Two Phase Flow In Pipes: What, Types, Different Pipes

This article discusses about two phase flow in pipes. The phases represent the state of matter. In a two phase flow the flow contains two states of matter mainly gas and liquid.

In this article we shall study how a two phase flow occurs in pipes. Heat transfer related to a two phase flow. We will also discuss the design of pipes which undergo two phase flow. Let us start our discussion with the definition of two phase flow.

What is a two phase flow?

As the name suggests, a two phase flow is a type of flow in which the contents that are flowing include two states of matter mainly gas and liquid.

The two phase flow can occur in many forms such as transitioning flows from pure liquid to pure gaseous states, separated flows as well as dispersed two phase flows. In dispersed two phase flows on phase is present in the form of bubble, particles, or droplets in carrier form.

Image credits: Rudolf Hellmuth, Two-phase Flow, CC BY-SA 4.0

Examples of Two phase flow

In large scale power systems, two phase flows were studied rigorously. Below are the examples or applications where two phase flow plays a very vital role in design process.

• Boilers – Pressurized water is passed through heated pipes and this water changes to steam while passing through the boiler. The boiler changes the phase of liquid water. During the phase transformation, the pipe will have a two phase flow that means gas and liquid phases will co exist.
• Nuclear reactors – In nuclear reactors, two phase flow is used to remove heat from the reactor core. In reactor core, the fuel is burnt. The fuel used is generally U-235.
• Cavitation – In pumps, when the operating pressure is nearly equal to the vapour pressure of the liquid, any increase in pressure will result into local boiling. This phenomenon of local boiling is called as cavitation.
• Electrolysis– Electrolysis is a technique which uses Direct Current (DC) to carry out a non spontaneous reaction.
• Clouds – We all have seen clouds up in the sky. They are aerosol consisting of a visible mass of liquid, droplets and other frozen particles that are suspended in the atmosphere.
• Groundwater flow – Groundwater is the water that flows beneath the surface of Earth. Two phase flow is used to study the movement of air and water in the soil.

Characteristics of a two phase flow

The list below tells us about the characteristics of a two phase flow:

• All the dynamical problems are made non linear due to surface tension.
• At standard temperature and pressure, the difference between the densities differ by 1000 for air and water.
• The speed of sound changes while passing through a phase change. Compressible effects come into play.
• The phase changes are not in equilibrium and they do not happen necessarily.
• The flow – induced pressure drops can cause further phase change in the system.
• Two phase flows can give rise to counter intutive negative resistance type instabilities.

Types of two phase flow

The types of two phase flows depends on the state of matter of the contents participating in the flow. The types of two phase flow are given in the list below-

• Liquid- liquid flow – In liquid liquid flow, the contents of the flow include two different types of liquid that are immiscible. The solution is such that one liquid floats on other liquid because of difference in densities and inability to make or break bonds.
• Gas-liquid flow – In gas liquid flows, the gas droplets are present on the surface of the liquid. These droplets move faster than the liquid resulting in a slug flow.
• Gas-solid flow– Gas solid flow as the name suggests has solid particles suspended in a gas. These solid particles create abrasive action. For examples large sized impurities present in air is an example of gas solid flow.
• Solid liquid flow– Solid liquid flow has solid particles suspended in liquid stream. The solid particles do not mix with the liquid. They are immiscible.

Two phase flow in vertical pipes

Two phase flows in vertical pipes refers to interactive flow of two distinct phases in common interfaces. Each of them having their own individual mass and volume.

To calculate the flow rate in vertical pipe, the formula given below shall be used-

Where,

H is the discharge height

Q is the flow rate

K is the coefficient ranging from 0.87-0.97

D is the diameter of pipe

Two phase flow regimes in horizontal pipes

The flow regimes in horizontal pipes can vary dependin upon the temperature and pressure drop inside the pipe. In the next section we shall discuss about the flow regimes and patterns found in the horziontal pipe for a two phase flow.

In two phase flows inside a horizontal pipe, the patterns observed are- bubbly flow, stratified wavy flow, slug flow, intermittent flow, stratified flow, plug flow, annular flow and mist flow. Other types of patterns maybe observed depending on special temperature ranges and pressure ranges.

Two phase flow and heat transfer

Heat transfer does not necessarily take place between two same phase substances. But the heat can be transferred between two different states of matter too.

Generally in heat exchangers, liquid-liquid flow is used to transfer heat. In two phase flows, the temperature of both the constituents are different and we consider the average temperature and pressure of the flow.

Heat transfer in heat exchangers

In heat exchangers, the heat is transferred between two fluids such that the working fluid heats up or absorbs heat from the fluid flowing in the system.

Heat exchangers are used in many applications such as rocket engines, cooling towers, water jackets, geysers etc. The main purpose of heat exchangers is to heat or absorb heat from the fluid flowing in the system. Sometimes it may be required to heat up the fluid and sometimes it may be needed to cool the fluid. Sometimes only latent heat is absorbed meaning the temperature of the fluid in system remains same but only the phase transformation takes place.