Y.C., G.L. Morrison and M. Behnia School of Mechanical and Manufacturing Engineering University of New South Wales Sydney 2052 AUSTRALIA E-mail: yens@student.unsw.edu.au Abstract The characteristics of falling film heat exchangers for pumped-circulation solar water heaters are investigated. Both falling film side and tank side heat transfer correlations have been developed from outdoor product tests. The TYPE 60 tank routine in the TRNSYS model was modified to incorporate the new non-dimensional correlations for predicting the performance of a solar water heater incorporating a falling film heat exchanger. The measurements and modelling show that the film heat exchanger can de-stratify the storage tank when the outlet temperature of the solar collector is below the temperature in the top of the tank. To overcome possible de-stratification, a collector flow rate controller is needed that is able to vary the flow rate to ensure a high collector return temperature relative to the top of the tank. 1. INTRODUCTION Solar water heating systems installed in locations where freezing is a problem require protection against freeze damage in the collector. Even in climates where the night-time temperature is above freezing, the radiation exchange between the collector and a clear cold sky may result in the collector riser tubes being damaged due to freezing. There have been a number of freeze protection techniques used in solar water heaters such as drain-down or drain-back systems, the use of tapered collector riser tubes and passive freeze protection. The most common and effective freeze protection system is to separate the potable water from the fluid in collector loop allowing the use of an antifreeze solution (propylene glycol/water mixture) as the collector fluid. For such systems, a heat exchanger is required between the closed loop collector circuit and storage tank. Numerous collectorloop heat exchanger configurations have been adopted, ranging from conventional shell and tube, inserted coil and side arm heat exchangers. However, they are all expensive to manufacture due to high cost of specially designed tanks that incorporate heat exchangers. Recently, the trend of collector loop heat exchanger design has expanded significantly in the Australian solar domestic water heating system market. A new and innovative collector loop heat exchanger has been proposed for pumped systems, which is known as a falling film heat exchanger. The falling film tank has a core tube down the centre of a water storage tank that is used as the heat exchanger (Figure 1a). The heat exchanger and tank are based on standard gas storage hot water tanks, which are very cheap as they are produced in large numbers. By adapting an existing tank, the manufacturing cost of the solar water heating system is substantially reduced because there is no additional cost for the heat exchanger. The falling film heat exchanger passes the collector loop fluid down the inside of the core tube as a thin film and transfers heat from the closed-loop collector fluid to the potable water in the tank. At night-time, such systems provide a reliable freeze protection because the collector fluid drains back to the core tube. Thus, there is no possibility of freeze damage in the collector riser tube. As the falling film tube extends from the top to the bottom of the storage tank, there is a possibility of de-stratification in the tank if the collector return temperature is less than the temperature in the top of the tank. This could occur in the afternoon when the collector outlet temperature is dropping or if an electric booster is used in the top of the tank.
This paper presents an evaluation of a falling film heat exchanger used for a pumped-circulation solar water heater. The characteristics of the falling film system and measurements of the heat transfer coefficients for the heat exchanger are presented. A TRNSYS model of a solar water heater incorporating a falling film heat exchanger is also outlined. (c) Figure 1 Construction of falling film heat exchanger and storage tank. Flow distributor fitted at top of core tube. (c) View down core tube. 2. FALLING FILM HEAT EXCHANGER The construction of the falling film heat exchanger and tank (270 L) is shown in Figure 1a. A flow distributor with four tangential distributor nozzles at top of core tube (Figure 1b) is used to establish the flow of the collector loop fluid as a thin film over the top inner surface of the core tube. The core tube is welded spirally to maintain the collector loop flow as a film over the core tube (Figure 1c). To evaluate the performance of a pumped-circulation solar domestic water heater with a falling film tank, tests were carried out under outdoor conditions in Sydney, Australia. The falling film tank is coupled to two steel flat plate collectors and 20% propylene glycol/water mixture was used as the working fluid in the closed loop collector and heat exchanger circuit. At the bottom of the core tube, a 200 mm depth of glycol mixture was used to avoid the cavitation in the pump located at the outlet of the core tube. Thermocouples were located both in the collector-heat exchanger circuit and in the core tank to determine the overall heat transfer characteristics of the falling film heat exchanger (Figure 2). Nine thermocouples were also installed on the tank side of the heat exchanger so that the falling film side and tank side heat transfer coefficients would be separately determined. Initially, measurements of conditions in the falling film tank system without a controller were conducted for heat up cycles from an initially cold tank (approximately 20 C in the early morning) and different constant collector flow rates under clear sky conditions. The purpose of these experiments was to develop both the falling-film side and natural-convective tank side mean heat transfer correlations which are required in order to model the long-term system performance using the TRNSYS solar simulation package (Klein, 1996). Clean Energy? Can Do! ANZSES 2006 2
Figure 2 Schematic diagram of monitoring points in the falling film system with a controller. The falling film tank was then operated with a switching controller, which used temperatures at various locations in the system to optimise the system performance. Controller temperature sensors were fitted in the collector outlet (hot sensor) and heat exchanger outlet (cold sensor), and on the outer tank wall. The collector flow rate is varied depending on the temperature rise (T rise ) across the collector loop. When there is available solar energy from the collector, the controller switches on the pump to circulate the hot collector fluid into the heat exchanger. At the start of the heating process, the pump operates with the maximum flow rate to flood the collector loop and then changes to a fixed T rise of approximately 10 K by varying the pump flow rate as shown in Figure 3. If the T rise is less than 3K or the temperature difference between the collector outlet and the tank top (T start ) is less than 5 K (under weak solar radiation conditions), the pump is turned off and the collector fluid is drained back to the core tube. At the same time, the non-filled steel collectors are heated up under stagnation conditions by the solar radiation and store the energy until the circulation pump is turned on again when the T start reaches 6 K. The controller sequences used in the falling film system also ensure a high collector return temperature relative to the top of the tank (T start approximately of 6 K) in order to maintain the thermal stratification in the storage tank. Figure 3 Controller sequences used in the falling film system across a clear day. Clean Energy? Can Do! ANZSES 2006 3
3. HEAT TRANSFER CHARACTERISTICS 3.1. Heat Transfer Rate and Tank Stratification The total heat transfer rate (Q & ) in a falling film heat exchanger can be evaluated from measurements of flow rate and temperature change across the heat exchanger, and expressed as: Q & = mc & (T T ) (1) p i o Figure 4a shows the useful thermal energy transferred from the collector fluid to the potable water in the tank for a pumped-circulation solar domestic water heater incorporating a falling film heat exchanger operated at constant flow rate under clear sky conditions in December. The system operated from an initially isothermal tank condition (25 C) with a constant collector flow rate of 2.7 L/min. Throughout the heat up cycle, it can be observed in Figure 4b that thermal stratification in the tank developed in the tank up to solar noon. In the afternoon a slight de-stratification can be seen in the top of the tank. For the same falling film tank system with a controller that regulates the collector flow rate as outlined in section 2 the results in Figure 5 shows that the tank stratification is maintained in the afternoon. Figure 4 Daily performance test of a falling film system without a controller from an initially isothermal tank under clear sky conditions (270 L tank, 3.6 m 2 solar collector, flow rate of 2.7 L/min). Total heat transfer rate. Tank temperature distribution. Figure 5 Daily performance test of a falling film system with a controller from an initially isothermal tank under clear sky conditions (270 L tank, 3.6 m 2 solar collector, varying flow rate of 2.5 to 3.2 L/min). Total heat transfer rate. Tank temperature distribution. For typical operation of a solar water heater, the top half of the tank may be heated by an electric boosting element or the tank may be stratified under clear sky conditions in the morning, and the system subsequently operates under cloudy conditions in the afternoon. In this case, the hot layer in the top half of the tank can be de-stratified by the cooler collector fluid. Figure 6a shows a falling film Clean Energy? Can Do! ANZSES 2006 4
tank system with a controller operating under cloudy conditions with an initially preheated top half of the tank. Under cloudy conditions, the controller monitored the collector outlet temperature and the tank top temperature and varied the collector flow rate so that the tank stratification was maintained as shown in Figure 6b. Figure 6 Daily performance test of a falling film system with a controller from an initially preheated top half tank under low solar radiation conditions (270 L tank, 3.6 m 2 solar collector, varying flow rate of 2.5 to 3.2 L/min). Operating conditions. Tank temperature distribution. The energy transfer capacity of a falling film heat exchanger can be characterised by the overall heat transfer coefficient (U) based on the log-mean temperature difference between the heat exchanger fluid and the tank water ( Δ T ) as follows: where lm Q& U = A Δ ΔT lm T lm (Ti = T t, 1 ) (T (Ti T ln (To T For a collector flow rate of 1.6 L/min, the overall heat transfer coefficient for the falling film heat exchanger (without a controller) was found to be 200 W/m 2 K to 250 W/m 2 K (Figure 7) under constant flow charging conditions. The corresponding falling film heat transfer coefficient and the natural convection coefficient on the tank side were determined based on the measured inner wall temperatures of the core tube and the tank core temperatures as shown in Figures 8a and 8b. These results show that the heat transfer coefficient on the film side is a factor of two to three times higher than the heat transfer coefficient on the tank side. As a result, the limiting factor on the performance of the heat exchanger is primarily the natural convection circulation in the storage tank. Measurements of the heat transfer characteristics were also conducted for collector flow rates from 1.6 L/min to 3.6 L/min. The overall heat transfer characteristics of the heat exchanger were found to be very similar due to the limitation of the natural convection heat transfer in the storage tank (Figure 7). For the falling film heat exchanger system with a controller, the overall heat transfer coefficient was also found to be in the range of 200 W/m 2 K to 250 W/m 2 K (Figure 7). In this paper, the development of heat transfer correlations for the falling film system is based only on measurements in with constant collector flow rate. o t, 1 t, 9 T ) ) t, 9 ) (2) Deleted: different 3.2. Falling Film Heat Transfer Correlation The long-term performance of a pumped-circulation solar water heater incorporating a falling film heat exchanger can be predicted using the TRNSYS solar simulation package. The detailed system simulation requires heat transfer correlations for the falling film covering the inner surface of the core tube and the natural convection circulation in the storage tank to compute the heat transfer from the collector fluid to the tank. The majority of the reported empirical falling film heat transfer correlations Clean Energy? Can Do! ANZSES 2006 5
are only applicable to gas-liquid flow in which the interfacial shear stress between the liquid film and gas stream is taken into account. Falling film heat exchangers have been widely used in coolers and evaporators. For the thin liquid film with an absence of a gas stream, there are only a limited number of previous analytical studies available on the flow and heat transfer in the single-phase falling film. Figure 7 Overall heat transfer coefficient of a falling film heat exchanger. Figure 8 Falling film heat transfer coefficient as a function of average film temperature. Heat transfer coefficient on the tank side as a function of average tank core temperature. Analytical solution of thin liquid film An analytical solution for the liquid film thickness (δ) was reported by Bird et al. (2002) for a smooth surface and laminar flow as given in equation (3). In this paper, the mean film thickness in the falling film heat exchanger is evaluated using equation (3). Several assumptions have to be made as following: 1) The film covers the entire inner surface of the core tube. 2) The flow is laminar. 3) As the core tube radius is large compared to the film thickness, the fully developed film thickness can be approximated to that of an infinitely wide wall. 4) The drag effect at the free surface of the film is neglected due to the absence of a gas stream. 5) The possibility of wavy flow is neglected. Under these assumptions the film thickness is given by equation (3). 3 μq δ = 3 (3) ρgb For the falling film heat exchanger, the Reynolds number (Re D ) is defined by equation (4), which is based on the hydraulic diameter (D) of the film and volumetric flow rate. Clean Energy? Can Do! ANZSES 2006 6
where, Q = volume flow rate in falling film (m 3 /s) μ = dynamic viscosity (kg/m.s) b = width of film (m) = πd g = gravitational acceleration (m 2 /s) d = diameter of core tube (m) = 197mm ρ = density (kg/m 3 ) 4ρQ Re D = (4) μb The Nusselt number (Nu δ ) for a thin liquid falling film with short contact times reported by Bird et al. (2002) is given in equation (5). 3 1 Nuδ = δ 2 1 3 Γ ( 4 3)( 9βH ) (5) where, H = height of core tube (m) Γ (4/3) = gamma function = 0.89297 2 β = μk ( ρ Cpgδ ) Comparison of analytical solution and measurements For collector flow rates from 1.6 L/min to 3.6 L/min, the measured Reynolds number (Re D ) is in range 100 to 600. The mean Nusselt number ( Nu f ) in a falling film heat exchanger based on equation 5 is compared to the experimental measurements in Figure 9. The analytic solution for Nu f from equation 5 is based on the theoretical film thickness (equation 3) whereas the measured Nu f is based on the hydraulic diameter of the wetted film (D = 4δ). If the analytic data for Nu f from equation 5 is scaled up four times, the predicted Nu f given by the analytical correlation (equation 5) over-estimates the measured values (in the range of 0.25 to 0.4) because the analytical solution is only valid for a short contact time falling film. Figure 9 Comparison of mean Nusselt number as a function of Reynolds number for analytical solution for laminar film flow and measurements. For the falling film heat exchanger in this study, a new mean Nusselt number correlation was developed by regression-fit to the measured data as given in the following equation, 0. 34 D 0. 33 Nu = 0. 102Re Pr (6) f for 5 < Pr < 13 and 100 < Re D < 600, with properties calculated at the average film temperature (20% propylene glycol/water mixture). Figure 8 also shows that the measured data of the falling film system when the flow controller is used matches the values determined from the steady flow testing. Clean Energy? Can Do! ANZSES 2006 7
3.3. Tank Side Heat Transfer Correlation On the tank side, the flow is driven by buoyancy forces, which distribute the heat over the height of the tank. The standard means of correlating natural convection heat transfer processes is to relate the Nusselt number (Nu t ) to the Rayleigh number (Ra) as the following non-dimensional form: n Nu t = CRa H (7) where, 3 gβδth Pr Ra H = 2 ν β = thermal expansion coefficient ΔT = temperature difference between the heat transfer surface and the tank contents (K) H = height of storage tank (m) = height of core tube C, n = constants The variation of measured mean Nusselt number ( Nu t ) with Rayleigh number (Ra H ) based on the tank height during heat up cycle testing is shown in Figure 10. The flow on the tank side of the outer surface of the falling film heat exchanger is observed to be turbulent. Natural convective heat transfer between the tank wall and the tank contents can be approximated by the vertical isothermal flat plate correlation (Cengel, 1998): 0 1 H For turbulent flow, Nu =.1Ra 3, 10 9 Ra H 10 13 (8) t As seen in Figure 10, the conventional turbulent correlation agrees with the measured data in the range 10 11 Ra H 10 12. Figure 10 Tank side measured Nusselt number and empirical turbulent isothermal vertical flat plate function versus Rayleigh number. 4. ANNUAL SYSTEM PERFORMANCE MODELLING The annual performance of a pumped circulation solar water heater incorporating a falling film heat exchanger requires the non-dimensional heat transfer correlations for the falling film side and the tank side. Models of heat exchangers in hot water storage tanks are available in system modelling programs such as TRNSYS (Klein, 1996) and T*SOL (Valentin 2004). The TRNSYS heat exchanger model (TYPE 60 stratified tank routine) is written in general with the tank side heat transfer coefficient specified in the standard Nusselt number as a function of Rayleigh number (eqn. 8) and the heat transfer coefficient on the collector side modelled using standard pipe flow correlations. For this study, the heat transfer coefficient on the collector side of the heat exchanger has been modified to include the new falling film heat transfer correlations obtained from measurements as given in equation 6. Clean Energy? Can Do! ANZSES 2006 8
Figure 11 Schematic diagram of TRNSYS model. Film nozzles located at the top of the core tube. Film nozzles located below the position of the auxiliary element. A TRNSYS model (Figure 11) based on correlations given by equations (6) and (8) was used to provide initial design sensitivity data for the falling film solar water heater. For a pumped system operating without an advanced controller, the falling film heat exchanger transfers heat to all sections of the tank over the depth of the film established in the core tube. If the film is started at the top of the core tube, then heat may be extracted from the top of the tank while adding heat to the bottom of the tank. If an in-tank electric boosting element is used and located in the middle of the tank, the position of the film nozzles should ideally be set below the position of the electric element. For daily and seasonal load conditions specified in Australian Standard AS4234, the simulations (Figure 12) indicate that the monthly energy savings of the falling film system for standard domestic hot water demand in Sydney (latitude of -34 ) are affected by the location of the film nozzles. The performance simulation also shows that the annual energy saving for a direct-coupled system is approximately 20 % higher than a falling film system with the film nozzles located below the position of the electric element. The performance of the falling film system with the specialized controller that avoids thermal destratification should give significantly better performance than the constant flow rate system performance shown in Figure 12. Figure 12 Energy savings for a falling film system for film nozzles positions shown in Figure 10 and a direct-coupled system. Clean Energy? Can Do! ANZSES 2006 9
5. CONCLUSION The heat transfer characteristics of a falling film heat exchanger used for a pumped-circulation solar water heater have been investigated under different outdoor and initial tank conditions. For the falling film system without a controller, the measured overall heat transfer coefficient was found to be 200-250 W/m 2 K for flow rates of 1.6 L/min to 3.6 L/min. Although the falling film side heat transfer coefficient is comparatively high, the factor limiting the performance of the heat exchanger is primarily the buoyancy driven circulation in the storage tank. Measurements also showed that thermal stratification in the storage tank could be maintained under low solar radiation conditions by a controller that modulates the collector flow. Non-dimensional heat transfer correlations of the falling film heat exchanger were determined based on measurements of the system operating with constant flow rate. The mean Nusselt number correlation for the falling film side was developed as a function of Reynolds number and Prandtl number, from a regression-fit to the measured data. For the natural convection tank side, the empirical turbulent isothermal vertical plate correlation was found to fit the observed data. A TRNSYS model of a falling film system was developed on the basis of the nondimensional correlations obtained from measurements. The annual performance of a falling film solar water heater with constant collector flow rate was shown to be effected by de-stratification if an electric booster was used in the top of the tank. It is anticipated that this can be overcome by modulating the collector flow rate so that the collector return temperature is always higher than the top of the tank.. 6. ACKNOWLEGMENT The falling film heat exchanger solar water heater evaluated in this paper was provided by Rheem Australia. 7. REFERENCES 1. AS4232 (1994), Solar water heaters-domestic and heat pump-calculation of energy consumption, Standard Australia. 2. Bird R.B, Stewart W.E and Lightfoot E.N (2002), Transport phenomena, Wiley. 3. Cengel, Y.A. (1998), Heat transfer: a practical approach, McGraw-Hill, USA, pp. 418. 4. Klein S.A et al. (2001), TRNSYS Version 15 User Manual, University of Wisconsin Solar Energy Laboratory. 5. Knudsen, S. and Furbo, S. (2004), Thermal stratification in vertical mantle heat exchangers with application to solar domestic hot-water systems, Applied Energy, 78, 257-272. 6. Morrison, G.L., Rosengarten, G. and Behnia, M. (1999), Mantle heat exchangers for horizontal tank thermosyphon solar water heaters, Solar Energy, 67, 53-64. 7. Rosengarten, G., Morrison, G.L. and Behnia, M. (2001), Mixed convection in a narrow rectangular cavity with bottom inlet and outlet, International Journal of Heat and Fluid Flow, 22, 168-179. 8. Shah L.J. (2000), Heat transfer correlation for vertical mantle heat exchangers, Solar Energy, 69, 157-171. 9., Y.C. and Morrison, G.L. (2005), Performance of a solar water heater with a vertical mantle heat exchanger, in Proceeding of ANZSES Annual Conference, December, Dunedin, New Zealand. 10. Valentin (2004), T*SOL Energies software. Clean Energy? Can Do! ANZSES 2006 10