Thermal Science & Engineering - short answer questions from AMIE exams (Summer 2019)
Explain the following in brief
The following equation is known as the inequality of Clausius. It provides the criterion of the reversibility of a cycle.
∮(dQ/T) ≤ 0
- If ∮(dQ/T) = 0 then the cycle is reversible
- If ∮(dQ/T) < 0 then the cycle is irreversible and possible
- If ∮(dQ/T) > 0 the cycle is irreversible and possible
In nucleate boiling, steam bubbles form at the heat transfer surface and then break away and are carried into the mainstream of the fluid. It is a type of boiling that takes place when the surface temperature is hotter than the saturated fluid temperature by a certain amount but where the heat flux is below the critical heat flux.
All heat is not equally valuable for converting into work. The heat that is supplied to a substance at a high temperature has a greater possibility of conversion into work than heat supplied to a substance at a lower temperature.
“Entropy is a function of a quantity of heat which shows the possibility of conversion of that heat into work. The increase in entropy is small when heat is added at a high temperature and is great when heat addition is made at a lower temperature. Thus for maximum entropy, there is minimum availability for conversion into work and for minimum entropy there is maximum availability for conversion into work”
“It is impossible to construct an engine, which while operating in a cycle produces no other effect except to extract heat from a single reservoir and do the equivalent amount of work”.
It says that in order to get net work from a device operating in the cycle (i.e. heat engine) it must have heat interaction at two different temperatures or with body/reservoirs at different temperatures (i.e. source and sink).
A radiation shape factor F12 (or view factor) is defined as the fraction of energy leaving surface 1 and reaching surface 2.
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