Numerical analysis to optimize the heat transfer rate of tube-in-tube helical coil heat exchanger

Abstract

Working towards the goal of saving energies and to make compact the design for mechanical and chemical devices and plants, the enhancement of heat transfer is one of the key factors in design of heat exchangers. In this process without application of external power we can enhance the heat transfer rate by modifying the design by providing the helical tubes, extended surface or swirl flow devices. Helical tube heat exchanger finds applications in automobile, aerospace, power plant and food industries due to certain advantage such as compact structure, larger heat transfer surface area and improved heat transfer capability. In this paper numerical study of helical coil tube-in-tube heat exchanger is done for different boundary conditions and optimizes condition of heat transfer is found out for different D/d ratio. The turbulent flow model with counter flow heat exchanger is considered for analysis purpose. The effect of D/d ratio on heat transfer rate and pumping power is found out for different boundary conditions. The D/d ratio is varied from 10 to 30 with an interval of 5. Nusselt number, friction factor, pumping power required and LMTD variation of inner fluid with respect to Reynolds number is found out for different D/d ratio. The optimize Reynolds number for maximum heat transfer and minimum power loss is found out by graph intersection methods. With increases in D/d ratio (inverse of curvature ratio) the Nusselt number will decreases and the outer wall boundary condition does not have any significant effect on the inner Nusselt number. The Darcy friction factor decreases with increase in Reynolds number. The Pumping power increases with increase in Reynolds number for all the condition of D/d ratio and for all the boundary conditions. Log mean temperature difference (LMTD) increases at a steady rate with increase in Reynolds number. The optimization point between Nu and f with respect to Re shifts toward the lower Reynolds number with increase in D/d ratio.