Who Wants to Be a CFD Expert? In the ME 566 course title, CFD for Engineering Design, what does the acronym CFD stand for? A.Car Free Day B.Cash Flow Diagram C.Computational Fluid Dynamics Lifelines Phone a Friend Ask the Audience

ME 566 Project 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:

Tasks You can choose a single angle of  5 ,  10 ,  15 , or  20  away from the perpendicular side inlets. 3 cases to study ++  12 3

Physical dimensions of the combustor The length unit is in mm X Y Z Y

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 4.56  10 4 Air Gas mixture

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). T side T axial Assume adiabatic at walls CFX-Pre: - Buoyancy Option: Non Buoyant - Heat Transfer Option: Thermal Energy

Project Report 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 X Y Z Y

Frequently Asked Questions (FAQs) How to create a side inlet with an angle  : 1.Draw a line with dimension L (where L = 15 mm) 2.Draw a line with dimension L/cos  (where  =  5  ~  20  ) 3.Join two lines together (Constraints  Coincident) 4.Specify the angle  between two lines (Dimensions  Angle)   L L/cos  3 Y* = 2 Y* = 1 Y* = 0 X* = 0  L

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/U ref and V/U ref from Tables 1A and 1B, where U ref = 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’/U ref and v’/U ref 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/U ref =  8.26E-1 at X* = 0.25 v’/U ref = 1.03E-2 at X* = 0.25 (V/U ref ) avg =  8.27E-1 (v’/U ref ) avg = 1.09E-2 Y* = 2 Y* = 1 Y* = 0 X* = 0X* = 0.5 X* Y*

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?

Order of Accuracy 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 [watts]. Q2: The order appears to decrease for the finer meshes. What numerical errors are affecting the solutions obtained with fine meshes?  x [m] [watts] Table 1

 : error, a: constant,  x: mesh spacing, n: order of accuracy The order of accuracy is first order  x (m) (watts) 1/21/2 x1/x2x1/x2 Order (n)  

Who Wants to Be a CFD Expert? The order of accuracy appears to decrease for the finer meshes. What numerical errors are affecting the solutions obtained with fine meshes? A.Discretisation error B.Round-off error C.Truncation error Lifelines Phone a Friend Ask the Audience  x (m)  Order (n) 

Numerical Errors (Sec. 10.2) 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 