ENG619: Aerodynamics and CFD Modelling – Engineering Assignment Case Study

Question Topic: Flow about a NACA12 aerofoil & CFD modeling of flow through a computer cooling axial fan.

Engineering Case Study Assignment

Case Study – 1
CFD Modelling: Flow about an aerofoil (NACA0012)

When the air flows over an aerofoil, it will suffer a loss of momentum as a result of the drag force acting on the aerofoil. This loss of momentum will be confined to the wake of the aerofoil. The wake behind the aerofoil is generated by the development of boundary layer on the surface of the aerofoil.

The aim of this case study is to enable students to adopt the CFD code – FLUENT, investigating the flow over a NACA0012 aerofoil. A sketch of the configuration of the flow used in this coursework is shown below.

Figure 1. Air flow about an airfoil

In order for you to use ANSYS 17 DesignModeler to create a mesh system for this study, the contour data of NACA0012 aerofoil with a chord length of 152.4 mm is provided which is tabled as follows:

 

You are required to conduct the following studies:

  1. Using the experimental data acquired in the win tunnel test, estimate the Reynolds number based on the chord length, Rec, and decide the use of a suitable turbulence modelling.
  2. Apply the oncoming velocity (inlet boundary condition) based on the experimental data to simulate the flow over the aerofoil. Generate the outputs of the velocity profiles at different locations, velocity vectors and static pressure distribution over the aerofoil.
  3. Determine the CL & CD of the aerofoil at various angles (choose appropriate angles) of incidence using the function – ‘REPORT’ in FLUENT. Compare the results obtained using FLUENT with those acquired using the Plint low speed wind tunnel. It should be noted that your CFD modelling is a 2-D model.

 

Case Study – 2

CFD modelling of flow through a computer cooling axial flow fan

As processors, graphics cards, RAM and other components in computers consume more powers with increasing demands for high performance, the heat generated by these components increases remarkably and has to be dissipated. One of the most important issues encountered in developing next generation computers is how to solve the cooling problem. These computer components need to be kept within a specified temperature range to prevent overheating, instability, malfunction and damage leading to a shortened component lifespan. In order to effectively cool these components, axial flow fans are currently used to remove the heat generated from the components and draw cooler air over them.

The axial-flow fans have blades that force air to move parallel to the shaft about which the blades rotate. Axial fans blow air along the axis of the fan, linearly, so called by their name. The axial flow fans used in computer cooling usually have diameters from 80-200 mm and deliver a pressure head up to 20 Pa. Figure 1 shows a typical axial flow fan used in computer cooling.

Figure 1. AcoustiFan™ DustPROOF – Premium Quality Quiet 60mm Multi-Purpose Cooling Fan

This assignment aims to investigate the flow through a computer cooling axial flow fan. An accurate numerical analysis of the flow phenomena in the axial flow fan is crucial for design of a high efficiency axial flow fan and reduction of aerodynamic noise emitted from the fan.

As shown in Figure 2, the axial flow impeller is mounted inside a circular tube of 64 mm in diameter with a tip clearance of 2 mm. The computer cooling axial flow fan is required to deliver a designed flow rate Q0 = 6×10-3 m3/s and a pressure head Ptotal = 10 Pa under the design condition (Note: These two parameters are designed parameters. Whether or not the fan will attain these design parameters depends on the fan actual working conditions). The following axial fan dimensions are given:

  • The hub diameter Dh = 30 mm;
  • The impeller diameter Dt = 60 mm;
  • The number of blades n = 6;
  • The hub width b = 12 mm;
  • The average blade installation angle βm = 25º;
  • The blade chord length = 15 mm
  • The blade thickness δ = 1 mm.

For simplicity of the design, the blade chord lengths along the radial direction have been assumed to be identical. The fan has been assumed to be mounted in a cylindrical tube with the diameter of 64 mm and the length of 80 mm.

Figure 2. An axial flow fan used for computer cooling

You are required to carry out the following CFD analysis using the CFD code ANSYS 17:

Assume that the properties of air to be the same as those default values set in ANSYS Fluent 17.0. The rotational angular velocity of the impeller ω is 1500 rpm.

(a) Present, respectively, the velocity distributions and static pressure profiles at different downstream locations of the axial fan;

(b) Discuss the simulation results in detail and your findings;

(c) Observe and explain your simulation results when the number of blades is increased to 8. Assume that the designed flow rate Q0 remains unchanged.

(d) Change the blade installation angle to 20º and conduct the simulation. Discuss the velocity and static pressure distributions behind the impellor.

 

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