8. Concluding remarks

Most of the correlations that could be met in literature are derived from the experimental studies, but unlike the common approach, some researchers try to extract the correlations from their numerical experience such as Kim et al. [2]. In the study, 39 different models are analyzed numerically by commercial CFD software where different geometric parameters and varying fluids are tried. It is also underlined that the earlier correlations are not sufficient to estimate the Colburn j-factor and friction factor when the blockage ratio (β) increases (β =12–27%). So there is a demand to find the answer of that question. The analysis are carried out by different turbulent models and the most suitable one is decided by comparing the findings with the most preferred correlations in literature [3, 4] and then the remaining calculations are performed to obtain a new correlation to attain the best fitting one not only for different fluids but also for different blockage ratios. The correlations are grouped as regarding to their blockage ratio (β). Since the blockage ratios below 20% are not efficient for offset strip fins and very high pressure drop could be seen beyond 35%, the blockage range investigated in the study is taken as 20–35%, the Colburn j-factor and friction f-factor can be correlated as in

As mentioned earlier, the blockage ratio is not the only objective of the study [2], but the Prandtl effect is also observed by the usage of different fluids such as water; the correlations according to Prandtl can also be found as in the lower part of Table 3. These correlations are compared by the Manglik and Bergles for the blockage ratio of 12 and 27% in

Figure 16. Comparison of Kim et al. correlation and Manglik and Bergles correlation for the blockage ratio of β = 12% and

Table 3.

52 Heat Exchangers– Advanced Features and Applications

β = 27% for friction factor [2].

In this particular chapter, the ultimate purpose is to inform and address in detail about the studies on offset strip fins heat exchangers that have been performed by the researchers for decades and also to call a strong attention to the superior advantages of their use. The basics of this structure are reported; in addition, the investigations found in the literature are summarized by considering their objectives. The parametric effects of the structural, experimental and numerical research of the fin under varying flow regimes and conditions, the evolution of heat transfer and friction factors under different flow conditions are all given with regard to the objective of the investigations, which are cited. The outcomes that have to be highlighted at the end of this section can be listed as in the following:


Other performance criteria derived and used for the offset strip fins are flow area goodness factor j/f, the ratio j/f 1/3 and thermal hydraulic performance factor JF. With regard to the investigations:

• JF factor could be useful for the water while j/f1/3 is more convenient for gas-oil liquid. In contrary to the given two factors, j/f couldn't be considered as a good performance criterion for fluids.

The reliable prediction of the heat transfer and pressure drop in offset strip fin is crucial. The correlations derived in the studies are presented together in Table 3 in order to provide a better comparison.

• With regard to the coverage, good agreement with the experimental data and mostly cited correlation pertains to Manglik and Bergles among the given equations.


#### Nomenclature

Comprehensive Study of Compact Heat Exchangers with Offset Strip Fin http://dx.doi.org/10.5772/66749 55


• Both of these factors (j and f) increase with the decrease of fin length.

distance decreases.

54 Heat Exchangers– Advanced Features and Applications

ment of the thickness.

factor j/f, the ratio j/f

criterion for fluids.

investigations:

comparison.

Nomenclature

• Colburn factor j-factor decreases, while the friction factor f increases when fin bending

• The flow streams could be illustrated depending on the calculated data, which provide to visualize the regime evolution. In addition, the recirculation zones emerge with the incre-

Other performance criteria derived and used for the offset strip fins are flow area goodness

• JF factor could be useful for the water while j/f1/3 is more convenient for gas-oil liquid. In contrary to the given two factors, j/f couldn't be considered as a good performance

The reliable prediction of the heat transfer and pressure drop in offset strip fin is crucial. The correlations derived in the studies are presented together in Table 3 in order to provide a better

• With regard to the coverage, good agreement with the experimental data and mostly cited

correlation pertains to Manglik and Bergles among the given equations.

Ac Heat transfer area of the cold side, m2 Ac,<sup>f</sup> Cross-sectional area of the fin, m2

Ah Heat transfer area of the hot side, m<sup>2</sup> At Heat transfer area of the tube, m<sup>2</sup> Cf Fin bending distance, mm

cp, <sup>c</sup> Specific heat of the cold stream, kJ/(kg °C) cp, <sup>h</sup> Specific heat of the hot stream, kJ/(kg °C)

Af Frontal area, m<sup>2</sup>

Cr Heat capacity ratio D Diameter, mm

De Hydraulic diameter, mm f Fanning friction factor

h Height of the fin, mm

f <sup>R</sup> Reference value of the friction factor

hc Heat transfer coefficient of the cold (external) stream, W/(m2 °C) hh Heat transfer coefficient of the hot (internal) stream, W/(m<sup>2</sup> °C)

1/3 and thermal hydraulic performance factor JF. With regard to the

Greek letters

