What is two-phase flow? In simple engineering terms, a proper two-phase flow definition is a physical condition where two different materials travel together. For example, water and steam moving in a boiler create this exact situation. The phases in two-phase flow are usually a liquid and a gas, like oil and natural gas. You can find this process in many industrial areas, from chemical plants to power stations.
Figure 1: An example of a two-phase flow condition generated by AI.
Predicting these two-phase flow regimes helps engineers build better heat exchangers, pipelines, and reactors. These physical patterns show how fluids mix inside a pipe. Many factors change these shapes. For instance, the direction of the pipe, the speed of the fluids, and the material properties all play a big role. Knowing these patterns allows designers to improve heat transfer and stop unwanted pressure drops.
Figure 2: The water boiling process creating a two-phase flow.
The Importance of a Flow Regime Map
Researchers use a flow regime map to predict these physical shapes. These charts use the speeds of the gas and the liquid to show what will happen inside a system. This step helps them avoid operational problems before building the actual machine. Plotting data for horizontal and vertical pipes is very common. The Mandhane map and the Taitel-Dukler map are two famous tools for this job. For example, the image below shows the Mandhane map. It predicts the multiphase flow regimes using the superficial speeds of the liquid and vapor.
Figure 3: The Mandhane model for predicting flow regimes (Reference: Emamzadeh M. & Issa R.I., 2013).
The Seven Flow Regimes
1. Bubble Flow
One of the most basic two phase flow regimes is bubble flow. In this physical process, tiny bubbles of gas scatter throughout a solid body of liquid. You will easily observe this regime at low gas flow rates in many industrial devices, such as heat exchangers and chemical reactors. A case in the point is shown belove, where our engineers have tried to model two-phase bubbly flow inside a column by means of Fluent.
2. Slug Flow
A slug flow happens when large bullet-shaped gas bubbles (known as Taylor bubbles) push through the pipe. Solid walls of liquid separate these large bubbles. Because of this heavy pushing, systems experience large pressure drops and heavy vibrations. Engineers pay close attention to this regime when designing pipelines and risers in the oil and gas industry. You can see the special pattern that happens when tiny gas bubbles and liquid drops through Slug flow in capillary microreactors CFD simulation.
3. Plug Flow
When the gas speed increases slightly, it pushes the fluid harder. This action creates elongated gas bubbles that sit near the top of the pipe. Because gravity pulls the heavier liquid down, the bottom of the pipe remains covered in a continuous liquid stream.
4. Annular Flow
Annular flow occurs when a thin layer of liquid travels along the inner wall of the pipe, while a fast gas core rushes through the center. This regime happens at very high gas speeds, which literally pushes the liquid outward against the walls. This pattern is common in vertical pipelines, such as those in nuclear reactors and oil production systems.
5. Stratified Flow
In a horizontal pipe, very low speeds cause the fluids to separate completely due to gravity. The heavy liquid phase always settles at the bottom of the pipe. Meanwhile, the lighter gas phase glides smoothly across the top.
6. Wavy Flow
When the gas speed increases over a stratified flow, the wind creates friction on the liquid surface. This friction generates waves that travel in the flow direction. The height of these waves depends on the speed difference between the two materials, but they rarely reach the top of the pipe.
7. Disperse Flow
Disperse flow is a condition where tiny particles, droplets, or bubbles randomly suspend inside a primary fluid, like air or water. You can see this highly mixed configuration in all types of two phase flow, including gas-liquid, gas-solid, and liquid-liquid systems.
Figure 4: Different flow regimes in a horizontal pipe (Reference: Rasul G. et al., 2020).
The Reynolds Number in Multiphase Systems
Calculating the Reynolds number is different when two materials mix. Because the materials interact, scientists approach the math in two ways:
- Separate Phase Model: You calculate the Reynolds number for the gas and the liquid individually. This uses the specific density, speed, and viscosity of each unique fluid.
The Reynolds number for each phase can be calculated based on the properties of each phase individually:
\[
\text{Re}_1 = \frac{\rho_1 u_1 D}{\mu_1}
\]
\[
\text{Re}_2 = \frac{\rho_2 u_2 D}{\mu_2}
\]
In the above equations, \(\rho_1\) and \(\rho_2\) are the densities of phases 1 and 2. \(\u_1\) and \(\u_2\) are the velocities of those phases. D is the characteristic length. \(\mu_1\) and \(\mu_2\) are the viscosities of phase 1 and 2.
- Mixture Model: If the fluids mix tightly, you calculate one combined Reynolds number. This approach uses an average density and average speed based on how much of each fluid is present in the pipe.
Understanding the Slip Ratio
In many multiphase flow regimes, the gas and liquid do not travel at the exact same speed. The slip ratio (or velocity ratio) is simply the gas speed divided by the liquid speed. If the two materials move together perfectly, the ratio is one. However, if the lighter gas moves faster than the heavy liquid, a velocity difference occurs. To read more about how this velocity difference is calculated in fluid dynamics, you can read our guide on the mixture model.
How to Model Multiphase Flow Regimes
When studying these physical patterns, we eventually need to bring their pipe designs into a CFD environment. Because every flow regime behaves differently, selecting the correct mathematical setup is highly important. For example, a separated stratified flow requires a completely different mathematical approach than a highly dispersed droplet flow. If you are preparing to set up a project and are not sure which physical model fits your system, read our complete multiphase flow modeling guide to learn about the different CFD approaches.
Conclusion
Understanding the various flow regimes is a core requirement for modern engineering. Whether dealing with oil pipelines, power generation, or chemical processing, these physical patterns present unique challenges. By studying bubble, slug, and annular shapes, designers can optimize heat transfer, reduce pressure drops, and ensure their machines operate safely.
Today, engineers use advanced software to predict these behaviors. If you want to learn how to analyze these patterns yourself, you can explore our multiphase CFD simulation tutorials.
Alternatively, if you need expert help with an industrial pipeline or reactor problem, our engineering team is here for you. You can easily book a CFD consultation Meeting to discuss your system, or directly order a tailored project to get fast, accurate data for your design.