Local heat transfer coefficient around a horizontal heating element in gas-solid fluidized bed

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1 Abstract Experiments have been carried out in a laboratory gas-solid fluidized bed heat exchanger. Heat transfer coefficient between immersed heated tube and bubbling fluidized bed is found experimentally around a tube at different angles. An experiment is performed with three different particle diameter and ten different superficial gas velocities. The bed particles used were Geldart B silica sand particles of diameter 200μm, 30μm and 00μm. Fluidizing media used was atmospheric air. The experimental results showed that the local heat transfer coefficient is maximum at the top (180 0 ) and minimum at the bottom ( 0 0 ) from direction of air. Heat transfer coefficient is increased with increasing the air velocity and it is found to decrease by increasing the particle size for Geldart B group particles. In order to predict the heat transfer coefficient from a horizontally immersed tube to fluidized bed directly, a correlation of Nusselt number has been proposed. The correlation predictions are compared with the experimental data of this works. It is observed that they show a good agreement with each other. Keywords: Fluidized bed, Heat transfer coefficient, Nusselt number 1. INTRODUCTION Local heat transfer coefficient around a horizontal heating element in gas-solid fluidized bed Anusaya M. Salwe University of pune,pune The key objectives of this work are to study the bubbling Gas-Solid fluidized bed in order to find the suitability of sand as a bed material in Gas-Solid fluidized bed and to find the local heat transfer coefficient around a horizontal heating element immersed horizontally in bubbling fluidized bed. An experiment has been carried out in a laboratory fluidized bed heat exchanger with atmospheric air as a fluidizing medium and silica sand of diameter 200μm, 30 μm and 00 μm diameter. Many researchers have studied the effect of various parameters like fluidizing velocity and particle diameter on the average heat transfer coefficient between the fluidizing bed and immersed heating element, but there is considerably less literature available on the heat transfer coefficient around the horizontal tube and fluidized bed.l.m. Armstrong, S. Gu and K.H. Luo [2] used two-fluid Eulerian Eulerian formulation incorporating the KTGF was applied to a tube-tobed reactor with one immersed tube and compared with the results in the literature. They showed that the heat transfer coefficient is maximum at the top of the tube.b.stojanovic, J. Janevski and M. Stojiljkovic [3] experimentally investigated the heat transfer by convection between an immersed tube and the fluidized bed on the laboratory scale apparatus. They also noted that heat transfer coefficient is maximum at the top of the tube.u.s.wankhede, D.D.Adgulkar [7] carried out an experiment to find the heat transfer from the immersed body in a two dimensional fluidized bed varying the gas velocity in the range of u/u mf from to 1. They compared the heat transfer behavior of Geldart group A, B, C,D particles. The simulated values of the heat transfer coefficient calculated for three positions on the circumference of the tube at the top (90), bottom (-90) and horizontal (0).They concluded, For the Group A material the heat transfer coefficient varies drastically with the gas velocity. The variation for the heat transfer coefficient for Group B with u/umf is linear. For Group C, because of cohesive nature of material shows increase in heat transfer coefficient between to 10 u/u mf and a decrease for upper position. They used Eulerian approach for fluid dynamics and heat transfer in fluidized beds and Gunn model with kinetic theory of granular flow is appropriate model for modeling heat transfer between multiple phases. Sung Won Kim, Jung Yeul Ahn, Sang Done Kim, Dong Hyun Lee [11] were carried out an experiments in a FBHE made of transparent acrylic plate. The effect of gas velocity on the average and local heat transfer coefficients between a submerged horizontal tube and a fluidized bed has been determined in a fluidized-bed-heat-exchanger of silica sand particles. They measured the heat transfer coefficient around the tube circumference by thermocouples and an optical probe. They concluded that the average heat transfer coefficient exhibits a maximum value with variation of gas velocity, the local heat transfer coefficient exhibits maximum values at the top of the tube (0 0 ) and the average heat transfer coefficient increases with increasing gas velocity toward a maximum value of the coefficient. 2.EXPERIMENTAL PROGRAMME Fig.1 shows an experimental set up. It consists of fluidizing column is of 9 mm internal diameter and 3 mm thickness and 1 m height, made up of plexiglass to aid visualization. A fine mesh of copper is used as a distributor plate. A brass tube of external diameter 10 mm and length 0 mm is fitted horizontally inside the column. The axis of brass tube is Volume 2, Issue 6, June 2013 Page 344

2 perpendicular to the axis of fluidizing column. A Cartridge heater of 6 mm outer diameter and 4 mm long is inserted inside the brass tube. Axis of brass tube is at the distance of 18 mm from the distributor plate. Fig 2 shows brass tube and cartridge heater. Fixed bed height was 22 mm. A constant heat input of 18.2W is given to the heater. Two nylon plugs of thickness 3mm each are fixed at the ends of brass tube to minimize the axial losses. It is assumed that heat flow is in only radial direction of the brass tube. All the readings are taken at the steady state. For one sample size and one velocity time taken was two hours. 1-Fluidizing column 2-Brass tube 3-Bed particles 4-Distributor plate -Orificemeter 6-Centrifugal blower 7-Inlet air flow 8-Manometer Fig 1 : Experimental Set-up 2.1 Instrumentation: A variable transformer and ammeter is used to give require input to the cartridge heater. Total Eight PT- 100thermocouples are used in the experimentation. Three of which are used to measure surface temperature on the circumference of the tube at different angles. Four thermocouples are used to measure bed temperature. One thermocouple is used to measure ambient temperature.fig.3 shows position of Thermocouples. A blower of the capacity 1. m 3 /min. is used to supply compressed air to the fluidizing column through diffuser and distributor plate. A butterfly valve mounted over an inlet pipe allows regulating the rate of flow of air. A differential manometer is connected across an orificemeter. Readings of the manometer is calibrated to get the velocity of the inlet air. Another differential manometer is connected to the fluidizing medium to find the pressure drop across the column. Fig. 2: Brass tube and Cartridge heater , 4, Fig.3: Position of Thermocouples Air Flow Fig.4: Side view of the brass tube showing angles at which temperature readings are taken Volume 2, Issue 6, June 2013 Page 34

3 2.2 Bed material: Bed/Particulate material-the bed material used in this work was silica particles that can be classified as Group B according to Geldart s classification. Standard Sieve analysis is carried out in laboratory for taking the bed sample size of silica sand. Density of sand is measured directly using an Archimedes principle. Properties of bed material are given in Table 1. Table 1: Properties of bed material Bed material Sand 1 Sand 2 Sand 3 Properties 3 RESULTS: D p (μm) ρ (Kg/m 3 ) U mf (m/s) U mb (m/s) Ut (m/s) ψ ε mf Fig.: Distribution of Heat transfer coefficient at different angle and D p =200 μm Fig.6: Distribution of Heat transfer coefficient at different angle and D p =30 μm Fig.7: Distribution of Heat transfer coefficient at different angle and Dp=00 μm Volume 2, Issue 6, June 2013 Page 346

4 4 CONCLUSION: 1) Silica sand is having good fluidizing properties. 2) Heat transfer coefficient between in immersed heated tube to the fluidized bed of silica is measured experimentally at different angles and from fig., 6 and 7, it is observed that it is maximum at and minimum at ) Heat transfer coefficient for the granular material like Silica of the size range 200µm to 00µm (Geldart B) ranges from 218 W/m 2 K to 692 W/m 2 K. 4) Heat transfer coefficient increases with decrease in particle diameter. ) From the experimental data and linear regression method, a Correlation is proposed to find Nusselt number directly. Nu = Re The Nusselt number calculated from experimentation and from correlation shows a good agreement with each other and gives 1 % error. REFERENCES [1.] Dr. Riyadh S. Al-Turaihi, 2012, Transient thermal behaviour of fluidized bed Column The Iraqi Journal for Mechanical and Material Engineering, Vol.12, No.1, [2.] L.M. Armstrong, S. Gu, K.H. Luo, 2010, The influence of multiple tubes on the tube-to-bed heat transfer in a fluidised bed International Journal of Multiphase Flow 36 (2010) [3.] B. Stojanovic, J. Janevski and M. Stojiljkovic,2009, Experimental investigation of thermal conductivity coefficient and heat exchange between fluidized bed and inclined exchange surface Brazilian Journal of Chemical Engineering, Vol. 26, No. 02, pp , April - June, [4.] Shu-qin WANG Xiao-jie ZHANG Yi-ming ZHANG, 2009 Numerical Calculation on Fluidized Property and Flow Field in small-scale Fluidized Bed /09/2009 IEEE. [.] Francesco Miccio, Andrea De Riccardis, Michele Miccio, 2009 Investigations on Heat Transfer Between a Bubbling Fluidized Bed and Immersed Tubes for Heat Recovery and Power Generation Combustion Colloquia 2009,Vol.1 PP1-4. [6.] Saeid. Rasouli, Mohammad.R. Golriz, 2009 Effect of Fins on Heat Transfer of Horizontal Immersed Tube in Bubbling Fluidized Beds Proceedings of the World Congress on Engineering 2009 Vol II. [7.] U.S.Wankhede, D.D.Adgulkar, 2008, CFD Simulations of Heat Transfer in a Bubbling Fluidized Bed for Different Materials First International Conference on Emerging Trends in Engineering and Technology, / IEEE. [8.] Nima Masoumifard, Navid Mostoufi, Ali-Asghar Hamidi, Rahmat Sotudeh-Gharebagh,2008, Investigation of heat transfer between a horizontal tube and gas solid fluidized bed, International Journal of Heat and Fluid Flow 29 (2008) [9.] Maan S. Al- Dabbagh, 200, Experimental Study of heat Transfer between the Shallow Fluidized bed and a Tube Bundle Immersed in it Al-Rafidain Engineering Vol.14 No [10.] Rahel Yusuf, Morten C. Melaaen and Vidar Mathiesen, 200, CFD Modeling of Heat Transfer in Gas Fluidized Beds Fourth International Conference on CFD in the Oil and Gas, Metallurgical & Process Industries. [11.] Sung Won Kim, Jung Yeul Ahn, Sang Done Kim, Dong Hyun Lee,2003 Heat transfer and bubble characteristics in a fluidized bed with immersed horizontal tube bundle International Journal of Heat and Mass Transfer 46 (2003) [12.] M.A. AL-Busoul, S.K. Abu-Ein, 2003, Local heat transfer coefficients around a horizontal heated tube immersed in a gas fluidized bed Heat and Mass Transfer 39 (2003) [13.] Yu Caiyuan, Zhang Yin, Wang Xizhong, 2002, Investigation of drying and heat transfer characteristics in agitation fluidized bed dryer, Proceedings of the 13th International Drying Symposium Beijing, China. [14.] Dr. Tahseen Al-Hattb, Dr. Riyadh S. Al-Turhee, Effect of particle size, flow velocity and heat flux on heat transfer coefficient in fluidized bed column The Iraqi Journal for Mechanical and Material Engineering, Special Issue (B). [1.] Francesco Di Natale, Amedeo Lancia, Roberto Nigro(2009) Surface-to-bed heat transfer in fluidized beds of fine particles, Powder Technology 19 (2009) [16.] Solid notes, George G. Chase, the University of Akron. [17.] H. S. Fogler and L. F. Brown [Reactors, Fluidized bed reactors, ACS Symposium Series,vol.168, p [18.] Jamshid Khorshidi, Hasan Davari, Fatemeh Deh bozorgi(2011) Model Making for Heat Transfer in a Fluidized Bed Dryer, J. Basic. Appl. Sci. Res., 1(10) , 2011 [19.] Achim Schmidt, Ulrich Renz (2000) Numerical prediction of heat transfer in fluidized beds by a kinetic theory of granular flows, Int. J. Therm. Sci. (2000) 39, [20.] Flow of fluids, 2010 Elsevier DOI: /B X [21.] Fundamentals of particle technology, an article from book Particle technology Volume 2, Issue 6, June 2013 Page 347

5 [22.] Francesco Di Natale, Piero Bareschino, Roberto Nigro (2010) Heat transfer and void fraction profiles around a horizontal cylinder immersed in a bubbling fluidized bed, International Journal of Heat and Mass Transfer 3 (2010) [23.] Xiaoying HU, Junjiao ZHANG, Changqing DONG, Yongping YANG (2010) Simulation of Dense Gas-Solid Fluidized BedBased on Two-Fluid Model, /10/$ IEEE [24.] Junjiao Zhang, Teng Zhang,Jingzhou Jiang, Wenyan Li, Changqing Dong,(2010) Studies of hydrodynamics of the fluidized bed reactor, 2010 International Conference on Digital Manufacturing & Automation [2.] N. Salehi-Nik, R. Sotudeh-Gharebag, N. Mostoufi. Zarghami, M.J. Mahjoob, 2009 Determination of hydrodynamic behavior of gas solid fluidized beds using Statistical analysis of acoustic emissions International Journal of Multiphase Flow 3 (2009) , [26.] Xiaoxin Wang, Youyi Guo,Pengcheng Shu, 2003, Numerical modelling of heat transfer between gas and solid in a vibrated fluidized bed, Applied Thermal Engineering 23 (2003) [27.] Yusumi Nagahashi, John R.Grace, Kok-Seng Limand Yutaka Asako, 2008, Dynamic Force Reduction and Heat Transfer Improvement for Horizontal Tubes in Large-Particle Gas-Fluidized Beds, Journal of Thermal Science Vol.17, No.1 (2008) 7783 Author Anusaya Salwe received her B.E.from Dr. Babasaheb Ambedkar marathwada university,aurangabad in 200.She is perusing her M.E. Heat Power from university of Pune. Her research interests include fluidized bed heat transfer and fluidized bed combustion. She has seven years of teaching experience to UG students. Volume 2, Issue 6, June 2013 Page 348