Chemoconvection of miscible solutions in an inclined layer

Abstract

This paper presents an experimental and numerical investigation of the chemoconvective flow of two miscible reacting solutions that generate a plain layer oriented at some angle to the gravity field. In the experiments, the aqueous solutions of nitric acid and sodium hydroxide were used. The system evolves from an initial state, in which each homogeneous solution occupies half of the layer, and the contact surface between the layers is flat. When the reacting solutions come into contact with each other, an acid-base neutralization reaction occurs, forming salt and water. The system configuration is chosen so that the acid solution of lower density rests on top of the denser base. This excludes the development of the Rayleigh–Taylor instability. The experiments were performed for such initial concentrations of the reagents at which a wave regime is realized in the system. The wave comprises a density jump rapidly advancing in the direction of gravity and separating the immobile alkali solution and the layer of acid and salt mixture, the convective motion of which feeds the frontal reaction with fresh acid. The visualization of the flow in the experiments was performed using a Fizeau interferometer. The numerical study of a complete three-dimensional problem was carried out using the ANSYS CFX hydrodynamic simulation program. We studied the flow evolution at a gradual increase in the angle of inclination from 0° to 70°. We found that the layer inclination leads to a significant change in the structure and intensity of the convective motion. Already at small inclination angles (up to 30°), the flow becomes three-dimensional, which makes the Hele-Shaw approximation inapplicable to this case. We show that there is a spontaneous stratification of salt and acid concentration fields in the cocurrent wave flow. With an increase in the angle of inclination, the wave velocity decreases, and chemoconvection in the cocurrent flow becomes less intense and acquires a certain vortex structure. At larger angles (from 50° to 70°), the wave front is strongly deformed or the wave breaks up. We demonstrate that an up-and-down fluid flow develops in the layer above the density jump. This flow eventually loses its stability with respect to the vertical rolls of solutal Rayleigh convection. There is a good agreement between the experimental measurements and the results of numerical simulation of the 3D problem.