Experimental investigation of air entrainment in a vertical liquid jet flowing down onto a rotating roll

The experimental results of air entrainment into a liquid jet flowing down onto a rotating roller were presented by means of the impinging jet method. The impinging jet was a substitute for a liquid curtain in an industrial c oating process. Roll velocity at the onset of air entrainment was measured for different flow rates, nozzle diameters, nozzle heights, impingement ang les, viscosities and surface tensions. The results indicated that the inertial force of the jet can delay the onse t of air entrainment and that it acts most effectively on the dynamic wetti ng line when the flow rate is set to make the wetting line locate at the im pingement point. The effect of inertial force decreased with further increa sing flow rate and the onset velocity reached a constant value which depend ed only on the physical properties of the fluid. The results of dimensionless analyses indicated that there are two critical values of the Reynolds number, i.e., the one at which the onset velocity h as a peak and another above which the onset velocity does not depend on the Reynolds number. In the Reynolds number above the second critical value, m ajor forces acting bn the dynamic wetting line were viscous drag force and surface tension, and the flow mechanism at the onset of air entrainment was the same as that in plunging tape flow. Application of the results to curtain coating flow indicated that the criti cal Reynolds number exists not only in plunging jet flow but also in plungi ng curtain flow. (C) 1999 Elsevier Science Ltd. All rights reserved.