Valve Lift and Mass Flow Rate Analysis of a FlowBench using Solidworks

Aim:- Valve Lift and Mass Flow Rate Analysis of a FlowBench using Solidworks

Objective:-

1. To obtain a plot of lift vs mass flow rate
2. To do a grid dependence test
3. To study the effect of valve lift on the mass flow rate

Introduction:-

An airflow bench is a device used for testing the internal aerodynamic qualities of an engine component and is related to the more familiar wind tunnel.

It is used primarily for testing the intake and exhaust ports of cylinder heads of internal combustion engines. It is also used to test the flow capabilities of any component such as air filters, carburetors, manifolds, or any other part that is required to flow gas. A flow bench is one of the primary tools of high-performance engine builders, and porting cylinder heads would be strictly hit or miss without it.

A flow bench consists of an air pump of some sort, a metering element, pressure and temperature measuring instruments such as manometers, and various controls. The test piece is attached in series with the pump and measuring element and air is pumped through the whole system. Therefore, all the air passing through the metering element also passes through the test piece. Because the volume flow rate through the metering element is known and the flow through the test piece is the same, it is also known. The mass flow rate can be calculated using the known pressure and temperature data to calculate air densities and multiplying by the volume flow rate.

Description:-

A homogeneous air-fuel mixture that enters the combustion chamber during the intake process can tremendously optimize the engine performance output. There are a number of factors that affect the mixture, but this study focuses on the flow through a cylinder head port. Since flow losses are caused by port expansion rather than contraction, the greatest flow loss in the intake port is due to the expansion of air out of the valve.

The airflow through the engine is directly controlled by the valve lift, so greater flow should occur at the maximum valve lift at least up to a certain point.

The design of the cylinder head for the automotive SI engine is important because it affects the performance, fuel economy, and emissions. Designing and developing an effective intake port is one of the major research areas in the SI engine since it determines the volumetric efficiency and in-cylinder charge motion of the spark-ignition engine, which influences the thermodynamic properties and is directly related to power output and emissions. In order to achieve higher thermal efficiency and better emission control, intake air motion should be enhanced during combustion.

Turbulence intensity, swirl, and tumble content in the incoming airflow to the combustion chambers are the key parameters in controlling the emission level. The postulations of several flow regimes are classified by the occurrence of flow separation at the valve face or valve seat. The separation at the valve face and seat has been shown to be crucial for the prediction of flow losses in intake regions. There could be a little variation of the flow with valve lift in the upstream part of the port, but ‘the flow pattern in the vicinity of the valve and inside the cylinder may vary with lift’.

Flowbench Model:-

Turbulence is something that is important for the proper mixing of air and fuel inside the combustion chamber. The inlet manifold configuration affects the turbulence of the fluid inside the cylinder. The turbulence inside the combustion chamber is recognized in two ways: 1. Swirling Motion; 2. Tumble Flow.

Swirl Flow: It is also known as the axis-symmetric flow. Here the conservation of angular momentum results in the creation of a free vortex flow in which circumferential velocity increases as the radius decrease and then decays to zero at r = 0 due to acting of viscosity. The strength of the swirl flow is calculated by the swirl ratio, a dimensionless parameter, a ratio of angular velocity to axial velocity.

Tumble flow: It is formed about the circumferential axis near the edge of the clearance volume in the piston crown or in the cylinder head, which is caused by squishing of the in-cylinder volume as the piston reaches near TDC. It is also known as vertical swirl or barrel swirl. During the compression stroke, swirl flow interacts with the tumble flow forming a squish air flow motion.

Procedure:-

1. Create the model with fully defined dimensions.
2. Give initial conditions and apply boundary conditions.
3. Create surface goals - Total Pressure & Mass flow rate.
4. Create cut plots to view the velocity profile for the varying values of the valve lift.
5. Run parametric study for varying values of mesh levels & obtain the best mesh value.
6. Run parametric study for varying values of valve lift with at best mesh level.
7. Analyze the plots & graphs generated.

Boundary Conditions for each Test:-

1. Internal flow study
2. Gases for flow – Air
3. Inlet total pressure - 150000 Pa (1.5 bar) at temperature 293.2 K
4. Outlet static pressure - 101325 Pa (1 bar) at temperature 293.2 K
5. Type of wall considered - Real wall.

Grid Dependency Test:- A parametric study for a given system by varying mesh levels and analyzing the output at the best mesh. Grid dependence test is a method in which the model is subjected to successive small cell sizes/mesh sizes for performing the calculations. When the element size decreases the percentage error in the results reduces or it means that the solution converges to a better solution. For the cylinder model with a constant lift of 2 mm, the mesh level is varied for 3 values.

It involves running simulations on successively finer mesh sizes to determine the effect of the mesh size on the accuracy of the result. Generally, 3 mesh sizes are chosen – Course, Medium, and Fine. These results are then compared to check the variation on values. The computed results show that we get finer results as increases the level of mesh. At a certain point, it is observed that the results do not change, so this further refinement of mesh will only lead to a waste of time and increase in run time.

Input Variables-

Level of mesh = 3,4,5

Inlet total pressure = 150000 Pa (1.5 bar)

Valve lift = 2 mm (0.002 m)

Output Parameters-

Mass flow rate (kg/s) at the face of lid2 (exit of the system)

Velocity in the system

Output Goals-

We can observe that with an increase in mesh level the pressure drop across the system reduces thereby the flow rate increases with fluctuation. By considering the above data the first 2 data points tend to converge to a better solution as the refining of cells increases. Hence we choose a mesh level of 5 for the parametric study to show the effect of valve lift on the mass flow rate.

Valve Lift Analysis:-

Input Variables-

Level of mesh = 5

Inlet total pressure = 150000 Pa (1.5 bar)

Valve lift = 1mm to 10mm (0.001 m to 0.01 m)

Output Goals-

The table clearly shows that as the valve lift increases, the mass flow rate of the fluid into the cylinder also increases. Valve Lift is the distance by which the valve is raised from the seated position. The camshaft controls the speed of the valve opening.

As the mass flow rate increases, the velocity of the fluid at the inlet also increases which results in a decrease in the swirl velocity and increases the discharge coefficient. This exhibits a better engine performance results.

Flow Trajectory:-

Pressure and Velocity Contours at 1 mm Valve Lift-

Pressure and Velocity Contours at 2 mm Valve Lift-

Pressure and Velocity Contours at 3 mm Valve Lift-

Pressure and Velocity Contours at 4 mm Valve Lift-

Pressure and Velocity Contours at 5 mm Valve Lift-

Pressure and Velocity Contours at 6 mm Valve Lift-

Pressure and Velocity Contours at 7 mm Valve Lift-

Pressure and Velocity Contours at 8 mm Valve Lift-

Pressure and Velocity Contours at 9 mm Valve Lift-

Pressure and Velocity Contours at 10 mm Valve Lift-

Mass Flow Rate vs Valve Lift Plot:-

As the valve lift increases, the mass flow rate also increases because the port opens and the air-fuel mixture flow increases. But the mass flow rate becomes constant, irrespective of the valve lift variation. In this graph, it is visible that the mass flow rate is constant after the valve lift of 5 mm. ( In the output goal table, the mass flow rate values are negative which shows that the gases are flowing out of the system.

Pressure vs Valve Lift Plot:-

As the valve lift increases, the pressure drops in the system with little variations as shown in the graph.

Velocity vs Valve Lift Plot:-

As the valve lift increases, the average velocity of the gases also increases because the port opens and the flow of air-fuel mixture increases. But the velocity becomes constant, irrespective of the valve lift variation with a little variation.

Conclusion:-

1. At first, the fluid velocity is maximum between the valve seat and port for a valve lift of 0.001 m, as it is quite obvious that the flow area is very small, satisfying the continuity equation. Now, as the valve lift increases, the velocity at that region decreases as the area between them increases.
2. . As the valve lift increases, the mass flow rate also increases but up to a certain limit. After that, it becomes constant irrespective of the increased valve lift.
3. The increase in mass flow rate leads to an increase in the velocity of the gases at the inlet, thus decrease in pressure, satisfying Bernoulli's Theorem.
4. The swirl and tumble ratio decreases with an increase in valve lift.

Reference:-

https://en.wikipedia.org/wiki/Air_flow_bench

https://journals.sagepub.com/doi/full/10.1155/2013/482317