Thermal engineering question bank with answers! This will help you to learn about the frequently asked questions on thermal engineering.
1. What is Microscopic and Macroscopic Approach of Thermodynamic Study?
There are two points of views:
1. Microscopic or Statistical Thermodynamics.
2. Macroscopic or Classical Thermodynamics, from which the behaviour of matter or working of a system can be studied.
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From microscopic point of view, it is considered that, the matter is not continuous, but it is made up of a large number of identical particles called molecules. For example- consider a gas in a cylinder as a system. Then this gas is made up of number of molecules.
Each molecule of a gas, at a given instant, has certain position, energy, etc., and for each molecule these changes very rapidly because of the collisions (striking’s). The behaviour of the gas is described by summing up the behaviours of each molecule. And for summing up the behaviour of molecules statistical methods are employed and hence it is also called as statistical thermodynamics.
In macroscopic point of view a certain quantity of matter is considered, and the events that are taking place at the molecular level are not taken into account. For example- let us consider a system of an IC engine consisting of a charge in the engine cylinder. At any instant the system has certain volume depending upon the position of the piston, this volume is easily measurable.
Another quantity to describe the system is the pressure of the gas inside the cylinder. A pressure gauge can be used to measure the same. Similarly temperature, chemical composition etc. may be described. Thus, in macroscopic point of view the system will be described by large scale properties.
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Though the approach based on microscopic point of view seems to be different, from that based on macroscopic point of view, but there exists a relationship between them. The relationship between macroscopic and microscopic point of view lies in the fact that the macroscopic properties are in fact the average properties of a large number of microscopic characteristics.
Hence, when both the methods are applied to a particular system, they give the same result.
2. What are the Instruments Used to Measure Pressure?
1. Barometer:
It is used to measure atmospheric pressure. Consider Hg in the container as shown in Fig. 1.31 (a) and (b). First of all, let a glass tube open at both the ends is immersed in the Hg. Then the atmospheric pressure acting on the surface of Hg level is same as shown.
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Now if the glass tube with one end sealed and evacuated to a state of vacuum is immersed then, due to atmospheric pressure acting on the surface of Hg, the Hg will rise in the glass tube and this will be equal to 760 mm at sea level.
This is the most common type of pressure gauge. This is used to measure the pressure inside the pipes, containers, vessels etc.
The basic element of this gauge is the tube A. It is elliptical in cross section and is bent into an arc of circle as shown in Fig. 1.32. End B of this tube is sealed, whereas open end C is connected to the connecting union D through which it can be mounted over the pipes or containers of whose pressures are to be measured.
If the pressure of the system is more than the atmosphere (i.e., +ve pressure) then the tube will tend to curl out. Conversely if the pressure of the system is less than atmosphere (i.e., –ve pressure) then the tube will tend to curl in. The change in pressure will therefore appear as the movement of the end B. This end B is connected to the quadrant gear F through the link E. The quadrant gear F engages with die small gear G, on to which a pointer H is attached.
Thus any movement due to change in pressure of the end B will be transmitted to the quadrant gear which will rotate the gear G and hence the pointer. The pointer rotates on the calibrated scale and thus gives the pressure readings directly.
These are used to measure the pressure inside the pipes, containers. It consists of a bent glass tube or a U-type glass tube which contains a liquid generally H2O or Hg. One end of the manometer is connected to the container or pipe of whose pressure measurement is to be done and the other end is generally open to the atmosphere. The U-tube with one end closed and evacuated to state of vacuum indicates the absolute pressure directly as shown in Fig. 1.33(c).
3. Explain the Changes in Form of Matter with a Suitable Diagram.
There are three forms of matter, viz.:
(i) Solid,
(ii) Liquid and
(iii) Gaseous.
In order to explain the changes in form of matter, a hypothetical experiment may be described with the help of Fig. 10.1.
As substance in the solid state is placed in a cylinder, which is fitted with a frictionless right fitting piston being loaded by a weight as shown in Fig. 10.1. Heat is supplied to the cylinder. The substance may change in volume during the heating process but the pressure is kept constant by the piston and weights on it. The change in temperature of the substance will be recorded by a thermometer.
At first, when the heat is added, the temperature of the solid rises and a certain value of temperature will be reached after which a considerable amount of heat is added, without a change in temperature. Through a sight window provided in the cylinder wall it can be seen that the solid is melting. When the solid has been completely transformed into liquid, further addition of heat again causes a rise in temperature.
This rise in temperature continues till a point is reached where further addition of heat does not change the temperature of the substance but on observation we see that the liquid is boiling. When the liquid has been completely transformed into a vapour, the temperature again rises when the heat is added. The relation between the heat added, entropy change and the temperature of the substance is shown in Fig. 10.2.
1 – 2 Shows the stage during which the rise in temperature of a substance in solid state takes place.
2 – 3 Shows the stage during which the substance is being transformed from a solid state to a liquid state without rise in temperature.
3 – 4 Shows the stage during which rise in temperature of the substance in liquid state takes place.
4 – 5 Shows the stage during which the substance is being transformed from a liquid state to a vapour state without rise in temperature.
5 – 6 Shows the stage, during which rise in temperature of the substance may take place in a vapour state.
If the hot vapour is cooled the curve will be exactly retraced with all changes taking place in reverse manner. If the weight W is increased and the experiment is repeated the same general behaviour is obtained but the temperature will be higher than that in the first case. If the weight is reduced still the behaviour will be the same but temperature will be lower than that in first case.
The above hypothetical experiment brings out certain important facts:
(i) When a solid melts the temperature remains constant and is always the same for a given substance at a given pressure. When a liquid freezes the temperature remains constant. This temperature is called the freezing point. The quantity of heat removed or added during the change of state is constant and is known as latent heat of fusion. The latent heat of fusion depends upon pressure.
(ii) If heating of water is continued after total melting, temperature increases till it starts boiling. The amount of heat added to one kg of water from freezing point to boiling point is called sensible heat.
(iii) When the liquid boils the temperature remains constant and is always the same for a given substance at a given pressure. If the pressure is increased, the temperature at which the change of state takes place also increases. Similarly when the vapour condenses the temperature also remains constant.
This temperature is called the boiling point or saturated temperature of the liquid. The definite quantity of heat removed or added during the change of state is called the latent heat of vaporisation. The latent heat of vaporisation depends upon pressure.
The freezing point, the latent heat of fusion, the boiling point and the latent heat of vaporisation vary widely with different substances.
When the pressure is very low, an interesting phenomenon is noticed. A solid when heated directly transforms into a vapour without passing through an intermediate liquid state. Such a phenomenon is known as sublimation. A familiar example is the conversion of solid CO2 (dry ice) directly into vapour. Figure 10.3 shows a temperature pressure curve.
In practice, steam is generated in a device called steam boiler.
4. Explain Fourier’s Law of Conduction.
The basic relation for heat transfer by conduction was proposed by French scientist JBJ Fourier in 1882.
It states that qk the rate of heat flow by conduction in a material, is equal to the product of the following three quantities:
(i) K- the thermal conductivity of the material
(ii) A – the area of the section through which heat flows by conduction and
(iii) dT/dx – the temperature gradient at the section i.e., the rate of change of temperature r with respect to distance in the direction of heat flow or
Composite Walls:
A composite wall, typical of the type used in a large furnace, is shown in Fig. 17.3. The inner layer, which is exposed to the high temperature gases, is made of firebrick. The intermediate layer consists of an insulating brick and is followed by an outer layer of ordinary red bricks.
The temperature of the hot gases is Ti and the unit surface conductance over the interior surface is h̅i. The atmosphere surrounding the furnace is at a temperature t0 and the unit surface conductance over the exterior surface is h̅0.
Under these conditions there will be a continuous heat flow from the hot gases through the walls to the surroundings. Since the heat flow through a given area A is the same for any section, we obtain
Conduction of Heat through Hollow Cylinder:
The heat flow through a cylinder is considered along radial direction. Therefore the Fourier equation of conduction for cylinder can be written as-
Figure 17.4 (a) shows hollow pipe having inside radius R1 and outside radius R2. Let the inside temperature be T1 and outside temperature T2. The conductivity of the pipe material is say K.
Consider an elementary ring at a radius of r and having a thickness dr as shown in Fig. 17.4(a). The heat flow equation through an element dr under steady state condition can be written as-
Where L is the length of pipe and A = 2πrL. Integrating this equation between the limits of R1 and R2 and T1 to T2 as the temperature changes from T1 and T2 through the thickness of the cylinder (R2 – R1),
5. Explain the Laws of Thermal Radiation.
It will be recalled that conduction and convection heat transfer depend on the existence of a medium to convey heat. The third mode of heat transfer, radiation, differs from the other two in that a medium is not required. Radiation is the term applied to the process by which energy is continually emitted from the surfaces of all bodies.
This energy is called radiant energy and is in the form of electromagnetic waves. These waves travel with the velocity of light and are transmitted through a vacuum as well as through substances which are transparent to them. In fact, transmission through a vacuum is better, since intervening media absorb some, if not all, of the radiant energy.
When the electromagnetic waves fall on the body which is not completely transparent to them, they are absorbed and there energy is converted to heat. The process by which radiant energy is converted to heat is not completely understood. However, it is known that all substances emit and absorb radiant energy at a rate which depends on the absolute temperature and physical properties of the substance.
The waves incident upon the surface of the substance may be partly absorbed, partly reflected and partly transmitted through the substance. The fraction of the radiant energy which is absorbed is called absorptivity (α). The fraction reflected is termed the reflectivity ρ and the fraction transmitted through the substance is known as the transmissivity τ. The relationship between the absorptivity, reflectivity and transmissivity is written as-
α + ρ + τ = 1
When the transmissivity is zero, as it is for most solids opaque to light, the substance is said to be opaque to radiation. Conversely, if transmissivity is unit, the substance is transparent to radiation. No substance is perfectly transparent but the less dense fluids, such as gases, have a high transmissivity.
An ideal reflector is a body whose surface reflects all radiant energy incident upon it. Highly polished surfaces are good approximations to an ideal reflector. An ideal absorber absorbs all radiant energy incident upon its surface and hence its absorptivity is unity.
Only the radiant energy absorbed can contribute to increasing the internal energy of the substance, and since the rate at which radiant energy is emitted depends on the temperature of the emitter, a good absorbed is also a good emitter. Therefore, we define an ideal radiator as a body which absorbs all radiant energy incident upon its surface. We see that the characteristics of an ideal radiator are an absorptivity of unity, and reflectivity and transmissivity of zero.
The rate at which energy is radiated at a wavelength λ, per unit area of surface is called monochromatic emissive power, Eλ. The total emissive power, E, is the energy radiated per unit time and per unit area of surface at all wavelengths.
The total emissive power of an ideal radiator may be found by integrating the monochromatic emissive power over all wavelengths.
Kirchhoff’s Radiation Law:
No true ideal radiator exists in nature, although some substances, such as lampblack and platinum black, are good approximations. Kirchhoff conceived of a method of producing close approximations to an ideal radiator through the use of a hollow enclosure.
A small hole is cut in a large hollow enclosure, such as a sphere, and the radiant energy incident upon this hole is partially absorbed by the inside surface of the enclosure. The rest of the radiant energy is diffused within the enclosure. The radiant energy entering through the hole continues to be absorbed reflected and reradiated, but only a very small amount finds its way out through the hole. Hence, the hole behaves very much like an ideal radiator.
Stefan Boltzmann’s Law of Radiation:
The quantity of energy leaving a surface as radiant heat depends upon the absolute temperature and the nature of the surface. A perfect radiator or black-body emits radiant energy from its surface at a rate qr given by –
qr = σA1T14 kJ/hr
Where A1 is the surface area in sq.m. T1 is the surface temperature in degrees Kelvin and σ is a dimensional constant and is known as Stefan-Boltzmann constant after two Austrian scientists, J. Stefan who in 1879 found the above equation experimentally and L, Boltzmann, who in 1884 derived it theoretically. Here 6 = 20.42 x 10-8 kJ/hr. m2 T4 = 5.671 W/m2.T4.
Stefan-Boltzmann Law of radiation states that any black-body surface above a temperature of absolute zero radiates heat at a rate proportional to the fourth power of the absolute temperature.
While the rate of emission is independent of the conditions of the surroundings, a net transfer of radiant heat requires a difference in the surface temperature of any two bodies between which the exchange is taking place. If the black-body radiates to an enclosure which completely surrounds it and whose surface is also black i.e. absorbs all the radiant energy incident upon it, the net rate of radiant heat transfer is given by-
qr = σA1 (T14 – T24 )
where T2 is the surface temperature of the enclosure in K.
Real bodies do not meet the specifications of an ideal radiator but emit radiation at a lower rate than the black- bodies. If they emit, at a temperature equal to that of a black-body, a constant fraction of the black-body emission at each wavelength, they are called Gray Bodies. The net rate of heat transfer from a gray body at T1 to a black surrounding body at T2 is-
qr = σA1ε1 (T14 – T24)
Where ε is the emissivity of the gray surface and is equal to the ratio of emission from the gray surface to the emission from a perfect radiator at the same temperature.
If neither of the two bodies is a perfect radiator and if two bodies possess a given geometrical relationships to each other, the net heat transfer by radiation between them is given by-
qr = σA1F1-2 (T14 – T24)
Where F1-2 is a modulus which modifies the equation for perfect radiators to account for the emissivities and relative geometries of the actual bodies.
6. Derive an Expression for Film Coefficient of Heat Transfer.
In heat exchangers, we are mostly concerned with walls separating liquids or gases from each other. In these cases we do not know the temperatures of both surfaces of the separating walls, but only the temperatures of the liquids or fluids on both sides of the walls. These temperatures are indicated as t1 and t2 as shown in Fig. 17.5. By measuring the temperature field in the liquids one obtains the curves shown.
The temperature gradient is confined to a relatively narrow layer of thickness δ quite close to the wall, whereas at a greater distance from the wall in most cases only small temperature differences exist. Simplified, the temperature curve can be replaced by the dashed broken line. This can be explained by assuming that a thin film of liquid (of the thickness δ’) adheres to (i) the wall whereas outside thin film, temperature differences vanish as a result of mixing motions of the liquid.
This picture over simplifies the actual process considerably, but it brings out the salient features. Within the film, the heat transfer takes place by conduction, as in a solid wall. The temperature in the film, therefore, is again linear, and the flow of heat follows the equation-
into which the thermal conductivity K of the liquid or of the gas and the thickness of the film δ’ are to be inserted.
From this equation the rate of heat flow Q can be calculated if the film thickness δ’ is known. The latter, however, depends to a great extent upon the external flow conditions, for instance upon the velocity with which the liquid flows along the wall, upon the shape of the wall, upon the structure of the wall surface and similar factors. In engineering practice it has become customary to calculate with the expression K/δ, rather (2) than directly with the film thickness δ’. This value is called film heat transfer coefficient and is designated by the letter h.
Thus the expression-
Q = hA (t – tw)
as formulated by Issac Newton is obtained. The value (1/hA) is called thermal resistance Rt, of the film heat transfer process-
7. Explain the Working of Air Pumps.
Air pump is a device which is used to remove air from the condenser and to maintain necessary vacuum in the condenser.
Air pumps are of two types:
(i) Dry air pump – Which removes only cold air
(ii) Wet air pump – It is used to remove cold air and condensate.
These pumps may be reciprocating or rotary type. We shall discuss now a very common form of reciprocating air pump called Edward’s air pump.
As shown in Fig. 21.12, it consists of head or delivery valves, one of them is shown at 1. These valves are provided in the cover 2, which on the top of the pump barrel 3. The piston 4 has the conical bottom 5 and the bottom of the pump casing also follows the conical shape as that of piston. Ports 6 are provided in the pump barrel near its bottom end. Connection with the condenser is made at 7.
During the downward stroke of the piston, a vacuum is produced in the barrel above the piston, the head valves being closed and as soon as the piston begins to uncover the ports 6, the air and water vapour from the condenser rush into the space above the piston. The condensate flows down in the pump casing. The conical end of downward moving piston displaces the condensate to the top of piston till the end of downward stroke.
Now during the upward stroke of the piston, some of the water flows down in the pump casing through the ports till the piston closes the ports. When the piston closes the ports the pressure of the charge above the piston will go on increasing and the charge of air, water vapour and condensate will be expelled through the head valves and then over the weir 8 on its way to the hot well. The weir ensures a head of water over the valves, so that if there is any leakage through these valves into the barrel, then it will be a leakage of water and not air.
The door 9 gives access to the head valves and 10 is the relief valve which opens to atmosphere, to release the pressure in case the pressure inside increase above the atmospheric pressure.
8. Describe about the Combustion Chambers in SI Engines with Diagram.
Different types of SI engine combustion chambers have been shown in Figs 26.7 and 26.8.
1. T-Head Combustion Chamber:
In this design the valves are on the opposite side as shown in Fig. 26.7 (a). This combustion chamber is not in much use as compression ratio cannot exceed a specified limit due to fact that this chamber facilitates large flame travel. This type of combustion chamber was first time used by Ford Company.
2. L-Head Combustion Chamber:
In this type of chamber both the inlet and outlet valves are on the same side as shown in Fig. 26.7 (b). We can remove cylinder head without disturbing valve assembly. This chamber has slow rate of combustion and has tendency to detonate because it has long air flow path and due to tendency to detonate, the compression ratio used is small. The major advantage of this type of chamber is that it has less maintenance that help using in those applications where regular checking is not feasible.
3. I-Head Overhead Valve Combustion Chamber:
In the I-head or overhead design, the valves are in the cylinder head. The design allows the use of large sized valves (resulting in improved volumetric efficiency. However as clearance volume increases, the compression ratio decreases. Different type of I-head combustion chambers are available as shown in Fig. 26.7(c), designs like bath tube design wedge designs etc. are available.
These chambers have both valves on the cylinder head. The spark plug is on the top side of the chamber. These chambers have high volumetric efficiency and pressure rise is also smooth and uniform. Therefore these can be operated on high compression ratio. These chambers also have a very less tendency to detonate as the flame travel is not long due to compact design in which all available space is used. Most of the cars in India use I-head or its variants.
4. F-Head Combustion Chamber:
The design of F-head chamber is slightly complex. Valve positioning is compromise between L- and I-heads. Modern F-head engines have exhaust valve in the cylinder head and inlet valve is in the cylinder block as shown in Fig. 26.7(c). The main disadvantage is that two separate camshafts actuate the inlet and exhaust valves.
The F-head chambers are quite compact. In these spark plugs are generally placed on the top of chamber. Due to location of the spark plug and compactness of the chamber such chambers have high efficiency, can operate on higher pressure and have almost negligible knocking tendency. However its complicated design makes it difficult to manufacturing.
5. Ricardo is Turbulent Combustion Chamber:
Ricardo is design is somewhat similar to L-L type chambers but with major improvements to increase turbulence and to reduce the flame travel. In this type of chamber position of valve is on the one side as in the L-L type. The combustion chamber is of semi-hemispherical shape located on the cylinder head. The flame speed is increased due to turbulence and hence the name.
In this type, due to peculiar shape of combustion chamber, more better is mixing of air and fuel which results in homogeneous and distributed fuel. This is used in many cars. This kind of chamber also has less tendency to detonate because of presence of higher flame speed and less flame travel.
9. Explain the Working of Lithium Bromide–Water Absorption System.
Boiling temperature of water at atmospheric pressure of 1.01325 bar is 100°C.
As usual for an absorption plant, following are the main components:
1. Evaporator
2. Absorber
3. Generator
4. Condenser
Additional Components used are:
5. A heat exchanger
6. Two fluid pump
7. A purge unit
8. A vacuum pump
9. An automatic decrystallization unit
10. A solution control valve
11. A steam or hot water valve
12. An educator and
13. A control centre
A typical water-lithium bromide system is shown in Fig. 36.37.
1. Evaporator:
Evaporators consist of tube bundles over which refrigerant water is sprayed and evaporated. The liquid to be cooled passes inside the tube.
2. Absorbers:
Absorbers are tube bundles over which the strong absorbent water is sprayed. Refrigerant vapour is condenser into the absorbent releasing heat to the cooling water passing through the tubes.
3. Generator:
Generators are tube bundles submerged in the strong solution heated by steam or hot water. Thus to have a boiling temperature of say 30°C, the pressure will have to be decreased to nearly 0-042 bar. Therefore, vacuum has to be produced if all the pressure is to be mentioned below atmospheric pressure.
When water is to be used as a refrigerant, the boiling or evaporating temperature cannot be reduced below 5°C. Therefore, if the refrigerator or evaporation temperature is above 5°C, then water can be used as a refrigerant. Then the absorbent for water must have a great affinity for water.
The substances which can be used as absorbent are common salt (sodium chloride), calcium chloride, lithium bromide etc. According to the tests made on these different salts, lithium bromide was found to possess the best overall characteristics required for an absorbent to be used in large capacity absorption chillers.
Lithium bromide is available in crystals and these crystals are dissolved in water and the mixture is called Lithium Bromide Solution which is used in the absorption chiller. The amounts of lithium bromide and water in a solution are measured by weights and not by volume 40% concentration means that 40% of the total weight is lithium bromide.
4. Condensers:
Condensers are tube bundles located over which vapours are present coming from the generators and the cooling water flows through the tubes. This cooling water first passes through the absorber.
5. Heat Exchangers:
These are of steel shelf and tube type and tube type heat exchangers.
6. Purge Unit:
This is used to remove non-condensable gases. These gases are present in small quantities and will rise the total pressure in the absorber sufficiently and the evaporator pressure changes thus changing the evaporating temperature. A very small pressure rise can cause appreciable change in the evaporating temperatures.
7. Expansion Device:
Normal mechanical expansion valves are not used in absorption units. The flow of refrigerant liquid to evaporator is controlled by an orifice or other fixed restriction between the condenser and the evaporator (just like capillary tube in small Household refrigerator).
8. Pumps:
Pumps which are used for recirculation are shown in Fig. 36.37.
10. How are Steam Turbines Compounded?
Impulse turbines require nozzles and pressure drop of steam takes place in nozzles. The steam enters the turbine with a high velocity. There is no pressure drop in the turbine. The velocity reduces as some of the kinetic energy of steam is used up in producing power. The steam velocity entering the turbine is very high if the pressure drop from boiler pressure to the condenser pressure takes place in a single nozzle row. The speed of rotation becomes exceedingly high—upto 30000 r.p.m. — if single row of turbine blading is used. Hence, this speed needs reduction to practical speed.
Compounding of the steam turbines is the utilisation of the energy in stages.
The methods used are:
1. Pressure Compounding (Reateu Stage):
The high rotational speed can be avoided by combining the impulse stages in series and on the same output shaft. The exhaust steam from the rotor blades of the first, stage enters the nozzle of the succeeding impulse stage. The nozzles in succeeding stage are built in and locked to fixed diaphragm. Each set of nozzles combined with one disc of moving blades is called a stage of a turbine.
Thus this arrangement amounts to splitting up the whole pressure drop from the steam chest pressure to the condenser pressure into a series of smaller pressure drops across the successive impulse stages.
The diaphragms separate one stage from the other and all the moving blades are mounted on wheels which are individually mounted on the output shaft. The expansion of steam takes place only in the nozzles. The blades of rotor change direction of steam, apply force and develop torque. With drop in pressure per stage, steam velocity is reduced. Thus shaft speed is reduced for maximum work done.
2. Velocity Compounding (Curtis Turbine):
This type of turbine consists of a set of nozzles and rows of moving blades fixed to the shaft and fixed blades fixed to casing. The entire expansion takes place in the nozzle and the high velocity steam parts with only a portion of the kinetic energy in the first set of the moving blades and then passes onto fixed blades or guide vanes where only change in direction of jet takes place without appreciable loss in velocity.
This jet then passes on to another set of moving vanes where further drop in kinetic energy occurs. By this method also high rotor speeds are reduced without sacrificing efficiency or work output.
The effect of velocity staging may be obtained with a single row of buckets by especially contrived means. In one such turbine, the blading is of the conventional type, but as the steam leaves the wheel the first time trough, it enters a reversing chamber that directs it back into the blades. Emerging for the second time from this row of blades, the steam may again be reversed and directed through the blades, thus providing three velocity stages. Turbines in which the steam enters a single wheel two or three times are called re-entry turbines.
3. Pressure Velocity Compounding:
This type of arrangement combines the pressure compounding and velocity compounding. First stage and second stage taken separately are identical to a velocity compounding explained before. But both stages taken together will mean that the pressure drop from the chest pressure to the condenser pressure occurs at two stages. This type of arrangement is very popular due to simple construction but its efficiency is very low.
11. Derive an Expression to Calculate Chimney Height in Steam Generator.
We know that,
Therefore, For Eqs (1) and (2) , one volume of O2 from air is used for the combustion of carbon in the fuel, produces one volume of gaseous products of combustion CO2. But one vol. of O2 from air produces 2 volumes of steam (H2O) in the gaseous products and since the hydrogen content in solid or liquid fuel is quite small, so steam formed is negligible compared to other constituents. N2 and other constituents does not flake part in combustion.
So, the assumption that the volume of air required for combustion is approximately the same as the volume of the products of combustion is fairly accurate.
