Know Your Thermodynamic Systems P M V Subbarao Professor Mechanical Engineering Department I I T...
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Transcript of Know Your Thermodynamic Systems P M V Subbarao Professor Mechanical Engineering Department I I T...
Know Your Thermodynamic Systems
P M V SubbaraoProfessor
Mechanical Engineering Department
I I T Delhi
A Thorough Understanding of Your System will Make you An Outstanding Innovator…
First Level of Knowledge about System : Substance & Purity
• Matter: Distinct chemical species.
– single component – pure substance
– two components – binary system
• A pure substance can become a binary system:
– example: water or steam under normal conditions is a single component; but at very high temperatures, H2O dissociates and becomes a two-component system – the components are H and O (binary)
• A binary system can be treated as a single component:
– O2 and N2 in air under normal conditions can be considered as a single component, “air”.
• Multi-component systems: three or more interacting chemical species
Nature of Substance working for Extrasomatic Cause
• Any substance working for Extrasomatic cause is titled as working susbstance in thermodynamics.
• Simple working Substance
• A simple system is one in which the effects of motion, viscosity, fluid shear, capillarity, anisotropic stress, and external force fields donot affect the primitive characteristics.
• Homogeneous Substance:
• A substance that has uniform thermodynamic properties throughout is said to be homogeneous.
• Pure Substance:
• A working substance which has a homogeneous and invariable chemical composition and may exist in more than one phase is titled as Pure Substance.
Superiority of A Thermodynamic Device
• A thermodynamic device can be identified as a superior device, if pure substance works inside this device to satisfy the extrasomatic needs with better efficiency & Eco-affability.
• Better efficiency is achieved when lower quantities of resource is consumed by this device.
• Eco-affability is achieved when lower levels of damage is made to surroundings by this device.
• How to select a right substance as working substance???
New Ideas for Same Desire
• Development of A Display system for A Computer
What fundamental quality of the substance is to be identified to select right candidate as working substance?
Second Level of Knowledge : Phases
• Phase: A phase is state of aggregation.
• A substance in a given phase will have the same physical properties throughout.
• How many different phases can we think of?
– Vapor, Liquid and Solid (many different forms depending on crystal symmetry)
New Phases
• Liquid Crystal
Nematic, Smectic and Cholesteric
• Plasma (electrons separated from nuclei)
• Super fluid (e.g., Liquid He)
• Glass (actually a fluid state)
• A system may have any number of solid phases, any number of liquid phases, but usually only one gas phase.
“ When you can measure what you are speaking about, and express it in numbers,
you know something about it; but when you cannot measure it in numbers, your knowledge
is of a meagre and unsatisfactory kind . . . ”.
•-- Lord Kelvin
Great Benefits through Quantitative Description of Change of Phase
Evaporator/Freezer
Condenser
Compressor
The Methodology of Quantification of Knowledge
• Thermodynamics deals with changes in the system.
• Measures for identifying these changes are necessary.
• This will help in estimating the ability of the system.
• The measures are generally based on some characteristics.
• The numerical values of characteristics will completely describe the system.
Numerical Description of Substance
• The behavior of a system may be numerically described in either a microscopic or a macroscopic point of view.
• Microscopic Description.
Microscopic Description of A System
• A System consists of Substance.
• A collection of Atoms or molecules.
• Each Atom/molecule can be described by its position, velocity, mass….
• Every characteristic of a substance is due to these kinematic properties of atoms/molecules.
• Collective description of atoms/molecules is the description of System.
• Each atom requires three equations for the description of position and three equations for the description of velocity.
• Computationally very intensive.
• Sciences namely, Kinetic Theory and Statistical Thermodynamics use Probability Theory to get a representative numerical description.
• This is a true description of system but impractical.
What is the Engineering Sense of this Peculiar Velocity?
• Useful in computing average kinetic energy of atoms in a system.
n
ii
n
iii
m
vm
1
1
2
21
• When atoms/molecules collide they tend on the average, to equalize kinetic energy spreads equally over all atoms.
• Same may happen when molecules of one system collide with molecules of another system.
• This describes a natural tendency of a system to reach equilibrium with surroundings.
• Can we promote this tendency?