Home | | **Gas Dynamics and Jet Propulsion** | Important Questions and answers: Gas Dynamics and Jet Propulsion

Mechanical - Gas Dynamics and Jet Propulsion -
BASIC CONCEPTS AND ISENTROPIC,
FLOW THROUGH DUCTS
NORMAL AND OBLIQUE SHOCKS,
JET PROPULSION,
SPACE PROPULSION.

**1.What is the basic difference between compressible and incompressible fluid flow?**

Compressible

1. Fluid velocities are appreciable compared with the velocity of sound

2. Density is not constant

3. Compressibility factor is greater than one.

Incompressible

1. Fluidvelocitiesaresmall compared with the velocity of sound

2. Density is constant

3. Compressibility factor is one.

**2. Write the steady flow energy equation for an adiabatic flow of air.**

In an adiabatic flow q = 0. Therefore energy equation becomes.

**3. Define the mach number in terms of bulk modulus of elasticity.**

Mach number is a non-dimensional number and is used for the analysis of compressible fluid flows.

**4. Explain the meaning of stagnation state with example.**

The state of a fluid attained by isentropically decelerating it to zero velocity at zero elevation is referred as stagnation state.

(e.g.) Fluid in a reservoir (or) in a settling chamber.

**5. Distinguish between static and stagnation pressures.**

In stagnation pressure state, the velocity of the flowing fluid is zero whereas in the static pressure state, the fluid velocity is not equal to zero.

**6. Differentiate between the static and stagnation temperatures.**

The actual temperature of the fluid in a particular state is known as **“static** **temperature”**whereas the temperature of the fluid when the fluid velocity is zero at zero** **elevation is known as **“stagnation temperature”**.

7. What is the use of mach number?

Mach number is defined as the ratio between the local fluid velocity to the velocity of sound.

i.e. Mach number

M = Local fluid velocity / Velocity of sound = c/ a

It is used for the analysis of compressible fluid flow problems. Critical mach number is a dimensionless number at which the fluid velocity is equal to its sound velocity. Therefore,

Crocco number is a non – dimensional fluid velocity which is defined as the ratio of

fluid velocity to its maximum fluid velocity.

**8. Write down the relationship between stagnation and static temperature interms of the flow, mach number for the case of isentropic flow.**

T0= stagnation temperature

T= Static temperature

M= Mach number.

**9.Give the expression of p/P0 for an isentropic flow through a duct.**

The expression of

**10. Name the four reference velocities that are used in expressing the fluid velocities in**

**non-dimensional form?**

**11. ****What are the different regions of compressible flow.**

The adiabatic energy equation for a perfect gas is derived in terms of fluid velocity © and sound velocity (a). This is then plotted graphically on the c- a co-ordinates, a steady flow ellipse is obtained.

The various regions of flow are:

i.Incompressible region (M ≈ 0)

(ii) Subsonic region (M < 1)

(iii) Transonic region (0.8 – 1.2)

(iv) Supersonic region (M > 1 and M < 5)

(v) Hypersonic region (M ≥ 5)

**12. Define M* and give the relation between M**

**and M*.**

It is a non-dimensional mach number and is defined by the ratio between the local fluid velocity to its critical velocity of sound / fluid.

**13. If an aeroplane goes to higher altitudes maintaining the same speed, the Mach**

**number will remain constant. Say true or false.**

False.

W.K.T. M = c/a

At higher altitude, the sound velocity ‘a’ will decrease and hence M will increase. Therefore, M is not constant.

**14. Show h – S diagram for the flow through a nozzle. Show how the stagnation**

**properties get affected.**

1 – 2’ = Isentropic expansion

1 – 2 = Adiabatic expansion

It is assumed that, the exit pressure is same for both cases. But stagnation pressure at the exit of the adiabatic process (P02 ) will be less than isentropic pressure (P02 ' ) . This is due to friction and irresversibilities. But stagnation temperature remains constant.

**15. A plane travels at a speed of 2400 KM/h in an atmosphere of 5**°**C, find the mach angle.**

**16.Define mach angle and mach wedge.**

Mach angle is formed, when an object is moving with supersonic speed. The wave propagation and changes are smooth. When an object is moving with hypersonic speed the changes are abrupt is shown in Fig. Hence for a supersonic flow over two – dimensional object **“mach wedge”** is used instead of **“mach cone”**.

**17. How will you illustrate the role of mach number as a measure of compressibility?**

If the flow is assumed to be incompressible, the value of pressure co-efficient (or) compressibility factor obtained by Bernoulli equation is unity.

By substituting different values of M, we can get different values of compressibility factor and is given in the table.

In the above table, when M increases, the compressibility factor also increases from the initial value 1. Thus the role of mach number is a measure of compressibility.

**18. What is meant by isentropic flow with variable area?**

A steady one dimensional isentropic flow in a variable area passages is called “variable area flow”. The heat transfer is negligible and there are no other irreversibilities due to fluid friction, etc.

**19. ****Define zone of action and zone of silence with neat sketch.**

**20.Find the sonic velocity in oxygen when it is at 110**°** C, ****γ****=1.4 and molecular weight 32.**

**21. Give the expression for ***T*0** /T and ***T*** /*** T **** for
isentropic flow through variable area in
terms of Mach number.**

**Sketch
the isentropic and adiabatic expansion process in P-V and T-S diagram}.**

**23. Represent the adiabatic flow through a
diffuser on T-S diagram. Label the different states, the initial and final
points.**

**24. Air from a reservoir is discharged through a
nozzle. Show the variation of pressure along the axis of the nozzle.**

**25. What will happen if the air flowing through a nozzle is heated?**

When the flowing air is heated in a nozzle, the
following changes will occur.

Velocity
of air will increase.

Increase
in temperature and enthalpy

Pressure
increases

Increase
in entropy

**26. Write
the Fliegner’s formula.**

**Write the
equation for efficiency of the diffuser.**

Diffuser efficiency = static pressure rise in the actual process
/static pressure rise in the ideal process

**28. What
is impulse function and give its uses?**

Impulse function is defined as the sum of pressure
force and intertia force. Impulse function F = Pressure force ρA + intertia force ρAc^{2}

Since the unit of both the quantities are same as
unit of force, it is very convenient for solving jet propulsion problems. The
thrust exerted by the flowing fluid between two sections can be obtained by
using change in impulse function.

**29. What
is chocked flow? State the necessary
conditions for this flow to occur in a**

**nozzle.**

When the back pressure is reduced in a nozzle, the
mass flow rate will increase. The maximum mass flow conditions are reached when
the back pressure is equal to the critical pressure. When the back pressure is
reduced further, the mass flow rate will not change and is constant. The
condition of flow is called “chocked flow”. The necessary conditions for this
flow to occur in a nozzle is

* The
nozzle exit pressure ratio must be equal to the critical pressure ratio where
the mach number M = 1.

**30. Draw the variation of P /P0
along the length of a convergent divergent device when it functions as (a)
diffuser, (b) nozzle and (c) venturi.**

Curves

a, b, c ⇒ venture

d, e ⇒ diffuser

g ⇒ nozzle

**31. Give
the expression for nozzle efficiency and diffuser efficiency with h – s
diagram.**

**32. Give
the important difference between nozzle and venturi.**

**NOZZLE**

1. The flow is accelerated
continuously i.e., Mach
number and velocity
increases continuously.

2. Used to
increase velocity and Mach number.

3. Generally convergent portion is
short.

VENTURI

1. The flow is accelerated upto M = 1
and then Mach number is decreased.

2. Used for flow measurement (discharge)

3. Convergent and divergent
portions are equal.

**33. What
is the normal shock?**

When the shock waves are right angles to the
direction of flow and the rise in pressure is abrupt are called normal shock
waves.

**34. What
is meant by normal shock as applied to compressible flow?**

Compression wave front being normal to the
direction of compressible fluid flow. It occurs when the flow is decelerating
from supersonic flow. The fluid properties jump across the normal shock.

**35. Shock
waves cannot develop in subsonic flow?
State the reason.**

Shocks are introduced to increase the pressure and
hence it is a deceleration process. Therefore, shocks are possible only when
the fluid velocity is maximum. In a subsonic flow, the velocity of fluid is
less then the critical velocity and hence deceleration is not possible. Thus,
shock waves cannot develop in subsonic flow.

**36.
Define strength of a shock wave.**

Strength
of a shock wave is defined as the ratio of increase in static pressure across the
shock to the inlet static pressure.

Strength
of shock = *p _{y} *−

^{ }

**36.
****Calculate the strength of shock wave when
normal shock appears at M = 2. **

From normal shock table M = 2, γ = 1.4. *p* *y*/Px = 4.5

Strength of shock = *p* *y* /px −
1 = 4.5 −
1 = 3.5

**38. Define oblique shock where it occurs.**

The shock
wave which is inclined at an angle to the two dimensional flow direction is
called as oblique shock. When the flow is supersonic, the oblique shock occurs
at the corner due to the turning of supersonic flow.

**39. Give
the difference between normal and oblique shock.**

**NORMAL SHOCK**

(a)
The shock waves are right angles to the direction of flow.

(b)
May be treated as one dimensional analysis.

**OBLIQUE SHOCK**

(a) The shock waves are inclined at
an angle to the direction of flow.

(b) Oblique shock
is two dimensional analysis.

**40. What is Prandtl-Meyer relation? What its significance?**

The
fundamental relation between gas velocities before and after the normal shock
and the critical velocity of sound is known as Prandtl-Meyer relation.

i.e., (i)
c_{x} x c_{y} = a*^{2} and (ii) M*_{x} x M* _{y}
= 1

it
signifies the velocities (before and after the shock) with the critical
velocity of sound and the product of mach numbers before and after the shock is
unity.

**41. Shown
a normal shock in h-s diagram with the help of Rayleigh line and Fanno line.**

**42. Define the term “Fanno flow”.**

A steady
one-dimensional flow in a constant area duct with friction in the absence of work
and heat transfer is known as **“fanno flow”.**

**43. Define Fanno line.**

The locus
of the state which satisfy the continuity and energy equation for a frictional

flow is known as **“fanno line”.**

**44. Give fanno line in h – s diagram with
isentropic stagnation line and show various**

**mach
number regions.**

The
equation which yields the fanno line for the given values of h_{0} and
G is called “fanno flow equation”.

**45. Explain briefly the chocking in fanno flow.**

In a
fanno line, any heating process (both subsonic and supersonic) will increase
the enthalpy, entropy and mass flow rate. This will go upto the limiting state
where mach number M* = 1. Further heating is not possible, because the entropy
change will be negative which violates the second law of thermodynamics. Hence
the mass flow rate is maximum at the critical state and is constant afterwards,
then the flow is said to be **“chocked flow”.**

**46. Give
two practical examples where the fanno flow occurs.**

Flow
occurs in gas ducts of aircraft propulsion engines, flow in air-conditioning
ducts

and flow
of oil in long pipes. etc.

**47.****Give the
effect of increasing the flow length after reaching critical condition in a
fanno flow. **

The mass flow rate will increase only upto the
critical condition and is constant afterwards. Therefore, if the length of pipe
is increased afterwards will not give any effect.

**48.Write
down the expression for the length of duct in terms of the two mach numbers M1
and M2 for a flow through a constant area duct with the influence of friction.**

**49.
Define isothermal flow with friction.
Give the applications.**

A steady one dimensional flow with friction and
heat transfer in a constant area duct is called isothermal flow with friction.
Such a flow occurs in long ducts where sufficient time is available for the
heat transfer to occur and therefore the temperature may remains constant.
Hence the friction factor may be assumed constant along the duct. The
applications of isothermal flow are oil or water flow in buried pipe.

**50.
****State assumptions made to derive the
equations for isothermal flow. **

i.
One dimensional flow with friction and heat
transfer.

ii.
Constant area duct

iii.
Perfect gas with constant specific heats and
molecular weights

iv.
Isothermal flow i.e., the temperature is constant

v.
On account of constant temperature the friction
factor may be assumed constant along the duct.

**Differentiate
between isothermal flow and fanno flow.**

**ISOTHERMAL**

a) Static temperature is constant

b) With heat transfer.

c) Flow occurs
in a long
ducts where sufficient
time is required
for heat transfer.

d) On account of constant temperature, the
friction factor is
assumed as constant.

**FANNO FLOW **

a) Static temperature is not
constant

b) Without heat transfer.

c) Long ducts are not
required.

d) Friction factor is constant.

**52.
Define the term “Rayleigh flow”.**

The one-dimensional flow in a constant
area duct with heat transfer and without

friction
is called **“Rayleigh flow”.**

**55.
Define Rayleigh line.**

The locus of the points of properties during a
constant area frictionless flow with heat exchange is called **“Rayleigh
line”.**

**54. What
is diabatic flow?**

It is the flow which deals with the exchange of
heat from the system in the absence of friction (Rayleigh flow).

**55.
****Give the assumptions made in Rayleigh flow. **

i.
Perfect gas with constant specific heats and
molecular weight.

ii.
Constant area duct,

iii.
One dimensional, steady frictionless flow with
heat transfer.

iv.
Absence of body forces.

**56.
****What do you understand by chocking in
Rayleigh flow. **

When the fluid is heated in a subsonic region, the
entropy increases and the mach number and fluid properties move to the right
unitil the maximum entropy is reached where M* = 1. When the fluid is heated in
a supersonic region, the entropy increases and the mach number and the fluid
properties move to the right until the maximum entropy is reached where M* = 1.
Further heating is not possible because, if it is heated the change in entropy
is negative which violates the second law of thermodynamics. Therefore, the
type of flow when the limiting condition M* = 1 is called **“chocked flow”.**

**57. Differentiate
between Fanno flow and Rayleigh flow.**

**FANNO
FLOW**

a) One dimensional
steady frictional flow.

b) Stagnation
temperature is constant.

c)
Because of considering
the wall friction forces it is
accurate.

d) Without
heat transfer.

**RAYLEIGH
FLOW**

a) One
dimensional steady frictionless
flow.

b) Stagnation temperature is not constant

c) Less accurate.

d) With heat transfer.

**58. What
is meant by a jet propulsion system?**

It is the
propulsion of a jet aircraft (or)other missiles by the reaction of jet coming
out with high velocity. The jet propulsion in used when the oxygen is obtained
from the surrounding atmosphere.

**59.
****How will you classify propulsive engines? **

The jet propulsion engines are classified into

i.
Air breathing engines and

ii.
Rocket engines which do not use atmospheric air.

**60.What
is the difference between shaft propulsion and jet propulsion?**

**SHAFT
PROPULSION**

a) The
power to the
propeller is transmitted through a reduction gear

b) At higher altitude, the performance is poor.
Hence it is suitable for lower
altitudes.

c) With increasing speeds and size of the aircrafts, the shaft propulsion engine becomes too complicated.

d) Propulsive efficiency is less.

** JET PROPULSION **

a) There is no reduction gear.

b) Suitable for higher altitudes.

c) Construction is simpler.

d) More.

**61.
****List the different types of jet engines. **

i.
Turbo-jet

ii.
Turpo-prop engine,

iii.
Ram jet engine,

iv.
Pulse jet engines.

**62.
****Define the principle of Ram jet engine. **

The principle of jet engine is obtained from the
application of Newton’s law of motion. We know that when a fluid is
accelerated, a force is required to produce this acceleration is the fluid and
at the same time, there is an equal and opposite reaction force of the fluid on
the engine is known as the **thrust**, and therefore the principle of jet
propulsion is based on the reaction principle.

**63.
****Give the components of a turbo jet. **

i.
Diffuser

ii.
Mechanical compressor,

iii.
Combustion chamber,

iv.
Turbine and

v.
Exhaust nozzle.

**64. Give
the difference between pulse jet and ram jet engine.**

**PULSE JET**

a)
Mechanical valve arrangements
are used during combustion.

b) The
stagnation temperature at the diffuser
exit is comparatively less.

**RAM JET**

a)
Works without the
aid of any
mechanical device and needs no
moving parts.

b) Since
the mach number in Ram jet engine
is supersonic, the
stagnation temperature is
very high.

**65. Give
the difference between turbojet and ram jet engine.**

**TURBO JET**

a)
Compressor and turbine
are used.

b)
Lower thrust and
propulsive efficiency at lower
speeds.

c)
Construction cost is more.

**RAM JET**

a)
Compressor and turbine
are not used
but diffuser and
nozzle are used.

b)
It provides high
thrust per unit
weight.

c)
In theabsenceof rotating
machines, the construction is
simple and cheap.

**66. What
is specific impulse?**

Specific impulse
is the thrust developed per unit weight flow rate through the propulsive
device. It is a useful performance parameter in aerospace propulsion systems.

I_{spc}= F/W

**67.Give
the difference between Jet propulsion and Rocket propulsion.**

**JET PROPULSION **

a) Oxygen
is obtained from
the surrounding atmosphere for combustion purposes.

b) The
jet consists of
air plus combustion products.

c) Mechanical devices are also used.

**ROCKET PROPULSION**

a)
The propulsion unit
consists of its
own oxygen supply
for combustion purposes.

b) Jet
consists of the exhaust gases only.

c) Mechanical devices are not used.

**68. What
is the difference between turbo prop engine and turbo jet engine.**

**TURBO – PROP**

a) The
specific fuel consumption based on thrust is low.

b) Propulsive
efficiency within the range of operation is higher.

c) On account of higher thrust at low speeds the
take-off role is short and requiring shorter runway.

d) Use
of centrifugal compressor stages increases the frontal
area.

e) Higher weight per unit thrust.

**TURBO – JET**

a) TSFC
is comparatively higher at lower speeds
and altitudes.

b)
Propulsive efficiency is low.

c)
Take –
off role is
longer and requiring longer run way.

d) Lower
Frontal area.

e) Lower
weight per unit thrust.

**69. Write the formula for propulsive efficiency
and define the same.**

The force
which propels the aircraft forward at a given speed is called **thrust**
(or)

**propulsive force**.

Propulsive
efficiency is defined as the ratio between propulsive power (or) thrust power
to the power output of the engine.

Effective
speed ratio (or) flight to jet velocity

**70. What is ram effect?**

When an
aircraft flies with high velocity, the incoming air is compressed to high
pressure without external work at the expense of velocity energy is known as “**ram
effect**”.

**71. Explain specific thrust as applied to jet
engines.**

Specific
thrust is defined as the thrust produced per unit mass flow rate through the
propulsive device.

F_{spec}
= F/m where, F = thrust and m = mass
flow rate

**72.
Differentiate between pressure thrust and momentum thrust.**

Pressure
thrust is mainly depends on the difference in pressure between the nozzle

exit
pressure and the ambient pressure and is given by

Pressure
thrust = (P_{e} – P_{a}) A

Momentum
thrust depends on the difference in velocity between the aircraft velocity

and jet
velocity is given by

Momentum
thrust = m (c_{j} – u) where,

P_{e} = nozzle exit pressure

P_{a} = ambient pressure

A
= Area of cross section at the nozzle exit

C_{j} = jet velocity and

u
= forward speed of aircraft

**73.
****What is “thrust augmentation”? **

To achieve better take-off performance, higher
rates of climb and increased performance at altitude during combat maneuvers,
there has been a demand for increasing the thrust output of aircraft for short
intervals of time. This is achieved by during additional fuel in the tail pipe
between the turbine exhaust and entrance section of the exhaust nozzle. This
method of thrust increases the jet velocity is called “**Thrust Augmentation**”.

**74. Why
after burners are used in turbojet engine?**

Exhaust gases from the turbine have large quantity
of oxygen, which can support the combustion of additional fuel. Thus if a
suitable burner is installed between the turbine and exhaust nozzle, a
considerable amount of fuel can be burned in this section to produce
temperatures entering the nozzle as high as 1900°C. The
increased temperature greatly augments the exhaust gas velocity, and hence
provides the thrust increase.

**75. Why a
ram jet engine does not require a compressor and a turbine?**

In general, the speed of a ram jet engine is
supersonic (the range of Mach number) is very high. At this flight speed the
contribution of the compressor to the total static pressure rise is insignificant.
Hence, arm jet engine does not require compressor and turbine.

**76.
Define Rocket propulsion.**

If the
propulsion unit contains its own oxygen supply for combustion purposes, the
system is known as “**Rocket propulsion**”.

**77. Define thrust for a rocket engine and how it
is produced.**

The force that propels the rocket at a given
velocity is known as thrust. This is produced due to the change in momentum
flux of the outgoing gases as well as the difference between the nozzle exit
pressure and the ambient pressure.

**78. What
are the types of rocket engines?**

Rocket
engines are classified in the following manner.

a)
On the basis of source of energy employed

i.
Chemical rockets,

ii.
Solar rockets

iii.
Nuclear rockets and

iv.
Electrical rockets

b)
On the basis of propellants used

i.
Liquid propellant

ii.
Solid propellant

iii.
Hybrid propellant rockets.

**Compare
solid and liquid propellant rockets.**

**SOLID PROPELLANT**

a) Solid fuels and oxidizers are used in rocket
engines

b) Generally
stored in combustion chamber (both oxidizer and fuel).

c) Burning
in the combustion chamber is uncontrolled rate.

**LIQUID PROPELLANT**

a) Liquid
fuels and oxidizers
are used.

b)
Separate oxidizer and fuel tanks are
used for storing purposes.

c)
Controlled rate.

**80.
****What are the types of liquid propellants used
in rocket engines? **

i. Mono
propellants

ii. Bi –
propellants

**Give two
liquid propellants.**

Liquid
fuels : Liquid
hydrogen, UDMH, hydrazine

Solid
fuels : Polymers,
plastics and resin material

**82. What
is mono-propellants? Give example.**

A liquid propellant which contains both the fuel
and oxidizer in a single chemical is known as “mono propellant”. e.g.,

i.
Hydrogen peroxide

ii.
Hydrazine

iii.
Nitroglycerine and

iv.
Nitromethane, etc.

**v. ****What is
bi-propellant? Give Example. **

**vi. ****Name some
oxidizers used in rockets. **

A liquid
propellant which contains the fuel and oxidizer in separate units is known as

bi-propellant.
The commonly used bi-propellant
combinations are:

OXIDIZER

a) Liquid
oxygen

b) Hydrogen
peroxide

c) Nitrogen
tetroxide

d) Nitric
acid

FUEL

a) Gasoline

b) Liquid
bydrogen

c) UDMH

d) Alcohol,
ethanol

**85.
****Name few advantages of liquid propellant
rockets over solid propellant rockets. **

i.
Liquid propellant can be reused or recharged. Hence it is economical.

ii.
Increase or decrease of speed is possible when it
is in operation.

iii.
Storing and transportation is easy as the fuel and
oxidizer are kept separately.

iv.
Specific impulse is very high.

**86.
****What is inhibitors? **

Inhibitors
are used to regulate (or prevent) the burning of propellant at some sections.

**87.
****Give the important requirements of rocket
engine fuels. **

i.
It must be able to produce a high chamber
temperature. It should have a high calorific value per unit of propellant.

ii.
It should not chemically react with motor system
including tanks, piping, valves and injection nozzles.

**88.
****What is meant by restricted burning in
rockets? **

In this case, the inhibition material (or)
restrictions prevent the propellant grain from burning in all directions. The
propellant grain burns only at some surfaces while other surfaces are prevented
from burning.

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