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Square Rod Cross-Flow — Isothermal

Average h and heat rate for an isothermal square prism in free-stream cross-flow.

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In turbulent flow h scales with velocity to the power 0.8 — doubling the air speed buys you roughly 75% more heat transfer, not double.

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Figures assume typical conditions and the stated method. For measured, guaranteed numbers on your plant, our engineers run site surveys, heat loss audits, and full process models.

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Square Rod in Cross-Flow — Isothermal

Calculates the average convective heat transfer coefficient and total heat rate for a square prism held at a uniform surface temperature in a uniform free-stream cross-flow.

Film Temperature

Fluid properties are evaluated at the film temperature:

Tf​=2Ts​+T∞​​

Reynolds Number

ReW​=νUW​

where W is the side length of the square cross-section.

Nusselt Number

Nu=0.102ReW0.675​Pr1/3

The constants C = 0.102, n = 0.675 are for a square rod with a face normal to the flow direction (Holman, Table 7-3).

Heat Transfer Coefficient and Heat Rate

h=WNu⋅k​ A=4WLr​,Q=h⋅A⋅(Ts​−T∞​)

Validity

  • Reynolds number: 5 000 – 100 000
  • Prandtl number: 0.6 – 50
  • Not suitable for liquid metals or very viscous fluids

References

  • Holman, J.P., Heat Transfer, 7th ed., McGraw-Hill, 1990, pp. 281–306.
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Frequently asked questions

Average h and heat rate for an isothermal square prism in free-stream cross-flow. Enter your inputs and press Calculate — the worked solution shows every step of the method with your numbers substituted in.

Fluid properties like viscosity and conductivity change strongly with temperature, so correlations specify evaluating them at the film temperature — the average of the surface and free-stream (or wall and bulk) temperatures. The calculator does this automatically through its built-in property database.

The underlying correlations are empirical fits to experimental data; ±10–25% is typical depending on geometry and Reynolds range. Treat the outputs as sizing estimates — for guaranteed numbers on a specific piece of plant, measurement or validated CFD is the next step.