Validation of the Gnielinski correlation for evaluation of heat transfer coefficient of enhanced tubes by non-linear regression model: An experimental study of absorption refrigeration system

The Wilson plot method has widely used to determine general heat transfer correlations, but this technique needs considerable experimental data. The Gnielinski correlation, however, simplifies the evaluation of the inside heat transfer coefficient of heating tubes only by using friction factor and hydraulic diameter. Nowadays, enhanced tubes, are used in an absorption refrigeration system to improve the thermal performance of the system. As these enhanced tubes have various heat transfer surfaces, the Wilson plot method based on numerous measured experimental data has been responsible for the evaluation of the Nusselt correlation; the validation of the Gnielinski correlation has not been found yet for enhanced tubes. The current study, thus, pursues to find the validity of the Gnielinski correlation to simplicate the assessment of the inside heat transfer coefficient of enhanced tubes over the Wilson plot method. First, a non-linear regression scheme developed by the Wilson plot method examined the Nusselt correlations of three different types of enhanced tubes including a smooth tube; friction factors of enhanced tubes, then, were measured to apply the Gnielinski correlation. Gnielinski correlation was modified using 103 data points and a new correlation was obtained, which showed a total mean error of 2.1%, and an agreement within +/- 5%.