Numerical Simulation of Fluid Flow and Heat Transfer in Shell Side of Shell and Tube Heat Exchanger

Xie Honghu Jiang Nan

(Institute of Chemical Machinery, South China University of Technology, Guangzhou 510640, China)



Abstract: In order to study the working mechanism of turbulent flow and heat transfer in the shell-side fluid flow of a longitudinal multi-spiral tube-and-tube heat exchanger, the FLUENT software is used to simulate the longitudinal multi-spiral flow under the condition that the set value of the shell-side fluid flow rate is constantly changing. Three-dimensional numerical simulation of the turbulent flow and heat transfer in the shell-and-tube heat exchanger was carried out. The temperature field, velocity field, particle trace map and shell-side heat transfer film coefficient distribution map of multi-spiral flow shell-and-tube heat exchanger under different shell-side fluid flow rates are obtained. According to the simulation results, the turbulent flow and enhanced heat transfer of the longitudinal multi-spiral shell-and-tube heat exchanger are discussed from several aspects. The simulation results were compared with the experimental results. The error between the two was about ±11%, which was in good agreement.

Key words: spiral twisted sheet; longitudinal multi-spiral flow shell and tube heat exchanger; three-dimensional numerical simulation

CLC number: TK 124 Document code: A Article ID: 1005-9954(2009)09-0009-04

The computational fluid dynamics simulation of the shell-and-tube heat exchanger without phase-change shell flow field was first proposed in 1974, but due to the limitations of computer and computational fluid dynamics conditions, the research progress is slow [1]. Since the 1980s, the numerical simulation study of heat exchangers has been carried out relatively quickly. For the numerical simulation of heat exchangers at home and abroad, more two-dimensional research is used [2]. In terms of three-dimensional research, domestic and foreign scholars have also done a lot of work, especially the numerical simulation of heat transfer performance of shell-and-tube heat exchangers with complex structures. Many foreign scholars use heat transfer tubes or tube inserts with complex structures. The simulation studies its effects on fluid flow and heat transfer, such as spiral grooved tubes, bellows, and interpolated spiral ties. However, foreign and domestic scholars rarely use numerical simulation methods to study the effect of inserts on the heat transfer performance of heat exchangers when they are inserted into the shell-and-tube heat exchanger shell rather than the tube.

Inserting a spiral twisted piece between the shell-side heat exchange tubes, the insertion of the spiral twisted sheet can effectively change the flow form of the shell-side fluid, so that the shell-side fluid generates a plurality of complex flow patterns from the spiral flow [3], effectively improving heat transfer. The fluid velocity of the wall of the tube bundle enables sufficient mixing of fluids at different shell radii to achieve enhanced heat transfer. In this paper, the three-dimensional numerical simulation of the shell-side turbulent flow and heat transfer of the new longitudinal multi-spiral flow shell-and-tube heat exchanger is carried out by FLUENT software. According to the simulation results, the shell-side fluid of the heat exchanger is enhanced by the spiral twist sheet. The mechanism of heat transfer has been discussed in a useful way.

1 simulation model

The heat exchangers used in the simulation are single-tube, single-shell and spiral-twisted structures. The heat exchanger is a square tube, and Figure 1 is a three-dimensional schematic diagram of the Pro/e of the spiral twisted sheet. Figure 2 is a schematic view showing the relationship between the heat exchange tube and the spiral twisted sheet.



Since the shell-side structure of the longitudinal multi-spiral tube-and-tube heat exchanger is relatively complicated, the tetrahedral mesh is used, and the tube process is divided by a hexahedral mesh. There are four types of boundary types in this model: inlet, outlet, pipe wall and shell wall [4-5]. When the mathematical form of the simulation model is established, it is mainly considered to set the continuity equation, the mass equation, the momentum equation and the energy equation for the fluid in the tube and shell processes to satisfy the control conservation. The turbulent k-ε model is further set because the shell-side fluid is in a turbulent state. After the relevant settings are completed, an iterative calculation is performed. When each iteration is about 210 times, the calculation converges and the residual curve is analyzed.

2 Analysis and discussion of numerical simulation results

A total of 7 sets of experimental data were simulated. The shell side was a hot fluid during the simulation. The inlet temperature was 60 ° C, the tube length was cold fluid, and the inlet temperature was 20 ° C. The tube flow rate is constant at 8 m3/h, and the shell flow fluid flow starts at 5 m3/h and then increases with a gradient of 1 m3/h up to 11 m3/h. The following is a simulation result obtained when the shell-side fluid flow rate is 9 m3/h.

2.1 Temperature vector field map

Figure 3 shows the partial layout of the temperature field in the radial section at Z = 600 mm (the ordinate temperature in the figure varies from 290 to 340 K). It can be seen from Fig. 3 that the temperature is sequentially decreased from the shell side to the tube length, and there is a temperature gradient. The change in tube temperature is the higher the temperature near the tube wall. A closer look at the shell-side fluid in Figure 3 reveals that due to the insertion of the helical twisted sheet, a temperature gradient occurs between the two parallel-inserted helical twisted sheets inside the shell-side fluid. This temperature gradient continues to heat transfer. At the contact of the pipe wall, the influence of the change on the heat transfer can be clearly seen from the figure, which is not available in the heat exchange of the ordinary light pipe shell-and-tube heat exchanger.



2.2 Axial section flow field

Shell-side fluid velocity vector field The ordinate in Figure 4 represents the shell-side fluid velocity, which varies from 1.52E-02 to 5.4E+02m/s. It can be seen from Fig. 4 that the shell fluid of the longitudinal multi-spiral flow shell-and-tube heat exchanger is highly violently mixed due to the spoiler effect of the spiral twisted sheet. And there is a very strong additional spiral flow [6], which enhances the separation of the boundary layer, increases the degree of turbulence between the shell-side fluids, and promotes the heat transfer between the tube and shell. The heat transfer efficiency is greatly improved, and the simulation results well verify the conclusions obtained from the experimental research.



2.3 axial direction fluid mass trace diagram

Figure 5 is a trace diagram of the fluid mass point when the shell-side fluid flows in the axial direction. The ordinate indicates that the shell-side fluid particle velocity varies from 0 to 1. 19 E+03 m/s. As can be seen from Fig. 5, the fluid also performs additional spiral flow while performing axial flow. The form of the spiral flow is very similar to the spiral structure of the spiral twisted sheet, because the spiral twisted sheet has a spiral flow guiding action, and the shell-side fluid flows along the surface of the spiral twisted sheet. Further research has found that the shell-side fluid does not spirally flow as a whole, but is divided into different streams that flow along different spiral twisted sheets.



Figure 3 shows the left, middle and right diagrams of the radial cross-section fluid mass point at the axial direction Z of the longitudinal multi-spiral flow shell-and-tube heat exchanger at 100, 600, 1 100 mm. It can be seen from Fig. 6 that the shell-side fluid always carries out respective longitudinal additional spiral flows around the twelve heat exchange tubes and under the direction of the flow of the spiral twist sheets. Further analysis shows that the degree of turbulence of the fluid particles is increasing along the axis.



2.4 Pressure drop distribution of shell fluid along the axial direction

Fig. 7 is a pressure drop distribution diagram of the shell-side fluid in the longitudinal multi-spiral flow shell-and-tube heat exchanger along the axial direction. In the figure, the ordinate indicates the pressure drop, which varies from 200 to 1 200 Pa, and the abscissa indicates that the heat exchanger axis is located at 0 to 1 200 mm. It can be seen from Fig. 7 that the pressure drop of the shell fluid in the axial direction has a periodicity, and the pressure drop trend line is composed of 12 small line segments. The total length of the spiral torsion is 1 200 mm and the pitch of the spiral is 100 mm, so the pitch of the spiral torsion is 12, which agrees well. The change of pressure drop along the axial direction of the shell-side fluid is mainly caused by the additional spiral flow of the fluid caused by the helical structure of the twisted piece. The additional spiral flow causes the shell-side fluid to flow at high speed along the surface of the spiral twisted piece, and the flow direction changes continuously. The turbulence intensity is intensified and the boundary layer separation is enhanced, resulting in a pressure drop in the axial direction. When simulated with a shell-side fluid flow of 9 m3/h, the simulated shell-side fluid has a pressure drop of approximately 750 Pa. The experimentally obtained pressure drop is 675 Pa, with an error of approximately 11%.



2.5 Heat transfer film coefficient distribution map

Fig. 8 is a distribution diagram of the heat transfer film coefficient of the shell side of the longitudinal multi-spiral tube-and-tube heat exchanger, and the ordinate indicates the coefficient of the heat transfer film, and the range of variation is 0-1. 35E+04W / (m2·K). It can be seen from Fig. 8 that the shell-side heat transfer film coefficient distribution is uneven, and the average value is about 5 500 W / (m 2 · K). When the shell-side fluid flow rate is 9 m3/h, the experimental shell-side fluid heat transfer film coefficient is 6 000 W / (m 2 · K), and the simulation results are about 9% smaller than the experimental results.



3 Comparison of experimental results with simulation results

The heat exchanger used in the experimental research in this paper has the same size parameters as the heat exchanger used in the simulation research. It is a shell-and-tube heat exchanger with a heat exchange tube length of 1 200 mm, an inner diameter of 15 mm and a shell inner diameter of 109 mm. The data collected by the experiment included shell flow, inlet and outlet pressure, temperature, tube flow, and inlet and outlet temperatures. The instrument used has an accuracy level of ±0. 2% temperature sensor Pt100, pressure sensor, flow sensor and so on.

Figure 9 is a comparison of the shell-pass heat transfer modulus coefficient h obtained by numerical simulation and experimental study. It can be seen from Fig. 9 that whether it is an experiment or a simulation, the obtained h increases with the increase of the Reynolds number Re of the shell, and the increasing trend of the two is consistent. Also, when Re is the same, the experimental value is always larger than the analog value. The shell-side fluid flow rate is 11 m3/h, and the h difference is the largest. The simulated h is about 11% smaller than the experimental value. The system error is mainly caused by improper installation of the sensor. The probes of the sensors are mounted near the axis of the heat exchanger shell, so that the measured data is one-sided and larger than the actual value.



4 Conclusion

Through the three-dimensional numerical simulation of the turbulent flow and heat transfer of the shell-side fluid in the longitudinal multi-spiral tube-and-tube heat exchanger, the temperature vector field, the velocity vector field of the shell-side fluid and the particle traces of the fluid flowing along the axial direction are obtained. . It can be seen from the simulation results that, due to the insertion of the spiral twisted sheet, the form of the shell-side fluid flow is similar to that of the spiral twisted sheet, and is divided into a plurality of streams which are respectively spirally flowed along the lead of the respective spiral twisted sheets. Due to the periodic spiral of the spiral twisted sheet structure, the fluid near the wall surface of the tube produces a distinct periodic spiral flow, which enhances the disturbance of the fluid near the wall of the tube bundle, increases the thermal diffusivity, and promotes fluid mixing. Moreover, the periodic spiral flow can effectively reduce the thickness of the boundary layer, especially the viscous underlayer, so that the heat transfer is enhanced, so that the heat transfer film coefficient of the shell-side fluid is greatly improved. The simulation results show that the new longitudinal multi-spiral flow shell-and-tube heat exchanger with simple structure and convenient disassembly has many advantages and has a broad application prospect.

The simulation results are compared with the experimental values. The error between the two is within ±11%, which is in good agreement and meets the engineering requirements.

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