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A NUMERICAL STUDY OF EFFECTS OF COMBUSTION CHAMBER

SHAPE ON PRE-CHAMBER COMBUSTION

S. Matsuo, Y. Kawabata, K. Okamoto and T. Amano

Tokyo Gas Co., Ltd.

Japan

ABSTRACT

To accomplish a 3-D simulation of pre-chamber combustion, the original turbulent

combustion model was developed. Two experiments, the performance test and thecombustion visualization one, were performed using a single-cylinder pre-chamber engine to

verify the applicability of the model. The history of the main chamber pressure, the flame jet

behavior and the flame propagation process were compared between the numerical and the

experimental results, and consequently, a good agreement was obtained.

Then, the effects of cavity area ratio on pre-chamber combustion were studied using the

two model engines with a bathtub type cavity. As a result, it is found that combustion in the

main chamber was promoted when the engine with the smaller cavity area ratio was used. In

addition, the effects of piston cavity type were also examined using the model engines with a

bathtub and a toroidal type cavity. As a consequence, it is found that combustion in the main

chamber was promoted for the toroidal type cavity compared with for the bathtub type one.

The causes of such combustion promotion could be explained with the differences of the

flame jet behavior and the flame propagation process induced by those of combustion

chamber shape factors.

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INTRODUCTION

Recently, interest on the energy saving and the less environmental harm of exhaust

are increasing. Thus, gas engines for co-generation systems are required to have higher

thermal efficiency and less harmful emissions such as NOx in their exhaust. For lean

burn method, the theoretical thermal efficiency is higher and NOx is less exhausted than

for the stoichiometric burn method because the ratio of the specific heat of the mixture

is higher and the flame temperature is lower, respectively. Hence, the lean burn method

is regarded as one of effective ways to meet these requirements.

Lean burn gas engines are categorized into two types, i.e. open chamber type andpre-chamber one. For the lean burn gas engines with a relatively large bore diameter,

which is larger than 150 mm in general, the latter type is adopted. The lean burn method

has the advantages mentioned above, however, it also has the weak points of low

ignitability and slow flame propagation. A pre-chamber engine can overcome these

problems because the flame jets with high energy play a role of ignition source of the

mixture in the main chamber and the turbulence produced by the flame jets eruption

promotes the flame propagation. Several researchers have investigated the operating

characteristics of pre-chamber lean burn gas engines[1]-[3].

For pre-chamber engines, combustion in the main chamber, which affects the

performance of the engine, depends strongly on the behavior of the flame jets and the

flame propagation process in the main chamber. In addition, those features are deeply

affected by the combustion chamber shape. Therefore, in order to achieve higher

performance of the engines, the combustion chamber shape needs to be optimized, and

relating parametric studies should be carried out. It takes much cost and time for the

experimental tests because the combustion chamber has many shape factors such as the

number of jet nozzles, diameter of the jet nozzle, cavity area ratio, piston cavity type

etc. Thus, numerical simulations are expected to be applied to such parametric studies to

perform an effective development of the engines.

However, it is difficult to simulate combustion by a flame jet and consequently, only

a few results of the simulations of pre-chamber combustion have been reported. While

flame jet is erupting with high velocity, extinction by flame stretch mechanism probably

occurs at the boundary between the flame jet and the surrounding mixture in the main

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chamber and consequently, the flame can not propagate to the directions perpendicular

to the flame jet axis. It is considered that one of the causes that the existing combustion

models can not simulate combustion by a flame jet is that they do not consider theextinction phenomenon.

In this study, in order to simulate pre-chamber combustion, we developed the

original turbulent combustion model (Turbulent Flame Speed Closure Model) in which

the extinction by flame stretch is considered, and tried to simulate the pre-chamber

combustion using the new model. The results of the present study could simulate well

the corresponding experimental results. Thus, we investigated the effects of cavity area

ratio and piston cavity type on pre-chamber combustion using the present combustion

model.

TURBULENT FLAME SPEED CLOSURE MODEL

Summary

In this combustion model, the fuel reaction rate can be determined using two

different mechanisms, auto-ignition and flame propagation schemes. The larger reaction

rate of these two mechanisms is the dominant one. Hence, the fuel reaction rate fuel

can be described using a maximum operator via:

{ }FPAIfuel opagationPrFlame,ignitionAutomax = (1)

The first scheme is built on the database of the reaction rate. The database is made by

using CHEMKIN (SENKIN). The auto-ignition reaction rate AI can be written as:

=

T

aexpTyya 6

aa

O

a

fuel

a

1AI54

2

32 (2)

where a1 to a6 are empirical coefficients, is the gas density, yfuel and yo2 are the

fuel and oxygen mass fractions, and T is the temperature, respectively.

The reaction rate of the flame propagation mechanism FP, the second one, can be

written as:

stTFP fcS = (3)

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where ST represents the turbulent burning velocity, c the reaction progress variable, which

is equivalent to the mass of combustion products divided by fully burnt (theoreticalmaximum) mass of combustion products, and fst stoichiometric mixture fraction.

The turbulent burning velocity ST [4] can be written as: