chapter 6 effect of swirl ratio on spray...

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CHAPTER 6

EFFECT OF SWIRL RATIO ON

SPRAY CHARACTERISTICS AND COMBUSTION

6.1. Introduction

Swirl is mainly used for getting the adequate fuel-air mixing rates. Air swirl is

generated with the support of a suitable inlet port and it is amplified at the end of the

compression stroke by forcing the air towards the cylinder axis into the bowl-in-piston

combustion chamber. Swirl is basically an organized rotation of air about the cylinder

axis. Though some decay of swirl occurs due to the presence of friction during the

cycle, intake generated swirl persists throughout the compression process as well as in

the combustion and expansion processes. The nature of the swirling flow in an actual

engine is extremely difficult to determine. Swirl ratio is defined as the solid-body

rotating flow, which has equal angular momentum to the actual flow, divided by

crankshaft angular speed. Swirl speed or velocity is the angular speed of the charge

about the cylinder axis (rad/sec).

From the literature survey, it was understood that the swirl in diesel engines is

an important parameter that affects the mixing rate of air and fuel, heat release rate,

emissions and overall engine performance. It was also observed that there is an

optimum level of swirl for particular combustion chamber geometry. If required, the

swirl level can be increased with an appropriate combustion chamber design. It was

observed that an increase in the swirl affects the fuel-air mixture. Finally, a CFD

simulation using Ricardo VECTIS is quite capable to investigate DI diesel engine

combustion with Ricardo two-zone flamelet (RTZF) combustion model (Wichman et

al., (2001) 146, Rahman et al., (2000) [147].

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In the present work, an attempt was made to understand the effect of swirl

ratio on spray and combustion and their influence on the emission levels of a DI

diesel engine using Ricardo VECTIS.

6.2 Best Swirl Ratio for 6 Holes Nozzle.

The present study was performed by considering six swirl ratios, viz., SR 0.5,

1.0, 1.5, 2.0, 3.0 and 4.0. The present study aims at determining “The effect of swirl

ratio on spray and combustion in a DI diesel Engine”. The effect of swirl ratio on

combustion was carried out with the support of the following plots viz., pressure, heat

release rate and temperature. The effect of swirl ratio on spray characteristics was

analyzed with the support of the following plots viz., spray penetration length, spray

angle and sauter mean diameter. The effect of swirl ratio on emissions was analyzed

with the support of the plots like viz., NOx, soot and CO.

6.2.1 Effect of Swirl Ratio on Pressure and Heat Release Rate.

Pressure and heat release rates against crank angle were drawn for all the swirl

ratios (SR) to study the impact of SR on pressure and heat release rate. This is shown

in figures 6.1 and 6.2. It is evident from the figures that the peak pressure is rising

from SR 0.5 to SR 1.5 beyond which it is falling. This is due to the fact that an

increase in SR promotes the mixing rate of air and fuel, which in turn advances the

start of combustion. This would have contributed for the rise of pressure. This trend is

observed for the SR 0.5, 1.0 and 1.5.

Reduced peak pressures were observed for the SR 2, 3 and 4. This is due to the

fact that, when the SR was increased beyond 1.5, the mixing rate of Air and Fuel is

increasing, which is affecting the combustion process. Increased swirl is not giving

sufficient scope for forming the appropriate air fuel mixtures that are favourable for

combustion. This is in agreement with the discussions of Heywood (1988) [57].

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Figure 6.1: Pressure Vs Crank Angle for 6 Holes Nozzle

Figure 6.2: Heat Release Rate Vs Crank Angle for 6 Holes Nozzle

It is evident from the figure.6.2 that HRR is more or less at a particular level

for the SR 0.5, 1.0 and 1.5; between 352 CA and 370 CA. This is due to the fact that

the air fuel mixture formation with reference to those crank angles is at the same rate.

A change in the behaviour in the rate of heat release for the SR 2.0, 3.0 and 4.0 was

observed. This change can clearly be observed especially for SR 4.0. This is due to

the fact that with an enhanced mixing rate most of the fuel would have confined to the

bowl.

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An increase in the HRR from 352 CA to 360 CA for SR 2.0, 3.0 and 4.0 was

observed. This is due to the enhanced mixing rates. This can clearly be understood

from the heat release rate plot (figure 6.2). This plot indicates that the HRR is smaller

for SR 0.5, 1.0, 2.0, 3.0 and 4.0 when compared with the SR 1.5 from 360 CA

onwards.

For the later part of the period it is observed that the heat release rate is a bit

higher for SR 3.0 and 4.0, when compared with the other swirl ratios. The same trend

was observed in the figure.6.1 i.e., the rise in peak pressure from SR 0.5 to 1.5 and

fall in peak pressure for SR 2.0, 3.0 and 4.0.

6.2.2 Effect of Swirl Ratio on Temperature.

Temperature against crank angle was drawn for all the swirl ratios. This is

shown in figure 6.3. From this figure it is found that the peak temperature is 1600K

for SR 1.5. From this figure, it is also evident that the peak temperature is rising as the

swirl ratio increases from SR 0.5 to 1.5. On further increase in swirl ratio from 1.5, it

is observed that the peak temperature is reducing.

The trends that were observed from the temperature plots are supporting the

results that were obtained from the pressure and heat release rate plots.

Figure 6.3: Temperature Vs Crank Angle for 6 Holes Nozzle

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6.2.3 Effect of Swirl Ratio on Swirl Speed.

Figure 6.4: Swirl Speed Vs Crank Angle for 6 Holes Nozzle

Swirl speed against crank angle was drawn for different swirl ratios. This is

shown in figure 6.4. From this figure, it is observed that with an increase in the swirl

ratio the swirl speed is increasing. It can be understood that a rise in the swirl ratio

rise the swirl velocity.

6.2.4 Effect of Swirl Ratio on Spray Tip Penetration, Sauter Mean Diameter and

Spray Angle.

Spray tip penetration lengths against crank angle was reported for all the swirl

ratios. This is shown in figure 6.5. The presence of swirl in the combustion chamber

affects the spray. As the swirl velocity increases air entrainment also increases. As the

air entrainment increases, it slows down the spray and makes the spray to bend more

and more in the swirl direction. Because of this, the spray penetration length reduces,

this is totally matching with the explanations of Heywood (1988) [57].

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Figure 6.5: Spray Tip Penetration Vs Crank Angle for 6 Holes Nozzle

Sauter Mean Diameter (SMD) against Crank angle is shown in figure 6.6.

Figure 6.6: Sauter Mean Diameter Vs Crank Angle for 6 Holes Nozzle

This provides the comparison of droplet SMD amongst the swirl ratios under

consideration. From this figure, it is evident that the SMD values are in decreasing

trend for any of the swirl ratios under consideration. Higher the swirl ratio, lower is

the SMD value. This is due to the fact that the air entrainment increases with an

increase in the swirl ratio. Because of that there would have been enhanced secondary

break up. This is well in agreement with the trends of Christopher et al., (1999) [150].

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Figure 6.7: Spray Angle Vs Crank Angle for 6 Holes Nozzle

Figure 6.7 shows the spray angle versus crank angle against the swirl ratios.

This figure provides comparison between the spray angles or spray cone angles

amongst the swirl ratios. It is observed that the spray angle is decreasing continuously

with an increase in the swirl ratio. The variation trend of cone angle is in agreement

with the conclusions drawn by Savoni et al.,(2001) [149]. Spray angles are observed

to lie between 14.5 and 16.9 degrees.

6.2.5 Effect of Swirl Ratio on Emissions – NOx, Soot and CO.

Figure 6.8 represents the NOx mass fraction against crank angle. It is observed

that NOx levels are high for the swirl ratios SR 0.5 and SR 1.5 when compared to all

the other swirl ratios. The SR 1.5 case can easily be understood for getting a high

value of NOx. In this case, as there is optimum mixing of air and fuel, the combustion

rate is predominant. This has resulted in higher rates of heat release, when compared

to all other cases of swirl ratios. This produced a maximum temperature higher than

those of all other swirl ratios. Figure.6.3 totally supports this explanation.

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Figure 6.8: NOx Mass Fraction Vs Crank Angle for 6 Holes Nozzle

Coming to the SR 0.5 case, as the swirl ratio is small, this might not be

sufficient for the formation of appropriate air and fuel mixture. This is giving a scope

for the mixture to get exposed to high temperatures and pressures. The NOx formation

will not depend only on the average temperature but also on the local temperatures.

The high temperatures prevailing at fewer favourable locations will cause the

production of NOx. Because of this at low SR more NOx formation takes place.

In all the other cases the swirl is contributing for the enhanced rate of air and

fuel mixture. Whereas, the available time is not sufficient for the formation of NOx.

Soot mass fraction against crank angle is shown in figure 6.9. It can be

observed that the trend is same for all the swirl ratios. And the levels of soot that has

formed is more or less same for all the cases.

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Figure 6.9: Soot Mass Fraction Vs Crank Angle for 6 Holes Nozzle

Basically the soot formation takes place in the diesel combustion environment

at temperatures between 1000K and 2800K, and at pressures higher than 5MPa till

10MPa, in presence of sufficient levels of air to burn the fuel completely.

Figure 6.10: CO Mole Fraction Vs Crank Angle for 6 Holes Nozzle

Figure 6.10 shows the variation of carbon monoxide with respect to the crank

angle for all the considered swirl ratios. Normally CO emissions are found to be very

low for diesel engines. It is observed that more or less same trend is observed for all

the considered swirl ratios. A small deviation is observed in the trend lines of SR 2.0,

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SR 3.0 and SR 4.0, beyond 390 CA. This is due to the enhanced mixing rates. The

cross over in this figure is on par with the trends of heat release rate plot shown in

figure 6.2.

Figure 6.11 show the distribution of air fuel mixture against different crank

angle across a plane for different swirl ratios. From this it can be observed that the

area showing the appropriate air-fuel ratio is increasing from SR 0.5 to 2. It is also

observed that this area is decreasing beyond SR 2.

Figure 6.12 shows the droplets distribution against various crank angles within

the combustion chamber for different swirl ratios. From this it can be observed that

the droplet diameters are decreasing from SR 0.5 to 2. It is also observed that the

droplet diameters are increasing beyond SR 2.

Figure 6.13 shows the distribution of temperature against various crank angles

across a plane for different swirl ratios. From this it can be observed that the

minimum value of temperatures are higher for SR 2. This is a clear cut indication for

improved combustion.

Figure 6.14 shows the distribution of NOx across a plane at different crank

angles against different swirl ratios. From this it can be observed that the minimum

levels of NOx for SR 2. This is a clear cut indication for the reduced emission levels.

Figure 6.15 shows the distribution of Soot across a plane at different crank

angles for different swirl ratios. There is noticed minimum deviation in soot levels.

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SR 0.5 , 356 CA SR 0.5 , 370 CA

SR 1.5 , 356 CA SR 1.5 , 370 CA

SR 2 , 356 CA SR 2 , 370 CA

SR 4 , 356 CA SR 4 , 370 CA

Figure 6.11: Distribution of Air-Fuel Ratio against Crank Angle for

6 Holes Nozzle

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SR 0.5 , 350 CA SR 0.5 , 370 CA

SR 2 , 350 CA SR 2 , 370 CA

SR 3 , 350 CA SR 3 , 370 CA

SR 4 , 350 CA SR 4 , 370 CA

Figure 6.12: Distribution of Droplets against Crank Angle for 6 Holes Nozzle

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Figure 6.13: Distribution of Temperature against Crank Angle for 6 Holes Nozzle

CRANK ANGLE 354 CRANK ANGLE 354 CRANK ANGLE 354

CRANK ANGLE 360 CRANK ANGLE 360 CRANK ANGLE 360

CRANK ANGLE 366 CRANK ANGLE 366 CRANK ANGLE 366

CRANK ANGLE 370 CRANK ANGLE 370 CRANK ANGLE 370

SR 0.5 SR 2 SR 4

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SR 0.5, 370 CA SR 0.5, 380 CA SR 0.5, 396 CA

SR 2 , 370 CA SR 2 , 380 CA SR 2 , 396 CA

SR 3 , 370 CA SR 3 , 380 CA SR 3 , 396 CA

SR 4 , 370 CA SR 4 , 380 CA SR 4 , 396 CA

Figure 6.14: Distribution of NOx against Crank Angle for 6 Holes Nozzle

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SR 0.5, 358 CA SR 0.5, 370 CA SR 0.5, 380 CA

SR 2 , 358 CA SR 2 , 370 CA SR 2 , 380CA

SR 3 , 358 CA SR 3 , 370 CA SR 3 , 380 CA

SR 4 , 358 CA SR 4 , 370 CA SR 4 , 380CA

Figure 6.15: Distribution of Soot against Crank Angle for 6 Holes Nozzle

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The noticed behaviour of air-fuel ratio (figure 6.11), droplet distribution

(figure 6.12), temperature distribution (figure 6.13), distribution of NOx (figure 6.14)

and distribution of Soot (figure 6.15) across a plane against various crank angles is

supporting the earlier explanation.

6.3 Conclusions

The effect of swirl ratio on the spray and combustion in a DI diesel engine has

been investigated numerically. The analysis was carried out for six swirl ratios. In the

process of identifying the optimum or best suited swirl ratio for the engine geometry

under consideration, though the peak pressure is observed for SR 1.5, taking NOx in

to account the best suited swirl ratio is concluded to be SR 2.0.