The heat-transfer capability is closely related to total heat
transfer coefficient and inner diameter. The relationship between
the heat transfer capability and dimensionless inner tube diameter
R1/R3 is shown in Fig. 12 on condition that the outer tube diameter
and interface wall thickness are fixed. The heat-transfer capability
first increases and then decreases gradually as the inner radius increases.
The optimal radius range is similar by employing the present
model and uniform wall heat flux model [20,21] while the
corresponding heat transfer capability of the present model is lower.
The gained optimal inner radius range is 0.6–0.7. This will provide
helpful guideline for design of double pipe heat exchanger.