Numerically-based parametric analysis of plain fin and tube compact heat exchangers

• The increase in efficacy of compact heat exchangers is numerically investigated.
• An analysis methodology based on local and global energy balances is provided.
• Quantitative assessment of the heat transfer streamwise-allocation is supplied.
• The energy transferred before the fluid reaches the fins and tubes is identified.
• Influence of Re number, tube diameter, fin spacing and tube layout is examined.

In the present study, the hydrodynamic and heat transfer characteristics of six-tube-row compact fin and tube heat exchangers have been investigated numerically by introducing a methodology of analysis based on local and global energy balances, from three-dimensional velocity and temperature fields. The aim is to analyze the influence of operating conditions and the geometry to design more efficient devices. Tube diameter, fin spacing and tube layout are the geometrical parameters; the over-tube fluid velocity, via Reynolds number, is used as the parameter of operation. Using the procedure, along with the concept of fraction of the total heat rate available for the specific device, the results have shown that, fluid velocity plays an important role, whereas the role of tube layout is minor, and that the effects of tube diameter and fin spacing are closely related to the magnitude of the fluid velocity. For small velocities, 99% of the heat rate that could be potentially achieved occurs closer to the inlet of the device, whereas for large velocity-values, nearly the entire length is necessary. In addition, the influence of the tube-diameter on both heat transfer and pressure drop is negligible at small velocities but progressively increases at bigger velocity-values; a similar –but less pronounced– effect is produced by fin spacing, where the influence of tube-diameter is relevant only for larger fin spacings. The approach introduced here is useful in providing, at the expense of some generality, high accuracy and clear information on the thermal convection process in these devices, since intermediate steps via Nusselt numbers and heat transfer coefficients are not needed.