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Dear Readers, Welcome to __ THERMODYNAMICS Interview Questions and Answers__ have been designed specially to get you acquainted with the nature of questions you may encounter during your Job interview for the subject of

Ans. ds=dQ/T Entropy is inversely proportional to the temperature so, as temp. Increases, entropy decreases.

Ans. Engine specifications are different in different manufactures like as Bore Diameter (CC), Ignition timing. Also the exhaust passage takes more responsible for sound.

Ans. 746.2 Watt

Ans. Rear wheel sprocket works under the principle of ratchet and pawl.

Ans. Octane No.- Octane number is defined as the percentage, by volume, of iso octane in the mixture of iso octane and h-heptane. It is the measure of rating of SI engine.

Cetane No.- Cetane number is defined as the percentage, by volume, of n-cetane in the mixture of n-cetane and alpha methyl naphthalene. It is the measure of rating of CI engine.

Ans. Differential mechanism

Ans. Natural gas (Gasoline)… at 20 Celsius

Ans. Divide the shaft diameter size by 5, it will give last two digit of the bearing no. and according to type of load we have to choose the type of bearing and that will give prior number of the bearing.

Ans. Improving the surface finish by Polishing & providing residual stress by Shot peening.

Ans. No, It will not work, as the Compression ratio of Petrol engine is 6 to 10 & that of Diesel engine is 15 to 22. Thus on such high compression, gasoline gets highly compressed & it may blast.

Ans. When a material is compressed in one direction, it usuallytends to expand in the other two directions perpendicular tothe direction of compression. This phenomenon is called thePoisson effect. Poisson’s ratio is a measure of the Poisson effect.

For rubber = 0.5

For steel = 0.288

For wood < 0.2

Thus Poisson’s ratio is higher in RUBBER.

Ans. The heat capacity of material is the amount of heat transformed to raise unit mass of a material 1° in temperature.

Ans. The specific heat of material is the ratio of the amount of heat transferred to raise unit mass of a material 1° in temperature to that required raise unit mass of water 1° of temperature at some specified temperature.

For most of the engineering purposes, heat capacities may be assumed numerically equal to specific heat values.

Ans. For an aqueous solution of salts, the specific heat can be estimated by assuming the specific heat of the solution equal to that of the water alone. Thus, for a 15% by weight solution of sodium chloride in water, the specific heat would be approximately 0.85.

Ans. Give examples of latent heats. For pure substances, the heat effects accompanying changes in state at constant pressure (no temperature change being evident) are known as latent heats. Examples of latent heats are the heat of fusion, vaporization, sublimation and change in crystal form.

Ans. Free energy (or Helmholtz function) is defined as f = u.T.s.

It is equal to the work during a constant-volume isothermal reversible nonflow process.

Free enthalpy (or Gibbs function) is defined as

g = h – Ts

(where u = internal energy, h = enthalpy, T = temperature, s = entropy)

Gibbs function is of particular importance in processes where chemical changes occur. For reversible isothermal steady-flow processes or for reversible constant-pressure isothermal nonflow processes, change in free energy is equal to network.

Ans. Isothermal process.

Ans. Volume remains constant.

Ans. Throttling process.

Ans. All the mechanical engineering systems are studied with the help of thermodynamics. Hence it is very important for the mechanical engineers.

Ans.** There are three laws of the thermodynamics:**

**First Law:** Energy can be neither created nor destroyed. It can only change forms.In any process in an isolated system, the total energy remains the same.

**Second Law: **When two isolated systems in separate but nearby regions of space, each in thermodynamic equilibrium in itself, but not in equilibrium with each other at first, are at some time allowed to interact, breaking the isolation that separates the two systems, and they exchange matter or energy, they will eventually reach a mutual thermodynamic equilibrium. The sum of the entropies of the initial, isolated systems is less than or equal to the entropy of the final exchanging systems. In the process of reaching a new thermodynamic equilibrium, entropy has increased, or at least has not decreased.

**Third Law: **As temperature approaches absolute zero, the entropy of a system approaches a minimum.

Ans. According to the laws of conservation of energy, “energy can neither be created nor be destroyed. It can only be transformed from one form to another.”

Ans. Yes definitely the boiler is a closed system.

Ans. It was being designed by Carnot and let me tell you that Carnot engine is an imaginary engine which follows the Carnot cycle and provides 100% efficiency.

Ans. Gibbs Helmholtz formula is the formula which forms the link between the thermodynamics and electromagnetism.

?Hs/R = [? lnp /? (1/T)]x

where: x – mole fraction of CO2 in the liquid phase

p – CO2 partial pressure (kPa)

T – temperature (K)

R – universal gas constant

a – mole ratio in the liquid phase (mole CO2 per mole of amine)

**The third law of thermodynamics states that:**

“The entropy of a perfect crystal of each element and a compound is zero at absolute zero.”

**Limitations: **If any disorder like impurity or imperfection is found in a substance then the entropy of such crystal is non-zero at 0 K. For example: The entropy of pure carbon dioxide and nitric oxide is zero at 0K. This shows that there exists disorder in the arrangement of such molecules.

This law is applicable only to pure compounds. Thus we can say that, this law is not applicable to glass which is a supercooled liquid. It is also not applicable to amorphous substance and supercooled solutions.

**Importance:**

With the help of this law Thermodynamic properties can be calculated and chemical affinity can be measured.

This law helps in explaining the behaviour of solids at very low temperature.

**Zeroth law:** If any two systems are in thermal equilibrium with the third system, then they are also in thermal equilibrium with each other.

**First law: **First law of thermodynamic states that energy can neither be created nor be destroyed but it can only be converted from one form to another.

**Second law:** This law states that “all processes in nature tend to occur with an increase in entropy and the direction of change always lead to the increase in entropy.”

**Third law:** This law states that “The entropy of a perfect crystal of each element and a compound is zero at absolute zero.”

**Isolated system: **A system that can neither exchange matter nor heat with the surrounding is known as an isolated system.

**For example:** Water placed in a vessel that is closed as well as insulated.

**Open system:** A system that can exchange both matter and energy with the surrounding is said to be an open system.

**For example:** A reaction taking place in an open vessel exchanges both energy and matter with the surrounding.

**Closed system:** A system that exchanges only energy and not matter with the surrounding is said to be a closed system.

**For example: **A reaction taking place in a closed metallic vessel.

This thermodynamic quantity states that the decrease in value during a process is equal to the useful work done by the system. It is denoted by G and the mathematical equation is:

G = H – TS

Where,

H = heat content

T = absolute temperature

S = entropy of the system

For isothermal process we have

G1 = H1 – TS1 for the initial state

G2 = H2 – TS2 for final stage

Therefore,

G2 – G1 = (H2 – H1) – T(S2 – S1)

Now,

?G = G2 – G1 is the change in Gibbs free energy

?H = H2 – H1 is the change in enthalpy of the system

?S = S2 – S1 Is the change in entropy of the system

**Thus the above equation becomes:**

?G = ?H – T?S is known as Gibbs-Helmoholtz equation.

ds=dQ/T. Entropy is inversely proportional to the temperature so, as temp. Increases, entropy decreases.

Engine specifications are different in different manufactures like as Bore Diameter (CC), Ignition timing. Also the exhaust passage takes more responsible for sound.

746.2 Watt

Rear wheel sprocket works under the principle of ratchet and pawl.

**Octane No:-**Octane number is defined as the percentage, by volume, of iso octane in the mixture of iso octane and h-heptane. It is the measure of rating of SI engine.

**Cetane No:-** Cetane number is defined as the percentage, by volume, of n-cetane in the mixture of n-cetane and alpha methyl naphthalene. It is the measure of rating of CI engine.

Differential mechanism

Natural gas (Gasoline)… at 20 Celsius

Divide the shaft diameter size by 5, it will give last two digit of the bearing no. and according to type of load we have to choose the type of bearing and that will give prior number of the bearing.

Improving the surface finish by Polishing & providing residual stress by Shot peening.

No, It will not work, as the Compression ratio of Petrol engine is 6 to 10 & that of Diesel engine is 15 to 22. Thus on such high compression, gasoline gets highly compressed & it may blast.

When a material is compressed in one direction, it usuallytends to expand in the other two directions perpendicular tothe direction of compression. This phenomenon is called the Poisson effect. Poisson’s ratio is a measure of the Poisson effect.

For rubber = 0.5

For steel = 0.288

For wood < 0.2

Thus Poisson’s ratio is higher in RUBBER.

A real gas behaves like an ideal gas in low pressure and high temperature conditions.

As per the second law of thermodynamics, any heat input to the system (Heat engine) cannot be converted completely into useful work. Some energy is lost and that is called 'unavailable work'. The amount of unavailable work increases as the entropy increases.

Heat transfer deals with the energy analysis which in transition and and depends on the modes of heat transfer like conduction, convection and radiation or combination of any modes. Heat transfer deals in non equilibrium domain and conditions while thermodynamics deals with study of system at equilibrium and does not depend on how heat transfer is calculated.

Not possible. Because thermal radiation becomes only zero at absolute zero temperature which can never be attained by the third law of thermodynamics.

There always a debate between differentiation of pipe and tube. Pipe is always designated by its Inside Diameter (ID) because main purpose of pipe is to carry fluid, while the

Tube is always designated by its Outside Diameter (OD). Pipe's thickness is sometimes called as schedule and tube's thickness is gauge.

As per the tolerance basis, tubes requires higher tolerances as to that of pipe and tubes are more costlier in manufacturing compared to pipe production.

The function of flywheel is to store the energy/power produced during the power stroke of an engine and this stored energy is used during remaining stroke to make piston up and down during working of engine while governor tries control the speed of an engine due to variation in load. It does this work by reducing or increasing the amount of fuel passing to the engine.

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