Question

Which machines are favored for energy conversion turbomachine or positive displacement machine'?

Answer 1

The machines whose functioning depend mainly on the change of volume of a certain amount of fluid within the machine are known as positive displacement machines. The word 'positive displacement' implies that there is a physical displacement of the boundary of a certain mass of fluid. This principle is utilized in practice by the reciprocating motion of a piston within a cylinder while entrapping a certain amount of fluid in it. So the word reciprocating is commonly used with the name of the machines of this kind.
Examples - Reciprocating engines, reciprocating pumps and compressors.

The machines whose functioning depend basically on the principle of fluid dynamics, are known as rotodynamic turbomachines. They are different from positive displacement machines in requiring relative motion between the fluid and the moving part of the machine. The rotating element of the machine usually consisting of a number of vanes or blades, is known as rotor or impeller while the fixed part is known as stator. Within the impeller, a change of angular momentum of fluid occurs imparting torque to the rotating member.
Exapmles - Francis and Kaplan turbines, centrifugal pumps and compressors.