A basic note on two-phase flow and slug flow

Difference between phase change and multiphase

Phase change

It is physical. process. Phase change is an energy-driven process. Change can take place in either direction. Water has so little energy at 0 degrees Celsius that it converts to ice, a lower energy phase. Water has so much energy at 100 degrees Celsius that it turns to vapour, a higher energy phase. Water's kinetic energy is so low at 0 degrees Celsius that molecules coalesce into solid ice, which acts as a centre for potential energy via H-bonds. Similarly, the kinetic energy is so high at 100 degrees Celsius that molecules break H-bonds and move to a stable high energy centre for kinetic energy, which is vapour.

In summary, a phase change occurs in order for a phase to maintain maximum stability in a given situation. At the phase boundary, both phases stay at equilibrium. For, a single component system as water with two phases, C=1, P=2, the degree of freedom F = C -P +2 = 1- 2 +2 =1. Only one independent variable, temperature or pressure, decides the location of the phase boundary in the phase diagram.

Multiphase

A multiphase flow is defined as one in which more than one phase (i.e., gas, solid and liquid) occurs.

Different regimes of two-phase flows

 (a) transient two-phase flow with the transition from liquid to vapour flow, (b) separated or segregated two-phase flow with large regions of gas, (c) dispersed two-phase flow with gas bubbles (or liquid droplets) dispersed in a liquid or solid particles or droplets dispersed in gas

Almost all oil and gas wells, as well as the associated pipeline systems that collect produced fluids and transport them to treatment facilities, exhibit multiphase flow. Because these fluids have significantly different physical properties, multiphase flow is much more complicated than single-phase flow. , The following major issues are associated with the main difficulties encountered in describing multiphase flow: (1) Concurrently flowing liquid and gas in a pipe can have different geometrical arrangements, known as flow patterns; (2) the flowing mixture is compressible, so its density changes with changes in pressure and temperature; and (3) in addition to fictional losses, a new type of energy loss is occurring. Slippage loss occurs as a result of the gas phase slipping through the liquid due to the large specific gravity difference between phases.

Most multiphase correlations assume a homogeneous flowing mixture and employ the general energy equation written for this hypothetical fluid.

Significance of prediction of flow patterns:  Flow in any channel requires design, development, and optimization. It is vital to predict the flow phases, like flow patterns primarily based on the interfaces formed between the phases. This know-how enables the prediction of the pressure drop and heat transfer characteristics based totally on the flow rate, channel size and operating conditions. From the pressure drop data, the acceptable float characteristics can be decided to decrease the incidence of corrosion, erosion, or scale formation, all of which can lead to excessive friction.

Slug flow

A Slug flow is a two-phase, flow, more particularly a liquid-vapor flow. Slug flow occurs due to two different densities of the phases.

In a multiphase flow in a pipe, the lighter fluid stays as bubbles dispersed within, and pushing along, the heavier fluid. The word slug normally refers to the heavier, slower-moving fluid, but sometimes to the bubbles of lighter fluid. There are also small bubbles within the liquid, but many of these have coalesced to form large bubbles until they span much of the pipe. In gas-liquid mixtures, slug flow is similar to plug flow, but the bubbles are generally larger and move faster. As flow rates increase, slug flow becomes churn flow. 

To summarize, the intermittent gas-liquid flow could be divided into two distinct systems: plug and slug flow. Each regime can provide its particular shape and hydrodynamics properties. The main characteristic of the slug flow is the intermittent passage of air-laden liquid eruptions followed by elongated air bubbles called cells. Interactions with neighbouring cells form or break liquid slugs

Different dimensions of the slug flow

Every fluid wants to reach the shape of a sphere because a sphere has the minimum energy. A sphere is the most stable shape for any fluid.

The problem is created by the surface tension at the interphase of two fluids. Surface tension is nothing but a stretch of the surface by the intermolecular forces at the interphase of two fluids. In the bulk, a liquid molecule experiences 360-degree pull from all sides.

Slug flow

Interestingly you can see bubbles of gases floating in the liquid causing a slug flow.

The obvious question arises how the light bubbles survive the hydrostatic pressure of the liquid inside the liquid. The answer is a gas has no option for any different shape. It will break and again will reform the sphere.

The answer is the surface tension

Surface tension

Gases have no surface tension because molecules are far apart to cause any wall tension.

This is making air or gas fold itself into its minimum energy shape which is a sphere.

Water cannot fold itself because of its large intermolecular forces pulling in different directions at 360 degree

Viscosity

There is another dimension. That is the effect of viscosity on gas bubbles..The shearing of the surface of the bubble by non-Newtonian liquids elongates the bubbles like a bullet that pushes a slug

Consequences of slug flow

Oil and gas transportation is an important activity for the oil industry worldwide. Pipelines are primarily used to transport oil and gas from wells or production areas to refinery facilities. These pipelines are subjected to severe environmental and operational stresses that can result in severe dynamic reactions that lead to pipe failure and fatigue.

Slug flow is one of the greatest threats to the pipeline industry

This flow system can cause rapid fluctuations in mass distribution and pressure disturbances along the pipe, which can lead to violent vibrations. These vibrations create transient stresses that lead to fatigue failure, localized buckling, and even excessive bending and failure. These fluctuating stresses in pipelines can significantly disrupt normal operation, accelerate pipeline corrosion, and cause significant damage to pipe walls. In fact, it has been reported that many accidents have occurred due to vibration caused by internal two-phase flow.

Effect of temperature on slug flow

The temperature would primarily influence the viscosity and solubility of the gas.

The following events are anticipated:

1. Viscosity: The viscosity of the liquid phase decreases as temperature rises. As a result, the liquid flows more easily and forms smaller slugs as the temperature rises.

2. Gas fraction: As the temperature rises, so does the gas fraction in slug flow. This is because the solubility of gas in liquids decreases with increasing temperature. The gas bubbles congregate and form larger pockets as the temperature rises, resulting in larger slugs.

3. Slug length and frequency: As temperature rises, the length of liquid slugs shortens, while slug formation frequency increases. This is due to the lower viscosity of the liquid phase, which allows it to flow. more easily and form smaller slugs.

4. Flow instability: At extremely high temperatures, the liquid evaporates rapidly, causing the liquid phase to decrease and the gas phase to increase. This destabilises the flow, which can lead to flow reversal or other system instabilities.

Credit: Google