3. Literature review
presents an overview of journals and articles relevant to this project.
Substantial studies of the background of turbocharged engines and how swirling
flows are being investigated, and the applications to CFD modelling of swirling
flows using different turbulent models are summarised.
The flow of
gasses within a turbo charged engines are significantly strong and pulsating.
Recent development to use turbochargers on engines has resulted in swirling
flow entering the catalyst. The aim of this project is to significantly create
a sufficient model in which the flow can be captured and analysed. Catalytic
converters are used to reduce the amount of emissions. The effect of swirl on
the flow distribution inside the catalyst has not been well understood.
3.1 Background Research
in turbocharged engines was attained in 1978 with the introduction of the first
turbocharged diesel engine. By means of the turbocharger, the diesel engine
efficiency could be increased, with almost petrol engine
“driveability”, and the emissions significantly reduced.
turbocharged engines are no longer seen from the performance outlook, but are
actually viewed as a means of reducing fuel consumption, consequently, Pollution
effecting the environment on account of lower carbon dioxide emissions.
Currently, the main reason for turbocharged engines is the use of the exhaust
gas energy to reduce fuel consumption and emissions. The first turbocharger
applications were constrained to very large engines, e.g. marine engines. In
the automotive engine, turbocharging started with truck engines. In 1938, the
first turbocharged engine for trucks was built by the “Swiss Machine Works
turbocharged engine also consists of a compressor and a turbine mounted on the
same shaft. Energy found in the exhaust gases of an Internal Combustion engine
is used to drive the turbocharger-turbine, which then turn drives its
compressor to raise the density of the air supplied to the engine. The energy
in the exhaust gas is used in running the impulse turbine.
3.2 Swirling flow distribution
flows have been commonly used for a number of years for the steadying of
high-intensity combustion processes. In general these swirling flows are wrongly
understood because of their complexity. A sourced journal describes the recent
progress in understanding and using these swirling flows. (N.Syred, J.M.Beér, 2003)
The main effects of swirl are to improve stability as a result of the formation
of “recirculation zones”.
flows the angular momentum creates a free vortex flow so that the velocity “V” increases
as the radius “r” decreases. To achieve an idyllic vortex, forces at the
circumference are at equilibrium with the radial pressure gradient.
flow through a pipe is a highly complex turbulent flow and is still challenging
to expect. An experimental investigation was performed to obtain systematic
data about the flow and to understand its concept.
important feature in swirling flows is the recirculation zone, there is usually
a significant amount of radial pressure gradient at a positon due to its
centrifugal effect which occurs at low swirl levels, however this is not strong
enough to cause a recirculation due to its axial pressure gradient. By increasing
the swirls there is an axial and tangential coupling between the components.
This allows the recirculation flow to be setup towards the centre of the flow
due to the fact that the pressure gradient along the axis of the flow can no
longer overcome the kinetic energy of the fluid.
important feature of swirling flows are vortex breakdown. Axial flow is
essential for the breakdown of the vortices. Usually within the breakdown of
the vortex, the vortex is weaker as well as the velocity gradient being lesser.
Harvey investigated swirling flows within a tube by
conducting experiments. By changing the level of swirl, he was able to find
that the vortex breakdown was a stage which occurred amongst weak swirling
flows, which showed no flow reversal, and rapid swirling flows which exhibited
great amounts of flow reversal as well as recirculation’s. ((Harvey, J.K.
The addition of swirl to the flow aids a radial pressure
gradient and centrifugal forces that force the air towards the diffuser wall;
this prevents the growth of boundary layer and prevents flow separation. Which
in time increases the efficiency and performance. Conversely adding swirl would
decrease the momentum which occurs near the centreline. This is expected due to the conservation of
mass principle (O Lucca-Negro, T O’Doherty, 2001)).
3.3 CFD modelling
CFD modelling uses a numerical method in solving flow of
fluid through various different geometries; by integrating the Navier stokes
equation using algorithms over a meshed area. Turbulence is the main cause for
using CFD mainly due to its complexity and to gain a better understanding. When
turbulent flow is pictured, rotating flows such as eddies are shown. CFD
modelling are best used due to their great reduction in time and cost as
opposed to experimental procedures, also the level of detail is unlimited
allowing a better understanding in the fundamentals of fluid flow.
3.4. Turbulence modelling
The Reynolds number in engines usually come to approximately
103 x 104 (Kandylas and Stamatelos, 1999). The flow that
occurs in a diffuser is usually turbulent, however the flow within a monolith
is laminar due to the fact that it has a relatively small diameter, yet the
flow in a diffuser affects the flow profile in a monolith, this is why it is
necessary to predict the nature of turbulence exhibited.
Turbulence only occurs once the inactivity of fluid
overcomes the viscous stresses and the laminar sub layer becomes unstable. Truly,
the NS equations show the linearity of all the terms. Laminar flow is so called
due to its smooth layers. The flow which is subjected to disturbances will
eventually allow the flow to become stable. However if the disturbances do not
decay fluctuations will occur.
Swirling flows with high viscosities are characterised as
nonlinear and highly fluctuating vorticity which consists of eddies. An eddy is
particles that move both laterally and longitudinally. Eddies that occur in turbulent
flow are 3 dimensional.
Turbulence flow of a system is extremely challenging as
turbulence models have not yet been recognised for all flows.
3.5 CFD Approach to modelling swirling flows
Large eddy simulation (LES) is an unsteady 3D method to simulation,
this is where the NS equations are filtered spatially. LES has been used hugely
in studies of turbulent flow. LES functions to resolve most of the turbulent
kinetic energy of the flow (K), while also modelling the dissipation (?).
In LES large eddies are only totalled directly which is why a low pass spatial
filter is applied to the Navier stokes equation hence formulating the 3D
unsteady formula, it can be written as
With being the density, is the filtered velocity, is the filtered pressure and is the molecular viscosity. (Yang, Zhiyin,
RANS gives the irregularity of the flow
statistically. The eddy viscotiy approximation is what forms the basis of the
RANS model. This approximation assumes how proportional the Reynolds stress
This also includes two models/equations,
k-e and k-w. With the k-e model the equations are solved using turbulent
kinetic energy “k” and the rate of turbulent dissipation “e”. However k-w
solves suing k and rate of specific dissipation rate w. K-e model assumes a scale for velocity of k. Moreover,
by solving the k and e the model then solves the Reynold stress functions, this
is why they are hugely complex and suffer great computational demand.
DNS (Direct numerical simulation) method
uses a calculation method without modelling as it directly solves the NS
equation. However it uses modelling due to the fact that the equations of
motions themselves contain a model. The viscous force is directly proportional
to the velocity difference; nonetheless there is no modelling about turbulence
which can be solved directly like the LES. It requires calculations with a mesh
that captures minimum sized eddies. Yet it is extremely difficult to use DNS
which makes it less practical, however researchers are able to examine the
characteristics of turbulence. The research benefits with DNS are that vortex
tubes for a turbulent flow show a similar velocity distribution to “burger”
Several numerical studies have also been performed as well
as experimental. Nickolaus and Smith studied highly analysed flows in
combustors. Using large eddy simulation (LES), they observed the flow features,
they also observed the (RANS) Reynolds averaged Navier stokes, however it was
found to be not as accurate as the LES, with incredible difference in the time
taken to run simulation. Undergoing the same mesh size LES model ran for
approximately 3 weeks while the RANS model took 1 day to converge (K.Mahesh,
G.Constantinescu, P.Moin, 2004). Furthermore LES holds a
greater accuracy over RANS, mainly with mixing of turbulence, however RANS uses
a more upwind numerical approach making the procedure more robust.
comparison of the temperature calculated contour for RANS, LES and experimental
using a lab.
LES is the
most suitable approach to analysing the structures in swirling flows; however
most of this simulation still focuses deeply on confined swirling flows at
moderate “Re”. Still RANS provides a practical approach to computing swirling
flows in complex geometries.
3.5. Summary/ Conclusion
(2018). History | BorgWarner Turbo Systems. Online Turbos.bwauto.com.
Available at: http://www.turbos.bwauto.com/products/turbochargerHistory.aspx
Accessed 22 Jan. 2018.