Question

Which of these gives the statement of one-dimensional steady-state diffusion problem?

(a) The diffusive flux of Φ leaving the exit face is the same as the diffusive flux of Φ entering the inlet face

(b) The diffusive flux of Φ leaving the exit face plus the diffusive flux of Φ entering the inlet face is equal to the generation of Φ

(c) The diffusive flux of Φ leaving the exit face minus the diffusive flux of Φ entering the inlet face is equal to the generation of Φ

(d) The diffusive flux of Φ leaving the exit face is the same in magnitude and opposite in direction as the diffusive flux of Φ entering the inlet face

I got this question during an online exam.

This is a very interesting question from FVM for 1-D Steady State Diffusion in division Diffusion Problem of Computational Fluid Dynamics

Answer 1

Right answer is (c) The diffusive flux of Φ leaving the exit face minus the diffusive flux of Φ entering the inlet face is equal to the generation of Φ

For explanation I would say: The diffusive flux of Φ leaving the exit face minus the diffusive flux of Φ entering the inlet face is equal to the generation of Φ. It constitutes the balance equation over the control volume. This ensures conservation.