Second law analysis of a gravity-driven liquid film flowing along an inclined plate

Abstract

The second law analysis of a gravity-driven liquid having variable viscosity and flowing along an inclined plate subjected to constant wall temperature or constant heat flux was studied. In this study, temperature-dependent viscosity was considered which made it difficult to solve the governing equations of velocity and energy analytically. The CFD package Fluent was used to find the velocity gradients and temperature gradients, and the results were used to find the entropy generation rate. The results for different cases of different inclination angles, different wall temperature, and different heat flux were analyzed and illustrated graphically. We found that the temperature-dependent viscosity has a big effect on the velocity profile and consequently on the entropy generation rate especially near the heated plate. In comparison with entropy generation for the case of constant physical properties, there is significant difference in the entropy generation rate. We found that the entropy generation rate for all cases has its maximum values near the plate surface and decreases sharply towards the free surface. It was observed that the entropy generation rate decreases in the longitudinal axial direction for all cases. It was shown that the irreversibility due to heat transfer dominates near the plate, and the irreversibility due to the fluid friction dominates near the free surface. Changing the inclination angle and identically changing the Peclet number has no significant effect in the entropy generation rate except near the plate surface. However, entropy generation rate increases with increasing plate temperature, increasing heat flux, or decreasing group parameter.