What Is Dry Adiabatic: Detailed Insight And Facts


This article discusses about what is dry adiabatic rate. Lapse refers to a decrease in quantity. Anything which lapses decreases in quantity or number. It represents negative slope in quantity vs time graph.

Lapse rate in adiabatic process refers to decrease in temperature in Earth’s atmosphere with altitude. Here, the temperature is the decreasing quantity. The rate at which it decreases is called as the lapse rate.

What is dry adiabatic rate?

Adiabatic process means a process in which there is no heat transfer across the walls of system and surroundings. Heat cannot escape the walls of adiabatic system and cannot enter through the walls of adiabatic system.

Dry represents the absence of water or moisture content in atmosphere. The rae of decrease in temperature with altitude when the air inside the system is dry is called as dry adiabatic rate.

How to calculate dry adiabatic lapse rate?

To calculate dry adiabatic lapse rate, one needs to know the gravitational pull and the specific heat at constant pressure of the local air.

Dry adiabatic rate can be calculated as-

Where,

The Greek symbol Gamma refers to lapse rate in SI units that is temperature, T divided by altitude, Z (in m).

g represents gravitational pull and Cp represents specific heat at constant pressure.

Dry adiabatic lapse rate formula

As discussed above, the formula for calculating dry adiabatic lapse rate requires local specific heat and gravitational pull.

The formula of dry adiabatic lapse rate can be given as-

Where,

The Greek symbol Gamma refers to lapse rate in SI units that is temperature, T divided by altitude, Z (in m).

g represents gravitational pull and Cp represents specific heat at constant pressure.

Is dry adiabatic rate constant?

The dry adiabatic rate can be considered constant because no heat is transferred from the moving parcel of air.

Even in the formula of dry adiabatic rate, we can see that gravity,g and specific heat, Cp remains constant.

What is moist adiabatic rate?

Moisture means anything having water content. More water content means more moisture.

Moist adiabatic rate means the rate of decrease of temperature with altitude when the air in the system has water content in it.

What is the formula for moist adiabatic rate?

The moist adiabatic rate can be given as-

Where,

Greek letter Gamma represents wet lapse rate

g is Earth’s gravitational acceleration

Hv is heat of vapourisation

R in the numerator represents specific heat of dry air

R in the denominator represents specific heat of wet air

Cpd is the specific heat of dry air at constant pressure

T is temperature in K

What is environmental lapse rate?

Just like dry adiabatic lapse rate and moist adiabatic lapse rate, environmental lapse rate is also related to rate of decrease in temperature.

The rate of decrease of temperature with altitude in a stagnant surrounding atmosphere is called as environmental lapse rate or ELR.

What is an adiabatic system?

A system is the three dimensional space that is taken under observation. The system is differentiated from the surroundings by walls or system boundary.

An adiabatic system is a system whose boundaries doesn’t allow heat to pass through it. This means that heat cannot enter or exit the system and stays constant.

Mathematically,

Del Q= 0

Where, Del represents change in quantity and,

Q represents heat inside the system.

What are different types of thermodynamic systems?

The type of boundaries decide the characteristic features of the system. On the basis of type of boundary of the system, thermdynamic systems can be classified into following types-

  • Open system– An open system is one in which both mass and heat transfer can take place. The boundary of system is such that it allows both mass and heat to enter or exit from the system. An example of open system is water reservoir which is open from top.
  • Closed system– A closed system is one in which no mass can enter but heat transfer can take place. An example of this system is water filled inside a plastic bottle.
  • Isolated system– In this system, no mass or heat transfer can take place. An example of this system is hot beverage stored in thermos flask.
  • Adiabatic system- In this system, mass can flow through the system but heat transfer cannot take place. An example of this system is Nozzle. In nozzle, hot gases enter through the inlet and exit from the outlet, the heat transfer does not take place across the walls of nozzle.

Mathematical representation of thermodynamic systems

The mathematical representations of different thermodynamic systems is given below-

  • Open system- In open system, both mass and heat transfer can take place.

So,

and,

  • Closed system- In closed system, only heat transfer takes place and no mass transfer takes place.

So,

This image has an empty alt attribute; its file name is gif.latex

and,

  • Isolated system- In isolated system, no heat transfer as well as no mass transfer can take place.

So,

and,

  • Adiabatic system- In adiabatic system, only mass transfer takes place and no heat transfer takes place.

So,

and,

Where,

m is the mass and Q is the heat content in the system.

Types of thermodynamic processes

There are different types of thermodynamic processes a working fluid may undergo. Different thermodynamic processes means different properties will be achieved by the working fluid.

  • Isothermal process- In isothermal process, the temperature of the system remains constant.

Mathematically,

Where, T is the temperature

  • Adiabatic process- In adiabatic process, the heat content of the system remains constant.

Mathematically,

Where, Q is the heat content

  • Isobaric process- In isobaric process, the pressure remains constant inside the system.

Mathematically,

Where, P is the Pressure in the system

  • Isochoric process- In isochoric process, the volume remains constant inside the system.

Mathematically,

V is the Volume

Work done in different thermodynamic processes

As the working fluid takes different path in different thermodynamic system, the amount of work done also changes from process to process

The work done by different thermodynamic processes are given below-

W = nRTln(V2/V1)

Where,

R is the universal gas constant

V2 and V1 are volume after the isothermal process and before the process respectively

  • Work done in isobaric process-
  • Work done in isochoric process-

W = nR(T2-T1)/γ-1

The Greek letter Gamma represents specific heat index

What is adiabatic index?

Adiabatic index is ratio of specific heat of gas at constant pressure to specific heat of gas constant volume.

Mathematically it can be given as,

γ = Cp/Cv

It is a very important ratio as it is used in finding the slopes and work done in different thermodynamic processes.

Graphical representation of different thermodynamic processes

The general equation of any thermodynamic process is given below-

PVn = C

Different thermodynamic processes can be plotted in graph as shown below-

what is dry adiabatic
Image: Graphical representations of thermodynamic processes

Image credits: toppr.com

The slopes of these processes is different because of adiabatic index, n.

Abhishek

I, Abhishek Khambhata, have pursued Btech in Mechanical Engineering. Throughout four years of my engineering, I have designed and flown unmanned aerial vehicles. My forte is fluid mechanics and thermal engineering. My fourth year project was based on performance enhancement of unmanned aerial vehicles using solar technology. I would like to connect with like minded people.

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