In Adiabatic heating, a matter is heated without adding any heat to the system, heating is simply due to the compression of volume of the matter.
When a gas is compressed by adiabatic processes just like in a diesel engine’s cylinder where the gas is pressurized and due to the work done by the surrounding, the temperature of the gas inside the cylinder rises and the process is known as Adiabatic Heating.
Adiabatic processes are those in which there is no heat transfer between the system and the surrounding. Adiabatic processes are generally visible in gases. Due to adiabatic heating temperature of gas increases with the increase in pressure.
What is Adiabatic Heating
Increase or decrease in temperature without adding or removing heat is called adiabatic heating or cooling.
Adiabatic Heating is an effect of Increase in internal energy of the system due to PdV work done by the surrounding on the system. Adiabatic Heating is also possible in an isochoric process
It can be demonstrated in a system with rigid walls which are impermeable to heat.
In the isochoric system since the walls are rigid, PdV work is nil or the system pressure is constant and no change in volume takes place. Now consider a viscous fluid is present in a system with rigid and thermally insulated wall, the energy from the surrounding is provided by stirring the viscous fluid. Since the stirring results in increase in temperature of the fluid which increases its internal energy.
The practical application of Adiabatic Heating is observed in a diesel engine where by compression the fuel vapor temperature is sufficiently increased to ignite it.
In general Adiabatic process is a thermodynamic process where no heat transfer takes place in between the system and surrounding. To prevent heat interaction, the whole system is insulated properly or the process is done so quickly so that there is no time for any heat transmission to take place even though there is no any thermal insulation.
Air is a mixture of different gases, undergoes both Adiabatic Heating and Cooling. If a gas is compressed during an adiabatic process, its temperature rises which indicates Adiabatic Heating. On the contrary if the gas is expanded during the adiabatic process, its temperature goes down refers to Adiabatic Cooling. We can see clearly both Adiabatic Heating and Cooling in Nature.
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In the above figure we can see Adiabatic Cooling or Heating of air occurs due expansion and compression of gases where neither heat is gained nor lost due to lack of time for heat exchange.
Where does adiabatic heating occur?
For adiabatic heating to occur, the system should be designed such that there is no heat loss from the system when work is done on the system by the surrounding.
A true adiabatic heating is thus possible, when the system is thermally insulated from the surroundings and energy is added to the system. This work may be a pressure volume work or a friction work.
In real life situations, this condition can occur if the PV work done is so fast that little or no time is available for the heat transfer to take place from system to the surroundings.
An example of such a process is observed in a 4 stoke compression ignition diesel engine, where the compression process takes place so quickly that no time is available for heat loss to take place to the surroundings. The resultant adiabatic temperature rise is so fast and so high that it leads to auto ignition of the fuel.
What is the process of Adiabatic heating?
The process of adiabatic heating takes place when work is done on the system by the surrounding.
For the process of adiabatic heating to take place, There are two ways in which energy can be converted to work in an ‘adiabatically isolated’ system. One is where the pdv work of compression is done on the system.
The compression process here is considered frictionless and the fluid being compressed has no viscosity. This type of work done is also called isentropic as no entropy is produced within the system. The second type of process is isochoric heating of fluid in a vessel with rigid walls.
The fluid considered here is highly viscous and heating is achieved by stirring of the fluid by providing an external energy source. Here since the walls are rigid and adiabatically isolated, there is no pdv work done and heat developed by stirring of the viscous fluid leads to increase in temperature or adiabatic heating.
Is heat absorbed in adiabatic process?
Adiabatic process is the one where either there is no source of heat dissipation to the surrounding or it is perfectly insulated. Hence, in an ideal adiabatic process there is no absorption of heat.
For a adiabatic process, the first law of thermodynamics transforms into:
dU= -PdV as dQ=0
dU is internal energy
PdV is pressure Volume work done
dQ is heat transfer with the surroundings.
How do you know if a process is Adiabatic?
Adiabatic process is an ideal process and cannot be achieved in real life. The processes in real life can only be approximately adiabatic.
In thermodynamics, for a process to be adiabatic, the system must be impermeable to heat. The energy transfer between the system and surrounding in a adiabatic process is only possible through work.
In reality this condition is hard to attain. However, if a process is carried out very rapidly such that there is no time available for heat to be dissipated, the process can be termed as approximately adiabatic. Here rapid is qualitative and not quantitative.
If the timescale during which the process occurs is small enough for an insignificant amount energy to be lost compared to gain or loss of internal energy of the system while work is done on or by the system, the process qualifies to be called an adiabatic process. An example of real life adiabatic process is cooling of hot magma as it rises up the surface from below the earth surface.
Adiabatic Heating Equation
In Adiabatic Heating the change in temperature of the system is mainly due to internal changes take place.
What is the difference between Adiabatic Heating and cooling?
Both Adiabatic Heating and Cooling occur frequently in a convective atmospheric current.
Major differences between these two phenomenons are listed below:
|Adiabatic Heating||Adiabatic Cooling|
|A temperature rise of gas is observed in Adiabatic Heating.||A temperature drop is observed during Adiabatic Cooling.|
|Air sinks and compresses.||Air rises and expands.|
|Due to high molecular collision temperature increases.||Due to less molecular collision temperature decreases.|
Due to oxidation some materials have an affinity to self heat.
Adiabatic self heating is the result of oxidation of a material, if the heat generated during oxidation is faster than the rate at which it is dissipated to surrounding, self heating results. The heat produced increases the temperature which enhances the oxidation process until the self ignition temperature is reached.
Self heating or spontaneous combustion of coal is due to its interaction with oxygen and its disability to dissipate the heat generated in this reaction.
Adiabatic Heating example
Adiabatic heating takes place if an ideal gas is compressed in a cylinder which is perfectly insulated. The above example is for ideal gas but the formulas derived based on above assumption can be put to practical use in many day to applications.
Examples of Adiabatic Heating are:
- The compression stroke of a diesel engine where the mixture of diesel and air is compressed leading to rise in temperature of the mixture causing auto ignition. The step occurs so quickly that no time is available for significant het loss from the surrounding.
- Another example of adiabatic heating is heating of air parcel in atmosphere as it slides down a mountain face rapidly. The gradual increase in atmospheric pressure as the air parcel goes down leads to decrease in the volume and increase of its internal energy. Here although the system is not insulated, because the mass of air can radiate this heat very slowly to the surrounding, the process is practically adiabatic.