GT3788VA Variable nozzle turbojet build

[racket]

Hi Mitch

As long as the accuracy is there you'll be able to tune her for best power :-)

Cheers
John

[mitch]

Yep! there is accuracy, the box was a last minute hacked together deal, as I didn't feel like going to the store to buy a larger box to fit all the pieces in, and I had lost the screws to hold it together, which is why I resorted to using electrical tape lol

[mitch]

Went to the turbo store to find a turbine for the free power section

Tacking the mounting strips onto the evaporator/ flametube base for holes to be drilled

Welded the strip onto the flametube, and bolted it to the evap/ flametube base (with nuts welded to the inside) 

[racket]

Hi Mitch

Nice before and after pic there :-)

Cheers
John

[mitch]

Time to finish the evaporator system, and with the information given by racket, I did some math to find the correct evap tube length and runner / branch length. (All math was done using the inner diameter of the piping being used)
With a .76 ID pipe as my evap main tube, I will use a length of 5.5". For the three branches/runners, I will be using .5" ID tubing, with a length of 3.53". This will create branches that are 70% of the length of the main tube, which is almost exactly the same ratio used by RCDon. I chose to use this ratio because it was proven to work fairly well for him.

[racket]

Hi Mitch

I don't like "trunk and branch" evaporators , theres always the possibility that there will be bias flow from the branches resulting in uneven combustion across the flametubes cross section, especially if the trunk is horizontal , not so bad if vertical .

Cheers
John

[mitch]

racket, what other type of evaporator would be possible to create?

[racket]

Hi Mitch

Have 3 separate evaporators , with fuel fed separately into each , that way there can't be any biasing .

Cheers
John

[mitch]

Got one of the branches welded to the trunk, had to end up going with the trunk and branch type design, as it is the simplest way to go, and I did not accommodate room for 3 individual evaporators in my mounting baseplate.

[racket]

Hi Mitch

As long as the combustor is vertical it should be OK .

With a trunk and branch arrangement the limiting flow area is the trunk inlet which needs to be ~10% of total hole area/inducer area , the airflow speeds in the branches will be considerably slower if they are relatively large in diameter and some biasing could occur.

Cheers
John

[mitch]

racket,
I made the inlet area about 10% of the total hole area, so I am okay in that area. However, I had planned on making the combustion chamber lay horizontally. Will I run into many problems with a branch arrangement and a horizontal combustion chamber?

[racket]

Hi Mitch

If you use a spray nozzle to supply fuel into the evaporator trunk like RC Don did , you probably won't have a problem , but if you use a solid jet of fuel ejected against the wall of trunk it could run to the bottom and go out one branch, ............even with the spray nozzle it could "wet" the wall to such an extent that it will puddle to the bottom of the trunk and behave the same as the solid jet of fuel, thats why I don't like trunk and branch systems , too many possibilities for problems .

Cheers
John

[mitch]

Racket, I will be using a "misting" nozzle that is rated at 14.5 gph flow at 100psi. My fuel pump maxes out at 100 psi, so I plan to run it around 50-70 psi to keep the internal temp of the pump down. In my previous short runs I didn't have any pooling problems running at 10 psi fuel pressure with a 4.5 gph nozzle. But with the larger nozzle and increased fuel pressures we'll have to see what Will happen!

[racket]

Hi Mitch

Yep , thats R and D , or should it be the other way around D estroy and R epair :-)

Just be careful not to restrict airflow into the evaporator by having the injection nozzle tip too close , no closer than approx. 1/3rd the evap bore diameter, so no closer than ~ 0.25 inch

Cheers
John

[mitch]

Got the evap assembly pretty much complete other than the top "plug" I'll have to weld in place
It came out alright, all the branches are fairly close to each other in dimensions, except one is about .20" longer than the other two. I am hoping that the amount of errors between the three is enough to even out the flow, not too worried about it though.