Experimental study of evaporation in a tubular solar still
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Experimental study of evaporation in a tubular solar still Document Transcript

  • 1. INTERNATIONALMechanical Engineering and Technology (IJMET), ISSN 0976 – International Journal of JOURNAL OF MECHANICAL ENGINEERING 6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME AND TECHNOLOGY (IJMET)ISSN 0976 – 6340 (Print)ISSN 0976 – 6359 (Online)Volume 4, Issue 2, March - April (2013), pp. 01-09 IJMET© IAEME: www.iaeme.com/ijmet.aspJournal Impact Factor (2013): 5.7731 (Calculated by GISI)www.jifactor.com ©IAEME EXPERIMENTAL STUDY OF EVAPORATION IN A TUBULAR SOLAR STILL Ajeet Kumar Rai*, Vivek Sachan, Bhawani Nandan Mechanical Engineering Department Sam Higginbottom Institute of Agriculture Technology and sciences, Allahabad *Corresponding author e-mail- raiajeet@rediffmail.com ABSTRACT This paper presents the experimental and theoretical work conducted at Allahabad, Uttar Pradesh, India to analyze the performance of a basin type tubular solar still. The tubular cover of the still was made of PVC sheet. Design and fabrication of the setup was done in the premises of SHIATS-DU, Allahabad. Outdoor experimentation has been carried out in the month of April 2012. The objective of the study was to determine a relation for predicting convective and evaporative heat transfer coefficients in a tubular solar still. The temperature dependent physical properties of enclosed vapor were considered. The temperatures and yields obtained were used to determine the values of constants C and n used in the expression Nu = C (Gr.Pr)n. A good amount of distillate was recorded. It was obtained that the proposed model gives closer results with the experimental observation than the model given by Dunkle. The performance of a tubular solar still is improved by 166% than that of a double slope solar still of the same basin area. Key words: heat transfer coefficients, tubular solar still. INTRODUCTION The availability of potable water is a main problem for the communities who live in arid new regions or especially for people in deserts. The availability of high intensity solar radiation in these areas makes the direct use of solar energy a promising option. The solar energy can be utilized to obtain drinking water from salty or brackish water through the use of solar still. Solar distillation is one of the available methods for water distillation. Solar 1
  • 2. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEMEstills of different designs have been proposed and investigated with a view to get greaterdistillate output. A basin type solar still is the most common among conventional solar stills.Many experimental and theoretical studies have been done on single slope solar still [1]. Theoldest, semi- empirical internal heat and mass transfer relation is given by Dunkle [2]. Thento predict the hourly and daily distillate output from the different designs of solar distillationunits, numerous empirical relations were developed. Most of these are based on thesimulation studies. Malik et al. [3] has considered the values of C = 0.075 and n = 0.33 for Gr> 3.2 x 105 as proposed by Dunkle. Clark [4] developed a model for higher operatingtemperature range ( ≥ 55 0C) in a simulated condition for small inclinations of the condensingsurface (β ≤ 15 0C). Clark [4] has observed that the coefficient of convective mass transferbecomes half that given by Dunkle [1]. Tiwari et. al. [5] developed a modified Nusseltnumber, precisely for a trapezoidal cavity, for evaluation of convective mass transfer in asolar distillation. A theoretical expression developed was validated by experiments but onlyfor temperatures greater than 60 0C. Later on Kumar and Tiwari [6] developed a thermalmodel to determine convective mass transfer for different Grashof numbers for solardistillation on a passive and active solar distillation system for only summer climaticconditions. Then Tiwari and Tripathi [7] developed a model for a high temperature range ofthe order of 80 0C but for an opaque, metallic, semi-cylindrical condensing cover made ofAluminium, which is not suitable practically for passive solar distillation in the field. Thecondensing cover developed may be suitable for either active solar distillation or for multi-source distillation units.In this communication, an attempt has been made to evaluate the convective mass transfer bya modified Nusselt number. The modified Nusselt number has been obtained by regressionanalysis using experimental data.EXPERIMENTAL SET-UP A prototype solar still having a horizontal tray which acts as absorber of 0.77 m 2was designed and constructed. Tray was constructed using galvanized iron sheet ofthickness 0.5 mm and later on painted in black. The tray is surrounded by tubular structuremade up of PVC sheet. The total area of the PVC cover is 3.52 m 2. The still is formed by atubular transparent surface made up of PVC sheet. Testing was performed by placing thetubular solar still operating in sunlight for a 24-h period. The work has led to thedevelopment of the tubular solar still and to a technical improvement. In order to achieve themaximum yield from the system, the still orientation should be the direction at which thehighest average incident solar radiation is obtained. Experimental investigation of the tubularsolar still has shown that the productivity of the system was substantially increased incomparison with that of the basin type solar still. The present study was concerned with thedesign of new TSS and development of theoretical models based on evaporation from thewater surface and based on condensation on the inner surface of the tubular cover.Copper – constantan thermocouples are used, along with a digital temperature indicator, torecord the glass temperature, water temperature and water vapor temperature in theexperimental setup. These thermocouples, over a prolonged usage period, tend to deviatefrom the actual temperature. Therefore, they were calibrated with respect to a standardthermometer. A view of the condensing chamber and photograph of the experimental set upare shown in figure1. 2
  • 3. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME Fig.1. Photograph showing experimental setup Tubular solar still model.ANALYSIS OF CONVECTIVE MASS TRANSFER The moist air above the water surface is freely convected to the condensing cover bythe action of a buoyancy force caused by density variation due to the difference between thewater surface and condensing cover. This process within the unit always happens in naturalmode. However the external heat transfer from condensing cover to the atmosphere takesplace outside the still and can either be under the natural or forced mode depending onambient conditions. The rate of heat transfer from the water surface to glass cover ( Qcw ) byconvection in the upward direction through humid fluid can be given by q cw = hcw (Tw − Tg ) (1)The coefficient hcw can be determined form the relation hcw dNu = = C (Gr. pr ) n (2) kThe expression for Gr and Pr are given as 3 2 2Gr = x 3 .ρ f .g.β .∆T µ f (3) C p .µ f Pr = (4) kf 3
  • 4. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME It is clear from the above equation that the value of hcw depends upon the values of twocoefficients namely, C and n. It had been observed from the different values of C and n forgiven models, for a particular range of Grashof number, that experimental and theoreticalvalues closely agree with a reasonable accuracy only for indoor simulation. However, foroutdoor experiments the deviation was more prominent between theoretical and experimentalvalues. Dunkle (1961), gave following expression for hcw for normal operating temperaturerange, 1/ 3  ( p w − p g )(Tw + 273) hcw = 0.884(TW − Tg ) +  (5)  (268.9 × 10 3 − p w ) The expression for hcw cannot be use for the situations not fulfilling conditions i.e. foroperating temperature of 75 °C, spherical, conical and higher inclined solar stills etc. Hencenew values of C and n need to be developed.In the present work, a thermal model will be developed and methodology is given hereunderto evaluate values of C and n. These are found by using experimental data of distillate output(mw), water temperature (Tw) and glass temperature (Tg).Malik et. al. (1982) have assumed that water vapour obeys the perfect gas equation and havegiven the expression for evaporative heat transfer rate (qew) as, qew = 0.0163 hcw ( PW − Pg ) => qew = hew (Tw-Tg) (6)Equation can be further written as K qew = 0.0163( PW − Pg )( )C ( Ra ) n (7) d where Ra = Gr.PrFurther, the rate of distillate output is evaluated by q m ew = ew × 3600 & (8) lEquation (4.6) after substituting qew from Eq. (8) becomes, K 3600 mew = 0.0163( PW − Pg )( )( & )C ( Ra ) n (9) d lThe above equation can be rewritten as, mew = R C ( Ra ) n & (10) 4
  • 5. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME & mew = C ( Ra ) n (11) R K 3600Where, R = 0.0163( PW − Pg )( )( ) (12) d lEquation (11) can be rewritten in the following form Y = aX b (13) & mew Where Y = ; X = Ra ; a = C ; b = n , REquation (13) can be reduced to a linear equation by taking log on both the side ln(Y ) = ln(a ) + b ln( X ) (14) Y = a + b X (15) Y = ln(Y ); a = ln(a ); b = b; X = ln( X ) (16)From Eq. (16), the values of coefficients a′ and b′ are calculated using regression analysis.The expressions for a′ and b′ are given by: N (ΣX Y ) − (ΣX )(ΣY ) b′ = (17) N (ΣX 2 ) − (ΣX ) 2 ΣY ΣX a′ = − b (18) N NWhere N is number of experimental observations.Knowing a′ and b′ from Equation (17) & (18), the value of C and n can be obtained by thefollowing expressions C= exp (a′) and n = b′ (19)The experimental method used is an indirect approach for estimating the convective heattransfer coefficient based on the mass of distillate collected from the still.RESULTS AND DISCUSSION Fig. 2 shows the solar intensity on a particular day in the month of April 2012. Themaximum intensity of 1190 W/m2 is received at 13:45 hrs. Water and cover temperatures arenearly equal at the start of the experimentation, but due to green house effect watertemperature is higher than the cover temperature which is shown in fig 3. 5
  • 6. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME Fig. 2 Variation of solar intensity on 18-04-12 Fig. 3 Variation of Temperature with timeFig.4 shows the variation of convective heat transfer coefficient obtained from present modeland than that of Dunkle model. hcw obtained from present model is higher than that of Dunklemodel. 6
  • 7. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME Fig.4. Variation in convective heat transfer coefficientEvaporative heat transfer coefficient is plotted in fig 5. hew obtained from present model ishigher than that of Dunkle model. It increases with time of heating and starts decreasing assolar flux declines after certain period of time. Maximum value of hew is at 13: 45 hrs. Thevalues of constants calculated for the present model are c = 1.017 and n = 0.210. Fig. 5 Variation in evaporative heat transfer coefficientDistillate output calculated by present model is having their values more closure to thepractical one than calculated using Dunkle model. This difference is due to the assumptionsmade by Dunkle. This is shown in fig.6. So the present model is more accurate to predict theperformance of a tubular solar still than using Dunkle model. Earlier work on double slopesolar still [9,10] has given a maximum daily distillate collected from the same size of basinarea is in the range of 1-2 kg/m2. Whereas from this tubular solar still total distillate collectedfrom 24 hour is 3.2 kg. Instantaneous efficiency is also plotted in fig 7. Maximum value ofinstantaneous efficiency is obtained as 79.63%. The overall efficiency the system is obtainedas 46.35%. 7
  • 8. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME Fig. 6. Variation in distillate output Fig. 7. Variation of instantaneous efficiency 8
  • 9. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEMECONCLUSION An experimental work has been conducted to find the performance of a Tubular solarstill. It is observed that the Dunkle model is not accurate in estimating the performance of asolar still, because of the assumptions made by Dunkle. From the present work it is thereforeinferred that the evaporative heat transfer coefficients are important for designng solardistillation systems. It is also observed that tubular solar still of the proposed design givesbetter results than the double slope slope solar still of the same basin area.REFERENCES 1. Tiwari G. N., Tiwari A, Solar Distillation Practice for Water Desalination Systems, Anamaya, New Delhi. 2. Dunkle, R.V., Solar water distillation: The roof type still and multiple effect diffusion still. Int. Development in Heat Transefer, ASME, Proc. Int. Heat Transfer. Part V, University of Colorado. 1961, P.895. 3. Malik, M.A.S., Tiwari. G.N., Kumar, A. and Sodha, M.S.,Solar Distillation. Pergamon Press Ltd, UK, 1982. 4. Clark, J. A., The steady state performance of a solar still. Solar Energy, 1990, 44, 43. 5. G. N. Tiwari, A. Minocha, P. B. Sharma and K. M. Emran, Simulation of convective mass transfer in a solar distillation process, Energy conv. Mgment., 38(8) (1977) 761- 770. 6. S. Kumar and G.N. Tiwari, Estimation of convective mass transfer in solar distillation system, Solar Energy, 57 (1996) 459-464. 7. G. N. Tiwari and R. Tripathi, Study of heat and mass transfer in indoor condition for distillation, Disalination, 154(2003) 161-169. 8. B. C. Nakra and K. K. Chaudhary, Instrumentation Measurements and Analysis, Ist ed., Tata McGraw Hill, New Delhi, 1985. P. 33. 9. Shukla S.K., and Rai A. K. 2008, Analytical Thermal Modelling of Double Slope Solar Still by Using Inner Glass Cover Temperature, Thermal Science: 12(3) 139-152. 10. Ajeet Kumar Rai, Ashish Kumar and Vinod Kumar Verma, “Effect of water depth and still orientation on productivity of passive solar still”, International Journal of Mechanical Engineering and Technology (IJMET), Volume 3, Issue 2, 2012, pp 740-753. Published by IAEME. 11. Ajeet Kumar Rai, vivek Sachan and Maheep Kumar, “Experimental Investigation of a double slope solar still with a latent heat storage medium”, International Journal of Mechanical Engineering and Technology (IJMET), Volume 4, Issue 1, 2013, pp 22-29. Published by IAEME. 12. Hitesh N Panchal, Dr. Manish Doshi Anup Patel and Keyursinh Thakor, “Experimental Investigation on Coupling Evacuated Heat Pipe Collector on Single Basin Single Slope Solar Still Productivity”, International Journal of Mechanical Engineering and Technology (IJMET), Volume 2, Issue 1, 2011, pp 1 - 9. Published by IAEME. 9