An Experimental Study on the Effect of Shape and Location of Vortex Generators Ahead of a Heat Exchanger

Al-Khwarizmi Engineering Journal Al-Khwarizmi Engineering Journal, Vol. 8, No. 2, PP 12- 29 (2012) An Experimental Study on the Effect of Shape and Location of Vortex Generators Ahead of a Heat Exchanger Wisam Abed Kattea Department of Machines and Equipment Engineering /University of Technology Email: (Received 15 September 2011; accepted 30 January 2012) Abstract An experimental study is carried out on the effect of vortex generators (Circular and square) on the flow and heat transfer at variable locations at (X = 0.5, 1.5, 2.5 cm) ahead of a heat exchanger with Reynolds number ranging from 62000< Re < 125000 and heat flux from 3000 ≤ q ≤ 8000 W/m2 . In the experimental investigation, an apparatus is set up to measure the velocity and temperatures around the heat exchanger. The results show that there is an effect for using vortex generators on heat transfer. Also, heat transfer depends on the shape and location. The circular is found to be the best shape for enhancing heat transfer at location [Xm=0.5 cm] distance before heat exchanger is the best location for enhancing heat transfer. The square is the best shape for enhancing heat transfer at location [Xm=2.5 cm] distance before heat exchanger is the best location for enhancing heat transfer. The results of flow over heat exchanger with vortex generators are compared with the flow over heat exchanger without vortex generators. Heat transfer around heat exchanger is enhanced (56%, 50%, 36%) at location (X=0.5, 1.5, 2.5cm) respectively by using circular vortex generators without turbulator and heat transfer around heat exchanger is enhanced (39%, 42%, 51%) at location (X=0.5, 1.5, 2.5cm) respectively by using square shape vortex generators without turbulator. Keywords: Vortex generator: VGs, NUX: Local nusselt number heat exchanger, vortex flow, heat transfer. 1. Introduction Convectional types of heat exchanger are externally plain tube in cross flow. They are widely used in chemical, petrochemical, automotive industry, cooling towers, heated pipes refrigerators of power plants as well as in applications for heating, refrigeration and air conditioning. In finned tube heat exchanger liquid or steam flows through the tube and gas through fin ducts, [1], [2]. Heat transfer is closely related to fluid dynamics. That is why heat transfer is considered simultaneously with fluid dynamics [3]. Heat transfer and fluid dynamic around curvilinear body as cylinder is complex process and need a big efforts to find out temperatures, pressure and velocity distribution. Therefore one must know what happens when the fluid flows over bluff bodies as sphere, wire and tube. The ability to manipulate a flow field to improve efficiency or performance is of immense technological importance. Flow control is one of the leading areas of research of many scientists and engineers in fluid mechanics. The potential benefits of flow control include performance and maneuverability, affordability, increased range and payload, and environmental compliance. The intent of flow control may be to delay/advance transition, to suppress / enhance turbulence, or to prevent/promote separation. The resulting benefits include drag reduction, lift enhancement, mixing augmentation, heat transfer enhancement, and flow–induced noise suppression. The objectives of Wisam Abed Kattea Al-Khwarizmi Engineering Journal, Vol. 8, No.2, PP 12 -29 (2012) flow control may be interrelated, leading to potential conflicts as the achievement of one particular goal may adversely affect another goal. For example, consider an aircraft wing for which the performance is measured by the improvement in lift-to-drag ratio. Promoting transition will lead to a turbulent boundary layer that is more resistant to separation and increased lift can be obtained at higher angle of incidence. The viscous or skin – friction drag for a laminar boundary layer can be an order of magnitude smaller than that for a turbulent boundary layer. However, a laminar boundary layer is more prone to separation resulting in a loss in lift and an increase in form drag. The performance of liquid-to-air and twophase-to-air heat exchangers is important in many applications, including thermal management and processing systems found in the air conditioning, automotive, refrigeration, chemical, and petroleum industries. Improving the performance of these heat exchangers can lead to a smaller surface-area requirement, reduced material cost, and a lower heat exchanger mass. Furthermore, improving heat exchanger performance can have a significant impact on the environment through improvements in energy efficiency. The total thermal resistance in these heat exchangers can be considered as the sum of three contributions: the liquid or two-phase convective resistance, the wall conductive resistance, and the air-side convective resistance. The air-side convective resistance is typically the dominant resistance to heat transfer,[4], and efforts to improve these heat exchangers should focus on the air - side heat-transfer behavior. The type of turbulator using heat exchanger is shown in Figure (1). [5] D e l t a w i n g D Rec Rec el tan tan ta gul gul w Vortex Generator. ar ar Fig. 1.Type of i win win n g glet g le t 1.1. Vortex Generation At the front of region of tube there is high heat transfer due to small boundary layer (B.L) thickness but there is a small heat transfer at the rear region especially at separation point. This region is therefore prime focus area for transport enhancement, [4]. The strategy proposed here involves placing the vortex generators (V.Gs) in front of or behind the tube to prevent growth of boundary layer (B.L) and to transfer the separation point to the rear region of tube in order to reject the minimum heat transfer region to the rear. The (V.Gs) are small plates placed in the stream flow of mixed flow, disturbing flow and controlling the growth of boundary layer, [6], has any shape; the common shapes of (V.Gs) (winglets) are circular and square. The (V.Gs) winglets affect both tube and fin, all previous studies were studied the effect of wing and winglet on heat transfer from the fin by generating a vortex due to pressure difference between front surface and back surface, this vortex will mix the hot fluid near the surfaces with cold main flow, this process enhances heat transition from the surfaces, [7]. The effect of winglet on tube guides the flow at high momentum in to low heat transfer region, [6].The size, shape and angle of attack of winglet determine the specific characteristics of the vortices generated in the flow, [8]. 1. 2. Application of VGs The advantages of these vortex generators or tabulators (turbulence promoters) are to: 1. Improve heat exchange in compact heat exchangers and electronic equipment packages or microelectronic devices in industrial application ,[9] like the wide use of plate-fin and fin-tube heat exchangers, for example, in dry cooling towers, in ch (...truncated)