Bubble-induced pseudo turbulence in laminar pipe flows

Turbulence intensities and Reynolds stresses of bubble-induced pseudo turbulence in air–water laminar bubbly flows in a vertical pipe of 20 mm diameter are measured at a low liquid Reynolds number, Re = 900. A pipe made of fluorinated ethylene propylene resin, which has the same refractive index as water, is utilized to enable the LDV measurement of Reynolds stresses. Experimental data show that (1) turbulence kinetic energy of bubble-induced turbulence is proportional to local void fraction, (2) the ratio among axial, radial and azimuthal components of turbulence intensity is 2:1:1, (3) Sato’s eddy viscosity model underestimates the Reynolds shear stress of bubble-induced pseudo turbulence, whereas it qualitatively well represents the Reynolds stress distribution, (4) the Reynolds shear stress of bubble-induced turbulence depend not only on velocity gradient but also on gradient of void fraction and (5) the dissipation process of bubble-induced turbulence is analogous to that of shear-induced turbulence.

► Bubble-induced turbulence is measured without interaction with shear-induced turbulence. ► Turbulence kinetic energy of bubble-induced turbulence is proportional to the void fraction. ► Axial intensity of bubble-induced turbulence is double of the other components. ► The Sato’s model underestimates Reynolds shear stress of bubble-induced turbulence. ► Reynolds shear stress of bubble-induced turbulence depends on the gradient of void fraction.