also used to support the inner tube. Non-Newtonian and Newtonian fluids are used as hot and cold fluids, respectively, through TTHC. Simulations are carried out considering (i) non-Newtonian fluid in the inner tube and Newtonian fluid in the annular section, and (ii) non-Newtonian fluid in the annular section and Newtonian fluid in the inner tube. In case of non-Newtonian fluids the power law index (n) is varied from 0.5 to 1.25 and Dean number is varied from 50 to 500 for laminar flow and 2000 to 10000 for turbulent flow. In turbulent flow study the Newtonian fluid was remained at 4000 Dean number. The influence of baffles in annulus on heat transfer and friction factor characteristics is studied for both parallel and counter flow configurations. It has been observed that the friction factor and Nusselt number in annulus with baffles is higher as compared to TTHC heat exchanger without baffles. The friction factor prediction in the inner tube of TTHC heat exchanger are validated with the empirical correlations of Mishra and Gupta (1979) and found in good agreement. In turbulent study when the power law fluids are passed through inner tube, there is a minor increase in heat transfer coefficient for all power law fluids. The heat exchanger analysis is also made by the effectiveness-NTU approach and it has been observed that effectiveness increases rapidly at small values of NTU up to about 2 whereas in turbulent flow study effectiveness values increase rapidly up to NTU value 1.5. Higher effectiveness values have been obtained for counter-current configuration as compared to the parallel flow configuration.