# MECHANICAL : Steam Engine, Generation Online Test

In a single stage impulse turbine, the maximum blading efficiency is obtained when
=cos
=cos2
=
=
When  is the nozzle angle and  is the blade speed ratio
Neglecting the effect of friction and assuming blades to be symmetrical, the maximum blading efficiency of a single stage impulse turbine is given by
2 cos2
cos2
Cos2

Wilson’s line is associated with
Governing of steam engines
Supersaturated expansion through a steam nozzle
Binary vapour cycle
Condensation of steam in the later stages of a steam turbine
The specific volume of supersaturated steam can be worked out by using the correlation
pv=0.01(H-464.1)
pv=0.1(H-464.1)
Pv1.3=0.3(H-464.1)
pv=0.15(H+464.1)
Where v is specific volume in m3/kg, H is enthalpy of steam in kcal/kg and p is the pressure in kgf/cm2
For supersaturated adiabatic flow through a nozzle, the pressure p and volume v are related by the expression
pv=c
Pv1.3=c
Pv1.135=c
Pv1.15=c
Because of sonic and supersonic speeds, the time taken by steam for its passage through nozzle is of the order of




The blade speed ratio defines the relation between
Relative velocities at outlet and inlet
Cosine of angles made by relative velocities at outlet and inlet
Blade speed and absolute steam velocity at inlet
Absolute steam velocities at inlet and outlet
The optimum value of nozzle angle lies within the range
20-25 degree
15-20 degree
10-15 degree
5-8 degree
If nozzle angle is 30 degree, the efficiency of an impulse turbine would be maximum when the blade speed equals
0.433 Ca1
0.5 Ca1
0.75 Ca1
0.866 Ca1
Where Ca1 is the absolute steam velocity at exit from the nozzle
If nozzle angle is 30o the De-Laval turbine will have a maximum efficiency of
0.43
0.5
0.75
0.875
For a fixed value of nozzle angle, maximum work would be produced when blade speed and absolute velocity of steam jet conform to the ratio
1:4
1:2
2:3
1:1
In single stage impulse turbine, the maximum rate of doing work per kg of steam per second is given by
0.5 Cb2
Cb2
2 Cb2
4 Cb2
Where Cb is the blade speed
A single stage impulse turbine with a diameter of 120 cm runs at 3000 rpm. If the blade speed ratio is 0.42, then the inlet velocity will be
79 m/s
188 m/s
450 m/s
900 m/s
For a single stage impulse turbine with nozzle angle 20o, rotor diameter 2m and speed 3000 rpm, the optimum velocity of steam in m/s is
334
356
668
711
Which of the following is wrong statement?
At Cb/Ca1=0, torque on the blades is minimum and no work is done
At Cb/Ca1=1, both torque and work done are zero
At Cb/Ca1=cos /2, maximum work output is obtained
At Cb/Ca1=cos /2, the diagram efficiency has a maximum value
Where  is the nozzle angle, Cb is the blade speed and Ca1 is the absolute velocity of steam jet issuing from the nozzle
A single stage impulse turbine is generally not used in practice because
It cannot generate more power
Heat energy is not utilized fully
It has very low efficiency
It needs large reduction gearing due to high speeds
Compounding of steam turbine is done to
Balance the rotor
Reduce the rotor speed
Connect the shaft of one turbine to that of another
Running speeds of steam turbines can be brought down to practical limits by which of the following method(s)? by using heavy flywheel by using a quick response governor by compounding by reducing fuel feed to the furnance Choose the correct answer using the codes given below:
3 alone
1, 2, 3 and 4
1, 2 and 4
2 and 3
Theoretically the work of two row Curtis wheel is………….times that of a simple wheel; blade speeds being equal in both the cases
2
4
6
8
Which of the following is a pressure compounded turbine?
Parsons
Curtis
Reteau
All of three
De-laval turbine is
Single rotor impulse turbine
Velocity-compounded impulse turbine
Pressure-compounded impulse turbine
Impulse-reaction turbine
De-laval turbine has a shaft speed of about
10,000 rpm
15,000 rpm
20,000 rpm
30,000 rpm
Which one of the following sketches represents an impulse turbine blade
(a)
(b)
(c)
(d)
As steam glides over the moving blades of a reaction turbine
Both pressure and velocity decrease
Both pressure and velocity increase
Pressure drops and velocity increases
Velocity decreases and pressure remains constant
The velocity here refers to the absolute velocity
What changes occur in the magnitude of pressure and velocity when steam flows through the fixed blades of a reaction turbine?
Velocity decreases and pressure remains constant
Both pressure and velocity decrease
Velocity increases and the pressure falls
Both pressure and velocity increase
Which aspect is not true in the control of a reaction turbine?
Steam is only partially expanded in the nozzle, the remaining expansion takes place in the rotor blades
Reaction turbine blades use aerofoil section and are asymmetrical
Different pressures exist on the two sides of the moving blades
The number of stages required for a reaction turbine are less than those for an impulse turbine of the same power
The essential merit of a reaction turbines lies in
Greater work output
High diagram efficiency
Blade speeds lie in the useful range
Capability to have expansions in stages
The degree of reaction is defined as the ratio of
Heat drop in moving blades to heat drop in the entire stage of reaction turbine
Heat drop in fixed blades to heat drop in the entire stage of reaction turbine
For a particular reaction turbine, the enthalpy drops in the moving blade and in the fixed blade are stated to be 15kJ/kg and 25kJ/kg respectively. The degree of reaction for the turbine is
0.375
0.5
0.625
0.67
Parson’s turbine is
A simple impulse turbine with degree of reaction R=0
An impulse-reaction turbine with degree of reaction R=0.5
An impulse-reaction turbine with degree of reaction R=0.9
A pure reaction turbine with degree of reaction R=1
When compared to stable flow, the available enthalpy drop for supersaturated flow of steam through a nozzle
Remains same
Increases
Decreases
Is unpredictable
In a single stage impulse turbine, the maximum blading efficiency is given by__________
Blading efficiency is also known as
Stage efficiency
Overall efficiency
Nozzle efficiency
Diagram efficiency
Blading efficiency is defined as the ratio of
Relative velocities at outlet and inlet
Blade speed and absolute steam velocity at inlet
Work done by steam on blades and the energy supplied to the blades
Cosine of the angles made by relative velocities at outlet and inlet
In an ideal impulse turbine, the
The absolute velocity at the inlet of moving blade is equal to that at the outlet
The relative velocity at the inlet of moving blade is equal to that at the outlet
Axial velocity at the inlet is equal to that at the outlet
Whirl velocity at the inlet is equal to that at the outlet
In an impulse turbine, the blade friction reduces the relative velocity of steam on passing over the blades by about
5-8%
10-15%
15-20%
20-30%
The effect of friction when steam flows across the blade passages is expressed by the ratio of relative velocities at outlet and inlet (K=Cr2/Cr1). The factor K is called
Friction factor
Utilization factor
When steam flows over moving blades of an impulse turbine
Pressure drops and velocity increases
Pressure remains constant and velocity decreases
Both pressure and velocity remain constant
Both pressure and velocity decrease
In the steam turbine embodying impulse principle, the expansion of steam takes place in
Stationary nozzles
Moving nozzles
Blades mounted on the wheel attached to the shaft
Partly in stationary nozzles and partly in moving blades
Which statement is not true for a steam turbine when compared to steam engines?
High thermodynamic efficiency
High power output per unit weight of machine
High power turbine units would need a heavier flywheel
Capacity to carry considerable overloads with only slight reduction in efficiency
A convergent diversity nozzle is said to be choked when 1. critical pressure is attained at the throat 2. velocity at the throat becomes sonic 3. exit velocity becomes supersonic Which of these statements are correct?
1, 2 and 3
1 and 2
2 and 3
1 and 3
At what location of a converging diverging nozzle, does the shock boundary layer interaction take place?
Converging portion
Throat
Inlet
Diverging prition
Shock waves involving an abrupt rise of pressure and increase of entropy generally occur
At entry to nozzle
In the convergent section of the nozzle
At the nozzle throat
In the divergent section of the nozzle
In flow through a convergent nozzle, the ratio of back pressure to the inlet pressure is given by the relation , if the back pressure is lower than that given by the above equation, then
The flow in the nozzle is supersonic
The shock wave exists inside the nozzle
The gases expand outside the nozzle
A shock wave appears at the nozzle exit
All of the following statements are correct, except
When the pressure at the throat of nozzle is well above critical pressure, the flow accelerates in the convergent part and decelerates in the divergent part
With the sonic velocity attained at throat, subsequent reduction in back pressure does not affect the flow condition in the convergent part
The nozzle is said to be under expanding when it operates above the design pressure
The expansion process occurring outside the nozzle is violent and irreversible
Which is not true regarding effect of supersaturated flow in a steam nozzle?
Reduction in heat drop
Reduction in exit velocity
Decrease in mass flow rate
Increase in entropy
Wilson’s line is associated with
Governing of steam engines
Supersaturated expansion through a steam nozzle
Binary vapour cycle
Condensation of steam in the later stages of a steam turbine
The specific volume of supersaturated steam can be worked out by using the correlation
pv=0.01(H-464.1)
pv=0.1(H-464.1)
Pv1.3=0.3(H-464.1)
pv=0.15(H+464.1)
Where v is specific volume in m3/kg, H is enthalpy of steam in kcal/kg and p is the pressure in kgf/cm2
For supersaturated adiabatic flow through a nozzle, the pressure p and volume v are related by the expression
pv=c
Pv1.3=c
Pv1.135=c
Pv1.15=c
Because of sonic and supersonic speeds, the time taken by steam for its passage through nozzle is of the order of




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Kalyan Sarkar
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