6340(Print), ISSN (Online) Volume 4, Issue 2, March - April (2013) IAEME AND TECHNOLOGY (IJMET)

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1 INTERNATIONAL Iteratioal Joural of Mechaical JOURNAL Egieerig OF MECHANICAL ad Techology (IJMET), ENGINEERING ISSN 0976 AND TECHNOLOGY (IJMET) ISSN (Prit) ISSN (Olie) Volume 4, Issue 2, March - April (2013), pp IAEME: Joural Impact Factor (2013): (Calculated by GISI) IJMET I A E M E EXPERIMENTAL STUDY OF EVAPORATION IN A TUBULAR SOLAR STILL Ajeet Kumar Rai*, Vivek Sacha, Bhawai Nada Mechaical Egieerig Departmet Sam Higgibottom Istitute of Agriculture Techology ad scieces, Allahabad *Correspodig author - raiajeet@rediffmail.com ABSTRACT This paper presets the experimetal ad theoretical work coducted at Allahabad, Uttar Pradesh, Idia to aalyze the performace of a basi type tubular solar still. The tubular cover of the still was made of PVC sheet. Desig ad fabricatio of the setup was doe i the premises of SHIATS-DU, Allahabad. Outdoor experimetatio has bee carried out i the moth of April The objective of the study was to determie a relatio for predictig covective ad evaporative heat trasfer coefficiets i a tubular solar still. The temperature depedet physical properties of eclosed vapor were cosidered. The temperatures ad yields obtaied were used to determie the values of costats C ad used i the expressio Nu = C (Gr.Pr). A good amout of distillate was recorded. It was obtaied that the proposed model gives closer results with the experimetal observatio tha the model give by Dukle. The performace of a tubular solar still is improved by 166% tha that of a double slope solar still of the same basi area. Key words: heat trasfer coefficiets, tubular solar still. INTRODUCTION The availability of potable water is a mai problem for the commuities who live i arid regios or especially for people i deserts. The availability of high itesity solar radiatio i these areas makes the direct use of solar eergy a promisig optio. The solar eergy ca be utilized to obtai drikig water from salty or brackish water through the use of solar still. Solar distillatio is oe of the available methods for water distillatio. Solar 1

2 stills of differet desigs have bee proposed ad ivestigated with a vi to get greater distillate output. A basi type solar still is the most commo amog covetioal solar stills. May experimetal ad theoretical studies have bee doe o sigle slope solar still [1]. The oldest, semi- empirical iteral heat ad mass trasfer relatio is give by Dukle [2]. The to predict the hourly ad daily distillate output from the differet desigs of solar distillatio uits, umerous empirical relatios were developed. Most of these are based o the simulatio studies. Malik et al. [3] has cosidered the values of C = ad = 0.33 for Gr > 3.2 x 10 5 as proposed by Dukle. Clark [4] developed a model for higher operatig temperature rage ( 55 0 C) i a simulated coditio for small icliatios of the codesig surface (β 15 0 C). Clark [4] has observed that the coefficiet of covective mass trasfer becomes half that give by Dukle [1]. Tiwari et. al. [5] developed a modified Nusselt umber, precisely for a trapezoidal cavity, for evaluatio of covective mass trasfer i a solar distillatio. A theoretical expressio developed was validated by experimets but oly for temperatures greater tha 60 0 C. Later o Kumar ad Tiwari [6] developed a thermal model to determie covective mass trasfer for differet Grashof umbers for solar distillatio o a passive ad active solar distillatio system for oly summer climatic coditios. The Tiwari ad Tripathi [7] developed a model for a high temperature rage of the order of 80 0 C but for a opaque, metallic, semi-cylidrical codesig cover made of Alumiium, which is ot suitable practically for passive solar distillatio i the field. The codesig cover developed may be suitable for either active solar distillatio or for multisource distillatio uits. I this commuicatio, a attempt has bee made to evaluate the covective mass trasfer by a modified Nusselt umber. The modified Nusselt umber has bee obtaied by regressio aalysis usig experimetal data. EXPERIMENTAL SET-UP A prototype solar still havig a horizotal tray which acts as absorber of 0.77 m 2 was desiged ad costructed. Tray was costructed usig galvaized iro sheet of thickess 0.5 mm ad later o paited i black. The tray is surrouded by tubular structure made up of PVC sheet. The total area of the PVC cover is 3.52 m 2. The still is formed by a tubular trasparet surface made up of PVC sheet. Testig was performed by placig the tubular solar still operatig i sulight for a 24-h period. The work has led to the developmet of the tubular solar still ad to a techical improvemet. I order to achieve the maximum yield from the system, the still orietatio should be the directio at which the highest average icidet solar radiatio is obtaied. Experimetal ivestigatio of the tubular solar still has show that the productivity of the system was substatially icreased i compariso with that of the basi type solar still. The preset study was cocered with the desig of TSS ad developmet of theoretical models based o evaporatio from the water surface ad based o codesatio o the ier surface of the tubular cover. Copper costata thermocouples are used, alog with a digital temperature idicator, to record the glass temperature, water temperature ad water vapor temperature i the experimetal setup. These thermocouples, over a prologed usage period, ted to deviate from the actual temperature. Therefore, they were calibrated with respect to a stadard thermometer. A vi of the codesig chamber ad photograph of the experimetal set up are show i figure1. 2

3 Fig.1. Photograph showig experimetal setup Tubular solar still model. ANALYSIS OF CONVECTIVE MASS TRANSFER The moist air above the water surface is freely covected to the codesig cover by the actio of a buoyacy force caused by desity variatio due to the differece betwee the water surface ad codesig cover. This process withi the uit always happes i atural mode. However the exteral heat trasfer from codesig cover to the atmosphere takes place outside the still ad ca either be uder the atural or forced mode depedig o ambiet coditios. The rate of heat trasfer from the water surface to glass cover ( Q ) by covectio i the upward directio through humid fluid ca be give by q = h T T ) (1) ( w g The coefficiet h ca be determied form the relatio h d Nu ) k = = C( Gr. pr (2) The expressio for Gr ad Pr are give as r ρ f. g. β '. T f G = x µ (3) C p.µ f Pr = (4) k f 3

4 It is clear from the above equatio that the value of h depeds upo the values of two coefficiets amely, C ad. It had bee observed from the differet values of C ad for give models, for a particular rage of Grashof umber, that experimetal ad theoretical values closely agree with a reasoable accuracy oly for idoor simulatio. However, for outdoor experimets the deviatio was more promiet betwee theoretical ad experimetal values. Dukle (1961), gave followig expressio for h for ormal operatig temperature rage, 1/ 3 ( pw pg )( Tw + 273) h = ( TW Tg ) + 3 (5) ( pw ) The expressio for h caot be use for the situatios ot fulfillig coditios i.e. for operatig temperature of 75 C, spherical, coical ad higher iclied solar stills etc. Hece values of C ad eed to be developed. I the preset work, a thermal model will be developed ad methodology is give hereuder to evaluate values of C ad. These are foud by usig experimetal data of distillate output (m w ), water temperature (T w ) ad glass temperature (T g ). Malik et. al. (1982) have assumed that water vapour obeys the perfect gas equatio ad have give the expressio for evaporative heat trasfer rate (q ) as, q h ) => q = h (T w -T g ) (6) = ( PW Pg Equatio ca be further writte as K q = ( P W Pg )( ) C( Ra) (7) d where Ra = Gr.Pr Further, the rate of distillate output is evaluated by q m& = 3600 (8) l Equatio (4.6) after substitutig q from Eq. (8) becomes, K 3600 m & = ( PW Pg )( )( ) C( Ra) (9) d l The above equatio ca be rritte as, m& = R C ) ( Ra (10) 4

5 m& R = C( Ra) (11) K 3600 Where, R = ( PW Pg )( )( ) (12) d l Equatio (11) ca be rritte i the followig form b Y = ax (13) m Where Y = & ; X = Ra; a = C; b =, R Equatio (13) ca be reduced to a liear equatio by takig log o both the side l( Y ) = l( a) + b l( X ) (14) Y + ' ' ' ' = a b X (15) ' ' ' ' Y = l( Y ); a = l( a); b = b; X = l( X ) (16) From Eq. (16), the values of coefficiets a ad b are calculated usig regressio aalysis. The expressios for a ad b are give by: N ( ΣX ' Y ') ( ΣX ')( ΣY ') b = (17) 2 2 N( ΣX ' ) ( ΣX ') ΣY ' ΣX ' a = b' (18) N N Where N is umber of experimetal observatios. Kowig a ad b from Equatio (17) & (18), the value of C ad ca be obtaied by the followig expressios C= exp (a ) ad = b (19) The experimetal method used is a idirect approach for estimatig the covective heat trasfer coefficiet based o the mass of distillate collected from the still. RESULTS AND DISCUSSION Fig. 2 shows the solar itesity o a particular day i the moth of April The maximum itesity of 1190 W/m 2 is received at 13:45 hrs. Water ad cover temperatures are early equal at the start of the experimetatio, but due to gree house effect water temperature is higher tha the cover temperature which is show i fig 3. 5

6 Fig. 2 Variatio of solar itesity o Fig. 3 Variatio of Temperature with time Fig.4 shows the variatio of covective heat trasfer coefficiet obtaied from preset model ad tha that of Dukle model. h obtaied from preset model is higher tha that of Dukle model. 6

7 Fig.4. Variatio i covective heat trasfer coefficiet Evaporative heat trasfer coefficiet is plotted i fig 5. h obtaied from preset model is higher tha that of Dukle model. It icreases with time of heatig ad starts decreasig as solar flux declies after certai period of time. Maximum value of h is at 13: 45 hrs. The values of costats calculated for the preset model are c = ad = Fig. 5 Variatio i evaporative heat trasfer coefficiet Distillate output calculated by preset model is havig their values more closure to the practical oe tha calculated usig Dukle model. This differece is due to the assumptios made by Dukle. This is show i fig.6. So the preset model is more accurate to predict the performace of a tubular solar still tha usig Dukle model. Earlier work o double slope solar still [9,10] has give a maximum daily distillate collected from the same size of basi area is i the rage of 1-2 kg/m 2. Whereas from this tubular solar still total distillate collected from 24 hour is 3.2 kg. Istataeous efficiecy is also plotted i fig 7. Maximum value of istataeous efficiecy is obtaied as 79.63%. The overall efficiecy the system is obtaied as 46.35%. 7

8 Fig. 6. Variatio i distillate output Fig. 7. Variatio of istataeous efficiecy 8

9 CONCLUSION A experimetal work has bee coducted to fid the performace of a Tubular solar still. It is observed that the Dukle model is ot accurate i estimatig the performace of a solar still, because of the assumptios made by Dukle. From the preset work it is therefore iferred that the evaporative heat trasfer coefficiets are importat for desigg solar distillatio systems. It is also observed that tubular solar still of the proposed desig gives better results tha the double slope slope solar still of the same basi area. REFERENCES 1. Tiwari G. N., Tiwari A, Solar Distillatio Practice for Water Desaliatio Systems, Aamaya, N Delhi. 2. Dukle, R.V., Solar water distillatio: The roof type still ad multiple effect diffusio still. It. Developmet i Heat Trasefer, ASME, Proc. It. Heat Trasfer. Part V, Uiversity of Colorado. 1961, P Malik, M.A.S., Tiwari. G.N., Kumar, A. ad Sodha, M.S.,Solar Distillatio. Pergamo Press Ltd, UK, Clark, J. A., The steady state performace of a solar still. Solar Eergy, 1990, 44, G. N. Tiwari, A. Miocha, P. B. Sharma ad K. M. Emra, Simulatio of covective mass trasfer i a solar distillatio process, Eergy cov. Mgmet., 38(8) (1977) S. Kumar ad G.N. Tiwari, Estimatio of covective mass trasfer i solar distillatio system, Solar Eergy, 57 (1996) G. N. Tiwari ad R. Tripathi, Study of heat ad mass trasfer i idoor coditio for distillatio, Disaliatio, 154(2003) B. C. Nakra ad K. K. Chaudhary, Istrumetatio Measuremets ad Aalysis, I st ed., Tata McGraw Hill, N Delhi, P Shukla S.K., ad Rai A. K. 2008, Aalytical Thermal Modellig of Double Slope Solar Still by Usig Ier Glass Cover Temperature, Thermal Sciece: 12(3) Ajeet Kumar Rai, Ashish Kumar ad Viod Kumar Verma, Effect of water depth ad still orietatio o productivity of passive solar still, Iteratioal Joural of Mechaical Egieerig ad Techology (IJMET), Volume 3, Issue 2, 2012, pp Published by IAEME. 11. Ajeet Kumar Rai, vivek Sacha ad Maheep Kumar, Experimetal Ivestigatio of a double slope solar still with a latet heat storage medium, Iteratioal Joural of Mechaical Egieerig ad Techology (IJMET), Volume 4, Issue 1, 2013, pp Published by IAEME. 12. Hitesh N Pachal, Dr. Maish Doshi Aup Patel ad Keyursih Thakor, Experimetal Ivestigatio o Couplig Evacuated Heat Pipe Collector o Sigle Basi Sigle Slope Solar Still Productivity, Iteratioal Joural of Mechaical Egieerig ad Techology (IJMET), Volume 2, Issue 1, 2011, pp 1-9. Published by IAEME. 9

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