Flow regime is the distinct patterns or behaviors that fluids exhibit as they flow through a particular environment, such as pipes, channels, or rivers. These regimes are characterized by the flow's key properties, including its velocity, turbulence, and overall behavior. The type of flow regime is determined by factors such as fluid velocity, density, viscosity, and the geometry of the channel.
Their are Three Primary Flow Regimes
Laminar Flow - In laminar flow, the fluid moves smoothly and orderly in parallel layers, with minimal mixing between layers. The motion is predictable, and turbulence is absent or minimal. Laminar flow typically occurs at low fluid velocities and is favored by fluids with low viscosity.
Transitional Flow - Transitional flow is a mix of laminar and turbulent characteristics. It involves intermittent fluctuations and can be less predictable than laminar flow. Transitional flow occurs as fluid velocity increases, leading to a transition from laminar to turbulent flow.
Turbulent Flow - Turbulent flow is characterized by chaotic and irregular fluid motion, with eddies, swirls, and increased mixing. Turbulence can result in enhanced heat and mass transfer. Turbulent flow typically occurs at higher fluid velocities and is more likely with fluids of higher viscosity.
The choice of which flow regime occurs depends on factors such as the Reynolds number which relates the inertial forces to the viscous forces in the fluid. The Reynolds number helps determine whether the flow will be laminar, transitional, or turbulent.
Understanding flow regimes is crucial in various fields, including fluid dynamics, engineering, and environmental science. Different flow regimes have distinct effects on the efficiency, heat transfer, and mixing characteristics of fluid systems, influencing the design and operation of various engineering applications such as pipelines, heat exchangers, and chemical reactors.
Reynolds Number is Used to Predict the Flow Regime of a Fluid
Laminar Flow (Low Reynolds Numbers) - Typically, Reynolds numbers less than about 2,300 indicate laminar flow for pipe flow. For other geometries, the transition may occur at different values.
Transitional Flow - The transition from laminar to turbulent flow is gradual and not precisely defined. It often occurs in the range of Reynolds numbers around 2,300 to 4,000 for pipe flow.
Turbulent Flow (High Reynolds Numbers) - Generally, Reynolds numbers greater than 4,000 (for pipe flow) indicate turbulent flow. The exact threshold can vary based on the geometry and specific conditions.
Other Flow Types
Bubble Flow - Bubble flow occurs when
liquid occupies the bulk of the cross-section and vapor flows in the form of bubbles along the top of the pipe. During this phase the vapor and liquid velocities are about the same.
Plug Flow - The bubbles in the pipe eventually coalesce as the vapor rate increases. Plug flow is similar to slug flow but liquid is the continuous phase along the bottom of the pipe.
Stratified Flow - As the vapor rate continues to increase, the plugs join and become a continuous phase. In fully stratified flow, vapor flows along the top of the
pipe and liquid flows along the bottom. The interface between the two phases is relatively smooth and the flow area occupied by each phase remains constant. In uphill flow, stratified flow rarely occurs with wavy flow being favored.
Wavy Flow - As the vapor rate increases even more in the pipe during stratified flow, the top most layer begins to form waves. As the vapor rate increases, the amplitude of the waves increases. Wavy flow can occur when flow moves uphill, in horizontal pipe or in downhill flow. Most often, it is found in piping that is horizontal but it may occur in uphill flow. When the flow is moving downhill, wavy flow can still occur but the amplitude of the waves is not as great.
Slug Flow - Slug flow occurs when the speed of the
vapor phase pushes the waves from the wavy flow regime onto each other. The waves grow until the liquid waves touch the top of the pipe and form frothy slugs. The velocity of frothy slugs and the "slugs" of vapor is faster than the liquid
velocity. Slug flow is found in lower velocities in piping running uphill than in horizontal piping. When running downhill, it takes higher vapor rates to establish slug flow than in horizontal pipe. Because the slugs are moving faster than the average liquid velocity, care should be taken to avoid slug flow around
fittings. Severe
water hammer may occur when changing flow direction when slug flow is occurring.
Annular Flow - A two-phase flow regime where the liquid forms a film of varying thickness along the wall of the pipe and the vapor phase flows at a higher speed down the middle of the pipe. The interface between the vapor and liquid phase is not entirely well defines. Part of the liquid is sheared off from the film by the vapor and is carried along in the core as entrained droplets. At the same time, turbulent eddies in the vapor deposit droplets on the liquid film. Due to the different forces on the fluid, the thickness of the liquid film is not constant across the cross section of the pipe. The effects of
gravity can cause the thickness of the fluid film towards the bottom of the annulus to be bigger than the top. Downstream of bends, most of the liquid will be at the outer wall.
Spray Flow - Also known as mist or dispersed flow occurs when two-phase flow where the liquid phase is the dispersed phase and exists in the form of many droplets, while the gas phase is the continuous phase. This occurs when the velocity of the vapor tears the liquid film away from the wall and is carried by the vapor as entrained droplets. Sprays are formed for industrial, commercial, agricultural, and power generation purposes by injection of a liquid stream into a gaseous environment. In addition, sprays can form naturally in a falling or splashing liquid.
Steady Flow - A condition where the fluid properties at any given point in a flow field do not change with time. In other words, in a system experiencing steady flow, the velocity, pressure, temperature, and other relevant parameters remain constant at any specific location over time. Steady flow does not mean that the flow cannot vary from one point to another within the system. Different points in the flow field can have different velocities, pressures, and other properties, but these properties remain constant at any fixed point over time.
Compressible Flow - Describes the behavior of fluids that experience changes in density as a result of variations in pressure and temperature. In compressible flow, the fluid density can change significantly, leading to variations in other fluid properties such as velocity and temperature. This is in contrast to incompressible flow, where the density of the fluid remains relatively constant.
Incompressible Flow - The density of the fluid remains constant or nearly constant throughout the flow process. This means that the fluid's volume does not change significantly under the applied conditions, and the variations in density are negligible. Incompressibility is often applied to liquids such as water and to gases under certain conditions. In incompressible flow, the change in pressure within the fluid is primarily due to changes in its
kinetic energy or
potential energy, rather than changes in density.
