cfd analysis of an energy scavenging axial flow micro turbine using automotive exhaust ...€¦ ·...

ISBN 978-93-5156-328-0 International Conference of Advance Research and Innovation (ICARI-2014)

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CFD Analysis of an Energy Scavenging Axial Flow Micro Turbine using Automotive Exhaust Gases Chitrarth Lav, Raj Kumar Singh Department of Mechanical and Automobile Engineering, Delhi Technological University, Delhi, India Abstract

This paper investigates the possibility of using the Micro Turbine as an energy scavenging device to generate back up power utilizing the energy of the waste exhaust gases from an automobile. An axial flow micro turbine is designed such that it may be fitted in the exhaust pipe of an automobile. The model is a 12 blade turbine of diameter 4 cm with a blade inlet angle of 00 with a blade outlet angle of 30 degrees. A front and rear plate is also modeled to provide a support for the turbine shaft. Calculations are done to model the blade profile and then a CAD model is developed on software DS SolidWorks. This model is analyzed on commercial CFD Solver Ansys Fluent using the Standard k-ε turbulence model. The simulations provide insights into the back pressure acting on the assembly as well as the turbulence characteristics of the flow.

1. Introduction Exhaust gases from an IC engine of an

automobile are discharged at a very high velocity from the cylinder at the final stroke. The velocities are in the range of 50 – 120 m/s. This high velocity is reduced before discharge to the atmosphere by using an expansion chamber called the muffler. Internal combustion engines are typically equipped with an exhaust muffler to suppress the acoustic pulse generated by the combustion process. A high intensity pressure wave generated by combustion in the engine cylinder propagates along the exhaust pipe and radiates from the exhaust pipe termination. The pulse repeats at the firing frequency of the engine which is defined by f = (engine rpm x number of cylinders)/120 for a four stroke engine. The frequency content of exhaust noise is dominated by a pulse at the firing frequency, but it also has a broadband component to its spectrum which extends to higher frequencies. Reduction in velocity is important otherwise there would an uncontrolled expansion at the outlet giving rise to shock waves. Since the flow velocity is quite high, it is possible to use a method to harvest this otherwise waste energy.

Harvesting the energy or scavenging can be done by using a micro turbine. This paper will investigate the possible application of installing a wind micro Corresponding Author E-mail address: All rights reserved: http://www.ijari.org

turbine for scavenging the exhaust gas energy.

The current scenario for energy scavenging is presented. For different applications, different methods can be used. Possible power sources can come from batteries, air/wind flow, solar, temperature, human power and vibrations. Figure 1 shows the different types of power sources and their applications.

Fig: 1 Comparison of potential power sources

Batteries Batteries are relatively inexpensive and can be disposed and replaced easily. Batteries might work for providing additional power for auxiliary

Article Info

Article history: Received 2 January 2014 Received in revised form 10 January 2014 Accepted 20 January 2014 Available online 1 February 2014 Keywords


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appliances in an automobile but they lose charge over time, which is not ideal. Constantly recharging the battery for backup power is a cumbersome process. Batteries pose a problem for weight gain since larger the amount of power required, more will be the weight and size of the battery. Batteries can also cause an environmental problem. Disposing of hundreds of batteries can leak lead and acid into the ground and water and also cause danger to human skin tissue. Thermal Energy Thermal energy can be generated by differences in temperatures between two surfaces by thermo-electric devices. The most common way to generate power from differences in temperature is through a thermoelectric or piezoelectric generator. Equation 1 shows the maximum efficiency of a thermal energy device. In general, the greater difference in temperature, the more power the system can produce.

ƞ = 푇 − 푇

푇

Previous Work on Energy Scavenging using Micro Turbines:

Limited work has been done with micro-turbines in an energy scavenging application. A miniature turbine was developed by the Micro and Precision Engineering Group at Katholieke Universiteit Leuven in 2005. It was tested with compressed air at 330°C (626°F) and produced 130,000 rpm. At 18% efficiency it outputs approximately 28 Watts of power. The air enters through a pneumatic connector and travels through a stationary nozzle. The nozzle deflects the air so that it hits the turbine blades tangentially. The air then leaves through the outlet disc (Micro and Precision Engineering Research Group, 2005). All of the parts except for the connector and the circlip are stainless steel. The turbine has a diameter of 10mm (0.394in) and the housing has a diameter of 15mm (0.591in) and a length of 25mm (0.984in) (Micro and Precision Engineering Research Group, 2005). 2. Theory 2.1 General Theory Let,

V1 be the inlet flow velocity to the blade Vb be the blade velocity V2 be the outlet velocity θ be the blade outlet angle

Since there is no relative movement along the axis of the turbine the change in the flow component of the velocity is zero, i.e.:

푉 = 푉 cos휃

(a)

(b)

Fig: 2. Blade Velocity Triangle (a) Inlet (b) Outlet

Also, the theoretical power (from the fundamental Euler Equation) due to the rotation of the turbine is given by:

푃 = 푚̇푉 푉 = 푚̇푉 sin휃 푉 Where, P = power output m = Mass flow rate = ρAV1 A = Area of the pipe through which the exhaust gases will flow. Substituting first equation in the second will yield:

푃 = 푚̇푉 tan휃푉 = 휌퐴푉 tan휃 푉 To establish a relationship between the inlet flow velocity and the blade velocity we consider the two velocity triangles. Here an assumption is made that Vr1 = Vr2 i.e. Blade friction factor is assumed as one due to small size of the blade. So by trigonometry:

푉 = 푉

푉 + 푉 = (푉 cos휃) + (푉 sin휃 − 푉 )


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푉 + 푉 = (푉 cos휃) + (푉 sin휃 − 푉 )

푉 = 푉 − 푉2푉 푠푖푛휃

Substituting the value of V2 from the first equation gives:

푉 = 푉 (sin휃)

2푉 tan휃 (cos휃) = 푉 tan휃

2푉 = 푉 tan휃

2

Thus the power output generated by the micro turbines will be:

푃 = 휌퐴푉 tan휃 푉

= 휌퐴푉 tan휃 푉 tan휃

2

= 휌퐴푉 (tan휃)

2 Whereas the total power available from the exhaust gas:

푃 =푚̇푉

2 = 휌퐴푉 푉

2 = 휌퐴푉

2 Thus the Coefficient of Performance for the turbine assembly can be given by:

퐶 = 푃푃 = (tan휃)

As can be seen the performance coefficient depends only on the blade outlet angle. *Calculations are presently done assuming compressed air so the properties of the gases are assumed as that of air. Also, the inlet velocity is chosen as 50m/s to the turbine while the blade outlet angle is set to 30 degrees. Also, the diameter of the micro turbine will depend on the size of the exhaust pipe. Considering lower segment cars, the diameter of the turbine and the pipe are assumed as 4 cm. So for Blade Velocity, Pressure Loss and Power Output:

푉 =푉 tan휃

2 = 50 ∗ 0.5 ∗ tan 30 = 14.43푚푠

푉 = 푟휔 = 0.02 ∗ 휔 = 14.43

휔 = 721.52푟푎푑푠 = 6890 푟푝푚

푃푟푒푠푠푢푟푒 퐿표푠푠 =휌(푉 tan휃)

2= 1.225 ∗ 50∗ (tan 30 ) ∗ 0.5= 510.41 푃푎

푃표푤푒푟 표푢푡푝푢푡 = 휌퐴푉 (tan휃)

2= 0.5 ∗ 1.225 ∗ 1.26∗ 10 ∗ 50 ∗ (tan 30 )= 32.1 푊

The above calculations are for a single value of flow velocity. In actual operation, the speed of the exhaust

gases will depend on the load on the engine so at different engine rpm the speed will be different and hence the power output. Below is a plot of the theoretical power outputs dependence on the inlet velocity keeping other factors such as blade diameter and blade outlet angle constant?

The rpm of the turbine blade varies linearly with the exhaust gas speed as shown in the following figures.

The variation of the power output v/s the turbine blade outlet angle is also studied. Using the established relationship between the power output and blade outlet angle for an exhaust flow speed of 50 m/s one obtains the following plot:

0100200300400500600

0 5 10 20 50 75 100 125

Pow

er O

utpu

t (W

)

Exhaust Speed (m/s)

0

5000

10000

15000

20000

0 25 50 75 100 125

Turb

ine

rpm

Exhaust gas velocity (m/s)

0

50

100

150

0 5 10 15 20 25 30 35 40 45

Pow

er o

utpu

t (W

)

Blade outlet Angle (deg)


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2.2 Governing Equations An incompressible Newtonian fluid viz air is

assumed and the continuity and momentum equations that were obtained after filtering were:

휕푣̅휕푥 = 0

휕푣̅휕푡 + ∇. (푣̅푣̅) = −

∇pρ

+∇. (τ̿)

ρ+ g

Where the stress tensor is given by:

휏̿ = 휇[(∇푣̅ + ∇푣̅ )− 23∇. 푣̅퐼]

Where μ is the molecular viscosity, I is the unit tensor while the second term on the right is the effect of volume dilation.

The Standard k-ε model was used for the simulations which is complete two equation turbulence model in which the solution of two separate transport equations allows the turbulent velocity and length scales to be determined independently. The turbulence kinetic energy k and the rate of dissipation are obtained from the following transport equations: 휕(휌푘)휕푡 +

휕(휌푘푢 )휕푥 =

휕휕푥 휇 +

휇휎

휕푘휕푥 + 퐺 − 휌휀

− 푌 + 푆 And,

휕(휌휀)휕푡 +

휕(휌휀푢 )휕푥

= 휕휕푥 휇 +

휇휎

휕휀휕푥

+ 퐶휀푘

(퐺 )− 퐶 휌휀푘 + 푆

In these equations Gk represents the generation of turbulence kinetic energy due to the mean velocity gradients. YM represents the contribution of fluctuating dilation in compressible turbulence to overall dissipation rate. σk and σε are the turbulent Prandtl numbers for k and ε respectively. Sk and Sε are the user defined source terms. The turbulent viscosity or eddy viscosity, μt, is computed by combining k and ε as follows:

휇 = 휌퐶푘휀

Where Cμ is a constant The model constants have the following default values: C1ε = 1.44, C2ε = 1.92, Cμ = 0.09, σk = 1.0, σε = 1.3

3. Methodology 3.1 Model Creation

The CAD model was created on DS Solid Works. The Figure 2 shows the CAD model of the turbine. Figure 3 and 4 show the front and end disks and the MT (Micro Turbine) Assembly.

Fig: 3. (a) CAD Model of turbine (b) Blade Profile

Fig: 4 (a) Front Disk (b) Rear Disk


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Fig: 4 CAD Assembly of Micro Turbine (MT) Assembly in Exhaust Pipe

The model is imported in the Ansys Workbench where the geometry is prepared for meshing. The model was meshed using a fully unstructured finite volume method using an independent patch conforming algorithm that produces grid where the mesh elements are tetrahedral. This meshed model is then imported in the solver Fluent. The meshed assembly of the model is shown in the Figure 5:

Fig: 5. Meshed model of micro turbine

The imported mesh is now setup. Different zones are setup with the corresponding boundary conditions as shown in the table below:

Zone Boundary Condition

Value

Inlet Velocity Inlet

50m/s (along x)

Outlet Pressure Outlet

0 Pa (Gauge

Pressure) Pipe Wall No slip

Nozzle Plate, Disc, Shaft

Wall No slip

Turbine Moving Wall

733 rad/s clockwise

Once the model is selected the meshed assembly is set to solve by setting a convergence criteria for the residuals at 1*10-6. The iteration limit was set to 600.

4. Results and Discussions The plot of the scaled residuals is shown in the

Figure 6 while the wall y plus of the MT Assembly is shown in Figure 7:

Fig: 6. Scaled Residuals

Fig: 7. Wall Y Plus for Micro Turbine Assembly

The model is now post processed to obtain the pressure contours on the turbine blades as well as the micro turbine assembly as shown in Figures 8 and 9

Fig: 8. Pressure Contour over MT


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Fig: 9. Pressure contour over MT assembly

Figure 10 shows the velocity contour on the Turbine Midplane.

Fig: 10. Velocity Contour of MT Midplane

The contours show the regions of higher velocity and lower velocity. There are three high velocity pockets as can be seen from the figure. These represent directly the openings of the front disk while the regions of lower velocity are those blades that are shielded by the front disk. The contours also show that per blade the velocity contours on each side of a blade have a velocity difference such that each set of contour imparts a pressure force on the blades in the anti-clockwise direction. For example, the top most blades have a region of lower velocity on the right side and correspondingly higher pressure than the pocket on the left of the blade. This pressure difference acts from the right to the left causing a force on the blade in the stated direction. Figure 11 shows the velocity streamlines for the MT assembly.

Fig: 11. Velocity Streamlines of MT Assembly

The pressure rise due to the presence of the micro turbine is an important factor to be considered. This rise in pressure attributes to back pressure and thus it is the aim to have as low a back pressure as possible. Figure 12 shows the pressure contours of the pipe inlet for both cases, when MT is not present in the exhaust (a) and when it is present (b)

(a)

(b)

Fig: 12. Pressure contour at pipe inlet (a) no MT present (b) MT is present

As can be seen from the Figure 12, due to presence of the micro turbine, the absolute pressure rises. The maximum pressure when the micro turbine isn’t present is 104140.59 Pa while it is 108236.41 Pa when the micro turbine is present. This accounts for a rise in pressure of 4095.82 Pa or 0.04 atm. This rise in pressure isn’t substantial enough thus eliminating the problem of back pressure.


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5. Conclusions The CFD analysis of an energy scavenging Micro Turbine has been carried out using the Standard k-ε turbulence model. The results obtained are

satisfactory in terms of the induced back pressure with a pressure drop of 0.1 atm only hence opening up the possibility of installing a MT in the exhaust pipe of an automobile for backup power generation.

References [1] “Cfd Analysis of Three-Dimensional Flows in A

Low Reynolds Number Microturbine”, Mohamed Omri and Luc G. Fréchette

[2] “Design Optimization of a Cost-Effective Micro Wind Turbine “, D.Y.C. Leung, Y. Deng, M.K.H. Leung

[3] “Simulation of the Aerodynamic Behaviour of a Micro Wind Turbine”, J. M. M. Monteiro1, J. C. Páscoa1 and F. M R. P. Brójo2

[4] “Design of a Micro-Turbine for Energy Scavenging from a Gas Turbine Engine”, Kalish A, Morrow. E. et al.

[5] SolidWorks User Guide [6] Ansys Fluent User Guide, 2009 [7] Ansys Fluent Theory Guide, 2009

cfd analysis of an energy scavenging axial flow micro turbine using automotive exhaust ...€¦ ·...

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