A correction approach of external wall surface temperature measurements in sub-millimeter and high length-diameter ratios tubes

The heat transfer characteristics of working medium in sub-millimeter and high length-diameter ratios tubes have broad application potential in various fields. However, accurately measuring the external wall surface temperature remains challenging due to several factors. In the non-axial end region, property variations of the working medium and non-uniform tube wall thickness complicate the measurement of external wall temperatures that reflect true heat transfer characteristics. In the axial end region, the external wall temperature is affected by the axial heat dissipation. This study introduces a correction approach for external wall temperature measurement. In the non-axial end region, multiple thermocouples are circumferentially welded to the tube surface. The heat transfer coefficients at these weld locations are averaged to represent the comprehensive heat transfer characteristics across the tube's cross-section. By combining these measurements with fluid temperatures that better reflect the local flow state, correction values for the inner wall temperature are derived, enabling accurate determination of the external wall temperatures. In the axial end region, corrections are applied by compensating for heat transfer to the condenser. Experimental validation demonstrates the effectiveness of the proposed method. After circumferential temperature correction, the average relative error of external wall temperature measurements decreases from −10.1 % to 1.1 %. Similarly, axial temperature correction reduces the average relative error from 32.4 % to 13 %. These results confirm the accuracy and robustness of the correction approach, offering a reliable solution for heat transfer characterization in sub-millimeter and high length-diameter ratios tubes.