Comparative assessment of classical heat resistance and Wilson plot test techniques used for determining convective heat transfer coefficient of thermo fluids

Abstract

The present study relates to testing the thermo fluids used for solar thermal power plants for their heat transfer coefficients. Two techniques – the Classical Heat Resistance Method and – the Wilson Plot Technique has been used to experimentally calculate the heat transfer coefficients. In comparison to the Classical Heat Resistance Method, the Wilson Plot Technique is able to cater to the transition flow regime as well as the two-phase fluids which is the nature of the existing commercially available solar grade heat transfer oil “based on diphenyl oxide-diphenyl chemistry” used for parabolic trough power plants. An analysis of repeatability and reproducibility of the two test techniques using the coefficient of variation and standard error of mean calculations shows that the heat transfer coefficients obtained through Wilson Plot Technique is within the experimentally acceptable range of 10% while those obtained from Classical Test Method falls beyond the acceptable range but are still within the workable range of 20%. The standard error of mean also exhibit similar results with the values in case of Wilson Method being much smaller than those in case of Classical Heat Resistance Method. Based on the test results obtained and for the specific geometry of the test setup, the authors have proposed an empirical relationship between the velocity of the oil flowing inside the tube and the mean temperature of the oil including a correction factor of 2.3. Finally, the convective heat transfer coefficient of oil hi up to the permissible operational temperature range of 400 °C of the solar grade oil was calculated based on the proposed empirical relationship. The Wilson Plot Technique shows better fit with the commonly used Dittus-Boelter equation of calculating the convective heat transfer coefficient of oils. The method proposed in the work shall help experimentally evaluate the new chemistries used for heat transfer oils for their convective heat transfer coefficients.