HYDROTHERMAL ANALYSIS OF THE ABSORBER TUBES USED IN LINEAR FRESNEL REFLECTOR SOLAR THERMAL SYSTEM

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1 Proceedings of the 21 st Ntionl & 10 th ISHMT-ASME Het nd Mss Trnsfer Conference Decemer 27-30, 2011, IIT Mdrs, Indi Pper ID: ISHMT_IND_17_034 HYDROTHERMAL ANALYSIS OF THE ABSORBER TUBES USED IN LINEAR FRESNEL REFLECTOR SOLAR THERMAL SYSTEM Sudhnsu S.Shoo* Suneet Singh Rngn Bnerjee Deprtment of Energy Science nd Engineering Indin Institute of Technology, Bomy, Mumi, Indi *Corresponding uthor: ABSTRACT This pper ddresses the hydrotherml modeling nd nlysis in the sorer tue of Liner Fresnel Reflector (LFR) solr therml system. Along with single phse flow nlysis, two phse slip flow model to study het trnsfer nd flow chrcteristics of wter-stem is lso studied. The present model cn e used to predict the vrition of ulk fluid temperture, vrition of net het flux to the fluid in tue, dryness frction vrition in two phse region, vrition of het trnsfer coefficient, pressure loss long the length under different vlues of mss flux s well s the het flux. The developed model cn e considered s n effective tool for sorer tue design nd cn e used to choose pproprite pressure nd mss flux under designed conditions. Keywords: LFR, Solr therml, two phse flow INTRODUCTION For hydrotherml nlysis the LFR sorer tue cn e divided into two prts. One is single phse region nd second one is convective flow oiling two phse region. Generlly, wter t tmospheric temperture enters the tue t high pressure (45 r in our cse) nd gets heted up due to incident het flux from reflective mirrors s shown in Fig. 1. Temperture of the fluid goes on incresing till the sturtion temperture (256 C) of wter t 45 r. The loction t which sturtion point is reched divides the tue into two regions. The region efore the mentioned loction is single phse region nd remining prt of the tue is two-phse region. Flow modeling in solr therml pplictions cn e found in the literture. (Odeh et l., 1998) (Reynolds et l., 2002) (Pye et l., 2004) (Eck et l., 2007). In ll cses, the single phse flow modeling ws found sed on constnt het flux input on the tue. But in solr systems prticulrly, Prolic Trough Solr Collector (PTSC) nd Liner Fresnel Reflector (LFR) systems, vrile net het flux is the sis on which flow occur in the sorer tue. Although incoming solr flux is sme throughout the length, ut due to vrile het loss cross the length, net het flux vries long the length of the receiver tue. We present here description of the stedy stte flow oiling in horizontl tue used in LFR solr system. The overll het loss coefficient ws found from the cvity het loss modeling which is then integrted with convective flow oiling model for the sorer tue to get the desired results. To find the pressure loss long the tue, two phse nlysis with slip flow model hs een used in two phse region. Being simple the model presented elow hs most chrcteristics fetures tht re needed to understnd the flow ehviour in the LFR sorer tues. SETUP DESCRIPTION The Fresnel Reflector system under considertion consists of trpezoidl cvity receiver (Fig.2) filled 1

2 with ir nd it houses eight prllel receiver tues for direct stem genertion, mde of SS304 mteril hving dimensions s given in tle 1. This receiver receives reflected rdition from eight prllel reflectors of 1.8 m width ech long the entire length of reflection. The reflector rrngements re shown in Fig.1 nd the detiled specifictions of the setup re given ppendix 1. The trpezoidl cvity consists of four sides. The ottom cover is mde of glss so s to llow the reflected rys to pss through it nd to minimize convective het loss outside of the cvity. Two side wlls nd top wlls re mde of steel sheets surrounded y thick insultors. Wter, while flowing through the pipes gets heted nd susequently ecomes vpor y receiving het flux from the mirrors plced on the ground. 1. All tues in the cvity receives sme mount of het flux. 2. One dimensionl het trnsfer nlysis is crried out. 3. Sucooled oiling portion hs een ignored. This model ws constructed y dividing the length of the sorer tue into N segments of equl length (Fig.3), with constnt thermodynmic property t the oundry fces. Although, concentrted flux incident on ll segments is constnt, ut, due to vrying het loss in xil direction the net het flux entering the tue vries long the length of the tue. And hence the temperture of the tue vries xilly. But for segment (which is resonly smll), constnt het flux condition my e ssumed. Mss, momentum nd energy lnce hs een pplied to ech segment for deriving necessry equtions. (Collier et l., 1996) (Duffie et l., 1991). Correltion sed pproch hs een pplied for finding overll het loss coefficient for the cvity (Tiwri et l., 1997) (Chpmn 1984), (Singh et l., 2010) which is required for energy lnce in flow nlysis in tue. The detils re omitted for the ske of revity. Figure 1. Collector set up Figure 2. Trpezoidl cvity with 8 tues MATHEMATICAL MODELING For the present hydrotherml nlysis of single sorer tue, following ssumptions re mde Figure 3. Schemtic of the one dimensionl het nd fluid trnsfer nd model. RESULTS AND DISCUSSION The current work focuses on the single phse region nd two phse region length in the sorer tue. Dryness frction vrition in two phse region, pressure drop nd convective het trnsfer coefficient 2

3 vrition in oth regions will e mentioned. The ovementioned dt hs een otined for three vlues of mss flux nmely 200,250,300 kg/m 2 s nd three vlues of Direct Norml Irrdition(DNI) i.e. 700,800,900W/m 2. These vlues re chosen keeping in mind tht dryness frction t exit should not go exceed 0.9. Inlet prmeters were kept constnt for ll clcultions, i.e. Temperture eing 35 C nd Pressure of 45 r. To incorporte het loss from the sorer tues, overll het loss coefficient were otined y considering tue temperture t five different points hving tempertures 75, 125, 175, 225, 275 C. Het loss from the cvity The sorer tues (crrying wter) in the trpezoidl cvity get heted due to the incident concentrted solr rdition. As it does so, it emits long wvelength rdition into the cvity. This rdition results in het loss from the tues. The emitted rdition is sored y inner cvity wlls nd glss cover t the ottom, which in turn rises their temperture. The resulting temperture grdients promote nturl convection within the cvity, which led to convective losses from the tues. Similrly, het losses from the out side glss cover het losses occur due to forced convection nd rdition. Fig.4 shows the plot of overll het loss coefficient s function of the outer surfce temperture of the sorer tue. Bsed on receiver re (sorer tue re), the overll het loss coefficient ws found. As totl het loss (sum of oth rditive nd convective components) increse with temperture, overll het loss coefficient lso increses with the temperture. The reltionship etween overll het loss coefficient nd temperture shown in Fig.4 is used to compute het losses t given temperture in the susequent nlysis (Flow nlysis in the tue). Flow nlysis in the tue Although het influx is sme throughout the length, the net het flux on the tue vries due to vrition in the het losses to the surroundings from the tue outer surfce. As het losses increse with the surfce temperture of the tue, one would expect het losses to vry in the single phse region nd remin constnt in the two-phse region of the tue. It is indeed the cse s shown in Fig.5 nd 5.The xil temperture vrition of fluid in the tue with constnt mss flux nd constnt het flux re shown in Fig.6 nd 6 respectively. As expected, it is seen tht for sme mss flux, nd for sme DNI, the oiling strts t smller length from the entrnce s mss flux decreses. The xil vrition (non dimensionl length) of stem qulity t constnt mss flux nd constnt DNI, re shown in Fig.7 nd7 respectively. It cn e seen tht for higher DNI, stem qulity t the outlet is higher. This is due to the fct tht for more net het flux to the sorer tue, evportion rte increses resulting in higher stem qulity t exit. However, for higher mss flux, more het is required for evportion, nd hence stem qulity decreses t the end of tue with incresing mss flux. Fig.8 nd 8 represent the het trnsfer coefficient vrition with respect to Reynolds numer in the single phse region for vrile DNI nd mss flux, respectively. The Reynolds numer (sed on the dimeter of the pipe) increses long the length of the tue due to decresing viscosity. It cn e seen from the grphs tht, s Reynolds numer increses, the het trnsfer coefficient increses. At given Reynolds numer, het trnsfer coefficient is higher for higher mss flux s well s higher DNI. Overll Het Loss Coefficient (W/m 2 K) U=2.55E-04T Tue Outer wll Temperture (K) Figure 4. Overll het loss coefficient t different tempertures of the sorer tue. w outer 3

4 Net het flux (W/m 2 ) Net het flux (W/m 2 ) Tue length (m) Tue length (m) Figure 5. Net het flux into the tue xilly Men fluid temperture (K) Men fluid temperture (K) Single phse region: vrile het flux Two phse region : constnt het flux Tue length (m) Single phse region: vrile het flux Two phse region : constnt het flux Tue length (m) Figure 6. Axil men fluid temperture Pressure drop (kp) Stem qulity (%) Single phse region: Vrile het flux x/l Two phse region:constnt het flux Figure 7. dryness frction vrition in the two phse region Het trnsfer coefficient (W/m 2 K) Het trnsfer coefficient (W/m 2 K) x/l Single phse region: vrile het flux Reynolds numer Single phse region: vrile het flux () Reynold numer Figure 8.Het trnsfer coefficient vrition w.r.t Reynolds numer Locl het trnsfer coefficient (W/m 2 K) Locl het trnsfer coefficient (W/m 2 K) Two phse region:constnt het flux Stem qulity Two phse region:constnt het flux Stem qulity Figure 9. flow oiling het trnsfer coefficient w.r.t. stem qulity Pressure (kp) pressure (kp) Single phse region:vrile het flux 4400 Two phse region :Constnt het flux Tue length (m) Single phse region: Vrile het flux 4400 Two phse region : Constnt het flux Tue length (m) Figure 10. Axil pressure distriution in the pipeline 4

5 Figure 9 presents the vritions of the clculted het trnsfer coefficients with the verge mss qulity x t three different DNI levels for pressure of 45r nd mss flux of 250kg/m 2 s. The results indicte tht t given qulity the het trnsfer coefficient increses with the increse of DNI. At given het flux the het trnsfer coefficient increses with the increse of stem qulity when the stem qulity is low nd then decreses in high stem qulity rnge s the qulity exceeds certin vlue. It is ecuse s qulity increses, void frction increses nd liquid film thickness ecomes thin nd ccordingly wll superhet decreses. The het trnsfer coefficient decreses with vpor qulity for ll het fluxes. The vritions of the locl het trnsfer coefficient with the stem qulity x t three DNI (200, 250 nd 300 kg/m 2 s) for pressure of 45r nd DNI of 800W/m 2 is shown in Fig.9. It cn e seen tht the het trnsfer coefficient increses with the increse of mss flow rte t given stem qulity. At given mss flow rte the het trnsfer coefficient increses with the increse of the stem qulity. As the stem qulity increses (x >0.5), due to less liquid in contct with wll, het trnsfer coefficient decreses. Fig. 10 nd10 indictes the xil pressure vrition in the whole tue length. In single phse region pressure drop is minly due to friction which is (lmost) independent of temperture nd hence independent of DNI. However, the frictionl pressure drop vries with mss flux significntly nd therefore vrition in the totl pressure drop in single phse region with vrile mss flux cn e seen in Fig. 10. The two-phse pressure drop increses significntly with stem qulity in the two phse region especilly for x > 0.3.At constnt mss flux, pressure drop increses s DNI increses. It is ecuse, the dry ness frction increses in high het flux cses nd frictionl pressure drop increses with respect to the stem qulity. In the constnt het flux cse, pressure drop increses s mss flux increses CONCLUSION Due to the use long tues used in LFR systems, the hydrotherml chrcteristics of LFR systems re unique in their chrcter. Moreover, input rdition distriution on the surfce of the tues s well s het loss chrcteristics re lso different for LFR systems. In order to understnd the ehviour of such systems the stedy stte hydrotherml nlysis of n LFR sorer tue ws studied. The performnce nlysis of single phse region nd two phse region were crried out. The net het flux ws considered vrile in nture in cse of single phse region nd constnt in two phse region. Vrition of men fluid temperture in the flow direction, dryness frction vrition long the two phse region ws found out. Pressure distriution nd convective het trnsfer coefficient were found out nd presented for single phse region nd two phse region. The pressure drops were found to e significntly higher in two phse region s compred to single phse region. It ws seen tht the two-phse pressure drop increses significntly with mss qulity in the rnge of x > 0.3. At constnt qulity, pressure drop increses s DNI decreses nd mss flux increse. Flow oiling het trnsfer coefficient in our cse ws found to e decresing trend fter x >0.5. Further work cn e focused on sucooled or prtil oiling nd three dimensionl flow nlysis for etter understnding of the flow ehviour in LFR systems. ACKNOWLEDGEMENTS We would like to thnk KG Design Services Privte Ltd., Coimtore nd Prnesh Krishnmurthy for providing the LFR setup dt. 5

6 REFERENCES ASHRAE. Hndook of fundmentls, Americn Society of Heting, NewYork. Chpmn, A.,J.,1984. Het trnsfer, Mcmilln Pulishing House, New York. Collier,J.G.,Thome, J.R.,1996. Convective oiling nd Condenstion, 3rd ed. Oxford Science, New York. Duffie, J.A., Beckmn, W.A., Solr Engineering of Therml Processes, second ed. Wiley Interscience, New York. Eck,M.,Uhlig,R.,Mertins,M.,Häerle,A.,Lerchenmüll er, H.,2007. 'Therml Lod of Direct Stem- Generting Asorer Tues with Lrge Dimeter in Horizontl Liner Fresnel Collectors, Het Trnsfer Engineering, 28(1), McLinden, M., O., Klein., S.,A., Lemmon, E.,W., REFPROP,Thermodynmic nd trnsport properties of refrigernts nd refrigernt mixtures, NIST stndrd reference dtse version Odeh,S.D, Morrison, G.L., Behni,M.,1998. Modeling of prolic trough direct stem genertion solr coleectors, Solr Energy, 62(6), Pye, J. D., Morrison, G.L., Mills,D., Le Lievre P, Behni.M.,2004. Stem-circuit modelling of the Compct Liner Fresnel Reflector, ANZSES Solr 2004, Perth, Austrli. Reynolds, D. J., Behni. M.,Morrison, G. L., A Hydrodynmic Model for Line-Focus Direct Stem Genertion Solr Collector, Proceedings of ANZSES Solr 2002, Newcstle, Austrli. Singh, P.L., Srviy,R.M, Bhgori,J.L., Het loss study of trpezoidl cvity sorers for liner solr concentrting collector. Energy Conversion nd Mngement, 51, Tiwri, G.N., Sunej,S., Solr therml engineering systems, Nros Pulishing House, New Delhi. APPENDIX-1 Proposed Specifictions of the LFR System Bottom width of the cvity Top width of the cvity Side length of the cvity Depth of the cvity No of tues in the cvity 8 Asorer tue inner dimeter Asorer tue outer dimeter Asorer length No of reflector mirrors 8 500mm 300mm 141mm 100mm 26.7mm 33.4mm 384m Reflector width 1.8m Positions of reflectors from 1m ground Positions of Cvity from the 13m ground Opticl Efficiency 80% 6

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