Bénard Cells

In Bénard cell experiments, structures emerge that increase the dissipation of the temperature gradient beyond that possible through conduction. The apparatus consists of a heated plate below, a cooling plate above, and a working fluid (air, silicon...) in between. Heat is transferred (Q=heat transfer) from the heated plate through the fluid to the cooling plate. Initially all dissipation (heat transfer) through the fluid occurs via conduction and molecule to molecule interaction. When the gradient reaches a critical level (Rayleigh number 1760) the transition to highly organized convection occurs. As the gradient is increased it becomes harder and harder (more work is required) to maintain the higher rate of dissipation . The further the system is moved away from its equilibrium state the more exergy is destroyed, the system produces more entropy, and more work is required to maintain it in it's nonequilibrium state. Due to the convective overturn most of the working fluid in the container becomes vertically isothermal (with little gradient) and only the boundary layers on the edge of the system carry the gradient. As the gradient is increased the boundary layers become thinner and more dissipation occurs.