Zhuojin Pipe Fitting Co., LtdExperimental Investigation of Axial Heat Conduction in Micro Steel Tubes Under Convective Heat Transfer
Literature Overview
This paper by Liu Zhigang, Xu Jianzhong, and Zhao Yaohua, published in the Journal of University of Shanghai for Science and Technology (2006, Vol. 28, No. 1, pp. 75–78), presents a systematic experimental study on the axial heat conduction behavior of stainless steel micro-tubes under forced convection conditions. The work addresses a fundamental yet practically significant question in micro-scale heat transfer: whether axial conduction along the tube wall can be neglected when calculating the apparent convective heat transfer coefficient. This is particularly relevant for engineers designing micro-channel heat exchangers, fuel cell cooling systems, and electronic cooling devices where stainless steel micro-tubes are commonly employed.
The experimental configuration involves stainless steel micro-tubes with an inner diameter of 168 μm and an outer diameter of 406 μm, yielding a wall thickness of approximately 119 μm. Two working fluids—distilled water and nitrogen gas—are used to establish liquid and gas flow regimes, respectively. Direct electrical resistance heating is applied to the tube wall, and the outer wall temperature field is measured using an infrared imaging system equipped with a dedicated magnification lens. Image processing and calibration corrections are applied to obtain precise wall temperature distributions, from which the axial heat conduction flux is derived.
Core Technical Findings
The study yields two principal conclusions that carry direct engineering implications:
- For liquid flow (distilled water): At constant Reynolds number and varying heating power, the axial heat conduction contribution along the tube wall is negligible when calculating the apparent convective heat transfer coefficient. This validates the common engineering practice of assuming one-dimensional radial heat transfer in micro-tube liquid cooling applications.
- For gas flow (nitrogen) under natural convection boundary conditions: The ratio of axial heat conduction to total heating power reaches up to 2.1%. This is a non-trivial contribution that, if ignored, would lead to systematic overestimation of the convective heat transfer coefficient by approximately the same percentage.
Key Experimental Parameters
| Parameter | Value |
|---|---|
| Inner diameter | 168 μm |
| Outer diameter | 406 μm |
| Wall thickness | ~119 μm |
| Working fluids | Distilled water, Nitrogen (N₂) |
| Heating method | Direct electrical resistance |
| Measurement technique | Infrared imaging with magnification lens |
| Axial conduction ratio (gas, natural convection) | Up to 2.1% |
| Axial conduction ratio (liquid) | Negligible |
Technical Interpretation and Engineering Implications
The physical mechanism behind these findings can be understood through the lens of thermal conductivity ratios. Stainless steel has a relatively high thermal conductivity (approximately 14–16 W/m·K at room temperature), which facilitates axial heat transport along the thin-walled micro-tube. However, the effectiveness of this axial conduction relative to convective heat transfer depends critically on the Biot number and the ratio of axial conduction resistance to convective resistance.
For liquid flow, the high convective heat transfer coefficient of water (typically 5,000–15,000 W/m²·K in micro-channels) overwhelms the axial conduction pathway, rendering the latter insignificant. In contrast, nitrogen gas has a much lower convective heat transfer coefficient (typically 500–2,000 W/m²·K in micro-channels), which allows axial conduction to play a measurable role. Under natural convection conditions, where the convective coefficient drops further, the 2.1% axial conduction contribution becomes engineering-significant.
Practical Design Recommendations
From a design perspective, the following guidelines emerge:
- Liquid-cooled micro-tube systems: The conventional assumption of radial-only heat transfer is justified, and axial conduction corrections are unnecessary for heat transfer coefficient calculations.
- Gas-cooled micro-tube systems: Engineers should apply axial conduction corrections, particularly under low-flow or natural convection conditions. A correction factor of approximately 2% should be considered for nitrogen-cooled micro-tubes.
- Temperature measurement methodology: The use of infrared imaging with magnification optics is validated as a reliable technique for micro-tube temperature field measurement, provided appropriate image processing and calibration corrections are applied.
Study Insights and Reflections
This research exemplifies the rigor required in micro-scale thermal engineering. The 2.1% axial conduction contribution, while seemingly small, can propagate through system-level thermal design calculations and lead to significant performance deviations in high-precision applications. The experimental approach—combining electrical resistance heating with infrared thermography—provides a non-contact, spatially resolved measurement method that is particularly well-suited for micro-tube applications where conventional thermocouple placement is impractical.
One area for further investigation would be the extension of these findings to higher temperature regimes where stainless steel thermal conductivity changes, and to different flow regimes (laminar vs. turbulent) at higher Reynolds numbers. Additionally, the influence of tube material properties (e.g., comparing austenitic vs. ferritic stainless steels) on axial conduction behavior would be valuable for material selection decisions in micro-heat exchanger design.