Calibrating for viewing angle effect during heat transfer measurements on a curved surface

Liquid crystal thermography (LCT) has been widely used for the determination of surface heat transfer distribution. However, this technique is sensitive to illumination and viewing angle and therefore limited to surfaces with only slight curvature. A liquid crystal calibration technique using true-colour image processing system has now been developed to alleviate the effect of viewing angle on oblique/curved surfaces. Application of the calibration and transient liquid crystal thermographic techniques and uncertainty analysis to a heated air slot jet impinging on a semi-cylindrical convex surface has been demonstrated. It is shown that the local heat transfer coefficient may be overestimated by up to 39.1% at a viewing angle of 69° from the normal under test conditions. However, the overall uncertainty in heat transfer coefficient can be significantly reduced from the maximum value of 36.3% to within 11.1% by using the implemented viewing calibration technique.