International Journal of Applied Engineering Research ISSN 0973-4562 Volume 3, Number 8 (2008), pp. 1091–1103 © Research India Publications http://www.ripublication.com/ijaer.htm
Experimental Studies on Heat Transfer and Friction Factor Characteristics of Thermosyphon Solar Water Heater System Fitted with Left-Right Twisted Tapes S.Jaisankara, T.K.Radhakrishnanb,*, K.N.Sheebac and S.Sureshd a,b,c
Department of Chemical Engineering, National Institute of Technology, Tiruchirappalli-620015, Tamil Nadu, India d Department of Mechanical Engineering, National Institute of Technology, Tiruchirappalli-620015, Tamil Nadu India E-mail:
[email protected] * Corresponding Author. E-mail:
[email protected]
Abstract Experimental studies on tubes fitted with Left - Right twisted tape inserts of various twist ratios to enhance the convective heat transfer rate are studied for thermosyphon solar water heater system. The swirl flow is induced by LeftRight twisted tape in clockwise and counterclockwise direction inside the riser tube which enhances the heat transfer. The various parameters such as Nusselt number, friction factor and thermal efficiency are studied. Empirical correlations developed for Nusselt number and friction factor for various LeftRight twist ratios (Y=3, 4, 5 and 6) are found to fit with the experimental data within ±6.36 % and ±5.69% respectively. The result shows that thermal performance of tube fitted with Left-Right twisted tape collector is better than the plain tube collector. The increased thermal performance of twisted tape collector is due to the introduction of swirl flow. Keywords: thermosyphon, Left-Right twist tape, Swirl flow, Reynolds number, friction factor, Nusselt number.
Introduction Solar water heaters are one of the predominant technologies for the extraction of solar thermal energy. Thermosyphon solar water heaters have been the most attractive option for hot water compared to forced circulation systems, because of ease of
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operation without the aid of external pumping and less maintenance cost. Hence, commercialization of thermosyphon solar water heating system must always focus on the improvement in its efficiency. Operating characteristics of thermosyphon solar water heaters contribute to the efficiency of the system to a greater extent, because the performance of the system is based on the mass flow rate of collector, absorber plate temperature and temperature rise of fluid from inlet to outlet [1]. The performance of thermosyphon solar water heaters varies significantly with absorber plate efficiency and loss factors. Unlike other parameters which greatly rely on the operating conditions like input energy and flow properties, plate efficiency factors were found to be dependent on the design of the system[2]. Mathematical derivations which are useful for the prediction of performance and comparison of different collectors were also worked out [3]. In addition to the design, constructional features of the system, flow properties of the working fluid have a major role to play over the thermal performance of the system. Experiments were conducted using different types of refrigerants as working fluid in two phase closed thermosyphon solar water heater system under various environmental conditions [4]. The heat transfer phenomena in solar water heaters can be enhanced by the use of twists inserted in the flow path of fluid inside the riser tubes which induces swirl flow. Hence twisted tapes act as turbulent promoters thereby improving the thermal performance of the system [5]. Various forms of turbulent promoters including twisted tapes, wire coils, ribs, axially supported discs were experimented for heat transfer augmentation. Among these Kenics twisted tapes were found to have pronounced effect on the heat transfer enhancement in laminar flow. Accordingly experiments were performed by using Kenics twisted tapes [6]. Furthermore, the performance of the Kenics static mixer was analyzed experimentally using Laser Doppler Anemometry and numerically using Lattice Boltzmann method and FLUENT software [7]. Investigation of heat transfer and friction factor characteristics of circular tube fitted with Right-Left helical screw inserts of equal and unequal length of different twist ratio had been reported by [8].Although different types of heat augmentation devices used in a fluid passage had been found to increase the heat transfer, no studies had been reported on heat transfer enhancement by these devices for thermosyphon water heating system. Therefore, the present investigation has been aimed at enhancing the heat transfer performance by using Left-Right twisted tapes in thermosyphon solar water heating system. Experimental Setup and Procedure The experimental setup consists of flat plate collector of 1 m2 aperture area connected with a well insulated storage tank of 100 liters capacity and named as plain tube collector as shown in Figure 1a. The cold water from the storage tank enters the collector from the lower header and is evenly distributed in the riser tubes. The riser tubes are brazed to the bottom of a black absorber plate and the absorbed solar energy is conducted to the riser tubes. The heat is transferred by convection from the tube wall to the fluid. Finally the hot water is collected from the upper header and stored in the insulated storage tank. The temperature difference in storage tank accelerates the
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driving force and the cycle is repeated until the temperature difference between the inlet and outlet of water is zero.
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Figure 1(a): Solar water heater (plain tubes) 1.Water inlet to collector 2.Absorber Plate Temperature 3.Absorber plate 4.Upper Header 5.Magnetic flow meter
7.Riser Tube Temperature 8.Wooden sides 9.Riser Tube 10.Lower Header ∆- Taps for measuring pressure drop
6.Glass wool supported by aluminium frame
O- Taps for measuring water inlet& outlet Temperature
Note: All Dimensions are in mm
Figure 1b: Solar water heater with twisted tapes
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Figure 1c: Cross section of the solar water heater (plain tube) A single transparent glass cover of 3mm thickness transmits the solar energy to the absorber plate. The collector and the pipe connections are well insulated to minimize the heat losses. Absorber plate, riser tubes and headers are made up of copper. Taps are provided to measure inlet and outlet temperature of water, absorber plate temperature, riser tube temperature and pressure drop in each riser tube. The twisted tape collector has the same design and dimensions as that of the plain tube collector and is fitted with Left-Right twisted tapes in the riser tubes as shown in Figure 1b.The cross sectional details of the plain tube collector are shown in Figure 1c. The detailed technical specifications of the collector and storage tanks are listed in Table 1. The twists are made from thin, flat strips of copper. It has a thickness of 0.3 mm and diameter of 11 mm. The twists are twisted through 1800 to form helices with twist ratio (length to diameter) of 3 to 6. Left and Right helices are connected alternatively for a length of 1000 mm and are named as Left-Right twist. Figure 2 shows the LeftRight twists for twist ratios 3 to 6.
Figure 2: Left-Right twisted tapes with various twist ratios 3,4,5 and 6 The work is carried out in Centre for Energy and Environmental Science & Technology (CEESAT) UK-India-REC project located at National Institute of Technology, Tiruchirappalli, India. Data were continuously recorded from the month of March to May 2007.Both the plain tube and tube fitted with Left-Right twisted tape collectors are kept in outdoor condition facing south direction with a tilt angle of 180. The experiment is carried out for the entire day. The storage tank is completely drained in the evening after the experiment and is refilled with clean tap water in the early morning. Calibrated RTD PT100 type temperature sensors of 0.10C accuracy are used to measure inlet, outlet, plate, ambient and tube temperature and is stored in Yokogowa temperature recorder. Flow rate in the collector loop is determined by
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magnetic flow meter. Solar radiation is measured by Kipp and Zonnen pyranometer and pressure drop inside the riser tubes by Yokogowa differential pressure Transducer. Characteristics of Thermosyphon Solar Water Heater with Plain Tube The characteristics of thermosyphon solar water heating system with plain tube, for typical sunny days are depicted in Figures 3a and 3b. The solar radiation is found to increase gradually and reach a maximum at 1.00 p.m. and later decrease gradually till 4.00 p.m. Mass flow rate also exhibits a similar trend. 1200.00
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(3b) Figures 3a, 3b: Characteristics of a thermosyphon solar water heater system (plain tube) Inlet and outlet temperature of water increases gradually until both the temperatures are equalized. The observation of this characteristic is divided into two phases. 9.00 a.m. to 1.00 p.m. is considered Phase 1 and 1.00 p.m. to 4.00 p.m. is considered Phase 2.
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Calculations The Nusselt number, friction factor and thermal performance are calculated for Phase 1 and Phase 2. Heat Transfer The heat transfer rate in the single riser tube is calculated using the fundamental equation [9]. (1) Q = mcp(Tout-Tin)=UoAo(Two-Tm) where 1/(UoAo)=1/(hiAi)+ln(Do/Di)/(2πkwL) (2) The internal convective heat transfer coefficient hi is determined by combining Equations (1) and (2). The experimental Nusselt Number is calculated by Nu=hiDi/k. All the thermo physical properties of the fluid are determined at the bulk mean temperature, Tm. Pressure drop The pressure drop for each riser tube over the length is measured by differential pressure transducer, from which the average pressure drop and the friction factor are calculated. f =(Di/L)(2ΔP/ρum2) (3) where ΔP is the pressure drop over the length L. Thermal Performance The thermal performance of the solar water heater is calculated using the[10] equation as shown η= FR(τα)-FRUL(Ti-Ta)/HT (4) The heat removal factor (FR), transmittance-absorptance product (τα) is calculated[11].The experimental uncertainty is calculated by [12]and[13] and error is within ±5.8%, ±8.0%, ±7.2% and ±9.7% for Reynolds number, friction factor, Nusselt number and efficiency respectively using uncertainty analysis. Results and discussion The results obtained in this study are presented, compared with available equations in the literature and suitable correlations are developed.
Heat transfer Plain tube collector The Nusselt Number for plain tube collector is calculated using the Sieder-Tate equation [14] for Phase1 and Phase 2. Nu=1.86 (Gz)1/3(μ/μw )0.14
(5)
The experimental Nusselt number is fitted with the above equation for Phase 1 and Phase 2 with the discrepancy of less than ±5.41%
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Effect of Left-Right Twist on Heat Transfer Enhancement The variation of Nusselt number with Reynolds number for the tube fitted with Left- Right twist of various twist ratios (Y=L/D) are as shown in Figures 4a and 4b. Nusselt number increases with increase in Reynolds number for various Left-Right twists and are compared to plain tube collector in Figure 4a. Nusselt number for tube fitted with Left- Right twist is higher than that for plain tube for a given Reynolds number. This is because of the fact that the periodic change and tangential direction of fluid flow magnifies the intensity of swirl in radial direction for every 33mm (Twist length 33 mm) and increases the hydraulic length for fluid flow. Hence the fluid particle mixing is more which causes an increase in Nusselt number for Left-Right twisted tape collector. 25
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Figure 4a: Variation of Nusselt number with Reynolds number for Left-Right twists of various twist ratios (Phase 1) 25
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Figure 4b: Variation of Nusselt number with Reynolds number for Left-Right twists of various twist ratios (Phase 2)
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It can also be inferred from Figure 4a, that Nusselt number is higher for twist ratio 3 for a given Reynolds number due to the increased intensity of swirl generation. As the twist ratio increases swirl generation gradually decreases thus minimizing the heat transfer. The decrease in Nusselt number with Reynolds number in Phase 2 is shown in Figure 4b. In Phase 2 the swirl flow decreases gradually due to decrease in solar intensity which minimizes the convective heat transfer. The following empirical correlations are developed for Phase1 and Phase 2 for various twist ratios. Nus =1.430 (Re)0.411 (Pr) -0.196 (Y) -0.103 (6) 1.263 0.357 0.073 Nus =0.00118 (Re) (Pr) (Y) (7) Where Nus is Nusselt number for twisted tapes. The fitted values of Nusselt number from Equations(6) and (7) are compared with the experimental values and are found to agree with the experimental data within ±3.89% and ±6.36 %.
Friction Factor Plain Tube Collector The experimental friction factor which is fitted with the Fanning equation for Phase1 and Phase 2 for plain tube collector and the discrepancy of less than ±14.97%. Effect of Left- Right Twist on Friction Factor The variation of friction factor with Reynolds number for Left -Right twist with different twist ratio are compared with plain tube collector as shown in Figures 5a and 5b. As evident from Figure 5a, the friction factor decreases with increase in Reynolds number. 0.14
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Figure 5a: Variation of Friction factor with Reynolds number for Left-Right twists of various twist ratios (Phase 1)
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Figure 5b: Variation of Friction factor with Reynolds number for Left-Right twists of various twist ratios (Phase 2) Also the Reynolds number for a twisted tape system is higher than that of a plain tube system because of the increase in surface area that enhances the fluid velocity simultaneously increasing the Reynolds number for a twisted tape system. Similarly the friction factor for Left- Right twist collector is higher than that for plain tube collector for a given Reynolds number, because of the flow mixing effects caused by the tangential, clockwise and counterclockwise movement of fluid and increased wetted surface area. Hence the velocity increases which affects the pressure loss of fluid flow near the tube wall. In twist ratio 3, the effect of mixing and wetted surface area is more. Hence, the friction factor for ratio 3 is higher and gradually decreases with increase in Reynolds Number. As the twist ratio increases the friction factor decreases.The observation from Figure 5b indicates that the friction factor increases with decrease in Reynolds number in Phase 2. This is due to the fact the solar radiation decreases gradually so that the swirling effect also decreases which would lead to an increase in friction factor. For various Y ratios, friction factor is compared with Reynolds number and correlations are developed for Phase1 and Phase 2 as follows: ft = 1.058 (Re)-0.291 (Y)-0.132 ft = 386.40 (Re)-1.068 (Y)-0.305
(8) (9)
Where ft is friction factor for twisted tapes. The fitted values of friction factor from Equations (8) and (9) are compared with the experimental values and are found to agree with the experimental data within ± 3.53% and ±5.69%.
Thermal Performance The heat transfer enhancement in Left-Right twisted tape collector is better than plain tube collector. The comparison of instantaneous efficiency of the twisted tape and plain tube collectors with local time is shown in Figure 6. In Phase 1, the efficiency of plain tube and twisted tape collectors increases gradually because of the heat
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enhancement. In Phase 2 the efficiency of all collectors decreases due to the decreased effect of heat transfer. It may also be noted from the above figure that the efficiency for plain tube collector is always lower than that for twisted tape collectors. 80
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Figure 6: Variation of efficiency with local time for various Left-Right twists and plain tube collector The efficiency for twist ratio 3 is higher than that for other twist ratios. If the twist ratio increases, the efficiency of the collector decreases. The swirl created is maximum in twist ratio 3 and hence the heat transfer effect is also maximum compared to other twists, which decreases as the twist ratio increases. The flow direction of fluid in the Left-Right twisted tape collector is in clockwise and counterclockwise with tangential direction. This increases the surface contact area and the particle mixing effect. Hence the fluid property changes which in turn accelerates the swirl generation and mass flow rate and increases the velocity thereby increasing the heat transfer and heat removal factor. 85
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Figure 7: Effect of twist ratio on average plate temperature in solar water heaters. The average plate temperature as a function of intensity of solar radiation in twisted tape collectors as well as plain tube collector is shown in Figure 7. The
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average value of plate temperature has been found to be higher in case of plain tube collectors and gradually decreases with decrease in twist ratio Y for the same value of solar intensity. Higher value of plate temperature in plain tube collectors shows that it is inefficient in convective heat transfer resulting in more heat losses leading to lower value of thermal efficiency. The lower value of parameter Y leads to lower value of plate temperature which consequently leads to higher value of thermal efficiency because of the effective convective heat transfer coefficient.
Conclusion The heat transfer and friction factor characteristics of the system with Left-Right twisted tape and plain tube collector has been studied. It is found that the enhancement of heat transfer in the Left-Right twisted tape system is higher than the plain tube collector. Experimental results for plain tube collector compared with available literature shows that the discrepancy within ±5.41% and ±14.97% for Nusselt number and friction factor respectively.The Left-Right twisted tape creates swirl flow in clockwise and counterclockwise direction in laminar regime which increases the heat transfer rate. As with all turbulent promoters, when the twist ratio decreases, the heat transfer rate and the pressure drop increases and hence the thermal performance also increases. This is verified by comparing with the plain tube collector under the same operating conditions. Table 1: Physical dimensions of solar water heating system Sl No
Design materials/parameters (a) Collector 1. Tilt Angle 2. Aperture area Ac 3. Collector glazing 4. Lower Header 5. Upper Header 6. Riser tubes 7. Absorber plate 8. Bottom Insulation 9. Side Insulation 10. absorptivity of absorber plate 11. Transmittance of glazing 12. Number of riser tubes b) Storage tank and piping
Specifications of materials 18o (South facing) 1m2 Single transparent glass of 3mm thickness ID 25.4mm ID 25.4mm OD 12.5mm, ID 11mm, Length 1000mm Width 120 mm, Length 1000mm 100mm glass wool 50 mm glass wool covered by aluminum frame 0.9 0.9 9
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Horizontal 100 litres 4 mm 50 mm ID 25.4mm 20 mm
Omenclature Ac Ai Ao Cp Di Do f ft FR Gz Ht hi k kw L m Nu Nus Pr Q Re Ta Tm Tin Tout Two um Ui Uo UL Y
Collector aperture area, m2 Inside surface area of the riser tube, m2 Outside surface area of the riser tube, m2 Specific heat, kJ/kg oC Inside diameter of riser tube, m Outside diameter of riser tube, m Friction factor for plane collector, dimensionless Friction factor for twisted tape collector, dimensionless Collector heat removal factor, dimensionless Graetz Number, dimensionless Total solar radiation, W/ m2 Average convective heat transfer coefficient W/ m2 oC Thermal conductivity of water, W/m oC Thermal conductivity of the riser tube wall, W/m oC Length of the riser tube, m Mass flow rate kg/s Nusselt number for plane riser tube, dimensionless, Nu = hi Di / k Nusselt number for twisted tape inserted riser tube (Swirl flow) Prandtl number, dimensionless Pr= Cpμ/k Heat transfer rate, W Reynolds number based on the internal diameter of the riser tube, dimensionless Ambient temperature, oC bulk mean temperature of fluid in the riser tube oC Average inlet temperature of water, oC Average outlet temperature of water, oC Average wall surface temperature outside riser tube section, oC Bulk average water velocity, m/s Overall inside heat transfer coefficient, W/ m2 oC Overall outside heat transfer coefficient, W/ m2 oC Overall heat loss coefficient, W/ m2 oC Twist ratio (length of one twist /diameter of the twist), dimensionless
Greek Letters
ρ μ μw
Density of water ,kg/ m3 Dynamic viscosity of water at bulk mean temperature ,Ns/ m2 Dynamic viscosity at wall temperature ,Ns/ m2
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Pressure drop of water, N/ m2 Transmittance-Absorptance product Collector efficiency
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[2]
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Zerrouki, A., Boumedien, A., Bouhadef, K., 2002, “The natural circulation solar water heater model with linear temperature distribution,” Renewable energy, 26, pp. 549-559. Bliss, R.W., 1959, “The Derivations of Several “plate-efficiency factors” useful in the design of flat-plate Solar Heat Collectors,” Solar Energy, 3, pp. 55-64. Parker, B.F., 1981, “Derivation of efficiency and loss factors for solar air heaters,” Solar Energy, 26, pp. 27-32. Esen, M., Esen, H., 2005, “Experimental investigation of a two-phase closed thermosyphon solar water heater,” Solar Energy, 79, pp. 459-468. Kumar, A., Prasad, B.N., 2000, “Investigation of twisted tape inserted solar water heaters-heat transfer, friction factor and thermal performance results,” Renewable Energy, 19, pp. 379-398. Joshi, P., Nigam, K.D.P., Nauman, E.B., 1995, “The Kenics Static Mixer: New Data and Proposed Correlations,” Chem.Eng.Journal, 95, pp. 259:265. Van Wageningen, W.F.C., Kandhai, D., Mudde, R.F., Van den Akker, H.E.A., 2004 “Dynamic flow in a Kenics Static mixer: An Assessment of various CFD Methods,” AIChE Journal, 50, pp. 1684-1696. Sivashanmugam, P., Nagarajan, P.K., 2007, “Studies on heat transfer and friction factor characteristics of laminar flow through a circular tube fitted with right and left helical screw-tape inserts,” Experimental Thermal and Fluid Science, 32(1), pp. 192- 197. Liau, Q., Xin, M.D.,2000, “Augumentation of convective heat transfer inside tubes with three- dimensional internal extended surface and twisted tape inserts,” Chem.Eng.Journal, 78, pp. 95-105. Hottel, H.C., Whillier, A., 1958, “Evaluation of flat plate collector performance,” Trans. Conf. on the use of solar energy, University of Arizona press, 2(1), pp. 74-104. Duffie, J.A., Beckman, W.A., 1974, “Solar Energy Thermal Processes”. Wiley- Interscience, New York. Coleman, H.W., Steele, W.G., 1989, “Experimental and Uncertainty Analysis for Engineers”. Wiley ,NewYork. ANSI/ASME, 1986, “Measurement uncertainty”, PTC 19.1-1985. Sieder, E.N, Tate, G.E., 1936, “Heat transfer and pressure drop of liquids in tubes,” Ind Eng. Chem., 28 , pp. 1429–1439.
