HEAT TRANSFER ENHANCED PARABOLIC TROUGH RECEIVER FOR DSG WITH CAPILLAR SYSTEMS

Transcription

1 HEAT TRANSFER ENHANCED PARABOLIC TROUGH RECEIVER FOR DSG WITH CAPILLAR SYSTEMS Rojas, M.E. Parabolic Trough Technology Group, PSA / CIEMAT, Av Complutense,, 8040 Mari, Spain Phone: ; Fax: , rojas@ciemat.es Abstract This paper gives some lights in the use of capillary pressure structures -porous coatings an microgroove channels- in enhancing heat transfer in horizontal steam generating channels, like solar parabolic trough receivers for Direct Steam Generation (DSG). On one han, porous coating moel has been simplifie by reucing the number of involve parameters. In previous analysis, the porous coating macroscopic characteristics, i.e., permeability, capillary raius an porosity, were impose in ifferent ranges with no connection. Several possible relations between them are shown epening on the porous meium configuration. These relations reuce in playing with parameters for rewetting pipes uner DSG conitions, resulting to be critical to etermine the configuration of the porous system or, what is the same, to check which of these relations ajust to experimental ata. On the other han, the improvement of wetting by capillary rise has been checke experimentally in a microgroove sample. Water temperatures from 100ºC up to 50ºC an the corresponing equilibrium pressures- have been covere. Data measurements showe an increase in wetting angle up to 00% against the non-improve pipe. Such improvement is higher at lower temperature range (100-00ºC) an lower at higher temperature range (up to 300ºC). 1 INTRODUCTION The benefits of substitution of thermal oil with water for electricity prouction in solar parabolic trough fiels have been pointe out in several papers (CIEMAT et al., 1997). It means to work with a water two-phase flow, which thermohyraulic characteristics have to be taken into account for the esign of the solar fiel. The nee of covering a wie range of flow masses to cope with variable solar insolation, makes to work uner low mass flows of water an steam, proviing an insufficient cooling of the horizontal pipes, when stratification occurs. Reuction of the influence region of such unesirable flow pattern in the flow map has been looke for by two methos: reucing hyraulic iameter by isplacers (Geskes, 1996) or irectly, combining seconary stage concentrator (CIEMAT et al., 1997) an changing the receiver pipe inclination (CIEMAT et al., 1997). Another way of assuring a goo cooling of the pipe receiver, although the regime flow is not change, is introucing internal capillary pressure structures -porous coatings an microgroove channels- (Rojas an Ajona, 1998). The introuction of such systems at pool boiling has alreay showe their high efficiency in heat transfer enhancement an in increasing the thermal stability of vertical an small iameter tubes (Webb, 1994; Malyshenko, 1996). But there is still a lack in funamental investigations of the hyroynamics an heat transfer in horizontal steam generating channels. In this sense, insie an european JOULE project (DISSphase II), several investigations are taking place. PHYSICAL MODELS Moel simulations for capillary systems are necessary to fin out the optimal system characteristics to rewet the internal absorber pipe uner stratifie DSG conitions. Initially, this is a multiphase flow problem, where the liqui film isplaces its vapour, but it can be treate like a single-phase flow problem, without losing accuracy, taking into account that the vapour velocity relate to the liqui is negligible an that the vapour pressure is constant in its efinition omain (Dullien, 199; Scheiegger, 1957). Therefore, the macroscopic moel is reuce to the momentum balance equation for the liqui film, where the inertial, gravitational, pressure graient, liqui-liqui viscous an soli-liqui viscous forces are taken into account.. 1 Porous Coating Moel Formulation For porous systems such an equation is known as the Darcy Brickman-Forchheimer equation. The mathematical form of this equation is ifferent epening on the simplifications assume. The normal simplification consiere is that the liqui-liqui viscous forces are negligible, because all the liqui is suppose to be at the same temperature. Washburn, Van Brekel & Heatjes (Dullien, 199) an Peng et al. (Peng an Peterson, 199) neglect, also, the inertial forces, while Szekeley et al (Dullien, 199), o consier them from observing a small initial liqui meniscus acceleration. The mathematical expressions for these inertial forces are usually given by experimental ajustments. The soliliqui viscous forces can be obtaine by the e Hagen- Poiseulle molecular flow equation (Cervantes e Gotari,

2 1983) or by the use of Darcy permeability concept (Peng an Peterson, 199; Bear, 199). The moel use in this analysis neglects inertial forces an uses the Darcy permeability coefficient for soli to liqui viscous forces. It consiers that the velocity of the liqui film is moifie when an external heat flux is applie 1. The preliminary results that have been alreay expose (Rojas an Ajona, 1998) impose the macroscopic characteristics of the porous coating, i.e., permeability, capillary raius an porosity, in ifferent ranges with no connection between them, leaing to a wie parameter matrix. It woul be esirable to fin out the relations between permeability, K, an capillary raius, r c, to some parameters well controlle by manufacturing processes, as porosity,, an particle size, s, can be.. Permeability Relations To establish the functional epenence between permeability an these mentione parameters two ways can be followe: From experimental ata (phenomenological moels). Although these approaches are base on experimental ajustments, theoretical consierations an aimensional analysis are use as well. Most of experimental relations foun in the literature are suitable for porous bes. Thus, Rumpf an Gupte state, for porous bes with spherical particles with a maximum raius relation of 7 an within a porosity range of 0.35 an 0.7, that s The most famous relation is the Ergun-Kozeny relation, for porous bes, 3 s C(1 ) where C can vary between 150 (Ergun s moel) an 180 (Kozeny s moel) From fixing the geometrical pattern of the porous system, i.e., the geometrical configuration of the system. The system can be consiere a channels group insie which the ifferent phases move. These channels can be straight tubes with constant cross section (capillaric moels) or with a ranom orientation (statistical moels). Both type moels work well for low porosity values. The other form to consier the system is by a simple (Dullien, 199) or complex (Du Plessis, 199) soli tube association, circulating the phases externally. Du Plessis, (Du Plessis, 199), istinguishe between consoliate, 1 Not all the authors consier such a change in liqui film velocity (Malyshenko, 1994) granular (Figure 1) an D porous meia. The permeability values he obtaine are: T cons cons consoliate conf. 4.69(1 T cons ) T 1 1 T granular conf. 36(1 T ) where T cons an T gran are the tortuosity for each configuration an gran an cons a geometrical parameter relate to s in a ifferent way for each geometrical pattern (see Figure 1) K/s² s (a) soli Du Plessis Consoliate Conf. Du Plessis Granular Conf. Ergun's Moel Kozeny's Moel cons Rump&Gupte's Moel Porosity s soli Figure 1: Porous meia configurations: (a) consoliate; (b) granular In Figure all the mentione correlations are shown. It shoul be stresse that is the Du Plessis consoliate configuration that gives higher permeability values. (b) Figure : Correlations comparison for permeability with particle size an porosity.3 Hyraulic Raius Relations In analogy with the establishe practice in hyraulics the capillary raius -or equivalent hyraulic raiusgoverning the flow rate through a porous system is assume to be twice the hyraulic raius, efine as the gr

3 flow cross-sectional area ivie by the wette perimeter. It means for porous systems twice the voi volume of meium ivie by the surface area of pores in the meium, (Dullien, 199), i.e., r c = r hyraulic = voi volume soli contact surface This analogy makes possible to relate capillary raius with porosity an porous particle size, epening on the porous meium configuration use. For granular an consoliate configuration the resulting hyraulic raius are shown in Figure 3. 3 EXPERIMENTAL SET UP. An experimental set up has been esigne for measuring the wetting capability of pipe samples with integrate capillary system -porous coate or microgroove- uner pool boiling DSG conitions, Figure 4, (Rojas an Ajona, 1998). It is base on the evelop of a thermal graient in the pipe ue to the existence of ry an wet regions. By comparing the theoretical cross-sectional thermal graient, ue to liqui rise by thermal water expansion, an the experimental thermal graient, where the wetting angle is higher ue to capillary forces, an improvement inex is obtaine. 6 5 Consoliate Conf. Granular Conf. (1) (1) (8) rh/s (7 () (3) (6) Porosity Figure 3: Correlations comparison for hyraulic raius with particle size an porosity (4) (5).4 Solar DSG-LS3 Design Introucing the relations for permeability an capillary raius in the porous coating moel for a LS3 receiver esign higher liqui film velocity, with no heat flux applie, is observe for consoliate porous meia than or granular configuration. It means that, although the hyraulic raius in consoliate meia is higher than in granular configuration, working against capillary rise, the improvement in permeability values compensates an even excees such an effect. To have a higher unheate case liqui film velocity gives that, for the same porous coating thickness, the maximum heat flux applicable is higher. With these relations the permeability an hyraulic raius ranges have been reuce, not been possible to combine ranom values that coul fix better for solar DSG conitions, as has been one up to now. Thus, for example, with a 10 µm particle size an a porosity of 0.7, if the porous coating is consiere as a consoliate system, it woul be possible to rewet the entire inner surface pipe (50 mm), but if it was similar to a granular configuration only the 5% of the pipe woul be rewette. It makes necessary to check experimentally the configuration followe by the porous. It is expectable to be given by the manufacturing process Figure 4: Schematic escription of measurement set up. (1)-feethroughs. ()-sample positioning bar. (3)-pipe sample. (4)-liqui water. (5)-pressurise chamber. (6)- thermocouples an secon power supply wires. (7)-first power supply heaters. (8)-security tare valve 3.1 Testing a microgroove sample A microgroove sample has been teste (Figure 5). It is a carbon steel tube (50/70 mm inner/outer iameter) with equilateral V-shape grooves (60º), machine into the inner surface with a milling machine. The epth grooves is 0.89 mm an the microgroove group has an apex angle of 0º. Seven thermocouples are fixe 7 mm in epth to the mile cross section of the sample in ifferent angular positions (Figure 5). The experiments were conucte for water over a wie range of pressure an temperature pool boiling conitions. The water base level has been chosen to just rise the inner surface pipe uner ambient conitions. When pressure an temperature stationary conitions were reache in the autoclave, for at least 0 minutes, the secon power supply was on. The transition from wet to ry zones is observe by a temperature step between thermocouples. The secon power voltage is as high as to observe a temperature step between 3 an 5 egrees. The

4 secon power heating voltage was recore to etermine the heat supplie to the test section. In Figure 6 the improvement obtaine for wetting angle in function of temperature by capillary rise is shown. Thermocouple Positioning -36º 36º 0º 7º -7º 90º 0º n Power Supply Wire by a small an low change in time in autoclave temperature, ue to the influence of secon power supply in water base liqui temperature. In any case, the step temperature is kept constant, although the mean temperature rises a little bit. From this figure it can be observe that for autoclave temperatures over 175ºC, the pipe is rewet up to the 80% of the whole perimeter. Although for lower temperatures the perimeter is rewet up to 60%, the improvement in the wet perimeter percentage is in 30 an 50%, except for 50ºC, where the improvement is just the 1%. For this temperature the improvement is lower, as was expecte by the moel, because the maximum heat sink capability of the liqui rise film is fixe by the capillary rise uner ambient conitions. The higher is the secon supply power, the earlier the liqui film water is evaporate. For increasing such a capillary rise capability the grooves characteristics have to be change. Experiments with other test samples will give the optimal configuration, although, just now, with this sample, it can be sai that there is an important behaviour improvement. 4 CONCLUSIONS Microgroove surface Figure 5: Equippe sample for measuring thermal angular graient uner pool boiling. Thermocouple positioning an secon power supply external spring The wetting angle error bans in experimental ata are given by the angular separation between thermocouples, fixe to 36º, while the temperature error bans are given Statement of geometrical pattern of porous coatings are necessary if it is intene to reuce their optimal properties to a few well-controlle parameters for manufacturing. in heat transfer enhancement an increasing thermal stability for horizontal steam generating channels. Although theoretical correlations can be applie, the optimal parameter for rewetting a horizontal pipe (in our case a pipe an heat flux istribution for LS3 esign) change in such a range for ifferent configurations that experimental checking with a real porous coate sample is unavoiable. Once the correlations are verifie, it woul be easy to fix the optimal parameter values. The rewetting capability by capillary forces in a microgroove sample, uner pool boiling, has been Wetting angle (º); 90º=top; - 90º=bottom Figure 6: Theoretical versus measure wetting angle Autoclave Temperature (ºC) measure theoretical

5 assure experimentally. Even when the microgroove esign is not optimise, an improvement in wet perimeter percentage is between 30% an 50%, maintaining in most the times the 80% of the perimeter wet. 5 REFERENCES 1. Bear, J. Moelling Transport Phenomena in Porous Meia. Heat an Mass Transfer in Porous Meia. Elsevier Science Publishers B.V., 199. Cervantes e Gotari, J. El caloriucto: un noveoso ispositivo para transmitir calor eficientemente. Revista e la Acaemia Nacional e Ingeniería, pp , Mexico, (1983) 3. CIEMAT; ZSW; CONPHOEBUS; INETI. Avance Receivers For Direct Solar Steam (ARDISS). Final Report, Contract N. JOU- CT Du Plessis, J.P. Pore-Scale Moelling for flow trough ifferent types of porous enviroments. Heat an Mass Transfer in Porous Meia. Elsevier Science Publishers B.V., Dullien, F.A.L. Porous Meia. Flui Transport an Pore Structure. Acaemic Press, Inc (199) 6. Geskes, P. Flow Patterns in an Annular Geometry of an Absorber Pipe with DSG by Water Injection. Convective Flow Boiling, Taylor &Francis (1996), Malyshenko, S.P, Boiling phenomena in Direct steam generators with porous coatings. Interim Report, IVTAN, Moscow (1996) 8. Peng, X.F.; Peterson, G.P. Capillary-inuce rewetting in a flat porous cover layer Int. J. Heat Mass Transfer, V.35, N., pp , (199) 9. Rojas, M.E. Ajona, J.I. Thermal graient avoiance uner stratifie two-phase flow in DSG by capillar structures, 9th International Symphosium on Solar Concentrating Technologies. Oeillo (France). June Scheiegger, A. The Physics of Flow through Porous Meia University of Toronto Press, Webb, R.L. 'Principles of Enhance Heat Transfer'.John Wiley & Sons,Inc. 1994