3. Literature review

This section

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

The discovery

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.

Today, the

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

Saurer”. (Systems,

2018).

A

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

studies

Swirling

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”.

In swirling

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.

Swirling

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.

One

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.

The second

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.

2003)) .

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

Numerical techniques

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

So: and

With being the density, is the filtered velocity, is the filtered pressure and is the molecular viscosity. (Yang, Zhiyin,

2014)).

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

such that:

.

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”

vortex.

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.

Figure shows

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

http://jestec.taylors.edu.my/Vol%209%20Issue%205%20October%2014/Volume%20%289%29%20Issue%20%285%29%20657-669.pdf

Systems, B.

(2018). History | BorgWarner Turbo Systems. Online Turbos.bwauto.com.

Available at: http://www.turbos.bwauto.com/products/turbochargerHistory.aspx

Accessed 22 Jan. 2018.

https://www.cambridge.org/core/journals/journal-of-fluid-mechanics/article/experimental-study-of-turbulent-swirling-flow-in-a-straight-pipe/CEC08397802E8F27F9D660CB36D04EFD

http://www.sciencedirect.com/science/article/pii/0010218074900571

https://www.sciencedirect.com/science/article/pii/S0360128500000228#bBIB5

http://www.cfd.com.au/cfd_conf97/papers/mey007.pdf

http://www.scielo.br/scielo.php?script=sci_arttext&pid=S2175-91462017000100091

https://www.sciencedirect.com/science/article/pii/S1875963716300672

https://www.sciencedirect.com/science/article/pii/S0307904X09004259

http://mafija.fmf.uni-lj.si/seminar/files/2006_2007/Turbulence_models_in_CFD.pdf

https://www.sciencedirect.com/science/article/pii/S0021999103006272

http://www.soton.ac.uk/~zxie/cv/ftac06_ms.pdf