Compilation of interview questions and answers on thermal engineering for engineering students.
1. What is meant by Quasi-Static Process in Thermodynamics?
Quasi means nearly or almost. So quasi-static process means nearly static process or nearly stationary process, or a process which proceeds with extreme slowness.
Quasi-static process proceeds from one equilibrium state to another equilibrium state till the end of the process i.e., all the states passed during the process are all equilibrium states.
In each state the process deviates from the equilibrium state by a very small amount and immediately attains its equilibrium.
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Let us consider a system consisting of a gas in a cylinder fitted with a piston. Let W be the weight kept on the piston which just balances the upward force exerted by the gas. The system is initially in the equilibrium state. During initial condition, let the properties be P1 V1 and t1. If the weight is removed then there will be an unbalanced force between the system and the surroundings and the piston will move up till it hits the stops.
Then again the system comes to another equilibrium state described by the properties P2, V2, t2 but the intermediate states passed through by the system are non-equilibrium states.
Now if the single weight on the piston is made up of many very small pieces of weights and these weights are removed one by one, then the departure of the state of the system from the thermodynamic equilibrium state will be very small. So, every state passed through by the system will be an equilibrium state.
A Quasi-static process is also called as Reversible process and is represented on the PV-diagram by a continuous line.
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In other words suppose when one weight say a is removed then the system will deviate from its equilibrium by a very small amount and immediately comes to an equilibrium state a and when weight b is removed, the system again deviates by a small amount and immediately comes to another equilibrium state b. Thus when these small weights are removed one by one the system proceeds from one equilibrium state to another equilibrium state till end, such a process which passes through all the equilibrium states is known as quasistatic process.
2. What are the Instruments Used in Thermodynamics? Derive its Efficiency Equations.
1. Heat Engine:
Heat engine converts heat energy into mechanical energy. Heat engine works on thermodynamic cycle like Otto, Diesel, Rankine in which there is a net heat transfer to the system and a network transfer from the system.
Heat engines are broadly classified into two types:
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(i) External combustion engines.
(ii) Internal combustion engines.
In a simple steam power plant an amount of heat Q1 is supplied from the furnace (Also called as high temperature reservoir) to the water in the boiler drum. An amount of heat Q2 is rejected to the low temperature reservoir, like a coolant in the condenser and in doing so, an amount of work W will be produced.
The thermal efficiency ηTh of the heat engine is defined as,
In case of IC engines combustion of air and fuel takes place inside the engine cylinder. The products of combustion will be directly acting on the pistons of the IC engines for producing the power – Now consider an IC engine.
“Heat reservoir is a body of infinite heat capacity, which is capable of absorbing or rejecting any quantity of heat without suffering any change in any of its thermodynamic properties”.
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A heat reservoir which supplies an amount of heat Q1 to the heat engine operating in heat engine cycle is called the High Temperature Reservoir (HTR) or Source.
The heat reservoir to which an amount of heat Q2 is rejected by the heat engine is called the Low Temperature Reservoir (LTR) or Sink.
3. Refrigerator:
“It is a device, which produces and maintains the temperature below atmospheric temperature” or
“It is a device which operating in cycle, maintains an enclosed space at a temperature lower than the temperature of the surroundings”.
Let a body A is to be maintained at a temperature t2, which is lower than the atmospheric temperature t1. Even though the body, A is insulated, there will be a heat leakage Q2, into the body because of the temperature difference between the body and the surroundings. In order to maintain the body A at constant temperature t2, same amount of heat is to be removed at the same rate, at which it is leaking into the body.
A refrigerator is a device which operating in a cycle, removes heat Q2 from the body a t2 and transfers this heat to the atmosphere at t1 by consuming an amount of work W.
In the refrigeration cycle, body A, Q2 and W are of prime importance. Just like the efficiency of heat engine cycle, here a parameter called co-efficient of performance is important. The copies defined by,
It is just opposite to the refrigerator i.e., “It is a device operating in a cycle, maintains a body say B at a temperature higher than the temperature of the surroundings”.
Because of the temperature difference between the body and the surroundings there will be a heat leakage Q1 from the body. The body will be maintained at constant temperature t1, if heat is supplied into the body at the same rate at which heat leaking out of the body.
A heat pump is a device which operates in a cycle extracts heat from the atmosphere and transfers an amount of heat Q1 to the body B and maintains the body B at constant temperature t1 in a heat pump, body B, Q1 and W are of prime importance.
3. State Second Law of Thermodynamics. Explain with Equations.
There are two important statements of second law.
They are:
1. Kelvin Planck’s Statement,
2. Clausius Statement,
1. Kelvin Planck’s Statement:
“It is impossible for a heat engine to produce network in a cycle, if it exchanges heat with a single reservoir” or
“Heat engine cannot produce work output, if it exchanges heat with a single reservoir”.
By practical experience, it is noted that total heat supplied cannot be converted into useful work W i.e., Q1 ≠ W. A heat engine can never be 100% efficient. But W < Q1 therefore there has to be heat rejection i.e., Q2 > 0.
Therefore for a heat engine to produce network in a cycle, it has to exchange heat with two reservoirs.
But if, Q2 = 0, then Q1 = W or ηTh = 100%, the heat engine will produce network in a complete cycle by exchanging heat with only one reservoir, thus it violates Kelvin Planck’s statement. Such a heat engine is called a Perpetual Motion Machine of the second kind. (PMM-2) Ref. Fig. 3.8. A PMM-2 is impossible and it is just a conceptual engine. All the attempts made so far to make such a machine have failed. Thus they show the validity of Kelvin Planck’s, statement.
2. Clausius Statement:
We know that, heat always flow from a hot body to a cold body. The reverse process never occurs by itself.
Clausius statement is given as, “It is impossible to construct a device, which operating in a cycle will produce no effect other than transfer of heat from a low temperature body to a high temperature body”. Or
“Heat cannot flow by itself from a cold body to a hot body, in order to achieve this some work must be expended”. For example- Refrigerator—in this case heat is removed from the cold body A and transferred to the atmosphere but by consuming work input W.
Equivalence of Kelvin Planck and Clausius Statements:
The Kelvin Plank and Clausius statement as follow:
(i) Kelvin Planck’s Statement:
“Far a heat engine to produce Wnet it has to exchange heat with two reservoirs”.
(ii) Clausius Statement:
“Heat cannot flow by itself from a cold body to a hot body. In order to achieve this some work must be expended”.
At first sight these two statements will appear to be two different statements and are unconnected. But it can be easily shown that, these are the two parallel statements of second law and are equivalent in all respects.
The equivalence of the two statements will be proved if we prove that violation of one statement results into the violation of the other.
(a) Violation of Clausius Statement Leads into the Violation of Kelvin Planck’s Statement:
Consider a cyclic heat pump P, which transfers heat from a LTR at t2 to HTR at t1 without consuming work input (i.e., W = 0), thus violating Clausius statement.
Now, let us assume a cyclic heat engine E which also operates between the same two reservoirs at t1 and t2 respectively. The rate of working of the heat engine is such that it draws an amount of heat Q1 from HTR equal to that discharged by the heat pump. Then the HTR may be eliminated and heat Q1 discharged by the heat pump may be directly fed to the heat engine. So, the heat pump P and heat engine E acting together will form a heat engine, operating in cycles and produce network by exchanging heat with one reservoir. This violates the Kelvin Planck’s statement.
(b) Violation of Kelvin Planck Statement Leads into the Violation of Clausius Statement:
Let us consider a PMM-2 (E), which produces Wnet in a cycle by exchanging heat with only one reservoir at t1 and thus violates Kelvin Planck’s statement.
Now, let us assume a cycle heat pump P extracting heat Q2 from LTR at t2 and supplying heat to HTR at t1 by consuming work W equal to that PMM-2 supplies in a complete cycle. So, E and P together will form a heat pump and producing the complete effect of transferring heat from LTR to HTR, without any external aid.
4. How is Steam Generated at a Given Pressure?
Consider one kg of water at 0°C. Also let us decide upon the pressure under which one kg of water is to be heated. Consider a cylinder with a frictionless piston loaded such that the pressure P under consideration is acting on water.
Heat is added to the water so that its temperature increases from 0°C to tsat (saturation temperature) at which the water boils at pressure P. Volume of water increases to Vf, This is shown in Fig. 10.4(a).
Heat supplied or added to 1 kg water to raise its temperature from 0°C to saturation temperature tsat is called sensible heat because it can be detected by the sense of touch and produces a rise in temperature to be seen on a thermometer. Generally this sensible heat is denoted by hf.
Note:
Vf means volume of fluid which is taken as liquid.
Heating is continued after the water reaches saturation temperature. Evaporation of water takes place and steam is generated at pressure P. This heating is continued till all the water is evaporated and the temperature of the steam is saturation temperature. Vg or Vsat denotes the volume of steam at this condition. The external work done during evaporation is given by W = P (Vsat – Vf) or p (Vg – Vf) with proper units.
Because the heat during this stage of evaporation cannot be recorded by a rise in temperature on the thermometer, it is called Latent heat (hidden heat).
The amount of heat required to evaporate 1 kg of water at saturation temperature to steam or vapour at saturation temperature is called Latent heat or enthalpy of evaporation and is generally denoted by hfg. This is shown in Fig. 10.4(b).
Once all the water is evaporated to saturated steam, if heat is added further, the steam behaves approximately as an ideal gas and the heating of saturated steam increases its temperature. The steam whose temperature is greater than saturation temperature at the given pressure, is called a superheated steam; and let its temperature be denoted by tsup. Its volume will also increase to Vsup.
The amount of heat required to raise the temperature of l kg steam from tsat to tsup is called superheat and the temperature difference called degrees of superheat.
5. Define of Heat Transfer. What are the Modes of Heat Transfer?
We have repeatedly used the term heat (Q) and heat exchanges between bodies without discussing the details of the phenomena by which heat is transferred.
Heat transfer is a name of general process applied to any device that effects a transfer of heat from one substance to another.
The various devices that employ the principles of heat transfer are:
(i) Steam boilers
(ii) Surface condensers, evaporators, closed feed water heaters and the automobile radiator etc.
Whenever a temperature gradient exists within a system, or when two systems at different temperatures are brought into contact, energy is transferred. The process by which the heat energy transport takes place is known as heat transfer.
Modes of Heat Transfer:
Heat transfer can be defined as the transmission of energy from one region to another as a result of a temperature difference between them. Since temperature differences exist all over the universe, the phenomena of heat flow over the universe, the phenomena of heat flow are as universal as those associated with gravitational attractions. Unlike gravity, however, heat flow is governed not by a unique relationship, but rather by a combination of various independent laws of physics.
The literature of heat transfer generally recognises three distinct modes of heat transmission:
(i) Conduction
(ii) Radiation and
(iii) Convection
Strictly speaking, only conduction and radiation should be classified as heat transfer processes, because only these two mechanisms depend for their operation on the mere existence a temperature difference. The last of the three, convection, does not strictly comply with the definition of heat transfer because it depends for its operation on mechanical mass transport also. But since convection also accomplishes transmission of energy from regions of higher temperatures to regions of lower temperatures, the term “heat transfer by convection”, has been generally accepted.
6. What is Convection Heat Transfer? Explain Newton’s Law of Cooling.
Convection heat transfer is defined as the transfer of heat through the agency of particles of fluid which receive heat from a high temperature source and move to the locality of a lower temperature sink to reject heat. The sink may be some body exposed to the fluid, or it may be cooler particles of fluid. It should be noted that convection heat transfer occurs by means of fluid motion, in contrast to conduction heat transfer, which takes place entirely by means of intermolecular energy transfers.
Convection heat transfer may be further classified according to the mode of motivating the flow. If the convection currents are caused by density differences brought about by temperature gradients within the fluid, the convection is said to be natural. If the flow is aided by some mechanical device such as a pump (fan) the resulting convection currents are forced.
Newton’s Law of Cooling:
The rate of heat transfer by convection between a surface and a fluid may be computed by the relation
qc = h̅CA.ΔT
Where qc = rate of heat transfer by convection (kJ/hr)
A = heat transfer area (m2)
ΔT = difference between the surface temperature Ts and the temperature of the fluid T∞ at some specified location (°C).
h̅c = average unit thermal convective conductance often called the surface coefficient of heat transfer or the convective heat transfer coefficient kJ/hr.m2.°C.
Thermal conductance K for convective heat transfer Kc = h̅c. A and thermal resistance
The relation expressed by this equation was originally proposed by the British scientist, Issac Newton in 1701. This is also known as Newton’s law of cooling. The evaluation of the convective heat transfer coefficient is difficult because convection is a very complex phenomenon.
7. Explain the Working of Two Stroke Spark Ignition (SI) Engine with Suitable Diagram.
In this engine the cycle is completed in two strokes of the piston or one revolution of the crankshaft (in four stroke engine crank rotates through two revolution in one cycle).
(a) Ports:
This engine has no valves but there are three ports inlet, exhaust and transfer ports as shown. These ports get opened and closed due to the position of the piston.
(b) Piston:
The piston has a deflector on its crown which prevents loss of fresh charge while allowing the burnt gases to be exhausted.
(c) Crank Case:
Charge is compressed in the crank case when the piston is coming down and is transferred to cylinder via. transfer port.
Working:
Its working is shown in Fig. 24.4.
(a) Starting from top dead centre position (TDC), as the piston goes downwards first the exhaust port gets uncovered. The burnt gases are going out through this port.
(b) With further downward movement of the piston the inlet port is opened. Fresh air fuel mixture is admitted into the crank case. The downward movement of the piston acts as pump and slightly compresses the fresh charge in the crank case.
(c) Further movement of the piston uncovers the transfer port allowing the air—fuel mixture charge to be admitted into the cylinder.
(d) When the piston is going upwards both the transfer and exhaust ports are covered and the charge is compressed. Just before the TDC, the spark plug gives spark to initiate combustion and the combustion develops fully when the piston is in the TDC. Due to high pressure and temperature of the gases the piston is pushed downwards.
(e) Points to be noted are that as the fresh charge is being admitted burnt gases are going out. Specific shape of the piston crown prevents to some extent that the fresh charge being let out along with the exhaust but total prevention is difficult.
8. Explain the Working of Electrolux Refrigerator.
This type of refrigerator takes advantage of the fact that the liquid ammonia evaporators very rapidly in hydrogen. For this refrigerator, no pump is required as the circulation is maintained by gravity and the heat is supplied by a small gas jet or electric heater.
Figure 36.36 shows a diagrammatic view of Electrolux refrigerating system. This system consist of an absorber A which contains a strong solution of ammonia dissolved in distilled water. When a gas burner B is lighted the circulation of the system commences because of the warming of ammonia solution in the boiler or heater D.
The strong ammonia solution now flows from A through the intercharger C, where it is heated by the hot work solution returning from D. It then passes around the heater D, which causes the ammonia vapour to be driven out of the solution. Now the released ammonia vapour passes through a rectifier, after which it enters the condenser E.
The object of the rectifier is to drain off any particles of water coming with the ammonia vapours. The hot weak solution left behind D drains back into the absorber A. In doing so it passes through the intercharger C, where it gives its heat to the strong solution flowing from A.
After condensation in the condenser, ammonia vapours forms the liquid and this ammonia liquid flows by gravity into the evaporator G. This is full of hydrogen into which the liquid ammonia evaporates. Liquid ammonia absorbs latent heat producing intense cold around G, which is situated in the food cabinet or any space which is to be refrigerated and thus the cabinet or space is cooled and the objects place in the cabinet or space are cooled.
The mixture of hydrogen and ammonia vapour now flows through the gas heat exchanger F, where it cools the fresh supply of hydrogen flowing into G. From F the NH3 ammonia vapour and hydrogen gas flows into the absorber A, where the hydrogen rises to the top and flows back to the evaporator G. Thus, the cycle is complete.
The main advantage of this Electrolux refrigerator is that there is no moving part like compressor, pump or fan and therefore there is no noise and no machinery to give mechanical troubles.
The coefficient of performance of this refrigerator is given by the equation-
Total pressure in the system is about 15 bar. H2 gas is present in the evaporator at about 11-12 bar pressure. The quantity of H2 is so proportioned that the practical pressure of NH3 within evaporator will be only a smaller portion of the total pressure for the pressure of NH3 vapours in the evaporator will only 4-3 bar. This low pressure causes NH3 to boil or evaporator at low temperatures. Here the Dalton’s law of partial pressure is applied.
Other substances that can be used in vapour absorption system are:
(i) Water as the refrigerant and lithium bromide salt solution as absorbent.
(ii) Water as the refrigerant and lithium chloride brine as the absorbent.
(iii) Methylene chloride as refrigerant and dimethyl ether of tatraethylene glycol as absorbent.
9. Explain Air Cycle System for Aircraft and Missiles with Diagram.
The advent of high speed passenger aircraft, jet aircraft and missiles has introduced the need for compact and simple refrigeration systems capable of high capacity with a minimum reduction of payload. When the power requirements needed to transport the additional weight of the refrigerating system are taken into account, air cycle system usually prove to be most efficient. In fact, some modern air refrigeration systems weight as low as 2 kg per ton of refrigeration.
In passenger aircraft and missiles following are the sources of heat coming into the cabins:
(i) Surface friction heat because of the high speeds of air-crafts or missiles.
(ii) Heat dissipated by electrical equipment used in the air craft and missiles.
(iii) Heat convected and radiated in addition to friction heat.
(iv) In passenger air-crafts, heat entering the cabins because of the hot tea, coffee and meals served.
The heat, thus entering the aircraft or missiles has to be removed by providing refrigeration.
The different systems of air refrigeration provided to the aircraft and missiles are:
(i) Simple air cycle-evaporating cooling system.
(ii) Bootstrap air-cycle cooling system.
(iii) Reduced ambient air-cycle system.
The schematic diagrams of these systems are given below without elaborating much the working of these systems.
1. Simple Air Cycle-Evaporating Cooling System:
Ram air is at lower temperature than compressed air and hence compressed air is cooled by ram air in heat exchanger. Ram air is inducted by a cooling fan. This fan is driven by the cooling turbine as shown in Fig. 36.16. Compressed air leaves the heat exchanger and is passed through the evaporator where water is evaporated and the heat required is taken from the air and is cooled. This air is expanded through a cooling turbine and this air is supplied to the cabin.
2. Bootstrap Air-Cycle Cooling System:
This system is shown in Fig. 36.17 (a) and (b). The cycle is shown on T-S diagram.
The cycle is shown on T-S diagram:
1- 2 Ramming effect.
2- 3 Main compression.
3- 4 Cooling in first H-E.
4- 5 Compression in secondary compressor.
5- 6 Cooling second HE.
6- 7 Expansion in cooling turbine.
3. Reduced Ambient Air-Cycle Cooling System:
This system is diagrammatically shown in Fig. 36.18 (a) and T-S diagram of Fig. 36.18 (b).
1- 2 Ramming effect.
2- 3 Compression in main compressor.
3- 4 Cooling in heat exchanger.
4- 5 Expansion in the cooling turbine.
10. Where to Place FWHS in the Cycle or What are the Pressures at which Steam is to be Bled from the Turbine, that will Result in the Maximum Increase in the Efficiency of FWHS?
To answer this let us consider for the sake of simplicity that only one FWH is used. The aim of using any feed water heater is to heat the feed water as close to the boiler temperature as possible. We may now place the feed water heater at position 1, 2 or 3. TB and TC are saturation temperatures at boiler and condenser pressures respectively. Now at position 1 the heat transfers are because of temperature differences TB – T1 and T1 – TC.
At position 3 the heat transfers are because of TB – T3 and T3 – TC. In either case one of the temperature differences is very large. To minimise the temperature differences, we have to select position 2 which is in the middle i.e., TB – T2 = T2 – TC.
Thus we find the temperature which is halfway between TB and TC and then obtain the saturation pressure at that temperature. Feed water placed at this pressure gives us optimum position of maximum efficiency. Note that the temperature at which steam is actually bled from the turbine may be superheated.
If two FWHS are used, the temperature range TB – TC is divided into three equal parts and for three FWHS it is divided into 4 equal parts.
Thus, in general, for n FWHS the optimum temperature rise per heater would be given by –
Consider for example a cycle where boiler pressure is 100 bar and condenser pressure is 0.08 bar and three FWHS are to be used. We find the pressures at which they are placed. From steam tables we get saturation temperatures at 100 bar and 0.08 bar. They are TB = 310.96°C and TC = 41.534°C respectively. From above Eq. (22.14) we have –
Thus the saturation temperatures of FWHS will be found by adding ΔTopt successively from TC = 41.534 onwards and then finding the corresponding saturation pressures from the steam tables. Table 22.1 summarises the results.
Thus, the FWHS will be placed at 34 bar, 9.2 bar and 1.35 bar respectively.
11. Explain the Working of Air Washer with the Help of a Suitable Diagram.
Air washer is a device that may be used for:
(a) Cooling and dehumidification
(b) Cooling and humidification
(c) Heating and humidification
(d) Humidification
(e) Adiabatic saturation
(f) Cooling.
The schematic diagram for air washer is shown in Fig. 37.7.
Following are the processes that can be carried out with the help of washer shown in Fig. 37.7 and they are shown on psychometric chart of Fig. 37.8.
They are:
i. 1-2a Sensible Cooling:
Water temperature is equal to dew-point temperature of air.
ii. 1-2b Adiabatic Saturation:
It takes place along the wet bulb temperature line. Water is at WBT
iii. 1-2c Cooling and Dehumidification:
Temperature of water is below the dew point temperature of air.
iv. 1-2d Cooling and Humidification:
When wet bulb temperature is less than water spray temperature and this spray water temperature is less than the dry bulb temperature of air, this cooling and humidification process takes place. The water is required to be heated externally.
v. 1-2e Humidification:
Water spray temperature is equal to the dry bulb temperature of air. The water externally heated. (Latent heat of evaporation)
vi. 1-2f Heating and Humidification:
If the spray water is heated and maintained at temperature higher than the dry bulb temperature of air entering the washer. In this case, the dry bulb temperature of air leaving the washer is more than the inlet temperature of air. Wet bulb is higher than the initial WBT and higher humidity ratio than the entering air.
12. What are the Different Types of Air Washers?
There are three different types of air washers.
They are:
1. Spray Type Air Washer:
There is no standardisation for air washers.
The essential requirements in the air washer operations are:
(a) Uniform distribution of the air across the spray chamber.
(b) An adequate amount of spray water broken up into fine droplets.
(c) Good spray distribution across the air stream.
(d) Sufficient length of travel through the spray and the wetted surfaces.
(e) The elimination of free moisture from the outlet air.
Refer Fig. 37.7 for the simple air-washer used in practice. A pumps recirculates water at a rate greater than the evaporation rate. Washers are commonly available from 0.8 to 118 m3\sec. capacity. This type of washer is used primarily as an evaporative cooler or humidifier.
2. High-Velocity Spray Type Air Washer:
Air velocity range is 6 to 9 m/s -12 m/s. Maximum capacity up to 70 m3/s.
These are generally rectangular in cross-section.
3. Cell-Type Air Washer:
These washers obtain intimate air-washer contact by passing the air through cells packed with glass, metal or fiber screens. Water passes over the cells arranged in tires. Behind the cells are blade- type or glass-mat eliminates. Most cell-type washers are arranged for concurrent air and water flow. They are also constructed for special duty with counter.
Current flow characteristics or in a combination of both arrangements, cell washers come to many sizes of insulated or un-insulated construction and standard washers are available up to 10 cells high by 12 cells wide with a capacity of up to 100 m3/s.
Atomisation of the spray water is not required in cell type washers, but good water distribution over the face of the cell essential. A saturation effectiveness of 90 to 97% is possible.