Investigations about ignition of the premixed fuel, propagation
of the combustion zone and extinction in the quenching zones require
detailed informations about the flowpattern, pressure and temperature
distribution. Moreover, the turbulent structure of the flow necessitates
the analysis of turbulence models with respect to their prediction
capabilities. The answers greatly rely on the particular geometry
and operation mode of the rotory engine.
In contrast to a stroke piston engine, four-stroke of the rotory engine
results from the special geometry of the piston, the trochoid, and
the rotor housing, the epitrochoid. Moreover, the basic dimensions
of the trochoid greatly affect all the engines characteristics, including
displacement, performance, durability and productivity. Schematically,
the operational sequences can be presented as follows:
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Corresponding to the symmetry of the piston, the different phases
of drawing in the premixed fuel (1-4), compression (5-7),
combustion (8-10) and expelling of the burnt gas (11-1),
occur displaced in time. This project, supported by the BMBF, objects
the modelling and simulation of the compression and combustion phases.
Experimental studies, numerical simulations and investigations of
stroke piston engines, predominantly calculated without friction,
have been carried out in the preceeding years. Experimental results
indicate, that higher engine speeds enhance the turbulence. This
improves air and fuel mixing. Numerical investigations on the effect
of various engine speeds show, that at higher engine speeds fuel
is consumed in a much shorter time period by the enhanced air and
fuel mixing. This shorter combustion duration provides much less
available time for soot and NOx formations. In addition, the enhanced
air/fuel mixing decreases soot and NOx by reducing the extent of
the fuel rich regions. Moreover, numerical results show, that engine
speed and fuel injection timing can be used to determine the optimum
conditions for increased engine power density with lower emissions
and acceptable engine and exhaust temperatures.
Comparable detailed investigations of a rotory piston engine, especially
concerning the combustion processes, haven't been carried out yet.
Experimental studies show, that emissions of CO or CO2 are the same
for both engines. Distinctions are known in NOx and CH emissions.
While the rotory engines produce 2/3 less NOx than piston stroke
engines, they comparatively produce more carbon monoxydes. This
behaviour results from the streched and rotating combustion chamber.
Another relying aspect presents the different temperature ranges
of both engines compared by measurements of emissions. Those temperature
distinctions are implied by the different dimensions and operation
modes of the engines as well as the propagation of combustion zones.
On account of the mentioned aspects it would appear, that experiences
and studies of piston stroke engines cannot be used, to understand
and describe a rotory piston engine in detail.
Objective:
- Investigation of following aspects:
- compressible and turbulent flow
- mixture composition of the fuel
- ignition and propagation of the combustion zone
- temperature distribution
- emission, especially concerning emission of carbon monoxydes
- Investigation of above mentioned aspects with respect to geometric
variation of the combustion chamber and fuel supply.
- Performance analysis under large variations of the rotational
speed of the engine.
- Analysis of different fuels such as diesel, cerosine or natural
gas.
Numerical Solution:
In the course of the preceeding project
for simulation a laminar premixed flame in a town gas burner, a
program package for solving the compressible Navier-Stokes equations
with a detailed reaction mechanism was developed in the object orientated
programmers language C++. This package is based on rectangular geometries
and cartesian grids. For discretization, a finite volume, collocated
grid method is applied.
In the meantime, this package has been extended for simulation the
compressible Navier-Stokes equations on non-rectangular geometries
and non-cartesian grids. Here, a special adapted structure describes
the complex geometry of the rotory engine for all phases of rotation.
Furthermore, instationary solution methods have been integrated,
which results, through the pressure correction equation, in a SIMPLE
and a SIMPLEC method for solution of the stationary and instationary
compressible Navier-Stokes equations respectively. An FAS multigrid
method is used to accelerate convergence. First results for two
positions of the piston for
present an isothermal flow simulation. First experiences with flow
simulations for different piston displacements show, that the strong
variation of the computational domain in time make high demands on
the underlying grid and solution methods. Block structured methods
have to be integrated to improve resolution.
Prospects:
- Solution methods for:
- block structured grids
- solving the compressible Navier Stokes equations on moving
grids
- Investigation of Favre averaged Navier-Stokes equations.
- k-eps model for simulation of turbulent flows.
- Investigation of different modifications of the k-eps model
as:
- Multiple Scale models (Hanjalic,Launder & Schistel)
- Modifications on the k-eps model for curvilinear domains proposed
by Gibson.
- k-eps model and Reynolds Stress Model proposed by Ha Minh
& Chassaing.
- Investigation of reduced reaction mechanisms for higher order
hydrocarbons, C12 H26.
- Simulation of the combustion process in the rotory engine.
- Turbulent combustion.
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