On correction factors in thermal-hydraulic correlations for compact fin-tube bundles

Heat exchanger designers need reliable thermal hydraulic correlations to optimize heat exchanger designs, particularly when a compact volume is desired. Current correlations for circular fin-tube bundles typically employ a flow velocity dependent term and a number of correction terms to account for geometry variations and other flow conditions. This paper evaluates five phenomena that influence thermal-hydraulic performance and discusses them in relation to current models for heat transfer and pressure drop. The varied parameters are the direction of heat flow (gas heating or gas cooling), the fin type (annular or helically wound), the fin efficiency, the fin pitch, the number of streamwise tube rows and the inlet turbulence level. A validated numerical model is used to produce Nusselt and Euler numbers for more than 30 helically wound fin-tube geometries in a staggered equilateral triangle layout. A few selected geometries are further analyzed in detail. Results indicate that a large gas-to-tube temperature difference, a low fin pitch or a high thermal effectiveness can lead to situations where some of the current models for heat transfer, pressure drop or fin efficiency perform poorly. Specifically, pressure drop is shown to be 25% higher for helical fin-tubes under gas heating conditions than for equivalent annular fin-tubes under gas cooling conditions, which is not considered by most correlations. Fin efficiencies can, further, be in error by more than 20%, which can partially be attributed to flow passing outside the fin diameter. More data is, however, needed to characterize these effects thoroughly in the general case.