American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) ISSN (Print) 2313-4410, ISSN (Online) 2313-4402 © Global Society of Scientific Research and Researchers http://asrjetsjournal.org/ Performance Evaluation of Solar Air Heater by Using W- Discrete Rib Pattern Alok Kumar Rohita*, A. M. Lanjewarb aPhD Scholar, Mechanical Engineering Department, AISECT University Bhopal, M.P.- 462024 India bAssistant Professor Maulana Azad National Institute of Technology, Bhopal, M.P. – 462003, India aEmail: alokrohit2007@gmail.com bEmail: lanjewar_atul@yahoo.com Abstract Solar air heater effectiveness enhancement is the major criterion today. For the enhancement of efficiency of solar air heater relative roughness pitch plays a very important role. An experimental investigation has been performed to know about heat transfer rate, thermo hydraulic performance and friction factor of W-Discrete ribs in the form of artificial roughness of solar air heater. The relative roughness pitch (p/e) of the ribs was taken as 6 and 10 to studies its effect. Keywords: W-discrete ribs; Reynolds number; Heat transfer enhancement rate; Solar air heater. 1. Introduction Artificial surface roughness is one of the active techniques of augmenting forced convection heat transfer. For the higher heat transfer rate the flow must me made turbulent at heat transferring surface. For the turbulent fluid flow some sort of external device is needed that may be in the form of fan, blower and compressor. But excessive turbulence leads to excessive power requirement for the proper supply of air through the duct. So it is desirable that the turbulence must be created only in the region very close to the heat transferring surface that is in the laminar sub layer only because it is the proper place of heat exchange but the fluid flow should not be unduly disturbed in order to avoid excessive friction losses. In solar air heater important parameters are shape of the roughness element, roughness element height (e) and pitch (p). ------------------------------------------------------------------------ * Corresponding author. 90 http://asrjetsjournal.org/ American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2016) Volume 16, No 2, pp 90-97 The above mention parameters are usually specified in terms of dimensionless parameter called as relative roughness height (e/D) and relative roughness pitch (p/e). Friction losses can be reduced by keeping relative roughness pitch within a certain limit. Reattachment point that is responsible for the enhancement of heat transfer rate for varying relative roughness pitch. Results shows that thermo-hydraulic performance of W-discrete rib for p/e as 10 are better than for p/e as 6. 2. Literature Review Bhagoria and his colleagues [1] worked on wedge shaped rib with parameter of p/e, e/D and ϕ of different dimensions. Sahu and Bhagoria [2] investigated on transverse broken rib with parameter of p/e, e/D and α. Prasad and Saini [3] worked on rib geometry of transverse continuous rib of parameter p/e and e/D. Varun and his colleagues [4] worked on rib geometry of combination of inclined and transverse rib with parameter of p/e and e/D. Momin and his colleagues [5] investigated on V-shaped rib geometry with parameter of p/e, e/D and α. Singh and his colleagues [6] worked on discrete V-rib with another different parameter i.e d/W , g/e, p/e, e/D, α. Karwa and his colleagues [7] investigated on geometry of chamfered rib with p/e, e/D and ϕ. Saini and Saini [8] investigated on expanded metal mesh with e/D, S/e, L/e, α. Lanjewar and his colleagues [9] worked on W- shaped rib with p/e, e/D and α. Bopche and Tandale [10] worked on U-shaped rib with p/e, e/D and α. Saini and Verma [11] studied on Dimpled shaped rib with parameters p/e, e/D. Jaurker and his colleagues [12] worked on Rib-groove with parameter of p/e, e/D and g/p. Karmare and Tikekar [13] worked on metal grit rib with p/e, e/D and α. Saini and Saini [8] worked on Arc shaped rib with e/D and α. Literature reveals no study has been done on W-discrete rib and is worth exploring. 3. Experimental Set-up An experimental set up is designed and fabricated to study effect of artificial roughed heat transfer and fluid flow characteristic in rectangular duct for range of parameters decided on the basis of practical considerations of system and operating conditions for different relative roughness pitch. The line diagram of experimental setup is shown in Fig.1. Experimental duct consists of wooden channel that includes five sections, namely smooth entrance section, roughened entrance section, test section, exit section and mixing chamber as outline by Duffie and Beckman [14]. G.I sheet of 20 SWG of 1.5 x 0.2 m2 size is used as an absorber plate and lower surface of plate is provided with artificial roughness in form of discrete W-shaped copper wires. An electric heater of dimensions identical to that of absorber plate is used to provide uniform heat flux to absorber plate up to maximum of 1500 W m-2. Power supply to heater is provided through variable transformers. Transformer enables heat flux applied to absorber plate to be varied as desired relative roughness pitch (p/e) is varied. Range of Reynolds number and relative roughness height (e/Dh) is chosen based on requirement of solar air heater. Parameters to be tested are Reynolds number and relative roughness pitch for given angle of attack and relative roughness height. The experimental result will be plotted as function of operating parameter like Reynolds number, relative roughness pitch. Roughened plate shown in Fig. 2 and Fig.3. 91 American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2016) Volume 16, No 2, pp 90-97 4. Result and Discussion Variation of Nusselt number and friction factor for of relative roughness pitch of 6 and 10 is shown in Fig. 4 and Fig. 5 Figure 1: Experimental Set-up Figure 2: Photograph of W-discrete ribs roughness plate 92 American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2016) Volume 16, No 2, pp 90-97 Figure 3: Enlarged view of cross-section of roughness plate Table 1: Roughness Parameters S.N O. PARAMETERS VALUES 1 Relative roughness height (e/Dh) 0.03375 2 Relative roughness pitch (p/e) 6, 10 3 Relative roughness length ratio(B/S) 6 4 Relative roughness staggering ratio (p’/p) 0.6 5 Relative roughness segment ratio (S’/S) 1 6 Plate length, L (m) 1.5 7 Reynolds number (Re) 4000 - 14,000 8 Angle of attack (α) 60o 9 Rib height (e) 1.5 mm 10 Duct aspect ratio ( W/H) 8 11 Hydraulic diameter, Dh (m) 0.044 Nusselt number increases with increase in Reynolds number due to progressive breaking of laminar sub-layer. Friction factor decreases with increasing Reynolds number due to increase in pressure drop. Heat transfer enhancement is accompanied with increase in pressure drop it is necessary to evaluate thermo-hydraulic performance. Thermo-hydraulic parameter is plotted against Reynolds number in Fig.6. Figure shows that thermo-hydraulic performance for p/e 10 is greater than thermo-hydraulic performance for p/e 6. Above results 93 American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2016) Volume 16, No 2, pp 90-97 are in accordance with the literature as optimum value occur for p/e 10 for rib roughness due to reattachment point considerations. 5. Conclusion In this paper an attempt has been made to report heat transfer and friction characteristics of artificial roughened duct of solar air heater. It is observed that artificial roughness is a good option to enhance the thermal performance of solar air heaters. After experimental investigation the successful results concluded are as follows: REYNOLDS NUMBER 2000 4000 6000 8000 10000 12000 14000 16000 NU SS EL T NU M BE R 0 20 40 60 80 100 120 p/e 6 p/e 10 SMOOTH PLATE Figure 4: Effect of Reynolds on Nusselt number with variation of relative roughness pitch as compare to smooth duct for W-discrete ribs flow arrangement. REYNOLDS NUMBER 2000 4000 6000 8000 10000 12000 14000 16000 FR IC TI ON F AC TO R 0.006 0.008 0.010 0.012 0.014 0.016 0.018 0.020 0.022 p/e 6 p/e 10 SMOOTH PLATE Figure 5: Effect of Reynolds on friction factor with variation relative roughness pitch as compare to smooth duct for W-discrete ribs flow arrangement. 94 American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2016) Volume 16, No 2, pp 90-97 1. Maximum thermo-hydraulic parameter with W-discrete rib is found with relative roughness pitch of 10. 2. It has been found with increase of Reynolds number the value of Nusselt number also increases. Maximum value of Nusselt number found for relative roughness pitch of 10. 3. Friction factor decreases with increase in Reynolds number. Maximum value of friction factor also occurs for relative roughness pitch of 10. 4. Thermo-hydraulic performance increases and attained maxima and then decreases. Thermo-hydraulic performance is higher for relative roughness pitch of 10 as compare to relative roughness pitch of 6. REYNOLDS NUMBER 2000 4000 6000 8000 10000 12000 14000 16000 (N u r /N u s )/( f r /f s )1/ 3 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 p/e 6 p/e 10 Figure 6: Effect of Reynolds number on Thermo-hydraulic parameter for relative roughness pitch as compare to smooth duct for W-discrete ribs flow arrangement. References [1] Bhagoria, J.L, Saini, J.S., Solanki, S.C., 2002.Heat transfer coefficient and friction factor correlations for rectangular solar air heater duct having transverse wedge shaped rib roughness on the absorber plate. Renewable Energy 25, 341-369. [2] Sahu MM, Bhagoria JL. Augmentation of heat transfer coefficient by using 90 broken transverse ribs on absorber plate solar air heater. Renew Energy 2005;30:2063-75. [3] Prasad BN, Saini JS. Effect of artificial roughness on heat transfer and friction factor in a solar air heater. Solar Energy 1988;41:555-60. 95 American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2016) Volume 16, No 2, pp 90-97 [4] Varun, Saini RP,Singal SK. Investigation of thermal performance of solar air heater having roughness element as a combination of inclined and transverse ribs on the absorber plate. Renewable Energy 2008:33:1398-405. [5] A.M.E. Momin, J.S. Saini, S.C. Solanki, Heat transfer and friction in solar air heater duct with v-shaped rib roughness on absorber plate, International Journal of Heat and Mass Transfer 45 (16) (2002) 3383-3396. [6] Singh S, Chander S, Saini JS. Heat transfer and friction factor correlation of solar air heater ducts artificial roughened with discrete V-down ribs. Energy 2011;36:5053-64. [7] R.Karwa, S.C. Solanki, J.S. Saini, Heat transfer coefficient and friction factor correlations for the transitional flow regimes in rib-roughened rectangular ducts, International journal of Heat Transfer 42 (9) (1999) 1597- 1615. [8] Saini RP, Saini JS. Heat transfer and friction factor correlation for artificially roughened ducts with expanded metal mesh as roughened element. Int J Heat Mass Tran 1997;40:973-86. [9] A. Lanjewar, J.L.Bhagoria, R.M. Sarviya, Experimental study of augmented heat transfer and friction in solar air heater with different orientations of W-Rib roughness. Exp Thermo Fluid Sci 2011;35:986-95. [10] Bopche SB, Tandale MS. Experimental investigation on heat transfer and friction characteristics of a tabulator roughned solar air heater. Int J Heat Mass Tran 2009;67:39. [11] Saini RP, Verma J. Heat transfer and friction factor correlations for a duct having dimple – shaped artificial roughness for solar air heaters. Energy 2008;133:1277-87. [12] Jaurker AR, Saini JS, Gandhi BK. Heat transfer and friction characteristic of rectangular solar air heater duct using rib-grooved artificial roughness. Solar Energy 2006;80:895-907. [13] Karmare SV, Tikekar AN. Heat transfer and friction factor correlation for artificial roughened duct with grit. Int J Heat Mass Trans 2007;50:4342-51. [14] Duffie JA, Beckman WA. Solar engineering thermal process. New York: John Wiley;1991. Appendix 1: Nomenclatures A Area of cross-section, m2 Cp Specific Heat of air at constant pressure Dh Equivalent diameter of duct, D = 4WH /2(W+H) α Rib angle of attack (o) 96 American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2016) Volume 16, No 2, pp 90-97 p Rib pitch, m e Rib height, m f Friction factor fr Friction factor of roughened duct fs Friction factor of smooth duct H Depth of duct L Duct length, m m Mass flow rate, kg/s Nu Nusselt number Nur Nusselt number of roughened duct Nus Nusselt number of smooth duct Re Reynolds number Dimensionless parameters e/Dh Relative roughness height p/e Relative roughness pitch, dimensionless B/S Relative gap width S’/S Relative roughness segment ratio p’/p Relative roughness staggering ratio Greek symbols α Angle of attack (o) 97