IJIRST International Journal for Innovative Research in Science & Technology Volume 1 Issue 11 April 2015 ISSN (online): 2349-6010 CFD Analysis of a 24 Hour Operating Solar Refrigeration Absorption Technology Arunkumar. H Department of Mechanical Engineering Saintgits College of Engineering, Kottayam, India Arun George Department of Mechanical Engineering Saintgits College of Engineering, Kottayam, India Benson P Sunny Department of Mechanical Engineering Saintgits College of Engineering, Kottayam, India Jesbin Antony Department of Mechanical Engineering Saintgits College of Engineering, Kottayam, India Abstract A computational fluid dynamics (CFD) model is used to investigate outlet temperatures in heat exchanger within generator by varying fluid velocities and also used to investigate outlet temperature in evaporator and heat flux rate at walls of evaporator. The solar refrigeration absorption system taken for analysis is a continuously operating refrigerant storage system. R- 717(Ammonia) is used as refrigerant. Aqua-ammonia vapor absorption refrigeration systems, which operate such that both, the generation of aqua ammonia vapors and the production of cold utilizing the generated aqua-ammonia vapors, take place at the same time are known as continuous-based operation systems. Model is completed using Solid works. Analysis is carried out in ANSYS 14. FLUENT is the software used to simulate fluid flow problems. It is generally used for computational Fluid Dynamics problems. It uses the finite-volume method to solve the governing equation for a fluid. It provides a wide field to solve problems. Numerical computations have been carried out to find coefficient of performance (COP). Variation of temperature at outlet of heat exchanger and evaporator are studied. Difference in maximum and minimum temperature at outlet of heat exchanger and evaporator at different fluid velocities are noted. The obtained profiles indicate variation in temperature of fluid. Graphs showing variation of COP with varying evaporator and generator temperatures are plotted. Air taken from outlet of evaporator via blower is used for refrigeration. Keywords: Coefficient of Performance; Computational Fluid Dynamics I. INTRODUCTION The excessive demand for air conditioning is as a result of extreme temperatures during summer. Thus, it is imperative to use refrigeration and air conditioning in all fields of life. By this 24 hour operation solar refrigeration absorption system, the electrical energy which is a high grade source can be saved from its use in comfort sector and being utilized in production sector. Some liquids like water have great affinity for absorbing large quantities of certain vapors (NH 3 ) and reduce the total volume greatly. The absorption refrigeration system differs fundamentally from vapor compression system only in the method of compressing the refrigerant. An absorber, generator and pump in the absorption refrigerating system replace the compressor of a vapor compression system. Out of the various renewable sources of energy, solar energy proves to be the best candidate for refrigeration and air conditioning because of the coincidence of the maximum cooling load with the period of greatest solar radiation input. Solar energy can be used to power a refrigeration system in two ways. First, solar energy can be converted into electricity using photovoltaic cells and is used to operate a conventional vapor compression refrigeration system. Second, solar energy can be used to heat the working fluid in the generator of vapor absorption system. The comparison showed that solar electric refrigeration systems using photovoltaic appear to be more expensive than solar thermal systems. Solar energy has a great potential renewable content that can be effectively utilized for refrigeration and air conditioning purposes using aqua ammonia vapor absorption system. However, the biggest challenge in utilizing solar energy, for uninterrupted cooling is its unavailability during the night time. The available technology for the utilization of solar energy in refrigeration and air conditioning purposes are continuous operating systems and intermittent operating systems. Aqua-ammonia vapor absorption refrigeration systems, which operate such that both, the generation of aqua-ammonia vapors and the production of cold utilizing the generated aqua-ammonia vapors, take place at the same time are known as continuous-based operation systems. The advantage of the continuous operating systems are that such systems have comparatively high COP and present a compact design. But intermittent systems have comparatively very low COP, possess a huge system size [1]. So we use a continuous operating system for our study. In this paper, design and CFD analysis of heat exchanger within generator and evaporator within the continuous based refrigerant storage system is done. Design of heat exchanger within generator and evaporator is done using SOLID WORKS 2013. ANSYS FLUENT 14 is the software used to simulate fluid flow problems. For all flows, ANSYS FLUENT solves All rights reserved by www.ijirst.org 191
conservation equations for mass and momentum. For flows involving heat transfer and compressibility, an additional equation for energy conservation is solved. II. RESEARCH METHODOLOGY The solar refrigeration absorption system taken for analysis is a continuously operating refrigerant storage system. R-717 (Ammonia) is used as refrigerant. Aqua-ammonia vapor absorption refrigeration systems, which operate such that both, the generation of aqua ammonia vapors and the production of cold utilizing the generated aqua ammonia vapors, take place at the same time are known as continuous-based operation systems. In this paper, we are considering design and analysis of heat exchanger with in the generator and evaporator within the solar refrigeration absorption system. Heat exchanger with in generator and evaporator are considered for design and analysis since these 2 parts play the most important role in deciding the performance of the solar refrigeration absorption system. Model of heat exchanger within generator and evaporator is done in Solid Works (2013). Solid Works is a solid modeler, and utilizes a parametric feature based approach to create models and assemblies. Solid Works files use the Microsoft structured storage file format. Analysis is carried out in ANSYS FLUENT 14. FLUENT is the software used to simulate fluid flow problems. It uses the finite volume method to solve the governing equation for a fluid. For all flows, ANSYS FLUENT solves conservation equations for mass and momentum. For flows involving heat transfer and compressibility, an additional equation for energy conservation is solved. These are governing equations of ANSYS FLUENT and it is shown below. 1) Navier Stokes Equation: 2) Continuity equation 3) Energy equation Numerical computations have been carried out to find coefficient of performance (COP). Where, T E is the evaporator temperature in K. T G is the generator temperature in K. T C is the condenser temperature in K. Fig. 1: Continuously operated refrigerant storage solar powered aqua ammonia vapor absorption refrigeration system. All rights reserved by www.ijirst.org 192
A. Modelling: 1) Heat Exchanger With in Generator: In heat exchanger within generator, brine in the inner tube is used to heat the aqua-ammonia refrigerant passing through the outer tube. The modeling of heat exchanger with in generator is done in Solid Works. It is done with the following specifications:- Table -1: Modeling specifications used in case of heat exchanger. Sl. No. Parameters Specifications 1 Type of heat exchanger Tube in tube 2 Type of flow Counter flow 3 Inner tube diameter 25mm 4 Outer tube diameter 40mm Fig. 2: Heat exchanger design using SOLID WORKS 2) Evaporator: Aqua ammonia is used to cool the air in the evaporator and air taken from the outlet of evaporator via blower is used for refrigeration. The modeling of evaporator is done in Solid Works. It is done with the following specifications:- Table -2: Modeling specifications used in case of evaporator Sl. No. Parameters Specifications 1 Inner pipe diameter 30mm 2 Wall length 75cm 3 Wall breadth 65cm 4 Wall depth 10cm Fig. 3: Model of evaporator designed using SOLID WORKS. B. Analysis: The analysis is done in ANSYS FLUENT 14. The first step of analysis involves insertion of an external geometry file of model which is modeledin Solid Works in IGES format. Second step is to give naming for each part and also represent whether part is a solid or fluid. Third step is the meshing of model. Meshing is done with appropriate mesher and sizing. Fourth step is to select material and activate the energy equation in addition to the default continuity and navier stokes equations. Fifth step is to apply All rights reserved by www.ijirst.org 193
suitable cell zone and boundary conditions. Next step is to intialize hybrid initialisation and final run calculation is done with appropriate number of iteration until convergence tolerance is obtained. 1) Meshing: The partial differential equations that govern fluid flow and heat transfer are not usually amenable to analytical solutions, except for very simple cases. Therefore, in order to analyze fluid flow, flow domains are split into smaller sub domains (made up to geometric primitives like hexahedra and tetrahedra in 3D and quadrilaterals and triangles in 2D). The governing equations are then discretized and solved inside each of these subdomains. Typically one of the methods is used to solve the approximate version of the system of equations: finite volumes, finite elements, or finite differences. Care must be taken to ensure proper continuity of solution across the common interfaces between two subdomains, so that the approximate solutions inside various portions can be put together to give a complete picture of fluid flow in the entire domain. The subdomains are often called elements or cells, and the collection of all elements or cells is called a mesh or grid. The origin of the term mesh (or grid) goes back to early days of CFD when most analysis were 2D in nature. For 2D analyses, a domain split into elements resembles a wire mesh, hence the name. a) Heat Exchanger within Generator: Meshing of heat exchanger is done in CFD with the following specifications as given below:- Table -3: Meshing Specifications of Heat exchanger Sl. No Specifications 1 Nodes 201683 2 Elements 171190. 3 Type of mesher Triangular surface mesher. Fig. 4: Meshing of heat exchanger within generator in CFD. The meshing of heat exchanger within generator in CFD is as shown in the figure 4.4. Coarse meshing is done in model within 201683 nodes and 171190 elements using a triangular surface mesher i.e tetrahedron meshing. b) Evaporator: Meshing of Evaporator is done in CFD with the following specifications as given below:- Table -3: Meshing Specifications of Evaporator Sl. No Specifications 1 Nodes 46496. 2 Elements 38285. 3 Type of mesher Triangular surface mesher. All rights reserved by www.ijirst.org 194
Fig. 5: Meshing of evaporator in CFD. The meshing of evaporator in CFD is as shown in the figure 5. Coarse meshing is done in model within 46496 nodes and 38285 elements using a triangular surface mesher i.e tetrahedron meshing. 2) Cell Zone and Boundary Conditions: The cell zone conditions involves the selection of the refrigerant or fluid needed from Fluent database. The boundary conditions includes the input parameters like velocity (in m/s) and temperature (in K) at inlet. a) Heat Exchanger within Generator: Applying cell zone and boundary condition as follows:- Brine: Velocity (in m/s) = 0.4, 0.5, 0.6. Inlet temperature = 360K. Aqua-ammonia: Velocity (in m/s) = 0.4. Inlet temperature = 300K. Solution using CFD Type of initialization Hybrid b) Evaporator: Applying cell zone and boundary condition as follows:- Air Velocity (in m/s) = 1 Inlet temperature = 300K. Aqua-ammonia Velocity (in m/s) = 0.4. Inlet temperature = 268K. Solution using CFD Type of initialization Hybrid All rights reserved by www.ijirst.org 195
III. RESULT AND DISCUSSIONS A. Heat Exchanger within Generator: Fig. 6: Temperature distribution of outlet aqua ammonia at fluid velocity of 0.4 m/s The figure 6 shows the temperature distribution at outlet of aqua ammonia of the heat exchanger within generator. From the figure, it is very clear that aqua ammonia gets heated up using the heat obtained from the brine which is passing through the inner tube of the counter flow heat exchanger. In this case brine has a velocity of 0.4 m/s. The minimum temperature of 317.591 K and maximum temperature of 330.27 K is observed at the outlet of aqua- ammonia at brine velocity of 0.4 m/s. Fig. 7: Temperature distribution of outlet aqua ammonia at fluid velocity of 0.5 m/s. The figure 7 shows the temperature distribution at outlet of aqua ammonia of the heat exchanger within generator. From the figure, it is very clear that aqua ammonia gets heated up using the heat obtained from the brine which is passing through the inner tube of the counter flow heat exchanger. In this case brine has a velocity of 0.5 m/s. The minimum temperature of 315.611 K and maximum temperature of 326.187 K is observed at the outlet of aqua ammonia at brine velocity of 0.5 m/s. All rights reserved by www.ijirst.org 196
Fig. 8: Temperature distribution of outlet aqua ammonia at fluid velocity of 0.6 m/s. The figure 8 shows the temperature distribution at outlet of aqua ammonia of the heat exchanger within generator. From the figure, it is very clear that aqua ammonia gets heated up using the heat obtained from the brine which is passing through the inner tube of the counter flow heat exchanger. In this case brine has a velocity of 0.6 m/s. The minimum temperature of 315.474 K and maximum temperature of 327.344 K is observed at the outlet of aqua ammonia at brine velocity of 0.6 m/s. From figure 5, 6, 7, it was understood that as fluid velocity increases temperature difference that is difference in maximum and minimum temperature first increases and then decreases. Difference in temperature is found to be 12.679 K, 10.576 K and 11.87 K at fluid velocities 0.4, 0.5 and 0.6 m/s respectively. Table -4: Temperature variations for different fluid velocities SL. no. Fluid velocity (m/s) Maximum temperature (K) Minimum temperature (K) Difference in temperature 1 0.4 330.27 317.59 12.679 2 0.5 326.187 315.61 10.576 3 0.6 327.344 315.47 11.87 It was also observed that highest outlet temperature of ammonia is obtained in case of fluid velocity = 0.4m/s and value is 330.27 K. From all the above information, it was observed that maximum heat transfer occurs in case of lowest velocity due to higher temperature difference between maximum and minimum. The Temperature distributions at the surfaces of the heat exchanger within generator at different brine velocities are shown below:- Fig. 9: Temperature distribution at fluid velocity of 0.4 m/s. All rights reserved by www.ijirst.org 197
The figure 9 shows the temperature distribution of the heat exchanger within generator. From the figure, it is very clear that aqua ammonia gets heated up using the heat obtained from the brine which is passing through the inner tube of the counter flow heat exchanger. The heat transfer between aqua ammonia is very clear from the above figure. In this case brine has a velocity of 0.4 m/s. The minimum temperature of 299.84 K and maximum temperature of 370.007 K is observed at the outlet of aqua ammonia at brine velocity of 0.4 m/s. Fig. 10: Temperature distribution at fluid velocity of 0.5 m/s. The figure 10 shows the temperature distribution of the heat exchanger within generator. From the figure, it is very clear that aqua-ammonia gets heated up using the heat obtained from the brine which is passing through the inner tube of the counter flow heat exchanger. The heat transfer between aqua ammonia is very clear from the above figure. In this case brine has a velocity of 0.5 m/s. The minimum temperature of 299.856 K and maximum temperature of 360.007 K is observed at the outlet of aqua ammonia at brine velocity of 0.5 m/s. Fig. 11: Temperature distribution at fluid velocity of 0.6 m/s The figure 11 shows the temperature distribution of the heat exchanger within generator. From the figure, it is very clear that aqua ammonia gets heated up using the heat obtained from the brine which is passing through the inner tube of the counter flow heat exchanger. The heat transfer between aqua ammonia is very clear from the above figure. In this case brine has a velocity of 0.6 m/s. The minimum temperature of 299.843 K and maximum temperature of 370.009 K is observed at the outlet of aqua ammonia at brine velocity of 0.6 m/s. All rights reserved by www.ijirst.org 198
SL. no. CFD Analysis of a 24 Hour Operating Solar Refrigeration Absorption Technology Table -5: Temperature distribution in heat exchanger surface at different velocities. Difference Fluid velocity (m/s) Maximum temperature (K) Minimum temperature (K) in temperature 1 0.4 370.007 299.84 70.167 2 0.5 360.007 299.856 60.151 3 0.6 370.009 299.845 70.166 Maximum Temperature is found to be 370.007, 360.07 and 370.009K at different fluid velocities 0.4, 0.5, 0.6 m/s respectively. Minimum Temperature is found to be 299.84, 299.856 and 299.843 K at different velocities. The difference in temperature is found to be decreases with fluid velocity and then decreases. B. Evaporator: Fig. 12: Temperature variation at air outlet The figure 12 shows temperature variation at air outlet in an evaporator. From the figure, it is very clear that air gets cooled using aqua ammonia within the evaporator and the maximum temperature is found to be 311.5 K and minimum temperature is found to be 268 K. From analysis we found that the temperature of air leaving the wall of evaporator is increased by 43.5 K. From this we can infer that a part of the heat of the inlet ammonia is given to the air by forced convection. Thus ammonia is cooled at the outlet of evaporator tube and thus sufficient cooling is produced. The temperature difference is found to be 43.5 K and thus refrigeration effect is obtained from cold outlet using a fan due to forced convection. Fig. 13: Total heat transfer in evaporator in watts. All rights reserved by www.ijirst.org 199
Figure 13 shows heat flux at walls in evaporator. The total heat transfer rate at walls is found to be 306.72 W. Thus refrigeration effect is produced and is taken from the cold outlet. C. Performance Curves: Table -6: Values of COP Vs Evaporator Temperatures Sl. No. Evaporator Temperatures (K) COP 1 268 0.15 2 272.3 0.172 3 276.7 0.195 4 281 0.223 5 285.4 0.26 6 289.7 0.31 7 294.1 0.38 8 298.4 0.485 9 302.8 0.67 10 307.1 1.05 11 311.5 2.403 Fig. 14: Performance curves for varying evaporator temperatures. Figure 14 shows performance curves for varying evaporator temperatures and generator temperature of 323.8 K. The figure implies as evaporator temperatures slightly increases and then increases to maximum, due to low heat transfer rate within evaporator. Table -7: Values of COP Vs Generator Temperature Sl. No. Generator Temperatures (K) COP 1 317.5 0.073 2 318.1 0.089 3 320 0.144 4 321.3 0.181 5 322.6 0.217 6 323.8 0.251 7 325.1 0.287 8 326.3 0.319 9 327.5 0.352 10 328.8 0.387 11 330.02 0.419 All rights reserved by www.ijirst.org 200
Fig. 15: Performance curve for varying Generator temperatures. Figure 15 shows performance curves for varying generator temperatures and evaporator temperature of 284.2 K. The figure implies as generator temperatures increases, COP linearly increases due to higher heat transfer rate in heat exchanger within generator. IV. CONCLUSIONS Following conclusions are obtained are as follows: 1) Total heat transfer rate at walls of evaporator is found out and is found to be 306.72 W. This is due to large temperature difference of 43.5 K in Evaporator. 2) Outlet temperatures of aqua ammonia in heat exchanger within generator are found out at different velocities 0.4, 0.5 and 0.6 m/s respectively. As fluid velocity increases temperature difference that is difference in maximum and minimum temperature first increases and then decreases. Difference in temperature is found to be 12.679 K, 10.576 K and 11.87 K at fluid velocities 0.4, 0.5 and 0.6 m/s respectively. It was also observed that highest outlet temperature of ammonia is obtained in case of fluid velocity =0.4m/s and value is 330.27 K. From all the above information, it was observed that maximum heat transfer occurs in case of lowest velocity due to higher temperature difference between maximum and minimum. 3) Performance curves against varying generator temperatures and varying evaporator temperatures are plotted. COP increases linearly with varying generator temperature because of higher heat transfer rate between brine and aqua ammonia within heat exchanger tubes. COP remains constant up to 300K and then increases with varying evaporator temperature because of lower heat transfer rate in evaporator compared to heat exchanger within generator. ACKNOWLEDGMENT The authors would like to acknowledge the support of Mechanical Engineering Department of Saintgits College of Engineering for conducting the present investigation. REFERENCES [1] Said A. M., Maged AI EI-Shaarawi, Muhammad Siddique U., (2012)., Alternative designs for a 24-h operating solar-powered absorption refrigeration technology, International journal of refrigeration, Vol. 35. pp. 1967-1977. [2] Cerezo. J, Bourouis. M, Manel. V, Alberto. C, Roberto. B, (2009)., Experimental study of an ammonia water bubble absorber using a plate heat exchanger for absorption refrigeration machines, Appl. Therm. Eng, Vol. 29. pp. 1005-1011. [3] De Francisco, Illanes. A., Tones. R., Castillo.J.L.M., De Bias, Prieto. E., Garcia. A., (2002)., Development and testing of a prototype of low power water-ammonia absorption equipment for solar energy applications, Renewable Energy, Vol. 25, pp. 537-544. [4] Sumayths. K, Huang, Z.C. Li, (2002)., Solar absorption cooling with low grade heat source a strategy of development in south china, Solar energy, Vol. 72(2), pp. 155-165. All rights reserved by www.ijirst.org 201