Numerical thermal analysis of helical-shaped heat exchanger to improve thermal stratification inside solar storage tank

Transcription

1 Ref: C0152 Numerical thermal analysis of helical-shaped heat exchanger to improve thermal stratification inside solar storage tank M. Imtiaz Hussain and Gwi Hyun Lee*, Department of Biosystems Engineering, Kangwon national university, Chuncheon-si, Gangwon-do, , Republic of Korea *Corresponding author: Tel: ; Fax: Abstract Thermally stratified solar storage tank is crucial to improve the efficiency of solar thermal energy system because of unstable behavior of energy source like solar radiation, in daily routine operation of heating load. This study presents the numerical transient analysis of helical-shape immersed heat exchanger to verify the effect of different operating conditions on thermal stratification and performance of solar storage tank. Different volumetric flow rates and inlet temperatures at heat exchanger side have been investigated to illustrate the impact of the variation of these parameters for the developed temperature field inside storage tank. The helical shape of the heat exchanger has shown good agreement with gravity and buoyant effects. The level of thermal stratification was increased by the reduction of entrainment effect by natural convection along the helical axis. The impact of this design of helical-shape heat exchanger on the degree of stratification was analyzed for flow rate of 2 to 6 L/min and inlet temperature of 45 to 65 ºC at a constant initial temperature of 25 ºC. Results indicate that, high and low flow rates do have adverse impact on the stratification process, but initial temperature difference does not have major effect on thermocline formation. In addition to this numerical parametric study can be used as an effective tool to optimize the whole thermal energy storage system at early design stage. Keywords: Transient analysis, Thermal stratification, Parametric study, Numerical simulation 1. Introduction Thermal stratification in thermal storage tanks has a noticeable effect on their efficiency and can use for a long time in low to medium applications. Andras (2013); investigated the new tube-in-tube helical flow distributor design to improve temperature stratification in hot water storage tank. The configuration of inlet devices improves the distribution of flow in the thermal storage tank and increases the efficiency and performance by restraining the mixing induced by water inflow (Shah & Furbo, 2003; Shin et al., 2004; Bahnfleth & Song, 2005; Hegazy, 2007; Chung, Cho, Tae & Yoo, 2008). Furbo, Andersen, Thur, Shah and Andersen (2005); analyzed the thermal performance of solar storage tank by changing the draw-off levels along the length instead of one fixed position and suggested a best position for drawoff level. To visualize the effects of tank geometry and operating conditions on the thermal stratification within a storage tank, seven three dimensional models have been numerically simulated by using the computational fluid dynamics program (ANSYS Fluent) with realistic boundary and initial conditions applied (Simon & Wenxian, 2009). Altuntop, Kilik, Ozceyhan Proceedings International Conference of Agricultural Engineering, Zurich, /7

2 and Kincay (2006); shown that the Reynolds number should be kept well below the transition region (<2100) to ensure laminar flow and to promote good thermal stratification. A model approach for the computational calculation of the time-dependent temperature distribution in stratified storage tanks based on the one-dimensional heat transport equation is presented by Steinert, Go ppert and Platzer (2013). In the present work, parametric numerical analysis of helical-shaped heat exchanger tank was carried out by varying the inlet flow rate and temperature of thermal fluid. Effect of operating conditions on thermal stratification in the storage tank was investigated by using ANSYS Fluent CFD software. Optimal design and operational parameters that support thermal stratification process were identified and analyzed. 2. Materials and methods 2.1 Methodology In order to analyze the temperature distribution along the length of thermal storage tank, a numerical simulation was carried out by using ANSYS Fluent 14.0 software. The grid independence test was performed to reduce the computational cost without compromising the accuracy of the solution. The three dimensional numerical analysis in a thermal tank is explained by using the Navier-strokes and temperature Equations from (1) to (5) as follows: ( ) ( ) ( ) (1) [ ( ) ] (2) [ ( ) ] (3) [ ( ) ] (4) [ ( ) ] (5) Where: r (m), (degree) and z (m) are the coordinates in the radial, tangential and veetical directions, (m/s), (rad/s), (m/s) are the velocity components in these directions, p (Pa) is the pressure, g (m/s 2 ) is the gravitational acceleration in the z direction, T (K) is temperature, (kg/m 3 ), (m 2 /s), and (m 2 /s) are the density, kinematic viscosity, and thermal diffusivity of fluid, respectively. 2.2 Numerical Procedure and boundary conditions In the present study, fluid solid interaction FSI heat transfer model is used by defining tank water as fluid and copper tube as solid in ANSYS design modular and then combines whole body as one part for simulation. Different convergence criteria of 10-3 and 10-6 for momentum and energy equations were defined in ANSYS solver. Some assumptions were considered during numerical modeling as follows: the temperature difference across the copper tube thickness is assumed to be negligible; the mode of heat transfer between copper tube and inside water is convection in nature. The adiabatic boundary condition was used around the all walls of the storage tank. The computational domain and meshing model have been created by using Pro-E wildfire and ANSYS ICEM software as shown in Figure 1. Proceedings International Conference of Agricultural Engineering, Zurich, /7

3 3. Results and Discussions The impact of inlet temperature variation of thermal fluid on the thermal stratification of storage tank at constant initial temperature was performed for three inlets fluid temperatures of 45, 55 and 65 ºC. The storage tank was charged at a constant flow rate of 2 L/min with a starting temperature of 25 ºC. Figures 2 to 4 show the temperature profile of the storage tank water by varying the inlet temperature of the working fluid at fixed flow rate of 2 L/min. As we can see, at low temperature difference corresponding to initial temperature there is very stable thermal stratification with no mixing along the length of storage tank. In addition, further increase in inlet temperature of the thermal fluid a slight decline in thermocline layer and mixing were observed at the top of the tank. But at a greater temperature difference with initial temperature prominent thermocline degradation and mixing can be observed. It can be explained as at higher temperature difference the axial wall conduction and thermal diffusion increased between the cold and hot water layers inside the storage tank, this leads to decrease in thermal stratification in a storage tank. Furthermore, the effect of volumetric flow rate of the incoming thermal fluid to the storage tank on the thermal stratification is analyzed as shown in Figure 5. Different volumetric flow rates of 2, 4 and 6 L/min were used with the storage tank water at initial temperature of 25 ºC and incoming inlet water at a temperature of 55 ºC. From this figure, the predicted temperature contours of hot water are shown in a circle made by heating coil in storage tank. It can be seen that inlet volumetric flow rate has a significant effect on the thermally stratified water layers inside the tank. Even at low flow rate of 2 L/min the incoming hot water moves along the side walls and radial direction downward to the cold water layers. By further increasing the flow rate of the incoming thermal fluid, the hot water penetrates into cold water layers along the axial direction, which results in dispersion of hot water before diffusion. This causes the mixing of tank top hot water and bottom cold water and reduces the phenomenon of thermal stratification inside the storage tank. In addition, it should be noted that these are the initial stage results as time increases the hot and cold water layers fill the top and bottom space of the tank while maintaining the thermal stratification. Figures 6 and 7 describe the variation of water temperature inside the thermal storage tank in function of time at different flow rate. When charging a tank over long period of time the deterioration of thermal stratification is more pronounced at low and as well as at high flow rates. Therefore, optimal flow rate can be found by adjusting the flow rate between high and low values. From these figures, the low flow rate of 2 L/min still shows better thermal stratification inside the storage tank as compared with high flow rate of 6 L/min. It is concluded that the optimal flow rate of incoming hot water could increase the phenomenon of thermal stratification over short and long time. 4. Conclusion In this paper, three dimensional numerical analysis of thermal storage tank equipped with helical-shaped heat exchanger have been performed to find the influence of different operating parameters. The main operating conditions such as the inlet flow rate and temperature of thermal fluid were considered during numerical simulation. It is found that the thermal stratification decreased with increasing the inlet temperature or temperature difference with an initial temperature of hot water at a constant flow rate. A high inlet volumetric flow rate deteriorated the thermally stratified layers with increasing the mixing phenomenon between hot and cold water layers in the tank. In addition, it is observed that by charging a tank over a long period of time, both low and high flow rates deteriorate the thermocline but the effect of high flow rate is more pronounced. Therefore, optimal flow rate decreased the effect of thermal diffusion and mixing, which result in enhanced thermal stratification. Proceedings International Conference of Agricultural Engineering, Zurich, /7

4 5. Acknowledgement This work was supported by IPET (Korea Institute of Planning and Evaluation for Technology in Food, Agriculture, Forestry and Fisheries, Project No ). 6. References Altuntop N, Kilik Z, Ozceyhan V, Kincay O. (2006). Effect of water inlet velocity on thermal stratification in a mantled hot water storage tank. International Journal of Energy Research, 30, András Zachár. (2013). Investigation of a new tube-in-tube helical flow distributor design to improve temperature stratification inside hot water storage tanks operated with coiled- tube heat exchangers. International Journal of Heat and Mass Transfer 63, Bahnfleth, W.P., Song, J., (2005). Constant flow rate charging characteristics of a full scale stratified chilled water storage tank with double-ring slotted pipe diffusers, Applied Thermal Engineering 25, Chung, J.D., Cho, S.H., Tae, C.S., Yoo, H. (2008). The effect of diffuser configuration on thermal stratification in a rectangular storage tank. Renewable Energy 33: Furbo, S., Andersen, E., Thur, A., Shah, L.J., Andersen, K. D. (2005). Performance improvement by discharge from different levels in solar storage tanks. Solar Energy 79, Hegazy, A.A. (2007). Effect of inlet design on the performance of storage-type domestic electrical water heaters, Applied Energy 84: Shah, L.J., Furbo, S. (2003). Entrance effects in solar storage tanks. Solar Energy 75, Shin, M.S., Kim, H.S., Jang, D.S., Lee, S.N., Lee, Y.S., Yoon, H.G. (2004). Numerical and experimental study on the design of a stratified thermal storage system. Applied Thermal. Engineering 24, Simon Ievers, Wenxian Lin (2009). Numerical simulation of three-dimensional flow Dynamics in a hot water storage tank. Applied Energy, 86, Steinert, P., Go ppert, S., Platzer, B. (2013). Transient calculation of charge and discharge cycles in thermally stratified energy storages. Solar Energy, 97, Proceedings International Conference of Agricultural Engineering, Zurich, /7

5 (a) Figure: 1 Describes the (a) CAD model Mesh model (a) Figure: 2 Temperature distributions at inlet temperature of 45 ºC (a) at heat exchanger surface & center plane at storage tank wall Proceedings International Conference of Agricultural Engineering, Zurich, /7

6 (a) Figure: 3 Temperature distributions at inlet temperature of 55 ºC (a) at heat exchanger surface & center plane at storage tank wall (a) Figure: 4 Temperature distributions at inlet temperature of 65 ºC (a) at heat exchanger surface & center plane at storage tank wall Proceedings International Conference of Agricultural Engineering, Zurich, /7

7 a) (c) Figure: 5 Temperature distribution inside the tank at different flow rates (a) 2 L/min 4 L/min (c) 6 L/min Figure: 6 Variation of temperature in function of time at different location inside the tank at 2 L/min Figure: 7 Variation of temperature in function of time at different location inside the tank at 6 L/min Proceedings International Conference of Agricultural Engineering, Zurich, /7