## Flow Regime

From a practical engineering point of view the flow regime can be categorized according to **several criteria**.

**All fluid flow** is classified into one of two broad categories or regimes. These two flow regimes are:

**Single-phase Fluid Flow****Multi-phase Fluid Flow**(or**Two-phase Fluid Flow**)

This is a **basic classification**. All of the fluid flow equations (e.g. **Bernoulli’s Equation**) and relationships that were discussed in this section (Fluid Dynamics) were derived for the flow of a **single phase** of fluid whether liquid or vapor. Solution of multi-phase fluid flow is **very complex and difficult** and therefore it is usually in advanced courses of fluid dynamics.

Another usually more common classification of **flow regimes** is according to the shape and type of **streamlines**. All fluid flow is classified into one of two broad categories. The fluid flow can be either laminar or turbulent and therefore these two categories are:

**Laminar Flow****Turbulent Flow**

**Laminar flow** is characterized by **smooth** or in **regular paths** of particles of the fluid. Therefore the laminar flow is also referred to as **streamline or viscous flow**. In contrast to laminar flow, **turbulent flow** is characterized by the **irregular movement** of particles of the fluid. The turbulent fluid does not flow in parallel layers, the lateral mixing is very high, and there is a disruption between the layers. **Most industrial flows**, especially those in nuclear engineering **are turbulent**.

The flow regime can be also classified according to the **geometry of a conduit** or flow area. From this point of view, we distinguish:

**Internal Flow****External Flow**

**Internal flow** is a flow for which the fluid is confined by a surface. Detailed knowledge of behaviour of internal flow regimes is **of importance in engineering**, because circular pipes can withstand high pressures and hence are used to convey liquids. On the other hand, **external flow** is such a flow in which boundary layers develop freely, without constraints imposed by adjacent surfaces. Detailed knowledge of behaviour of **external flow** regimes is **of importance especially in aeronautics** and **aerodynamics**.

### Single-phase Fluid Flow

Classic study of **fluid dynamics concentrates** on the flow of a **single homogeneous phase**, e.g., water, air, steam. All of the fluid flow equations and relationships discussed normally in this section are for the flow of a single phase of fluid whether liquid or vapor.

When at certain important locations in fluid flow systems the simultaneous flow of liquid and gas occurs, the problem must be solved **as two-phase flow**. The relatively simple relationships used for analyzing single-phase flow are **insufficient** for analyzing two-phase flow.

### Two-phase Fluid Flow

By definition, **multiphase flow** is the interactive flow of **two or more** distinct phases with common interfaces in, say, a conduit. Each phase, representing a volume fraction (or mass fraction) of solid, liquid or gaseous matter, has its **own** properties, **velocity**, and **temperature**.

A multiphase flow can be simultaneous flow of:

- Materials with
**different states or phases**(e.g. water-steam mixture). - Materials with
**different chemical properties**but in the same state or phase (e.g. oil droplets in water).

There are many combinations in industrial processes, but the **most common** being the simultaneous flow of **steam and liquid water** (as encountered in **steam generators** and **condensers**). In reactor engineering a great deal of study has been performed on the nature of **two-phase flow** in case of a loss-of-coolant accident (**LOCA**), which is an accident of importance in reactor safety and in all thermal-hydraulic analyses (**DNBR analyses**).

### Characteristics of Multiphase Fluid Flow

All multiphase flow problems have features which are characteristically different from those found in single-phase problems.

- In the case of steam and liquid water the
**density**of the two phases**differs by a factor of about 1000**. Therefore the influence of gravitational body force on multiphase flows is of much greater importance than in the case of single-phase flows. - The
**sound speed**changes dramatically for materials undergoing phase change, and can be orders of magnitude different. This significantly influences a**flow through an orifice**. - The relative
**concentration**of different phases is usually a dependent parameter of great importance in multiphase flows, while it is a parameter of no consequence in single-phase flows. - The change of phase means flow-induced pressure drops can cause further phase-change (e.g. water can evaporate through an orifice) increasing the relative volume of the gaseous, compressible medium and increasing efflux velocities, unlike single-phase incompressible flow where decreasing of an orifice would decrease efflux velocities.
- The spatial distribution of the various phases in the flow channel strongly affects the flow behavior.
- There are many types of instabilities in multiphase flow.

### Laminar Flow

In fluid dynamics,** laminar flow** is characterized by **smooth or in regular paths** of particles of the fluid, in contrast to **turbulent flow**, that is characterized by the **irregular movement** of particles of the fluid. The fluid flows in **parallel layers** (with **minimal lateral mixing**), with no disruption between the layers. Therefore the laminar flow is also referred to as **streamline or viscous flow**.

The term streamline flow is descriptive of the flow because, in laminar flow, layers of water flowing over one another at **different speeds** with virtually no mixing between layers, fluid particles move in definite and observable paths or streamlines.

When a fluid is flowing through a **closed channel** such as a pipe or between two flat plates, either of two types of flow (laminar flow or turbulent flow) may occur depending on the **velocity**, **viscosity** of the fluid and the **size of the pipe (or on the Reynolds number)**. Laminar flow tends to occur at lower velocities and high viscosity.

### Turbulent Flow

In fluid dynamics, **turbulent flow** is characterized by the **irregular movement** of particles (one can say **chaotic**) of the fluid. In contrast to laminar flow the fluid **does not flow in parallel layers**, the **lateral mixing is very high**, and there is a disruption between the layers. Turbulence is also characterized by **recirculation, eddies**, and **apparent randomness**. In turbulent flow the speed of the fluid at a point is continuously undergoing changes in both **magnitude and direction**.

Detailed knowledge of behaviour of turbulent flow regime is of importance in engineering, because **most industrial flows**, especially those in nuclear engineering **are turbulent**. Unfortunately, the highly intermittent and irregular character of turbulence **complicates all analyses**. In fact, turbulence is often said to be the “**last unsolved problem in classical mathemetical physics**.”

The main tool available for their analysis is **CFD analysis**. CFD is a branch of fluid mechanics that uses numerical analysis and algorithms to solve and analyze problems that involve **turbulent fluid flows**. It is widely accepted that **the Navier–Stokes equations** (or simplified **Reynolds-averaged Navier–Stokes equations**) are capable of exhibiting turbulent solutions, and these equations are the basis for essentially all CFD codes.

### Internal Flow

In fluid dynamics, internal flow is a flow for which the fluid is **confined by a surface**. Detailed knowledge of behaviour of internal flow regimes is of importance in engineering, because circular pipes can withstand high pressures and hence are used to convey liquids. Non-circular ducts are used to transport low-pressure gases, such as air in cooling and heating systems. The internal flow configuration is a convenient geometry for heating and cooling fluids used in energy conversion technologies such as nuclear power plants.

### External Flow

In fluid dynamics, **external flow** is such a flow in which **boundary layers** develop freely, without constraints imposed by **adjacent surfaces**. In comparison to internal flow, external flows feature **highly viscous effects** confined to rapidly growing “**boundary layers**” in the entrance region, or to thin shear layers along the solid surface. Accordingly, there will always exist a region of the flow outside the boundary layer. In this region velocity, temperature, and/or concentration does not change in and their gradients may be neglected.

This effect causes the **boundary layer** to be expanding and the boundary-layer thickness relates to the fluid’s kinematic viscosity.

This is demonstrated on the following picture. Far from the body the flow is nearly inviscid, it can be defined as the flow of a fluid around a body that is completely submerged in it.

**Reactor Physics and Thermal Hydraulics:**

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