Compilation of lab viva questions and answers on thermal engineering for engineering students.
1. What are the Units of Measurement Used in Thermodynamics?
Nowadays generally SI (System International) system of units is used.
The basic units in this system are:
Basic Units:
The dimensions of all other quantities are derived from these basic units which are:
A proper understanding of units of Force, Pressure, Temperature, Work, Heat and Power is necessary for solving numerical problems in Thermal Engineering.
1. Force:
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The unit of force in SI units is Newtons (N). A force of 1N produces an acceleration of 1 m/sec2 when applied to a mass of 1 kg.
If the mass of 1 kg is allowed to fall freely under the action of standard gravitational force, it is accelerated at the rate of 9.806 m/sec2 (9.81 m/sec2) and we have,
Force = 1 kg x 9.81 m/sec2 = 9.81 N.
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∴ It follows that the weight of 1 kg mass equals 9.81 N and since the weight is the force,
1 kgf = 9.81 N
2. Weight:
Weight of a body is the force exerted on its mass due to gravity.
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3. Density:
Density is defined as mass per unit volume of the substance. It is denoted by ρ
ρ = mass/volume = kg/m3
Since unit of volume is m3 and mass is kg.
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4. Work (Mechanical):
When a force F acts on a body and causes displacement through a distance dl in the direction of force, then the work is said to be done. This work is equal to the product of force and distance moved.
W = F x dl
If force F is in Newton and distance dl is in metre, then the resultant unit of W.D. will be Nm and 1 Nm = 1 joule.
5. Power:
It is defined as the rate of doing work.
or Power = W.D/Time taken
If the unit of W.D. is in joules and the time taken is in seconds, then the unit of Power is J/sec.
And the rate of doing work of 1 J/sec is called watt (W)
∴ Power = J/sec or W
Earlier there used to be another unit of Power known as Horse Power (H.P.)
1 HP = 746 watts = 0.746 kW
6. Pressure (P):
Pressure is defined as the force per unit area. Thus if F is the force applied to an area A then,
P = F/A and if F is in newton and A is in m2 then the unit will be N/m2
This unit of pressure is called as Pascal (Pa).
2. Name Few Mountings and Accessories of a Boiler.
Mountings:
Mountings are the fittings mounted on the pressure part of the boiler. Mountings are necessary for the safe and efficient operation of the boiler. Without mountings boiler cannot work safely. Special provisions are made on the pressure part to mount the mountings.
The term mountings refers to the items such as –
(1) Safety valves –
(a) Dead weight safety valve
(b) Spring loaded safety valve
(c) High steam and low water alarm.
(2) Water level indicators
(3) Fusible plug
(4) Pressure gauges
(5) Steam stop valve
(6) Feed check valve
(7) Blow-off valve etc.
The above mountings are usually installed in accordance with IBR.
Accessories:
Accessories are used to improve the efficiency of the boiler plant. Without accessories boiler can work safely. These are not mounted on the boiler, but these are connected in the boiler circuit.
Usually following accessories are provided with the boiler:
1. Superheater
2. Economiser
3. Air-preheater
4. Steam injector
5. Feed pump
6. Steam trap
7. Induced draught fan
8. Forced draught fan etc.
3. Why is a Rope Brake Dynamometer Used?
It is generally used for the determination of brake power in the laboratories, since it is easy to fabricate and cheap.
It consists of a belt or rope wrapped round the flywheel of an engine of whose Brake power is to be measured. The upper end is connected to the spring balance suspended from overhead and the lower end carries the load W.
4. What are the Components of Water Cooling System in CI Engines?
Water Cooling System mainly consists of:
1. Radiator:
It mainly consists of a upper tank and lower tank and between them is a core. The upper tank is connected to the water outlets from the engines jackets by a hose pipe and the lover tank is connect to the jacket inlet through water pump by means of hose pipes.
There are 2-types of cores:
(a) Tubular
(b) Cellular as shown.
When the water is flowing down through the radiator core, it is cooled partially by the fan which blows air and partially by the air flow developed by the forward motion of the vehicle.
It is to be noted that radiators are generally made out of copper and brass and their joints are made by soldering.
2. Thermostat Valve:
It is a valve which prevents flow of water from the engine to radiator, so that engine readily reaches to its maximum efficient operating temperature. After attaining maximum efficient operating temperature, it automatically begins functioning. Generally it prevents the water below 70°C.
Figure 30.7 shows the Bellow type thermostat valve which is generally used. It contains a bronze bellow containing liquid alcohol. Bellow is connected to the butterfly valve disc through the link.
When the temperature of water increases, the liquid alcohol evaporates and the bellow expands and in turn opens the butterfly valve, and allows hot water to the radiator, where it is cooled.
3. Water Pump:
It is used to pump the circulating water. Impeller type pump will be mounted at the front end. Pump consists of an impeller mounted on a shaft and enclosed in the pump casing. The pump casing has inlet and outlet openings.
The pump is driven by means of engine output shaft only through belts. When it is driven water will be pumped.
4. Fan:
It is driven by the engine output shaft through same belt that drives the pump. It is provided behind the radiator and it blows air over the radiator for cooling purpose.
5. Water Jackets:
Cooling water jackets are provided around the cylinder, cylinder head, valve seats and any hot parts which are to be cooled. Heat generated in the engine cylinder, conducted through the cylinder walls to the jackets. The water flowing through the jackets absorbs this heat and gets hot. This hot water will then be cooled in the radiator.
6. Antifreeze Mixtures:
In Western countries if the water used in the radiator freezes because of cold climates, then ice formed has more volume and produces cracks in the cylinder blocks, pipes, radiator. So, to prevent freezing antifreeze mixtures or solutions are added in the cooling water.
The ideal antifreeze solutions should have the following properties:
1. It should dissolve in water easily.
2. It should not evaporate.
3. It should not deposit any foreign matter in cooling system.
4. It should not have any harmful effect on any part of cooling system.
5. It should be cheap and easily available.
6. It should not corrode the system.
No single antifreeze satisfies all the requirements.
Normally following are used as antifreeze solutions:
(a) Methyl, ethyl and isopropyl alcohols
(b) A solution of alcohol and water
(c) Ethylene Glycol
(d) A solution of water and Ethylene Glycol
(e) Glycerin along with water etc.
5. What are the Components of NC Machine?
Main components of an NC machine are as follows:
(a) Programme of Instruction:
(i) It is part programme which contains coded instructions to execute the operation in a cycle of operation. Numbers, letters and symbols logically organized to direct a machine tool to the required job are called NC programme. The instructions are in the form of numerals, letters and symbols.
(ii) Programme Media:
Punched tapes are most widely used media for programming in NC machines. Tapes are made of paper, paper-aluminum sandwich or plastic. Paper tapes are inexpensive but do not last long. The programme instructions can also be given on the magnetic tapes. Punched tapes are preferred to the magnetic tapes due to- Low cost of tape, Low cost of punching machine and the corresponding reader, Ease of detecting error or damage on a punched tape.
(b) Machine Controller Unit:
The controller unit consists of hardware and electronics to read and interpret the programme of instructions and convert it into mechanical inputs to the machine tool.
Typical elements of a controller are as below:
(i) Programme Reader:
The tape reader and winds the punched tapes.
Depending on the tape used the programme readers are:
i. Punched Tape Reader:
The readers can be electromechanical, photoelectrical or pneumatic type. Depending whether there is hole on the tape electrical signals are generated in all these readers. For example in the photovoltaic reader when light passes through the perforated tape and falls on the photocell. This is turn would give the programmed signal to operate the machine.
ii. Magnetic Tape Reader:
It consists of a magnetic head which reads as well encodes the instructions. For reading propose the tape is moved across the magnetic head. An emf would be induced in the winding which is used for controlling.
(c) Machine Controller:
The controller gets signals from the reader and controls various activities of the machine to execute the instructions. The controller consists of control panel. Decoder, feed rate generator, interpolator and auxiliary rate function.
(d) Decoder:
It converts the signals from the reader to operate or switch on a particular machine tool function. The decoders have relays to operate the switches.
(e) Buffer Storage:
If the tape speed is not sufficient it is necessary to have memory storage device for execution of the programme. Buffer storage is used to temporarily store data received from the decoder.
(f) Interpolator:
As the name suggests it is to interpolate the data when needed. Interpolation is required for computing the intermediate points of a curve while machining on NC machines.
6. What are the Various Lines Drawn on the Psychometric Chart?
The mass basis of the chart is 1 kg. dry air plus associated water vapour. The various lines drawn on the psychometric chart are discussed now with reference to Fig. 37.5.
(a) Constant Dry Bulb Temperature Lines:
All vertical lines perpendicular to the horizontal d.b.t. scale are constant temperature lines. (-5°C to 43°C). Fig. 37.5 (a).
(b) Constant Specific Humidity Lines:
All horizontal lines perpendicular to the vertical specific humidity scale are constant specific humidity lines. (kg)vapour per (kg) dry air [0 to 0.026]. Fig. 37.5(b).
(c) Saturation Curve:
The extreme left hand curve is the saturation curve which represents state of saturated air at different values of d.b.t. (DBT). It is a 100% relative humidity line or a curve. Fig. 37.5 (c).
(d) Constant R.H. Lines:
Suppose we want to construct a constant RH line of ɸ = 50%. We can select dry bulb temperatures t1, t2, t3 etc. and find out the corresponding saturation pressures ps1, ps2, ps3 etc. from the Steam Tables. Since pv = ɸ, ps = 0.5 ps, the values of pv1, pv2, pv3 etc. are readily calculated. Since w = 0.622 pv, we can find out w1, w2, w3 etc. for ɸ = 50%. The P – Pv curve passing through (t1, w1), (t2, w2), (t3, w3) is the required ɸ = 50% line or curve. Fig. 37.5 (d).
(e) Constant Enthalpy Lines:
The enthalpy lines are inclined straight lines and uniformly spaced as shown in Fig. 37.5 (e). These lines are parallel to wet bulb temperature lines and are drawn up to saturation curve. Most of these lines coincide with bulb temperature lines. Fig. 37.5 (e).
(f) Constant Wet Bulb Temperature Lines:
The wet-bulb temperature lines are inclined lines and non-uniformly spaced as shown in Fig. 37.5 (f). At any point on the saturation curve, the dry bulb and wet-bulb temperature are equal. The values of wet bulb temperature are generally given along the saturation curve of the chart Fig. 37.5 (f).
(g) Constant Specific Volume Lines:
The constant specific volume lines are inclined straight lines and are uniformly spaced as shown in Fig. 37.5 (g). These lines are drawn up to saturation curve. The values of specific volume represented by them are generally given at the base of the chart. Fig. 37.5 (g).
(h) Vapour Pressure Lines:
The vapour pressure lines are horizontal and uniformly spaced. Generally, the vapour pressure lines are not drawn in the main chart. But a scale showing vapour pressure in mm Hg is given on the extreme left side of the chart shown in Fig. 37.5 (h).
(i) Sensible Heat Factor (SHF) Scale:
The scale on the extreme right is the sensible heat factor, SHF, scale which is drawn with reference to the point shown as a dark circle at 50% RH line, (near 27° DBT). When this point is joined to the appropriate value on the SHF scale, the resulting line represents the inclination of all heating or cooling processes of moist air which are carried on according to that particular value of SHF.
The sensible heat factor (SHF) is defined as the ratio of the sensible heat (added or removed) to the total heat in the process.
7. How to Find Indicated Power (IP) of the Engine?
Generally, indicated power is calculated or determined if an indicator is connected to the engine and thereby an indicator diagram is available.
If an engine is not provided with an arrangement of connecting an indicator and only brake dynamometer is available, then we can determine an indicated power by carrying out a test on the engine. There are three methods to find an indicated power of the engine.
They are:
1. Willan’s Line Method:
The engine is tested at constant speed but at different loads and the corresponding fuel consumption is measured. From this data, the fuel consumption per hour at each load is calculated. A graph between fuel consumption per hour and Brake Power (BP) is now plotted as shown in Fig. 25.9.
The relation between BP and fuel per hour is represented approximately by a straight line indicating that BP is proportional to fuel consumption per hour.
It will be seen, with the line intersecting Y-axis at A, that some fuel is consumed even at no-load i.e., zero BP. The power developed at the piston head by the fuel consumed at zero BP is just sufficient to overcome the friction.
The friction power (FP) can thus be estimated from the constant of proportionality i.e., slope of the straight line. It has been pointed out earlier that at constant speed running, the friction power does not change with the load, but remains approximately constant.
Then the friction power FP is the negative intercept of the straight line on X-axis.
∴ Indicated Power developed at any particular load
= BP + FP.
IP = BP + FP
This test is applicable only to compression Ignition engines. (CI Engines/Diesel or oil engine)
2. Morse Test for Multi-Cylinder Petrol Engine:
This method is also based on the assumptions that the mechanical losses at constant speed are independent of load.
Requirements for the test are:
1. Multicylinder Engine
2. Constant speed at all loads
3. Throttle full – open.
Procedure:
The engine is run at constant speed during the period of test. Initially, the brake power BP developed by the engine with all its cylinders firing or working is measured with the help of dynamometer. Now, No. 1 cylinder of the engine is cut out or the spart plug is short circuited so that no ignition takes place in that cylinder and consequently the engine speed falls off. The engine speed is brought back to its original value by adjusting the load on the engine. The (BP), now developed by the engine is less than that developed with all the cylinders firing and the difference (BP – BP1) is equal to the IP of the cylinder No. 1. The same procedure is repeated with the remaining cylinders to get the IP of the engine.
Let there be 4-cylinders numbered 1, 2, 3 and 4. Knowing IP = BP + FP.
∴ When all cylinders are working, we have
(IP1 + IP2 + IP3 + IP4) – (FP1 + FP2 + FP3 + FP4) = BP1 + BP2 + BP3 + BP4 = BP …(i)
With cylinder No. 1, cut out we have
(IP2 + IP3 + IP4) – (FP1 + FP2 + FP3 + FP4) = BP1 …(ii)
∴ Equation (i) – (ii),
IP1 = BP – BP1 …(iii)
In the same way, IP2, IP3 and IP4 can be found.
and Mech. Eff. = IP/BP
where IP = IP1 + IP2 + IP3 + IP4
3. Motoring Test:
Engine is run at no load and constant speed to get the steady-state conditions—particularly all temperatures (cooling water outlet temperature, lubrication oil temperature etc.). Once this state is attained the engine is stopped. The engine is connected to an electric motor having either watt-meter (energy meter) or Voltmeter and Ammeter.
The engine is run by electric motor at the same conditions as before and again the same steady state conditions. The energy supplied by motor to run the engine is the energy or power required to overcome the friction power. Thus friction power at particular speed of the engine is obtained. After friction power is obtained, IP can be calculated and hence the mechanical efficiency is calculated.
8. Explain Joule’s Experiment on Thermodynamics.
Joule carried out experiments in 1843, which led to the formulation of the First Law of Thermodynamics. He took a known quantity of water in a rigid vessel, which was insulated adiabatically from the surroundings. The vessel was fitted with a paddle wheel and a thermometer.
Now let a certain amount of work W1-2 be done upon the closed system by the paddle wheel. The quantity of work can be measured by the fall of weight W which drives the paddle wheel.
Let t1 be the initial temperature of water before work transfer and after work transfer let the temperature rise to t2. So, the system is changing its state from state (1) to state (2) and the process 1-2 undergone by the system.
Now let the insulation be removed. Then the system will interact by heat transfer (i.e., heat will be dissipated from the system to the surroundings) till the system comes to original temperature t1.The amount of heat transfer Q2- 1 from the system during the process 2-1 can be estimated – (Q2- 1 = mCp ΔT).
The system thus undergoes a cycle, which consists of a definite amount of work input W1-2 to the system, followed by a heat transfer Q2- 1 from the system.
Joule, repeated this experiment for different weights and different heights and in each case he found that, network transfer during a cycle is proportional to net heat transfer,
i.e., ∑ δW ∝ ∑ δQ
Or in other words when a closed system undergoes any cyclic process, the cyclic integral of work is proportional to the cyclic integral of heat. This is known as the first law of thermodynamics for a closed system undergoing a cycle.
Thus, the I-Law states the general principle of conservation of energy i.e., Energy can neither be created nor be destroyed, but energy can be converted from one form to the other.
9. How are Air Conditioning Systems Classified?
Air conditioning systems can be classified in a number of ways such as:
I. The Cooling or Heating Fluid That is Used:
Under this category, there are three possible groups in regard to the fluid used:
(a) All Air Systems:
These systems use only air for cooling and heating.
(b) All Water (Hydronic) Systems:
These systems use only water for cooling and heating.
(c) Air-Water Combination Systems:
These systems use both water and air for cooling and heating.
II. Unitary and Central Systems:
(a) A unitary system uses packaged equipment. This is most, if not all, of the system components (fan, coils, refrigeration equipment) are furnished as an assembled package from the manufacturer.
(b) A central or built system is one whose components are furnished separately and installed and assembled by the contractor.
III. Single-Zone or Multiple Zone Systems.