Chemical compression with nitrous

[syler]

Suppose in front of your intake sits an intercooler device. use an oil cooler, trans cooler, what ever. You shoot your nitrous through it turning your intake air ice cold which of course makes it dense. Then route it to your AB as an oxidizer or to your chamber if required. Of course more heat should be expected so maybe an AB made with fins for strength and heat dissipation. Or, if you want, just pump CO2 through your IC and run it to waste. 

Good for short bursts of power obviously. Anyway, if you cut your air temp in half, you effectively move twice as much.

Just a half baked idea.

[racket]

Hi Syler

Inlet cooling does work to increase power/thrust , BUT , we need to be careful about ice formation, that "intercooler/oilcooler heat exchanger you mention would very quickly be choked by ice .

Remember density changes with absolute temperature , to get double the density we'd need to drop the ingoing temp from STP 288 Kelvin to 144 kelvin or ~ minus 130 C ......or ~90 horsepower of energy removed per pound of airflow per second.

Then theres the problem of turbine stage configuration , large mass flow changes with large T2 temperature drops will require massive fuel burn increases , then the turbine stage will need to be increased in flow areas to accommodate the extra volume from the increased mass flow during inlet cooling ...........and the requirement for less pressure drop because of the lowered specific power requirement with cold inlet air just adds another "variable" ...........lotsa complications :-(

Inlet "augmentation " is generally only used to cool the inlet air to just above freezing to minimise the risk of ice damage to those fast moving compressor blades , the major benefit is cooling the air from/by evaporation during the compression process , the latent heat of evaporation of the water or water/alky mix within the compressor reduces the work of compression allowing less pressure drop across the turbine wheel and more pressure and temperature downstream for thrust/power production .

Inlet cooling has been discussed a number of times and the general consensus is thats its complicated and doesn't give a big enough increase in power output for the work involved in fitting it ...............our smallish compressor wheels with very short duration residence time within the wheel limits the effectiveness of augmentation compared to either a 2 stage centrif aero engine or one with a large number of axial stages with several times the residence time which optimises evaporation when taken in conjunction with the high air temps experienced in a high pressure ratio engine .

Our lowish pressure ratios, with ~half the static temperature rise within the comp wheel , never sees "boiling point" temps within the wheel, only temps up to ~90 - 95 deg C , sufficiently below the point where the advantage of evaporative phase change will reduce compression power requirements ..............most of the water droplet goes straight through the compressor wheel and is evaporated downstream of it .

Some inlet injection of water/alky during hot dry weather will make a difference , but even the full sized aero engines are only looking at at 10% or so improvement, but even that small amount might make the difference between getting off the ground or not , so is worth the complication for them .

For us DIY'ers , extra mass flow from a bigger turbo is the easiest route to go , like with IC engines "there ain't no substitute for cubic inches"

Cheers
John

[syler]

Ice? Seriously? I'm not planning on emptying a 200lb tank at 30,000 feet in the air in a single shot. Let's talk about all else being equal - not about using a turbo twice the size or a 9 stage axial compressor. I'm talking about short bursts of modestly cooled intake air, plus putting nitrous through the system. I'm trying to power a go cart - not the next space shuttle.

[racket]

Hi Syler

OK, you want to power a kart ..............so what sized turbo do you want to use ??

Have you done any rough calculations on mass flow , thrust , fuel burn etc ??

Having used nitros in a bike engine , it was more than 30 years ago, but I sorta remember it makes them run kinda hot .................now if the same thing happens with a turbine engine you'll melt the turbine wheel unless theres more dilution air used to bring the gas temps down to the same pre nitros injection level .

A short squirt of nitros and fuel at the comp inlet would give an instantaneous boost to thrust , but you'd probably send the comp wheel into surge from turbine stage choking if too much is injected :-( ............nitros on its own will probably only lower power output due to the lowered T I T from the lowered comp inlet temp .

Turbine engines are "tuned" by adjusting "hole sizes" ..............firstly compressor wheel inlet hole , then flametube hole areas , flametube cross section , turbine scroll "A" in the A/R, or throat area of the NGV stator, then the flow area of the turbine wheel especially the exducer "hole" area , then the hole area of the jet nozzle ...............all these "holes" have to be the right size for the engine to work correctly , after all theres no "sealed" parts in a turbine , if we blow in the front , air comes out the back , its only a tube with various sized holes at different stations .

Now lets talk about nitros shot sizes ..............if your turbo is flowing 1 pound of air per second , thats a comp wheel with a ~2.5 inch dia inducer .............the injected nitros will turn to a gas at ~minus 130 F - minus 90 C , so theres roughly ~110 centigrade degrees difference between the nitros and ambiant air

Lets assume an ~ 4:1 ratio of air to nitros , so 0.25 lbs of nitros/sec - 15 lbs/min , this should get the inlet temp down to roughly freezing point from say ~20 deg C ambiant ...........that will produce an ~7 or 8% increase in mass flow due to the increased density , the cooler inlet air will require a bit less compressor horsepower and there'll be a slight increase in pressure ratio produced for the same comp tip speed ............I'd say overall maybe a 10% increase in thrust .

The turbine inlet temperature ( T I T ) will need to be maintained the same as pre nitros as its already at its structual safe limits when burning only ~30% of the ambiant air oxygen , any extra oxygen from the nitros will simply go straight on through the motor , there will be more oxygen to burn in the afterburner , but again due to the "square root" component in the calcs the increase isn't going to be great .

Its very difficult to " hot up" a turbine engine , they already run as hot ( T I T ) and as fast ( tip speed) as possible , reducing internal losses is one avenue to improve power , but again its rather limited .............you're thinking "automotive" again if you think its easy to get a 50% power increase from a turbine engine

Cheers
John