Visual froth height measurements and the clear liquid heights on 20% free area dual-flow trays, operated at total reflux, have been obtained on the homogeneous system of water-steam, on the heterogeneous systems of ethyl acetate-water and ethyl acetate-ethanol-water, and in the homogeneous region near the homogeneous ternary azeotrope of that ternary system. The experimental results were obtained under distilling conditions at a total pressure of 101.3 kPa for a range of liquid and vapor densities and vapor velocities. The churn flow conditions on the dual-flow trays at total reflux were found to have the characteristic of constant volume fraction liquid in the froth for all systems investigated: Phi (L) = 0.291; h(f) < 0.300. A correlating equation for the froth height that contains the impact pressure was found for all systems investigated to be h(f) = 2.625(1/2 rho (V)u(2))rootp(V)/rho (L) with h(f) < 0.300. A dimensionless correlating form, based on a modified Froude number, was found to be suitable for all systems: Fr-M = 280.0 x 10(-6)(rho (V)/rho (L))(-1/2) with hf < 0.300. The clear liquid height was found for all systems to follow h(CL) = Phi (L)h(f) with h(f) < 0.300. The dimensionless correlation for froth heights for all systems could be converted to a flooding vapor velocity relationship at total reflux for dual-flow trays by u(fl) = 0.0283 rootZ(rho (V)/rho (L))(-3/4) with Z < 0.300.