Comparison of G-Hot Plate and Three Point Methods to Measure Thermal Conductivity of Fibrous Insulation

Laboratory methods for steady-state and transient testing are two well-known approaches for measuring the heat transfer coefficient. Transient methods are more widely used due to their lower cost and broader temperature range. However, their application in fibrous insulation is more limited due to the low thermal conductivity and non-homogeneous nature of these materials. Guarded hot plate and three-point methods are two significant transient techniques for measuring the heat transfer coefficient in fibrous insulations. This paper compares these two testing methods, with the primary evaluation criterion being the minimum variance in the final experimental results. The variance of the final results is inversely proportional to the determinant of the sensitivity matrix. To compute the sensitivity matrix, the transient temperature distribution across the insulation thickness is first calculated using an initial estimate of the heat transfer coefficient, obtained from other experimental sources. Changes in the number and placement of thermal sensors, the insulation geometry, and the test duration affect the determinant of the sensitivity matrix. The optimal experiment is determined by identifying the maximum determinant of this matrix. Studies show that in the three-point method, the optimal setup consists of two thermal sensors placed on an insulation layer with a thickness of 2 cm, over a duration of 100 seconds. In contrast, in the guarded hot plate method, the optimal setup includes two thermal sensors on insulation with a height of 5 cm, over a 300-second testing period. The analysis indicates that the variance of the final results in the three-point method is lower than in the guarded hot plate method, which can significantly enhance the accuracy of heat transfer coefficient measurements.