Characterization of intermittent gas-liquid flow subregimes

As it is well known the so-called intermittent gas-liquid flow, which occurs under several circumstances in industrial applications is usually divided into two subregimes, depicted in figure 1: the plug or elongated bubble flow, in which slugs entrain no gas, and the slug flow, in which slugs entrain many small gas bubbles. Although these regimes are quite similar to each other, their fluid dynamic characteristics are very different indeed, and greatly influence such quantities as the pressure drops and the slug velocity.

Method

In order to distinguish the two subregimes from a quantitative point of view, the gas phase characteristic function (the so-called phase density function) was measured in several positions and for different flow conditions by means of single-fiber optical probes, which are sensitive to the refractive index of the surrounding medium, as shown in figure 2.

Figure 3 shows the Pg time series measured for two flow conditions corresponding to plug flow and slug flow, respectively: in the plot on the left, as the probe gets through a liquid slug Pg = 0, while in the one on the right Pg fluctuates between 0 and 1, suggesting the existence of small bubbles entrained in the slug body.
This behaviour is highlighted in the Fourier space, an can be related to a discontinuity in a parameter defined as the mean power spectral density over the discrete frequency spectrum:

where Dt is the inverse of the sampling rate and N the number of samples. The relationship between the mean value of the power spectral density and the flow pattern is due to the different frequency ranges of the characteristic structures of the flow (low frequency for large gas pockets and high frequency for small bubbles): thus, as the flow pattern changes from plug flow to slug flow and the small bubbles appear, the mean frequency value grows.

Results

For each value of the liquid phase superficial velocity, the mean frequency values were plotted as a function of the gas superficial velocity. Figure 4 shows that over a narrow range of gas superficial velocities the partial derivative of the mean frequency displays an abrupt change in value with respect to the gas superficial velocity itself. Such an abrupt change can be related to the transition from plug flow to slug flow: in fact, the high frequencies of the small bubbles characterizing slugs cause the mean frequency to grow with a different slope than in the case of plug flow. The two trends were expressed in an analytical form by linear best fit of the experimental data on a semilogarithmic chart, and the transition from plug flow to slug flow was assumed to occur at the intersection between the two lines.
The experimental results are compared in Figure 5 with the flow pattern map proposed by Mandhane et al. (1974): the overall agreement is good, although slight discrepancies can be guessed close to the transitions from intermittent to dispersed flow and from intermittent to stratified flow. The proposed experimental criterion has the undoubted advantage of lying on a quantitative basis, which makes it a more reliable tool for the transition boundary definition. Experiments carried out in a pipe of 0.06 m i.d. in the same flow conditions indicate that the influence of the pipe diameter on the transition is negligible.