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.