How's about a corner with a table for two?
Where the music's mellow and some gay rendezvous
There's no chance romancing with a blue attitude
You've got to do some dancing to get in the mood.

("In The Mood," by Glenn Miller, 1944)

FLOWING FLUID REGIMES: LAMINAR FLOW AND TURBULENT FLOW

A schematic that attempts to illustrate the differences between the Laminar, Transitional, and Turbulent Flow Regimes.

There are two Flowing Fluid Regimes that help to characterize the behavior of a fluid while it flows through the Piping Network and the Stationary and Rotating Equipment in a processing plant.

The two Flowing Fluid Regimes are Laminar Flow and Turbulent Flow.

Fluids are categorized as being "Laminar" or "Turbulent" based upon 4 factors which are mentioned toward the end of this PTOA Segment. The Velocity Factor of a fluid's PV Flowrate is the most influential factor regarding which regime best characterizes the flowing fluid.

The flow regime exiting the pipe is Laminar at first and transitions into a Turbulent Flow which is characterized by the helter-skelter flow of the water droplets.

Guess what?

The Process Operator cannot impact which Flowing Fluid Regime that is a-happening in them thar pipes!

The Process Operator has so little influence over the Flowing Fluid Regime that Your Mentor has always wondered why the subject is part of every brick-and-mortar college Process Technology curriculum.

Determining the Flowing Fluid Regime was the job of the E&C company that designed and built the processing facility.

On the other hand ...

The Process Operator or Instrumentation Technician who is aware of Flowing Fluid Regimes will be enlightened as to why several feet of linear pipe must precede the installation of most types of Flow Detection and Measuring Devices so that the PV Flowrate can be accurately measured.

Furthermore, understanding the attributes of Turbulent Flow will help explained why the Turbulent Flow Regime is required for efficient operation of Shell and Tube Heat Exchangers.

For the above two reasons Your Mentor decided to devote some paragraphs to the exploration of Laminar Flow and Turbulent Flow.

THE LAMINAR FLOW REGIME

The Laminar Flow Regime Velocity Profile

Laminar Flow has the slower velocity of the two Flowing Fluid Regimes.

The Laminar Flow Regime is characterized by smooth, linear flow which appears to flow in layers. The fastest Laminar Flow velocity is at the center of the pipe. Flowing "layers of fluid" can be imagined as concentric circles of sequentially slower-flowing fluid which radially surround the fastest flowing fluid at the center of the pipe. The slowest moving fluid is the fluid that interacts with the interior of the pipe wall.

The "Laminar Flow Velocity Profile" that develops as a fluid is flowing in the Laminar Flow Regime was first featured in PTOA Segment #159.  The fluid appears to be "cone-shaped" in the front.

Fluid flowing through a transparent pipe is photographed in the Laminar Flow Regime (Top Photo), Transitional Flow Regime (Middle Photo), and Turbulent Flow Regime (Bottom Photo).

The nearby photo shows a liquid flowing through a transparent pipe. The top photo is a snapshot of a Real-World Laminar Flow Velocity Profile.

The nearby colorful pipe flow is an illustration of the Laminar Flow Velocity Profile which would be observed from the front of the pipe.

The red flow circle at the center of the pipe has the fastest velocity.

The yellow flow area that radially surrounds the red area would have the next fastest velocity.

The green flow area that radially surrounds the yellow area would have a slower velocity than the yellow velocity area ... but a greater velocity than ...

the light blue velocity area which radially surrounds the green area.

This radial light blue velocity area has the slowest fluid velocity area because it interacts with the dark blue interior wall. 

The slowest fluid velocity is always the fluid that is closest to the interior pipe wall. The fluid closest to the pipe wall creates an interface with the wall which leaves a film of fluid on the interior surface of the pipe.

The Laminar Flow Regime Must Be Established
For Accurate PV Flowrate Detection and Measurement 

A Turbine Flowmeter.

PTOA Readers and Students will soon learn the form and function of the devices that detect and measure the PV Flowrate.

Prior to reaching the PV Flowrate Detector, the fluid flowing through the Piping Network must be established in the Laminar Flow Regime. PV Flowrate Detectors/Measurers cannot accurately determine a PV Flowrate if the fluid is swirling around prior to being measured.

Uh-oh! Fred is looking confused.  He wants to know what is the PV Flowrate if the front "nose" of the fluid is moving faster than the rest of the fluid?

GREAT QUESTION, FRED!

The PV Flowrate Detector is detecting the PV Flowrate of the "bulk fluid." The velocity of the "bulk fluid" is where the vertical line is drawn in the nearby graphic. The averaged velocity at this vertical, dashed line is labelled V_avg.

Establishing the Laminar Flow Regime requires a specific length of straight pipe inserted in front of (i.e., "upstream") of the PV Flowrate Detector.  Additionally, a specific length of straight pipe must exist after ("downstream") of the PV Flowrate Detector.

For accurate flow measurement, this Turbine Flowmeter requires a straight length of pipe equal to 10 pipe diameters upstream of the flowmeter and a straight length of pipe equal to 5 pipe diameters downstream of the flowmeter.

How much straight pipe must be piped upstream and downstream of the PV Flowrate Detector?

The manufacturer of the PV Flowrate Detector will specify the required amount of pipe in terms of "Pipe Diameters."  In the nearby graphic, a straight length of pipe equal to 10 Pipe Diameters must be upstream of the Turbine Flowrate Meter.  Downstream of the Turbine Flowrate Meter a straight length of pipe equal to 5 Pipe Diameters must be piped into the Piping Network.

THE TURBULENT FLOW REGIME

The wake created by a motorized boat is in the Turbulent Flow Regime.

Turbulent Flow does not at all flow in smooth concentric circles that emanate from the radially center fastest velocity. A fluid flowing in the Turbulent Flow Regime flows forward but contains swirling eddies that mix up the flow and constantly change the magnitude and direction of the flowing fluid.

Most flow observed in nature can be classified in the Turbulent Flow Regime.

Most flow observed in nature can be categorized in the Turbulent Flow Regime.

Atmospheric currents and ocean currents are examples of Turbulent Flow.  Likewise, the flow of rivers can typically be classified in the Turbulent Flow Regime.

Man-made examples of Turbulent Flow include the wake of a motorboat and the intentionally designed airflow around the wing of an aircraft. Turbulent Flow creates the "lift" needed to allow the aircraft to become airborne.

The gas flow through a Gas Turbine IS NOT Laminar! The bulk of the fluid moves forward with rapid changes in Intensity and direction. PV Flowrate measurement is not accomplished within the structure of the GT.

The fluid flow through Rotating Equipment like Pumps and Gas Turbines is in the Turbulent Flow Regime; the bulk of the fluid has a forward moving velocity, but swirls and eddies within the fluid constantly change both its magnitude and direction.

The Relationship Between Turbulent Flow and Heat Exchange

As was mentioned above, the Laminar Flow Regime is characterized by a film on the interior walls of a pipe.

Brilliant PTOA Readers and Students ... meaning those who are reading the PTOA Segments in the intended, sequential order ... completely understand the theory and mechanics of Conduction Heat Transfer because the subject matter was featured in PTOA Segment #62. The industrial application of Conductive Heat Transfer was featured in PTOA Segment #68.

The film created by the Laminar Flow Regime would effectively increase the thickness of the wall through which heat must be transferred in a Shell and Tube Heat Exchanger.

Those same brilliant PTOA Readers and Students understand the role Conduction Heat Transfer plays in the Shell and Tube Heat Exchangers featured in PTOA Segment #78.

The Turbulent Flow Regime characterizes the Shell Side and Tube Side Flow through this Heat Exchanger.

The velocity of the fluids flowing through a Heat Exchanger must be in the Turbulent Flow Regime to enhance efficient heat transfer. The Turbulent Flow Regime constantly pushes fresh fluid against the interior metal pipe which is where the Conduction Heat Transfer occurs.

The higher the velocity through the Heat Exchanger, the greater the turbulence and the more efficient the rate of thermal heat transfer.

Otherwise stated: To enhance efficient heat transfer within the Shell and Tube Heat Exchanger, the designed PV Flowrates of both fluids will be in the Turbulent Flow Regime.

The upper limit of a fluid's velocity ... hence, the upper limit of a fluid's turbulence... is determined by good ole Pressure Drop (aka, ΔP).

Brilliant PTOA Readers already learned in PTOA Segment #165 how a flowing fluid causes Pressure Drop as it flows through a pipe. Too much Pressure Drop results in additional costly Pumps and Compressors to restore the PV Pressure that is required to make fluids move.

Additional Pumps and Compressors are expensive to purchase and maintain. Thus, the velocity of a fluid as it flows through a Heat Exchanger must be optimized to balance efficient heat transfer without creating too much Pressure Drop. This optimization is the job of the E&C company that designed the Heat Exchanger.

 

REYNOLDS NUMBER AND THE RELATIONSHIP BETWEEN
LAMINAR, TRANSITIONAL, AND TURBULENT FLOW REGIMES

The Reynolds Number for Laminar Flow is below 2000. The Reynolds Number for Turbulent Flow is above 4000. The Reynolds Number for Transitional Flow is between 2000 and 4000. The Reynolds Number is a dimensionless number (meaning all the units cancel out while it is calculated).

As was mentioned previously, a fluid flowing in the Laminar Flow Regime has less velocity and a fluid flowing in the Turbulent Flow Regime has more velocity.

As the velocity of a flowing fluid increases, it can move from the Laminar Flow Regime through a Transitional Flow Regime and ultimately to the Turbulent Flow Regime.

Laminar Flow (Re=2000) at top of photo. Turbulent Flow (Re-4000) at bottom of photo.

The Reynolds Number (Re) is a dimensionless number that is used to classify Flowing Fluid Regimes.

• A Re below 2000 is in the Laminar Flow Regime.
• A Re above 4000 is in the Turbulent Flow Regime.

 

The Transitional Flow Regime is flow which is characterized by a Re above 2000 and below 4000.

The Reynold's number equation is shown in the nearby box.  This dimensionless number can be calculated by multiplying the Fluid Density and the Fluid Velocity and the Pipe Diameter together. The resulting numerator is divided by the Fluid's Dynamic Viscosity. Voila! All the units cancel out and the Re is a dimensionless number!

PTOA Readers and Students should not stress about calculating a Reynolds Number.

However, the design engineer for a Shell and Tube Heat Exchanger should be able to glance at the Reynolds Number equation and deduce that the higher the Dynamic Viscosity of the flowing fluid, the more Fluid Velocity will be required to drive the Reynolds Number into the Turbulent Flow Regime. In summary, the Dynamic Viscosity of the flowing fluid has a significant impact on the Pressure Drop observed through a Heat Exchanger.

The Moody Diagram graphs Reynolds Number (X axis) as a function of Friction Factor and Relative Roughness of the interior of the Pipe. The Reynolds Number and Relative Roughness can be determined. Then the Friction Factor can be interpolated to predict the Pressure Drop through the Piping Network.

A dude named Moody had the wherewithal to assimilate the known research regarding fluid dynamics into The Moody Diagram.

A colorful Moody Diagram appears nearby. The Reynolds Number appears on the X Axis.

On this Moody Diagram, the Laminar Flow Regime is shaded green. The Transition Flow Regime is shaded tan. The Turbulent Flow Regime is shaded pinkish red.

An example of a Relative Roughness chart for several Piping Materials.

The Moody Diagram plots the Reynolds Number as a function of the Friction Factor on the left Y-axis. Determining the Friction Factor is required to estimate Pressure Drop.

On the right Y-axis, the Moody Diagram plots the Reynolds Number as a function of the Relative Roughness (of the pipe interior through which the fluid is flowing). Relative Roughness is represented by the Greek letter Epsilon and can be determined from a table of Relative Roughness. An example of a table of Relative Roughness for various pipe manufacturing materials is shown nearby.

After determining the Relative Roughness of the pipe and the Reynolds Number, the Friction Factor can be interpolated from the Moody Chart, hence making it possible to predict the Pressure Drop that will be caused by the flowing fluid as it flows through the Piping Network and associated Rotating and Stationary Equipment.

 

TAKE HOME MESSAGES: Another way to classify how fluids are flowing is by Flowing Fluid Regime. The two Flowing Fluid Regimes are the Laminar Flow Regime and the Turbulent Flow Regime.

Laminar Flow is a slower velocity flow. A Laminar Flow Velocity Profile develops wherein the fastest fluid flows at the center of the pipe and slower fluid flows in concentric circles from the radial center.  The slowest fluid interfaces with the interior wall of the pipe through which the fluid is flowing.

Turbulent Flow constantly changes intensity and direction although the bulk of the fluid moves forward.

A fluid with a Reynolds Number of 2000 or lower is in the Laminar Flow Regime.  A fluid with a Reynolds Number of 4000 and higher is in the Turbulent Flow Regime. The Transitional Flow Regime is between 2000 and 4000.

The Reynolds Number depends on 4 characteristics of a flowing fluid as is featured in this PTOA Segment.  PTOA Readers and Students, Process Operators, and Instrumentation Technicians have no control over which Flowing Fluid Regime is taking place in the Piping Network of a processing facility and thus do not need to stress over calculating the Reynolds Number.

The content in this PTOA Segment is informational; the intent is to make the Process Operator and Instrument Tech aware of the classification of fluid flow into the Laminar and Turbulent Flow Regimes. The Process Operator and Instrument Tech cannot impact the Fluid Flow Regime of the fluid flowing through pipes. The Engineering and Construction (E&C) Company that designed and built the processing facility ensures that the Laminar Flow Regime is established prior to PV Flowrate detection and measurement.   The E&C Company also ensures that the Turbulent Flow Regime is established in Heat Exchangers. In the processing facility all flow through Rotating Equipment is in the Turbulent Flow Regime. The design of Turbulent Flow in the processing facility requires optimizing fluid velocity and Pressure Drop. 

Turbulent Flow exists in most observed flows of nature.  

©2023PTOA Segment 0236

PTOA PV FLOWRATE FOCUS STUDY AREA
PV FLOWRATE FUNDAMENTALS FOCUS STUDY

You need to login or register to bookmark/favorite this content.