4. Heat and mass transfer enhancement of outward convex corrugated tube heat exchangers

#### 4.1 Enhancement methods

Heat transfer enhancement methods are classified into three classifications: active, passive, and compound. The active methods include electrostatic and

magnetic fields, induced pulsation, mechanical aid, vibration, and jet impingement. These methods require external activating power to enhance the heat transfer [3–6]. Passive methods modify the geometrical structure to expand the effective surface area to disturb the actual boundary layer. Compound methods combine the two heat transfer augmentation methods to increase heat transfer performance. In the above-mentioned methods, passive methods have attracted significant attention from researchers and engineers since they are user-friendly and affordable. Extensive research has been devoted to develop highly efficient heat transfer components to better understand the physical mechanisms and optimal parameters of passive heat transfer augmentation methods.

The heat transfer enhancement mechanism in the corrugated tube is described as follows. The periodically corrugated structure on the tube wall arouses periodic alteration of velocity gradient, leading to adverse and favorable pressure gradient locally. The recurrent alternation of axial pressure gradient induces the secondary disturbance, and then the produced intensive eddy destroys the flow boundary layer. The eddy also increases the turbulence intensity of the flow. The disturbance caused by corrugated structures thus increases the heat transfer coefficient drastically.

#### 4.2 Tube side enhancement

Figure 12 shows the effect of Re on Nuc with various p/D and H/D. The Nu<sup>c</sup> tends to increase linearly with the increasing Re with a fixed structure of the corrugated tube. This behavior occurs because the increases of flow velocity break wall thermal boundary layer and could obtain higher convective heat transfer coefficient. Moreover, with the decreasing p/D and increasing H/D, the values of the Nuc increase.

In order to compare the performance between corrugated tube and smooth tube, the ratio of Nu in the corrugated tube to that in the smooth tube (Nuc/Nus) is adopted to indicate the relative grow rate of heat transfer performance. Figure 13 shows the effect of Re on Nuc/Nus with various p/D and H/D, and the figure exhibits that with the increase of Re, Nuc/Nus declines deceleratedly. Moreover, the Nuc/Nus increases with the decreasing p/D and increasing H/D.

#### 4.3 Shell side enhancement

Figure 14 shows the effect of Re on Nuc with various p/D and H/D in the shell side. Compared with Figure 12, the changing tendency of Nuc along with Re, p/D,

Figure 12. Effect of Re on Nu with various p/D and H/D in the tube side. (a) various p/D (b) various H/D.

Heat and Mass Transfer in Outward Convex Corrugated Tube Heat Exchangers DOI: http://dx.doi.org/10.5772/intechopen.85494

Figure 13.

Effect of Re on Nuc/Nus with various p/D and H/D in the tube side. (a) various p/D (b) various H/D.

#### Figure 14.

Effect of Re on Nu with various p/D and H/D in the shell side. (a) various p/D (b) various H/D.

and H/D is consistent, but the Nu<sup>c</sup> in the shell side is obviously higher than in the tube side.

Figure 15 shows the effect of Re on Nuc/Nu<sup>s</sup> with various p/D and H/D in the shell side. It can be found when com ared with Figure 13, the changing tendency of Nuc/Nu<sup>s</sup> along with Re, p/D, and H/D is also consistent, but the Nuc/Nu<sup>s</sup> in the shell side is obviously higher than in the tube side.

Figure 15. Effect of Re on Nu with various p/D and H/D in the shell side. (a) various p/D (b) various H/D.

Figure 16. Effect of Re on η with various p/D and H/D in the tube side. (a) various p/D (b) various H/D.

#### 4.4 Overall enhancement

Generally, heat transfer enhancement accompanies with a penalty of flow resistance when a heat transfer enhancement component (corrugated tube in this paper) is utilized in a heat exchanger compared to the smooth tube. Therefore, an assessment criterion needs to be constructed to evaluate the overall heat transfer performance for the investigated corrugated tube. The function of overall heat transfer performance is adopted as follows:

$$\eta = (\text{Nu}\_{\text{c}}/\text{Nu}\_{\text{s}}) / \left( f\_{\text{c}}/f\_{\text{s}} \right)^{1/3} \tag{17}$$

Figure 16 indicates the effect of Re on overall heat transfer performance (η) with various p/D and H/D in the tube side of outward convex corrugated tube. The figure displays that with the increase of Re, η declines deceleratedly. This is because the Nuc/Nus gradually decreases along with increasing Re. In addition, with the increase of p/D, η decreases when Re < 30,000, but increases when Re > 30,000. This can be explained from the fact that decreasing extent of Nuc/Nus is larger than that of fc/fs with increase in p/D when Re < 30,000, but lower when Re > 30,000. Moreover, the η decreases obviously with the increasing

Figure 17. Effect of Re on η with various p/D and H/D in the shell side. (a) various p/D (b) various H/D.

Heat and Mass Transfer in Outward Convex Corrugated Tube Heat Exchangers DOI: http://dx.doi.org/10.5772/intechopen.85494

H/D, and the decreasing extent from H/D = 0.02 to H/D = 0.06 is more obvious than that from H/D = 0.06 to H/D = 0.10. This variation is quite intuitive because of the fact that increasing extent of Nuc/Nus is larger than that of fc/fs along with increasing H/D.

It can be observed from Figure 17 that the changing trend of η with various p/D and H/D in the shell side is almost the same from the tube side. However, the values of η in the shell side are larger than in the tube side. Therefore, the overall heat transfer enhancement in the shell side is superior to the tube side.
