Internal convection in tubes — The convective heat-transfer coefficient becomes essentially independent of the tube length-to-diameter (L/D) ratio when the flow is in which regime?

Introduction:
In internal forced convection, local and average heat-transfer coefficients depend on whether the flow and thermal boundary layers are still developing (entrance region) or fully developed. Designers often prefer regimes where correlations are insensitive to L/D for predictable performance, especially in long tubes and exchangers.

Given Data / Assumptions:

• Single-phase flow in a circular tube.
• Constant wall heat flux or temperature (standard boundary conditions).
• Comparing laminar vs. turbulent and developing vs. fully developed behavior.

Concept / Approach:

In fully developed turbulent flow, vigorous mixing diminishes axial development effects; average Nusselt number correlations (e.g., Dittus–Boelter, Gnielinski) are largely independent of L/D (outside short entrance lengths). In laminar flow, while a fully developed limit exists (Nu constant), practical exchangers often operate with significant entrance effects, and heat-transfer predictions remain sensitive to L/D unless the tube is very long. The question asks when the coefficient is 'not affected by L/D', which most clearly holds in fully developed, highly turbulent regimes.

Step-by-Step Solution:

Verification / Alternative check:

Heat-transfer textbooks show Nu independent of length for fully developed turbulent flow; entrance length is short (~10–60 diameters) compared to typical exchanger tubes.

Why Other Options Are Wrong:

A/B/C: Laminar/transition regimes commonly display stronger L/D sensitivity unless very long. E explicitly refers to the developing region where L/D matters.

Common Pitfalls:

Assuming laminar fully developed always applies in practice; many laminar applications are entrance-dominated.

Final Answer:

highly turbulent (fully developed turbulent region)