Flow conditioning
Flow conditioning ensures that the "real world" environment closely resembles the "laboratory" environment for proper performance of inferential flowmeters like orifice, turbine, coriolis, ultrasonic etc.
Types of flow
Basically, Flow in pipes can be classified as follows –- Fully developed flow
- Pseudo-fully developed flow
- Non-swirling, non-symmetrical flow
- Moderate swirling, non-symmetrical flow
- High swirling, symmetrical flow
Types of flow conditioners
- Those that eliminate swirl only
- Those that eliminate swirl and non-symmetry, but do not produce pseudo fully developed flow
- Those that eliminate swirl and non-symmetry and produce pseudo fully developed flow
A flow straightener, sometimes called a honeycomb, is a device used to straighten the air flow in a wind tunnel. It is a passage of ducts, laid along the axis of main air stream to minimize the lateral velocity components caused by swirling motion in the air flow during entry. The cross-section shapes of these "honeycombs" may be of square, circular and regular hexagonal cells.
A low-cost handmade flow straightener
A low-cost flow straightener can be constructed using drinking straws, as they have low cost and good efficiency. The MythBusters television show used such a construction for their wind tunnel, as did an experimental wind tunnel at MIT. The straws should be cut to equal size and placed in a frame.Effectiveness of honeycomb
The effectiveness of honeycomb, in reducing the swirl and turbulence level, is studied by simulating the flow field using standard k-ε turbulence model in commercial computational fluid dynamics. CFD is the most precise and economical approach to estimate the effectiveness of a honeycomb.Computational model
A computational domain of honeycomb is created as shown in Fig. 1
We know computationally, it is very difficult to provide the realistic non-uniform flow at the entry of honeycomb as experienced in the experiments. Such random inlet conditions would essentially simulate the realistic case in which air can enter the honeycomb from any direction and at any level of turbulence. Therefore, special domain is designed for introducing practical inlet condition
Meshing of Computational Models
The solid model of honeycomb is meshed in GAMBIT 2.3.16. As shown in Fig. 2. A structured rectangular mesh is used for the simulation with square honeycomb configuration. Governing equations for mass and momentum conservations for subsonic flow along with the equations for turbulence and porous flow are solved for the honeycomb using commercial CFD. RANS type RNG k-ε model is used for the turbulence modeling.
Boundary Conditions
The separate domain created upstream of the honeycomb is provided with various inlet conditions to arrive at the disorderly motion at the exit, which should be given as an inlet to the honeycomb cells. This essentially simulates the more realistic case that the flow can enter into the honeycomb from any direction. Specifications of this inlet along with other necessary boundary conditions are mentioned here. Flow at the inlet of the honeycomb shall necessarily have turbulent and swirling motions. Therefore, in order to incorporate these requirements, a separate fluid domain is constructed.
Top and bottom circular faces are considered as inlet to this domain to get a flow field with higher magnitude of lateral velocity. This domain is provided with vertical and horizontal cylinders as an obstruction to the inlet to produce sufficient swirling at the exit of this section. A tetrahedral mesh as shown in Fig. 3 with tetrahedral elements is generated for this geometry. The number of nodes are 1,47,666. Three faces of this configuration are specified as inlets with velocity boundary conditions. Fluid velocity at these inlet faces has been so taken that averaged mean velocity at the outlet is 1 m/s, which is in the operational wind tunnel.
A pressure outlet boundary condition is used at exit of the settling chamber where pressure at outlet is set to zero for gauge pressure.
It is always possible to predict the entire flow field by meshing whole fluid domain; however simulation for the prediction of entire flow field using symmetry boundary condition. This approach reduces the mesh requirement and computational efforts. Therefore, symmetry boundary is used at the periphery of the computational domain.
All the solid boundaries in the computational domain are specified as viscous walls with no-slip wall boundary condition.
Turbulence intensity profile at the exit of turbulence model is shown in Fig. 4. This figure shows the turbulence intensity and which is maximum at the center and at the walls is around 16-18%, now this profile is incorporated inside the honeycomb as shown in Fig. 2, the profile of turbulence intensity comes out from the honeycomb is shown in Fig. 5. In this profile we can see that the turbulence intensity is reduced from 30% to 1.2% at the center and 16% to 3.5%, it means the honeycomb effectiveness is very high which is around 96%.
Natural gas measurement
that carries a lot of liquids with it is known as wet gas whereas natural gas that is produced without liquid is known dry gas. Dry gas is also treated as to remove all liquids. The effect of flow conditioning for various popular meters which is used in gas measurement is explained below.Pipe flow conditions
The most important as well as most difficult to measure aspects of flow measurement are flow conditions within a pipe upstream of a meter. Flow conditions mainly refer to the flow velocity profile, irregularities in the profile, varying turbulence levels within the flow velocity or turbulence intensity profile, swirl and any other fluid flow characteristics which will cause the meter to register flow different than that expected. It will change the value from the original calibration state referred to as reference conditions that are free of installation effects.Installation effects
Installation effects such as insufficient straight pipe, exceptional pipe roughness or smoothness, elbows, valves, tees and reducers causes the flow conditions within the pipe to vary from the reference conditions. How these installation effects impact the meter is very important since devices which create upstream installation effects are common components of any standard metering design. Flow Conditioning refers to the process of artificially generating a reference, fully developed flow profile and is essential to enable accurate measurement while maintaining a cost-competitive meter standard design. The meter calibration factors are valid only of geometric and dynamic similarity exists between the metering and calibration conditions. In fluid mechanics, this is commonly referred to as the Law of Similarity.Law of similarity
The principle of Law of Similarity is used extensively for theoretical and experimental fluid machines. With respect to calibration of flowmeters, the Law of Similarity is the foundation for flow measurement standards. To satisfy the Law of Similarity, the central facility concept requires geometric and dynamic similarity between the laboratory meter and the installed conditions of this same meter over the entire custody transfer period. This approach assumes that the selected technology does not exhibit any significant sensitivity to operating or mechanical variations between calibrations. The meter factor determined at the time of calibration is valid if both dynamic and geometric similarity exists between the field installation and the laboratory installation of the artifact.A proper manufacturer's experimental pattern locates sensitive regions to explore, measure and empirically adjust. The manufacturer's recommended correlation method is a rational basis for performance prediction provided the physics do not change. For instance, the physics are different between subsonic and sonic flow. To satisfy the Law of Similarity the in situ calibration concept requires geometric and dynamic similarity between the calibrated meter and the installed conditions of this same meter over the entire custody transfer period. This approach assumes that the selected technology does not exhibit any significant sensitivity to operating or mechanical variations between calibrations. The meter factor determined at the time of calibration is valid if both dynamic and geometric similarity exists in the "field meter installation" over the entire custody transfer period.
Velocity flow profile
The most commonly used description of flow conditions within the pipe is the flow velocity profile. Fig. shows the typical flow velocity profile for natural gas measurement. The shape of the flow velocity profile is given by the following equation,----
The value of n determines the shape of the flow velocity profile. The eq. can be used to determine the flow profile's shape within the pipe by fitting a curve to experimentally measured velocity data. In 1993, the transverse flow velocities were being measured within the high pressure natural gas environment using hot wire technology to accomplish the data fit. A fully developed flow profile was used as the reference state for meter calibration and determination of Coefficient of Discharge. For Reynolds Number to n is approximately 7.5; for Re of, n is approximately 10.0 where a fully developed profile in a smooth pipe was assumed. Since n is a function of Reynolds Number and friction factor, more accurate values of n can be estimated by using the eq.,
----
Where, f is friction factor. A good estimate of a fully developed velocity profile can be used for those without adequate equipment to actually measure the flow velocities within the pipe. The following straight pipe equivalent length in eq. was utilized to ensure a fully developed flow profile exists.
----
In eq. the pipe lengths required is significant, hence we need some devices that can able to condition the flow over a shorter pipe length allowing metering packages to be cost competitive and accurate. Here the velocity flow profile is generally three-dimensional. Normally the description requires no axial orientation indication if the profile is asymmetric and if it does exists, then axial orientation with respect to some suitable plane of reference is required. Asymmetry exists downstream of installation effects such as elbows or tees. Usually, the velocity flow profile is described on two planes 90° apart. Using the latest software technology a full pipe cross sectional description of the velocity profile is possible provided sufficient data points are given.