Thermal Response Characteristics of Micro Steel Pipe Piles Under Winter Conditions

Literature Overview

This research by Xu Jian and colleagues from Henan Polytechnic University investigates the thermal response characteristics of micro steel pipe energy piles under winter operating conditions. Combining field test data with numerical simulation methods, the study examines the effects of fluid flow velocity and pile arrangement configuration on heat exchange efficiency and thermal stress development. The research provides critical data for the design of ground-source heat pump systems utilizing steel pipe piles as heat exchangers.

Thermal Performance Characteristics

Parameter	Value/Range	Engineering Significance
Optimal flow velocity	0.51 to 0.77 m/s	Maximum heat exchange efficiency per unit pumping power
Maximum axial additional tensile stress	53.8% of concrete tensile strength design value	Within safe limits, no structural failure expected
Thermal stress-temperature relationship	σ_T = 110ΔT (MPa/°C)	Design equation for thermal stress calculation
Heat exchange efficiency trend	Nonlinear increase with velocity, then plateau	Diminishing returns above optimal velocity

Numerical Model and Thermal Analysis

The numerical model captures the coupled thermal-mechanical behavior of the micro steel pipe pile system. Key thermal parameters include:

Heat exchange mechanism: Convective heat transfer between the circulating fluid and the steel pipe inner wall, followed by conductive heat transfer through the steel wall and surrounding concrete to the ground
Winter operating mode: The system extracts heat from the ground, causing the pile temperature to decrease below the ambient ground temperature
Thermal stress development: As the pile cools, differential contraction between the steel pipe and surrounding soil/concrete generates axial tensile stresses in the pile

The thermal stress equation σ_T = 110ΔT provides a direct design tool. For a temperature difference of 10°C between the pile and surrounding ground, the induced thermal stress reaches 1100 kPa, which must be accounted for in the pile's axial capacity verification.

Engineering Design Implications

For steel pipe manufacturing and system design:

Pipe specification: Micro steel pipes used as energy piles typically have outer diameters of 32-57mm with wall thicknesses of 2.0-3.0mm. The material grade should be Q235 or Q345 with appropriate corrosion protection.
Flow velocity optimization: Operating within the 0.51-0.77 m/s range maximizes the ratio of heat extracted to pumping energy consumed. Exceeding this range increases pumping costs without proportional thermal gains.
Thermal stress management: The 53.8% utilization of tensile capacity under maximum thermal loading leaves adequate safety margin. However, combined thermal and mechanical loading scenarios require careful verification per GB 50007 foundation design codes.
Pile arrangement: The configuration of micro piles affects the thermal interaction between adjacent piles. Dense arrangements may cause thermal depletion zones that reduce long-term system efficiency.

Quality Control and Monitoring

Inspection Item	Method	Acceptance Criteria
Steel pipe wall thickness	UT measurement	Within ±10% of nominal
Weld quality (if applicable)	RT or PT	No discontinuities per GB/T 3323
Pipe straightness	Visual and gauge measurement	≤ 1/1000 of length
Thermal performance verification	Field temperature logging	Within 15% of design heat flux

Study Insights

The research demonstrates that micro steel pipe energy piles represent a practical solution for integrating geothermal energy extraction with structural foundation systems. The thermal stress findings confirm that properly designed systems operate within safe stress limits even under extreme winter conditions. The nonlinear heat exchange efficiency curve emphasizes the importance of hydraulic optimization in system design, as over-designed flow velocities waste pumping energy without improving thermal performance. For steel pipe suppliers, this research highlights the need for consistent wall thickness control and straightness specifications, as these geometric properties directly affect the convective heat transfer coefficient and overall system efficiency.