Fig. 10 Time-domain diagram of differential pressure pulsation under different spray densities. (a) F s = 1.2 (m/s*(kg/m3)0.5) and (b)F s = 2.0 (m/s*(kg/m3)0.5) in overflow distribution; (c) F s = 1.2 (m/s*(kg/m3)0.5) and (d)F s = 2.4 (m/s*(kg/m3)0.5) in spray distribution.
Fig. 10 shows the time-domain diagram of the pulsation signal under different spray densities with the gas-phase kinetic factorsF s = 1.2 (m/s*(kg/m3)0.5) andF s = 2.0 (m/s*(kg/m3)0.5). In Fig. 10a, the pressure difference amplitude does not increase with the increasing spray density. The fluctuation of the pressure difference decreases as the spray density increases. The flow pattern is changed to a BFF with the spray density of L W < 104 m3/(m2*h). At this point, the gas phase perforated flow is hindered by the liquid layer at the sieve holes. With the decreasing liquid phase spray density, the liquid layer becomes thinner. The blocking effect on the gas-phase flow is weakened, and the influence of the gas flow gradually dominates. The film rupture decreases and condensation film reconstruction increases, resulting in pressure loss. The flow pattern changes into CPF with the spray density of L W = 104 m3/(m2*h). Then, the liquid layer of the unit thickens, and the liquid flow is more stable. The blocking effect of the liquid film on the gas-phase perforated flow at the sieve holes reaches the maximum, and the proportion of the gas-phase perforation decreases, causing the decrease of pressure difference fluctuation.
As shown in Fig. 10b, the gas-phase kinetic factor increases. The pressure difference first increases with increasing liquid spray density and then stabilizes in the range of 40 Pa. The flow pattern corresponds to the DMF with the liquid spray density of L W = 26 m3/(m2*h). Then, the liquid layer of the sieve holes is dispersed by the airflow. The gas-phase proportion of the perforated flow increases, and the gas-liquid interaction is strong. The pressure difference fluctuates greatly; however, the blocking effect on the gas phase is weak owing to the small liquid spray density. In addition, the pressure amplitude is small. With the liquid spray density of L W = 52 m3/(m2*h), the liquid layer on the surface of the unit thickens, and the flow pattern is unchanged. However, the resistance of the gas-phase perforated flow increases, causing an increase in the pressure difference. When the spray density is L W > 52 m3/(m2*h), the flow pattern changes to CPF. The gas phase of the perforated flow is hindered, and the pressure difference becomes larger. The gas phase is dominated by the rotational flow. The flow becomes stable, and the pressure difference fluctuation gradually decreases.
Fig. 10c, d shows the time-domain diagram of the differential pressure pulsation signal under different liquid spray densities with the gas-phase kinetic factors F s = 1.2 (m/s*(kg/m3)0.5) andF s = 2.4 (m/s*(kg/m3)0.5). The pressure difference amplitude increases with the liquid spray density concentrated in the range of 0–10 Pa, and the pressure difference fluctuates greatly. The corresponding flow patterns within the operating range are film and jet flow. Based on the characteristics of the flow type, the spray columns hit the surface of the unit, forming a dispersed liquid film and jet stream. In this process, the liquid phase loses energy during impact, dispersion, convergence, injection, etc., resulting in a strong pressure difference fluctuation. However, the effect on the liquid phase and the pressure difference is small, because of the low velocity of the gas phase.
As shown in Fig. 10d, the gas-phase kinetic factor becomes larger. The pressure difference amplitude increases significantly with the spray density, while the pressure difference fluctuation changes similarly. Under this operation, the flow patterns are converted into JMF, the rate of collapse and reconstruction of the liquid layer above the sieve holes is directly affected by the spray density. The increase in spray density increases the rate of liquid layer reconstruction, which increases the resistance of the gas-phase perforated flow, and the pressure difference becomes larger. Because of the consistent flow patterns in the operation range, the gas-liquid interaction intensity and pressure difference fluctuation are similar.

3.2.2. Frequency domain analysis for pressure pulsation signal

The distribution of the power spectral density (PSD) under different kinetic energy factors of the gas phase in the overflow and spray distributions are shown in Fig. 11, and the PSD values increase with the gas-phase kinetic factor. According to the characteristics of each flow pattern, under the joint action of the perforated and mixing flow of gas-liquid, the first and second main frequencies and corresponding density values will change correspondingly. The variation of the PSD with the gas-phase kinetic energy factor with a liquid phase spray density ofL W = 78 m3/(m2*h) is shown in Fig. 11a. When the spray density is F s ≤ 1.2 (m/s*(kg/m3)0.5), the flow pattern is BFF. The main frequencies change in the range of (2.52 Hz–5.28 Hz), and the PSD values change within (0.0040 dB/Hz–0.0070 dB/Hz). The liquid is coated on the surface of the unit, while the gas phase mainly flows through the unit in the form of rotational flow. The gas phase in the perforated flow is minimal, and the mixture strength of the gas phase is lower. The pressure signal carries less energy, and the PSD values are small under each main frequency. For increasing gas kinetic factors, the values of the factor are in the range of 1.6–2.0 (m/s*(kg/m3)0.5). The two main frequencies are slightly reduced, and the PSD values are slightly increased within (0.0098 dB/Hz–0.0226 dB/Hz), corresponding to the main frequencies within (2.48 Hz–4.72 Hz). Then, the flow pattern is transformed into a continuous flow, and the driving force of the gas phase begins to dominate. The gaps in the liquid layer above the sieve holes keep changing alternately. The gas-phase ratio of perforated flow increases, and the perforation flow of the gas-phase and gas-liquid mixing strength become larger. For the gas kinetic factorF s > 2.0 (m/s*(kg/m3)0.5), the flow pattern is dispersion-mixing flow. The frequency range of the main frequencies begins to expand (2.44 Hz–5.4 Hz), and the PSD values increase (0.0098 dB/Hz–0.0226 dB/Hz). The liquid layer on the unit is dispersed by the airflow, which further enhances the renewal frequency of the liquid film, while the droplets on the back of the unit are constantly mixed. The gas-liquid interaction is intense, and the energy of the main frequencies are increased.
The variation of the PSD with gas-phase kinetic factors under the liquid spray density L W = 156 m3/(m2*h) in spray distribution for the liquid phase is shown in Fig. 11b. For factorF s ≤1.6 (m/s*(kg/m3)0.5), the two main frequencies change in the range of (2.48 Hz–5.16 Hz), and the PSD varies in (0.0109 dB/Hz–0.0203 dB/Hz). The flow pattern on the unit is FJF. The stability of the liquid layer above the sieve holes is poor, and the perforated flow of the gas phase occupies a certain proportion. At this moment, the velocity of the gas phase is low, and the resistance of perforated flow for the gas phase is large. The perforated strength for the gas phase shows minimal change, and the PSD values of the two main frequencies are more stable within the operating range. For the increasing gas kinetic factor range F s ≥ 2.0 (m/s*(kg/m3)0.5), the variation of the two main frequencies expand to the high frequency range (2.36 Hz–5.36 Hz). The PSD values are improved (0.0234 dB/Hz–0.0758 dB/Hz), and sub-peaks appear in the high-frequency range. In this operating range, the flow pattern changes to JMF, and the liquid layer above the sieve holes begins to blow away. The gas-phase perforation strength and two-phase mixing increases, and the flow field is chaotic on the reverse of the unit. Moreover, the frequency energy, number of sub-peaks, and energy for the pressure signal increases significantly with the increasing gas-phase kinetic factors. The gas-liquid contact degree is further strengthened.