Second law of Thermodynamics

Second law of Thermodynamics

There are two classical statements of the second law of thermodynamics

1) Kelvin – Planck statement

2) Clausius statement

Kelvin – Planck statement

“It is impossible to construct a device which will operate in a cycle & produce no effect other than the raising of a weight and the exchange of heat with a single reservoir”

i.e., it is impossible to construct an engine which will operate in a cycle will produce no effect other than the transfer of heat from a single thermal reservoir and the performance of an equivalent amount of work”.

No actual or ideal engine operating in cycles can convert into work all the heat supplied to the working substance, it must discharge some heat into a naturally accessible sink because of this aspect and the second law is often referred as the law of degradation of energy.

The statement implies that it is impossible to construct a heat engine that working in a cyclic process can absorb an amount of heat from a high temperature reservoir and can do an equivalent amount of work.  In other words it is not possible to construct a heat engine having thermal efficiency of 100 percent.

Clausius Statement 

It is impossible to construct a heat pump which operating in a cycle will produce no effect other than the transfer of heat from a low temperature thermal reservoir to a higher temperature thermal reservoir.

That is in order to transfer heat from a low temperature thermal reservoir to a high temperature thermal reservoir work must be done on the system by the surroundings.

Although the Kelvin – Planck and Clausius statements appear to be different, they are really equivalent in the sense that a violation of one statement involves violation of the other.

Although the Kelvin – Planck and Clausius statements appear to be different, they are really equivalent in the sense that a violation of one statement involves violation of the other.

Thermodynamic System

A Thermodynamic system is defined as a quantity of matter or a region in space upon which attention is concentrated in the analysis of a problem. Everything external to the system is called the surrounding or environment. The system is separated from the surrounding by the system boundary. Boundary may be either fixed or moving.  A system and its surrounding together comprise a universe.

Open System: The open system is one in which matter crosses the boundary of the system. There may be energy transfer also. Most of the engineering devices are generally open systems.

 Ex: An air compressor in which air enters at low pressure and leave at high pressure and there is energy transfer across the system boundary.

Closed System: A closed system is a system of fixed mass. There is no mass transfer across the system boundary.  

Ex: A certain quantity of fluid in a cylinder bounded by a piston constitutes a closed system.

Isolated System:  The isolated system is one in which there is no interaction between the system and surrounding. It is of the fixed mass and energy and there is no mass or energy transfer across the system boundary.

Homogeneous and Heterogeneous system:

A quantity of matter homogeneous throughout in chemical composition and physical structure is called a phase. Every substance can exist in any one of the three phases viz. Solid, Liquid or  gas. 

A system consisting of a single phase is called a homogeneous system while a system consisting of more than one phase is known as a heterogeneous system.

Fundamental Concepts of Fluid Mechanics

·         Mechanics : Deals with action of forces on bodies at rest or in motion.

·         State of rest and Motion: They are relative and depend on the frame of reference.  If the position with reference to frame of reference is fixed with time, then the body is said to be in a state of rest.  Otherwise, it is said to be in a state of motion.

·         Scalar and heater quantities: Quantities which require only magnitude to represent them are called scalar quantities.  Quantities which acquire magnitudes and direction to represent them are called vector quantities.

Eg: Mass, time internal, Distance traveled à Scalars

            Weight, Displacement, Velocity à Vectors

·         Velocity and Speed: Rate of displacement is called velocity and Rate and distance traveled is called Speed.

Unit: m/s

·         Acceleration: Rate of change of velocity is called acceleration. Negative acceleration is called retardation.

·         Momentum: The capacity of a body to impart motion to other bodies is called momentum.

The momentum of a moving body is measured by the product of mass and velocity the moving body

Momentum = Mass x Velocity

Unit: Kg m/s

·       Newton’s first law of motion: Every body continues to be in its state of rest or uniform motion unless compelled by an external agency.

·       Inertia: It is the inherent property the body to retain its state of rest or uniform motion.

·       Force: It is an external agency which overcomes or tends to overcome the inertia of a body.

·       Newton’s second law of motion: The rate of change of momentum of a body is directly proportional to the magnitudes of the applied force and takes place in the direction of the applied force.

·       Mass:   Measure of amount of matter contained by the body it is a scale of quantity.

                        Unit: Kg.   

                                                                   

·       Weight:  Gravitational force on the body. It is a vector quantity.

                        F = ma

                        W = mg

                        Unit: newton (N)                    g = 9.81 m/s²

·       Volume:  Measure of space occupied by the body.

                        Unit: m³

                                1 m³ = 1000 litres

·       Work:  Work done = Force x Displacement  à Linear motion.

                  Work done = Torques x Angular displacement  à Rotatory motion.

Unit: Nm or J

·       Energy:  Capacity of doing work is called energy.

Unit: Nm or J

FLUID MECHANICS

·       Matter: Anything which possess mass and requires space to occupy is called matter.

·       States of matter:

Matter can exist in the following states

¨      Solid state.

¨      Fluid state.

¨      Solid state: In case of solids intermolecular force is very large and hence molecules are not free to move. Solids exhibit definite shape and volume. Solids undergo certain amount of deformation and then attain state of equilibrium when subjected to tensile, compressive and shear forces.

¨      Fluid State: Liquids and gases together are called fluids. Incase of liquids Intermolecular force is comparatively small. Therefore liquids exhibit definite volume. But they assume the shape of the container

            Liquids offer very little resistance against tensile force.  Liquids offer maximum resistance against compressive forces. Therefore, liquids are also called incompressible fluids. Liquids undergo continuous or prolonged angular deformation or shear strain when subjected to tangential force or shear force. This property of the liquid is called flow of liquid. Any substance which exhibits the property of flow is called fluid. Therefore liquids are considered as fluids.

            In case of gases intermolecular force is very small. Therefore the molecules are free to move along any direction. Therefore gases will occupy or assume the shape as well as the volume of the container.

         Gases offer little resistance against compressive forces. Therefore gases are called compressible fluids. When subjected to shear force gases undergo continuous or prolonged angular deformation or shear strain. This property of gas is called flow of gases. Any substance which exhibits the property of flow is called fluid. Therefore gases are considered as fluids.

GASEOUS FUELS

Natural gas. The main constituents of natural gas are methane (CH4) and ethane (C2H6).

It has calorific value nearly 21000 kJ/m3. Natural gas is used alternately or simultaneously with oil for internal combustion engines.

Coal gas. Mainly consists of hydrogen, carbon monoxide and hydrocarbons. It is prepared by carbonisation of coal. It finds its use in boilers and sometimes used for commercial purposes.

Coke-oven gas. It is obtained during the production of coke by heating the bituminous coal. The volatile content of coal is driven off by heating and major portion of this gas is utilised in heating the ovens. This gas must be thoroughly filtered before using in gas engines.

Blast furnance gas. It is obtained from smelting operation in which air is forced through layers of coke and iron ore, the example being that of pig iron manufacture where this gas is produced as by product and contains about 20% carbon monoxide (CO). After filtering it may be blended with richer gas or used in gas engines directly. The heating value of this gas is very low.

Producer gas. It results from the partial oxidation of coal, coke or peat when they are burnt with an insufficient quantity of air. It is produced in specially designed retorts. It has low heating value and in general is suitable for large installations. It is also used in steel industry for firing open hearth furnaces.

Water or illuminating gas. It is produced by blowing steam into white hot coke or coal.

The decomposition of steam takes place liberating free hydrogen, and oxygen in the steam combines with carbon to form carbon monoxide according to the reaction.

The gas composition varies as the hydrogen content if the coal is used.

Sewer gas. It is obtained from sewage disposal vats in which fermentation and decay occur. It consists of mainly marsh gas (CH4) and is collected at large disposal plants. It works as a fuel for gas engines which in turn drive the plant pumps and agitators. Gaseous fuels are becoming popular because of following advantages they possess.

Advantages :

1. Better control of combustion.

2. Much less excess air is needed for complete combustion.

3. Economy in fuel and more efficiency of furnace operation.

4. Easy maintenance of oxidizing or reducing atmosphere.

5. Cleanliness.

6. No problem of storage if the supply is available from public supply line.

7. The distribution of gaseous fuels even over a wide area is easy through the pipe lines and

as such handling of the fuel is altogether eliminated.

8. Gaseous fuels give economy of heat and produce higher temperatures (as they can be preheated in regenerative furnances and thus heat from hot flue gases can be recovered).