MODELLING AND SIMULATION OF FLOW THROUGH A FLOWBENCH
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.
There are several possible types of metering element in use. Flow benches ordinarily use one of three types: orifice plate, venturi meter, and pitot/static tube, all of which deliver similar accuracy. Most commercial machines use orifice plates due to their simple construction and the ease of providing multiple flow ranges. Although the venturi offers substantial improvements in efficiency, its cost is higher.
Airflow conditions must be measured at two locations, across the test piece, and across the metering element. The pressure difference across the test piece allows the standardization of tests from one to another. The pressure across the metering element allows calculation of the actual flow through the whole system. The pressure across the test piece is typically measured with a U tube manometer while, for increased sensitivity and accuracy, the pressure difference across the metering element is measured with an inclined manometer. One end of each manometer is connected to its respective plenum chamber while the other is open to the atmosphere.
Ordinarily, all flow bench manometers measure in inches of water although the inclined manometer's scale is usually replaced with a logarithmic scale reading in the percentage of total flow of the selected metering element which makes flow calculation simpler. The temperature must also be accounted for because the air pump will heat the air passing through it making the air down stream of it less dense and more viscous. This difference must be corrected for. Temperature is measured at the test piece plenum and at the metering element plenum. Correction factors are then applied during flow calculations. Some flow bench designs place the air pump after the metering element so that heating by the air pump is not as large a concern. Additional manometers can be installed for use with hand held probes, which are used to explore local flow conditions in the port.
The air flow bench can give a wealth of data about the characteristics of a cylinder head or whatever part is tested. The result of main interest is bulk flow. It is the volume of air that flows through the port in a given time. Expressed in cubic feet per minute or cubic meters per second/minute. Valve lift can be expressed as an actual dimension in decimal inches or mm. It can also be specified as a ratio between a characteristic diameter and the lift L/D. Most often used is the valve head diameter. Normally engines have an L/D ratio from 0 up to a maximum of 0.35. For example, a 1-inch-diameter (25 mm) valve would be lifted a maximum of 0.350 inch. During flow testing the valve would be set at L/D 0.05 0.1 0.15 0.2 0.25 0.3 and readings taken successively. This allows the comparison of efficiencies of ports with other valve sizes, as the valve lift is proportional rather than absolute. For comparison with tests by others the characteristic diameter used to determine lift must be the same. Flow coefficients are determined by comparing the actual flow of a test piece to the theoretical flow of a perfect orifice of equal area. Thus the flow coefficient should be a close measure of efficiency. It cannot be exact because the L/D does not indicate the actual minimum size of the duct.
Using extra instrumentation (manometers and probes) the detailed flow through the port can be mapped by measuring multiple points within the port with probes. Using these tools, the velocity profile throughout the port can be mapped which gives insight into what the port is doing and what might be done to improve it.
Of less interest is mass flow per minute or second since the test is not of a running engine which would be affected by it. It is the weight of air that flows through the port in a given time. Expressed in pounds per minute/hour or kilograms per second/minute. Mass flow is derived from the volume flow result to which a density correction is applied. With the information gathered on the flow bench, engine power curve and system dynamics can be roughly estimated by applying various formulae. With the advent of accurate engine simulation software, however, it is much more useful to use flow data to create an engine model for a simulator.
OBJECTIVES:
1) To create a 3D Model of the flowbench and to run the CFD flow simulation for the intake port in SOLIDWORKS FLOWSIMULATION.
2) To Perform a Grid dependency test for a particular valve lift and to select mesh based on the results for further analysis.
3) To perform a parametric study to determine the effect on the mass flow rate with different valve lifts and to plot the result.
4) To obtain velocity cutplot, pressure cutplot contour, and flow trajectories contour for different valve lifts.
USER INPUTS:
GEOMETRY CREATION:
First, the sketch for the cylinder is made using a circle tool and then it is extruded using extruded boss/base option to get a 3d cylinder. The front plane is selected to draw lines and the reference plane is used to draw a circle and sweep it along the line drawn to create a port now to give the entire geometry a thickness, shell option is used and a hollow cylinder and port are obtained. The reference axis is made along the cylindrical face in which the valve will be placed. Now in order to make the valve, the section view is used to view the cut section of the flowbench, and using the line tool we will draw half section of the valve and revolve it along its axis to get a 3d valve. And the lids are made to get watertight boundaries in order to perform internal flow simulations.
3D MODEL OF FLOWBENCH:
SECTIONAL VIEW:
GENERAL SETTINGS:
Analysis Type: Internal
Steady-state analysis
Fluids:
Liquids > Water
Since the initial temperature is T=293.2K and Pressure P=101325Pa
Density=1000kg/m^3
Dynamic viscosity=1e-3
Flow Type= Laminar and Turbulent.
Wall conditions: Adiabatic Wall
Roughness: 0micrometer
Initial conditions:
Pressure: P=101325Pa
initial temperature is T=293.2K
MESH:
After the Mesh/ Grid dependency test is done and a Mesh is Selected based on the results.
BOUNDARY CONDITIONS:
Inlet boundary:
Total Pressure = 2bar = 202650 Pa
Outlet boundary:
Static Pressure = 1bar = 101325 Pa
Flowbench wall:
Real Wall condition
GRID DEPENDENCY TEST:
This test is performed by taking the Case 1 at valve lift of 1mm.
Project name > New Parametric study
Input Variables:
Global Mesh > Initial mesh level and Local Mesh Refinement > Fluid cells Refinement is selected
Output variable:
Here cutplots, Global goals, Surface goals, and flow trajectory are selected as output variable
Scenario:
RESULTS AND PLOTS FORM GRID DEPENDENCY TEST:
GOALS:
MASS FLOW RATE AT INLET V/s NUMBER OF ITRATIONS:
MASS FLOW RATE AT OUTLET V/s NUMBER OF ITERATIONS:
DESIGN POINT 1:
Level of initial mesh - 1
Level of Refining the Fluid Cells - 1
PRESSURE CUTPLOT:
VELOCITY CUTPLOT:
DESIGN POINT 2:
Level of initial mesh - 2
Level of Refining the Fluid Cells - 1
PRESSURE CUTPLOT:
VELOCITY CUTPLOT:
DESIGN POINT 3:
Level of initial mesh - 3
Level of Refining the Fluid Cells - 2
VELOCITY CUTPLOT:
DESIGN POINT 4:
Level of initial mesh - 4
Level of Refining the Fluid Cells – 3
PRESSURE CUTPLOT:
VELOCITY CUTPLOT:
DESIGN POINT 5:
Level of initial mesh - 5
Level of Refining the Fluid Cells – 3
PRESSURE CUTPLOT:
VELOCITY CUTPLOT:
DESIGN POINT 6:
Level of initial mesh - 6
Level of Refining the Fluid Cells – 3
PRESSURE CUTPLOT:
VELOCITY CUTPLOT:
DESIGN POINT 7:
Level of initial mesh - 6
Level of Refining the Fluid Cells – 4
PRESSURE CUTPLOT:
VELOCITY CUTPLOT:
From the mass flow rates of inlet and outlet V/s iterations plot we can see that for the coarser mesh, that is Design point 1,2,3 and 4 the values vary largely and from the design point 5, 6 and 7 converge to certain value of 0.03kg/s
From the velocities and pressure cut plots of design point 1,2,3 and 4 we can observe that they are quite non-uniform and choppy, but from design point 5 we can observe that the cutplots become more uniform and in case of design point 6 and 7 the cutplots further improve.
We would use the Mesh of design point 5 having Level of initial mesh -5 and Level of Refining the Fluid Cells -3 for further study as it is best in terms of the results and the less amount of computational time required.
PARAMETRIC STUDY:
VARIATION OF THE VALVE LIFT TO GET DIFFERENT MASSFLOW RATES:
Project name > New Parametric study
Input Variables:
Here the dimension which location of valve from the inner top face (valve lift) is selected
Output Variables:
Here various Cutplots, Global goals, Surface goals, and flow trajectory are selected as output variable.
Scenario:
Here the dimension which location of valve from the inner top face (valve lift) is selected as the input variable having range of values from 1mm to 10mm having an increment of 1mm.
RESULTS AND PLOTS:
GOALS:
Here we get mass flow rate at inlet and outlet of the flowbench, -ve values of the outlet shows that the mass is leaving the system.
MASS FLOW RATE AT INLET V/s NUMBER OF ITERATIONS:
MASS FLOW RATE AT OUTLET V/s NUMBER OF ITERATIONS:
DESIGN POINT 1: VALVE LIFT OF 1mm
PRESSURE CONTOUR:
PRESSURE CONTOUR WITH PROBE
VELOCITY CONTOUR:
VELOCITY TRAJECTORY:
ANIMATION:
PRESSURE TRAJECTORY:
ANIMATION
DESIGN POINT 2: VALVE LIFT OF 2mm
PRESSURE CONTOUR:
VELOCITY CONTOUR:
VELOCITY TRAJECTORY:
PRESSURE TRAJECTORY:
DESIGN POINT 3: VALVE LIFT OF 3mm
PRESSURE CONTOUR:
VELOCITY CONTOUR:
VELOCITY TRAJECTORY:
PRESSURE TRAJECTORY:
DESIGN POINT 4: VALVE LIFT OF 4mm
PRESSURE CONTOUR:
VELOCITY CONTOUR:
VELOCITY TRAJECTORY:
PRESSURE TRAJECTORY:
DESIGN POINT 5: VALVE LIFT OF 5mm
PRESSURE CONTOUR:
VELOCITY CONTOUR:
VELOCITY TRAJECTORY:
PRESSURE TRAJECTORY:
DESIGN POINT 6: VALVE LIFT OF 6mm
PRESSURE CONTOUR:
VELOCITY CONTOUR:
VELOCITY TRAJECTORY:
PRESSURE TRAJECTORY:
DESIGN POINT 7: VALVE LIFT OF 7mm
PRESSURE CONTOUR:
VELOCITY CONTOUR:
VELOCITY TRAJECTORY:
PRESSURE TRAJECTORY:
DESIGN POINT 8: VALVE LIFT OF 8mm
PRESSURE CONTOUR:
VELOCITY CONTOUR:
VELOCITY TRAJECTORY:
PRESSURE TRAJECTORY:
DESIGN POINT 9: VALVE LIFT OF 9mm
PRESSURE CONTOUR:
VELOCITY CONTOUR:
VELOCITY TRAJECTORY:
PRESSURE TRAJECTORY:
DESIGN POINT 10: VALVE LIFT OF 10mm
PRESSURE CONTOUR:
VELOCITY CONTOUR:
VELOCITY TRAJECTORY:
PRESSURE TRAJECTORY:
MASS FLOWRATE AT OUTLET:
Here the negative sign indicates that mass is leaving the control volume and for plotting we will be considering the absolute values.
MASSFLOWRATE AT OUTLET V/S VALVE LIFT:
CONCLUSION:
1) From the Velocity cutplots we can observe that there is an increase in the velocity when the fluid reaches the valve lip region since the area across which the fluid is required to pass reduces, and also as the valve lift increases the velocity also increases.
2) From the Pressure cut plot of the first case, we can observe that there is not much significant pressure drop across the valve and hence our valve design is good since the significant pressure drop causes an increase in the pumping cost.
3) From the mass flow rate at the outlet plot, we can observe that the mass flow rate increases almost linearly as the valve lift increases, because from the equation, the density is constant and the velocity at the inlet is also constant, and from Area, the diameter of the port is constant the only thing changing is the valve lift so the mass flow rate varies linearly with valve lift.
4) The plot of mass flow rate at the outlet is very important since it is going to tell us how much amount of air is going inside the system, which affects the Air-fuel Ratio and compression rate, and we use all this information to tune in our engine correctly so that correct amount of valve lift gives us the required amount of the fuel inside the engine in order to get the correct of air-fuel mixture.