CFD ANALYSIS OF A PARALLEL FLOW HEAT EXCHANGER
1Deobrat Kumar Bhaskar, M.Tech, Student
2Dr. Anoop Kumar Pathariya,
1,2Technocrat Institute of Technology, Bhopal 1. INTRODUCTION
It is realized that heat exchangers are utilized to move heat between two liquids at various temperature. Essentially there are three methods of warmth move for example conduction, convection and radiation. If there should be an occurrence of warmth exchanger, radiation doesn't assume a significant part in contrast with conduction and convection. Conduction happens when the warmth streams from a higher temperature liquid to the lower temperature liquid through a strong divider.
To augment this conductive warmth move the strong segment should be dainty and comprised of an exceptionally conductive material. The biggest commitment to move of warmth in a warmth exchanger is made by the convection. The power convective warmth move follows starting with one moving stream then onto the next. The warmth moves from the moving stream to the strong segment divider by convection and afterward directed to the next surface of the divider, and eliminated by the progression of the other moving stream.
This keeps up the temperature slope between two moving liquids. To enhance the efficiency, heat exchangers must be designed in such a manner that it posses’ maximum surface area of the wall between two moving stream and minimum resistance to fluid flow through the heat exchanger. Its performance can also be affected by the addition of fins or corrugation in one or both directions of fluid flow, which increases surface area or induce turbulence.
Improvised heat transfer technologies have been widely applied to heat exchanger applications like in refrigeration, automobile and process industries, etc in the recent years. The performance of a heat exchanger can be improved by economical design of the heat exchanger. With the use of heat transfer enhancement techniques, economic design of heat exchanger is obtained and thereby reducing the size
which generally leads to energy, material and cost savings.
1.1 Double Pipe Heat Exchanger Applications
Concentric heat exchangers are commonly used in applications that involve relatively low flow rates and high temperatures or pressure for which they are best suited.
Concentric tube heat exchanger mostly used for material processing, food preparation, and air conditioning.
Advantages
The main advantage of a concentric tube heat exchanger is its simple design.
The concentric tube heat exchanger has a low installation cost, ease of maintenance and flexibility.
It is ideal for fluids that cause fouling.
Disadvantages
Length of tube can be extended up to a limit.
The rate of heat transfer is low in concentric tube heat exchangers.
It posses high cost when considering amount of heat transfer.
1.2 Objective of work
In present work I have taken the concentric line heat exchanger with hole in internal line. The hole plate is given at the center of the warmth exchanger. The punctured plate with 1mm thickness is set to be 5 openings. The fluid benzene is chosen liquid to cool to wanted temperature and set it up for external annulus pipe in concentric line heat exchanger while water is chosen as a cooling medium through which wanted temperature of Benzene is accomplished.
The water is arrangement in the inward line of the warmth exchanger.
2. LITERATURE SURVEY
M. Sheikholeslami3 et al [2016]
tentatively examined heat move and constrain misfortune in an air to water twofold line heat exchanger. Average roundabout ring (TCR) and punctured roundabout ring (PCR) turbulators are put in annular line. The examinations are directed for various overseeing boundaries to be specific; wind stream Reynolds number (6000–12,000), pitch proportion (1.83, 2.92 and 5.83) and number of punctured opening (0, 2, 4 and 8).
Connections for rubbing factor, Nusselt number and warm execution are introduced by trial information. Results showed that utilizing PCRs prompts get lower heat move upgrade than the CRs due to decrease of convergence point between the speed and the temperature field. Warm execution increments with increment of N yet it diminishes with increment of Reynolds number and pitch proportion. The impacts of the pitch proportion and number of punctured opening on stream and warmth move qualities are thought of. The connections of the Nusselt number, erosion factor and warm execution are introduced.
P.V. Durga Prasad4 et al [2015]
presented trapezoidal-cut twisted tape insert in a double pipe U-tube heat exchanger Experimental data is generated at flow rates ranging from 0.0333 kg/s to 0.2667 kg/s. Experimental data is generated with water and nano-fluid for Reynolds number in the range 3000<Re<30000, the Nusselt number of entire pipe for 0.03% concentrations of nano-fluid with trapezoidal-cut twisted tape inserts of H/D = 5 is enhanced by 34.24% as compared to water.
The friction factor of entire pipes for 0.03% concentration of nano-fluid with trapezoidal-cut twisted tape inserts of H/D=5 is enhanced by 1.29 times as compared to water. Convective heat transfer, friction factor as well as thermal performance factor tends to an increase by increasing Al2O3 concentration of nano-fluid and twist ratio of trapezoidal- cut tape inserts.
Ojha Pramod Kailash5 et al [2015] utilized warmth exchanger having tube with balance and without balance.
The balances were taken as semi- roundabout sort masterminded in exchanging route with dividing of 50mm.
The blades were just given on the internal
cylinder to making disturbance of cold water. The quantities of blade were 18 and its tallness and thickness 10 and 1.6mm individually. Various boundaries were acquired and thought about for basic internal cylinder and finned tube.
They found For = 17161.05, the general warmth move coefficient utilizing semi- round balance diminishes is more than 300 % which shows that enormous expansions in by and large warmth move territory.
Nonetheless, the warmth move improvement coefficient acts oppositely, as this amount increments with increments in warmth move zone. For mass stream rate 0.3832 kg/sec, the declines in convective warmth move coefficient for the virus side is around 220
% more than for basic cylinder consequently indicating better warmth move. While for more sizzling side the worth increments around 125%.
The expansions in rubbing factor for the virus side for finned tube is around 140 % more than straightforward cylinder for = 15954.24 while for the hot side the estimation of erosion factor remains roughly same for both cylinder.
The pressing factor drop because of contact in virus side for the finned tube is around 450 % more than the straightforward cylinder.
Tzu-Chen Hung 6 et al [2015]
used computational fluid dynamics (CFD) to simulate a three-dimensional concentric high temperature heat exchanger. They used helium gas and molten salt as hot and cold streams respectively in a counter-flow mode. Flow fields and heat transfer characteristics of the two concentric channels are examined extensively.
A design with an optimal performance of the heat exchanger is achieved by maximizing the effectiveness of the heat exchanger (ε-NTU method) using the following parameters as optimizing variables; the width of the flow channel, the length, pitch, thickness, and angle of the fins. The number of CFD simulation are substantially reduced by Taguchi method, and the optimal configurations of the concentric high temperature heat exchanger are found with a channel width of 1 mm, a fin length of 11 mm, an angle of fin of 2.6, and a fin thickness of 1.125 mm.
3. CFD MODELLING
3.1.1 Computational Fluid Dynamics (CFD)
CFD is a computational technology that enables the engineer to study the dynamics of things that flow. Using CFD, one builds a computational model that represents a system or device to be studied. Then the fluid flow physics is applied to this virtual prototype, and the software outputs a prediction of the fluid dynamics. A simplified and straight definition of the computational fluid dynamic would be that, CFD is, in part, the art of replacing the governing equations of fluid flow with numbers, and advancing these numbers in space and/or time to obtain a final numerical description of the complete flow field of interest [11].
The importance of CFD for the development of the fluid dynamic is enormous and it is seen as its “third dimension”, together with the experiments and the pure theory. CFD is a sophisticated analysis technique that not only predicts fluid flow behaviour, but also the transfer of heat, mass, phase change, chemical reaction, mechanical movement, and stress or deformation of related solid structures. These characteristics have rapidly transformed CFD into a very popular tool in engineering analysis.
3.1.2 Advantages of using CFD Methods 1. There are many devices and systems that are very difficult to prototype. Often, CFD analysis shows on part of the system or phenomenon happening within the system that would not otherwise be visible through any other means. CFD gives one a means of visualizing and enhanced understanding of the designs.
2. CFD is a tool for predicting what will happen under a given set of circumstances. One provides the input variables and it provides the outcomes. In a short time, one can predict how a design will perform, and test many variations until an optimal result is obtained. To achieve these in physical
prototyping and testing (done in the past) would require a huge amount of time and labour. The foresight gained from CFD helps the engineer to design better and faster.
3. The better and faster design or analysis leads to shorter design cycles. Time and money are saved.
Products get to the market faster.
Equipment improvements are built and installed with minimal downtime. Thus, CFD is a tool for compressing the design and development cycle.
There are a number of different solution methods, which are used in CFD codes.
This numerical technique is in fact the chosen one for almost all the CFD commercial software.
The fundamental basis of any CFD problem is the Navier-Stokes equations, which define any single-phase fluid flow.
These equations can be simplified by removing terms describing viscosity to yield the Euler equations. Further simplification, by removing terms describing vorticity yields the full potential equations. Finally, these equations can be linearized to yield the linearized potential equations.
Historically, methods were first developed to solve the Linearized Potential equations. Two-dimensional methods, using conformal transformations of the flow about a cylinder to the flow about an airfoil were developed in the 1930s. The computer power available paced development of three-dimensional methods.
4. RESEARCH METHODOLOGY 4.1 Physical Models
The point of this undertaking is to explore the punctured cylinder designs a twofold line heat exchanger. With the expansion of this punctured plate inside the internal line of concentric cylinder heat exchanger lead to changes the speed and pressing factor of liquid in inward line. It additionally changes the warmth move rate. The designs of punctured cylinder concentric sort heat exchanger as demonstrated in the fig.
Figure 1 Concentric Tube 4.2 Geometrical Parameters
Table 1 Geometrical specification
Materials Iron
Inner tube Internal Diameter 21 mm Inner tube External Diameter 25.4 mm Outer tube Internal Diameter 41 mm Outer tube External Diameter 48 mm Perforated plate Thickness 1 mm
Length of Heat Exchanger 1000 mm 4.3 Discretization
The area is disretizised for mathematical exploring to the issue. It requires a decent quality lattice in the computational space to fruitful mathematical examination. The more modest the size of the component close to the punctured divider, the more definite and exact stream construction will be caught. Notwithstanding, for the 3D recreation, a little change in the size of component will lead a significant expansion in the quantity of components.
That will brings about a huge increment of computational time. To decrease computational time and stream balance in y and z hub, the recreations are performed for half of computational space by evenness plane. The speed documented at y=0 mm and z=0 mm the
symmetric limit condition in applied. The computational space is isolated into four zones. Water Zone (cold liquid), inward cylinder zone with punctured plate, fluid Benzene (hot liquid) Zone and external line zone.
The name determination is indicated with water and fluid benzene for channel and leave condition. The connective interface has been arrangement between two zones so information can be moved. Non uniform cross section is utilized to discretize the computational area. Near the outside surface of inner tube and inside surface outer tube inflation layer is provided. The number of inflation layer 3 and maximum thickness 0.001 are provided in the mesh control. As shown in fig.
Figure 2 Meshed Section
A Fine no uniform mesh is employed in computational domain. At water domain and inner pipe with perforated plate is meshed with tetrahedron element and liquid Benzene & outer pipe meshed with quadrilateral element.
Figure 3 Mesh structure of the computational domain
The total numbers of mesh in the computational domain and in the different zones are Listed in Table 2
Table
2
Number of elements in three different levels of mesh Node Element Element TypeWater 17091 44610 Tetrahedron
Benzene 34080 28202 Quadrilateral Inner Pipe 14098 48470 Tetrahedron
Outer pipe 8640 4063 Quadrilateral
4.3.1 Transport Equations for the Standard k-ε Model
In the present study, RNG version of turbulence model is used to model the flow regime. The RNG-based- turbulence model is derived from the instantaneous Navier-Stokes equations, using a mathematical technique called
"renormalization group'' (RNG) methods. It is similar in form to the standard model, but includes the following refinements:-
1. The RNG model has an additional term in its equation that significantly improves the accuracy for rapidly strained flows.
2. The effect of swirl on turbulence is included in the RNG model, enhancing accuracy for swirling flows.
3. The RNG theory provides an analytical formula for turbulent Prandtl numbers, while the standard model uses user- specified, constant values.
4. While the standard model is a high-Reynolds-number model, the RNG theory provides an analytically-derived differential formula for effective viscosity that accounts for low-Reynolds-number effects.
4.4 Numerical Procedure and Computational Methodology
The administering differentials are transport conditions. They are changed over to logarithmic conditions to address mathematically. The limit condition cell zone condition has been indicated.
Subsequent to applying the limit condition reference esteem set for fluid
benzene. The pressing factor speed coupling calculation with SIMPLE plan is applied .In pressing factor speed coupling pressure, force, fierce active energy and tempestuous dispersal rate condition set to be second request upwind displayed.
At that point the arrangement control and mixture introduction of the arrangement must be given before the cycle begins.. The arrangements consecutive calculation (pressure based solver) utilized in the mathematical calculation requires less memory than that of the thickness based solver.
Thinking about no thick warming.
Besides, the union basis of 10-4 is picked for determined boundaries aside from the energy where an estimation of 10-6 is utilized. Mathematical reproduction are done for punctured plate with hole measurement of 5mm, 4mm and 2 mm for laminar stream conditions by picking suitable mathematical estimations of boundaries.
5. RESULTS ANALYSIS
The numerical investigating is carried out with laminar flow in the inner pipe. The Reynolds no. is less than 2300. The Reynolds number Various from 65 to 80.
Although the Reynolds number is less but is it sufficient to capture the effect of perforating material in the pipe.
The mass flow rate of Benzene it kept constant value 0.05 kg/s as it is specified for achieving the cooling of benzene for desired limit of temperature.
The mass flow rate of water has been varied with certain diameter of perforating plate hole. The programme is converged in 50 iteration as shown in the fig 04.
Figure 4 convergence graph The accompanying diagram and
temperature dispersion distinguished at different area during mathematical examination. The fig 4 shows the temperature forms at the mid area of the warmth exchanger. It is the area where I have embedded the punctured plate with
5 openings. It very well may be seen that with the addition of plate the more temperature angle at the area. Course of temperature inclination advance toward the hole territory.
Figure 5 Temperature Contours at perforating plate In Fig 5 shows temperature contours at
the location 800mm from inlet of water. It is location where no perforation is
provided, It is clearly visualised that less temperature gradient in inner pipe toward centre of pipe. The temperature gradient exists only near the wall of inner tube.
Figure 6 Temperature Contours 800 mm from water Inlet The fig 6 and 7 shows speed profile at the
district of puncturing plate. It is plainly shows that puncturing opening prompts the adjustment in the speed profile in the line as contrast with other area of the line. The adjustment in the speed would cause to change the warmth move coefficient thus it increment the warmth
move rate from water to benzene fluid (heat retaining liquid). Along these lines the more temperature slope in puncturing area as contrast with other locale of the line.
Figure 7 Velocity at Perforating Region.
Figure 8 Variation in Velocity at Perforating Region Fig 7 shows the graph between mass flow
rate of water and its heat transfer coefficient with diameter of perforation on perforating plate. It shows that heat transfer coefficient increases with mass flow rate of water for particular perforating diameter. For 3 mm perforation diameter shows more heat transfer coefficient as compare to other
4mm and 5 mm diameter. It show that as we decrease the perforation diameter the heat transfer coefficient increases and it lead to increase the heat transfer rate from water, Hence it increases the cooling rate of benzene.
Figure 9 Graph between Heat transfer coefficient and mass flow rate of water
Figure 10 Graph between Re no. and Mass flow rate of water The fig 10 shows diagram between
Average Reynolds number and mass stream pace of water with various hole width in puncturing plate. It portrays that as the increment of mass stream pace of water Reynolds no. is additionally
expanded. The Reynolds number is practically same for3 mm and 4 mm hole width. The adjustment in Reynolds no.
would increment in warmth move rate from water.
Figure 11 Graph between Benzene outlet temperature and Mass flow rate of water The fig 11 shows benzene outlet
temperature with perforation diameter.
For perforation diameter 3mm shows minimum temperature of benzene.
Although 4mm diameter is almost same as for 3 mm perforation diameter.
6. CONCLUSION AND SCOPE FOR FUTURE WORK
The current work can additionally be reached out with more number of hole plates. The hole width in puncturing plate
might be changed at various area. In this work I have considered the round hole has been utilized however it very well may be altered with scored roundabout opening in the hole plate. It might build the tempestuous and thus heat move.
These variation puncturing plates can be utilized for heat move enlargement concentrates likewise in refrigeration framework.
REFERENCES
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