Foams and Microfoams
Superficial Gas Velocity for Onset of Foaming
Pneumatic foams do not form for any arbitrarily small gas flow rate. Laimbock (1998) has observed that a minimum superficial gas velocity should be reached to initiate foaming of molten glass. The same observations have been made for different aqueous solutions (Jeelani and Hartland, 1990 and Ghag et al., 1998), as well as for molten steel. Figure 2 shows a typical plot of the steady-state foam thickness as a function of the superficial gas velocity and demonstrate the existence of a minimum superficial gas velocity for onset of foaming jm.
Whether foam is desirable or not, it is of fundamental and practical interest to understand the foaming process and to predict the conditions under which foam starts forming in order to operate a process under the most desirable conditions, the minimum superficial gas velocity for onset of foaming should be determined as a function of the thermophysical properties of the two phases and the operating conditions. However, to the best of our knowledge, no quantitative analysis for predicting jm is available in the literature.
Figure 1: Schematic of the behavior of a foaming solution as the superficial gas velocity is increased (a) bubbly flow without foam, (b) onset of foaming, (c) developed foam layer.
This project aims to better understand and predict the conditions for onset of foaming of semi-batch foams by developing a quantitative self-contained model giving the minimum superficial gas velocity as a function of the thermophysical properties of the two phases and the operating conditions.
L. Pilon and R. Viskanta, 2004. Minimum Superficial Gas Velocity for Onset of Foaming. Chemical Engineering and Processing, Vol.43, No.2, pp.149-160. doi:10.1016/S0255-2701(03)00012-6