Experimental Study of Heat Transfer Enhancement in a Tilted Semi-Cylindrical Cavity with Triangular Type of Vortex Generator in Various Arrangements. #1 Korake Supriya S, #2 Dr Borse Sachin L #1 P.G. Student, Department of Mechaniacal Engineering,S.P.University,Pune,R.S.C.O.E Pune,Maharashtra,India-411033 #2 Department of Mechaniacal Engineering, S.P.University,Pune,R.S.C.O.E Pune,Maharashtra,India-411033 ABSTRACT This paper presents an experimental study of heat transfer enhancement in a tilted semi-cylindrical cavity with Triangular Type of Vortex Generator in Various Arrangement. To study heat transfer in semi-cylindrical cavity measurements were carried out on smooth surfaces for different tilt angles. Here for rough surface triangular type of Vortex Generators are used. Roughness may increase the blockage effect on the flow that can increases the turbulence intensity resulting in a higher heat transfer. The results of heat transfer measurements on smooth surfaces are compared with Triangular Type of Vortex Generator in a tilted semi-cylindrical cavity. The Triangular Type of Vortex Generators with different inclination angles are placed along the length of the cavity. Semi-cylindrical cavity is tilted with different angles such as 90 0, 60 0,30 0,-90 0,- 60 0. Keywords Heat transfer, Semi-cylindrical cavity, Vortex generators,tilt angles I. INTRODUCTION Natural convection is one of the most important economical and practical methods of cooling and heating. The thermal control in many systems is widely accomplished by applying natural convection process due to its low cost, easy maintenance and reliability. Natural convection on a surface depends on the geometry of the surface as well as its orientation. It also depends on the variation of temperature on the surface and the thermo physical properties of the fluid. Natural convection heat transfer from an inclined surface and in open cavities are getting more attention due to the importance of such geometry in solar thermal central receiver system. It also helps in aircraft-brake housing system, refrigerators, pipes connecting reservoirs of fluids at various temperatures, fire research, electronic cooling, energy-saving household refrigerators, electronic equipment cooling, etc. Natural convection heat transfer from an inclined surface and in cavities is used in various engineering devices. II. LITERATURE REVIEW Experimental and numerical works were carried out to better understand the heat transfer mechanism inside cavity previously. Research on study of heat transfer measurements for smooth and rough tilted semi-cylindrical cavities by Walid Chakroun et al [2001] shows that when the cavity is placed in an upward position, high convection current occurs. Roughness helps to trip the boundary layer and adds more turbulence to the flow resulting in increase in heat transfer. Previous researcher Shuang-Ying Wu done study on The convection heat loss from cavity receiver in parabolic dish solar thermal power system can significantly reduce the efficiency and consequently the cost effectiveness of the system. It is important to assess this heat loss and subsequently improve the thermal performance of the receiver. The study of natural convection heat transfer between parallel plates by changing the inclination and plate spacing were investigated, [N. Onur, et al 1997] it can be stated that Nusselt number depends on the separation distance between the plates. Researcher S.Siddiqa, S.Asghar, M.A. Hossain done investigation deals with study of laminar natural convection flow of a viscous fluid over a semi-infinite flat plate inclined at a small angle to the horizontal with internal heat generation and variable viscosity. 2015, IERJ All Rights Reserved Page 1
Experimental Setup The experimental setup is as shown in fig.1 & 2 Fig.1 Experimental Setup with triangular type vortex generators with inline arrangement Fig.2 Experimental Setup with triangular type vortex generators with staggered arrangement no. 2 The Semi-cylindrical cavity is mounted on a stand. It is supported by two arms. The design of arms and the stand are to minimize the disturbance of the airflow and it will ensure good physical stability. The cavity can be rotated about its longitudinal axis. About the vertical axis the angle of rotation was measured. The semi-cylindrical cavity is made up of aluminium material. The dimensions of cavity are as radius R is of 0.15 m and length l is of 0.9 m and the thickness of cavity is 0.004 mm. The dimensions are chosen in the way that it will give two dimensional flow inside the cavity. The heat is supplied to the cavity by using silicon rubber heaters. The heating pads are self-adhesive and fixed at back of the cavity. Outside of the cavity there are different layers such as first layer is of glass wool of 0.02 m thick, then 0.03 m thick layer of hard polystyrene insulation and then again the 0.02 m thick layer of glass wool is placed at the top. This is a smooth semi-cylindrical cavity. Fig.3 Cross-section of the Semi-cylindrical cavity. A triangular type Vortex generators are placed for rough semi cylindrical cavity along the length of cavity. The Vortex generators are fixed to the upper part of the surface such that it will act as a rough surface. 2015, IERJ All Rights Reserved Page 2
Vortex generators Vortex generators (VG s) are the most commonly known as passive flow control devices and are already used in different industries. Although VG s come in various shapes and sizes, in general a vortex generator is build up as a small vertical plate positioned at an angle with respect to the local free stream flow. There are different types of Vortex generators Fig.3 different types of Vortex generators Fig 4. Dimensions of vortex generator In this experimental set up triangular type Vortex generators are placed for rough semi cylindrical cavity along the length of cavity with various arrangement with different angles as shown in fig 1 & 2. The Length is taken in proportion of the height of the Vortex generator (L/H). Also in every arrangement they are placed with some angles such as 90 0, 60 0,30 0 Electric circuit and voltage regulator are provided to control heat input. The thermocouples are connected at different point to measure the temperature. All the thermocouples are connected to the digital recorder. The readings are taken at different interval until steady state is reached. The readings were taken at different tilt angles of the cavity at a various positions of Vortex generators and recorded. Nusselts number calculation The average Nusselt number from the cylindrical cavity is defined as below- hr Nu (1) k Where, h is the average heat transfer coefficient between the cavity and surroundings, R is the radius of the cavity and k is the thermal conductivity of air in the cavity. For better understanding Eq. (1) can be rewrite in terms of the local heat transfer coefficient h as- 2015, IERJ All Rights Reserved Page 3
H 1 R N u hds (2) H k 0 Where H R. When the cavity is heated at constant heat flux, the local heat transfer coefficient is given by the equation as - qc" h (3) ( TwT) Where Tw is the local temperature of the cavity surface and qc is heat transfer rate per unit area of the cavity surface due to convection. As we are dividing the wall into five equal sections, Equations. (2) and (3) can be added to give 5 R qc" Nu (4) 5k i T 1 ( Twi ) Where i is the order of various sections of the heated surface. To calculate qc IV = HL (qc + qcd + qr ) (5) Where V I is the Voltage and current supplied. qcd is heat lost due to conduction, qr heat lost due to radiations. The heat lost due to radiation from hot surface is calculated as below 4 4 1 1 qr" [ Tw 273] ( T 273) ( ) Fpa (6) Where, Tw is the average temperature of the cavity surface, is its emissivity and Fpa is the configuration factor between the plate and the aperture. The conduction heat loss is the heat loss through hard wall insulation and is given by the equation askin ( Ti To) qcd" R Ln( ro/ ri) (7) Where kin is the thermal conductivity of hard insulation. Ti and To are average inner and outside surface temperatures of the hard wall insulation, respectively. By considering equation. (6) and (7), the Eq. (2) for Nusselts number can be written as- Nu R 5k 5 i1 IV kin ( Ti To) { HL R Ln( Ti To) [( Tw 273) 4 ( T 273) 4 1 ]( ) (8) 1 1 Fpa III. RESULTS The study shows various results for semi-cylindrical cavity with smooth as well as for rough surface with aluminium rods. The Nusselt number is depend on average heat transfer coefficient, radius of the cavity and thermal conductivity of air in the cavity. So from the experiment we can obtain Nusselt numbers with various time intervals at tilt angles such as 0 0, 90 0, 60 0, 30 0,-90 0, -30 0, - 60 0 Fig 5 to Fig 7 shows various graph of Nusselt number Vs required Time at different tilt angles such as 0 0,-30 0, - 60 0 At above range of inclination of angles Buoyancy mechanism is strong. The heat transfer rate for roughness surface is increased over smooth surface. At minimum angle of inclination it will causes increase in heat transfer rate but as inclination increases heat transfer rate increases in average. Here in Fig 5-7 time history of average nusselt numbers are shown for rough as well as smooth surface. 2015, IERJ All Rights Reserved Page 4
Fig.5 Nusselts number Vs time intervals at tilt angle 0 0 for smooth and rough surface Fig.6 Nusselts number Vs time intervals at tilt angle -30 0 for smooth and rough surface Fig.7 Nusselts number Vs time intervals at tilt angle -60 0 for smooth and rough surface 2015, IERJ All Rights Reserved Page 5
Fig. 8 represents the time required to achieve steady state for both smooth as well as rough semi-cylindrical cavities. Steady state is recorded when the average Nusselt number remains constant for at least 120 minutes. It shows that the rough cases require more time to achieve steady state than smooth surfaces. When the cavities are facing down more time is required to achieve steady state for both. Fig.8 Effect of various tilt angles with respect to time required to achieve steady state for semi cylindrical cavity. IV. CONCLUSIONS 1) Two competing effects are present with the existence of roughness. Roughness produces drag causing blockage effect to take place resulting in a less heat transfer. Roughness also increases the turbulent intensity on the surface, which causes heat transfer to increase. Both of each effect is a function of tilt angle 2) While considering roughness, the height of roughness element as compare to boundry layer thickness is an important factor. 3) Rough surface helps to trip the boundary layer and adds more turbulence to the flow resulting in increase in heat transfer rate at different angles. 4) Above conclusions are based on rough surface with aluminium rods. Rough surface with triangular type vortex generator gives more heat transfer enhancements. REFERENCES Walid Chakroun, Mir Mujtaba A. Quadri, (2002), Heat transfer measurements for smooth and rough tilted semi-cylindrical cavities, Int. J. Therm. Sci. 41, pp 163 172. Shuang-Ying Wu Lan Xiao a, Yiding Cao b, You-Rong Li a, The convection heat loss from cavity receiver in parabolic dish solar thermal power system N. Onur, M. Sivrioglu, M. K. Akta (1997), An experimental study on the natural convection heat transfer between inclined plates (Lower plate isothermally heated and the upper plate thermally insulated as well as unheated), Heat and Mass Transfer 32, pp 471-476. N. Onur, M. Sivrioglu, M. K. Akta (1997), An experimental study on the natural convection heat transfer between inclined plates (Lower plate isothermally heated and the upper plate thermally insulated as well as unheated), Heat and Mass Transfer 32, pp 471-476. B. Premachandran, C. Balaji, (2006),correlation for mixed convection heat transfer from converging, parallel and diverging channels with uniform volumetric heat generating plates, International Communications in Heat and Mass Transfer 33, pp 350 356. M. Havet, D. Blay (1999), Natural convection over a non-isothermal vertical plate, International Journal of Heat and Mass Transfer 42, pp 3103-3112. Walid Chakroun, Effect of Boundary Wall Conditions on Heat Transfer for Fully Opened Tilted Cavity S.Siddiqa, S.Asghar, M.A. Hossain Natural convection flow over an inclined flat plate with internal heat generation and variable viscosity. 2015, IERJ All Rights Reserved Page 6