Effect of viscosity on entropy generation for laminar flow in helical pipes

Abstract

Entropy generation for fully developed laminar flow in a helical pipe carrying high viscous fluid under constant temperature boundary conditions is investigated analytically. This work focuses on geometrical, fluid, and thermal aspects and their influence on irreversibilities in helical coils. The effect of viscosity on the irreversibilities and its influence on the operating parameters of the helical coil are studied with the second law of thermodynamics. The most commonly used relationships for estimating viscosity change due to temperature are selected for analysis. The entropy generation and avoidable exergy destruction in each case are presented. Bejan number is plotted for varying viscosities under different wall temperatures for both heat transfer to and from the fluid. The thermodynamic potential of improvement based on avoidable and unavoidable exergy destruction concepts showed that the potential of improvement for heating and the cooling condition is considerable for a given operating condition in helical tubes. The selected model for estimating viscosity influences the optimum operating wall temperature, thereby giving an insight into a selection of a proper viscosity model. The optimum helical number is not affected by fluid properties and wall temperature. The heat transfer to pumping ratio is evaluated and it is found that the optimal value is influenced by the change in viscosity.

Fluid Flow in Helically Coiled Pipes

“…Further studies on curved pipes and injunctions were reported by Ko [160] and Sanchez et al [161], respectively. Entropy generation in helical pipe flows has instead been analyzed by Shokouhmand and Salimpour [162], Satapathy [163], Bahiraei et al [164], Ahadi and Abbassi [165], Dizaji et al [166], Kurnia et al [47], Huminic and Huminic [167] and more recently by Pendyala et al [168] and Prattipati et al [56]. Evidently, in the last 10 or more years, there has been an ever-increasing interest to evaluate the thermal performance of helically coiled pipes (and other systems) using the second law of thermodynamics [169].…”

“…An important result form these calculations is that entropy generation from heat transfer is about two orders of magnitude higher than from viscous dissipation. In a more recent work, Prattipati et al [56] analytically investigated the entropy generation for fully developed laminar flow of a highly viscous fluid in a helically coiled pipe under uniform wall-temperature boundary conditions. They found that, for glycerol as the working fluid, frictional effects produce large irreversibilities compared to lighter fluids, as is the case in water.…”

“…Investigations on multiphase flow in helical pipes are by far much more scarce (see, for instance, Colombo et al [52] and references therein). On the other hand, analytical and semi-analytical works based on perturbation methods [53,54], asymptotic analysis [55], entropy generation analysis [56] as well as physical bounds on the flow rate and friction factor in pressure-driven flows through helical pipes using a background method formulation [57] have also been reported in the open literature.…”