Fluid Mechanics Applications/A2MA33: How fluid flows in a combustion chamber

Introduction

Friends, now get ready to visualize fluid flow in combustion chambers.This project discusses air, fuel, and exhaust gas motion that occurs within the cylinders during the compression stroke, combustion stroke, and power stroke of the cycle. Before proceeding I would like to introduce you with the basic terms of this project.

combustion

In general we can say that combustion is a process in which fuel is burned to produce heat. Or Combustion is an chemical reaction between substances, usually including oxygen & usually accompanied by the production of heat and light in the form of flame. The rate at which the reactants combine is quite high, in part because of the nature of the chemical reaction itself and in part because more energy is generated than can escape into the sink with the result that the temperature of the reactants is raised to accelerate the reaction even more. A common example is a lighted match. When a match is struck, friction heats the head to a temperature at which the chemicals react and generate more heat than can escape into the air, and they burn with a flame. If a wind blows away the heat or the chemicals are moist and friction does not raise the temperature sufficiently, the match goes out. Properly ignited, the heat from the flame raises the temperature of a nearby layer of the matchstick and of oxygen in the air adjacent to it, and the wood and oxygen react in a combustion reaction.

combustion chamber

Combustion chamber RD-45F

The combustion chamber is that part in which fuel supplied by feeding nozzles, is mixed with air flow coming coming from the compressor and burns producing heat to obtain a gas stream to a temp as much as possible as required by engine.

combustion chamber's design has an important influence upon the engine efficiency and its knock properties. The design of combustion chamber includes shape of combustion chamber, the location of the sparking plug and the disposition king plug and the disposition of inlet and exhaust valves. due to the wide importance of design of combustion chambers it has been a subject of considerable amount of research and development since last fifty years. It has resulted in raisin fifty years.

The basic requirements of a good combustion chamber are to provide

1. High power output High power output
2. High thermal efficiency and low specific fuel consumption
3. Smooth engine operation Smooth engine operation
4. Reduced exhaust pollutants

Higher power output requires the following

1. High compression ratio.

The compression ratio is limited by the phenomenon of detonation. Detonation depends on the design of combustion chamber and fuel quality. Any change in design improves the anti- quality. Any change in design that improves the anti-knock characteristics of a combustion chamber permits the use of a higher compression ratio which should result in higher output and efficiency.

1. Small or no excess air.
2. Complete utilization of the air –no dead pockets.
3. An optimum degree of turbulence.

Turbulence is induced by inlet flow that enhances the volumetric efficiency.

High Volumetric Efficiency requires the following

This is achieved by having large diameter valves with ample clearance round the valve heads, proper valve ample clearance round the valve heads, proper valve timing and straight passage ways by streamlining the combustion chamber so that the flow is with lesser pressure drop.This means more charge per stroke and proportionate increase in the power output.Large valves and straight passageways also increase the speed at which the maximum power is obtained. This further increases the power by increasing the displacement per minute.

High thermal efficiency requires the following

• High compression ratio: discussed above
• A small heat loss during combustion:

This is achieved by having a compact combustion chamber which provides small surface volume ratio.The other advantage of compact combustion chamber is reduced flame travel.a given turbulence, this reduces the time of combustion and hence time loss.

• Good scavenging of the exhaust gases

Smooth engine operation requires the following

1. Moderate rate of pressure rise during combustion.
2. Absence of detonation which in turn means:
3. proper location of spark plug & exhaust valve.
4. satisfactory cooling of these two which is the hottest part.

or

Types of fluid flow

Laminar and turbulent flows

Firstly we should know the types of fluid mtion: there are two types of fluid motion characterized by REYNOLD'S NO.

1. laminar &
2. turbulent

laminar flow

The flow separates into "layers" that slide relative to one another without mixing. If we introduce a coloured stream into the laminar flow, the colour will stay in the stream. The flow is called steady. Laminar flow might be represented by a set of lines known as streamlines (flow lines).

turbulent flow

A vigorous mixing of the fluid occurs. A complex flow pattern changes continuously with time. The velocity of the particles at each given point various chaotically with time. The Velocity profile for the turbulent flow of a non viscous liquid erratic motion of the fluid often shows small whirlpool circles called eddies or vortices. Eddies absorb a great deal of energy due their rotational kinetic energy. A coloured dye added to a stream will readily disperse.

• A transition from laminar flow to turbulent flow occurs

very suddenly as the flow rate increases. The flow becomes unstable at some critical speed.

• Turbulent flow occurs when there are abrupt

boundary surfaces. The flow of blood through a normal artery is laminar. However, when irregularities occur the flow becomes turbulent and it can be heard with a stethoscope.

• When the flow becomes turbulent there is a

decrease in the volume flow rate.

• when the fluid flows around an object, the shape of the object is an important parameter in determining the type of flow.

In the combustion chambers the fluid(air-fuel mixture) flows in a "turbulent manner" so that better air fuel mixing takes place and volume efficiency increased. Due to high velocity involved, all flows into, out of and within the cylinders are turbulent except those flows in the corners and crevices where the close proximity of the walls dampens out the turbulent. Turbulence in a cylinder is high during intake and decreases as flow rate slows near BDC and increases again during compression as swirl, squish, tumble increases near TDC. The motion that occurs within the cylinder is quite important to speed evaporation of the fuel, to enhance air fuel mixing and to increase combustion speed and efficiency. In addition to the normal desired turbulence, a rotational motion called swirl is generated on the air-fuel mixture during intake. Near the end of the compression stroke, two additional mass motions are generated: squish and tumble squish is a redial motion towards the central line & tumble is a rotational motion around circumferential axis.

Fluid motion within the combustion chamber is described by following

TURBULENCE

As discussed above it is due to the high velocity of the flow. As a result of turbulence, thermodynamic transfer rates within an engine are increased by an order of magnitude. Heat transfer, evaporation, mixing, and combustion rates all increase when the fuel is burned, the high turbulence near TDC is very desirable for the combustion process. It breaks up and spreads the flame front many times faster than that of a laminar flame. The air-fuel is consumed in a very short time, and self-ignition and knock are avoided. A maximum tumble is achieved at about 120º after TDC. It may be due to the jet air flow during the valve opening that the normal tumble decreases in the beginning and increases into the early part of the compression stroke. or

SWIRL

there is also also a desired kind of motion called swirl motion in which air-fuel mixture enters tangentially to the combustion chamber and it moves in a rotational manner. so this kind of motion gradually enhances the air-fuel mixture and obviously increases volumetric efficiency in that way and is very desired for better efficient engines. so different combustion chamber geometries are designed to allow this kind of motion for better efficiency. Swirl is defined as the large scale vortex in the incylinder fluid with the axis of rotation parallel to the piston axis. swirl is considered to be as two dimensional solid body rotation, persists through compression and combustion process. The decay of swirl in an engine cylinder during the compression process is relatively small so that the overall angular momentum of the swirl vortex is almost conserved.

SQUISH & TUMBLE

Another kind of motion is squish motion. This is also a great importance assuming the piston moving upward towards the TDC.As it reaches the top dead centre the clearance volume suddenly decreased.As in the air filter the clearance valve is shorter clearance volume and the bigger one is in the combustion chamber so as it reaches the TDC the clearance volume reduces the shorter one and no longer the bigger one and so the air-fuel mixture from the bigger clearance volume has to moved in smaller clearance volume because there is no space left and as the air fuel mixture moves readily in to the smaller clearance volume as we know its a compression stroke so the combustion occurs here. spark plug ignites the air fuel mixture here and the burnt. As the piston moves downward towards the power stroke, the clearance volume would again increase and the burnt gases would again occupying the bigger clearance volume and this kind of motion is called as reverse squish. There is also a secondry effect of reverse squish referred as secondry swirll and is generally called as tumble. Tumble action caused by squish as piston approaches TDC. Tumble is a rotational motion about circumferential axis near the age of clearance volume in the piston bowl or in the cylinder head.

DIVIDED COMBUSTION CHAMBERS

Some combustion chambers are divided into parts means that some engines have divided combustion chambers, generally with about 80% of the clearance volume in the main chamber above the piston and about 20% of the volume as a secondry chamber connected through a small orifice.

CREVICE AND BLOWBY

In the combustion chamber of an engine there are tiny crevices that fill with air, fuel, and exhaust gas during the engine cycle. These crevices having the clearance between the piston and cylinder walls (about 80% of total), imperfect fit in the threads of the spark plug or fuel injector (5%), gaps in the gasket between head and block (10- 15%), and unrounded corners at the edge of the combustion chamber and around the edges of valve faces. Although this volume is on the order of only 1-3 % of the total clearance volume, the flow into and out of it greatly affects the overall cycle of the engine. The blowby loss ie due to the leaking of gas flow through crevices or gaps between the piston, piston rings and cylinder walls. The gas generally leaks or flows through them to the crankcase. although crevice volume is small enough in comparison to total combustion chamber volume, the flow into and out of it affects combustion and engine emissions. some of the gas flow in the crevice between the piston and cylinder walls gets past the piston into the crankcase where it raises the crankcase pressure and contaminates the lubricating oil.