Experiment on the Flow Characteristics of Film Self-Excited Flapping Jet

A flexible film fixed at one end is installed at the exit of the jet nozzle. Under the condition of sufficient flow rate, the jet and the flexible film are mutually induced to produce self-excited flapping. Aiming at this phenomenon, a new self-excited flapping jet mixing technology was proposed. By means of a fluorinated ethylene propylene (FEP) film with fixed length L=(0.5~2)D and thickness δ=50 μm on a tapering nozzle with diameter D=40 mm, the pressure loss caused by smooth tapering nozzle and film flapping was measured using a pressure differential meter. The film motion was displayed and the flapping amplitude (A) was determined with a laser light source and a high-speed camera. The influences of film length and Reynolds number (Re) on A were investigated, where the measured flapping amplitude (A) and frequency (f) were used as the characteristic scales of Strouhal number (St=fA/U_o, U_o is jet exit velocity). Under Re=3×10⁴, the distribution of the axial velocity along the jet centerline was measured at different film lengths by using hot-wire anemometer, and then the turbulence intensity, probability density function and other quantities were calculated and analyzed. Moreover, digital iterative filtering was used to obtain the integral, Taylor and Kolmogorov scales along the jet centerline. These experimental results show that the turbulence intensity in the flapping jet is higher than that in the free jet, which means that the former has a stronger large-scale entrainment capacity to the surrounding fluid. However, the turbulent characteristics of the flapping jet are different at a variety of film lengths. Within the film length range in the present experiment, the film with length L=1.25D performs best in jet mixing. By investigating the probability density function and its skewness factor (Su) and flat factor (Fu), it is found that compared with the free jet, the velocity distribution in the flapping jet approaches the Gaussian distribution much quicker, which means that the flapping jet not only enhances the large-scale entrainment, but also promotes the small-scale mixing.