Finned surface pressure drop

For plate-fin heat exchangers in single-phase flow, the heat transfer coefficients are related to the developed heat transfer surface, and the area ratio must be taken into account. As related to the projected surface, the overall heat transfer coefficient is very high. Heat transfer and pressure drop can be estimated from correlations (43 44), but these correlations give only an estimate of the performance, because local modification of the fin geometry will affect heat transfer and pressure drop. 

In this section we limited our consideration to tube banks witli base surfaces (no fins). Tube banks with finned surfaces are also commonly used in practice, especially when the fluid is a gas, and heat transfer and pressure drop correlations can be found in the literature for tube banks with pin fins, plate fins, strip fins, etc. 

When extended surfaces such as fins are used to enhance natural convection heat transfer between a solid and a fluid, the flow rate of the fluid in the vicinity of the solid adjusts itself to incorporate the changes in buoyancy and friction. It is obvious that this enhancement technique will work to advantage only when the increase in btroyancy is greater than the additional friction introduced. One does not need to be concerned with pressure drop or pumping power when studying natural convection since no pumps or blowers are used in (his case. Therefore, an enhancement technique in natural convection is evaluated on heat transfer performance alone. 

Determine the surface geometrical properties on each fluid side. This includes the minimum free flow area A , heat transfer surface area A (both primary and secondary), flow lengths L, hydraulic diameter Dh, heat transfer surface area density P, the ratio of minimum free flow area to frontal area o, fin length and fin thickness for fin efficiency determination, and any specialized dimensions used for heat transfer and pressure drop correlations. 

The next problem is the calculation of the flue gas pressure drop as it crosses the selected finned tube convection section. Using the design information for tubes per row, number of rows, tube spacing, fin and tube geometry and fin pitch, compute the net free volume and friction surface of the convection section. 

From inspection of Fig. 4.50, the thermal conductance (solid line) increases as the number of fins increases but at the cost of an increasing pressure drop (dotted line). As stated previously, a low parasitic pressure drop will increase overall cycle thermal efficiency. A pressure drop of 1% was budgeted arbitrarily for the core heat transfer surface, which results in a thermal conductance of 11.3 kW/K for a fin count of 420 (fin spacing of 6 mm) as shown on Fig. 4.50. Thus for a core power level of 500 kW, the metal temperature at the fin root would be 44 K higher than the working fluid. If a lower pressure drop is considered to potentially improve the overall cycle thermal efficiency, the core temperatures would increase to accommodate the lower thermal conductance of the heat removal finned aimulus. 

Reference 9- 58 provides a summary of open literature for heat transfer and pressure drop correlations/test data for offset strip and wavy fin surface geometries. Only one set of test data for a fixed offset strip geometry have been identified for HeXe gas mixtures in the PrandtI/Reynolds regime being considered for the recuperator. Reference 9- 59 provides a summary of test data for various gas mixtures of HeXe for an interrupted plate fin geometry. 

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