Flow in an elbow with a throttle valve- Transient with valve motion from 0-25-0˚.

Flow in an elbow with a throttle valve- Transient with valve motion from 0-25-0˚.

A Butterfly valve is a quarter-turn rotational motion valve, that is used to stop, regulate, and start flow.

Butterfly valves are easy and fast to open. A 90° rotation of the handle provides a complete closure or opening of the valve. Large Butterfly valves are usually equipped with a so-called gearbox, where the handwheel by gears is connected to the stem. This simplifies the operation of the valve, but at the expense of speed.

Butterfly valves has a short circular body, a round disc, metal-to-metal or soft seats, top and bottom shaft bearings, and a stuffing box. The construction of a Butterfly valve body varies. A commonly used design is the wafer type that fits between two flanges. Another type, the lug wafer design, is held in place between two flanges by bolts that join the two flanges and pass through holes in the valve's outer casing. Butterfly valves are especially well-suited for the handling of large flows of liquids or gases at relatively low pressures and for the handling of slurries or liquids with large amounts of suspended solids.

Typical applications of Butterfly valves

A Butterfly valve can be used in many different fluid services and they perform well in slurry applications. The following are some typical applications of Butterfly valves:

• Cooling water, air, gases, fire protection etc.
• Throttling service involving low differential pressures, as in cooling water or air supply system.
• Slurry and similar services.
• Vacuum service.
• High-pressure and high-temperature water and steam services.

Advantages of Butterfly valves

• Compact design requires considerably less space, compared to other valves.
• Light in weight.
• Quick operation requires less time to open or close.
• Also available in very large sizes.
• Low-pressure drop and high-pressure recovery.

Disadvantages of Butterfly valves

• Throttling service is limited to low differential pressure.
• Cavitation and choked flow are two potential concerns.
• Disc movement is unguided and affected by flow turbulence.

Domain:

Some Important concept:

Stagnation Pressure: In fluid dynamics, a stagnation point is a point in a flow field where the local velocity of the fluid is zero. Stagnation points exist at the surface of objects in the flow field, where the fluid is brought to rest by the object. The Bernoulli equation shows that the static pressure is highest when the velocity is zero and hence static pressure is at its maximum value at stagnation points. This static pressure is called the stagnation pressure. The Bernoulli equation applicable to incompressible flow shows that the stagnation pressure is equal to the dynamic pressure plus static pressure. Total pressure is also equal to dynamic pressure plus static pressure so, in incompressible flows, stagnation pressure is equal to total pressure. In compressible flows, stagnation pressure is also equal to total pressure provided the fluid entering the stagnation point is brought to rest isentropically.

Choked flow: Choked flow is a fluid dynamic condition associated with the venturi effect. When a flowing fluid at a given pressure and temperature passes through a constriction (such as the throat of a convergent-divergent nozzle or a valve in a pipe) into a lower pressure environment the fluid velocity increases. At initially subsonic upstream conditions, the conservation of mass principle requires the fluid velocity to increase as it flows through the smaller cross-sectional area of the constriction. At the same time, the venturi effect causes the static pressure, and therefore the density, to decrease at the constriction. Choked flow is a limiting condition where the mass flow will not increase with a further decrease in the downstream pressure environment for a fixed upstream pressure and temperature. For homogeneous fluids, the physical point at which the choking occurs for adiabatic conditions, is when the exit plane velocity is at sonic conditions; i.e., at a Mach number of 1. At choked flow, the mass flow rate can be increased only by increasing density upstream and at the choke point. The choked flow of gases is useful in many engineering applications because the mass flow rate is independent of the downstream pressure, and depends only on the temperature and pressure and hence the density of the gas on the upstream side of the restriction. Under choked conditions, valves and calibrated orifice plates can be used to produce a desired mass flow rate.

Problem Statement: To simulate and analyse flow in Elbow with Throttle valve or Butterfly valve and mainly check for pressure built-up around the throttle body.

Mesh:
Size: 2e-3mm
Fixed Embedment: two layers with scale of two.

Mesh at 0 milli sec fully Open (0 degree)

Mesh at 2 to 4 milli sec partially closed (25 degree)

Mesh from 4ms to 8ms open fully again (0 degree)

Mesh when its open completely (slice view along z axis)

Boundary Conditions:

Inlet: Inflow Pressure: 150000 Pa

Temperature: 300K

Specie: Air (with O2 and N2 at usual concentrations)

Turbulent Kinetic Energy: 0.02% intensity

Turbulent Dissipation: 0.003 m

Outlet: Outflow Pressure: 100000 Pa

Backflow temperature: 300K

Backflow specie: Air (with O2 and N2 at usual concentrations)

Turbulent Kinetic Energy: 0.02% intensity

Turbulent Dissipation: 0.003 m

Elbow Wall: Fixed/Stationary Wall using law of wall for velocity, temperature.

Throttle Valve/Butterfly Valve: Rotating Wall with law of wall used for temperature, velocity.

Rotation of the valve defined in throttleposition.in

File throttleposition.in is defined as follows:

TEMPORAL

SEQUENTIAL

second Angle

0.0 0.0

0.002 25.0

0.004 25.0

0.008 0.0

0.01 0.0

Solver Details:

Physical Model: RANS- Reynolds Averaged Navier Stokes

Turbulence Model: K-Epsilon RNG

Solver: Steady-State solver, Full Hydrodynamic, Compressible.

Simulation time: 15000 cycles

Initial time step: 1e-9

Minimum time step: 1e-9

Maximum time step: 1.0

Discretisation Schemes:

Navier Stokes Solver Scheme: PISO

Navier Stokes Solver Type: Pressure-Based

Convective Flux Scheme: Flux blending

Specie: Flux blending

Turbulence: Flux blending

Initialization: domain: Air

Pressure: 101325 Pa

Temperature: 300K

Grid Size: 2e-3mm.

Results:

Inferences and conclusion:

• This simulation uses fixed embedment with 2 layer and at scale of 2, this allows refinement of the mesh near the rotating wall (butterfly valve) at it moves from 0 degree to 25 degree, this allows more fruitful data around this boundary.
• This flow is a pressure driven flow that is high total pressure is defined at the inlet boundary and the domain is at lower pressure, once the flow starts the total yields higher velocities at inlet.
• In the course of the fluid flow when it encounters the valve which is initially in open position, some high- and low-pressure zones form which can be seen from the following figure.

• The red region shows high pressure of about 1.5e5 Pa and blue region shows low pressure 1e4 Pa, now this pressure is not much of concern because the pressure is not low enough to cause cavitation, moreover the fluid used here is air so there is no threat of cavitation.
• The figure below shows the pressure around the valve body when its partially closed.

• Red region shows high pressure of around 1.6e5 Pa, this is because of the streamlines whose direction is changed or is restricted, while the other half experiences pressure that is in the range of 1 to 3 bar, this is because of the streamlines in this part have less restriction or experience smaller change in direction, also the velocities in this very region is very high (to maintain the mass flow rate). See the next image.

• Similar is the case with back side of the valve, the only difference is that instead of high-pressure low pressure appear on the valve body/ surface and extreme velocities around that. See the next figure.

• The blue region shows the low pressure zone on the surface of the valve, now if you looks closely around this very region the velocity streamline show deep red colour shows the extreme velocities in this region and remember these velocities are higher than those seen on the other side of the valve as seen earlier. Lets looks at the velocity vectors in next image.

• The velocity vector shown above shows that there is certain level of swirl in the domain which mainly emerges from the interaction of the flow with the partially closed valve.
• We can also see the deep red coloured velocity vector showing the extreme velocity of about 390 m/s.
• If we look at the pressure contour the blue marked region has pressure as low as 25000 Pa which is actually very low which could have given rise to cavitation if the fluid used was some liquid, although as air is used here there will be no cavitation for now.
• From one of the graphs shown earlier we can see the pressure drop across the valve is found around 13698 Pa. This pressure varies a little as compared to the steady sate simulation. Now this drop in pressure across the valve can be accounted by the drop in head due to pipe bend, due to frictional losses in the pipe and various other.
• The most important aspect we are looking for in this simulation is the pressure built up in and around the valve and check for the possibility of cavitation, because we aim to find any such possibility and find out way to avoid it as cavitation is undesirable for any such case of flow through valves.
• For this simulation as air was used as a fluid of flow the possibility of cavitation can be ruled out but if same results were obtained for water or some other liquid then the low pressure regions had to rectified either by improving the design parameters of the valve or by improving the flow characteristics like the inlet total pressure extra.

References:

1. Fluid Mechanics and Hydraulic Machine, Sukumar Pati, IIT Kharagpur- Mechanical Engineering Department.
2. Computational Fluid Dynamics Analysis of Butterfly Valve Performance Factors, Adam Del Toro, Utah State University.