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Cfd analysis of shell and tube heat exchanger.

INTRODUCTION

Shell-and-tube heat exchangers (STHXs) are used in many industrial areas, such as power plant, chemical engineering, petroleum refining, food processing, etc. Among different types of heat exchangers, more than 35-40% of heat exchangers are of the shell-and-tube type because of their robust geometry construction, easy maintenance and possible upgrades. Baffle is an important component on shell-side of STHXs. Besides supporting the tube bundles, it diverts fluid flow and form flow passage for the shell-side fluid in conjunction with the shell. The segmental baffle is the most commonly used baffle, which forces the shell-side fluid going through in a zigzag manner, to improve the heat transfer. This type of heat exchanger has been well-developed and probably is still the most commonly used type of the shell-and-tube heat exchanger. The major disadvantages of the conventional shell-and tube heat exchangers with segmental baffles are large shell-side pressure drop, a dead zone forming in each compartment between two adjacent segmental baffles, leading to an increase of fouling resistance and the dramatic zigzag flow pattern causes high risk of vibration failure on tube bundle.

To overcome the above-mentioned drawbacks of the conventional segmental baffle, a number of different improved structures were proposed for the purposes of higher heat transfer coefficient, low possibility of tube vibration, reduced fouling factor and reduced pressure drop. Over the past decades, various kinds of baffles have been developed, for example, the H-baffles, the deflecting baffles, the overlap helical baffles, and the rod baffles. Liu et al [5] compared the overall performance of shell and tube heat exchanger using rod baffles and rod-vane baffles and they found that the rod-vane baffles gives better overall performance of heat exchanger. Peng et al [7] studied experimentally shell and tube heat exchanger with continuous helical baffle and compared the result with the segmental baffle shell and tube heat exchanger and they found that the use of continuous helical baffles resulted in nearly 10% increase in heat transfer coefficient for the same pressure drop. Wang et al [8] experimentally compared the flower baffle heat exchanger and segmental baffle heat exchanger and found that the use of flower baffle resulted in 60% increase of nusselt number to pressure drop ratio which gives better overall performance. Li and Kottke [3] studied the effect of baffle spacing in shell and tube heat exchanger and found that for same Reynolds number, the pressure drop and average heat transfer are increased by increasing the baffle spacing. Radojkovic et al [6] carried out the experimental study on the influence of baffle cut on heat exchanger performance and fount that the baffle cut of 22% gives optimal heat efficiency. Jian-Fei-Zhang et al [4] carried out simulation of shell and tube heat exchanger with helical baffles using commercial codes of GAMBIT 2.3 and FLUENT 6.3. The helix angle considered for analysis is 40degree and the results show a reasonable agreement with the experimental result. Dong et al [1] carried out numerical and experimental investigation of shell side characteristics for ROD baffle heat exchanger and found that the numerical result had reasonable agreement with the experimental result.

Therefore, the main objectives of this study are to develop a new type of baffle to overcome the deficiencies mentioned above and to numerically investigate its performance.

Modelling For Simulation:

2.1. Computational Model:

The computational model of an existing and modified STHXs are shown in Fig. 1 and 2 respectively, and the geometry parameters are listed in Table 1. The whole computational domain is bounded by the inner side of the shell and everything in the shell is contained with the domain.

To simplify numerical simulation while and keep the basic characteristics of the process, following assumptions are made: (1) constant thermal properties are maintained for the shell-side fluid; (2) the heat transfer processes and fluid flow are in steady-state and turbulent; (3) the leak flows between baffle and tube and that between baffle and the shell are neglected; (4) Neglect the natural convection induced by the fluid density variation.; (5) the constant tube wall temperatures are kept in the whole shell side; (6) the whole heat exchanger is well-insulated hence the heat loss to the environment is totally neglected.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

2.2. Governing Equations and Boundary Conditions:

The standard k-[epsilon] model is adopted because it can provide improved predictions of near-wall flows and flows with high streamline curvature. The governing equations for the mass, momentum, and energy conservations, and for k and [epsilon] can be expressed as follows:

Mass:

[[partial derivative])[rho][u.sub.i])/[partial derivative][x.sub.i]] = 0.

Momentum:

(2) [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII].

Energy:

(3) [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII].

Non-slip boundary condition is applied on all solid surfaces within the computational domain and the inner wall of the shell. In the near-wall region, the standard wall function method is used to simulate the flow. The mass-flow-inlet and outflow boundary condition [2] are applied on the inlet and outlet sections, respectively. The temperature of tube walls are set as constant and their values are 450K. The shell wall of heat exchanger is set as fully insulated. Heat conduction of baffles in heat exchanger is considered by using shell conduction in thin-walls model in FLUENT. The conductive-320 oil is taken as working fluid for shell side of heat exchanger in simulation and thermo physical properties of the fluid are listed in Table 2.

2.3. Grid Generation and Numerical Method:

The three-dimensional model is then discretized in GAMBIT using tetrahedral and hexahedral mesh which are accurate and involve less computation effort. Fine control on the meshes near the wall surface allows capturing the boundary layer gradient accurately. The heat exchanger is discretized into solid and fluid domains in order to have better control over the number of nodes. The fluid mesh is made finer then solid mesh for simulating conjugate heat transfer phenomenon. Once the meshes are checked for free of errors and minimum required quality it is exported to ANSYS FLUENT. The generated grid model is shown in Fig.3.

[FIGURE 3 OMITTED]

RESULTS AND DISCUSSION

The simulation results for 0.5 kg/s mass flow rate for existing and modified models (with 10[degrees], 15[degrees], 20[degrees] and 25[degrees] baffle cut) are obtained. It is seen that the temperature gradually increases from 300 K at the inlet to 329 K at the outlet of the shell side. The average temperature at the outlet surface is nearly 328 K for all the three models. The pressure drop is less for 25[degrees] baffle cut compared to other models due to smoother guidance of the flow. The maximum velocity is nearly equal to 3.89 m/s for all the three models at the inlet and exit surface and the velocity magnitude reduces to zero at the baffles surface as shown in fig.4. It can be seen that compared to segmental baffle and modified 10[degrees] baffle cut, 15[degrees], 20[degrees] & 25[degrees] baffle cut, provide a smoother fluid flow.

From the CFD simulation results, for fixed tube wall and shell inlet temperatures, shell side outlet temperature and pressure drop values for varying fluid flow rates are shown in Table 3 and in Fig.5 and 6. It is found that the shell outlet temperature decreases with increasing mass flow rates as expected even the variation is minimal as shown in Fig. 7. It is found that for three mass flow rates 0.5 kg/s, 1 kg/s & 2kg/s there is so much effect on outlet temperature of the shell when the baffle cut is increased from 10[degrees] to 25[degrees]. However the shell-side pressure drop is decreased with increase in baffle cut i.e., as the cut angle is increased from 10[degrees] to 25[degrees]. The pressure drop is increased by 10% for heat exchanger with 10[degrees] baffle cut angle and pressure drop is decreased by 11% for heat exchanger with 15[degrees] baffle cut, 17% for heat exchanger with 20[degrees] baffle cut and by 22% for heat exchanger with 25[degrees] baffle cut compared to segmental baffle heat exchanger as shown in Fig. 8. Hence it can be observed that shell and tube heat exchanger with 25[degrees] baffle cut angle results in a reasonable pressure drop. Hence it can be concluded shell and tube heat exchanger with 25[degrees] baffle cut angle results in better performance compared to 10[degrees], 15[degrees], 20[degrees] cut angles and segmental baffles.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

Conclusions:

The shell side of small shell-and-tube heat exchanger is modeled with sufficient data to resolve the flow and temperature fields.

For the given geometry the mass flow rate must be below 2 kg/s, if it is increased beyond 2 kg/s the pressure drop increases rapidly with little variation in outlet temperature. The pressure drop is increased by 10%, for heat exchanger with 10[degrees] baffle cut angle, pressure drop is decreased by 11%, for 15[degrees] baffle cut, 17% for 20[degrees] baffle cut and by 22% for 25[degrees] baffle cut angle.

The maximum baffle cut angle can be 20[degrees], if the angle is beyond 25[degrees], the outlet temperature of shell side fluid decreased, so the baffle cannot be used effectively.

Hence it can be concluded shell and tube heat exchanger with 20[degrees] baffle cut angle results in better performance compared to existing segmental baffle, 10[degrees], 15[degrees] and 25[degrees] baffle cut angles.

REFERENCES

[1.] Dong, Q.W., Y.Q. Wang, M.S. Liu, 2007. 'Numerical and experimental investigation of shell side characteristics for ROD baffle heat exchanger', Applied thermal engineering, 28: 651-660.

[2.] FLUENT 6.3 user's guide, FLUENT Inc., 2006 section 12.4.2, 7.5, 7.10 and 7.13.1.

[3.] Huadong Li and Volker Kottke, 1998. 'Effect of baffle spacing on pressure drop and local heat transfer in shell and tube heat exchangers for staggered tube arrangement', International journal of heat mass transfer, 41: 1303-1311.

[4.] Jian-Fei Zhang, Ya-Ling He, Wen-Quan Tao, 2009. '3D numerical simulation on shell-and-tube heat exchangers with middle-overlapped helical baffles and continuous baffles--Part I: Numerical model and results of whole heat exchanger with middle-overlapped helical baffles', International journal of heat and mass transfer, 52: 5371-5380.

[5.] Liu, Z.C., W. Liu, Y.S. Wang, S.Y. Huang, 2009. 'Numerical investigation for flow and heat transfer in longitudinal-flow tube bundle of shell and tube heat exchanger', Turbulance Heat and mass transfer, pp: 1-10.

[6.] Nenad Radojkovic, Gradimir Ilic, Zarko Stevanovic, Mica Vukic, Dejan Mitrovic, Goran Vuckovic, 2003. 'experimental study on thermal and flow processes in shell and tube heat exchangers', Mechanical Engineering, 1: 1377-1384.

[7.] Peng, B., Q.W. Wang, C. Zhang, 2007. 'An experimental study of shell and tube heat exchangers with continuous helical baffles', Journal of heat transfer, 129: 1425-1431.

[8.] Yingshuang Wang, Zhichun Liu, Suyi Huang, Wei Liu, Weiwei Li, 2011. 'Experimental investigation of shell and tube heat exchanger with a new type of baffle' Heat mass transfer, 47: 833-839.
Nomenclature

B           Baffle spacing, mm
Cp          Specific heat, J/kg K
ID          Inside diameter of shell, mm
[d.sub.i]   Inner diameter of tube, mm
[d.sub.o]   Outer diameter of tube, mm
k           Turbulent kinetic energy
L           Effective length of tube, mm
N           Number of tubes
[DELTA]p    Shell side pressure drop, kPa
[t.sub.p]   Tube pitch, mm
[epsilon]   Turbulent energy dissipation
[lambda]    Thermal conductivity, W/m K
[mu]        Dynamic viscosity of fluid, kg/m s
[rho]       Density of fluid, kg/[m.sup.3]


(1) K. Chandrasekar, (2) B. Bala Murali, (3) T. Prabhakaran, (4) S. Jayachandran

(1,2,3,4) Assistant Professors, Department of Mechanical Engineering, SNS College of Engineering, Coimbatore.

Received 25 January 2016; Accepted 28 April 2016; Available 5 May 2016

Address For Correspondence:

K. Chandrasekar, Assistant Professors, Department of Mechanical Engineering, SNS College of Engineering, Coimbatore.

E-mail: nkchandru@gmail.com
Table 1: Geometric Parameters Of Shell And Tube Heat Exchanger

Item                        Dimension and
                            Description
Shell parameters
Inner diameter, ID          90mm
Material                    Steel
Tube parameters
Inner diameter, [d.sub.i]   16mm
Outer diameter, [d.sub.o]   20mm
Effective length, L         600mm
Number of tubes             7
Tube pitch                  30mm
Material                    Steel
Baffle parameter
Baffle pitch                86mm
Number                      6
Thickness                   5mm
Material                    Steel

Table 2: Thermophysical Properties Of Oil

Parameter                        Value

Specific Heat, Cp                2270.1 (J/kgK)
Dynamic Viscosity, [mu]          0.0095 (kg/ms)
Density, [rho]                   826.1 (kg/[m.sup.3])
Thermal Conductivity, [lambda]   0.132 (W/mK)

Table 3: Outlet Temperature & Shell-Side Pressure Drop
Values For Various Baffle Cut Degree And Mass Flow Rates

                 Mass flow rate 0.5 kg/s

Baffle cut       [DELTA]P   Outlet
                 (kPa)      temperature
                            (K)

Segmental type   18         328
10[degrees]      20         329
15[degrees]      16         328
20[degrees]      15         329
25[degrees]      14         322.5

                 Mass flow rate 1.0 kg/s

Baffle cut       [DELTA]P   Outlet
                 (kPa)      temperature
                            (K)

Segmental type   74         326
10[degrees]      81         327
15[degrees]      63         326
20[degrees]      60         327
25[degrees]      58         318

                 Mass flow rate 2.0 kg/s

Baffle cut       [DELTA]P   Outlet
                 (kPa)      temperature
                            (K)

Segmental type   297        324
10[degrees]      327        326
15[degrees]      254        326
20[degrees]      239        326
25[degrees]      227        314
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Article Details
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Author:Chandrasekar, K.; Murali, B. Bala; Prabhakaran, T.; Jayachandran, S.
Publication:Advances in Natural and Applied Sciences
Date:May 15, 2016
Words:2223
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