Thermodynamics Notes

–Engineering Thermodynamics–

Ø Pressure, temperature and volume control the physical porperties of a perfect gas.

Ø Boyle’s law, Charles’ law and Gay-Lussac lawas are applicable for the behaviour of a perfect gas.

Ø The unit of temperature in S.I. unit is Kelvin.

Ø The unit of time in S.I. unit is second.

Ø The unit of length in S.I. unit is meter.

Ø The unit of energy in S.I. unit is Joule.

Ø According to Gay-Lussac law for a pefect gas, the absolute pressure fo given mass varies directly as absolute temperature, is volume is kept constant.

Ø An dieal gas as compared to the real gas at very high pressure occupies more volume.

Ø General gas equation- PV = mRT

Ø Gas law are applicable for gases alone and not to vapours.

Ø According to Dalton’s law, the total pressure of the mixture of gases is equal to sumof the partial pressure of all.

Ø Oxygen, Nitogen, Hydrogen can be regarded as gas so that gas laws could be applicable, within the commonly encountered temperature limits.

Ø The unit of pressure in the S.I. unit is Pascal.

Ø One property at one of the states, provided all other properties are known.

Ø A closed system is one in which mass does not cross boundaries of the system, through energy may do so.

Ø Temperature of gas is produced due to kinetic energy of molecules.

Ø According to kinetic theory of gases, the absolute zero temperature is attained when kinetic energy of the molecules is zero.

Ø Kinetic theory of gases assumes that the collisions between the molecules are perfectly elastic.

Ø The pressure of a gas in terms of its mean kinetic energy per uint volume E is equal to 2E/3.

Ø Superheated vapour behaves approximately as a gas.

Ø Absolute zero pressure will occur whn molecular momentum of the system becomes zero.

Ø No liquid can exist as liquid at zero temperature.

Ø The unit of power in S.I. unit is Watt.

Ø The condition of perfect gas vaccum i.e. absolute zero pressure can be attained at a temperatue of -273.16 degree calcius.

Ø Specific heat of air at constant pressure is equal to 0.24

Ø Intensive property of a system is one whose value does not depend on the mass of the system, like temperature and pressure etc.

Ø Characterstics gas ocnstant of a gas is equal to Cp-Cv=R

Ø The behaviour of gases can be fully determined by 3 laws.

Ø The ratio of two specific heats of air is equal to 1.41

Ø Boyle’s law i.e. PV=Constant is applicable to gases only small range of pressure.

Ø Joule’s law states that the internal energy of a gas is a function of temperature.

Ø The same volume of all gases would represent their molecular weight.

Ø Regnault’s law states that the specific heat of a gas remains constant at all temperatures and pressures.

Ø An open system is one in which both energy and mass cross the boundaries of the system.

Ø According to the Charles’ law all perfect gases change in volume by 1/273th of their origional volume at 0 degree celcius chenge in temperatue when pressure remains constant.

Ø Gases have two values of specific heat.

Ø Accrding to Avogadro’s hypothesis the molecular weights of all the perfect gases occupy the same volume under conditiions of pressure and temperature.

Ø Extensive property of a system is one which depends on the mass of system.

Ø Work done in a free expansion process is zero.

Ø The statement that molecular weights of all gases occupy the same volume is known as Avogadro’s hypothesis.

Ø To convert volumetric analysis to gravimetric analysis, the relative volume of each constituent of the flue is multiplied by its molecular weights.

Ø If a gas is heated against a pressure, keeping the volume constant, then work done will be equal to zero.

Ø An isolated system is one in which neither mass or energy crosses the boundaries of the system.

Ø Properties of substance like pressure, temperature and density, in thermodynamics coordinates are point function.

Ø Heat is not a property of the system.

Ø According to Avogadro’s law for a given pressure and temperature, each molecule of gas occupies  some volume.

Ø Mixture of ice and water from a heterogeneous system.

Ø Pressure, temperature, volume, density, internal energy, enthalpy and energy are the property of a system.

Ø On weight basis, air contains 23 parts of Oxygen.

Ø Heat is not the intensive property.

Ø Thermal conductivity is not a path function.

Ø Work done in an adiabatic process between a given pair of end state depends on the end states only.

Ø Heat and work are path function.

Ø Volume is constant for a mole for most of the gases at a given temperature and pressure.

Ø A reversible polytropic process can be described by the equation PVⁿ=C

Ø Some heat transfer take place for reversible polytropic process.

Ø Specific heat of air at constant volume is equal to 0.17

Ø Cp-Cv = R/J

Ø The value of polytripic exponent n is the reversible polytropic process usually varies between 1.2 to 1.4

Ø If the value of n is high in the polytropic process, then the compressor work between given pressure limits will be less.

Ø The value of n=1 in the polytropic process indicates it to be isothermal process.

Ø Solids and liquids have one value of specific heat.

Ø The index of compression n tends to reach ratio of specific heats when process is isentropic and specific heat does not change with temperture.

Ø Change in enthalpy of a system is the heat supplied at constant pressure.

Ø The NTP stands for normal temperature and pressure.

Ø The heat exchange process in which the product of pressure and volume remains constant is known as hyperbolic process.

Ø The internal energy of a system is a function of only absolute temperature.

Ø In an isothermal process, the thermal energy of gas molecules ramins constant.

Ø Zeroth law of thermodynamics states that if two systems are both in equilibrium with a third system, they are in thermal equilibrium with each other.

Ø If a certain amount of dry ice is mixed with same amount of water at 80 degree celcius the final temperature of mixture will be zero degree celcius.

Ø The bsis for mesuring thermodynamic property of temperature is given by zetoth law of thermodynamics.

Ø One watt is equal to 1 Nm/sec

Ø Work done is zero for constant volume, for free expansion and throttling.

Ø For dry steam gas laws can be used with minimum error.

Ø 1 Calorie = 0.427 Kgm

Ø On volume basis air contains 21 parts of Oxygen.

Ø Universal gas constant is defined as equal to product of the molecular weight of the gas constant.

Ø The value of the product of molecualr weight and the gas charactersitc constant for all the gases in S.I. units is 83.14 J/K mol K

Ø The molecular weight expressed in gm of all gases at NTP occupies a volume of 22.4Litres.

Ø All engineering processes are irreversible.

Ø For water, the internal energy and enthalpy are function of temprature only.

Ø In a free expansion process work done is zero but hear increases.

Ø If a gas vapour is allowed to expand through a very minute aperature then such a process is known as throttling.

Ø The specific heat of air increases with increase in temperature.

Ø If a fluid expands suddenly into vaccum through an orifice of large diamension, then such a process is called free expansion.

Ø Enthalpy remains constant during throttling process.

Ø  Isothermal and adiabatic processesa are thrrmodynamically reversible.

Ø Throttling is irreversible process.

Ø In order that a cycle be reversible –
_ Free expansion or friction resisted expansion/compression process should not be encountered.
_ When heat is being absorbed, temperature of hot source and working substance should be same.
_ When heat is being rejected, temperature of cold source and working substrance should be same.

Ø For a thermodynamic process to be reversible, the remperatue difference between hot body and working substance should be zero.

Ø Minimum work in compressor is possible when the value of adiabatic index n is equal to 1.

Ø The more effective way of increasing efficiency of carnot engine is to decrease lower temperature.

Ø Entropy change depends on heat transfer.

Ø For reversible adiabetic process, change in entropy is zero.

Ø Isochoric process is one in which no mechanical work is done by the system.

Ø When a gas flows through a very long pipe of uniform corss-section the flow is approximately isothermal.

Ø The work done is the expansion of a gas from volume V1 to V2 under constant pressure P is equal to P(V2 - V1)

Ø An expansion process as per law PV=Constant is known as hyberbolic process.

Ø Under ideal conditions, isothermal, isobaric, isochoric and adiabatic processes are quasi-static processes.

Ø During throttling process enthalpy remains constant.

Ø When liquid boils at constant pressure, the enthalpy increases.

Ø Maximum work by an expansion of a gas in a closed system is possible when takes place at constant pressure.

Ø Enthalpy is the sum of internal energy, the pressure and volume product.

Ø Heat energy stored in the gas and used for raising temperature of a gas is known as intermal energy.

Ø Accoring to first law of thermodynamics total energy of a system remains constant.

Ø Energy can neither be created nor destroyed but can be converted from one form to other if inferred from first law of thermodynamics.

Ø First law of thermodynamics is furnishes the relationship between heat, work and properties of system.

Ø Change in enthalpy in a closed system is equal to heat transferred if the reversible porcess takes place at constant volume.

Ø The first law of thermodynamics was developed by Joule.

Ø According to first law of thermodynamics heat and work are mutually convertible.

Ø Total heat of a substance is also called as enthalpy.

Ø Addition of heat at constant pressure to a gas results in raising its temperature and doing external work.

Ø The temperature in a process in which work is done by expanding a gas under adiabatic condition will decrease.

Ø The work given out duing expansion process in a closed system will increase when the value of n decrease.

Ø The work required for compression in a closed system increases when value of n increases.

Ø Carnot Cycle –
_It is represented as a standard of perfection.
_It provides concept of maximising work output between two temperature limits.
_The degree of perfection of other engine can be compared with carnot cycle.
_It has maximum efficiency for reversible cycle.
_It’s efficiency depends upon temperature range of operation.

Ø Measurement of operation of temperature is based on zeroth law of thermodynamics.

Ø Carnot cycle efficiency is maximum when final temperature is 0 degree Kelvin.

Ø If a heat engine attains 100% thermal efficiency, it violates second law of thermodynamics.

Ø If heat be exchanged in a reversible manner, entropy will change accordingly.

Ø If a system after undergoing a series of processes, returns to the initial state then sum of heat and work transfer will be zero.

Ø According to Clausius Statement –
_ Heat flows from hot substance to cold substance unadided.
_ Heat can flow from cold substance to hot substnace with the aid of external work.

Ø A Machine that continuously creates its own energy represents the perpetual motion of the first kind.

Ø An actual engine is to be designed having same efficiency as the carnot cycle. Such a proposition is impossible.

Ø In a carnot cycle, heat is transferred at constant temperature.

Ø Change of entropy depends upon change of heat.

Ø The value of Joule-Kelvin coefficient for an ideal gas is zero.

Ø The value of entropy at 0 degree celsius is taken as zero.

Ø In a cycle the heat is rejected at constant pressure.

Ø Steam flow through a nozzle is considered as adiabatic process.

Ø Internal energy of a substance depends on temperature.

Ø A diathermic well is one which permits thermal interaction.

Ø An adiabatic well is one which prevents thermal interaction.

Ø The door of a running refrigerator inside a room was left open then the room will be gradually warmed up.

Ø Compressed air coming out from a punctured football becomes cooler.

Ø Water contained in a beaker can be made to boil by passing steam through it at a pressure greater than atmospheric pressure.

Ø Heat can be converted into work.

Ø Heat rays can ve converged at one point by concave mirror.

Ø Water expands when cooled below 4 degree celcius.

Ø Heat may transfer by any of three modes.

Ø During throttling process heat exchange does not place, no work is done by expanding steam and there is no change of internal energy of steam.

Ø The energy of molecular motion appears as heat.

Ø A sudden fall in the barometer reading is a sign of storm.

Ø Calorie is a measure of quantity of heat.

Ø The first law of thermodynamics is the law of conservation of energy.

Ø A perpetual motion machine is a hypothetical machine whose operation would violate tha laws of thermodynamics.

Ø Kelvin Plank’s law deals with conversion of heat into work.

Ø According to Clausis statement of second law of thermodynamics heat can’t be transferred from low temperature to high temperature source without the aid of external energy.

Ø A frictionless heat engine can be 100% efficient only if its exhaust temperature is 0 degree kelvin.

Ø Carnot cycle consists of two isothermals and two isentropics.

Ø Tha ratio of actual cycle efficienry to that of the ideal cycle efficiency is called efficiency ratio.

Ø The constant volume cycle is also called Otto cycle.

Ø Air refrigerator work on Reversed Joule cycle.

Ø Rankine cycle consists of five processes.

Ø Relative efficiency of a gas power cycle is given as the ratio of actual thermal efficiency to air standard efficiency.

Ø It is proposed to make a direct heat-to-heat converter out of an elementary system which absorbs heat while doing isothermal work exactly equal to the heat absorbed, therby keeping internal energy constant. Such a system is not possible.

Ø Maxwell’s thermodynamic relation are applicable to chemical system in equilibrium.

Ø An engine working between positive temperatures can not be a heat pump.

Ø A refrigerator working between positive temperatues must take work out of work reservoir.

Ø Thrmal power plant works on Rankine cycle.

Ø Otto cycle consists of two isentropisc and two constant columes processes.

Ø The efficiency of a carnot cycle is depends on temperature of source and sink.

Ø For same compression ratio and for same heat added Otto cycle is more efficient than Diesel cycle.

Ø The efficiency of Carnot cycle is maximum for reversible engine.

Ø For the same compression ratio, the efficiency of dual combustion cycle is less than otto cycle and greater than diesel cycle.

Ø Carnot cycle is a reversible cycle.

Ø Diesel cycle consists of two isentropics, one constant volume and one constant pressure processes.

Ø If both Stirling and Carnot cycles operates within the same temperature limits then efficiency of Stirling cycle as compared to Carnot cycle is equal.

Ø Stirling and Ericsson cycles are reversible cycle.

Ø The working substance for acarnot cycle is ideal gas.

Ø A cycle consisting of two adiabatics and two constant pressure processes is known as Joule cycle.

Ø Reversed joule cycle is called Brayton cycle.

Ø Brayton cycle sonsists of two isentropics and two constant pressure processes.

Ø The cycle in which heat is supplied at constnat volume and rejected at ocnstant pressure is known as Atkinson cycle.

Ø The efficiency of Diesel cycle with decrease in cut off increase.

Ø The ideal efficiency of a Brayton cycle without regeneratio with increase in pressure ratio will increase.

Ø The ideal efficiency of a Brayton cycle with regeneration, with increase in pressure ratio will decrease.

Ø Reversed-Brayton cycle is used for air craft refrigeration.

Ø Gas turbine cycle consists of two isentropics and two constant pressure processes.

Ø The thermodynamic difference between a Rankine cycle working with saturated steam and the Carnot cycle is that heat is supplied to water at temperature below the maximum tempereture of the cycle.

Ø The heat addition in dual combustion cycle is done at partly at constnat pressure and partly at constant volume.

Ø Diesel cycle efficiency is maximum when the cut off is zero.

Ø The pressure ratio in case of Bell-coleman cycle is of the order of 10-15.

Ø Stirling cycle consists of two isothermals and two constant volumes processes.

Ø Atkinson gas engine has a special feature that isentropic compression is on a short stroke and the expansion on longer stroke.

Ø The ideal efficiency of an Ericsson cycle with perfect regenration and operating between two given temperature limits is equal to carnot cycle.

Ø In a Carnot engine, when the working substance rejects its heat to sink, the temperature of sink remains same.

Ø The ideal efficiency of Joule cycle operating between two given temperature limits is less than carnot cycle.

Ø The ideal efficiency of a Striling cycle with perfect regeneration and operating between two given temperature perature limits is equal to Carnot cycle.

Ø The concept of regeneration is used in Stirling and Ericsson cycles.

Ø A Bell-coleman cycle is a reversed Joule cycle.

Ø Ericsson cycle consists of two isothermals and two consrnat pressure processes.

Ø A petrol engine theoretically operates on constant volume cycle.

Ø A diesel engine theoretically opeartes on constant pressure cycle.

Ø A high speed diesel engine theoretically operates on mixed cycle of constant pressure and constant volume.

Ø A constant pressure gas turvine works on the Brayton cycle.

Ø According to Kelvin-Plank’s statement, it is impossibe to construct a heat engine which operates in a cycle and receives a given amount operates in a cycle and receives a given amount of heat from a high temperature body and does equal amount of work.

Ø A steam nozzle converts heat energy into kinetic energy.

Ø According to Clausius statement it is impossible to construct a device which operates in a cycle and produces no effect other than the reansfer of heat from cooler body to hotter body.

Ø Under condition that hot resevoir is at negatice temperature, it is possible that heat be abstraced from hot source, as well as cold source and positive work be done.

Ø According to Pettlier Thomson effect when two dissimilar metals are heated at one end and cooled at other, e.m.f. is developed proportional to dieefrence of temperature at two ends.

Ø All diesel cycles except slow speed engine use mixed dual.

Ø Second law of thermodynamics defines entropy.

Ø Gas turvine works on constant pressure cycle.

Ø Cycle used in thermal power plant is Rankine.

Ø An isentropic process on T-S diagram is represented by vertical line.

Ø Expansion in nozzle is a adiabatic process.

Ø Air standard efficiency of a diesel cycle is dependent upon ratio of specific heats, cut off ratio and chemical equilibrium.

Ø Entropy is called the property of the system because it has a single value at each equilibrium state.

Ø When two gases suddenly mix up with each other then resultant entropy of the system will increase.

Ø If hot water and cold water are mixed, then the entropy of the system will increase.

Ø When a process undergoes a complete cycle then the change of entropy will be zero.

Ø If air is compressd adiabatically from atmospheric condition in a cylinder having compression ratio of 6 then pressure at the end of the compression shall be greater than 6 ata.

Ø A process which undergoes enegy loss due to friction is called irreversible process.

Ø Density is not an extensive property.

Ø Internal energy and enthalpy of an idle gas are funcitons of temperature only.

Ø A process occurs spontaneously of its entropy increases.

Ø The areas under the curve on T-S diagram represents the heat transfer for reversible processes.

Ø Work is considered to be superior from of energy as compated to heat energy because while work can be fully converted into heat, heat can’t be fully converted into work.

Ø Triple point of a pure substance is a point of which solid, liquid and vapour phases exit together.

Ø A substance above critical temperature exists as supersaturated fluid.

Ø One kilowatt hour of energy is equal to 3600KJ.

Ø All the reversible engines working between the same temperature limits have same efficiency.

Ø It is not possible to liquefy hydrogen at room temperature by application of pressure because its critical temperature is lower than room temperature.

Ø All gases behave ideally under low pressure conditions.

Ø Otto cycle is more efficient than Diesel cycle for a given compression ratio.

Ø RMS velocity of hydrogen gas at NTP is 1839 m/sec.

Ø Atom is defined as the element made up of minute and chemecally indivisible particles.

Ø Element is defined as the substance which has not been resolved into simper from.

Ø Work done during a process can be determined by integration of pdV when the process is quasi-static.

Ø During a process on the closed system, its internal energy increases by twice the units than the heat aded to it. It is possible due to performing of shaft work on the system.

Ø Partial pressure of a component in a mixture is pressure of the component itself if it was present in the entire volume at the same temperatue.

Ø Siblimation curve on p-t diagram for all substances possesses the positive slope.

Ø The process of sublimation is found to occur in the case of solid Carbondioxide.

Ø Vaporisation curve on p-t diagram for all substance possesses positive slope.

Ø When a vapour condenses into a liquid it evolves heat.

Ø Sometimes ice is white in colour. It is due to presence of dissolved air/gases and impurities.

Ø Bomb calorimeter is used to measure the calorific value of solid or liquid fuel.

Ø Avogadro’s hypothesis is concerned with the combustion of gaseous fuels.

Ø The area under a curve on T-S diagram represents heat transfer during reversible process.

Ø Submerged combustion involves combustion inside the liquid medium.

Ø All the commercial liquid fuels are derived from crude oil.

Ø Specific fuel consumption is defined as fuel consumed per hour per BHP.

Ø Flameless combustion means catalytic combustion.