Some fundamental questions on turbomachinery

[mech-eng]

I try to understand topic of turbomachinery. Would someone like to make some guidance?

In that page I cannot understand some parts.

1) What does it mean by "above two conditions" in "Viscous effects must unfortunately be neglected, as it
is generally impossible to satisfy the above two conditions and have equal Reynolds numbers in the
model and prototype. I cannot see any so-called "two conditions" there.

 

[Nidum]

The illustration is very hard to read .

 

[mech-eng]

The illustration is very hard to read .

I am searching for a link, containing it. Maybe when you download it it would seem better.

Source is Fluid Mechanics by Streeter/Wylie/Bedford and page 506, 11.1 Homologuos Units: Specified Speeds

Thank you.

 

[Nidum]

Can you photograph the page instead of scanning it ?

 

[mech-eng]

It was already a photograph but I have a different idea I can upload the page in Spanish and then write the words and some simple equations. Equations can be seen in Spanish version as well? May do it?

Thank you.

 

[boneh3ad]

It is written literally in the first sentence on that page. It tells you two conditions that must be satisfied in order to use a scale model of an engine. It then says that you usually can't meet both of those requirements and match Reynolds numbers, so generally you can't consider viscous effects in such tests.

 

[jack action]

#1: geometric similitude

#2: geometrically similar velocity vector diagrams at the entrance to, or exit from, the impellers.

 

[mech-eng]

Can you photograph the page instead of scanning it ?

It is written literally in the first sentence on that page. It tells you two conditions that must be satisfied in order to use a scale model of an engine. It then says that you usually can't meet both of those requirements and match Reynolds numbers, so generally you can't consider viscous effects in such tests.

1) What does geometric similitudes refer to in that context? Similitudes between what and what? Reality and model?

2) Why is it important to match Reynolds numbers?

Thank you.

 

[boneh3ad]

I'd suggest you might be a little ahead of yourself in reading about turbomachinery if you aren't familiar with Reynolds number yet.

And it says it all right there: geometric similitude and geometrically similar velocity fields. I'm not really sure what you are missing here. Are you aware of the topic of the discussion on that page?

 

[Nidum]

What does geometric similitudes refer to in that context? Similitudes between what and what? Reality and model?

Two engines with the same geometry but different sizes . Usually means a full size engine and a scaled down version . The scaled down version is used for research and development prior to designing and building the full size version .

The scaled down version is not always a complete engine . Quite commonly scaled down versions of sub assemblies and individual components are used .

There is another way of doing scaled down tests . Full size engine components are used but with reduced running conditions . For instance useful information about performance of turbine blades can be obtained by testing them in a moderate speed cold air flow .

 

[jack action]

1) What does geometric similitudes refer to in that context? Similitudes between what and what? Reality and model?

What @Nidum said in post #10.

And concerning similar velocity vector diagrams, this is what it refers to.

2) Why is it important to match Reynolds numbers?

https://en.wikipedia.org/wiki/Reynolds_number#Similarity_of_flows