Modelling and simulation of flow through a flowbench Using SolidWorks

AIM: To create a 3D model of a flow bench and run flow analysis.

OBJECTIVES :

1. To obtain a plot of valve lift versus mass flow rate.
2. Find the effect of valve lift on the mass flow rate.
3. To run a parametric study for various valve lift.

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. 

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 the 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 downstream 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 handheld probes, which are used to explore local flow conditions in the port.

The airflow bench can give a wealth of data about the characteristics of a cylinder head or whatever part is tested. The result of the 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 inches. 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 that 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, the 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.

PROCEDURE :

• Geometry Modelling -
• At first, 3D model of the flow bench is made. It comprises of making a cylinder, an inlet channel, a valve. For making a cylinder, a circle was drawn and extruded to get a desired cylindrical shape. For making inlet channel, in the front plane of the cylinder, lines were drawn for a path to be swept. After that, a reference plane was inserted by selecting the lines and it's initial point to make the required plane. In this plane, a circle was drawn and it was sweep tool is used along the lines to make an inlet channel.
• Using the Shell tool, holes were provided at the two regions, i.e, inlet, and outlet of flow bench. For making a valve, a reference plane is inserted using an axis parallel to inlet channel and right plane. An approximate geometry of valve was made and was revolved around it's axis. It has to be taken care that all the geometries are properly constrained and was saved.

• Flow Simulation Setup - 
• After sketching, flow simulation was setup and lids were given at two openings and following parameters were given as inputs:
• Analysis Type: Internal,
• Type of fluid: Air,
• Type of flow: Laminar and Turbulant,
• Wall conditions: Adiabatic with 0 micrometer roughness,
• Initial conditions: Pressure($101325Pa$), Temperature($293.2K$).
• Boundary conditions were given to the surface of inlet and outlet regions with Total pressure of $202650Pa$ or  $2$bar and Static pressure of $50662.5Pa$ or $0.5$bar.
• As a part of challenge statement, different grid sizes from 3 to 6 were given at a particular valve lift to simulate and deciding which will be best for rest of simulation and was run to get cut plots for pressure and velocity.

• Case 1 (For grid mesh size -3):
• Pressure Cut Plot -

• Velocity Cut Plot -
• 

• Case 2 (For grid mesh size -4):
• Pressure Cut Plot -

• Velocity Cut Plot -

• Case 3 (For grid mesh size -5):
• Pressure Cut Plot -

• Velocity Cut Plot -

• After these simulations,it was decided to go for most fine mesh quality, and mesh size 6 will be used for parametric studies. In addition to that, a local mesh surrounding valve faces were also selected to get many accurate results as smaller the mesh size, more accurate is the result but on the other hand processing time will increase. Moreover, coarse mesh size may give inaccurate results as it can be shown in the given table. As the mesh size became fine from coarse, the value of average mass flow rate coming from the outlet for a particular valve lift decreases or it converges towards more accurate results.

• After choosing correct mesh size, a parametric study was setup and for input conditions the distance between valve and cylinder head were given in the range of $1mm$ to $8mm$ with 8 steps in between them. For output, all cutplots, flow trajectories, surface goal of mass flow rate from outlet were given and were made to run.

 RESULTS :

• Case 1 ( For Valve lift $1mm$):
• Pressure Cut Plot -

• Velocity Cut Plot -

• Flow Trajectories for Pressure and Velocity-

• Case 2 ( For Valve lift $2mm$):
• Pressure Cut Plot -

• Velocity Cut Plot -

• Flow Trajectories for Pressure -

• Case 3 ( For Valve lift $3mm$):
• Pressure Cut Plot -

• Velocity Cut Plot -

• Flow Trajectories for Pressure -

• Case 4 ( For Valve lift $4mm$):
• Pressure Cut Plot -

• Velocity Cut Plot -

• Flow Trajectories for Pressure -

• Case 5 ( For Valve lift $5mm$):
• Pressure Cut Plot -

• Velocity Cut Plot -

• Flow Trajectories for Pressure -

• Case 6 ( For Valve lift $6mm$):
• Pressure Cut Plot -

• Velocity Cut Plot -

• Flow Trajectories for Pressure -

• Case 7 ( For Valve lift $7mm$):
• Pressure Cut Plot -

• Velocity Cut Plot -

• Flow Trajectories for Pressure -

• Case 8 ( For Valve lift $8mm$):
• Pressure Cut Plot -

• Velocity Cut Plot -

• Flow Trajectories for Pressure :-

• Case 9 ( For Valve lift $9mm$):
• Pressure Cut Plot -
• 
• Velocity Cut Plot -
• 
• Flow tracjectory for pressure :- 
• 
• Case 10 ( For Valve lift 10 mm):
• Pressure Cut Plot -
• 
• Velocity Cut Plot -
• 
• Flow Trajectories for Pressure -
• 
• Output Data of Goals -

Here mass flow rate was the surface goal from outlet of flow bench. Negative sign indicates that mass is leaving the control volume and is just sign convention. For plotting graph between valve lift and mass flow rate this negative sign is removed.

CONCLUSION :

1. As the valve lift increases, the mass flow rate coming out of the outlet also increases simultaneously, as fluid rushes into the cylinder and exit from the outlet. 
2. From velocity cut plots, it can be concluded that as valve lift increases the velocity of fluid entering into the cylinder also increases and the same thing can also be said for pressure cut plots. In addition to that it can be noted that as valve lift increases the flow trajectories of pressure and velocity became uneven, it may be due to sudden rushing of fluid into the cylinder causing it to have uneven pathlines. 

REFERENCES :

1. Air flow bench

2. Engine Port Flow Simulation