Yasser Siddiqi

MCQs An isothermal expansion process for a vapor on a Ts

diagram is shown by

A horizontal line

A vertical line

Downward sloping curve

Upward sloping curve

The heat supply process in a diesel cycle is

isochoric

isobaric

isothermal

isentropic

If the maximum temperature of Source is 200oC and of the sink is 100oC, the Carnot efficiency of the cycle will be

0.5

Less than 0.5

Greater than 0.5

None of the above

In a Otto cycle, if the compression ratio decreases, its efficiency

Decreases

Increases

Remains same

None of the above

In an Electrolux refrigeration system, a weak solution of ammonia in water flows from

Separator to Absorber

Absorber to Separator

Generator to Separator

Evaporator to Absorber

Refrigerating effect is the heat removed in which of the following pieces of equipment;

Evaporator

Condenser

Compressor

None of the above

Which of the following parts is found in a Petrol engine but not in a Diesel engine;

Piston

Engine bearing

Fuel injector

Spark plug

During one revolution of the crankshaft, the four stroke engine completes;

Half stroke of piston

One stroke of piston

Two strokes of piston

Four strokes of piston

When a car is pulled with a rope, the rope is subjected to;

Normal tensile stress

Normal compressive stress

Shear stress

None of the above

The normal stress acting in the direction of the axis of a slender member is called;

Bearing stress

Shear stress

Axial stress

None of the above

COP for a reverse heat engine is always

Greater than one

Less than one

zero

None of the above

The most efficient possible heat engine cycle is

Otto cycle

Brayton Cycle

Carnot Cycle

None of the above

At compressor inlet in a vapor compression cycle, the desired condition of the vapor is;

Dry saturated vapor

Wet vapor with high wetness fraction

Wet vapor with low dryness fraction

None of the above

On a Ts diagram for a single phase fluid which line slopes less steeply (tends towards horizontal)

Constant Volume line

Constant Pressure line

Constant Temperature line

None of the above

During the heating process in a gas turbine, the pressure of the working fluid;

Decreases

Increases

Remains same

None of the above

A good refrigerant has a

Boiling point below target temperature

A low density in vapor form

A low specific enthalpy of vaporization

None of the above

Air fuel mixture in a Petrol engine is prepared inside the;

Governor

Cylinder

Carburetor

Fuel Pump

SAQs Define second law of Thermodynamics in context

of reversed heat engine.

It is impossible to construct a device that operating in a cycle will produce no effect other than transfer of heat from a cooler to a hotter body.

Define unit of refrigerating capacity.

The standard unit of refrigeration in vogue is ton refrigeration or simply ton denoted by the symbol TR. It is equivalent to the production of cold at the rate at which heat is to be removed from one US tonne of water at 32oF to freeze it to ice at 32oF in one day or 24 hours. Thus

1 TR = ( 1 x 2,000 lb x 144 Btu/lb ) / 24 hr

= 12,000 Btu/hr = 200 Btu/min

Why carburetor is not used in a Diesel engine?

Carburetor is used in a petrol engine to prepare air fuel mixture to enter into the cylinder during a suction stroke. Whereas in a diesel engine only air is sucked in which is compressed to a relatively high pressure and temperature and then diesel is sprayed into it through a fuel injector.

Differentiate between a normal and a shear stress.

The stress on a surface is an internally distributed force system that can be resolved into two components: normal (perpendicular) to the imaginary cut surface, called normal stress, and tangent (parallel) to the imaginary cut surface, called shear stress.

What is the primary difference between a refrigerator and a heat pump?

Both the heat pump and the refrigerator are actually reversed heat engines. A reversed heat engine lifts heat energy from a low temperature energy reservoir and throws it into high temperature energy reservoir. The difference between the two is the difference of purpose. A refrigerator is used to produce cold i.e. lift heat energy from low temperature energy reservoir whereas a heat pump is used to produce heat i.e. throw heat into high temperature energy reservoir.

What is the purpose of using Hydrogen gas in Electrolux refrigeration system?

Hydrogen gas is filled in the evaporator-absorber circuit of Electrolux refrigeration system at a partial pressure slightly less that total pressure of the system to allow liquid nitrogen flowing from condenser to evaporator to expand and attain thermodynamics properties corresponding to that low partial pressure.

What is ‘deflector piston’ and in which type of engine it is used?

The deflector piston is used in a two stroke engine. When air fuel mixture enters the cavity between piston and cylinder head, it finds exhaust port open. To minimize leakage of fresh charge through the exhaust port, the head of the piston is designed in a way to deflect fresh charge towards top of the cylinder. In this way, the fresh charge helps in pushing out the burnt gases, a process known as scavenging.

What is the difference between the pressure and the stress.

Pressure is generally applied by some fluid on its solid boundary, whereas stress is induced inside a solid body when it is subjected to some external loading.

Define the Second Law of Thermodynamics in the context of a Forward Heat Engine.

It is impossible for a heat engine to produce a net work output in a complete cycle if it exchanges heat with a single reservoir.

How do you define ‘work ratio’ for the Braytoncycle?

For Brayton cycle, work ratio is defined as Net work output divided by Gross Work output.

Net Work output = Turbine work – Compressor work

Gross Work output = Turbine work

What is ‘under-cooling’ in a vapor compression cycle? Show diagrammatically.

When a refrigerant is condensed in a condenser it reaches saturated liquid line. To make use of the temperature difference between condensed refrigerant and the cooling water, the condensation process in further extended in the liquid region. This extension of cooling process in the liquid region is called under-cooling.

Define bearing stress?

The compressive normal stress that is produced when one real surface presses against another is called the bearing stress.

Numericals 1 In a gas turbine unit, air is drawn at 1.02 bar and 15oC,

and is compressed to 6.12 bar. Calculate the thermal efficiency and the heat supplied of the ideal Braytoncycle, when the maximum cycle temperature is limited to 800oC. (Take Cp = 1.005 kJ/kg K and γ = 1.4 for air)

Pressure ratio. rp = P2/ P1 = 6.12/1.02 = 6

For an isentropic process

T1/T2 = (P1/P2)ɣ-1/ɣ

T2 = T1 (P2/P1) ɣ-1/ɣ = 288 (6) 0.4/1.4 = 480.5 K

Thermal efficiency, η = 1 – 1 / rpɣ-1/ɣ = 40.1%

Heat supplied, Q23 = Cp(T3 – T2) = 1.005(1073 – 480.5) = 595.46 kJ/kg

Q2 All members of the truss shown in the figure have a

cross-sectional area of 500 mm2. Determine the axial stress in CD and EF.

Constructing equilibrium equations for forces in vertical direction at point D, we get

NDC sin 45o – 21 kN = 0 or NDC = 29.7 kN

Axial stress, σDC = NDC / Am = 29.7 x 103 / 500 x 10-6 = 59.4 x 106 N/m2 = 59.4 MPa (T)

Taking moments about point C, we get

-21 kN (2m) – NEF (2m) = 0

Or NEF = - 21 kN

Axial stress, σEF = NEF / Am = -21 x 103 / 500 x 10-6 = -42 x 106 N/m2 = 42 MPa (C)

Q 3 Air enters the compressor of a gas turbine at 85 kPa

and 0 oC. If the pressure ratio is 6 and the maximum temperature is 1000 oC, find (a) the thermal efficiency and (b) the heat supplied for the associated Braytoncycle. (Take Cp = 1.005 kJ/kg K and γ = 1.4 for air)

For an isentropic process

T1/T2 = (P1/P2)ɣ-1/ɣ

T2 = T1 (P2/P1) ɣ-1/ɣ = 273 (6) 0.4/1.4 = 455.5 K

Thermal efficiency, η = 1 – 1 / rpɣ-1/ɣ = 40%

Heat supplied, Q23 = Cp(T3 – T2) = 1.005(1273 – 455.5) = 821.58 kJ/kg

Q 4 Truss analysis showed the forces at joint A given in

figure. Determine the sequence in which the three members at joint A should be assembled to minimize the shear stress in the pin.

Three different sequences are possible for the given joint i.e. BCD, BDC, and CBD.

We have to compute shear stress at two different sections for every possible configuration and compare it with the other.

Possible configuration Shear stress at sec 1 Shear stress at sec 2

BCD 32.77/Ap 22.94/Ap

BDC 32.77/Ap 40/Ap

CBD 40/Ap 22.94/Ap

Where Ap is cross-sectional area of pin that is a constant. Comparing the above-mentioned three configurations it is evident that configuration BCD is the preferred one since it results in lesser value of maximum shear stress.

Q 5 A maximum temperature of 1600 oC is possible in an

Otto Cycle in which air enters the compression process at 85 kPa and 30 oC. Find the heat addition and the thermal efficiency of the cycle, if the compression ratio is 6. (Use Cv = 0.718 kJ/kg K and γ = 1.4 for air)

For an isentropic process from 1 to 2

T1 / T2 = (v2/v1)ɣ-1

T2 = T1 (v1/v2) ɣ-1 = 303 (6)0.4 = 620.4 K

Thermal Efficiency, η = 1 – 1 / rvɣ-1 = 1 – 1 / 60.4 =

51.1%

Heat Supplied, Q = Cv (T3 – T2) = 0.718 (1873 –620.4) = 899.3 kJ/kg

Q 6 Two possible joint configurations are to be evaluated.

The forces on joint in a truss were calculated and a magnified view is shown in the figure. The pin diameter is 20 mm. Determine which joint assembly is better by calculating the maximum shear stress in the pin for each case.

Area of the pin = Ap = (π/4) * d2 = 3.14 x 10-4 m2

We have to compute shear stress at 3 sections of both configurations i.e. between every two consecutive forces.

SectionShear stress in

Configuration 1Shear stress in

Configuration 2

1 32.68/Ap 32.68/Ap

2 58.31/Ap 43.97/Ap

3 30/Ap 30 / Ap

Shear stress is maximum at section 2 in both of the configurations. Comparing the two values it is concluded that Configuration 2 is preferred since it results in lesser value of maximum shear stress.