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3 computational model and experimental verification

3.1 Calculation Model

Due to the current computer hardware and software limitations, accurate simulation of the heat exchanger as a whole is far from difficult. Wang calibration and other studies using the "flow cell" approach to establish the calculation model can solve the vertical flow heat exchanger shell numerical simulation of flow and heat transfer [6]. Clamping structure has the traditional rod baffle heat exchanger similar to the characteristics of the cycle, so cycle model used to characterize the fully developed flow field characteristics. Based on the periodic unit flow model to Figure 1, shown as the basic unit of the shaded cross-section, taken along the flow direction of a geometric cycle model (Figure 2).This model used the momentum and energy equations are second-order upwind discretization scheme, the pressure-velocity coupling the SIMPLE algorithm. As a result of variable non-linear relationship between the strong, iterative selection-relaxation method.

Flow channels in cell cycle of iterative solution. Computational domain to take the entire unit flow, the grid number of 5.2 ¡Á 10 5, along the flow direction of the standard grid and non-uniform grid block division, local refinement refined.

Boundary conditions: for a given mass flow rate cycle model imports and temperature; wall temperature is constant, non-penetration and no-slip conditions. Model includes a heat exchanger shell-side flow, Re varied from 1 000 to 15 000, covering the common shell-flow velocity and diameter composite structures of different sizes.

3.2 Experimental setup

Reducing experimental device is a clamp-type heat exchanger piping, internal diameter of 147 mm, large pipe diameter 19 mm, small tube diameter of 14 mm, baffle grid spacing of 100 ~ 200 mm; shell-work media the air, the working medium for the over-tube of water vapor in order to ensure a constant wall temperature. Experimental setup shown in Figure 3.

3.3 Result Analysis

Table 1 is part of the heat transfer and pressure drop data. As can be seen from Table 1, calculated results and experimental values or less, the maximum difference of 6.7% in heat transfer; pressure drop difference between the more, the maximum difference of 11.8%. The main reason for the discrepancy between the experimental model analysis of small, and the "flow cell" hypothesis more different conditions. According to the laboratory analysis of the accumulation of industrial data, the experimental model due to the size and impact of imports and exports, the possible maximum error of 5.6% drop, heat transfer error can be ignored; Therefore, the experimental device has a high experimental accuracy. The numerical simulation results were compared with experimental results, numerical simulation has a higher accuracy, can be used in different-diameter pipe clamp structure of the shell fabric heat transfer and flow forecasts.

4 Results and analysis of field synergy

This article uses the cell flow model, the influence of boundary layer heat transfer performance at the main analysis, the use of the energy equation and the boundary layer equations derived in the holding structure in the horizontal straight pipe and the convergence of the first two parts of the velocity and temperature gradient field synergies.

In the two-dimensional incompressible fluid flow, ignoring the longitudinal heat conduction, the energy equation

Using the FLUENT software on the shell side of the clamp structure simulation, to obtain any point within the shell fluid flow rate, size, direction and distribution of information, and can be vectors, contour maps and other graphic displays. In this paper the boundary conditions: medium is air; mass flow rate of 0.020 kg / s; Re of 3853; baffle grid spacing of 50 mm. Clamping structure extraction unit shell-side flow and temperature fields of information for analysis.

Figure 4 shows the fluid flow through the baffle element in time, will produce vortex behind the baffle element. Figure 5 point A in Figure 4 enlarged map, you can clearly see, because the presence of reflux, the rate of recirculation zone with the main area opposite direction; fluid level in the straight pipe through the next baffle element when the fluid will after a convergence stage, flow, flow rate changes.




These kinds of, Rex, x Nu, Pr, respectively, the average Reynolds number along the tube length, the average Nusselt number, Prandtl number. To simplify calculations, engineering calculations desirable average flow Reynolds number and average Nusselt number.

As can be seen from the above derivation, the boundary layer heat transfer tubes in the middle of the analysis, the rear of the baffle straight pipe gate play a major role in heat transfer, and its diffuser field synergy angle ¦Â <¦Â level of straight pipe, we can see in accounts for the main heat diffuser portion of the best heat transfer; clamping structure which is a major reason for enhanced heat transfer. This conclusion and the flow field, temperature field analysis is consistent; other tubes have similar conclusions.

Figure 6 is through the development of custom programs to extract the speed - temperature gradient field synergy angle cloud. In order to obtain a clear display of the results, the angle shows the range of 75 ¡ã ~ 110 ¡ã, other angles are not shown. For x = 0 cross section, from the figure we can clearly see that in the back near the baffle element, speed - a large temperature gradient field synergy angle, heat transfer is poor (more than 90 ¡ã). This part is extremely small, does not affect the heat transfer. Over this period, the field synergy between the good, the area has good heat transfer performance; With the development of flow and thermal boundary layer development, field synergy gradually worse, until the next baffle element. This is consistent with the conclusions derived earlier, but also with the flow field, temperature field analysis is consistent. The figure also clearly shows the clamping structure for enhanced heat transfer irreplaceable role.


From the launch of two regional co-angle formulas, you can analyze hatched side of the flow field synergy angle ¦Â with the variation of Re. As Re increases, the shell side of the mean flow field synergy angle ¦Â has increased.

5 Conclusion

In this paper, the establishment of computational fluid dynamics (CFD) "unit flow channel" model, the structure of different-diameter piping, heat exchanger shell side of the clamp single-phase turbulent flow in the flow field, temperature field coupled three-dimensional numerical simulation. Confirmed by experimental comparison:

(1) numerical simulation of the model to accurately reflect changes in the shell-side flow and heat transfer laws.

(2) confirmed the structure of different-diameter piping is an excellent gripping longitudinal flow of the supporting structure.

(3) the method by numerical simulation analysis of the velocity vector and temperature gradient vector and the angle between the two fluid heat transfer capability of the role, structure of shell-derived field synergy grip angle formula, confirmed the structure with more good market synergies. This method is also suitable for other similar studies.

References:

[1] Qi-Wu Dong, Jin-Xing Wu, Min-Shan Liu, etc. support plate heat exchanger shell Numerical prediction [J]. Pressure vessel, 2003,20 (8): 4-7.

[2] Dong Qi-wu, Liu Min-shan, Zhao Xiao-dong.Research on the Characteristic of Shellside Support Structures of

Heat Exchanger with Longitudinal Flow of Shellside Fluid [J]. IASME Transaction, 2005,2 (8): 1491 ~ 1498.