(88d) A Numerical Revisit on Mass Transfer Experiments at a Free Surface in a Turbulent Open Channel Flow

This study deals with mass transfer mechanism into a turbulent liquid at a shear-free gas-liquid interface (or free surface) in an open channel. The author carried out several laboratory experiments on the mass transfer in the early 1990's [1], and found that the normalized mass transfer coefficient, Sh (Sherwood number), is proportional to about 1 power of the Reynolds number. The laboratory experiments did not measure three-dimensional turbulence structures in turbulent water, therefore, interactions of turbulence with the free surface still remain unknown. A numerical revisit on mass transfer experiments in the turbulent open channel flows has been attempted in the present study to confirm suitability of the previous laboratory experiments on the mass transfer mechanism. In addition, details of the three-dimensional turbulence structures in the turbulent liquid which boost mass flux at the free surface are explored. The results of the present numerical study show that the numerically predicted mass transfer coefficients by the DNS technique agree well with our previous laboratory measurements [1], as shown in Fig. 1. However, the predicted Sh in the present DNS is proportional to 3/4, power of Rem, rather than 1 as found in the laboratory experiments. The difference of the exponent of Rem could be a reason of underestimation of Sh in the numerical predictions for larger Reynolds number turbulence. The details of the discrepancy between the laboratory and numerical experiments on the prediction of the mass transfer coefficient, as well as three-dimensional interactions of turbulence with the free surface, will be given in the conference. [1] Komori, S., Nagaosa. R. and Murakami, Y.: AIChE J. 37, 957 (1990).

Figure 1: The comparison of the normalized mass transfer coefficients Sh in both the laboratory experiments (open circles) and the numerical predictions (closed circles). The dashed line indicate that Sh is proportional to 3/4 power of Rem.