Figure 297.1 shows six commonly used surface geometries for plate-fin, compact heat exchangers. Typical fin pitches are 300 to 800 fins per meter, although as many as 1,200 fins per meter are used in automotive applications. Because of the small hydraulic diameter and low density of gases, these surfaces are usually operated with 500 < Re_Dh < 1,500 (hydraulic diameter basis). Although increased performance will exist at higher Reynolds numbers (turbulent regime), fan-power limitations generally limit operation to the above low-Reynolds number range. To be effective, an enhancement technique must be applied to low-Reynolds number flows.

The heat transfer and friction data are normally presented in the form j = St Pr^(2/3), and f versus the Reynolds number (Re) based on the hydraulic diameter. This approach is somewhat arbitrary, because several variations of one basic type of surface geometry will generally not correlate on the j and f versus Re basis. This is because geometric variables, other than the hydraulic diameter, may have a significant effect on surface performance. For example, in laminar duct flows, j and f are influenced by the channel shape, or aspect ratio. Because the values of j, f, and Re are dimensionless, the test data are applicable to surfaces of any hydraulic diameter, provided that geometric similarity is maintained.

A standard reference for the heat transfer and friction data of plate-fin heat exchanger surfaces is the book by Kays and London (1984), Compact Heat Exchangers. This book gives j and f versus Reynolds number plots for 52 different plate-fin surface geometries. Similar data are also included for tube banks, fin-tube heat exchangers, and crossed-rod matrices. However, the Kays and London (1984) data are all prior to 1964. Creswick et al. (1964) present a very complete report of existing data as of 1964, some of which does not appear in Kays and London. Since the original publication of the Kays and London book in 1964, additional performance data have been published. Recent additions include the following:

1. Perforated surfaces (Mondt and Siegla, 1974; Shah, 1975; Shah, 1975; Liang and Yang, 1975; Pucci et al., 1967)
   
   
2. Offset-strip fins for gases (Shah, 1975; Liang and Yang, 1975; Pucci et al., 1967; London and Shah, 1968)
   
   
3. Offset-strip fins for liquids (London and Shah, 1968; Sparrow et al., 1977; Mochizuki and Yagi, 1977; Dubrovskii and Fedotva, 1972)
   
   
4. Louvered fins (Smith, 1972; Wong and Smith, 1966; Davenport, 1983; Aoki et al., 1989; Fujikake et al., 1983; Chang and Wang, 1996)
   
   
5. Pin fins and wire screens (Theoclitus, 1966; Hamaguchi et al., 1983; Torikoshi and Kawabata, 1989)
   
   
6. Wavy and herringbone fins (Kays and London, 1984; Goldstein and Sparrow, 1977; Ali and Ramadhyani, 1992; O'Brien and Sparrow, 1982; Sparrow and Hossfeld, 1984; Molki and Yuen, 1986; Dong et al., 2007)
   
   
7. Vortex generators (Brockmeier, 1993; Tiggelbeck et al., 1994)
   
   
8. Metal foam (Kim et al., 2000; Klett et al., 2000; Bhattacharya and Mahajan, 2002; Asby et al., 2000; Calmidi and Mahajan, 2000)

A compilation of j and f versus Re plots is not presented here because of space limitations. Such data are readily available in the book by Kays and London (1984) and the above references. Webb and Kim (2005) provide detail on many of the studies listed above. Substantial work has been done to develop correlations of j and f versus Re. This work is discussed in Section 301.