Proceedings of 41 st The IIER International Conference, New York, USA, 23 rd October. 2015, ISBN: 978-93-85832-18-5 19 HEAT TRANSFER ENHANCEMENT USING CONCAVE AND RIBBED SURFACES IN LAMINAR FLOWS ONUR YEMENICI Abstract — In the present study, the heat transfer enhancement of laminar air flows over concave and ribbed surfaces have been investigated experimentally in a wind tunnel. The experiments were performed over a flat plate, a curved surface with a radius of 2.54 m in concave curvatures and a ribbed surface has a rib aspect ratio (the ratio of the rib width to the rib height)of 1.2 for the free stream velocity of 2 m/s. A constant-temperature hot wire anemometer and copper-constant thermocouples were used to the velocity and temperature measurements, respectively. The results show that the heat transfer rates are increased up to 60% by the concave and 175% by the ribbed surface comparing to the flat plate. It is noted that ribs give the higher heat transfer augmentation than concave surface. Keywords — Heat Transfer Enhancement, Laminar Flow, Ribbed Surface, Concave Surface. I. INTRODUCTION The heat transfer enhancement by the ribbed and concave surfaces is of great importance for engineering applications such as turbine blades, heat exchangers, electronic cooling systems, biomedical devices and power plants.Therefore, there have been several previous investigations on the laminar flows over concave andribbed surfaces such as; Leung et al. [1] investigated experimentally the convective heat transfer and pressure-drop characteristics for laminar flows over rectangular ribs and recorded an increase of 133% with traverse rectangular ribs.Chen and Wang [2] carried out an experimental study on forced convective laminar and turbulent flow in a channel with blocks in tandem and they explained the effects of separation on flow and heat transfer characteristics.Kim and Anand [3] worked out numerical simulation of laminar and turbulent heat transfer in two-dimensional ribbed channel and reported that Nusseltnumber of block surfaces increased with increasing Reynolds number.Nakajima et al. [4] presented numerical results simulating a three-dimensional laminar separated flow and heat transfer around staggered surface-mounted rectangular blocks in a plane channel.Davalath and Bayazitoglu [5] solved the two-dimensional, conjugate heat transfer problem for laminar flow over an array of three obstacles and indicated that the heat flux distributions at the rear surfaces of blocks were much smaller than those at the front and top surfaces. On the other hand, Crane and Sabzvari [6], Umur [7], Thomann[8], Mayle et al. [9] and Simonich and Moffat [10] carried out a number of investigations over concave surfaces and showed that concave curvature caused higher Stanton number than flat plate values. In this paper, the effect of the surface shapes on heat transfer enhancement was examined experimentally. The measurements were performed for the free stream velocities of2 m/s which correspond to the laminar flow case, the rib aspect ratio of 1.2and the concave surface radius of 2.54 m. II. EXPERIMENTAL SET-UP AND METHOD Experiments were performed in an open circuit and blowing-type, low-speed wind tunnel, as shown in Fig.1. The tunnel run by a 5.7 kW axial fan and has 30 m/s maximum velocity with a turbulent intensity of 0.7%. Air velocity was adjusted by a butterfly valve. Air passes through a metal duct having a length of 1.07m and a honey comb with a cross- sectional area of 0.305x0.305 m 2 to obtain smooth streamlines and to reduce turbulent level. After the honey comb, a nozzle that has 1.5:1 contraction ratio and 0.2x0.305 m 2 exit area was used to prevent boundary layer separations and accelerate flow. A straight duct of 0.4 m length was installed before the test section to eliminate the contractions effects of the nozzle and achieve smooth streamlines. The test section has an initial area of 0.2x0.305 m 2 , length of 0.8 m and plexiglass side surface to provide full visibility of the flow area. Fig. 1. Wind tunnel and test section. The experiments were carried out over a flat, a ribbed and a concave surfacewhich have 0.24 mspanwise wide and 0.0008 m thick, as shown in Fig. 2. The streamwise distance (L) is 0.75 m in flat plate, 1.25 m in concave surfacewith a radius of 2.54 m and 0.45 m in ribbed surface. The ribbed surface with a sequence of 6 ribs has a rib height of 0.025 m and width of 0.03 m, a distance between ribs of 0.03 m and a distance up to the first rib of 0.06 m. These rib dimensions and numbers were chosen to obtain the same total surface area of the concave and flat surfaces. The experiments were performed over the all surface for the free stream velocity of 2 m/s.DANTEC digital constant temperature