The effect of inclination on the convection of mercury in a small chamber heated from below was studied. Mercury's low viscosity and high thermal conductivity makes it a low Prandtl number fluid. Because it is opaque, traditional optical techniques for visualizing flow states were not possible. Instead, we made use of the variation of the speed of sound with temperature. The time between sending an ultrasound pulse and receiving its echo was converted to a temperature measurement. Flow states were deduced from the temperature distributions. A phase diagram of flow states for different angles of inclination and different Rayleigh numbers was produced.
In the horizontal chamber with low temperature differences across the mercury layer, cellular convection rolls parallel to shorter side of the chamber are observed. With higher temperature differences, they are not observed. As the angle of inclination is increased, the range in which cellular convection rolls are observed decreases. In the ranges where rolls are not observed, it is likely that the rolls have changed their orientation by 90 degrees, changing from transverse to longitudinal. Our measurement technique does not allow visualization of longitudinal rolls. Linear stability theory indicates that longitudinal rolls occur when thermal effects are important, while transverse rolls occur when hydrodynamic effects are important. The flow state in which there are no visible transverse rolls is found to be more efficient at heat transfer than the transverse roll state. This supports the idea that at higher temperature differences, when no transverse rolls are observed, longitudinal rolls are present.