Experimental Study of Free Convection Heat Transfer From Array Of Vertical Tubes At Different Inclinations

Transcription

1 Experimental Study of Free Convection Heat Transfer From Array Of Vertical Tubes At Different Inclinations A.Satyanarayana.Reddy 1, Suresh Akella 2, AMK. Prasad 3 1 Associate professor, Mechanical Engineering Department, M.J.College of Engineering and Technology, Hyderabad, India 2 Principal, Sreyas Institute of Engineering & Technology, Hyderabad, India 3 Professor, Mechanical Engineering Department, Osmania Engineering College, Hyderabad, India Abstract Free convection heat transfers from array of vertical tubes were experimentally studied for different inclination angles. The inclination angles were changed from angles with steps of 15 degrees. The experiments were carried on specially developed facility to perform constant heat flux and the temperatures were measured by thermocouples. The electrical input to the heater was controlled by dimmer stat and is measured by wattmeter. The experiments are carried for Rayleigh no (Ra) from 4.5x10 4 to 8.5 x10 8. The effects of array inclination and Rayleigh number on the temperature distribution were investigated. The local Rayleigh numbers and Nusselt numbers (Nu) were estimated along the tube length. The experimental results show that for higher values of Rayleigh numbers and higher inclination angles the temperature differences from the tube to atmosphere air decreases. Keywords Constant Heat flux, vertical tube array, Inclination angle, Bulk mean temperature I. INTRODUCTION Natural convection heat transfer from a vertical array of tubes and fins is of great importance in engineering application, such as radiators, distribution transfer cooling systems, oil cooling systems, condensers of refrigerators and cooling of electronic devices. Any small increase in heat transfer rate decreases the power consumption and increases the life of the equipment. In many engineering situations the equipments are placed at different geographical locations which are not accessible for regular maintenance and which require cooling of the surfaces continuously and free convection heat transfer process is preferred for these practical applications. Natural convection heat transfer from cylinders and array of horizontal cylinders was done by Eckert & Soehngen (1) and M.Sadegh Sadeghipour, M. Ashegfhi (2) The study included heat transfer from horizontal cylinders of small diameter with rectangular grid. Lieberman and Gebhart (3) experimentally investigated the heat transfer from heated wires with uniform flux. Heat transfer from vertical array of horizontal elliptic cylinders was studied by Annunziata O Orazo & Lucia Fontana (4) and Yousefi & Ashjaee (5). 253 Many theoretical and experimental studies were done on vertical array of horizontal cylinders. Natural convection heat transfer from vertical plate and cylinders are different than heat transfer from horizontal cylinders. When the fluid flows over a surface, boundary layer is formed. Boundary layer plays an important role in flow over a flat plate and vertical cylinder. Boundary layer of natural convection flow has extensively been studied for long years. Many experimental and theoretical investigations have been carried out to study the behavior of the development of boundary layer flow on plates and cylinders Natural convection from vertical slender circular cylinders has been studied for many years. Typical of the experimental studies of natural convective heat transfer from vertical circular cylinders are those of Jarall and Campo [6], Welling et al. [7] and Fukusawa and Iguchi [8].Cylinders with exposed top inclined at an angle to the vertical have received attention by Oosthuizen [9, 10]. Natural convection heat transfer from a single vertical tube is different from to a bundle of vertical tubes. Due to the development of the boundary layer, the heat transfer from the tubes varies for different centre to centre spacing, aspect ratio and inclination of tube array. S.A.Nada and M.Mowad (11) performed an experimental study on free convection from a vertical and inclined semi circular cylinder at different orientations at constant heat flux. The experiments were carried at four inclination angles (0 0, 30 0, 45 0 and 60 0 ) of the semi circular cylinder. The results showed that the average Nusselt number increases as the inclination angle of the semi circular cylinder increases. Literature survey shows minimum work in the study of free convection heat transfer from vertical tube bundles. The aim of the present work is investigate the effect of inclination angle of the vertical tube bundles on the Nusselt number and Rayleigh numbers. The experimental study is performed on vertical array of 3 cylinders of 30 mm diameter, 480 mm height and with a centre to centre distance of 30 mm. The study is carried out for different inclination angles of tube bundles at different Rayleigh numbers.

2 II. EXPERIMENTAL SETUP AND PROCEDURE (i) Experimental setup The experimental set up consists of three hollow mild steel cylinders of 30mm internal diameter with 2mm thickness and 480 mm length are welded to a header of 36X36 mm square pipes as shown in figure (1). The aspect ratio (tube length to diameter ratio) L/D is maintained at 16.The header is connected to the tank of size 200x100x700(LBH) mm. Mineral oil is filled in the tank up to the header pipe top level. The tank is fitted with an immersion heater of 500 W capacity. The input to the heater is regulated by a dimmer stat and the input power is measured by a wattmeter. The Mineral oil picks up the heat from the heater and hot oil rises to the top due to the buoyancy effect. The hot oil enters the top of the tube and moves down through the tube giving away the heat to the atmosphere air by free convection. The heat transfer from the tube to the atmosphere is by convection and radiation. The radiation heat transfer losses are calculated at the mean temperature of the test setup for each inclination. The free convection heat transfer plays an important role and conduction the least. To record the tube surface temperatures, eight k-type thermocouples are inserted in to the tube and temperatures are measured with a spacing of 60mm along the tube height and ambient temperature is measured by the ninth thermocouple The thermocouples are connected to 16 channel temperature scanner, data logger and with the help of acquisition software the temperatures readings are recorded in the computer. The complete experiment is conducted with minimum disturbance from air movement so that free convection heat transfer prevails. Fig 1. Schematic diagram of tube array at an angle (ii) Experimental procedure The aim of the experiment is to measure the temperatures along the length of the tube for the desired constant heat flux and estimate the Grashof number and Rayleigh number for four inclinations of the tube bundles, namely 0 0,15 0,30 0 and 45 0 inclination from the vertical. The heater is adjusted for the desired power input with the help of dimmer stat. The experiment is allowed to run till the steady conditions are reached, that is when the temperature change is within C in 10 minutes of thermocouple reading of temperatures. When steady state conditions are reached, eight temperature readings along the tube height and ambient temperature and power input to the heater are recorded by the data acquisition system. The experiments are repeated for a fixed heat flux for four different inclinations of tube array. The experiments are repeated for different ambient temperatures to test the repeatability of the results. 254

3 III. DATA REDUCTION For every experimental condition constant power is supplied to maintain fixed heat flux to the setup. The temperatures are measured at eight locations along the tube and the ambient temperature. The heat input to the tube assembly = V I (1) Where V is the voltage applied I is the current applied The total heat transfer from the tubes = Q convection + Q radiation (2) The radiation heat transfer from the tubes are estimated by Q radiation= σ A ε (T 4 f - T 4 ) (3) Where σ is the Stefan Boltzmann constant ε emissivity of the mild steel pipe T f Bulk mean temperature of the air The net heat transfer from the tubes by natural convection is given as Q convection =h A (T f -T ) (4) Where h is the heat transfer coefficient W/m 2 K The average temperature along the tube is calculated. The bulk mean temperature or film temperature (T f ) is calculated as the average of ambient temperature and average temperature along the tube length. All the properties of air are calculated at the film temperature T f The local heat transfer is calculated as h x = Q conv / (T i -T ) (5) Where Ti is the local surface temperature on the tube r The local Nusselt number is calculated as Nu x = h x x / k (6) Where x= distance measured from the lower end of the tube to the thermocouple position along the tube The local Grashof and local Rayleigh numbers are calculated as Gr x =g β x 3 (T i -T ) / ν 2 (7) Where β = 1/ T f thermal expansion coefficient ν = Kinematic viscosity of the air Ra x = Gr x * Pr x (8) IV. RESULTS AND DISCUSSIONS The experiment is conducted with constant heat flux for all the inclinations. The temperatures are measured along the length for all the experiments. In this experiments hot mineral oil will flow from the top of the tube to the bottom of the tube due to the free convection heat transfer from the tube to the atmosphere Fig.2. Temperature variation along the tube at different inclinations It is found that the temperatures along the tube increases for all the inclinations. It is observed from fig.2 that the temperature increases up to a certain length and after that it shows a sign of decrease in temperature. This can be attributed to the turbulence of the thermal boundary layer with the increase of vertical distance and also because the flow is closer to the critical Rayleigh number. This is more prominent at higher inclination angles of the tube array. The temperature variation along the tube with Rayleigh numbers for different inclinations is shown in fig 3.The variation of temperature along the tube for different inclinations indicates that the temperature difference between tube wall and atmospheric air temperature decreases as the tube inclination increases. This trend indicates the disturbance of thermal boundary layer. Small inclinations have no effect on temperature difference. 255

4 Fig.3. Temperature variation with Rayleigh no. for different inclinations Variation of Nusselt number along the tube length is shown in fig.4. From the above figure it shows that the local Nusselt number increases along the tube length and increases as the inclination angle increases. This attributes the decrease of boundary layer effect on the heat transfer. The relation between Rayleigh number and Nusselt number were shown in fig.5. The Local Nusselt number increases with the increase of Rayleigh number and as the inclination angle increases the Nusselt increase is much steeper. Experimental data were fitted into the an empirical relations as Nu=1.03 Ra which follows the trend of Nu=C Ra n ( 12). Fig.4 Nusselt no. along the tube for different inclinations Fig.5. Rayleigh no Vs Nusselt no for different inclinations 256

5 V. CONCLUSIONS Free convection heat transfer from vertical array tubes at constant heat flux have been studied experimentally investigated. Experiments have been performed on three tubes placed vertically with spacing equal to the tube diameter and studied the effects of inclination on Nusselt number and Rayleigh number. It was observed that the temperatures along the tube array increase up to a certain height and decreases for all inclinations and this decrease is prominent at high inclinations due to the turbulence generation. It is found that for higher values of Rayleigh numbers and higher inclination angles the temperature differences from the tube to atmosphere decreases, due to the disturbances of turbulent boundary layer. The local Nusselt number of tube array increase with increase in Rayleigh number, and at higher inclinations the increase of Nusselt number is very significant. Nomenclature T Temperature (K) T f V I Film temperature Voltage applied (volts) Current applied (Amperes) Β = 1/ T f Thermal expansion coefficient g Gravitational acceleration (m/s 2 ) h Local heat transfer coefficient (W/m 2 K) k Thermal conductivity of air (W/m K) ν Kinematic viscosity of air (m 2 /s) Gr x Local grashof number Nu x Local Nusselt number Pr x Local Prandle number Ra x Local Rayleigh number REFERENCES [1] E.R.G. Eckert, E.E. Soehngen, Studies on Heat Transfer in Laminar Free Convection with the Zehnder-Mach Interferometer, AF Technical Report, 5747, USAF Air Material Command, Wright Paterson Air Force Base, Ohio, 1948 [2] M.Sadegh Sadeghipour, M.Ashefhi, Free Convection Heat Transfer From Arrays of Vertically Separated Horizontal Cylinders At Low Rayleigh Numbers, International Journal of Heat and Mass Transfer, Volume 37, Issue 1, January 1994, Pages [3] Lieberman, B. Gebhart, Interaction in natural convection from an array of heated elements, experimental, Int. J. Heat Mass Trans. 12 (1969) [4] Annunziata D Orazio, Lucia Fontana, Experimental study of free convection from a pair of vertical arrays of horizontal cylinders at very low Rayleigh numbers,, International Journal of Heat and Mass Transfer 53 (2010) [5] Yousefi a, M. Ashjaee Experimental study of natural convection heat transfer from vertical array of isothermal horizontal elliptic cylinders.experimental Thermal and Fluid Science 32 (2007) [6] Jarall S, Campo A (2005) Experimental study of natural convection from electrically heated vertical cylinders immersed in air. Exp Heat Transfer 18(3): [7] Welling I, Koskela H, Hautalampi T (1998) Experimental study of the natural-convection plume from a heated vertical cylinder. Exp Heat Transfer 11(2): [8] Fukusawa K, Iguchi M (1962) On optical measurements of natural Convection along vertical cylinder. J Mech Lab Japan 16(3): [9] Oosthuizen PH (1979) Free convective heat transfer from vertical cylinders with exposed ends. Trans Can Soc Mech Eng 5(4): [10] Oosthuizen PH (2007) Natural convective heat transfer from a vertical cylinder with an exposed upper surface. In: ASME/JSME thermal engineering summer heat transfer conference, ASME, Vancouver, BC, Canada, pp [11] S. A. Nada,a and M. Mowad Free convection from a vertical and inclined semicircular cylinder at different orientations, Alexandria Engineering Journal, Vol. 42 (2003), No. 3, [12] H.R.Nagendra, M.A.Tirunarayana and A.Ramachandran,. Free convection heat transfer from vertical cylinders part1; Power law surface temperature variation, Nuclear Engineering and design 16 (1970) , North-Holland publishing company, Amsterdam 257