What is Work and Heat Transfer? 

It is generally related to thermal engineering which tells about the use, creation, conversion and transfer of heat energy between the systems. It is also considered that to transfer heat from one source to another, transfer of masses is also required. There are various mechanisms by which, transfer of heat takes place i.e. thermal conduction, convection, radiation or by change of phase. All these mechanisms have their own specific features which sometimes occur simultaneously within the same system. 

Heat transfer can be understood with the analogy to mass transfer. For example, to rooms attached with a duct, one filled with air and the other being vacuum, the flow is from filled room to empty room, under the influence of pressure difference which is due to the mass of air. 

It uses second law thermodynamic for heat transfer and two bodies under heat transfer tries to reach thermal equilibrium. At constant temperature when heat transfer takes it is known as an isothermal process like during boiling.  

Modes of Heat Transfer 

Various energy conversion methods can be utilised for working of the heat exchanger like solar energy to heat, nuclear energy to heat, chemical energy to heat etc. 

• Advection: It is generally defined as the mechanism by which the transport of fluid takes place from one place to another. This is completely dependent upon the motion as well as the momentum of the fluid. The fluid can be either hot or cold and it can be seen by keeping hot water into the bottle and heating the bed and can also be seen in the movement of an iceberg which is done to change the current of the ocean. One of the practical applications of this can be seen in thermal hydraulics. The mathematical expression for this is given by- 

${\text{Ф}}_{\text{q}}{\text{= vρc}}_{\text{p}}\text{ΔT }$

Where Ф_qis the heat flux whose unit is W/m². 

ρ is the density whose unit is Kg/m³. 

            c_p is the heat capacity of the system at constant pressure (J/Kg.K). 

            ΔT is the temperature change (Kelvin). 

            v is the velocity which is in m/s. 

• Conduction: This is generally defined as the process of transfer of energy when the systems are in contact with each other physically. During this process, while in contact the atoms of one particle vibrate against the other which causes the electrons to move from one place to another. Thermal contact of bodies, physically, is necessary for conduction. Fluids and gases are considered to be less conductive than solids. One of the common examples of conduction can be seen when a cold water bottle is touched by a person with his hands. During this, the heat is transferred from the warm hand to the cold water bottle. When the hand is kept away from the bottle but with some distance, a very low amount of conduction takes place as air is considered to be the poor conductor of heat. Steady-state conduction is defined as the amount of heat entering the system is equal to the amount of heat coming out of the system which leads to the change in temperature to zero. This can be better understood with the example of heat flow from the walls of a house when the outside temperature is too low. In this case, the temperature inside the house is high, while the temperature outside the walls is very low. 
• Convection: This is generally defined as the process of transfer of energy between an object and the environment due to the movement of fluid. This movement of fluid can be due to the external forces or due to the buoyant forces which occur when the fluid is provided with heat causing it to expand. In this process, it is considered that some amount of movement of heat takes place due to diffusion. On the other hand, sometimes the fluid is forced to move with the help of a pump, fan, or some type of mechanical means. This is generally the process in which heat transfer takes place in a mass. The movement of fluid takes place in bulk which helps in transferring the heat between solid and liquid. This is the most important form of heat transfer between the liquid and the gases. This is quite similar to the process of advection in a way that it is also the process in which heat transfer takes place via fluid but the only difference in convection is that, there is a mass flow of fluid in this and this process can be better understood as the sum of total heat which is transferred with both the advection as well as the diffusion. When the external forces with the help of pumps or fans are given to the masses of fluids which causes an induced convection current in the fluid, such type of convection is said to be the forced convection. Boiling water is a classic example of convection. 
• Radiation: This is the process in which the heat transfer takes place with the help of photons present in the electromagnetic waves in presence of any kind of transparent medium, which can be anyone between the solid, liquid, or gas. In this, the movement of molecules or atoms takes place randomly. These contain charged electrons as well as protons, and this movement of particles will lead to the emission of radiation. This is basically for those objects which have a high-temperature difference. 

 Work Transfer 

In this, the work done is defined as the energy which is transferred by the system in the surroundings, which requires a certain amount of force and displacement as well. The whole process consists of macroscopic pushes as well as pulls. And the system is considered to be in equilibrium condition when there is no net force and as well as no torque on it. In thermodynamics, it is believed that the effect of the work on the things which are external to the system helps in raising a certain amount of weight. This can be understood with the help of the example in which a pulley is connected to the motor and as soon as the motor starts, causing the effect on the pulley to raise the weight. So here external force is applied with the help of a pulley and the work is done when the weight connected to the pulley is raised to a certain height. This concept of thermodynamical work is different from mechanical work in a way that it includes the other types of transfer of energy as well. Some examples of work transfer: 

 Shaft work 

• Paddlewheel work or the stirring work 
• Flow work, which is related to the open system.  

Application 

 The heat exchanger works on the principle of heat transfer, for example, heat exchangers are used in power plants to produce steam. In a superheater heat exchanger, the major mode of transfer is thermal radiation.  

Context and Applications    

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for  

• Bachelors in Science (Physics) 
• Masters in Science (Physics) 
• Bachelors in Technology