Performance Investigation of Artificially Roughened Duct used in Solar Air Heaters

Transcription

1 International Performance Journal Investigation of Product of Artificially Design Roughened Duct used in Solar Air Heaters January-June 2011, Volume 1, Number 1, pp Performance Investigation of Artificially Roughened Duct used in Solar Air Heaters Tarun Mahajan 1, Ranjit Singh 2 and Brij Bhushan 3 Department of Mechanical Engineering, Beant College of Engineering and Technology, Gurdaspur, Punjab , India, 1 tarun_29@yahoo.co.in. ABSTRACT: Experimental investigation has been reported in the present paper for fully developed turbulent flow in the rectangular duct having absorber plate roughened by fixing plain-woven square wire mesh. Effect of artificial roughness on heat transfer coefficient and friction has been investigated for a range of system and operating parameters. It has been observed that roughened absorber plate results into higher heat transfer coefficient at the cost of frictional penalty. In order to predict performance of the system, Nusselt number and friction factor correlations have been developed by using experimental data. Keywords: Solar air heater, artificial roughness, heat transfer coefficient, Nusselt number and friction factor. Nomenclature A c cross-sectional area of duct, m 2. A o area of orifice plate at the throat, m 2. C d coefficient of discharge. C p specific heat of the air, J kg 1 K 1. D hydraulic diameter of duct, m. e height of roughness element, m. f f s friction factor (dimensionless). friction factor for smooth plate (dimensionless). g acceleration due to gravity, m 2 s 1. 45

2 International Journal of Product Design H height of the duct, m. h t difference of manometric fluid levels in micro-manometer, m. h w difference of water column in U-tube manometer, m. h heat transfer coefficient, W m 2 K 1. k thermal conductivity of air, W m 1 K 1. L length of the test section, m. m mass flow rate of air, kg s 1. Nu Nusselt number (dimensionless). Nu s Nusselt number for smooth plate (dimensionless). p pitch of roughness element, m. Pr Prandlt number (dimensionless). P o pressure drop at the orifice plate, Nm 2. P t pressure drop at the test section, Nm 2. q heat transfer rate, W. Re Reynolds number (dimensionless). T i inlet air temperature of the test section, K. T o outlet air temperature of the test section, K. T am mean temperature of air, K. T pm mean temperature of absorber plate, K. V velocity of air, ms 1. W width of the duct, m. Greek Letters ρ density of air, kg m 3. ρ k density of manometric fluid in micro-manometer, kg m 3. ρ w density of water, kg m 3. β ratio of orifice and pipe diameter (dimensionless). θ inclination of U-tube manometer, degree. µ dynamic viscosity of air, kg/s-m. 46

3 Performance Investigation of Artificially Roughened Duct used in Solar Air Heaters 1. INTRODUCTION Energy has been needed and used by human being at an increasing rate for his substance and well being on the planet earth. Continuous use of fossil fuels have resulted energy crisis and environmental threats. Now the need has been felt to have inexhaustible and clean energy resources. Solar energy is considered to be primary energy source for all forms of energy. It is free and available almost everywhere. Simplest method to utilize solar radiation is to convert it into thermal energy for heating applications by using solar collectors. Solar air heaters because of their intrinsic simplicity and minimum use of materials are cheap and are used for many applications like space heating, timber seasoning, crop drying etc. Thermal efficiency of solar air heaters in comparison of solar water heaters is generally considered poor because of their inherently low heat transfer capability between the absorber plate and air flowing in the duct. Methodology of creating artificial roughness on the surface of absorber plate is considered to be an effective technique for enhancing heat transfer coefficient in order to increase heat transfer rate between absorber plate and air flowing through the duct. However, it results into an increase in friction loss. Therefore, turbulence must be created only in the region very close to the absorber surface i.e. in laminar sub-layer only. It can be produced by several methods such as by wire fixation in the form of transverse continuous ribs, transverse broken ribs, inclined and V-shaped or staggered ribs; rib formation by machining process in the form of chamfered ribs, wedge shaped ribs, combination of different integral rib roughness elements and by using expanded metal mesh ribs as has been described by Bhushan and Singh [1]. Prasad and Mullick [2] utilized artificial roughness in the duct used in solar heater in the form of small diameter wires to increase heat transfer coefficient. Prasad and Saini [3] investigated fully developed turbulent flow in the duct with a small diameter protrusion wire on the absorber plate. Muluwork et al. [4] compared thermal performance of staggered discrete V-apex up and down with corresponding transverse staggered discrete ribs. Karwa et al. [5] performed experimental study to predict effect of rib head chamfer angle and duct aspect 47

4 International Journal of Product Design ratio on heat transfer and friction loss in a rectangular duct roughened with integral chamfered ribs. Bhagoria et al. [6] performed experimental investigation to determine the effect of relative roughness pitch, relative roughness height and wedge angle on heat transfer and friction loss in a solar air heater roughened duct having wedge shaped rib roughness. Momin et al. [7] experimentally investigated effect of geometrical parameters of V-shaped ribs on heat transfer and fluid flow characteristics in rectangular duct used in solar air heaters. Jaurker et al. [8] experimentally investigated heat transfer and friction characteristics of rib-grooved artificial roughness. Varun et al. [9] presented a review on artificial roughness investigations reported in literature. It has been observed from various experimental investigations on artificial roughness that creating artificial roughness on absorber plate is a tedious task and may not be economically feasible. A suitable geometry of roughness element therefore needs to be selected, which besides being easily available should be easy to fix on the absorber plate. In the same direction, Saini and Saini [10] used expanded metal mesh to create artificial roughness on absorber plate and investigated effect of system and operating parameters on heat transfer and friction loss. Similar to expanded metal mesh, plain-woven square wire mesh available in common market can be used for generating artificial roughness on the absorber plate. It has been observed that heat transfer and pressure drop data for such type of a wire mesh is not reported in the literature. Therefore, it was decided to carry out experimental investigation to study performance of artificially roughened duct with plain-woven square wire mesh of two types. In the present paper an experimental investigation has been reported, in which heat transfer and pressure drop data have been generated for artificially roughened duct and used to develop Nusselt number and friction factor correlations for predicting performance of the system. 2. EXPERIMENTAL SET-UP AND PROCEDURE In order to carry out present experimental investigation, a test rig was designed and fabricated as per guidelines proposed in literature 48

5 Performance Investigation of Artificially Roughened Duct used in Solar Air Heaters for similar experimental investigations. Schematic and photogaraphic view of experimental set-up are shown in ig. (1) and ig. (2) respectively. It consists of an air duct along with temperature and pressure drop measuring instruments, electric heater assembly, pipe line, flow measuring device, centrifugal blower and gate valves. Air duct comprised 20 mm thick wooden ply board and size of the duct was mm. Length of entry, test and exit sections were kept 900 mm, 1000 mm and 500 mm respectively. Length of converging plenum on exit side of the duct was 600 mm. An electric heater having size of mm was fabricated by combining series and parallel loops of heating wire on mica sheet to get uniform heat flux. Backside of the heater was insulated with glass wool to minimize the heat loss. Heater was placed on top of the duct and variac was provided to control electric supply to it. igure 1: Schematic of Experimental Set-Up. 49

6 International Journal of Product Design igure 2: Photographic View of Experimental Set-Up. Absorber plate being a 20 SWG GI sheet of mm size was painted black on heater side and artificial roughness was provided on air duct side by fixing wire mesh. Mass flow rate of the air was measured by means of an orifice-meter connected with a U- tube manometer. low of air through duct was smoothly controlled by means of two gate valves provided at entry and exit of centrifugal blower. Thermocouples made from copper-constantan wire were used to measure temperature of air and absorber plate at different locations as shown in igs. (3) and (4). igure 3: Different Locations of Thermocouples used to Measure Absorber Plate Temperature. 50

7 Performance Investigation of Artificially Roughened Duct used in Solar Air Heaters igure 4: Different Locations of Thermocouples used to Measure Air Temperature in the Duct. igure 5: Photographic View of P-1 and P-2 Type of Plain-Woven Square Wire Mesh used in Experimental Investigation. Micro-manometer was used to measure pressure drop across test section of the duct. Photographic views of two types of wire mesh used in the present experimental investigation are shown in ig. (5). Specifications of wire meshes are given in Table 1. Table 2 shows range/value of parameters used in present experimental investigation. Wire mesh was glued on to the underside of the absorber plate and ends of the wire mesh were fixed with screws to ensure good contact between the absorber plate and wire mesh. Measuring instruments like orifice meter, temperature indicator, temperature selector switch, micro manometer and U-tube manometer were properly checked and calibrated before starting the experimentation. All joints in the test rig were thoroughly checked to avoid any leakage. ive values of flow rate were used for each set and following data were collected at an interval of one hour in each set of experimentation: (i) Pressure drop across orifice meter to measure flow rate of air. (ii) Pressure drop across test section of duct. 51

8 International Journal of Product Design (iii) Temperature of absorber plate at various locations in test section of the duct. (iv) Temperature of air at various locations in test section of the duct. (v) Voltage and current supplied to electric heater. Table 1 Specifications of Wire Mesh used as Roughness Element. Plate No. Geometry Specifications e(mm) p(mm) P-1 Plain-woven square wire mesh P-2 Plain-woven square wire mesh Table 2 Range/Value of Parameters. S. No. Parameter Range/Value 1. Reynolds number Mass flow rate of air kg/s 3. Duct aspect ratio (W/H) Relative roughness height (e/d) 0.022, Relative roughness pitch (p/e) 1.75, 5.65 Accuracy of experimental data was verified by conducting experiments for a conventional smooth duct. Nusselt number and friction factor values were determined from experimental data and compared with the values obtained from the following Nusselt number and friction factor correlations reported by Momin et al. [7] for rectangular smooth duct. where Modified Dittus-Boelter correlation for Nusselt number is Nu s = Re 0.8 Pr 0.4 2R av D e ( H / W 1) = H / W 52 2R av Modified Blasius equation for friction factor is D e (1) f s = Re (2)

9 Performance Investigation of Artificially Roughened Duct used in Solar Air Heaters igure 6: Comparison of Experimental and Predicted Data of Nusselt Number for Smooth Absorber Plate. igure 7: Comparison of Experimental and Predicted Data of riction actor for Smooth Absorber Plate. 53

10 International Journal of Product Design ig. (6) and ig. (7) shows comparison of experimental and predicted data of Nusselt number and friction factor for smooth absorber plate. A reasonably good agreement between experimental and predicted data ensures accuracy of the data being collected from self designed and fabricated experimental set-up. Error analysis based on the procedure described by Holman [11] has been carried out to find out uncertainties in measured/calculated values of experimental data. Uncertainty in Reynolds number, Nusselt number and friction factor values has been estimated as 1.65%, 1.73%, and 3.28% respectively. 3. DATA REDUCTION ollowing equations were used for calculating pressure drop across test section and orifice plate ( P t and P o ), mass flow rate of air (m ), velocity of air (V), heat transfer rate (q), heat transfer coefficient (h), Nusselt number (Nu) and friction factor (f): Also P t = ρ k g( h t )...(3) P o = ρ w g ( h w )...(4) m = C d A o V = m ρ A Therefore, from Eqs. (7) and (8) c 2ρ po Sinθ 4 1 β (5)...(6) q = m C p (T o T i )...(7) q = ha c (T pm T am )...(8) h = q A ( T ) T c pm am...(9) Where T pm and T am are mean temperature of absorber plate and air. These were determined from temperature values recorded for absorber plate and air at different locations along test section of the

11 Performance Investigation of Artificially Roughened Duct used in Solar Air Heaters duct. Reynolds number, Nusselt number and friction factor values were calculated by using the following relationships: Re = VD ρ µ...(10) Nu = hd k 2 PD t f = 2 4ρLV...(11)...(12) 4. RESULTS AND DISCUSSION Presence of wire mesh in the flow regimes cause vortices, help in increasing turbulence and hence enhancing heat transfer as well as friction loss. Effect of various flow and roughness parameters on heat transfer and friction characteristics has been investigated and results have been reported and discussed in the present section. ig. (8) represents variation of heat transfer coefficient as a function of mass flow rate of air. It can be observed that for a given type of wire mesh, heat transfer coefficient increases monotonously with increase of mass flow rate of air for smooth as well as roughened plate. Experimental data show that heat transfer coefficient for roughened plate is considerably higher as compared to smooth plate. It may happen due to turbulence caused by breakage of laminar sub-layer with application of artificial roughness. It has also been observed from these results that P-1 type of plain-woven square wire mesh has higher heat transfer coefficient than that of P-2 type of plain-woven square wire mesh. ig. (9) shows variation of Nusselt number as a function of Reynolds number for smooth and roughened absorber plates. Test results show that enhancement in Nusselt number for roughened plates is less at lower values of Reynolds number. However, enhancement in Nusselt number increases at higher rate for higher values of Reynolds number for both types of roughened absorber plates. It may be due to higher local distur-bances and secondary flow formation by roughness elements in flow regimes as reported by Saini and Saini [10]. 55

12 International Journal of Product Design ig. (10) shows variation of pressure drop as a function of mass flow rate for smooth and roughened absorber plates. Due to local disturbances and generation of secondary flows by roughened elements, increase in pressure drop has been observed as compared to smooth plate. ig. (11) shows variation of friction factor as a function of Reynolds number for smooth and roughened absorber plates. riction factor decreases with increase of Reynolds number as has been expected. With increase of Reynolds number boundary layer thickness decreases and roughness element begins to project beyond laminar sub-layer. This reduction in the boundary layer thickness increases heat transfer coefficient as well as friction loss as reported by Karwa et al. [5]. The shedding of vortices also causes additional loss of energy resulting in increased friction loss. It has also been observed in the present experimental investigation that heat transfer coefficient and friction loss decrease with increase in relative roughness pitch (p/e) as reported by Verma and Prasad [13]. igure 8: Variation of Heat Transfer Coefficient with Mass low Rate for Smooth and Roughened Absorber Plates. 56

13 Performance Investigation of Artificially Roughened Duct used in Solar Air Heaters igure 9: Variation of Nusselt Number with Reynolds Number for Smooth and Roughened Absorber Plates. igure 10: Variation of Pressure Drop with Mass low Rate for Smooth and Roughened Absorber Plates. 57

14 International Journal of Product Design igure 11: Variation of riction actor with Reynolds Number for Smooth and Roughened Absorber Plates. 5. DEVELOPMENT O NUSSELT NUMBER AND RICTION ACTOR CORRELATIONS It has been observed that Nusselt number and friction factor are strong functions of Reynolds number. unctional relationships for Nusselt number and friction factor can therefore be written as: Nu = λ(re)...(13) f = λ(re)...(14) As per procedure described by Singh et al. [12] and using Sigma plot software following Nusselt number and friction factor correlations were developed corresponding to experimental data as shown in igs. (8) and (10) in order to predict performance of the system: for P-1 type of plain-woven square wire mesh Nu = Re (15) f = 68 Re (16) and for P-2 type of plain-woven square wire mesh Nu = Re (17) f = Re (18) 58

15 Performance Investigation of Artificially Roughened Duct used in Solar Air Heaters igure 12: Comparison of Predicted and Experimental Data of Nusselt Number for P-1 Type of Plain-Woven Square Wire Mesh. igure 13: Comparison of Predicted and Experimental Data of Nusselt Number for P-2 Type of Plain-Woven Square Wire Mesh. 59

16 International Journal of Product Design igure 14: Comparison of Predicted and Experimental Data of riction actor for P-1 Type of Plain-Woven Square Wire Mesh. igure 15: Comparison of Predicted and Experimental Data of riction actor for P-2 Type of Plain-Woven Square Wire Mesh. 60

17 Performance Investigation of Artificially Roughened Duct used in Solar Air Heaters igs. (12), (13), (14) and (15) show comparison of experimental data and that predicted from above developed Nusselt number and friction factor correlations for both types of roughened absorber plates. Average absolute percentage deviations between experimental and predicted values of Nusselt number and friction factor for P-1 type of plain-woven square wire mesh have been found to be ± 2.5 % and ± 16 % respectively, whereas the corresponding values for P-2 type of plain-woven square wire mesh have been found to be ± 3.5 % and ± 14 % respectively. 6. CONCLUSIONS An experimental investigation on performance of artificially roughened duct used in solar air heaters has been reported in the present paper. Effect of artificial roughness (created by fixing plainwoven square wire mesh on absorber plate) on heat transfer and friction has been investigated for Reynolds numbers range of It has been observed that roughened absorber plate results into higher heat transfer coefficient at the cost of frictional penalty. In order to predict performance of the system Nusselt number and friction factor correlations have been developed by using experimental data. REERENCES [1] B. Bhushan, R. Singh, A Review on Methodology of Artificial Roughness Used in Duct of Solar Air Heaters, Energy, 35 (2010) pp [2] K. Prasad, S.C. Mullick, Heat Transfer Characteristics of a Solar Air Heater Used for Drying Purposes, Appl. Energy, 13 (1983), pp [3] B.N. Prasad, J.S. Saini, Effect of Artificial Roughness on Heat Transfer and riction actor in Solar Air Heater, Solar Energy, 41 (1988), pp [4] K.B. Muluwork, J.S. Saini, S.C. Solanki, Studies on Discrete Rib Roughened Solar Air Heaters, In: Proceedings of National Solar Energy Convention-98, Roorkee, India, pp , [5] R. Karwa, S.C. Solanki, J.S. Saini, Heat Transfer Coefficient and riction actor Correlations for the Transitional low Regime in Rib Roughened Rectangular Ducts, International Journal of Heat and Mass Transfer, 42 (1999), pp [6] J.L. Bhagoria, J.S. Saini, S.C. Solanki, Heat Transfer Coefficient and riction actor Correlations for Rectangular Solar air Heater Duct Having Transverse 61

18 International Journal of Product Design Wedge Shaped Rib Roughness on the Absorber Plate, Renewable Energy, 25 (2002), pp [7] A.M.E. Momin, J.S. Saini, S.C. Solanki, Heat Transfer and riction in Solar Air Heater Duct with V-shaped Rib Roughness on Absorber Plate, International Journal of Heat and Mass Transfer, 45 (2002), pp [8] A.R. Jaurker, J.S. Saini, B.K. Gandhi, Heat Transfer and riction Characteristics of Rectangular Solar Air Heater Duct using Rib-Grooved Artificial Roughness, Solar Energy, 80 (2006), pp [9] Varun, R.P. Saini, S.K. Singal, A Review on Roughness Geometry used in Solar Air Heaters, Int. J. Solar Energy, 81 (2007), pp [10] R.P. Saini, J.S. Saini, Heat Transfer and riction actor Correlations for Artificially Roughened Ducts with Expanded Metal Mesh as Roughened Element, International Journal of Heat and Mass Transfer, 40 (1997), pp [11] J.P. Holman, Experimental Methods for Engineers, Tata McGraw-Hill, New Delhi, [12] R. Singh, R.P. Saini, J.S. Saini, Nusselt Number and riction actor Correlations for Packed Bed Solar Energy Storage System Having Large Sized Elements of Different Shapes, Solar Energy, 80 (2006), pp [13] S.K. Verma, B.N. Prasad, Investigation for the optimal Thermo-Hydraulic Performance of Artificially Roughened Solar Air Heaters, Renewable Energy, 20 (2000), pp