Why reaction machines are more efficient than impulsive machine?
Answers
Answered by
0
The answer to that depends completely on the operating conditions under which the two types of turbines are used.
For any given turbine type (i.e., impulse or reaction), the quantity that generally has the highest impact on efficiency (to first order) is the turbine ‘velocity ratio’, usually written as ‘U/C’ in the literature.
In the term ‘U/C’, U represents the turbine blade translational velocity (i.e., a local radius multiplied by a rotational speed). This obviously means that U varies continuously along the blade span. For most of these discussions, U is generally taken to be that at the blade mid-span (although you should certainly check this if you are using someone else’s data). C in the above expression represents an isentropic ‘spouting velocity’ that is obtained by expanding the turbine working fluid across the turbine inlet-to-outlet conditions.
For a typical full admission impulse turbine (and I mean one that is large enough to avoid the performance losses that are unique to ‘small’ turbines) the efficiency generally peaks at or around a U/C value of 0.45. When a reaction turbine is used at a U/C value of less than 0.45, its efficiency is generally lower than that of the impulse turbine.
As we allow the turbine U/C value to increase (either by increasing blade speed, or by changing the turbine conditions such that C decreases) the reaction turbine efficiency will generally exceed that of the impulse turbine at a U/C of somewhere between 0.45 and 0.50. At U/C of something like 0.55, the reaction turbine will (normally) have a noticeably higher efficiency.
So, as you can see, you can think of the impulse and reaction turbine types as being sort of ‘complimentary’ to each other. Each can perform with high efficiency, but the two types operate at their best at different sets of conditions.
Happy Learning
For any given turbine type (i.e., impulse or reaction), the quantity that generally has the highest impact on efficiency (to first order) is the turbine ‘velocity ratio’, usually written as ‘U/C’ in the literature.
In the term ‘U/C’, U represents the turbine blade translational velocity (i.e., a local radius multiplied by a rotational speed). This obviously means that U varies continuously along the blade span. For most of these discussions, U is generally taken to be that at the blade mid-span (although you should certainly check this if you are using someone else’s data). C in the above expression represents an isentropic ‘spouting velocity’ that is obtained by expanding the turbine working fluid across the turbine inlet-to-outlet conditions.
For a typical full admission impulse turbine (and I mean one that is large enough to avoid the performance losses that are unique to ‘small’ turbines) the efficiency generally peaks at or around a U/C value of 0.45. When a reaction turbine is used at a U/C value of less than 0.45, its efficiency is generally lower than that of the impulse turbine.
As we allow the turbine U/C value to increase (either by increasing blade speed, or by changing the turbine conditions such that C decreases) the reaction turbine efficiency will generally exceed that of the impulse turbine at a U/C of somewhere between 0.45 and 0.50. At U/C of something like 0.55, the reaction turbine will (normally) have a noticeably higher efficiency.
So, as you can see, you can think of the impulse and reaction turbine types as being sort of ‘complimentary’ to each other. Each can perform with high efficiency, but the two types operate at their best at different sets of conditions.
Happy Learning
Similar questions