Modelling and simulation of flow through a flowbench

OBJECTIVE

To model and simulate the flow of air through a flowbench.

INTRODUCTION

The internal combustion (IC) engine is designed to produce power from the energy that is
contained in its fuel and for the fuel to burn air(oxygen) is necessary. The quantity of air(oxygen) decides the quantity(power) and the quality(exhaust gases) of the combustion process. Airflow rate depends on the geometry, size of inlet port and the inlet valve design. Flowbench is a device which helps us to understand how the fluid flows inside the cylinder through inlet port and valve to find the mass flow rate.

THEORY

Combustion is a chemical process in which a substance reacts rapidly with oxygen and gives off energy in the form of heat. The original substance is called the fuel and the source of oxygen is called the oxidizer. An internal combustion engine (ICE) is one where the ignition and combustion of the fuel occur within the engine itself. The engine then partially converts the energy from the combustion to work. The engine consists of a fixed cylinder and a moving piston. The expanding combustion gases push the piston, which in turn rotates the crankshaft. Ultimately, through a system of gears in the powertrain, this motion drives the vehicle’s wheels. The combustion is the chemical reaction between the air and fuel generally composed by a mixture of different hydrocarbon molecules.

Depending on the amount of oxygen(air) present the combustion process can be complete or incomplete.

Complete combustion reaction:`C_xH_y+nO_2rarraCO_2+bH_2O+\"energy\"`. Here n number of oxygen molecules are available for fuel and a number of carbon dioxide molecules and b number of water molecules are obtained along with the release of energy.

Incomplete combustion reaction:`C_xH_y+mO_2rarrpCO_2+qH_2O+rCO+sHC+\"energy\"`. Here m (less than n) number of molecules of oxygen is available for fuel and p number of carbon dioxide molecules, q number of water molecules, r number of carbon monoxide molecules and s number of hydrocarbon are obtained along with the release of energy.

The energy released in case of incomplete combustion is less compared to complete combustion and also exhaust contains gases that are harmful to the environment.

An air flowbench is a device used for testing the internal aerodynamic qualities of an engine component so that we can understand how and the rate at which air flows through an engine. It is used primarily for testing the intake and exhaust ports of cylinder heads of an internal combustion engine. It is also used to test the flow capabilities of any component such as air filters, carburettors, 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.

METHODOLOGY

Generation of an inlet valve and cylinder model:

A cylinder with a bore of 0.1 m and the stroke length of 0.1 m and an inlet valve of suitable dimension is generated using solid works.

Setup:

Since we need a closed volume for analysis, we define lids at the openings which are, intake port opening and cylinder bottom dead centre.

Boundary conditions:

Inlet Pressure: 10123 Pa

Outlet Pressure: 10123 Pa

Grid Dependency Test:

Grid convergence is the term used to describe the improvement of results by using successively smaller cell sizes for the calculations. A calculation should approach the correct answer as the mesh becomes finer, hence the term grid convergence. The normal CFD technique is to start with a coarse mesh and gradually refine it until the changes observed in the results are smaller than a pre-defined acceptable error.

We perform a What-If Analysis of the parametric study where we set the level of global mesh to one and increment the value until a grid independent solution is obtained.

Flow simulation:

To get the flow simulation during the decent of the inlet valve, we again perform What-If Analysis of the parametric study. We increase the distance between the top surface of the inlet valve and the bottom surface of the engine top dead centre. Mass flow rate and velocity plot are calculated at different positions of the valve.

RESULT

Grid Dependency Test:

Goal (Value)	Design Point 1	Design Point 2	Design Point 3	Design Point 4	Design Point 5	Design Point 6	Design Point 7
Level of initial mesh (Automatic Mesh) [ ]	2	1	3	4	5	6	7
SG Mass Flow Rate 1 [kg/s]	-0.045578011	-0.04067533	-0.057728749	-0.054504999	-0.055247214	-0.053110362	-0.054244342
SG Mass Flow Rate 2 [kg/s]	0.045316137	0.039648672	0.056255999	0.051796252	0.054600592	0.052816292	0.053705917

Lift v/s mass flow rate:

Goal (Value)	Design Point 1	Design Point 2	Design Point 3	Design Point 4	Design Point 5	Design Point 6	Design Point 7	Design Point 8
D1@[email protected] [m]	0.001	0.002	0.003	0.004	0.005	0.006	0.007	0.008
Level of initial mesh (Automatic Mesh) [ ]	4	4	4	4	4	4	4	4
SG Mass Flow Rate 1 [kg/s]	-0.022810012	-0.035227099	-0.04514932	-0.053651027	-0.055684894	-0.057957323	-0.059065906	-0.062315483

Mass flow rate is negative because mass is leaving the system

CONCLUSION

Grid Dependency Test:

We can observe from the graph that finer the mesh mass flow rate is varying. But after 4th level result almost remains constant. Thus we choose 4 as the grid size level which gives an optimal solution in an optimal period.

Lift v/s mass flow rate:

As the lift increases the flow rate also increases as the area for the air to flow increases and losses(due to contraction and expansion) decreases.