Structure of Growing Double-Diffusive Convection Cells

When a fluid with a vertical solute gradient is heated from a sidewall, layers of convection cells form along the wall. For an aqueous solution of common salt (i.e., fixed values of Pr and τ), the convection cells will form along the vertical heated wall for values of π(= α(q/k) / β(−dS/dz) ) greater than ∼0.28. This paper reports the experimental investigation of the structure of the growing convection cells when a uniform heat flux is applied at the vertical wall. A series of tests was conducted using a small tank (23 cm high × 16 cm deep × 20 cm wide). Measurements of the vertical temperature distribution in the fluid at five different locations were taken continuously along with shadowgraph pictures to monitor the growth of the convection cells. Based on the set of data thus obtained, the following characteristics of a growing convection cell were found: (a) A convection cell with a vertical height, K, grows laterally into the quiescent fluid at a constant speed, U, and the Reynolds number of a moving front, defined as UH/ν, changes linearly with π on a log-log scale for our experimental range of π = 0.3 – 10. (b) The vertical averaged temperature inside a growing cell is a linear function of the distance from the heated wall over a major portion of the cell. (c) The lateral temperature gradient inside a cell decreases with time, and is proportional to the inverse of the elasped time.