Study & Analysis of Convergent Divergent Nozzle
ABSTRACT
Convergent Divergent
nozzles are used for high velocity applications where supersonic flows are
desired. With the increasing demand for fuels it has become imperative to
extract as much performance as possible from these devices with the existing
technology. Convergent Divergent nozzles have been used in aircrafts for thrust
generation, in combustion applications for efficient mixing and combustion of
fuels etc. However supersonic flow raises a few problems like the generation of
shock waves in the flow domain which result in loss of thrust. This effect is a
major concern in rocket applications. Since the atmospheric pressure changes
with altitude the Convergent Divergent nozzle cannot always work at its design
conditions. Hence during lift off the rockets are generally overexpanded and
shocks are present in the Convergent Divergent nozzle. These shocks can be
influenced by introduction of swirling flows at the inlet. Swirling flows
introduce an additional tangential velocity to the flow. Swirling flows are
known to increase the central axial velocity in the Convergent Divergent nozzle
and in turn improve the thrust obtained up to a certain extent.
INTRODUCTION
A nozzle is a device designed specially with
a primary motive to change the flow characteristics such as velocity and
pressure. In 1890, Carl Gustaf Patrik de Laval developed a first
convergent-divergent (CD) nozzle which has the ability to increase a steam jet
to a supersonic state. This nozzle is known as a de-Laval nozzle and later it
was used for rocket propulsion. An American engineer Robert Goddard was the
first to integrate a de-Laval nozzle with a combustion chamber which increases
the rocket efficiency and attaining the subsonic and supersonic velocities and the gas flow through a de- level nozzle is
isentropic (less friction and less heat dissipation).
Nozzle is basically used to convert pressure energy to
kinetic energy the flow in a nozzle is very rapid.
The major effect of Mach number and nozzle pressure
ratio (NPR) on mass flow rate maximum pressure and maximum velocity and on
maximum force are studied using fluent analysis.
Nozzles are widely
used in some types of steam turbines and rocket engine nozzles. It also sees use
in supersonic jet engines.
Similar flow properties have been applied to jet streams within astrophysics.
OBJECTIVE
The main objective of
this project is to analyse how the introduction of swirl at the inlet in a CD
nozzle affect the shocks formed during an overexpanded flow.
The effect on shock is
further studied on how it affects the performance of the CD nozzle and the
change in thrust obtained for a range of swirl number representing different
strengths of swirling flows.
The study also aims at
analysing the Mach number changes in a completely expanded flow with the variation
of swirl number.
TYPES OF NOZZLE
- Convergent nozzle :– It is a smoothly varying cross- sectional area duct which is used for
accelerating a steadily flowing fluid. As a fluid enters the smaller
cross-section, it has to speed up due to the conservation of mass. To
maintain a constant amount of fluid moving through the restricted portion
of the nozzle, the fluid must move faster.
- Convergent–divergent nozzle :– This type of nozzle is a modification of the convergent
type where is a divergent section
which acts as an accelerator for supersonic flow. It is
used to accelerate a compressible fluid to supersonic speeds in the axial
(thrust) direction, by converting the thermal energy of the flow
into kinetic energy.
- Steam nozzle :– The steam nozzle
is a passage of varying cross-section by means of which a part of the
enthalpy of steam is converted into kinetic energy as the steam expands
from a higher pressure to a lower pressure.
- Flow nozzle :– The flow nozzle
is generally used for measuring the flow of steam as well as
non-viscous, erosive and high-velocity media. It can be used in a wide
variety of applications that include steam, air, water, vapour, gas,
chemical substances and high temperatures.
During pre–processing
•
The
geometry (physical bonds) of the problem is defined.
•
The
volume occupied by the fluid is divided into discrete cells (the mesh) the mesh
may be uniform or non-uniform.
•
The
physical modelling is defined for example – the equation of motion ,radiation
,species conservation.
•
Boundary
conditions are defined this involves specifying the fluid Behaviour and properties at the boundaries of the problem the initial
condition are also defined.
Ø The simulation is start and the equation are solved
iteratively as a steady – state or transient.
Ø Finally a postprocessor is used for the analysis and
visualization of the resulting solution.
Procedure of working methodology
•
Modelling
of nozzle geometry
•
Surface
split the nozzle
•
Meshing
and their controlling operation
•
Boundering
condition defined
•
Contour
and X-plots
Geometry
•
The
geometry branch contains the part that makes up the model . In mechanical there
are three type of bodies which can be analyzed .
•
Solid
bodies are 3D or 2D volume or area.
•
Surface
bodies are only areas.
•
Lines
bodies only curves.
Modelling
Convergent –divergent nozzle physically distinguished
by it area ratio ,the ratio of exit area and throat area . Flow condition
determine by operating pressure if the speed of gas is much lesser than the
speed of sound of gas is much lesser than the speed of sound . The density
remains constant and the velocity of the flow will be increase.
LITERATURE REVIEW
When nozzles were
invented, their purpose was primarily to change the characteristic of the flow
such as an increase in pressure or velocity. In 1890 Swedish engineer and
inventor Karl Gustaf Patrik de Laval developed a convergent-divergent nozzle
that had the capacity to increase a steam jet to a supersonic state. This
nozzle was termed as de Laval nozzle and later was used for rocket propulsion.
An American engineer
Robert Goddard would be the first to integrate a de Laval nozzle in connection
with a combustion chamber, increasing efficiency and achieving supersonic
velocities in the region of Mach 7.
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