A. Pressure drop in banks of finned tubes

Pressure drop in banks of finned tubes is a function of the tube geometric parameters as illustrated in Figure 16076.2. The velocity profiles and boundary-layer thicknesses around the tubes are determined by the parameters illustrated and, of course, by fluid properties and velocity. Significant differences are noted between fluid flows on the first-row tubes and those in the inner rows. A high degree of turbulence exists inside the bank, in the intertube and interfin spaces. In the first two rows, separation of the boundary layer at φ ≈ 90° (see Figure 16076.1) is followed by a circulation zone. Effects of the turbulence on boundary-layer separation are observed from the third or fourth row onward. Comparisons with tubes in the first and the second rows suggests that the separation point for an inner tube is shifted downstream, and that the circulation zone is smaller in volume and more complex in structure. The lower the fin height h and the longer the fin spacing, the more is the flow on a finned tube similar to that on a plain tube. Conversely, the higher and closer the fins, the more the flow is similar to that in a slot.

Pressure drop over a finned-tube bank is a function of flow velocity, bank geometry, fin geometry, and fluid physical properties:

\[\label{eq1} \Delta p=f(u,d,s,h,\delta_1,\delta_2,s_1,s_2,z,\eta,\rho)\tag{1}\]

Fin thickness is often found to have no significant effect on the pressure drop, and a simplified dimensionless form of Equation 1 may be written: