Report No. SH-08-0201-B1-V1.01 - Final Report - INFLUENCE OF TURBOCHARGER FOULING ON FUEL CONSUMPTION Customer : Innospec Ltd. Thiesstr. 61 D-44649 Herne Germany Contractor : MET Motoren- und Energietechnik GmbH Erich-Schlesinger-Straße 50 D-18059 Rostock Germany Editors : Prof. Dr.-Ing. habil. S. Bludszuweit Dr.-Ing. H. Stückrad Dipl.-Ing. C. Escher Date : 01.07.2008
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Report No. SH-08-0201-B1-V1.01
- Final Report -
INFLUENCE OF TURBOCHARGER FOULING ON FUEL CONSUMPTION
Customer: Innospec Ltd. Thiesstr. 61 D-44649 Herne Germany Contractor: MET Motoren- und Energietechnik GmbH Erich-Schlesinger-Straße 50 D-18059 Rostock Germany Editors: Prof. Dr.-Ing. habil. S. Bludszuweit Dr.-Ing. H. Stückrad Dipl.-Ing. C. Escher Date: 01.07.2008
SH-08-0201-B1-V1.01 INFLUENCE OF TURBOCHARGER FOULING ON FUEL CONSUMPTION
Fig. 21 Fuel consumption changes due to changes in turbine efficiency ............................. 24
Fig. 22 Influence of turbocharger efficiency on fuel consumption [5]................................. 25
LIST OF TABLES
Tab. 1 Pressure loss and isentropic efficiency of the turbine part of a turbo charger for
different boundary conditions .................................................................................. 22
Tab. 2 12-Technical data of a 12-cylinder gas engine......................................................... 23
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1 INTRODUCTION
A turbocharger is a piece of equipment installed in diesel engines to increase the power output
of the engine by increasing the mass of air entering the engine. Basically, the turbocharger
consists of a gas turbine driven by the waste energy of the engine exhaust gas, which is di-
rectly coupled to a centrifugal blower (gas compressor). It is known that modern turbochargers
with their high efficiency can take from the exhaust gas stream a sufficient amount of energy
to make available the required air flow for the combustion as well as the scavenging air of the
cylinder. The blower compresses the intake air above atmospheric pressure, greatly increasing
the volumetric efficiency beyond that of naturally aspirated engines. Compressor and turbine
have the same shaft. A key advantage of turbochargers is that they offer a considerable im-
provement in engine efficiency and an increase in engine power with only a slight increase in
weight.
The compressor increases the pressure of the air entering the engine, so a greater "charge"
(fuel/air mixture) enters the engine in the same time interval (the increase in fuel is required to
keep the same mixture ratio). This greatly improves the volumetric efficiency of the engine.
The increase in pressure is called "boost". The energy from the extra fuel leads to more overall
engine power. A disadvantage of high boost pressures for internal combustion engines is that
compressing the inlet air increases its temperature. The higher temperature is a volumetric
efficiency downgrade which can be alleviated by cooling the inlet air before it is fed into the
cylinders, using an intercooler or a charge air cooler (a heat-exchange device).
However, there are some parasitic losses due to heat and exhaust backpressure from the tur-
bine, so turbochargers are generally only about 80% efficient because it takes some work for
the engine to push those gases through the turbocharger turbine (which is acting as a resis-
tance in the exhaust stream). Recently intensive analysis has been carried out as one can addi-
tionally increase the efficiency of the turbochargers to reduce engine exhaust emissions.
This can be achieved by compressing the fresh air before the cylinder to a very high pressure
before expanding the highly compressed air into the cylinder. As a result, heat is extracted, as
is well known, from the cylinder contents and peak temperatures are lowered during the com-
bustion process leading to reduced NOx formation.
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1.1 Initial Situation - Motivation for the Project
The previously described positive effects of the turbocharger on the engine process can only
occur during the engine operation if the efficiency of the turbocharger is not reduced under
real conditions by fouling. The components of the turbines of turbochargers operate in a par-
ticularly harsh environment. All surfaces that come in contact with the exhaust gases invaria-
bly become rough with service. Normally the main cause of the fouling is the incomplete
combustion of heavy fuel oil. The danger of fouling is greater, the more large diesel engines
are operated in the lower partial-load operational range without the use of combustion improv-
ing fuel additives. The degree and the nature of roughness due to mechanisms such as erosion,
fuel deposition, pitting, and spallation of thermal-barrier coatings depend on the fuel (HFO)
burnt, the operating conditions, and service time. This material degradation in the form of
surface roughness is known to increase surface skin friction and heat transfer in a significant
way. For a given cooling management, increase in surface heat transfer increases material
temperature, which hastens further material degradation.
It is well known that keeping the foils of a turbine free-flowing helps to maintain pumping
efficiency and subsequently helps to keep thermal efficiency and output capacity of the engine
high. Otherwise if a turbocharger is used with very rough nozzle ring and turbine blades the
fuel consumption and the exhaust temperature will increase significantly, which in turn can
possibly lead to engine and turbocharger break-down.
The turbine nozzles are the components that will have the most effect on efficiency where
surface finish (roughness) is concerned. Although both nozzles and turbine blades have the
highest ratio of surface area to the cross-sectional area of the exhaust gas path, the turbine
blades have a lower differential velocity between the gas and surface, making them less af-
fected by roughness than are the nozzles. Also, the higher the pressure of the gas, the more
important roughness becomes.
Fig. 1 shows clearly the surface state of a nozzle ring in service in comparison to a factory-
new component. Clearly deposits and roughness on a millimetre scale can be recognised.
In Fig. 2 the deposits on the rotor blades of a used turbine are shown after washing. Also here
the thickness of the deposits partially lies in the scale of millimetres.
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Fig. 1 Nozzle ring a) brand-new b, c) after use under severe service conditions
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Fig. 2 Deposits on rotor blades used under severe service conditions a), b), c) after washing
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1.2 Terms of Reference
In order to estimate the efficiency losses of turbo chargers during a service cycle, it is neces-
sary to estimate the skin friction and heat transfer augmentation generated by various types of
surface roughness. So the aim of the current project was to demonstrate the influence of the
typical changes of surface roughness under service conditions to the pressure drop inside the
turbine of a turbocharger using state of the art numerical (CFD) simulation and analysis tech-
niques on
a) strongly simplified geometries taking into consideration the real roughness of a surface.
b) the real complex geometry of a turbocharger turbine using simplified roughness models.
2 SOLUTION STRATEGY – STEPS OF THE INVESTIGATION
The solution of the problem is planned in 3 steps:
Step 1:
The first step of the investigations is the preliminary analysis of a 2D geometrical model. The
geometrical model is based on real roughness conditions similar to experimentally determined
roughness data. Primarily, the model is used for the comparison of different mesh generation
strategies and the evaluation of the influence of different turbulence models. The results of the
simulation runs will be compared to results taken from well known reference sources (see
[4]). For the comparison cases nearly identical geometrical proportions and boundary condi-
tions were used. The main objectives of the investigations are
• the optimization of the necessary refinement of the numerical mesh and
• the choice of the turbulence model for further investigations.
Step 2:
The second step of investigation work is characterized by the creation of a three-dimensional
geometrical model of a real surface topography based on measured data. The three-
dimensional surface topography is formed by a texture consisting of repetitive or random de-
viations from the nominal surface, whereas surface roughness consists of the finer irregulari-
ties which are present. The coarser elements of the surface texture are waviness, lay and flaws.
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The universally and internationally recognized parameter of roughness is Ra and is defined as
the arithmetic average deviation of the surface from a mean line or centreline.
The above three-dimensional geometrical model of the real surface roughness will be mapped
on a numerical mesh and using boundary conditions accordingly a 3D CFD model will be
created. Using the CFD model the pressure loss along the actual rough test surface will be
determined and compared to results which were obtained with the application of
• a hydraulically smooth surface
• a surface with an equivalent sand roughness varying the grain size
As a result of the investigations an equivalent sand grain roughness size can be derived en-
tirely from roughness geometry for further use with complex geometries.
Step 3:
In step 3 a three-dimensional geometrical model representing the complex geometry of the
stator (nozzle ring), rotor (turbine blades) and housing of a real turbine stage of a turbocharger
is mapped on a numerical mesh and simulated under real boundary conditions by using state
of the art CFD tools. The calculation effort of the model as a result of the necessary resolution
of the geometry does not permit taking into consideration the actual roughness of the surface,
so it is necessary to fall back on the model of equivalent sand grain roughness. While doing so
the values of sand grain size determined in step 2 will be used for the CFD model. For com-
parison purposes a simulation case with a hydraulically smooth surface will be calculated as
well. As result of the calculations the influences of surface roughness of the nozzle ring, tur-
bine blades and the housing on the pressure drop over the turbine and with it the turbine effi-
ciency may be evaluated. By means of the last findings an approximation of the changes in
fuel consumption caused by a changed turbine efficiency of the turbocharger can be calcu-
lated.
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3 RESULTS
3.1 Strongly Simplified 2D Model of a Rough Surface
270 mm270 mm
Roughness Profile
0.00.0 0.2
3.6
8.8
10.7
13.3
9.6
8.3
6.7
5.4
14.8
18.4
0.0
5.0
10.0
15.0
20.0
below -2.5mm -2.5mm to -2.0mm -2.0mm to -1.5mm -1.5mm to -1.0mm -1.0mm to -0.5mm -0.5mm to 0.0mm 0.00mm to 0.5mm 0.5mm to 1.0mm 1.0mm to 1.5mm 1.5mm to 2.0mm 2.0mm to 2.5mm 2.5mm to 3.0mm above 3.0mm
Roughness Magnitude [mm]
Freq
uenc
y [%
]
Avarage Value of Roughness Magnitude: 0.424 mm
Fig. 3 Surface roughness profile (see [4])
Fig. 4 Detail of a numerical mesh with mesh adaptation
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Fig. 5 Pressure and velocity distribution; turbulence model: k-ε
Fig. 6 Pressure and velocity distribution; turbulence model: SST
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Fig. 7 Pressure and velocity distribution; turbulence model: SA
Fig. 8 Pressure and velocity distribution published in [4]
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The principal possibilities of the entire mapping of a rough surface on a numerical mesh and a
simulation with modern CFD solvers were examined in a two-dimensional model. Fig. 3
shows the enlargement of a representative sectional view of a rough surface and the statistical
distribution of the roughness. A detail of the numerical mesh is drawn in Fig. 4. With the help
of the scale the refinement (high resolution) of the grid used can be read. Simulation calcula-
tions with different turbulence models were carried out. Fig. 5 to 7 show the velocity and
pressure distributions in the vicinity of the test case for the following turbulence models:
• k-ε Fig. 5,
• SST Fig. 6,
• S-A Fig. 7.
After analysis of the calculations carried out and comparison of the skin friction coefficients
to results which were taken from available literature (see Fig. 8), the Spallart-Allmaras (S-A)
turbulence model proved to be suitable for use with further investigations of rough surfaces.
3.2 Simplified 3D Model of a Rough Surface
Boundary Conditions:Inlet - velocity: 10 m/sOutlet - pressure: 0 PaBottom - wall: smooth / rough surfaceAll other sidesof the fluid domain: symmetry b. c.
Inlet
Outlet
Wall (hydraulically smooth)Rough Surface
Wall (hydraulically smooth)
Inlet
Outlet
Wall (hydraulically smooth)Rough Surface
Wall (hydraulically smooth)
Fig. 9 Simplified 3D model of a rough surface; geometry and boundary conditions
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Numerical mesh (detail)Roughness profile Height max = 2.8 mmHeight min = 0.0 mmHeight average = 1.4 mm
Fig. 10 Simplified 3D model of a rough surface; roughness profile and mesh detail
Fig. 15 Rotor-stator flow model and boundary conditions
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∆ pstat(inlet-outlet) = 307193 Pa ; ∆ ptotal(inlet-outlet) = 291921 Pa
Pressure [Pa] Velocity in Stationary Frame [m/s]
ωω ωωa)
∆ pstat(inlet-outlet) = 347582 Pa ; ∆ ptotal(inlet-outlet) = 322590 Pa
Pressure [Pa] Velocity in Stationary Frame [m/s]
ωω ωωb)
Fig. 16 Pressure and velocity in stationary frame distribution a) surface: hydraulically smooth; turbulence model: transition-model b) surface: deposit height 0.1 mm, sand roughness 2.6 mm; turbulence model: k-ε
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Pressure [Pa]Pressure [Pa]
ωωa)
Pressure [Pa]Pressure [Pa]
ωωb)
Fig. 17 Pressure distribution a) surface: hydraulically smooth; turbulence model: transition-model b) surface: deposit height 0.1 mm, sand roughness 2.6 mm; turbulence model: k-ε
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Velocity in Stationary Frame [m/s]Velocity in Stationary Frame [m/s]
ωωa)
Velocity in Stationary Frame [m/s]Velocity in Stationary Frame [m/s]
ωωb)
Fig. 18 Distribution of velocity in stationary frame a) surface: hydraulically smooth; turbulence model: transition-model b) surface: deposit height 0.1 mm, sand roughness 2.6 mm; turbulence model: k-ε
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In the third investigation step the flow conditions inside a characteristic axial rotor stator tur-
bine were simulated. The CAD data of the turbocharger turbine geometry were placed at
MET’s disposal by ABB Turbo Systems AG. Normal forces (pressure) and shear stresses act
on the turbine blades depending on the geometrical conditions of the turbine housing, Rey-
nolds number and the surface roughness. The simulation geometry consists of a single stator
blade passage and two rotor blade passages. The rotor is modelled as rotating domain; the
stator is stationary attached in space and time. Periodic boundaries are used to allow only a
small section of full geometry to be modelled. Hydraulically smooth as well as rough surfaces
were simulated, whereas the surface roughness was modelled by using the equivalent sand
grain roughness approach.
First of all a numerical mesh of the fluid domain without deposits on the turbine blade sur-
faces was generated and the following 2 different surface roughness conditions were investi-
gated:
case 1: hydraulically smooth surface
case 2: equivalent sand roughness 2.6 mm
In the following the numerical mesh was modified taking into consideration deposits on the
turbine blade surfaces. The following combinations of surface roughness and deposits were
calculated:
case 3: deposit (0.1 mm) + hydraulically smooth surface
Within the scope of the present study flow simulations were carried by using state of the art
CFD software with the help of simple geometrical models (flat plate: 2D, 3D) taking into con-
sideration the real roughness as well as the complicated geometry (turbine stage of a turbo-
charger) by means of a model approach for the roughness (equivalent sand grain roughness).
The wall functions of the flow are appropriate when the walls can be considered as hydrauli-
cally smooth. For rough walls, the logarithmic profile still exists, but moves closer to the wall.
The roughness height specified is the equivalent sand grain roughness; this is not exactly
equal to the real roughness height of the surface under consideration. Wall friction depends
not only on roughness height but also on the type of roughness (shape, distribution, etc.);
therefore, the appropriate equivalent sand-grain roughness had to be determined in such a way
that the calculated pressure loss simulating the real rough surface of a flat plate is equal to the
pressure loss using an equivalent sand grain roughness approach. So the results of the 3D
CFD simulations of the flat rough plate were used to determine the necessary parameter of the
roughness height for the sand grain roughness model.
Analysis of the simulations proves the following statements:
• Flow disturbances originate in the area close to the wall.
• Pressure losses due to friction and the additional production of turbulence are irre-
versibly transformed to thermal and sound energy.
• Thus these parts of the energy balance are not available as useful output (drive of the
turbine) any more.
• An additional reduction of the cross-sectional area due to deposits leads to an increase
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in the flow speed in these areas. The losses to be overcome thereby increase as well.
The following example calculation for a 12-cylinder gas engine based on measurements made
by MET demonstrates the influence of the assumed change of turbine efficiency on the fuel
consumption of the engine. Tab. 2 shows the technical data of the engine examined. The
change of effective power vs. the varying turbine efficiency is shown in Fig. 19. Fig. 20 shows
the specific fuel consumption dependence on turbine efficiency. Assuming as a starting point
a turbine efficiency of 0.54, this means a specific fuel consumption of 239.9 g/kWh. If the
efficiency deteriorates by 9 percentage points to 0.45, a specific fuel consumption of
250.0 g/kWh can be expected. This corresponds to an additional fuel consumption compared
with the starting point of 4.21% (see Fig. 21).
Tab. 2 12-Technical data of a 12-cylinder gas engine
Bore diameter D [m] 0.3550Stroke s [m] 0.3810Conrod length l [m] 0.9400Conrod ratio λ [-] 0.2027Piston displacement Vh [m³] 0.0377Cylinder number z 12Revolutions per minute n [rpm] 405
2300.0
2350.0
2400.0
2450.0
2500.0
2550.0
2600.0
2650.0
2700.0
0.45 0.50 0.55 0.60 0.65 0.70 0.75
Turbine Efficiency [-]
Effe
ctiv
e Po
wer
[kW
]
Fig. 19 Effective power of a12-cylinder gas engine as a function of the turbine efficiency
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