Turbo Shaft Lawn Tractor Build

[Colin Heath]

Jan 13, 2020 14:43:01 GMT -5 racket said:

Perfectly OK , they're assembled dry to make sure the wheels don't hit the housings , they need a lot of bearing clearance otherwise we couldn't get the required oil flow for cooling the bearings or sufficient control of rotor dynamics which can be very complex ..................shove some oil in it and alls well :-)

Yep , keep flanges thick , especially on the hot bits

Thanks for the reassurance John, I hoped that was the case. Every day is a school day

This flange will be 9mm once I skim in flat from 10 after welding into bottom of combustion chamber 

I am also thinking of rounding the joint between the 2 ports to reduce the flat surface for the heat and flow to hammer into. I was going to make sharp or split entry of flame tube but the thin material would probably be more of a risk from heat and bits falling off.

[Colin Heath]

I can't get the online flame tube calculator to work for me and use a MAC most of the time so have done an excel spreadsheet using same formulas.

If interested please let me know so it can be checked for errors and then shared in a way possible on here.

Cheers,

Colin

[Colin Heath]

So here is screen shot of flame tube calcs and image of flat sheet layout.

There are on 12 holes instead of 22 for primary as the rest of the area will be spread for swirl vanes for injector and a few small cooling holes.

This is first stab in the dark as i don't know what makes good hole sizes for single point kerosene spray injection. I tried to scale of John's previous link to image of his tube as a start.

Please ignore spacing from edges of sheet as it will be moved around to ensure even spacing once a design is agreed to be workable.

I have made flame tube the 414mm long and not included the cone which will reduce down to square slip joint into combustion chamber flange.

Primary holes start 80mm down from top and tertiary 80mm from bottom 

I'm assuming I am missing lots from calculation here such as what fuel amounts will be required per minute, which is calculated from flow rate of compressor to then gibe correct AF ratio etc?

I'm hoping to run between 2-10 bar so I can use a standard bosch fuel pump but not sure if that will atomise enough for pure kerosene starts?

[racket]

With your Primary hole axial distance from the top , it'll depend on your spray angle , generally positioning the row of holes at the point where the spray would theoretically impact the wall works OK .

LOL..........theres whole texts written about flametube holes , but what you have should work .

You'll be needing ~1 lb/min of fuel , maybe a tad more depending on exactly how much air you flow which will depend on the turb stage scroll sizing etc etc etc, as well as the max turb temp you wish to use

[finiteparts]

If you read Lefebvre's book, you will see that he suggests 6 to 8 primary holes for can style combustors based on the work Westinghouse published in the mid-1950s.

The logic is that the larger primary holes provide higher jet momentum, helping to maximize the mixing energy in the PZ. These jets do this by energizing the recirculation flow provided by the primary swirler, without being so large that they block the flow in the PZ from also flowing out of the PZ. Too many holes can actually reduce the amount of recirculating flow in the PZ because of the old saying: " What comes in is set by what can get out".

Fewer-larger sized holes provide a primary zone with a wider operability. Conversely, a higher number of smaller sized holes provides a more uniform mixture in the PZ, but this increased homogeneity actually reduces the stability of the PZ near the rich or lean blowout conditions.

Good luck,
Chris

[slittlewing]

This is what is meant by the spray hitting the primary holes - you can work out where it will hit the wall for say, an 80deg oil burner nozzle (it’s the triangle in the cutaway):






I read a comment of yours earlier about using two sets of primary holes. Racket advised me to do this so I think I went with 6x large/medium/small holes for primaries. I was also advised to put holes in for top cap cooling which you can see right below the cap (you may not have a top cap depending on design).




guys such as Andy M have used 2->10bar Bosch pump successfully and fired up on kerosene with it, so that’s the route I am taking (having initially started on propane up until now). I am under the impression you need reasonable starting rpms/pressure though 

cheers

scott 

[Colin Heath]

Jan 13, 2020 17:34:47 GMT -5 racket said:

With your Primary hole axial distance from the top , it'll depend on your spray angle , generally positioning the row of holes at the point where the spray would theoretically impact the wall works OK .

LOL..........theres whole texts written about flametube holes , but what you have should work .

You'll be needing ~1 lb/min of fuel , maybe a tad more depending on exactly how much air you flow which will depend on the turb stage scroll sizing etc etc etc, as well as the max turb temp you wish to use

Thanks John,

I will move the holes to align with contact with flame tube wall once I have selected nozzle. From a quick google it looks like 1lb/min converts to just over 7gph so will aim for somewhere around there for nozzle.

[Colin Heath]

Jan 13, 2020 23:02:10 GMT -5 finiteparts said:

If you read Lefebvre's book, you will see that he suggests 6 to 8 primary holes for can style combustors based on the work Westinghouse published in the mid-1950s.

The logic is that the larger primary holes provide higher jet momentum, helping to maximize the mixing energy in the PZ. These jets do this by energizing the recirculation flow provided by the primary swirler, without being so large that they block the flow in the PZ from also flowing out of the PZ. Too many holes can actually reduce the amount of recirculating flow in the PZ because of the old saying: " What comes in is set by what can get out".

Fewer-larger sized holes provide a primary zone with a wider operability. Conversely, a higher number of smaller sized holes provides a more uniform mixture in the PZ, but this increased homogeneity actually reduces the stability of the PZ near the rich or lean blowout conditions.

Good luck,
Chris

I have been looking for a good book to read so perfect timing. I am reading the model jet engines by Kamps currently and it's a nice intro so far for my level at present.

So like all engineering there is a compromise between performance and reliability Thanks for the detailed reply and I think I am making sense of it.

[Colin Heath]

Jan 14, 2020 8:08:55 GMT -5 slittlewing said:

This is what is meant by the spray hitting the primary holes - you can work out where it will hit the wall for say, an 80deg oil burner nozzle (it’s the triangle in the cutaway):






I read a comment of yours earlier about using two sets of primary holes. Racket advised me to do this so I think I went with 6x large/medium/small holes for primaries. I was also advised to put holes in for top cap cooling which you can see right below the cap (you may not have a top cap depending on design).




guys such as Andy M have used 2->10bar Bosch pump successfully and fired up on kerosene with it, so that’s the route I am taking (having initially started on propane up until now). I am under the impression you need reasonable starting rpms/pressure though 

cheers

scott 

Thanks Scott,

Nice drawing there and they make the point perfectly. I'm guessing that having the large, medium and small holes covers a few bases and stands more chance of operating over wider range of conditions?

I will stay with the high pressure fuel pump then and there seem to be a few cheap bosch copies so might give one of those a go for experimenting, then change to better if needed later.

[Colin Heath]

A small amount of progress today as I am tagging the jobs onto end of the paying stuff.

I cut the flange / combustion tube end cap out of 10mm stainless with the CNC Plasma and welded in 4 off bolts to mount on turbo (No room to insert bolts through back of turbo).

Once bolted back on I will port the turbine chamber inlet to match as there is some sharp overhang ripe for overheating.

[Colin Heath]

All the tube fittings for feed from compressor to combustion chamber have arrived and I have also received the oil pump.

It's an escort oil pump and got the idea from Azwoods build. I will machine up a plate that will both provide threaded ports flow and return and also double as motor mount for a belt drive arrangement.

I have ordered an EGT gauge and sleeved probe and need to get a boost gauge also.

Collection of parts is going well but so busy engineering for others that this isn't getting much time at the moment. I am welding up a turbo exhaust tomorrow though so it's related

[Colin Heath]

I’m thinking of air start for turbine and am wondering what pressure is needed to achieve start speeds?

I have spun things to crazy speeds with a standard compressed air supply but others here are talking of high pressure bottles etc.

Now the greater the pressure in the vessel versus discharge atmosphere the the greater the velocity but so we need to go that high?

I’m assuming if a standard tank would work then it would be used. My supply is max 11bar in the workshop but maybe need to look at diving bottle pressures?

I was going to use nitrogen but then figured that won’t really help with starting combustion haha.

[racket]

Hi Colin

If you use workshop air compressor then you'll need a larger bore supply to get sufficient energy transfer than with a dive bottle , a 3/8" bore airline and "nozzle" impacting your turbine wheel inducer might be sufficient , the ball bearing turbos cope OK with low pressure air start , but the "brass bush" ones have more initial drag , though at max RPM theres less of a difference in "drag" .

Cheers
John

[Colin Heath]

Jan 18, 2020 16:14:44 GMT -5 racket said:

Hi Colin

If you use workshop air compressor then you'll need a larger bore supply to get sufficient energy transfer than with a dive bottle , a 3/8" bore airline and "nozzle" impacting your turbine wheel inducer might be sufficient , the ball bearing turbos cope OK with low pressure air start , but the "brass bush" ones have more initial drag , though at max RPM theres less of a difference in "drag" .

Cheers
John

Thanks John, my compressor works at 11 bar so maybe I need to hook up oil pump when running and give it a try.

[racket]

Yep , a bit of experimenting required