Gas dispersion phenomena at a narrow slot submerged in a liquid

Abstract When air or helium was injected upwards into water or methanol through long, narrow slots of width 50 ⩽ w ⩽ 250 μm formed in a horizontal surface, small bubbles were generated spontaneously and continuously at an array of discrete sources distributed distance λ apart along the slot. The dimensionless source spacing was found by experiment to be λ w = 17.2(ϱ l /ϱ g ) 0.16 We −0.25 , where (ρ l /ρ g ) is the liquid—gas density ratio and We is the modified Weber number for the slot flow. ( We = U 2 s w ϱ l /σ, where U s is the superficial gas velocity in the slot and σ is the liquid interfacial tension). The existence and spacing of the sources is attributed to the formation of curvature-dependent Rayleigh-Taylor nodes at the “most dangerous” wavelength on the gas—liquid interface in the slot. A new analysis of the node wavelength is presented for this interface configuration. Bubble volume V b was found to increase with U s up to a critical value of U s given for water by the dimensional equation U s = 115ϱ g −0.12 w −0.068 , beyond which lateral coalescence occurred between neighboring growth sites with a consequent breakdown of the discrete source array.