Effect of orifice geometry on bubble formation in melt gas injection to prepare aluminum foams

Abstract

The bubble formation process at submerged orifices with different geometry is investigated in the preparation of aluminum foams by gas injection method. The bubble profile on a horizontal plate is calculated by quasi-static analysis through Laplace equation. The bubble formation process is then distinguished into three stages: nucleation stage, growth stage and detachment stage in wetting and less wetting conditions based on the force balance analysis. In addition, the bubble size at high Reynolds number is obtained by considering the contribution of buoyancy, pressure force, inertial force, drag force and surface tension based on the three stages of bubble formation. The bubble size is confirmed to be sensitive to the equivalent contact angle, which consists of two terms including the contact angle and the wedge angle. Therefore, the wedge angle is introduced in the design of gas outlet orifices for the purpose of decreasing bubble size generated. The experimental study is conducted at three different types of stainless steel orifices under constant gas flow rates (0.05–2 L/min). It is clarified that the orifice geometry and the orifice size are both responsible for the cell size of aluminum foams. The experimental results for three different types of orifices show a consistent trend with the theoretical predictions at various gas flow rates. In the design of orifices to generate small bubbles in the melt, the wedge angle that coordinates with the contact angle is thus suggested.