Jet Impact Heat Transfer Characteristics of Hot-Rolled Seamless Steel Pipes

Literature Overview

Published in Machinery Design and Manufacture (2025, Issue 4), this research from the State Key Laboratory of Rolling and Automation at Northeastern University addresses a critical gap in seamless steel pipe production technology: the uniformity of controlled cooling for annular cross-sections. The study is supported by the National Natural Science Foundation of China (Steel Joint Fund Key Project U1860201 and Youth Fund Project 51804074). The researchers developed a finite element model of annular jet impact cooling and employed orthogonal experimental design to systematically investigate the influence of nozzle diameter, jet flow rate, inclination angle, and jet number on cooling uniformity.

Technical Significance and Background

Controlled cooling technology, already well-established in flat steel product production, represents a transformative opportunity for seamless steel pipe manufacturing. However, the annular cross-section of seamless pipes presents unique challenges that flat products do not encounter. The cooling uniformity requirement is stringent because non-uniform cooling in a pipe wall creates differential thermal stresses that can lead to:

• Residual stress concentrations at the pipe wall
• Uneven microstructure development across the wall thickness
• Dimensional distortion including ovality development
• Variations in mechanical properties along the pipe circumference

The inability to achieve uniform cooling has been the primary barrier to implementing online controlled cooling in seamless pipe production lines, limiting the ability to optimize mechanical properties through thermal processing.

Key Process Parameters and Optimal Ranges

The study identifies the following optimal parameter ranges through combined numerical simulation and experimental validation:

The critical performance metric is the range (extreme difference) of average heat transfer coefficients across different zones of the pipe cross-section. The study demonstrates that proper matching of jet flow rate to nozzle diameter can control this range within 1000 W/(m²·K). This represents a significant engineering achievement because it establishes quantifiable design criteria for cooling system optimization.

Heat Transfer Mechanism Analysis

The annular jet impact cooling process involves complex fluid-structure interactions. When a jet impinges on a curved cylindrical surface, the flow behavior differs substantially from flat-surface impingement. Key phenomena include:

1. Impingement zone: Direct jet impact creates a high-velocity stagnation point with maximum heat transfer coefficient. The jet spreads radially along the curved surface.
2. Wetting zone: Downstream of the impingement point, the boundary layer develops and heat transfer decreases. On a curved surface, the boundary layer separation behavior differs from flat plates.
3. Turbulent intensity effects: The study specifically examines turbulent intensity as a governing parameter. Higher turbulent intensity enhances near-wall mixing and promotes heat transfer, but excessive turbulence can cause jet detachment from the surface.

The inclination angle of 0–10° represents an optimal compromise. At zero degrees (normal impingement), the heat transfer coefficient is maximized at the stagnation point but coverage is limited. At larger angles, coverage improves but the normal component of heat transfer decreases. The 10° maximum maintains sufficient normal velocity component while extending the wetted area along the pipe circumference.

Engineering Application Considerations

For seamless pipe production plants considering implementation of online controlled cooling, several practical considerations emerge from this research:

• Nozzle array design: The 8–12 jet configuration must be arranged to provide uniform circumferential coverage. For typical seamless pipe diameters (50–300 mm), this translates to specific nozzle spacing requirements that must account for manufacturing tolerances and alignment accuracy.
• Flow rate matching: The 6–8 L/min range is specific to the pipe diameters and materials studied. Scaling to larger diameters or different steel grades requires re-evaluation of the optimal flow rate to maintain the target heat transfer coefficient range.
• Process integration: Online cooling must be synchronized with the rolling speed to maintain consistent cooling rates. The nonlinear mathematical model developed in this study provides the basis for developing control algorithms that adjust jet parameters in real-time based on rolling speed and material temperature.
• Material-specific optimization: Different steel grades (carbon steel, low-alloy steel, stainless steel) have different cooling sensitivity windows. The optimal cooling rate for austenite transformation control in low-alloy steels differs significantly from that required for grain refinement in carbon steels.

Nonlinear Mathematical Model and Predictive Capability

The study's development of a nonlinear mathematical model relating average cooling rate to process parameters represents a valuable engineering tool. Such models enable:

• Rapid evaluation of alternative cooling configurations without physical testing
• Optimization of cooling parameters for specific material targets (e.g., achieving a target microstructure or mechanical property)
• Integration into process simulation frameworks for full production line modeling
• Development of feedback control systems that maintain target cooling rates despite process disturbances

Summary

This research provides a systematic framework for optimizing annular jet impact cooling of hot-rolled seamless steel pipes, establishing that nozzle diameter of 3–4 mm, flow rate of 6–8 L/min, inclination angle of 0–10°, and 8–12 jet streams achieve cooling uniformity within 1000 W/(m²·K). The nonlinear mathematical model developed enables predictive process design, and the findings directly address the long-standing challenge of implementing controlled cooling in seamless pipe production to achieve uniform microstructure and mechanical properties across the pipe wall.