Numerical Investigations on the Thermo-hydraulic Characteristics of Conical Spiral Tubes

Abstract

Heat exchangers are widely used in various industries and efficiency in its performance is crucial. Curved tube heat exchangers find applications in various industries, including chemical processing, power generation, HVAC (heating, ventilation, and air conditioning), refrigeration, and waste heat recovery. The curved tubes create turbulent flow patterns and enhance heat transfer within the heat exchanger. The secondary flow induced by the curved geometry helps prevent the accumulation of deposits on the tube internal surface, thereby reducing the likelihood of fouling and maintaining higher heat transfer efficiency over time. Besides conical spiral tubes generally exhibit higher mechanical strength compared to helical tubes which makes it suitable for applications requiring durability and structural integrity. In the present study, numerical study results are presented for fluid flow and heat transfer characteristics in laminar and turbulent flow under constant wall temperature heating conditions for various geometries of the conical spiral tubes. In the laminar flow, the secondary flow patterns identified can be categorized according to Dean and Lean type circulation. The CFD results for the pressure drop show that with an increase in the cone taper angle (α) of the conical spiral tube, the fanning friction factor (fCS) increased for the given inlet velocity of the flow. The uniform distribution of pressure in the outer tube bend due to formation of boundary layer in the outer half of the tube section is observed. Under the influence of this boundary layer, distribution of velocity and temperature and different patterns of secondary flow are influenced in a section. This formation of boundary layer near the outer tube bend reduce the pressure variation and heat transfer at the outer tube wall. The local friction factor (fФ) at the wall of the outer tube bend was found to decrease with an increase in the cone taper angle (α). This may be attributed to the deformation formation of fluid boundary layer formed near the outer bend of tube. The heat transfer study found that with the increase in curvature ratio and cone taper angle of a conical spiral tube, the Nusselt number increased in the conical spiral tube. The study found that for Reynolds number up to 400 and cone taper angles up to 40⁰ of conical spiral tubes have lower heat transfer compared to straight tubes. The compatative study of conical spiral tube with elliptical cross-section showed that the value of Reynolds number at which flow characteristics changes is found to increase for elliptical cross-section with aspect ratio 0.5. Nusselt number values are found be the highest in the coil VII with an elliptical cross-section with aspect ratio of 0.5 both in laminar and turbulent regimes. The results are reversed when the coil oriantion is vertical. The CFD results for turbulent flow reveal that the total pressure drop and the local cross-flow pressure loss coefficient increased with Reynolds number, cone taper angle and curvature ratio. For the given curvature radius and tube diameter of the conical spiral tube, there exists an optimum Reynolds number at which the dimensionless cross-flow pressure coefficient has an optimum value. For the usual practically applicable curvature ratio of less than 0.1 in curved tubes, the pressure drop prediction using the proposed correlation was found to be within ±10% of the numerical simulation results. The heat transfer study found that local circumferential and coil heat transfer increases with the increase in cone taper angle, curvature ratio (d/D), pitch and Reynolds number. The CFD results showed that the local circumferential and total coil heat transfer increased with the increase in cone taper angle and Reynolds number. The fully developed Nusselt number is found to be achieved by angular length of 210⁰ from inlet plane in the coil. A correlation to predict Nusselt number for turbulent flow in conical spiral tube for various cone taper angle and curvature ratio is proposed on the basis of numerical analysis and found to be within ±10% of the numerical simulation results. Conical spiral tubes have better thermo-hydraulic performance in comparison with helical tube in the turbulent flow regime. The conical spiral tubes have better thermo-hydraulic performance (Thermo-Hydraulic Performance Parameter –THPP) in comparison with helical tube in the turbulent flow regime. When comparing aspect ratios of 0.5 and 2.0 for elliptical cross-section tubes, it is found that the tubes with the aspect ratio 0.5 have a lower pressure drop but worse heat transfer rates when the coil orientation is vertical. For coils with horizontal orientation showed that in the considered Reynolds number range, coil with elliptical cross-section with aspect ratio 0.5 showed the highest pressure drop and heat transfer. The values of fc for coil with elliptical cross section with aspect ratio 2.0 are found to fall between values of fc for the coil with circular cross-section and the coil with elliptical cross-section with aspect ratio 0.5.