CFD Introduction using Ansys Fluent

ANALYSIS OF GEAR PUMP USING ANSYS FLUENT

SAVANI BHAVIK B.

INTRODUCTION What is CFD?CFD: A methodology for obtaining a discrete solution of real world fluid flow problems.Discrete solution: Solution is obtained at a finite collection of space points and at discrete time levels

CFD IN A NUTSHELL

Solving the governing equation of fluid by using computational methods under various physical conditions (e.g. heat transfer, radiation, electromagnetic field, nuclear reaction…etc.) to study different phenomena encountered in the numerous branches of physical science.

CFD APPLICATIONAerospace Automotive

Chemical Processing

Polymerization reactor vessel- prediction of flow separation and residence time effects.

Marine

CFD APPLICATIONTurbomachinery Semiconductor

Industry

Civil Engineering

BUILDING BLOCKS

Geometry & grid generation

module

Problem setting module

Solution Module

Visualization Module

What to: Presume• Boundary / initial

conditions • Physical models

& what to: Neglect• Physical phenomena (i.e. heat transfer,

chemical reaction…etc)• Properties (i.e. Compressibility, real gas…

etc)

& what to : Calculate • Unknowns • Relationships between field

variable

Solution methods & algorithms • Spatial / temporal discretization

schemes • Convergence criterion • Interpolation methods

METHODOLOGY During Preprocessing

The geometry of the problem is defined The volume occupied by the fluid is divided into discrete cells.

The mesh may be uniform or non-uniform The physical modelling is defined – for eg. The equation of

motion + enthalpy + radiation + species conservation Boundary conditions are defined. This involves specifying the

fluid behavior and properties at the boundaries of the problem. For transient problems, the initial conditions are also defined

The simulation is started and the equation are solved iteratively as a steady-state or transient

Finally a postprocessor is used for the analysis and visualization of the resulting solution.

FINITE VOLUME METHOD USED IN DISCRETIZATION

The finite volume method (FVM) is a common approach used in CFD codes, as it has an advantage in memory usage and solution speed, especially for large problems, high Reynolds number turbulent flows, and source term dominated flows (like combustion).

In this method the governing partial differential equations are recast in the conservative form and then solved over a discrete control volumes and thus guarantees the conservation of fluxes through a particular control volume.

Here Q is the vector of conserved variables,  F is the vector of fluxes V is the volume of the control volume element, and A is the surface area of the control volume element.  The finite volume equation yields governing equations in the form:

FINITE ELEMENT METHOD The finite element method (FEM) is

used in structural analysis of solids, but is also applicable to fluids.

It is much more stable than the finite volume approach. However, it can require more memory and has slower solution than the FVM.

FINITE DIFFERENCE METHOD

The finite difference method (FDM) has historical importance and is simple to program.

It is currently only used in few specialized codes, which handle complex geometry with high accuracy and efficiency by using embedded boundaries or overlapping grids (with the solution interpolated across each grid).

PURPOSE OF PRESENTATIONGEAR PUMP A gear pump uses the

meshing of gears to pump fluid by displacement.

There are two main variation: external gear pump and internal gear pump

Gear pump are positive displacement meaning they pump a constant amount of fluid for each revolution.

THEORY OF OPERATION As the gears rotate they separate on the intake

side of the pump, creating a void and suction which is filled by fluid. The fluid is carried by the gears to the discharge side of the pump, where the meshing of the gears displaces the fluid.

Mechanical clearances are small, which prevents the fluid from leaking backwards.

Rigid design of the gears and houses allow for very high pressures and the ability to pump highly viscous fluids.

Internal Gear Pump

External Gear Pump

PROCESS IN SOLVING PROBLEM Set up a problem using the 2.5D dynamic re-

meshing model. Specify dynamic mesh modeling parameters. Specify a rigid body motion zone. Specify a deforming zone. Use prescribed motion UDF macro. Perform the calculation with residual

plotting. Post process using CFD-Post.

PROBLEM STATEMENT The current setup is an external gear

pump which uses two external spur gears.

Gears rotate at constant rate of 100 rad/s.

Fluid- oil (Density- 844 kg/m3 & Viscosity – 0.02549 kg/m-s)

Mass flow rate of fluid in and out of the pump is of interest.

PROBLEM SCHEMATIC

MESH

PROBLEM SETUP General setting- Solver

1. Type- pressure-Based2. Velocity Formulation- Absolute 3. Time – Transient

Models- Viscous - Realizable k-epsilon with standard wall function

PROBLEM SETUP Material

1. Material – oil2. Density -844 kg/m33. Viscosity – 0.02549 kg /m-s

Cell Zone conditions1. Each cell zone, select material as oil

PROBLEM SETUP Boundary conditions (Inlet and outlet

conditions are set)1. Inlet- Default Pressure 2. Outlet – Gauge Pressure (101325 Pa)

Define UDF 1. UDF macro is written for the gear sets to

rotate in opposite directions at constant rate of 100 rad/s

2. UDF is loaded and compiled

PROBLEM SETUP Mesh Motion Setup

1. Dynamic mesh Mesh Method:

Smoothing Remeshing

Remeshing Method 2.5D Parameters

Min. Length Scale – Default

Max. Length Scale – Default

Size Remeshing Interval - 1

2. Motion of gear 1 Zone – Gear 1 Type – Rigid body Motion UDF/ profile –

gear1::libudf C.G (X,Y,Z)- (0, 0.085,

0.005)3. Motion of gear 2

Zone – Gear 2 Type – Rigid body Motion UDF/ profile –

gear2::libudf C.G (X,Y,Z) – (0, -0.085,

0.005)

PROBLEM SETUP4. Motion of symmetry-1

gear_fluid Zone – sym1-gear_fluid Type – Deforming Geometry Definition – Plane Point on Plane (X ,Y, Z) – (0,

0, 0.01) Plane normal (X ,Y, Z) – (0,

0, 1) Meshing Options:

Methods: Smoothing & Remeshing

Zone Parameters Min. Length Scale – 0.0005 Max. Length Scale- 0.002 Max. Skewness – 0.8

5. Motion of symmetry-2 gear_fluid

Zone – sym1-gear_fluid Type – Deforming Geometry Definition – Plane Point on Plane (X ,Y, Z) – (0,

0, 0) Plane normal (X ,Y, Z) – (0,

0, 1) Meshing Options:

Methods: Smoothing Zone Parameters

Min. Length Scale – 0.0005 Max. Length Scale- 0.002 Max. Skewness – 0.8

PROBLEM SETUP Dynamic mesh

Time step size – 5e-6 No. of steps – 1000 Integrate

Preview control Time step size – 5e-6 No. of steps – 1000

SOLUTION Solution methods

Retain defaults Solution controls

Relaxation factor of pressure- 0.4

Momentum – 0.5 Turbulent K.E – 0.7 Turbulent dissipation

rate – 0.7 Turbulent viscosity –

0.75

Solution initialization TKE – 0.1 Gauge pressure – 101325

Pa Initialize the solution

Run Solution (Run calculation for 150 time steps) Time step size- 5e-6 No. of time steps – 3000 Max iterations / time step

– 40 Calculate

POSTPROCESSING Velocity Contour

Pressure Contour

CONCLUSION The accuracy of the computed flow rate

for a given model increases as the outlet pressure decreases.

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