Fig. 8 Jet and mixed flow. (a) The front side; (b) The reverse side; (c) Diagram of JMF.
The flow pattern of the JMF is shown in Fig. 8. The operating conditions are L W = 156 m3/(m2*h), F s= 4.0 (m/s*(kg/m3)0.5). The gas kinetic energy factor increases. The liquid film on the unit surface is blown away by the air jet stream, and the distribution becomes more unevenly distributed. As shown in Fig. 8b, the liquid phase blown out by the gas airflow at the back of the unit is ejected. The dispersed droplets and columns flow disorderly and mix. The flow mechanism of the flow pattern is similar to that of the DMF.

3.2. Interaction intensity and operating conditions for different flow patterns

First, the acquired differential pressure signal is processed to remove the singularity, and the moving average method is used to denoise the signal to obtain the time-domain characteristic diagram of the differential pressure signal29. Second, the Welch average period method, where the Hanning window is adapted is used to estimate the frequency spectrum density (PSD) of the de-noised differential pressure signal30. Finally, combined with the domain signal and power spectrum density, the interaction intensity of the gas-liquid is analyzed, and the operating field under different liquid distributions of flow patterns can be divided 31.

3.2.1. Time-domain analysis for the pressure pulsation signal

Fig. 9 and Fig. 10 show the time domain signals of the differential pressure of all the flow patterns. The pressure signals of the gas phase in the time domain under overflow distribution are shown in Fig. 9a, and the liquid spray density is L W = 78 m3/(m2*h). With the increasing kinetic energy factor of the gas phase, the average amplitude of the pressure difference pulsation presents a stepped-upward trend. For the range of the kinetic energy factor of the gas phaseF s ≤ 1.2(m/s*(kg/m3)0.5), the pulsation change in the pressure difference is stable. In this operating range, the flow pattern is a bilateral flow. The surface and sieve holes of the unit are covered by the liquid film. The gas phase mainly flows through the unit in the form of rotational flow. In this case, the gas velocity is low, and the interaction between the gas and liquid phases is weak; therefore, the pressure difference fluctuation is weak.
When the gas-phase kinetic factor F s rises to 1.6 (m/s*(kg/m3)0.5), the pressure difference amplitude increases as the gas velocity increases, and the flow pattern of the unit starts to change into CPF, under the squeezing influence of the gas phase. At this time, the gas phase perforation is still hindered by the liquid layer at the sieve holes, and the proportion of the rotational liquid phase is large. Although the gas-liquid two-phase interaction is slightly strengthened, the overall gas velocity is relatively small, and the pressure difference fluctuation is stable. When the kinetic factor increases toF s = 2.4 (m/s*(kg/m3)0.5), the gas phase propulsion becomes prominent, and the flow pattern changes from CPF to DMF. The liquid layer above the sieve holes is continuously blown out by the airflow and replenished immediately. The gas-liquid phase is mixed on the back of the unit. The interaction force between the two phases is increased, resulting in a sharp fluctuation in the pressure difference signal.
The time-domain signal changes of differential pressure pulsation are shown in Fig. 9b. The liquid spray density is L W= 156 m3/(m2*h). The pulsation amplitude of the differential pressure signal gradually increases with the increasing gas-phase kinetic energy factor, which is similar to that under the overflow distribution. The flow pattern changes into the FJF when the gas phase kinetic energy factor F s ≤ 1.6 (m/s*(kg/m3)0.5). At this point, the gas-phase velocity is small, and the influence of the gas phase decreases. The regular jet flow of the liquid is dominant on the reverse side of the unit, and the mixing degree of gas-liquid is low. The fluctuation of the differential pressure signal is stable.