Gas-liquid flow visualization

The intermittent gas-liquid flow is characterized by high pressure and flow rate fluctuations, so that an extremely careful design of the pipeline components (valves, orifices, etc.) is required; unfortunately, very little is known about the flow structure inside such devices, so that very often they turn out to be not so efficient as those used for single-phase flow. Although the instantaneous visualization of the whole flow field gives the most detailed information about the flow structure, the deduction of the overall flow behavior is often difficult: therefore, sometimes a time average flow structure representation can be much more useful.

Method

The experiments were carried out on air-water flow at atmospheric pressure and temperature in a horizontal pipe with a sharp-edged sudden area contraction with inner diameters of 0.08 m upstream of the contraction and 0.06 m downstream, so that the area ratio was 0.56; the pipe geometry is schematically depicted in figure 1. Single fiber optical probes were introduced into the test section at a distance from the inlet where the flow could be considered fully developed. Thanks to micrometric screws, the probe could move along the diameter of the pipe cross-section (figure 2); by rotating the pipe around its axis, it was possible to reach any point of the cross-section.

The instrument returns a DC binary output, which was sampled at a frequency of 1 MHz by a digital data acquisition system; this binary output represents the so-called phase density function.The phase density function Pg, measured by the optical probes, cannot adequately represent the flow structure because of its fluctuations; more useful is the so-called local void fraction eg, which comes from Pg after time averaging:
(1)
where T is the measurement duration. As shown in figure 3, if T is large enough eg becomes a steady quantity, so that the values measured in different positions can be compared with one another.
In order to reconstruct the flow structure, the void fraction distribution over the whole pipe cross-section is required. Since the pipe geometry and the action of the gravity force cause the flow to be symmetrical with respect to the vertical, the void fraction was measured only in 60 points uniformly distributed over half cross-section: the probes were moved in steps of 0.075 i.d. along five diameters with angular spacing of 20°. The local void fraction was measured in seven cross-sections of the horizontal pipe, which were placed 27.5, 15, 1.1 diameters upstream and 1.2, 6, 20, 48 diameters downstream of the contraction; this choice allowed a description of the contraction effects on the flow structure along the whole pipe: in particular, one cross-section was placed 1.2 diameters downstream, where the vena contracta can be observed in single-phase flow and is supposed to exist in two-phase flow.
The experiments were performed with a constant liquid mass flow rate of 3 kg/s and a gas fraction of volume flow ranging from 0.23 to 0.87; according to the Mandhane flow pattern map these flow conditions are located across the transition boundary between the plug flow regime and the slug flow regime.

Results

The local void fraction distribution provides a time average description of the flow structure over a given cross-section. The cross-sectional area occupied by the gas grows with the gas fraction of volume flow, but the two quantities are not proportional, so that the gas and the liquid have different velocities. Figure 4 shows the flow structure evolution upstream of the sudden contraction (above) and in the downstream pipe (below), for a gas fraction of volume flow equal to 0.52. As it was expected, the larger amount of gas flows in the upper region of the pipe: the void fraction is maximum in the highest point of the pipe cross-section, and monotonically decreases as one moves downwards, until a region occupied only by the liquid is reached. The flow structure is always stratified, that is, the points of the pipe cross-section in which the void fraction has the same value are placed along almost horizontal lines: this means that the gravity force, which keeps the two phases separated, is prevailing on turbulence, which is responsible for their mixing.

The sudden contraction considerably affects the gas distribution in both the upstream and the downstream pipe, and its effect grows more and more as the flow approaches the singularity. The obstacle encountered by the flow causes the level of the incompressible phase in the stratified region between two slugs to raise, so that the cross-sectional area available for the gas flow gets smaller; this can be observed for a gas fraction of volume flow up to about 70%, when the gas flow rate becomes too large to allow an area reduction.
In the downstream pipe, the situation is symmetrical to the upstream pipe, from a qualitative standpoint: the area available on average for the gas flow is minimum just after the fitting, and grows as the flow goes on. Indeed, a migration of the air towards the lower part of the cross-section can be noticed. Far away from the contraction, the void fraction keeps almost constant in the upper part of the pipe, and abruptly slopes down to zero in the lower part: this is due to the fact that in this cross-section plug or slug flow patterns are well established, and no more feel the contraction influence. The same things can be observed in the cross-section 27.5 diameters upstream. The change in flow structure along the downstream duct is accompanied by a deceleration of the lower density phase; hence a void fraction increase and a velocity ratio decrease can be observed. Figure 5 compares the two cross-sections placed 1.1 diameters upstream and 1.2 diameters downstream, for two different values of the gas fraction of volume flow: the iso-voidage lines show how the percentage of the cross-sectional area occupied by the liquid is higher in the downstream duct, while the gas is pushed upwards. The observed behavior of the two-phase mixture after the sharp-edged abrupt contraction suggest that a vena contracta similar to that of single-phase flows is unlikely to exist: in fact, the sudden growth of the area occupied by the liquid and the consequent reduction of the area occupied by the gas determine a higher value of the velocity ratio around the fitting, while the existence of a vena contracta requires a homogeneous flow, with a velocity ratio close to one.