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Who Wants to Be a CFD Expert?PowerPoint Presentation

Who Wants to Be a CFD Expert?

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Who Wants to Be a CFD Expert?

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- In the ME 566 course title, CFD for Engineering Design, what does the acronym CFD stand for?
- Car Free Day
- Cash Flow Diagram
- Computational Fluid Dynamics

- Lifelines
- Phone a Friend
- Ask the Audience

- Mixing in a Model Combustor
- You are to determine the effect that the angle of the side inlets has on mixing in the combustor.

T.-M. Liou and Y.-Y. Wu,

Experimental Thermal and Fluid Science 1992, 5:401-409.

- Tasks
- You can choose a single angle of 5, 10, 15, or 20 away from the perpendicular side inlets.
- 3 cases to study

+

1

2

3

- Physical dimensions of the combustor
- The length unit is in mm

15

120

15

15

Y

Y

30

1.58

X

Z

2000

- Experimental setup
- Hydrocarbon gas mixture is supplied through the axial inlet
- Air is supplied through the two side inlets
- The combustor bulk mean velocity is 23.9 m/s
- The combustor Reynolds number is 4.56104

Air

Gas mixture

Air

- Comments
- The experimental results can be used as a guideline.
- To augment the experimental findings on mixing, you can set the temperatures of the side inlet and axial jets to different values and use temperature as a passive fluid marker (i.e. solve the thermal energy equation with no buoyancy force).

Tside

Assume adiabatic at walls

CFX-Pre:

- Buoyancy Option:Non Buoyant

- Heat Transfer Option: Thermal Energy

Taxial

- You report should include
- a comment on the validity of the CFD simulation for the perpendicular angle based on comparison to experimental data
- a clear statement of your findings on the effect of the side inlet angle on mixing within the combustor
- a clear presentation of the computational model’s geometry, boundary conditions and properties. Wherever possible, quantify the influence that the two most significant model parameters have on your solution

- You report should include
- a description of the basis of your geometry and mesh designs
- a comment of the applicability of the k model for this flow
- a statement on the degree of convergence obtained by the solver

15

120

15

15

Y

Y

30

1.58

X

Z

2000

Y* = 2

L

Y* = 1

Y* = 0

X* = 0

- How to create a side inlet with an angle :
- Draw a line with dimension L (where L = 15 mm)
- Draw a line with dimension L/cos (where = 5 ~ 20)
- Join two lines together (Constraints Coincident)
- Specify the angle between two lines (Dimensions Angle)

4

L/cos

2

L

3

1

- What fluid properties should be used for hydrocarbon gas mixture?
- You can assume the gas mixture has the same properties as air

- What entrance velocity should be used?
- One way to find out entrance velocity is to read U/Uref and V/Uref from Tables 1A and 1B, where Uref = 23.9 m/s

- What turbulence intensity and length scale values should be used?
- One way to find out turbulence intensity is to read u’/Uref and v’/Uref from Tables 1A and 1B
- For length scale, select the turbulence option: Intensity and Auto Compute Length

- Inlet velocity and turbulence intensity for the side inlet can be found from Table 1A at Y* = 2
- For example, at Y* = 2:
- V/Uref = 8.26E-1 at X* = 0.25
- v’/Uref = 1.03E-2 at X* = 0.25
- (V/Uref)avg = 8.27E-1
- (v’/Uref)avg = 1.09E-2

X* = 0

X* = 0.5

Y* = 2

Y* = 1

Y*

Y* = 0

X*

- How to define velocity at the inlet with side angle?
- Mass and Momentum Option Cart. Vel. Components u = -Vsin (m/s), v = -Vcos (m/s), w = 0 (m/s)

V

V

Vcos

Vsin

Questions?

- A certain discretisation scheme is used to estimate the temperature in a rectangular fin. The computations are done with a uniform mesh having a spacing of x. Table 1 shows the variation of the predicted heat flux at the base of the fin, , with the mesh spacing.
- Q1: Estimate the order of accuracy of the numerical method. The exact heat flux is 214.3 [watts].
- Q2: The order appears to decrease for the finer meshes. What numerical errors are affecting the solutions obtained with fine meshes?

Table 1

- : error, a: constant, x: mesh spacing, n: order of accuracy
- The order of accuracy is first order

- The order of accuracy appears to decrease for the finer meshes. What numerical errors are affecting the solutions obtained with fine meshes?
- Discretisation error
- Round-off error
- Truncation error

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- Round-off error
- Round-off errors are the result of the computational representation of real numbers by means of a finite number of significant digits, which is termed the machine accuracy.

- Discretisation error / Truncation error
- The discretisation error is associated with the neglected contributions due to the truncation of high-order terms.

Taylor series expansion