Published on June 6, 2016
slide 1: International Journal of Engineering Research Science IJOER ISSN: 2395-6992 Vol-2 Issue-5 May- 2016 Page | 272 Experimental study of effect of Reynolds number and Nusselt Number on the performance of flat-plate solar Air Heater having artificial roughened Rhombus shape absorber plates Pankaj Kumar 1 S.C.Roy 2 M.K.Paswan 3 Ujjwal Kumar Nayak 4 124 Department of mechanical engineering B.I.T.Sindri Dhanbad-828123 Jharkhand India 3 Department of Mechanical Engineering N.I.T. Jamshedpur-831014 Jharkhand India Abstract — An experimental investigation has been carried out for a range of system an operating parameters in order to analyse effect of artificial roughness on heat transfer in flat plate solar air heater having rhombus shape sheet metal as roughness geometry. Duct has an aspect ratio W/H of 7 relative pitch p/e range of 40 to 60 and Reynolds Number R e range from to 5100 to 28000. A considerable increase in heat transfer has been observed. Keywords — Solar air heater Centrifugal Blower Artificial roughness. I. INTRODUCTION The use of artificial roughness on a surface is an effective technique to enhance heat transfer to fluid flowing in a duct. This roughness can be provided by sand blasting fixing wires wire mesh or by providing roughness in the form of ribs dimples protrusion etc. reported by Dippery and Sabersky 1963 Sheriff and Gumley 1966 Saini and Saini 1997 Saini and Verma 2008 Hans et al. 2009 and Bhushan and singh 2011. Several investigations have been carried out to study the effect of artificial roughness on heat transfer used in compact heat exchanger by Elyyan et al. 2008 and Webb 1994 and in solar air heater by Momin et al. 2000 Varun et al. 2008 Singh et al. 2011 Lanjewar et al. 2011 Jauker et al. 2006 and Layek et al. 2007.The roughness destroy the laminar sublayer and create turbulence in the flow. The turbulence leads to increase in pumping power which is required for flow of air in the duct. Therefore roughness is created in such a region which is near to the absorber plate i.e. laminar sublayer only. The roughness was first used in solar air heater and resulted in better heat transfer in comparison to that in conventional solar air heater by Prasad and Mullic 1985. Prasad and Saini 1988 studied the effect of roughness and flow parameters on heat transfer for transverse ribs. It was observed that maximum heat transfer occurred near to the reattachment points. The maximum enhancement in Nusselt number was reported to be 2.38 times over smooth duct.Verma and Prasad 2000 has been carried out experimental study for thermo hydraulic optimization of the roughness and flow parameters for Reynolds number R e range of 5000-20000 relative roughness pitch P/e range of 10-40 and relative roughness height e/D h range of 0.01-0.03. The optimal thermohydraulic performance was reported to be 71. Karwa et al. 1999 has been experimentally investigated the effect of repeated rectangular cross-section ribs on heat transfer for duct aspect ratio W/H range of 7.19-7.75 P/e value of 10 e/D h range of 0.0467-0.050 and R e range of 2800-15000. The enhancement in the Stanton number was reported to be 65-90 Gupta et al. 1997 experimentally investigated the effect of e/D h inclination of rib with respect to flow direction and Reynolds number R e on the thermohydraulic performance of roughened solar air heater. The detailed studies on roughness geometries used in solar air heater ducts are also available in Varun et al. 2007 Hans et al. 2009 and Bhushan and Singh 2010. The application of artificial roughness in the form of rhombus shape on absorber plate is attractive roughness geometry for solar air heater due to its less complicated manufacturing process. In this paper experimental data has been collected by performing experiment to see the effect of roughness parameters rhombus shape on heat transfer. 1.1 Roughness parameters Rhombus shape sheet metal roughness elements have been generated on the absorber plate to create roughness in the duct. A schematic and pictorial view of roughness geometry is shown in fig. 1a and b. The roughness parameters in non-dimensional form have been expressed as relative roughness pitch P/e. The rhombus shape of roughness was produced on the underside of the absorber plate. The range of roughness parameters and operating parameters is given in Table1. For studying the effect of P/e on heat transfer roughened plates were experimentally investigated at various mass flow rates. slide 2: International Journal of Engineering Research Science IJOER ISSN: 2395-6992 Vol-2 Issue-5 May- 2016 Page | 273 A DIAGRAM OF THE ABSORBER PLATE B PICTORIAL VIEW OF THE ABSORBER PLATE FIG. 1. A DIAGRAM OF THE ABSORBER PLATE. B PICTORIAL VIEW OF THE ABSORBER PLATE TABLE 1 VALUES OF FLOW AND ROUGHNESS PARAMETER S.No. Parameters Range 1 Aspect ratio W/H 1 2 Relative roughness pitch P/e 40-60 3 Reynolds number 5100-27000 Nomenclature A area of absorber plate m 2 D equivalentdiameter of the air passage m e height of roughness element e + roughness Reynolds number e/D relative roughness height f r friction factor for roughened duct f s friction factor for smooth duct h convective heat transfer coefficient W/m 2 k H height of duct m I intensity of solar radiation W/m 2 m mass flow rate of air kg/s Nusselt number for roughened absorber plate Nu s Nusselt number for smooth absorber plate P r prandtl number slide 3: International Journal of Engineering Research Science IJOER ISSN: 2395-6992 Vol-2 Issue-5 May- 2016 Page | 274 p roughness pitch m p/e relative pitch roughness R e Reynolds number t f average fluid temperature K t i air inlet temperature K t o air outlet temperature K t p average plate temperature K W width of duct m II. EXPERIMENTAL SETUP AND PROCEDURE FIG. 2.1 SHOWS THE SCHEMATIC DIAGRAM OF THE EXPERIMENTAL SET-UP USED. 1 Flow straighteners 2 Plane solar air heater 3 Roughened solar air heater 4 Outlet headers pipe fittings 5 Manometers 6 Selector switches 7 Centrifugal blower 8 Auto transformer R reference junction icebath Major components of this set-up are: a Flow Straighteners. b Test section i.e. plane and roughened collectors c Out let headers and pipe fittings. d Flow meters provided with U-tube manometers e Thermocouples with selector switches. f Centrifugal blower. g 5 h.p. 3-phase electric motor. Two- dimensional fully developed flow was obtained by sucking atmospheric air through flow straighteners by means of blower. The air thus passes through the test section i.e. solar air heaters one with rhombus shape roughened and the other plane Flat-Plate collector and flow meters before exhausting into the atmosphere. Thermocouples were used to measure absorber and air temperature at different location in the solar air heaters as flow progresses. The output of the thermocouple fed to digital micro voltmeter displays directly the temperature values. Mass flow rate of air through these collector ducts were measured by two orifice meters provided with U-tube manometers. slide 4: International Journal of Engineering Research Science IJOER ISSN: 2395-6992 Vol-2 Issue-5 May- 2016 Page | 275 For each experimental run initially all the instruments viz. manometer milli-voltmeter U-tube manometer Blower and electric circuit were checked for their correctness and all joints were carefully checked to avoid any air leakage. Data was recorded under quasi-study state when there is no appreciable change in temperature for 10-15 min conditions for the air temperature at different points on the duct and temperature of absorber plate at 05 different locations. Data were taken at the regular interval of 1hour and accordingly the pressure drop across orifice meter has been measured with the help of U-tube manometer. III. RESULTS AND DISCUSSION The total 30 numbers of test runs raw experimental data were collected for 3 set of roughened solar air heater as well as smooth one. For a particular test run mass flow rate in the roughened and smooth collector remained the same. Table 2 represents the roughness and flow parameters investigated. The raw experimental data were reduced to work out for the values of the results with respect to heat transfer. Fig. 3a and Fig. 3b shows the data of Solar Intensity and Time and Temperature and Time respectively. Fig. 3c represents the heat transfer results in the form of Nusselt number for roughened and smooth absorber plates. The analytical and experimental values of Nusselt number for roughened and smooth absorber plates has been compared. The analytical value of Nusselt number is calculated by using the equation given by Prasad and Saini 1988 as: Nu s 0.14Pr 0.5 R e 1 2 Where The experimental value of the heat transfer coefficient for roughened and smooth absorber plates have been calculated by using the relation: mc p t o -t i hAt p -t f 3 Nusselt number is calculated for roughened and smooth absorber plates with the help of equation 4 by using the relation Nu hD/k 5 The experimental and analytical values of Nusselt number for roughened and smooth absorber plates is represented in Figure 3.c for a given value of e/D equal to 0.0493 at different values of P/e of 40 50 and 60. FIGURE 3A FIGURE 3B slide 5: International Journal of Engineering Research Science IJOER ISSN: 2395-6992 Vol-2 Issue-5 May- 2016 Page | 276 FIGURE 3C Fig.3 c has been drawn to analysis the effect of the roughness parameter p/e on Nusselt number in roughened as well as smooth absorber plate at the same value of Reynolds number. In this figure the effect of roughness parameter p/e on heat transfer is shown for a given value of e/D. It is clear from the figure that the value of Nusselt number increases with decrease in the value of roughened parameter p/e. The value of Nusselt number also increases as the values of Reynolds number increases and also at the faster rate than that in smooth absorber plate. It can be also seen that the experimental and analytical results on heat transfer is have been found to be compare good. It is found for a particular value of Reynolds number at 10000 the values of Nusselt numbers are 7582 and 98 for p/e of 60 50 and 40 respectively in roughened absorber plate whereas 55 for smooth absorber plate. In figure present result is compare with prasad and saini but for higher values of Reynolds number. Within the tested range of investigation the value of the Enhancement factor /Nu s is found to be in the range of 1.5 to 2.1 for Reynolds number of range 5100 to 18000.But after that it decreases. IV. CONCLUSION The effect of roughened geometry on heat transfer leads the following conclusions: 1. Heat transfer for flat plate solar air heater is represented in the form of Nusselt number. 2. Solar air heater having artificial roughened absorber plate have high rate of heat transfer than that of smooth solar air heater. 3. The rate of enhancement of heat transfer of roughened flat plate solar air heater strongly depends on Reynolds number and relative roughness pitch p/e.It is worthy to note here that the best Enhancement Factor is found in the range of 12500 to 18000 Reynolds number. 4. At higher Reynolds number i.e. above 18000 the Enhancement Factor decreases considerably. REFERENCES 1 Aharwal K.R. Gandhi B.K. Saini J.S. 2008. 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