Thermodynamics & Heat transfer

                        The science of thermodynamics deals with the amount of heat transfer as a system undergoes a process from one equilibrium state to another, and makes no reference to how long the process will take.

* Thermodynamics deals with equilibrium states and changes from one equilibrium state to another. Heat transfer, on the other hand, deals with systems that lack thermal equilibrium, and thus it is a non-equilibrium phenomenon.

* Therefore, the study of heat transfer cannot be based on the principles of thermodynamics alone.

* However, the laws of thermodynamics lay the framework for the science of heat transfer.

                       The science of heat transfer deals with the determination of the rates of energy that can be transferred from one system to another as a result of temperature difference.

* The basic requirement for heat transfer is the presence of a temperature difference.

* The second law requires that heat be transferred in the direction of decreasing temperature.

*  The temperature difference is the driving force for heat transfer.

*  The rate of heat transfer in a certain direction depends on the magnitude of the temperature gradient in that direction.

*   The larger the temperature gradient, the higher the rate of heat transfer.

Energy can exist in numerous forms such as:
– thermal,
– mechanical,
– kinetic,
– potential,
– electrical,
– magnetic,
– chemical, and
– nuclear.

*  Their sum constitutes the total energy E (or e on a
unit mass basis) of a system.

*  The sum of all microscopic forms of energy is called
the internal energy of a system.

Energy Transfer
• Energy can be transferred to or from a given mass by two mechanisms:
      – heat transfer, and
      – work.

• The amount of heat transferred during a process is denoted by Q.

• The amount of heat transferred per unit time is called
heat transfer rate, and is denoted by  Q.

The total amount of heat transfer Q during a time interval Δt can be determined from

       Δt

Q = ∫  Qdt (J)

      0

• The rate of heat transfer per unit area normal to the direction of heat transfer is called heat flux, and the average heat flux is expressed as

q  = Q/ A   (W/m²)