International Conference of Advance Research and Innovation (-014) 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 Article Info Article history: Received January 014 Received in revised form 10 January 014 Accepted 0 January 014 Available online 1 February 014 Keywords 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 1 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 10 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)/10 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 159
International Conference of Advance Research and Innovation (-014) 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 thermoelectric 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 = T T T 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 005. It was tested with compressed air at 330 C (66 F) and produced 130,000 rpm. At 18% efficiency it outputs approximately 8 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, 005). 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 5mm (0.984in) (Micro and Precision Engineering Research Group, 005).. Theory.1 General Theory Let, V 1 be the inlet flow velocity to the blade V b be the blade velocity V 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.: V = V cos θ (b) (a) Fig:. 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: P = m V V = m V sin θ V Where, P = power output m = Mass flow rate = ρav 1 A = Area of the pipe through which the exhaust gases will flow. Substituting first equation in the second will yield: P = m V tan θ V = ρav tan θ V 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 V r1 = V r i.e. Blade friction factor is assumed as one due to small size of the blade. So by trigonometry: V = V V + V = (V cos θ) + (V sin θ V ) 160
International Conference of Advance Research and Innovation (-014) V + V = (V cos θ) + (V sin θ V ) V = V V V sinθ Substituting the value of V from the first equation gives: V (sin θ) V = V tan θ (cos θ) = V tan θ = V tan θ V Thus the power output generated by the micro turbines will be: P = ρav tan θ V = ρav tan θ V tan θ = ρav (tan θ) Whereas the total power available from the exhaust gas: P = m V = ρav V = ρav Thus the Coefficient of Performance for the turbine assembly can be given by: C = P = (tan θ) P 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: V = V tan θ = 50 0.5 tan 30 = 14.43 m s V = rω = 0.0 ω = 14.43 ω = 71.5 rad = 6890 rpm s Pressure Loss = ρ(v tan θ) = 1.5 50 (tan 30 ) 0.5 = 510.41 Pa Power output = ρav (tan θ) = 0.5 1.5 1.6 10 50 (tan 30 ) = 3.1 W 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? Power Output (W) The rpm of the turbine blade varies linearly with the exhaust gas speed as shown in the following figures. Turbine rpm 600 500 400 300 00 100 0 0000 15000 10000 5000 0 0 5 10 0 50 75 100 15 Exhaust Speed (m/s) 0 5 50 75 100 15 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: Power output (W) 150 100 50 0 Exhaust gas velocity (m/s) 0 5 10 15 0 5 30 35 40 45 Blade outlet Angle (deg) 161
International Conference of Advance Research and Innovation (-014). Governing Equations An incompressible Newtonian fluid viz air is assumed and the continuity and momentum equations that were obtained after filtering were: v x = 0 v t +. (v v ) = p ρ Where the stress tensor is given by:. (τ ) + + g ρ τ = μ[( v + v ). v I] 3 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: (ρk) + (ρku ) t And, = (ρε) + (ρεu ) t = μ + μ σ k + G ρε Y + S μ + μ σ ε ε + C k (G ) C ρ ε k + S In these equations G k represents the generation of turbulence kinetic energy due to the mean velocity gradients. Y M represents the contribution of fluctuating dilation in compressible turbulence to overall dissipation rate. σ k and σ ε are the turbulent Prandtl numbers for k and ε respectively. S k and S ε are the user defined source terms. The turbulent viscosity or eddy viscosity, μ t, is computed by combining k and ε as follows: k μ = ρc ε Where C μ is a constant The model constants have the following default values: C 1ε = 1.44, C ε = 1.9, C μ = 0.09, σ k = 1.0, σ ε = 1.3 3. Methodology 3.1 Model Creation The CAD model was created on DS Solid Works. The Figure 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 16
International Conference of Advance Research and Innovation (-014) 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: 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: 6. Scaled Residuals 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 Value Condition Inlet Velocity Inlet 50m/s (along x) Outlet Pressure Outlet 0 Pa (Gauge Pressure) Pipe Wall No slip Nozzle Plate, Wall No slip Disc, Shaft Turbine Moving Wall 733 rad/s clockwise 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 163
International Conference of Advance Research and Innovation (-014) 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 1 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) Fig: 9. Pressure contour over MT assembly Figure 10 shows the velocity contour on the Turbine Midplane. (a) 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. (b) Fig: 1. Pressure contour at pipe inlet (a) no MT present (b) MT is present As can be seen from the Figure 1, 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 10836.41 Pa when the micro turbine is present. This accounts for a rise in pressure of 4095.8 Pa or 0.04 atm. This rise in pressure isn t substantial enough thus eliminating the problem of back pressure. Fig: 11. Velocity Streamlines of MT Assembly 164
International Conference of Advance Research and Innovation (-014) 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 References [1] Cfd Analysis of Three-Dimensional Flows in A Low Reynolds Number Microturbine, Mohamed Omri and Luc G. Fréchette [] 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. Brójo 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. [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, 009 [7] Ansys Fluent Theory Guide, 009 165