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IJIRST –International Journal for Innovative Research in Science & Technology| Volume 1 | Issue 11 | April 2015 ISSN (online): 2349-6010
All rights reserved by www.ijirst.org 191
CFD Analysis of a 24 Hour Operating Solar
Refrigeration Absorption Technology
Arunkumar. H Benson P Sunny
Department of Mechanical Engineering Department of Mechanical Engineering
Saintgits College of Engineering, Kottayam , India Saintgits College of Engineering, Kottayam , India
Arun George Jesbin Antony
Department of Mechanical Engineering Department of Mechanical Engineering
Saintgits College of Engineering, Kottayam , India 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 (NH3) 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
CFD Analysis of a 24 Hour Operating Solar Refrigeration Absorption Technology (IJIRST/ Volume 1 / Issue 11 / 034)
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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, TE is the evaporator temperature in K.
TG is the generator temperature in K.
TC is the condenser temperature in K.
Fig. 1: Continuously operated refrigerant storage solar–powered aqua–ammonia vapor absorption refrigeration system.
CFD Analysis of a 24 Hour Operating Solar Refrigeration Absorption Technology (IJIRST/ Volume 1 / Issue 11 / 034)
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Modelling: A.
Heat Exchanger With in Generator: 1)
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
Evaporator: 2)
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.
Analysis: B.
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
CFD Analysis of a 24 Hour Operating Solar Refrigeration Absorption Technology (IJIRST/ Volume 1 / Issue 11 / 034)
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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.
Meshing: 1)
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.
CFD Analysis of a 24 Hour Operating Solar Refrigeration Absorption Technology (IJIRST/ Volume 1 / Issue 11 / 034)
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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.
Cell Zone and Boundary Conditions: 2)
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
CFD Analysis of a 24 Hour Operating Solar Refrigeration Absorption Technology (IJIRST/ Volume 1 / Issue 11 / 034)
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III. RESULT AND DISCUSSIONS
Heat Exchanger within Generator: A.
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.
CFD Analysis of a 24 Hour Operating Solar Refrigeration Absorption Technology (IJIRST/ Volume 1 / Issue 11 / 034)
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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.
CFD Analysis of a 24 Hour Operating Solar Refrigeration Absorption Technology (IJIRST/ Volume 1 / Issue 11 / 034)
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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.
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Table -5:
Temperature distribution in heat exchanger surface at different velocities.
SL.
no. Fluid velocity (m/s) Maximum temperature (K) Minimum temperature (K)
Difference
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.
Evaporator: B.
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.
CFD Analysis of a 24 Hour Operating Solar Refrigeration Absorption Technology (IJIRST/ Volume 1 / Issue 11 / 034)
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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.
Performance Curves: C.
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
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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.
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