Heat Transfer Phenomena in Transitional Flow Regimes

Heat transfer in transitional flow regimes occurs where fluid motion evolves from orderly laminar behaviour to chaotic turbulence, typically at Reynolds numbers around 2 300 to 4 000. In this intermediate state, convective heat‐transfer coefficients and pressure‐drop characteristics depart from classical laminar or turbulent correlations, exhibiting pronounced sensitivity to buoyancy forces, surface heating and geometric features. Small perturbations—such as surface inserts, corrugations or changes in channel inclination—can precipitate an early onset of transition, markedly enhancing the Nusselt number while incurring a friction‐factor penalty. Understanding these phenomena is critical for optimising engineering systems ranging from compact heat exchangers and nuclear reactor coolant channels to solar air heaters, where maximising thermal performance without excessive pumping power is essential. Recent advances in high‐resolution flow visualisation, precision thermal measurement and numerical methods calibrated for transition-aware turbulence models are beginning to resolve the complex interplay of inertial, viscous and buoyant effects. Such insights are guiding the development of refined correlations and predictive tools that bridge the gap between laminar and turbulent heat‐transfer models, with global significance for energy‐efficient design and process intensification.

Research from Nature Portfolio

Recent studies have experimentally examined the influence of inclination and surface inserts on transitional heat transfer. An investigation into inclined air heat exchangers fitted with spring turbulators revealed that inclination angles as low as 15° can advance the onset of transition under uniform surface heating, yielding up to 100 per cent enhancement in Nusselt number compared to plain channels, with a corresponding rise in friction factor. This work delineated the Reynolds number ranges over which laminar, transitional and turbulent regimes prevail at varied heat fluxes and inclinations, providing essential benchmarks for thermo‐hydraulic design.

Heat Transfer Phenomena in Transitional Flow Regimes publication trend

The graph below shows the total number of articles in heat transfer phenomena in transitional flow regimes across all publications each year (not limited to Nature Index journals).

Technical terms

Transitional flow regime: The intermediate state between laminar and turbulent flow, characterised by intermittent vortical structures and heightened sensitivity to perturbations.

Reynolds number: A dimensionless parameter expressing the ratio of inertial to viscous forces, used to predict the flow regime.

Nusselt number: A dimensionless heat‐transfer coefficient representing the ratio of convective to conductive heat transfer across a boundary.

Mixed convection: Heat transfer resulting from the combined effects of forced flow and buoyancy-driven motion.

Friction factor: A dimensionless measure of pressure loss due to viscous effects in internal flow.

References

1. Thermal and flow dynamics of an inclined air heat exchanger equipped with spring turbulators in the transition flow regime. Scientific Reports (2024).
2. The local heat transfer characteristics associated with mixed convective developing flow through a horizontal tube exposed to a uniform wall temperature boundary condition. International Journal of Thermal Sciences (2024).
3. The Influence of Forced Convective Heat Transfer on Hybrid Nanofluid Flow in a Heat Exchanger with Elliptical Corrugated Tubes: Numerical Analyses and Optimization. Applied Sciences (2022).