Modelling and simulation of flow through a flowbench
AIM
To perform a flow simulation on flow bench and obtain the following:
1) Grid dependency for selected valve lift
2) Effect of valve lift on ass flow rate
THEORY
An air flow 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.
Working principleof flow-bench
A flow-bench needs a source of airflow, a system to measure the actual air flow, a method to control the test pressure of air being applied and an area to mount the device under test. The test piece is mounted and a certain pressure, called test pressure (eg 28” of water) is applied to it and the resulting air flow measured.
The chosen way to measure the air flow is by measuring the pressure drop across an ‘orifice’ with a known characteristic. Greater air flow will cause a greater pressure drop. Simple ‘water column’ manometers are used to measure the test pressure and the flow rate, no need for complex tables or spreadsheets. This method is not only easy but it’s very accurate, it is independent of all the atmospheric variables (air temp and pressure) that plague other methods. Any variations are applied to the test piece as well as the measuring system (it’s called ‘ratiometric’ measuring). It is the method also used by the industry standard flow-bench’s from the Super-flow company.
To cover a full range of testing accurately we need several orifices as each has a limited range of flows it can be used for.
PROCEDURE
1) On the top plane a cylinder of diameter --- is made and extruded to a length of ---
2) Front plane is selected and a path is drawn for the inlet. A reference plane is created and a circle of diameter --- is drawn. Using swept boss/base, it is extruded along the path drawn.
3) Once the geometry is done, the shell feature is used to create a shell of 1 mm between the inlet and the outlet.
4) Using reference created at the inlet path, a reference plane is created on which the inlet valve is drawn. After providing the necessary dimensions, the revolved boss/base is used to create the valve.
5) The part is saved and a flow simulation is set up using the wizard. Internal flow is selected and air is selected as the flowing medium.
6) For the boundary conditions, total pressure is provided at the inlet and the static pressure is provided at the outlet. Here, total pressure at the inlet is equal to 101325 Pa and the static pressure at the outlet is equal to 37025 pa.
7) A mesh of grid size 4 is created and presure and velocity cut plots and flow trajectories are created.
Grid dependency test
A new parametric study is selected and the grid size is given s the input variable. It is varied from 2-4.
For the output, we obtain the pressure and the velocity plots at different grid sizes and compare them to determine the optimum grid size.
Effect of valve lift on mass flow rate
Once the optimum grid size is determined, using that grid, a new parametric study is created and the valve lift dimension is varied from 2mm to 8mm with 7 discrete values.
Corresponding to different valve lifts, the mass flow rate goal plot is provided as the desired output to study the effect of valve lift on the mass flow rate.
RESULTS
Grid dependency test
Grid size = 2
Grid size = 3
Grid size = 4
Effect of valve lift on mass flow rate
OBSERVATIONS AND CONCLUSION
On observing the pressure and velocity plots for different grid sizes, we can see that the flow gets more refined with increase in the grid size. Therefore, we can say that the larger grid size is better, that is, more refined mesh provides better results. Hence we select grid size of 4.
Now, from the plot between valve lift and the mass flow rate, we can observe that with increase in the valve lift, the mass flow rate is decreasing. However, the value of mass flow rate is negative, which implies that the fluid is leaving the system. Therefore, the mass low rate is increasing with increase in the valve lift which is true because with higher valve lift, more area would be provided for the air to flow. Thus, mass flow rate would increase.
REFERENCES