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Basic Engineering Thermodynamics U3MEA02. Prepared by Mr.N.Dilip Raja, Assistant Professor, Department of Mechanical Engineering, VelTech Dr. RR & Dr. SR Technical University. UNIT - 1. SYSTEM.
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Basic Engineering Thermodynamics U3MEA02 Prepared by Mr.N.Dilip Raja, Assistant Professor, Department of Mechanical Engineering, VelTech Dr. RR & Dr. SR Technical University.
SYSTEM • In thermodynamics – “system is a closed region in space or a body upon which experiments or study is conducted”. • Types of system • Open system : Energy transfer and mass transfer take place. Eg: Pump, compressor, turbine. • Closed system: Only energy transfer take place but no mass transfer. Eg: Earths atmosphere, inflated baloon • Isolated system: Neither energy transfer nor mass transfer take place. Eg: Flask
Surrounding: Every thing apart from system is called as surroundings • Universe: Both system and surrounding together is called as universe • Boundary: The invisible layer which separates system and surrounding is called boundary • Control volume: The maximum volume occupied by a system is called control volume
PROPERTIES • In thermodynamics, properties are the quantities used to determine the state of a system • Types of properties • Intrinsic properties: These depend upon mass of the system. Eg: mass, density, specific heat, etc. • Extrinsic properties: These do not depend upon mass of the system. Eg: pressue, temperature, time, etc.
State: In thermodynamics state is the term used to denote the present conditions of the system • Process: If a system experiences changes in state, then it is called as process • Cycle: A series of process in which the initial and final states are the same is called as a cycle. • Types of cycle • Open cycle • Closed cycle
Equilibrium: In thermodynamics equilibrium is a term used to determine whether there is a process taking place in a system. If there is no changes in states of a system, then it is said to be in equilibrium • Types of equilibrium • Chemical • Mechanical • Thermodynamic
HEAT • Heat is the form of energy transfer taking place in a system by virtue of temperature difference. • It is denoted by the symbol ‘Q’. • Its unit is J (joules). Rate of heat transfer is ‘W’. • Sign conversion + for heat given to a system - For heat taken from a system
WORK • Work is the form of energy transfer taking place in a system because of change in volume. • It is denoted by the symbol ‘W’. • Its unit is J (joules). • Sign conversion + for work taken from a system - For heat given to a system
INTERNAL ENERGY • The energy available within a system is called as internal energy • It is denoted by the symbol ‘U’. • Change in internal energy is denoted by the symbol ‘ U’. • Its unit is J (joules).
ENTHALPY • Enthalpy is a measure of the total energy of a system. It includes the system's internal energy, as well as its volume and pressure. • It is denoted by the symbol ‘H’. • Change in internal energy is denoted by the symbol ‘ H’. • Its unit is J (joules). • H = U + pV
LAWS OF THERMODYNAMICS • Zeroth law of thermodynamics: • First law of thermodynamics: • For open system: W = Q • For closed system: Q = W + U
SECOND LAW OF THERMODYNAMICS • Kelvin statement: It is impossible, by means of inanimate material agency, to derive mechanical effect from any portion of matter by cooling it below the temperature of the coldest of the surrounding objects. • Clausius statement: Heat can never pass from a colder to a warmer body without some other change, connected therewith, occurring at the same time.
CARNOT THEOREM • Carnot’s theorem(1824) is a principle that limits the maximum efficiency for any possible engine. The efficiency solely depends on the temperature difference between the hot and cold thermal reservoirs. Carnot's theorem states: • All irreversible heat engines between two heat reservoirs are less efficient than a Carnot engine operating between the same reservoirs. • All reversible heat engines between two heat reservoirs are equally efficient with a Carnot engine operating between the same reservoirs. • In his ideal model, the heat of caloric converted into work could be reinstated by reversing the motion of the cycle, a concept subsequently known as thermodynamic reversibility. Carnot however further postulated that some caloric is lost, not being converted to mechanical work. Hence no real heat engine could realise the Carnot cycle reversibility and was condemned to be less efficient.
The Carnot cycle when acting as a heat engine consists of the following steps: Reversible isothermal expansion of the gas at the "hot" temperature, T1 (isothermal heat addition or absorption). During this step (1 to 2 on Figure 1, A to B in Figure 2) the gas is allowed to expand and it does work on the surroundings. The temperature of the gas does not change during the process, and thus the expansion is isothermal. The gas expansion is propelled by absorption of heat energy Q1 and of entropy from the high temperature reservoir. Isentropic (reversible adiabatic) expansion of the gas (isentropic work output). For this step (2 to 3 on Figure 1, B to C in Figure 2) the piston and cylinder are assumed to be thermally insulated, thus they neither gain nor lose heat. The gas continues to expand, doing work on the surroundings, and losing an equivalent amount of internal energy. The gas expansion causes it to cool to the "cold" temperature, T2. The entropy remains unchanged. Reversible isothermal compression of the gas at the "cold" temperature, T2. (isothermal heat rejection) (3 to 4 on Figure 1, C to D on Figure 2) Now the surroundings do work on the gas, causing an amount of heat energy Q2 and of entropy to flow out of the gas to the low temperature reservoir. (This is the same amount of entropy absorbed in step 1, as can be seen from the Clausius inequality). Isentropic compression of the gas (isentropic work input). (4 to 1 on Figure 1, D to A on Figure 2) Once again the piston and cylinder are assumed to be thermally insulated. During this step, the surroundings do work on the gas, increasing its internal energy and compressing it, causing the temperature to rise to T1. The entropy remains unchanged. At this point the gas is in the same state as at the start of step 1.
ENTROPY • Entropy is a measure of the number of specific ways in which a system may be arranged, often taken to be a measure of disorder, or a measure of progressing towards thermodynamic equilibrium. The entropy of an isolated system never decreases, because isolated systems spontaneously evolve towards thermodynamic equilibrium, which is the state of maximum entropy. • Entropy was originally defined as
IDEAL GAS • An ideal gas is a theoretical gas composed of a set of randomly moving, non-interacting point particles. The ideal gas concept is useful because it obeys the ideal gas law, a simplified equation of state.
IDEAL GAS LAW • The ideal gas law is the equation of state of a hypothetical ideal gas. It is a good approximation to the behaviour of many gases under many conditions, although it has several limitations. It was first stated by Emile Clapeyron in 1834 as a combination of Boyels law and Charles law. The ideal gas law is often introduced in its common form: • where P is the absolute pressure of the gas, V is the volume of the gas, n is the amount if substance of gas (measured in moles), T is the absolute temperature of the gas and R is the ideal, or universal, gas constant.
Boyel’s law: or • Charle’s law or • Gay Lusac’s law or • Dalton’s law of partial pressure • Amagat’s law of partial volume
REAL GAS • Real gases – as opposed to a perfect or ideal gas – exhibit properties that cannot be explained entirely using the ideal gas law. To understand the behaviour of real gases, the following must be taken into account: • compressibility effects; • Variable specific heat capacity • Van der Waals forces • non-equilibrium thermodynamic effects; • issues with molecular dissociation and elementary reactions with variable composition.
Van der Waals equation The van der Waals equation is an equation of state for a fluid composed of particles that have a non-zero volume and a pairwise attractive inter-particle force (such as the Van der Waals force).
COMPRESSIBILTY FACTORY • The compressibility factor (Z), also known as the compression factor, is the ratio of the molar volume of a gas to the molar volume of an ideal gas at the same temperature and pressure. It is a useful thermodynamic property for modifying the ideal gas law to account for the real gas behaviour. • The compressibility factor is defined as
JOULE THOMSON EFFECT • In thermodynamics, the Joule–Thomson effect or Joule–Kelvin effect or Kelvin–Joule effect or Joule–Thomson expansion describes the temperature change of a gas or liquid when it is forced through a valve or porous plug while kept insulated so that no heat is exchanged with the environment.[This procedure is called a throttling process or Joule–Thomson process.[ At room temperature, all gases excepthydrogen, heliun and neon cool upon expansion by the Joule–Thomson process. • The effect is named for James Prescott Joule and William Thomson, 1st Baron Kelvin, who discovered it in 1852 following earlier work by Joule on Joule expansion, in which a gas undergoes free expansion in a vacuum. • In the Joule experiment, the gas expands in a vacuum and the temperature drop of the system is zero, if the gas is ideal. • The throttling process is of the highest technical importance. It is at the heart of thermal machines such as refrigerators, air conditioners, heat pumps, and liquefiers.Furthermore, throttling is a fundamentally irreversible process. The throttling due to the flow resistance in supply lines, heat exchangers, regenerators, and other components of (thermal) machines is a source of losses that limits the performance.
JOULE THOMSON CO-EFFICIENT • The rate of change of temperature with respect to pressure in a Joule–Thomson process (that is, at constant enthalpy ) is the Joule–Thomson (Kelvin) coefficient . This coefficient can be expressed in terms of the gas's volume , its heat capacity atr constant pressure, and hat capacity at constant pressure, and its coefficient of thermal expansion as:
Clausius–Clapeyron relation • The Clausius–Clapeyron relation, named after Rudolf Clausius and Benoit Paul Emile Clapeyron,is a way of characterizing a discontinuous phase transition between two phase of matter a single constituent. On apressure - temperature (P–T) diagram, the line separating the two phases is known as the coexistence curve. The Clausius–Clapeyron relation gives the slope of the tangents to this curve. Mathematically,
STEAM • Steam is the technical term for water vapour , the gaseous phase of water, which is formed when water boils. • Technically speaking, in terms of the chemistry and physics, steam is invisible and cannot be seen; however, in common language it is often used to refer to the visible mist of water droplets formed as this water vapour condenses in the presence of (cooler) air. • Water boils at a lower temperature than the nominal 100 °C (212 °F) at standard temperature and pressure. • If heated further it becomes superheated steam.
TYPES OF STEAM • Steam is traditionally created by heating a boiler via burning coal and other fuels, but it is also possible to create steam with solar energy.] Water vapour that includes water droplets is described as wet steam. • As wet steam is heated further, the droplets evaporate, and at a high enough temperature (which depends on the pressure) all of the water evaporates and the system is in vapour-liquid equilibrium. • Superheated steam is steam at a temperature higher than its boiling point for the pressure which only occurs where all the water has evaporated or has been removed from the system.
