Questions and Answers in Thermodynamics

Questions and Answers in Thermodynamics Based on Basic Engineering Thermodynamics by T.Roy Chowdhury, Tata McGrawHill Inc.,1988

- D Viswanath

Contents

Contents

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1 Q and A in Aerodynamics and Thermodynamics

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1.1

Aerodynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1.2

Thermodynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1.3

Propulsion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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References

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Chapter 1 Q and A in Aerodynamics and Thermodynamics 1.1

Aerodynamics

1.2

Thermodynamics

(1) What is thermodynamics? Thermodynamics can be defined as the science that deals with the interaction between energy and material systems. It is important in analysis and understanding of compressible flow. Classical thermodynamics deals with systems in thermodynamic equilibrium. (2) Define a perfect gas. A gas is a collection of particles (molecules, atoms, ions, electrons etc.)which are more or less in random motion. Due to electronic structure of these particles, a force field pervades the space around them. The force field due to one particle reaches out and interacts with neighboring particles and vice versa. Hence these fields are called intermolecular forces. If the particles of the gas are far enough apart, the intermolecular forces is small and can be neglected. A gas in which the intermolecular forces are neglected is defined as a perfect gas. 1

(3) Give the equation of state for a perfect gas. For a perfect gas, pressure, density and temperature are related through the equation of state given by p = ρRT

(1.1)

where R is the specific gas constant=287 J/(kg.K). Its counterpart is pv = RT

(1.2)

where v is volume =1 rho (4) Define internal energy. The sum of all the energies of all the molecules in a finite volume of gas is defined as the internal energy of the gas denoted E. The internal energy per unit mass of gas is defined as the specific internal energy denoted e. (5) Define enthalpy A quantity related to specific internal energy is the specific enthalpy denoted by h and defined as h = e + pv

(1.3)

(6) Define calorifically perfect gas. For a perfect gas, internal energy and enthalpy are functions of temperature only. e = cv T

(1.4)

h = cp T

(1.5)

where cv is specific heat at constant volume and cp is specific heat at constant pressure. A perfect gas where cv and cp are constants is defined as calorifically perfect gas. (7) What is the difference between external combustion process and internal combustion process? 2

In external combustion process, working fluid is entirely separated from the air-fuel mixture whereas in internal combustion process, the working fluid consists of the products of combustion of the air-fuel mixture. Examples: External-steam engines, steam turbine, nuclear power plant, closed cycle gas turbine etc. Internal - reciprocating type IC engines and rotary type open cycle gas turbines. (8) What are the two types of reciprocating IC engines? Spark-ignition and compression-ignition IC engines. (9) List the series of operations in two and four stroke cycle. Four stroke cycle consists of Suction stroke, compression stroke, power or expansion stroke and exhaust stroke and takes two revolutions of crank shaft. In two-stroke, suction and exhaust strokes are omitted and the power and compression strokes are completed in one revolution of crank shaft. Theoretically, power obtained from two-stroke is twice that obtained from four stroke. (10) Explain refrigeration process. The process of refrigeration consists of removal of heat from a low temperature region and transfer of heat to a high temperature region. It works mainly on two processes namely - vapor compression and vapor absorption. (11) How many laws are there in thermodynamics? What do they deal with? There are four laws of thermodynamics. Zeroth law introduces the concept of temperature. First law introduces the concept of internal energy. Second law introduces the concept of entropy. Third law enables the evaluation of absolute entropy. There is no mathematical proof for these laws. (12) Define system. A system is a prescribed region of space or a fixed quantity of matter. (13) Define boundary. The system is surrounded by an envelope known as boundary.

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(14) Define surroundings or environment. Everything external to the boundary or adjacent to it is called the surroundings or environment. (15) Define universe. The combined system and environment is known as universe. (16) What are the three classes of systems in thermodynamics? Closed system, Open system and isolated system. (17) Define a closed system. If the boundary of a system permits energy to cross it (i.e., heat and work) but is impervious to the flow of matter i.e., no mass crosses the boundary of the system, the system is a closed system. (18) Define an open system. Open system is one in which matter, work, and heat cross the boundary of the system. (19) Define an isolated system. An isolated system is one in which matter, work or heat cannot cross the boundary of the system. Thus it is independent of all changes that can take place in the environment. (20) Define an adiabatic process. An adiabatic process is one in which no heat is added to or taken away from the system. If a system is thermally insulated from its surroundings it becomes an adiabatic system. It can however exchange work with the surroundings. (21) Define path. Path is the complete series of states through which the system passes during a change from one given state to another state. (22) Define process. The transformation of a system from one fixed state to another is called a process. (23) Define a quasi-static process. If a process is carried out in such a way that the deviation from thermal equilibrium is infinitesimal, such a process is called quasi-static. 4

(24) Define a reversible process. A reversible process is one in which no dissipative phenomena occur. The effects of viscosity, thermal conductivity and mass diffusion are absent. Hence all the states of the system pass through quasi-static process, i.e., a succession of equilibrium states. If the system passes through a series of non-equilibrium states during a process where effects of viscosity, thermal conductivity and mass diffusion are present, then the process is called irreversible. (25) List out the various thermodynamic processes. Isobaric (constant pressure) Isothermal(constant temperature) Isochoric(constant volume) Isentropic(reversible and adiabatic) polytropic throttling processes. (26) Define an isentropic process.

(27) What is an isocline?

(28) Define a cycle. A cycle is a process whose initial and final states are the same. Thus all the properties of the working fluid have the same values at the beginning and end of a cycle. (29) What are the two types of cycles? Thermodynamic and mechanical cycle. A thermodynamic cycle is one in which the chemical composition of the working fluid during a process does not change. Thus all the properties in the initial and final states remain unchanged. Example: Water that circulates through a steam power plant. Refrigerant that passes through a refrigeration plant. A mechanical cycle is one in which the properties of the working fluid change during the process. 5

Examples: In internal combustion engine, air and fuel are burnt in the engine and converted into combustion products at the end of the cycle. Thus the properties of the working fluid change and their end states are not the same. (30) Define work. Work is a form of energy. It is equal to the product of force and distance moved in the direction of force. (31) Define power. Power is defined as rate at which work is done. (32) Define heat. Heat is a form of energy which crosses the boundary of the system due to the difference of temperature between the system and its environment. When heat flows into a system it is positive and vice versa. (33) Define entropy.

(34) Define enthalpy.

(35) Define calorie. A calorie is the quantity of heat required to raise the temperature of one gram of water from 14.5 deg C to 15.5 deg C at a pressure of one standard atmosphere. (36) Define conduction mode of heat transfer.

(37) Define convection mode of heat transfer.

(38) Define radiation mode of heat transfer.

(39) Define property. Property is defines as any observable characteristic of the system. Example pressure, volume, temperature (thermodynamic properties). Two types of properties that cannot be measured or observed are internal energy and entropy. 6

(40) What are the categories of thermodynamic properties? Intensive and extensive. (41) Define pressure. Pressure is defined as the normal force exerted by a system on a unit area of its boundary. (42) Define standard atmospheric pressure. The standard atmospheric pressure is defined as the pressure exerted by a column of mercury 760 mm high. (43) State the Zeroth Law of Thermodynamics. The zeroth law states that if a system A is in thermal equilibrium with systems B and C, then the systems B and C are in turn in thermal equilibrium. On the basis of this law, it is possible to compare the temperatures of two bodies B and C with the help of a third body A (example thermometer). Hence the law forms the basis of temperature measurement. (44) Define a pure substance. A pure substance in thermodynamics is defined as one that has invariable chemical composition in all phases, solid, liquid and gas. (45) State the first law of thermodynamics. When a closed system undergoes any cyclic process, the cyclic integral of work is proportional to the cyclic integral of heat. In simple terms, the net work input to the system is always proportional to the heat transferred out of the system. Thus heat and work can be measured in same units. From first law-energy can neither be created nr destroyed. It only transforms from one form to another. (46) Define energy. Energy of the system E is the difference between heat transferred to the system Q and work done by the system W. (47) What is the limitation of first law of thermodynamics.

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(48) What are the two important statements of second law of thermodynamics. Kevin-planck statement and Clausius statement. (49) State Kevin-Plank’s Law. It is impossible to construct an engine which while operating in a cyclic process, will produce no effect other than transfer of heat from a single thermal reservoir and the performance of an equivalent amount of work. In simple terms, it is impossible to construct a heat engine with 100 per cent efficiency. The rest of the heat must be rejected to a low temperature reservoir and hence for the operation of a heat engine at least two reservoirs at different temperatures are essential. (50) State Clausius’s Law. It is impossible to construct a heat pump which while operating in a cyclic process, will produce no effect other than the transference of heat from a low temperature thermal reservoir to a high temperature thermal reservoir. This implies that in order to transfer heat from a low temperature reservoir to high temperature thermal reservoir, work must be done on the system by the surroundings. (51) State the third law of thermodynamics. The third law states that when a system is at zero absolute temperature, the entropy of the system is zero. (52) Explain change in entropy Entropy is disorder. In an irreversible process, change in entropy dS is greater than In a reversible process, dS =

δQ T

δQ . T

Hence effect of irreversibility is to increase entropy. For isolated system δQ = 0. Hence (dS)isolated ≥ 0, i.e., entropy either increases or remains constant for isolated system. (53) State Clausius Inequality.

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(54) State Carnot’s theorem.

(55) Explain Carnot’s cycle.

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1.3

Propulsion

(1) Distinguish between solid, liquid and hybrid propellants. (2) Distinguish between turbojet, ramjet and scram jets. (3) Distinguish between piston engines, turbo-propeller engines and turbojet engines. (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) 10

(22) (23) (24) (25)

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References

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