ADIABATIC COMPRESSION AND EXPANSION OF GASES.

PRESENTED BY: -GOHAR REHMAN.OMAIR ALI.ZAIN INAM KHAN.MUHAMMAD WASEEM.DATE:-24 nov 09

Overview

Adiabatic process.Adiabatic law of Compression and Expansion.Compression of a gas.Expansion of a gas.Applications.

Adiabatic processIt is a thermodynamic process in which no

heat is transferred to or from the system.

Any process that occurs within a container which is a good thermal insulator is adiabatic.

Adiabatic processStrictly speaking adiabatic processes

do not exist, since one can not provide absolute thermal isolation.

In this sense adiabatic process is an idealization. However, there are situations when it is a good approximation to treat a process as adiabatic. These situations are

Adiabatic processThe system is fairly well thermally isolated.

The process is so fast that heat has no time to escape or to enter the system.

Neither the system is isolated, nor the process is very fast, but the system is very large.

Adiabatic process equation

LAW OF ADIABATIC COMPRESSION AND EXPANSION.

“Any gas will cool that is allowed to expand freely from higher pressure to lower pressure without transfer of external energy to it. Similarly a gas will heat if compressed from lower to higher pressure in the absence of transfer of energy from gas”.

ADIABATIC EXPANSION AND COMPRESSION OF AIR .

Rising air experiences a drop in temperature .

The decrease in temperature is due to decrease in pressure at higher altitudes.

If the pressure of surrounding air is reduced air parcel will expand.

When air expands molecules of air do work this will affect air parcels temperature

Energy can only be utilized to do work for expansion or to maintain temperature but cannot be used for both.

ADIABATIC COMPRESSION OF AIRTotal amount of energy remains same as none is

added or lost.

If air is compressed air volume decreases and it will heat

up.

If air parcel is forced to descend then it will heat up without taking heat from outside.

CARNOT CYCLE:-

4-1

CARNOT CYCLEReversible Isothermal Expansion (process 1-2): 

Heat transfer between the heat source and the cylinder occurs with an infinitesimal temperature difference. Hence, it is a reversible heat transfer process. Gas in the cylinder expands slowly, does work to its surroundings, and remains at a constant temperature TH. The total amount of heat transferred to the gas during this process is QH.

CARNOT CYCLEReversible adiabatic expansion (process 2-3):

The heat source is removed, and the gas expands in an adiabatic manner. Gas in the cylinder continues to expand slowly, do work to its surroundings till the temperature of the gas drops from TH to TL. Assuming the piston moves frictionless and the process to be quasi-equilibrium, the process is reversible as well as adiabatic.

CARNOT CYCLEReversible isothermal compression (process 3-4):

The cylinder is brought into contact with a heat sink at temperature TL. The piston is pushed by an external force and which does work on the gas. During the compression, the gas temperature maintains at TL and the process is a reversible heat transfer process. The total amount of heat rejected to the heat sink from the gas during this process is QL.

CARNOT CYCLEReversible adiabatic compression (process 4-1):

The heat sink is removed and the gas is compressed in an adiabatic manner. Gas in the cylinder continues to be compressed slowly, accepting work from its surroundings till the temperature of the gas rises from TL to TH. The gas returns to its initial state, which completes the cycle

ANY QUESTIONS?