ME 2301 - THERMAL ENGINEERING Two Marks Questions With Answers 2014

Anna University, Chennai

SRINIVASAN ENGINEERING COLLEGE PERAMBALUR -621212

ME 2301 - THERMAL ENGINEERING

UNIT 1 – GAS POWER CYCLES

PART A

1. Define air standard efficiency. (June 2004)

When the engine is working with air as the medium, then the efficiency of the engine is said to be air standard efficiency.

2. Define relative efficiency. (Dec 2006)

Relative efficiency is the ratio between actual thermal efficiency and air standard efficiency.

hRelative =

Actual thermal efficiency

Air standard efficiency

3. What are the assumptions made in analysis of air standard cycle? (May 2007)

(i) There is no chemical reaction taking place when the heat is supplied or rejected.

(ii) The physical constants of the gas in the cylinder are the same as that of air at moderate temperature.

(iii) The processes like compression and expansion are adiabatic and they are frictionless.

(iv) The gas used in the process is a perfect gas. (obeys gas laws).

4.Which air standard cycle (Otto/ Diesel/ Dual) is more efficient for the same heat input? Justify.

Among Otto, Diesel and Dual combustion cycles, there is least heat rejection in Otto cycle. Hence Otto cycle has more efficiency.

5. List out the important factors which are used for comparison of Diesel, Otto and Dual cycles.

(i) Compression ratio. (ii) Maximum pressure (iii) Heat supplied

(iv) Heat rejected

(v) Net work done.

6. Sketch the P-v and Ts diagram of Diesel cycle. (May 2007)

7. Define mean effective pressure of Otto cycle. (Dec 2007)

Mean effective pressure is defined as the constant pressure acting on the piston during the working stroke. It is the ratio between workdone and stroke volume.

8. How does the change in compression ratio affect the air standard efficiency of an ideal

Otto cycle? (May 2008)

The expression for the air standard efficiency of an Otto cycle is given by,

hOtto =1 -

1

Rg -1

R – Compression ratio

γ – Ratio of specific heats

From the above expression the efficiency increases with the increase in the value of R compression ratio, which means that the efficiency could be improved, by increasing the compression ratio. But increasing the compression ratio also has a considerable extent due to practical difficulties.

9. Mention four thermodynamic processes involved in diesel cycle. (May 2008)

(i) Isentropic compression

(iii) Isentropic expansion

(iv) Constant volume heat rejection

10. Give the expression for pressure ratio for maximum work in Brayton cycle. (June 2009)

rp = ç

1

 2
T ö 2(g -1)

÷

è T1 ø

 p
 p
r = p2

1

UNIT II (INTERNAL COMBUSTION ENGINES)

1. What is the function of camshaft and crank shaft? (May 2007)

Camshaft converts the rotary motion of cam into linear motion of the follower. It operates the inlet and outlet valves through rocker arm.

Crank shaft converts the reciprocating motion of the piston into rotary motion.

2. List out the effects of detonation. (May 2007)

(i) Noise and roughness

(ii) Mechanical damage to the engine parts

(iii) Carbon deposit on cylinder walls

(iv) More heat transfer

(v) Reduced efficiency and power.

3. Define nozzle efficiency. (May 2007)

Nozzle efficiency is the ratio of actual enthalpy drop to the isentropic enthalpy drop.

Nozzle efficiency = Actual enthalpy drop

Isentropic enthalpy drop

4. What is the function of push rod and rocker arm? (Nov 2007)

The motion of the cam is transmitted to the valve through the push rod and rocker arm. These links combinedly known as valve gear.

5. What are the basic requirements of a fuel injection system of a diesel engine. (Nov 2007)

(i) Constant supply of fuel from cycle to cycle operation

(ii) Uniform and constant supply of fuel from cylinder to cylinder

(iii) Atomize the fuel to the required level

(iv) Beginning of injection at proper timing

6. What is the effect of friction on the flow through a steam nozzle? (May 2007)

(i) The expansion is no more isentropic and enthalpy drop is reduced thereby resulting in exit velocity.

(ii) The final dryness fraction of steam will be increased. (iii) The specific volume of steam will be increased.

7. What is splash lubrication? (Dec 2008)

Splash lubrication is applicable in small sized internal combustion engines. In this system, the oil from the sump is splashed to the moving parts of the engine with spool. A small hole is drilled in the crank shaft and the oil is forced through this hole to the bearings.

8. What do you mean by supersaturated flow? (Dec 2008)

The steam may not have time to condense and remains dry, when the expansion through the nozzle is very rapid. This type of flow is called supersaturated flow.

9. What is the importance of delay period in CI engine combustion phenomena?

If the delay period is more, more diesel fuel will be injected in the combustion chamber and pressure rise will be more. This leads to diesel knock.

But delay period is needed to disperse and atomize the fuel in the air for complete combustion. Hence an optimum and required time should be given for the delay period depending on the combustion characteristics of the engine.

10. How CI combustion chambers of diesel engine classified?

(i) Non turbulent type – Open combustion engine

(ii) Turbulent type – (a) Turbulent chamber (b) Pre-combustion chamber (iii) Energy cell

UNIT III – STEAM NOZZLES AND TURBINES

1. What is critical pressure ratio of a steam nozzle? (May 2008)

Critical pressure is the ratio of the outlet pressure to inlet pressure only when the mass flow per unit area is maximum.

2. What is the need for compounding in steam turbines? (May 2008)

In a simple impulse turbine, if the steam is expanded from the boiler pressure to condenser pressure in one stage the speed of the rotor becomes tremendously high which crops up practical complicacies. There are few methods of reducing the speed to the lower value, all these methods utilize a multiple system of rotor in series keyboard to a common shaft. This is known as compounding.

3. Give the working principle of an impulse turbine. (May 2007)

The operation of impulse turbine depends on the dynamic action of steam expanding through nozzle. The enthalpy of steam is first converted into kinetic energy in nozzle. The high velocity of steam impinges on the curved blade fixed on the circumference of rotor. The rotor charges to the flow direction of steam which causes the force to be exerted on the rotor.

4. What is blading efficiency? (Dec 2007)

Blading efficiency is also known as diagram efficiency. It is defined as the ratio of work done on the blade to the energy supplied to the blade.

5. What is the effect of friction on the flow through a steam nozzle? (Dec 2007)

(i) The expansion process will not be isentropic and enthalpy drop will get reduced, which further lead to reduced exit velocity.

(ii) The final dryness fraction will increase.

(iii) The specific volume of the steam will increase.

6. What is the function of governor in steam turbine?

The governor in the steam turbine maintains a constant rotor speed, even during the varying load condition.

7. What is the effect of superheating in Rankine cycle?

The effect of superheating increases the mean temperature of heat addition and hence the cycle efficiency.

8. Explain supersaturated flow (or) Meta stable flow in steam nozzles. (Apr2003)

When the superheated steam is expanded in the nozzle the condensation should occur in the nozzle. Since the steam has a great velocity, the condensation does not take place at the expected rate. So the equilibrium between the liquid and vapour phase is delayed and the steam

continues to expand in a dry state. The flow of steam expanding like this is called supersaturated flow or meta stable flow.

9. What is Wilson line?

The limiting condition of under cooling at which condensation commences and is assumed to restore conditions of normal thermal equilibrium is called “Wilson line”.

10. State the necessity of governing of steam turbine. (Apr 2011)

Governing is to maintain the speed of the turbine fairly constant at varying load conditions.

UNIT IV – AIR COMPRESSORS

1. How is air compressors classified?

Air compressors are classified into,

(i) Reciprocating compressor and rotary compressor. (ii) Single stage and multistage compressor.

(iii) Single acting and double acting compressor.

2. What is meant by FAD (Free Air Delivery)?

Free air delivered is defined as the actual volume ratio of air reduced to atmospheric condition and usually expressed in m3/min.

3. What is meant by perfect inter-cooling?

If the temperature of air leaving the intercooler is equal to the original inlet temperature the inter-cooling is known as perfect inter-cooling. By having inter-cooling, we can approach the isothermal process. So the isothermal efficiency will be increased by perfect inter-cooling.

4. Define clearance ratio of an air compressor.

The ratio between clearance volume to swept volume is called clearance ratio (k).

k = Clearance v olume

Swept volume

5. Define volumetric efficiency of an air compressor.

Volumetric efficiency of an air compressor is the ratio of free air delivered to the displacement of the compressor.

Effective swept volume

Free air delivered

hVol =

=

Swept volume Displacement volume

6. Explain the working principle of rotary compressor.

In rotary compressor the air is entrapped between two sets of engaging surfaces and the pressure either by back flow of air (roots blower) or by both squeezing action and backflow of air (vane type).

7. What are the various methods to improve the isothermal efficiency?

The various methods for improving the isothermal efficiency are, (i) Spray injection

(ii) Water jacketing

(iii) Inter-cooling

(iv) External fins

8. Which type of compression is the best one in reciprocating compressor? State the reason.

Isothermal compression is the best one in reciprocating compressor as it requires less power to drive the compressor.

9. What factors limit the delivery pressure in a reciprocating compressor?

(i) The size of the cylinder will be too large for very high pressure.

(ii) Due to compression, there will be a rise in the temperature of the air. So the delivery pressure is limited, so that rise in temperature of air is not going beyond limit and size of cylinder is not too large.

10. What is the effect of clearance upon the performance of air compressor? (May 2003)

(i) It is necessary to avoid the piston hitting the cylinder end.

(ii) For the movement of valves, a space to be left when the piston travel upwards. Hence clearance volume is provided.

(iii) The maximum compression pressure is also controlled by clearance volume.

11. What are the advantages of multistage compression with intercooling over single stage compression for the same pressure ratio. (May 2003)

(i) The workdone per kg of air is reduced in multistage compression with intercooler when compared with single stage compression for the same delivery pressure.

(ii) There will be increase in volumetric efficiency also.

12. Classify the various types of air compressors. ( Nov 2003)

Air compressors are basically classified into positive displacement air compressors and dynamic air compressors.

Positive displacement air compressors are further classified into

(i) Reciprocating air compressor

(ii) Rotory

(a) Roots blower (b) Screw type (c) Vane type

Dynamic air compressors are further classified into

(i) Centrifugal air compressor (ii) Axial flow air compressor.

13. State the effect of clearance on work done in a reciprocating compressor. (Apr 2011)

(i) Actual suction volume decreases

(ii) Mass of air is reduced

(iii) Volumetric efficiency decreases

UNIT V- REFRIGERATION AND AIR CONDITIONING

1. What is commonly used unit of refrigeration?

The „Ton of Refrigeration‟ is the commonly used unit for refrigeration. It is defined as the quantity of heat required to extract the heat from 1000 kg of water at 0o C into 1000 kg of ice at

0o C within 24 hours.

1 Ton of refrigeration ≈ 3.5 kW

2. Distinguish between summer air conditioning and winter air conditioning.

In summer air conditioning the air gains both sensible and latent heat. Hence the conditioning of air is done by both cooling and dehumidification. In winter air conditioning, heating and humidification is done to the air.

3. Define RSHF line. (May 2003)

It is Room Sensible Heat Factor (RSHF) line. This line is drawn parallel to the base line in the psychrometric chart.

4. Define by pass factor of a heating coil. `(Apr 2011)

The ratio of the difference between the mean surface temperature of the coil and leaving air temperature to the difference between the mean temperature and the entering air temperature

is known as by pass factor.

BPF = td 3 - td 2

td3 - td1

5. What is the effect of sub cooling a refrigerant in a vapour compression cycle? `(Apr 2011)

(i) Refrigerating effect is increased

(ii) COP of the plant is increased.

(iii) Liquid refrigerant below the condensing temperature.

6. What is dew point temperature? (Nov 2010) It is the temperature of air when the water vapour present, begins to condense. It is measured by thermometer.

7. Define the COP of refrigeration. (Nov 2010) The coefficient of performance is the ratio of heat extracted in the refrigerator to the work done on the refrigerant.

8. What is the basic difference between vapour compression and vapour absorption refrigeration system? (Apr 2008) In vapour compression system a compressor is used, hence it is noisy. In vapour absorption system compressor is not used, hence it is noiseless.

Vapour absorption differs from vapour compression system with a principle that, it uses heat energy instead of mechanical energy.

9. Define relative humidity and wet bulb temperature.

Relative humidity is the ratio of mass of water vapour in the air in a given volume at a given

temperature to the mass of water vapour contained in the same volume at same temperature when the air is saturated.

Wet bulb temperature, is the quantity of temperature measured by the thermometer

when the bulb of the thermometer is wrapped with a wet cloth.

10. Differentiate between heat pump and refrigerator. (Apr 2003)

Heat pump takes the atmospheric air and supplies the heat to the air and the heat is rejected to the space to be warmed.

Refrigerator extracts the heat from the space to be cooled and heat is given out to the atmosphere.