Compilation of basic interview questions on thermodynamics for engineering students.

1. State Zeroth Law of Thermodynamics.

When the two bodies one hot and the other cold, are placed in contact with each other, then the hot body loses heat and becomes colder and the cold body gains heat and becomes hotter, and this process continues till the thermal equilibrium is reached. In this state the two bodies will be at the same temperature.

From this a law is deduced known as Zeroth law of thermodynamics. It states that, “When a body A is in thermal equilibrium with the body B and also separately with the body C, then the bodies B and C will be in thermal equilibrium with each other. On the basis of this law, it is possible to compare the temperatures of B and C and it can be said that the temperatures of B and C are equal without actually making any contact between them. This law thus forms the basis of temperature measurement – by comparison method.

If A is thermal equilibrium separately with B and C then B and C will also be in thermal equilibrium.

2. How will you Classify Thermodynamic System?

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Thermodynamic system is classified as:

1. Closed or Non-Flow system

2. Open on flow system and

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3. Isolated system

In closed or non-flow system the boundary allows the energy to flow in or flow out i.e., the energy crosses the boundary of the system. Secondly, the boundary does not allow the mass to cross the boundary of the system.

When the energy crosses the boundary of the system, the boundary may or may not move. The process that will take place in this closed or Non-flow system is called Non-Flow Process.

The energy equation for this Non-Flow Process is, with usual relations, as –

In open or Flow system, both energy and mass will cross the boundary of the system. This open or flow system can further be classified as Steady-Flow System or Unsteady-Flow-System.

At this stage, we are required to consider steady flow system. In this steady flow system the rate of mass flow and energy flow into the system is equal to the rate of flow of mass and energy out of the system i.e., m1 = m2, and E1 = E2. This means that there is no accumulation of mass as well as energy, within the system.

The process that takes place in a Steady-Flow-System is called Steady Flow Process. Steady flow energy equation can be written as –

3. What is the Influence of Heat Losses or Gains on the Surface of Conducting Body?

The equations of conduction heat transfer presented so far have not included the influence of heat losses or gains at the surfaces of the conducting body. When such concurrent heat flow must be considered, the surface coefficient of heat transfer h enters the basic equations. Also it is seen that the overall heat transfer resistance of a plane wall is determined principally by the greatest single resistance.

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If this resistance is one of the convection resistances, the heat flow through the wall can be increased by putting fins (extended surfaces) on the surface where this large resistance occurs. Such extended surfaces are widely used, for example, in economizers in steam power plants, convectors for steam and hot water heating systems, or electrical transformers, for the cylinders of air-craft engines, two wheeler engines etc.

There are many forms and shapes of these extended surfaces. Some of these are as follows:

i. A Thin Rod:

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This is transferring heat at its surface to a surrounding fluid and which is connected at its base to a heated wall. 

ii. The Rectangular Fin:

This is the simplest case for the plane finned surface. As long as the height of the fins on a tube is comparatively small with respect to the tube diameter, the formula derived from the plane wall can also be used for the tube. 

iii. Straight Fins of Triangular Profile:

In determining the optimum fin, the question arises as to whether or not weight advantage can be gained by using a shape other that rectangular for the fin cross-section. Generally, a straight fin of triangular cross-section is considered.

The mathematical treatment in this case is similar to the case of the fin of rectangular cross-section except that the area normal to the heat flow is a function of the distance along the fin, decreasing as the fin length increases.

iv. Cylindrical Fins:

Fins which are arranged around tubes are called cylindrical fins and are quite important from an engineering point of view.

Here again the treatment is substantially the same as for the rectangular fin except that the area must be allowed to vary with the radius.

4. Explain the Stages of Combustion in SI Engines.

(a) Theoretical pressure time or—crank angle curve would consist of adiabatic compression (a – b), heat addition at constant volume—virtually instantaneously (b – c) and expansion (c – d) as given in Fig. 26.2.

This is idealised situation and actual pressure time curve is quite different due to:

(i) Combustion taking finite time for completion

(ii) Presence of ignition lag.

(b) The combustion phenomenon in SI engines can be divided into three distinct phases:

1. Phase I—Ignition lag or preparation phase

2. Phase II—Flame propagation throughout the combustion chamber

3. Phase III—After burning

1. Phase I — Ignition Delay/Ignition Lag:

Ignition lag is not a period of inactivity but it is preparation phase during which pre-flame chemical reactions take place in which growth and development of nucleus of flame occurs. It depends on various factors like nature of fuel; temperature, pressure and presence of residual exhaust gases.

Ignition delay can be defined as the time be­tween the initiation of the spark and distinct rise in pressure that can be seen from separation from the motoring curve. (Motoring curve is the pressure versus crank travel curve when the engine cylinder is not firing—shown by dotted lines in Fig. 26.3.

When the spark is occurs at point A (Fig. 26.3) the pre-flame reactions commence and the flame in the vicinity of the spark plug spreads very slowly and not much energy is released. The pressure rise is quite insignificant. But once the flame front gets established it accelerates at a very high speed. Point B where appreciable pressure rise can be seen marks the end of the ignition lag phase. Figure 26.3 shows ab as region of ignition lag or stage I.

Duration of the ignition lag is very small—0.15 to 2 millisecond and may correspond to 350 crank angle for 2 ms at 3000 rpm. It is somewhere about 10-200 crank angle rotation but depends on the RPM. In this region the pressure produced is around 1% of max combustion pressure and volume taken by burnt portion is approximately 5% of the total volume of combustion chamber.

2. Second Phase — Flame Propagation:

In this phase the flame spreads and consumes the charge in the combustion chamber. This stage starts from point B as shown in Fig. 26.3 and ends at point C. In this stage the flame front travels with a maximum speed. This is the main stage of combustion accompanied by maximum release of energy. This is a primarily me­chanical stage and is quite simple.

The starting point of second stage is where ignition lag ends at B and appreciable rise of pressure can be seen on the indicator diagram. It ends at point C where the maximum pressure is attained. As in this stage the flame front moving at high velocity keeps the pressure and tempera­ture rising till they reach maximum values point C. The burnt and un-burnt mixtures are compressed continu­ously with accompanying temperature rise.

The flame front travels in a spherical shape. The main cause of high-speed travel of flame is turbulence in the engine. In this zone the reaction rate plays only a small role. Turbulence would depend on the velocity of the mixture entering the cylinder during intake, shape of the piston and cylinder head etc. The turbulence and hence the flame speed can be increased by adjusting these factors.

3. Third Phase — After Burning:

Point C marks the completion of the second stage and represents the end of flame travel. However the energy continues to be released till point d although the burning has ceased. The reaction of gases continues with the release of additional energy. The heat of the fuel has been liberated because even after the passage of flame the reaction continues during this stage of ‘After Burning’. Around 10% to 12% of heat may be evolved in this stage.

5. What are the Drawbacks (Disadvantages) of Conventional Ignition Systems?

Following are the drawbacks of conventional ignition systems:

(i) Because of arcing, pitting of contact breaker point and which will lead to regular maintenance problems.

(ii) Poor starting- After few thousands of kilometres of running, the timing becomes inaccurate, which results into poor starting (Starting trouble).

(iii) At very high engine speed, performance is poor because of inertia effects of the moving parts in the system.

(iv) Sometimes it is not possible to produce spark properly in fouled spark plugs. In order to overcome these drawbacks Electronic Ignition system is used.

6. What are the Advantages of Electronic Ignition System?

Following are the advantages of electronic ignition system:

1. Moving parts are absent—so no maintenance

2. Contact breaker points are absent—so no arcing

3. Spark plug life increases by 50% and they can be used for about 60000 km without any problem.

4. Better combustion in combustion chamber, about 90-95% of air fuel mixture is burnt compared with 70-75% with conventional ignition system.

5. More power output.

6. More fuel efficiency.

7. Name the Important Types of Superchargers Used in Engines.

Following are the important types of superchargers which are generally used for supercharging:

1. Reciprocating Compressors:

Reciprocating air compressor takes air from the atmosphere during the suction stroke and it will be delivered to the engine after increasing the pressure. This type of reciprocating air compressors are very rarely used because of their heavy weight and more bulk. However they are used with some stationary engines.

2. Vane Blower:

It consists of a rotating disc with radial slots into which vanes can slide. The disc will be rotating eccentrically in the casing. When the disc is rotated, the vanes will move outwards and press against the casing due to centrifugal action. And the air entering from the atmosphere will be trapped between the vanes and casing and will be supplied to the engine after increasing the pressure.

3. Roots Blower:

It essentially consists of two intermeshing lobes rotating in the casing. Air from the atmosphere enters at the inlet and the pressure of air increases because of squeezing action of lobes rotating in the casing. And the high pressure air is supplied from the delivery end.

4. Screw Compressor:

It has helical screws that match and mesh with the helix of the screw. The air is taken in from one side. The entrapped air progressively passes through the narrow passage ways formed by the locks and cover.

When the entrapped air reaches the wall it gets compressed and is taken out of the outlet valve. This process happens in a fraction of a second because the screw is rotated at a very high speed.

5. Centrifugal Compressors:

It consists of an impeller rotating in the casing as shown. In this case air enters axially at the centre or at the hub of impeller and will be turned through right angles because of blades of impeller. And in doing so it imparts high velocity to the air because of centrifugal action. The pressure of this high velocity air is increased as it flows through the diffuser and this high pressure air is supplied to the engine.

If the centrifugal compressors are driven by the exhaust gas driven turbines, then they are known as Turbochargers. Nowadays these are most commonly used for supercharging the engines. These turbochargers utilise the energy of exhaust gases for supercharging purpose.

8. What are the Methods of Refrigeration?

There are different methods of refrigeration.

They are:

1. Refrigeration by non-cyclic methods-

(a) Refrigeration by using ice or snow

(b) Refrigeration by evaporation

2. Refrigeration by cyclic methods or by means of mechanical or artificial systems as-

(a) By using non-condensable gases such as air, CO2 and SO2

(b) By using condensable vapours-

(i) Vapour compression refrigeration system

(ii) Vapour absorption refrigeration system

3. Other methods of refrigeration-

(a) Magnetic refrigeration system

(b) Vortex tube system

(c) Thermo-electric refrigeration system

(d) Steam-jet refrigeration system

(e) Ultra-sound refrigeration.

9. What are the Advantages, Disadvantages and Uses of NC Machines?

Advantages of NC Machines:

Advantages of NC machine are as below:

1. Increased productivity

2. Fewer rejections

3. Job accuracy

4. Lower tooling cost

5. Easy design changes

6. Less number of jigs and fixtures needed

Disadvantages of NC Machines:

1. Higher initial investment

2. Higher maintenance cost

3. Need for having programmer

Uses of NC Machines:

It is-possible to have NC and CNC machines from simple single spindle drilling to complex machining centers like:

(a) Metal Cutting Operations:

Turning, drilling, boring, grinding, milling etc.

(b) Production Job:

Where geometry is complex and need many operations, works where high accuracy is needed.

10. What are the Effects of Operating Variables on Thermal Efficiency of Gas Turbines?

The thermal efficiency of the Gas-turbine plant taking into consideration the compressor and turbine efficiencies, will be given by an expression given below-

Therefore, the thermal efficiency of an actual simple cycle gas turbine depends upon the following operating variables:

(a) Pressure ratio rp= P2/P1

(b) Turbine isentropic efficiency ηt

(c) Compressor isentropic efficiency ηc

(d) Turbine inlet temperature T3

(e) Compressor inlet temperature T1

(f) Specific fuel consumption

1. Effect of Variation of Turbine Inlet Temperature and Pressure Ratio:

Figure 34.19 shows the variation of turbine inlet temperature T3 and pressure ratio rp = P2/P1.

As the turbine inlet temperature T3 increases, the thermal efficiency increases with other variables being held constant. For each value of turbine inlet temperature there is an optimum pressure ratio for maximum thermal efficiency. For lower value of turbine inlet temperature the thermal efficiency first increases and after reaching a maximum value it decreases rapidly.

For higher values of turbine inlet temperature the peaks of the curves are flatter giving a greater range of optimum value of pressure ratio. The turbine inlet temperature is limited by turbine blading material. Similarly specific output increases first and then decreases as the pressure ratio increases.

2. Effect of Variation of Turbine and Compressor Efficiency:

Figure 34.20 shows the effect of compressor and turbine efficiencies. From the equation for thermal efficiency of the Brayton Cycle it is very clear that the thermal efficiency is very much sensitive to the variations of turbine and compressor efficiencies. As the turbine and compressor efficiencies increase, the thermal efficiency of the cycle increases. There is an optimum pressure ratio for each set of component efficiency.

3. Effect of Variation of Compressor Inlet Temperature:

Figure 34.21 shows the effect of compressor inlet temperature. If the air inlet temperature is lowered, the thermal efficiency increases keeping the same pressure ratio. Peaks are at a higher pressure ratio and have a flatter curve giving optimum value of a pressure ratio at a greater range.

4. Effect of Specific Fuel Consumption of Thermal Efficiency:

Figure 34.22 shows for a certain turbine inlet temperature T3, the effect of regenerative, simple and complete cycle on the specific fuel consumption. It suggests that for all values of pressure ratios the fuel consumption is minimum in complete cycle i.e., cycle with regeneration, reheating and intercooling.

11. Compare between Steam-Turbine and Steam Engine.

Steam turbines are compared with steam engines as given below:

1. In steam engine, reciprocating motion has to be converted into rotary motion by connecting rod and crank but in steam turbine direct rotational motion is available. Steam engines are inherently low speed engines because of the balancing problems. As against this, steam turbines being rotary ones there are no balancing problems.

2. It is difficult to couple steam-engine to electric generator. Steam turbines are most suitable to be coupled to electric generators so turbines are used for power generation.

3. In steam-engine, steam gets mixed with oil, so condensate is not suitable as feed-water. For steam turbine, internal lubrication is not required so steam is free from oil and condensate can be used as boiler feed.

4. The wear and tear and maintenance cost of steam engine is more than turbine.

5. It is difficult to design large units for steam-engine. But for steam-turbines large units (250 MW to 500 MW) are feasible.

6. The thermal efficiency of steam-turbine is more than that of steam engine.

12. Name the Types of Boilers Used.

Boilers are broadly of the following types:

1. Utility or Power Generation Boilers:

(a) Subcritical boilers – They operate at a pressure below critical pressure of steam i.e., 221.2 bar. They usually operate in a pressure range of 130 – 180 bar and with a maximum steam temperature of 540°C.

(b) Supercritical boilers – They operate at a pressure above critical pressure of steam. These usually operate in the pressure range of 240-300 bar and temperature 600°C. The steam generation capacity of modern boilers range from 500 T/hr to 5000 T/hr.

2. Industrial Boilers:

They are relatively low pressure, low capacity boilers which operate at a pressure range of a few bar to 100 bar and capacities ranging from few tons to 500 T/hr.

The steam is either used for power generation on small scale or for process heating in chemical, textile industries and in refineries or for heating purpose.

3. Marine Boilers:

They are used on ships to drive turbines. Their pressure range is normally 60-100 bar and temperature 550°C.

4. Waste Heat Recovery Boilers:

These are used both in industries and in power plants. They use waste heat such as from a blast furnace, exhaust from the gas turbine, or waste heat from industrial and chemical processes. The waste heat is passed over a heat exchanger surface to produce steam or hot water for the required use.

13. How are Heat Exchangers Classified?

A heat exchanger is an apparatus in which the processes of heating or cooling occur i.e., heat is transferred from one substance to another. There is a large number of different heat exchanger or heat exchange apparatuses varying both in application and design.

In respect to the principle of operation, heat exchangers may be divided in three categories as follows:

1. Direct Contact Heat Exchanger:

The simplest type of heat exchanger is a container in which a hot and a cold fluids are mixed directly. In such a system both fluids will reach the same final temperature and the amount of heat transferred can be estimated by equating the energy lost by the hotter fluid to the energy gained by the hotter fluid to the energy gained by the cooler one. Open feed water heaters, desuperheaters, water cooling towers, scrubbers and jet condensers are the examples of heat transfer equipment employing direct mixing of fluids.

2. Recuperative Heat Exchangers:

In heat exchangers of the recuperative variety, hot and cold fluids flow simultaneously through the heat exchanger and the heat is transferred through a wall separating the fluids. This group unites such heat transfer equipment such as steam boilers, water heaters, condensers etc. There are many forms of such equipment ranging from a simple pipe— within a pipe with a few sq.m of heat transfer surface upto complex surface condensers and evaporators with many hundreds of sq.m of heat transfers surface.

In between these extremes is a broad field of common shell and tube exchangers. These units are widely used because they can be constructed with large heat transfer surfaces in a relatively small volume, can be fabricated from alloys to resist corrosion, and are suitable for heating, cooling, evaporating or condensing all kinds of fluids.

3. Regenerative Heat Exchanger:

A regenerative heat exchanger is an apparatus in which one and the same heating surface is alternately exposed to the hot and cold fluids. The heat carried by the hot fluid is taken away by and accumulated in the walls of the apparatus and is then transferred to the cold fluid flowing through the heat exchanger. Regenerators of open health and glass-melting furnaces, air preheaters of blast furnaces are some of the specimens of regenerative heat- exchange equipment.

The process of heat transfer in recuperative and regenerative heat exchangers is invariably bound with the surface of solid. That is why they are known as surface exchanger.

The special names given to heat exchanger are usually determined by their application and designation for example, steam boilers, furnaces, water heaters, evaporators, super-heaters, condensers etc. In spite of the great variety of shapes, layouts, principles of operation and working media, heat exchange apparatuses—ultimately serve one and the same purpose—transfer of heat from one, hot fluid to another cold fluid. The design fundamen­tals are, therefore common to all.

The complete design of a heat exchanger can be broken down into three major phases:

1. The thermal analysis

2. The preliminary mechanical design

3. Design for manufacture.

Here the emphasis will be on the thermal design. This phase of the design is primarily concerned with the determination of the heat-transfer surface area required to transfer heat at a specified rate for given flow rates and temperatures of the fluids.

The mechanical design involves considerations of the operating temperatures and pressures, the corrosive char­acteristics of one or both fluid, the relative thermal expansions and accompanying thermal stresses, and the relation of the heat exchanger to other equipment concerned.

The design for manufacture requires the translation of the physical characteristics and dimensions into a unit, which can be built at a low cost. Selection of materials, seals, enclosure, and the optimum mechanical arrangement have to be made and the manufacturing procedures must be specified.

14. What are the Applications of Air Conditioning?

Air conditioning is basically divided in two types:

1. Comfort air conditioning

2. Industrial air conditioning

We will enumerate the applications accordingly.

1. Comfort Air Conditioning:

Following are the applications for comfort air conditioning:

1. Comfort air conditioning had its first major use in motion picture theaters (early 1920s).

2. Multiform office buildings

3. At the end of 1920s came the introduction of the first self-contained room air conditioners

4. Apartments

5. Hotels, restaurants and night clubs

6. Hospitals

7. Departmental stores

8. Drug stores

9. Dress shops

10. Grocery shops

11. Banks

12. Dance halls and skating rinks

13. Transportation: trains, air planes, buses, automobiles, trolleys and many more applications can be listed.

2. Industrial Air Conditioning Applications:

In this case, human comfort element is not considered.

Following are some of the applications of industrial air conditioning:

1. Textile industry

2. Printing industry

3. Drugs and chemicals

4. Libraries and museums

5. Metal working

6. Laboratories and many more.

15. What are the Disadvantages of Air Washers in Comparison with Cooling Coil?

In comparison with the cooling coil, the air washer has a number of disadvantages:

(a) It is more bulky.

(b) Cost wise it is more expensive as far as maintenance is concerned.

(c) Water is required in greater quantity.

(d) Corrosion is a greater risk.

(e) Rusting, deposition of scales in the chilling plant is also a great problem. Because of rusting and deposition of scales, heat trouser efficiency is greatly affected and cost of refrigeration plant is increased.

The air washers allow humidification of air and therefore we can think of steam injection for humidification and it is safe.