Axial flow compressor design

[andreas]

Hello guys,

I just found your forum and i signed up! Its amazing what you have gathered in here! Congratulations to you all.
My name is Andreas and i am from Greece. I have finished studying Mechanical Engineering in Athens and recently (i mean several months) i started designing my first axial flow compressor.

Because my field of study in University was construction, i didn't have much knowledge on aerodynamics so i borrowed many books on compressor from friends of mine that were in different field.

After a biiiiiiiiiiiig excel file and many calculations i have some numbers that i would like to share with you so that you can tell me if i am heading right or completely wrong.

For input parameters i set compressor compression ratio of 2.5 with constant outter diameter design. The outter dia is 0.2m and design RPM is 25000. all velocities are subsonic so no shock losses are present. To accomplish that i have assumed overall efficiency of 85% and i have calculated that i need a 6stage compressor. After the calculations i get appx 1.2 stage pressure ratio and after 6th stage i have 3.xxx bar pressure. Diffusion factors are at the rate of <0.45max and de Haller ratio all >0.73. 

After that stage of calculations i have all diameters and blade angles and i procced with blade angle corrections. I assume an aspect ratio of 1.5 at inlet and 1 at compressor outlet and blade thickness/chord ratio of 0.08 at root and 0.05 at the tip. For the blade design i have used DCA airfoils because i could find the correction factors online. 

My issue till now is regarding the correction. Blade solidity plays an important role in incidence and deviation angles. For solidities of 1.2/1.45/1.75 (tip/mean/root) i get incidences <-5degrees and deviations <8degrees. If i increase solidity they tend to decrease but i think 1.75 solidity is already too high. For calculated solidity of 0.6-1.2 i get crazy numbers for incedence and deviation correction.

Can anyone help me out about what to do?

What's a proper value of correction? I mean if i get incidence of -5deg and deviation of 9deg the overall camber increases by 14degrees. Isn't that too high. I think that something i am doing wrong.

Thank you in advance for your answers!

Andreas

[rythmnbls]

Hi Andreas,

Welcome to the forum.

As you already know axial comp design is a massive undertaking, I have a monster spreadsheet which is probably similar to yours in size.

How are you calculating your pitch chord (s/c) ratios ? That's probably the most crucial number to get right. In my spreadsheet all deviation angles are calculated from s/c

I used Howell's approximation to calculate s/c, all other calcs are based on data from the axial design chapter in Gas Turbine Theory by Cohen and friends.

I've also noticed that at the blade tip these numbers are generally not useful, but at root and mean diameters they make more sense.

Here's link to a screenshot of the first stage deviation calcs of my spreadsheet to give you an idea of how mine looks. ( btw, it could be completely wrong )

This is for a wheel of 100mm in dia with 25mm long blades.



Regards,

Steve.

[andreas]

Sept 8, 2015 11:56:28 GMT -5 rythmnbls said:

Hi Andreas,

Welcome to the forum.

As you already know axial comp design is a massive undertaking, I have a monster spreadsheet which is probably similar to yours in size.

How are you calculating your pitch chord (s/c) ratios ? That's probably the most crucial number to get right. In my spreadsheet all deviation angles are calculated from s/c

I used Howell's approximation to calculate s/c, all other calcs are based on data from the axial design chapter in Gas Turbine Theory by Cohen and friends.

I've also noticed that at the blade tip these numbers are generally not useful, but at root and mean diameters they make more sense.

Here's link to a screenshot of the first stage deviation calcs of my spreadsheet to give you an idea of how mine looks. ( btw, it could be completely wrong )

This is for a wheel of 100mm in dia with 25mm long blades.



Regards,

Steve.

Hi Steve,

First i calculate the chord length by the aspect ratio. Then i set the number of blades on the disk and it calculates the blade  spacing and s/c and then i continue to calculate deviation and incidence. Simply by adjusting the number of blades, i adjust the s/c factor and so i change the corrections. 

By the way, for incidence and deviation i am using these formulas below.

postimg.org/image/dzsfhaold/

After i play with the number of blades for a little and i see some values of incidence and deviation i don't know what values are best. More blades, mean higher solidity and less deviation and vice versa. As you can see in the image below, these are the values i have calculated right now.

One question. After correcting the angles b1 and b2 with the incidence and deviation, am i done? Changing these values doesn't mean that angles a1 and a2 also change? 


postimg.org/image/q47oya1hd/

[rythmnbls]

My basic understanding of pitch / chord is, the more the the gasses are deflected the lower the number. Which makes sense when you think about it. More gas deflection requires more blades to ensure/control that deflection. If you look at the 1241.pdf file on this page jetandturbineowners.proboards.com/thread/503/links on page 75 you will see how Howell's approximation for pitch / chord based on gas deflection is calculated. I would recommend you calculate s/c first based on deflection, then deviations.

One question. After correcting the angles b1 and b2 with the incidence and deviation, am i done? Changing these values doesn't mean that angles a1 and a2 also change?

I believe you are done. Again I could be wrong, I've never built a working axial comp ( although I'm tempted to try, senility or a delusional state must be setting in )

Regards,

Steve.

[andreas]

Sept 8, 2015 18:32:13 GMT -5 rythmnbls said:

My basic understanding of pitch / chord is, the more the the gasses are deflected the lower the number. Which makes sense when you think about it. More gas deflection requires more blades to ensure/control that deflection. If you look at the 1241.pdf file on this page jetandturbineowners.proboards.com/thread/503/links on page 75 you will see how Howell's approximation for pitch / chord based on gas deflection is calculated. I would recommend you calculate s/c first based on deflection, then deviations.

One question. After correcting the angles b1 and b2 with the incidence and deviation, am i done? Changing these values doesn't mean that angles a1 and a2 also change?

I believe you are done. Again I could be wrong, I've never built a working axial comp ( although I'm tempted to try, senility or a delusional state must be setting in )

Regards,

Steve.

Hi Steve,

I read the pdf about the solidity and deviation from Howell and after applying it and comparing it with the calculations i had done before, i notice some big differences.
Below is an example.

For the hub station of a 2nd stage rotor the specifications are these:
b1=42.72
b2=7.82
camber=34.89=b1-b2
solidity=c/s=1.65

using howell's equations i get deviation = 13.01deg and incidence = -5.46deg  b1=50.02deg & b2=20.84deg (i am confused about how he calculates the angle b2  b2=b2'+deviation. Since deviation is positive, he had to subtract it right?)
using the equations i showed you above i get deviation 9.05deg and incidence = -4.15deg   b1=46.86deg & b2=-1.20deg.

[rythmnbls]

I've done an example by hand using the method from Gas Turbine Theory with your numbers as an example, I also punched them into my spreadsheet to verify the values.



Hopefully it will be a bit clearer now.

Regards,

Steve.

[andreas]

Sept 9, 2015 18:16:24 GMT -5 rythmnbls said:

I've done an example by hand using the method from Gas Turbine Theory with your numbers as an example, I also punched them into my spreadsheet to verify the values.



Hopefully it will be a bit clearer now.

Regards,

Steve.

Hi Steve,

Thank you very much for your time. I see how you've done it and its clear now. One last thing... You haven't calculated any incidence as i see so b1 is equal to the original b1 right?

[rythmnbls]

Hi Andreas,

The Gas Turbine Theory calculations all assume zero incidence for b1

Regards,

Steve.

[andreas]

Sept 10, 2015 10:23:02 GMT -5 rythmnbls said:

Hi Andreas,

The Gas Turbine Theory calculations all assume zero incidence for b1

Regards,

Steve.

Hi Steve,

After applying the stats to my deviation calculation spreadsheet (and this time keeping the b1 angle same) i get a deviation of 8.41degrees so its pretty close to what you got. 
How it comes the assumption for 0 incidence? I had never heard of that. All design projects that i had read on the web, calculate incidence.

[rythmnbls]

I was quoting the book, see screenshot.