THERMAL SCIENCE

CHAPTER-1 CONCEPT & DEFINITION

Thermodynamics

                Thermodynamics is the branch of science which deals with energy (heat) transfer and its effect on state or condition of a system. Essentially thermodynamics pertains to the study of:

1.   Interaction of system & surrounding:-

      It relates the changes which the system undergoes to the influences to which it is put.

2.   Energy & its transformation:-

                                Energy inter-conversion in the form of heat & work. James Joule had proved with his well known experiment that mechanical work can be converted into heat energy. The credit for using heat by converting into work goes to James Watt who produced the first "Steam Engine" & paved the way for industrial revolution.

3.   Relationship between heat, work & physical properties.

                Such as pressure, volume & temperature of the working substance (water for steam engine, petrol for bike etc) employed to obtain energy conversion.

                Thermodynamics has been excellently defined as the science of three "Es" namely Energy, Entropy & Equilibrium. The principles & concepts of thermodynamics are important tools in the innovation, design, development & improvement of engineering processes, equipment & devices which deals with effective utilization of energy.

The applications of engineering thermodynamics in the field of energy technology are as follows:-

1.    Power producing devices for e.g. internal combustion engine (ICG) & gas turbine, steam & nuclear power plant.

2.    Power consuming devices for e.g. fan, blower, compressor, refrigeration & air conditioning plants.

3.   Chemical process plants & direct energy conversion devices. A large number of processes in various fields such as agriculture, textiles, dairy, drugs & pharmaceutical industries are also governed by thermodynamics principle.

Macroscopic & Microscopic approach.

or Classical & Statistical  Thermodynamics.

                There are two approaches for investigating the behavior of a system.

Macroscopic approach.

It is also known as classical thermodynamics which is concerned with gross or overall behavior of matter.  In classical thermodynamics ,the analysis of thermodynamics  systems is explained with the measurable property like pressure ,volume, temperature  etc. It doesn’t explain the structure of matter.(i.e No attention is focused on the behavior of individual particles constituting the matter.)The volume considered is very large compared to molecular dimension & the system is regarded as bulk (whole/single).The study is made of overall effect of several molecules. The behavior & activities of the molecules are averaged. Only a few variables are needed to describe the state or condition of matter. The properties like pressure, temperature etc. needed to describe the system can be easily measured & felt by our senses. The particles of system required simple mathematical formula for analyzing the system for e.g. Piston cylinder assembly of an IC engine, volume occupied by the gas for each position of piston etc. It is based on the concept of continuum. The macroscopic form of energy are those a system possess as a whole with respect to some outside reference frame, such as potential and kinetic energies.

Microscopic point of view

This is also known as statistical thermodynamics and is directly concerned with structure of matter. It focuses on statistical behavior of mass consisting numerous individual molecules and correlates macroscopic properties of the matter with molecular configuration and intermolecular forces. It explains the structure of matter. Few coordinates are not sufficient to describe this system. Concept  of continuum is not valid for this viewpoint. The microscopic form of energy are those related to the molecular structure of a system and the degree of molecular activity, and they are independent of outside reference frames. The sum of all the microscopic forms of energy is called the internal energy of a system.

 

Thermodynamics system:-

It is defines as a definite area or a space where some thermodynamic process takes place. It is a region where thermodynamic process are studied. A thermodynamic system has it’s boundary and anything  outside the boundaries is called it’s surrounding. These boundaries may be fixed like that of a tank enclosing a certain mass of compressed gas, or movable like boundary of a certain volume of liquid in a pipe line.

                                                                                                                    

               

Thermodynamic system represents a prescribed & fixed quantity of matter under consideration to analyze a problem; to study the changes in its properties due to exchange of energy in the form of heat & work. The system may be quantity of steam, a mixture of vapor & gas or a piston cylinder assembly of an IC engine & its contents.

                For the description of thermodynamic system some of the following quantities need to be specified.

1.    Quantity as well as composition of matter.

2.    Measurable properties such as pressure, temperature & volume of the system

3.    Energy of the system

                The combination of matter & space, external to the system that may be influenced by the changes in the system is called surrounding or environment. The thermodynamic system & surrounding are separated by an envelope called boundary of the system. The boundary represents the limit of the system and may be either real or imaginary and may change shape, volume, position or orientation relative to observer for e.g. an elastic balloon which is initially spherical in shape may change into cylindrical shape or some other geometrical shape or may get squeezed to reduce volume during a certain period. As such, the boundary of the gas in the balloon couldnot retains the same size and shape. Further, the boundary may be diathermal or adiabatic depending upon whether it allows or not exchange of energy in the form of heat. The walls of the boundary which does not let heat transfer to take place across them are named as adiabatic. In contrast the walls that do allow heat interaction across them are called diathermic. The diathermic system can be classified into.

A.   Closed system

A close system can change energy in the form of heat & work with its environment but there is no mass transfer across the system boundary. The mass within the system remains the same and constant, though its volume can change against flexible boundary further the physical nature & chemical composition of the mass may change. Thus, a liquid may evaporate, a gas may condense or a chemical reaction may occur between two or more components of the system for e.g.

Cylinder fitted with movable piston.

1.                        Motor car battery.

2.                        Pressure cooker

3.                        Refrigerator

4.                        Ice-cream greaser

5.                        Bomb Calorimeter etc.

B.   Open system:

A open system has mass exchanged with the surrounding along with transfer of energy in the form of heat and work. The mass within the system doesn't necessarily remain constant. It may change depending upon mass inflow & mass outflow for e.g.

1.         Water wheel.

2.         Motorcar engine:-

         The engine initially draws charge (mixture of air and petrol) and finally exhausts the spent up gases to the surrounding atmosphere. The mass flows across the engine boundary.

3.         Steam generator (Boiler):-

                A boiler is a device which converts water from liquid vapor freeze. Here consist of system change, water froze into and steam froze out of the system.

Most of the engineering devices are open system.

C.   Adiabatic system:

                There exists wall or boundary which doesn't allow heat transfer to take place across them, no matter how not one member is compared to other. Such boundaries are named as adiabatic. An adiabatic system is thermally insulated from its environments. It is enclosed by adiabatic walls and can exchange energy in the form of work only for e.g. a pipe carrying heat enclosed by thermal insulation.

D.  Isolated system:

An isolated system is a fixed mass and energy; it exchanges neither mass nor energy with another system or with surrounding. An isolated system has no interaction with the surrounding, it neither influences the surrounding, nor it is influenced by it. When a system and its surrounding are taken together they constitute an isolated system. The universe can be considered as an isolated system & so is the fluid enclosed in the perfectly insulated closed vessel (thermos/flask)

E.   Homogeneous & heterogeneous:

A system consisting of single phase is called homogeneous system for e.g.

1.    Ice, water, dry saturated steam.

2.    A mixture of air and water vapor

3.    Mixture of ammonia in water

A system whose mass content is non uniform through i.e. it consists of more than one phase is called heterogeneous system for e.g.

1.    A mixture of ice and water

2.    A mixture of water & gasoline

3.    A mixture of water & mercury

Thermodynamic property

                Thermodynamic property refer to the characteristic which can be used to describe the condition or state of the system for e.g. temperature, pressure, chemical composition, color, volume, energy etc. The salient aspects of thermodynamic properties are:-

a)        It is a measurable characteristic describing system & helps to distinguish one system to another

b)        It has a definite unit value when the system is in a particular state

c)         It is dependent only on the state of the system, it does not depend on path or route. The system follows to attain the particular state.

d)        It's differential is exact

If 'P' is the thermodynamic property with dP representing its differential change, then the integral between initial state 1 to final state 2 of system will have only one value given by 'O₂'

Since, the thermodynamic property is the state of a system; it is referred to as point a function or state function. There are two kinds of thermodynamic property namely Intensive and extensive.

  a)Intensive Property:

It is independent of the extent or the mass of the system. Its value remains same whether consider whole the system or part on it for e.g. pressure, temperature, density, viscosity, composition, thermal conductivity, electrical potential etc.

b) Extensive property:

It depends on the mass or extent of the system. Its value depend on how big portion of the system is being considered for e.g. energy, enthalpy (heat contained), entropy (disorder), volume etc.

Specific property:-

                An extensive property expressed per unit mass of the system is known as specific property for e.g. specific volume, specific energy, specific entropy, specific enthalpy etc.

Equilibrium:-

                Equilibrium is the concept associated with the concept of tendency for spontaneous change in the value of any macroscopic property of the system when it is isolated from its surrounding.

1.       Mechanical equilibrium (Equality of pressure)

                Condition or state in which there is no unbalance force within the system and nor its boundaries. Mechanical equilibrium implies uniformity of pressure i.e. there is one value of pressure for the entire system; diffusion takes place to wipe out the unbalance and attain the state of mechanical equilibrium.

2.       Chemical Equilibrium:-

                A system in chemical equilibrium may undergo a spontaneous change of internal structure due to chemical reaction or diffusion (transfer of matter) for e.g. there will be spontaneous change in the properties of the mixture of oxygen & gasoline once it is ignited. Chemical equilibrium represents that condition a state of the system when all chemical reaction in it have ceased & there is no mass diffusion.

3.       Thermal Equilibrium

A system is said to be in thermal equilibrium, when is no temperature difference between the parts of the system or between the system and surroundings.

4.       Thermodynamic Equilibrium

                A system which is simultaneously in a state of mechanical equilibrium, chemical equilibrium & thermal equilibrium is said to be thermodynamic equilibrium.

⋆ State, path, process cycle.

Consider a system constituting by a gas enclosed in a piston cylinder assembly of reciprocating machine. Corresponding to the position of the piston at any instant, the condition of the system will be prescribed by pressure, volume & temperature of the gas. When all such properties have a definite value, the system is said to be exist at definite state. State is thus condition of the system identified by thermodynamic property.

When a piston move outwards the properties of the system change (pressure decreases, volume increases). Any such operation in which properties of the system change is called change of state. The locus of series of states through which a system passes is going from initial state to final state constitutes the path.

A complete specification of the path is referred to as process.

                               When a system in a given state undergoes  series  of processes such that the final state is identical with initial state, a cyclic process or a cycle is said to be be executed. The cycle 1-A-2-B-1 consist of 1-A-2 & 2-B-1(process), cycle 1-C-2-B-1 consist of 1-C-2 & 2-b-1 (process).

 

 

Quasi-state or Quasi Equilibrium process

              

  Some unbalanced potential must exist either within the system or between a system & its surrounding to promote the change of state during a thermodynamic process. Consider a system of the gas in cylinder fitted with piston  upon which are placed many small pieces of weights.

                The upward force exerted by the gas just balances the weight on the piston. The system is initially in equilibrium in equilibrium state indentified by pressure P₁, Volume V₁ & Temperature T₁. When these weights are removed slowly one at a time, the unbalanced potential is infinitesimally small. The piston will slowly move upward & at particular instant of piston travel, the system would be almost closed to the state of equilibrium.  The departure of system from thermodynamic equilibrium state will be infinitesimal small. Every state passed by the system will be in equilibrium state.  The locus of series of such equilibrium state is called Quasi-static or Quasi Equilibrium process.

Thus, when the process is carried out in such a way that at every instant, the system deviation from the thermodynamic equilibrium is infinitesimal, then the process is known as quasi-static or quasi equilibrium process. It is a very slow process and each state in the process may be considered as an equilibrium state. It is also known as reversible process.