Re: Motor Jets and thermojets.


"Eunometic" <eunometic@xxxxxxxxxxxx> wrote in message
news:539e1328-355d-4170-a055-8ae7eba9a805@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Sep 5, 6:09 am, "Keith Willshaw" <keithnos...@xxxxxxxxxxx> wrote:


Indeed and that is what piston engines do: throw away exhaust energy.

Not if fitted with a turbocharger, in that case some the energy is used
to drive the supercharger.

About 15% to 20% appears to be recoverable if the R-3350 is taken as
an example.

The R-3350 used turbo-compounding and didnt fly at 100,000 ft

<snip>


Marginal at best , the formula in the simple case is jet
thrust=mass*velocity

The Merlin is dumping its exhaust into atmosphere whose pressure is at
about 0.5 atmospheres, If it is dumping the same amount of exhaust
into air at 0.01 ata it is going to come out faster and produce more
thrust.

Feel free to prove it isnt marginal by doing the math - show all working



Of course the best way to use this lowered pressure is to use a turbo-
supercharger.

A simplistic assessment, In fact for high speed flight the jet thrust
might be more useful as it will not fall off in the same way the power
delivered by the propeller does

It depends, if the power is desired for a motor jet it might be more
useful

So instead of using the jet thrust directly you propose to drive a turbine
that drives a fan to produce jet thrust !

Pardon me while I larf




Also note that the ambient pressure is 0.15psi so the engine will also
recover pressure since there is essentially zero back pressure on the
exhaust.

Not much - try running a diesel engine on air at 15 psi

Diesels are started on less.

Indeed , look at the power output of an alternate electric starter


<snip>


The only practical way to do this would be install a steam turbine.
This is often done in land based diesel installations but has
been deemed a trifle impractical for aircraft

Several kW at 600C is a rather good heat source.

Not when considering the weight penalty


It don't take it too seriously but there are other cycles: the
stirling cycle or even the closed brayton cycle if other forms of
coolant than water are considered.

And practical issues are ignored.

Steam turbines and boilers suitable for aircraft use are in fact very
light and compact.

Thats a very strange definition of light and compact you have there.

It is the condensers that
are the problem so one could recover a little in say the wing surfaces
or leading edges if they were not yet in use.
Extracting mechanical energy from the coolant probably reduces the
radiator area.

No it doesnt. The mechanical energy lost ends up as more heat which
has to be discarded. See Entropy



If an intercooler is used such that after a compression of 10:1 the
air is cooled.to say 330K and only then further compressed 10:1 then
their is a considerable reduction in work required.

You'll have to demonstrate those figures are accurate, I dont believe em
and you'll still need an aftercooler.

Inter coolers are a very accepted and widely used in both piston and
gas turbine engines.

No shit sherlock

Just repeat my calculations only for two 10:1 compression ratios in
series instead of one 100:1.

Repeat after me 'the physical laws governing the energy required to compress
gas dont care how many stages you use'

Assume that intake air is at 240K in both cases and assume that the
intercooler cools the exit
air of the intermediat stage.

Bad assumption number 1, the air temperature at 100,000 ft is
less than 240k, at 70,000 ft its -57 C so the value is closer to210 K


The original for a 100:1 ratio:
Equation 1:
Power required to compress gas is
W = Cp(T2-T1)

WRONG.

There are no mass or time terms here, this cannot be valid formula for power
and you are ignoring Cv so this is assuming isentropic compression


This is a lot less than the original 880hp required admittedly there
are slight pressure losses in the inter cooler and after cooler due to
friction but they are considered worthwhile. It should be possible to
circulate coolant water in the stator blades thus avoiding pressure
loss of an inter cooler entirely.

Guess what happens when you cool a gas sherlock - the pressure
DROPS. You cant apply formula for adiabatic compression
when the heat generated is being extracted from the system

Your statement would get you a fail on a thermodynamics course



The gas equations dont change just because you do
the compression in two stages

The work done is purely a ratio of the inlet temperature, therefore if
you can lower inlet temperature you reduce work.
If the work is divided up between 12 stages of 1.46 compression with
an inter cooler between each stage to keep temperature below 330K the
power required is even lower. I get 34kW per stage and only 408kW/
550hp.

WRONG in oh so many ways

When you compress a gas you increase its internal energy.
This shows up as an increase in pressure and temperature

See the ideal gas laws

You have been using formulae based on isentropic compression
where the assumption is that no heat flow outside the system
occurs and thus the internal energy is a constant

However when you cool the gas in those intercoolers you
throw away some of that internal energy, the net effect is
to INCREASE the energy required to reach the final state

See what happens when you make bad assumptions


There are some pressure losses but they are not so
significant. This gives us a net power of 1000-671 = 330hp.
Intercooling is worthwhile and even gas turbines can benefit from it.

Inter cooling is worthwhile because it reduces the outlet gas
temperature which is essential for a piston engine and it reduces
the physical size of the compressor as the density in secondary
stages and burners is higher. This can increase power output but unless
you can recuperate that heat the energy is lost reducing thermal
efficiency

We've already given it away so no need to recuperate, the limitation
in petrol engines is octane rating and the limitation in gas turbines
is inlet temperature. Diesels are more tolerant of high inlet
temperatures.

Lots more errors

1) Look up the word recuperate - it means recover and use the
heat instead of throwing it away

2) Diesels are HIGHLY sensitive to inlet temperature
thats why turbodiesels have intercoolers and aftercoolers


In an ideal world the gas turbine doesn't need inter cooling but Gas-
turbines with high pressure ratios can use an intercooler to cool the
air between stages of compression, allowing you to burn more fuel and
generate more power.

Just like internal combustion engines

Remember, the limiting factor on fuel input is
the temperature of the hot gas created, because of the metallurgy of
the first stage nozzle and turbine blades. One turbine using this
cycle is the General Electric LM1600 / Marine version

A well known phrase or saying involving Grandmothers Sucking Eggs
springs to mind.


As for their usefulness for gas turbines while they are widely
used in land and marine based applications their weight rules
them out of aviation usage


You'd bet wrong

I bet its more a maintenance issue than a weight issue. Imagine a RB.
211 or Trent. If the IP compressor stator
blades are hollow and water cooled then the pressurized water can be
flashed into steam in the hollow cowling to take advantage of all of
that bypass air, alternatively sodium-Potassium cooling or heat pipes
might be used to 'wick' the heat to the inner cowling.

Trouble is none of those assumptions are even close to reality

Take a look at such an installation - I have installed several

Ok, I did make an minor error in the scenario of a purely after cooled
engine, the compressor power is 880hp leaving only 120hp.

You made lots more errors than that

However in the case of effective inter cooling between 10:1 and 10:1
stages only between 700hp is required if perfect inter cooling is used
the power requirement is only around 550hp

The errors in your assumptions have been displayed

Perfect inter cooling is the situation where the gas is never allowed
to rise in temperature in subsequent stages.

And is non existent as it would require 100 efficiency in the
exchanger. If your cooling air is at 210 K there is no way you
can cool the compressed air enough to reach the same
temperaure

It can be accomplished by integral inter cooling of an axial
compressor. Cooling a water jacket is normal on an
axial compressor, in this case cooling of the stator blades would be
required as well. This can be accomplished with no
pressure loss at all.

This is lucidrous, you are no claiming to be able to make a machine
with 100% efficiency that can cheat the gas laws

<snip>


Lets get back to reality. The single stage supercharger on the Merlin 20
required almost 150hp to drive it at full power and even then the power
output fell off rapidly at altitude.

Actually it gained power before falling off with the fall off not
becoming rapid until after full pressure altitude which in itself
varied as to supercharger gearing and settings.

You are confusing engine power output with the power demands of the
supercharger


The two stage supercharger at full output in high altitude flight
absorbed
around 400 hp and its power output at 35,000 ft was still much lower
than at sea level

The Merlin 61 supercharger required 800hp to deliver air with
24" boost. You are specifying close to 29" boost

Of course an engine running at 24 inches boost is delivering more than
1 atmosphere boost and the Merlin is more than a 1000hp engine. In
English units: 1 inHg = .491098 psi, or 2.036254 inHg = 1 psi so this
is 12 inches boost on top of 14 ie 1.88 ata.

Wrong

One atmosphere = 29.92 inches hg

24 inch of boost is less than one atm


The Merlin engine is not that optimized for high altitude as it is set
up for power from a small block, it has a relatively low compression
ratio of 6.3:1 which limits its expansion ratio and on top of that
while the two stage supercharger was supposedly efficient for its day
it is unlikely to have exceeded 70% by much.

Another ASSumption

You also haven't
specified the operating altitude,

I dont need to, the power to provide 24" boost doesnt vary that much
it just want usable low down

Even the Merlin supercharger wasn't
that powerful. When the Germans finally introduced two stage
superchargers DB605L and Jumo 213E1 they had critical altitudes of
around 9600m (nearly 32,000ft)

A LONG way short of 100,000 ft


Once you figure in all the losses the power required to deliver sea
level pressure air at 100,000 ft going to require more power
than the engine can produce.

With inter cooling we have around 291hp net available, assume some
additional parasitic losses in pumping coolant water and some pressure
losses and we are down to say 200hp.

Only if you use the wrong formulae and ignore the fact that you
dont have isentropic compression. In the real world this is
simply the wrong answer

We still have 300lbs jet thrust or 200hp that can easily be recovered
by an exhaust gas turbine.

You might if the engine could run - it cant


I don't think their is going to be a problem achieving

85% efficiency in the supercharger and even if that efficiency isn't
quite achieved the use of inter-cooling will substantially cut the
work required down

The work done compress n kg of gas from Pressure 1 to Pressure
2 doesnt change just because you use an intercooler.

It does for the subsequent stage because the work of compression is
always dependent on the absolute inlet temperature.

Not when dealing with non isentropic compression


What it does do is help the efficiency of your compressor
by increasing density.

Yes it reduces the size of the turbo machinery and allows it to run
slower but it also reduces the work required of any down stream stage
in achieving a certain compression ratio.

Wrong, you have not accounted for the energy lost by throwing away
energy in the intercoolers

If you need to produce compressed air at a certain temperature it is
better to intercool than aftercool.

For mechanical reasons


On top of that we have hundreds of hp available in the exhaust for use
in either a turbine for either turbo compounding or turbo-
supercharging that would boost our power from 220-330 by about 20% of
1000hp ie 200hp to 440hp-530hp as exhaust gas turbines in turbo
compounded engines recover about 20% of exhaust gas energy which is
almost the same as shaft hp. In our case I think this would be much
more due to the very low backpresssue,

Nonsense, you have already accounted for that energy by giving yourself
the extra 100hp in the engine and you have already boosted the inlet
pressure with your supercharger. The very act of fitting a turbocharger
RAISES the backpressure. This negates your hoped for 100hp
and also loses you the jet thrust

Let me repeat:
worst case scenario with an 85% efficient compressor and aftercooler
is 880hp required power which leaves 120hp net power.


<snip repeated nonsense>

http://www.wipo.int/pctdb/images1/PCT-PAGES/1997/371997/97031192/97031192.pdf

Did you actually read this paper ?

It clearly refers to the use a heat recuperation system
to recover the energy for efficiency. This is a LAND BASED system
and they are suggesting using the heat from the intercoolers via
a recuperator with augmentation to recover the heat energy extracted
in the intercooling process.




So at 100,000ft it is possible to produce usefull levels of power
albeit with superchargers and radiators and even more with a turbo.
The supercharger isn't going to be to big and heavy as the IC engine
doesn't need to much air and the lightly loaded first stages aren't at
too much pressure.

More wishful thinking.

I've done the maths,

Using the wrong assumptions and formulae

without inter cooling there is just enough power
to produce a tiny net output of 12% which would reduce a little when
after cooler pump losses are considered. With inter cooling there is
a significant net output of around 29% of sea level output with 45%
being the theoretical limit.

parasitic pressure losses in the inter cooler are not that significant
while the water pump power required is no more than the coolant
required in normal engine cooling as it is in the same order of
magnitude.

Statements made with absolutely zero basis


<snip>

While we are talking about intercoolers bear in mind
that the intercooler on the Merlin XX required around 30 gallons of
water per minute to reduce the charge temperature to a reasonable
level.

Merlin XX is a single stage Merlin?

With a relatively small supercharger

How much did the coolant pump for the remainder of the engine require?

An irrelevant question when discussing th supercharger



Feel free to list the engines with mechanical superchargers and
exhaust turbines that produced significant jet thrust

CW R-4360 VDT (Variable Discharge Turbine) several hundred pounds jet
thrust
http://www.time.com/time/magazine/article/0,9171,887932,00.html
Napier Nomad 2 (without reheat) about 318lbs

Interesting example. The Nomad 1 had 300 lbs of thrust from a 3000hp
engine with a ceiling of 35,000 ft so a pro-rata conversion would
suggest 100lbs of thrust and of course not even Napier claimed a
ceiling of 100,000 ft

The Nomad 2 was given an extra row on the exhaust turbine to provide
more power for the 12 stage axial compressor and so lost that jet thrust.

The engine was so complex and heavy compared with the turboprop alternatives
that its development was abandoned

It seems that all that is required is a variable area nozzle.

Not to any sane individual






Now take a look at the combined weight of your diesel
engine, intercoolers and compressor and compare that
with a gas turbine of the same power
Sure, the gas turbine might be better but there is not indication that
for subsonic aircraft that piston engined aircraft can't fly as high
as jets though they do seem to use turbochargers. Technically the
heat exchangers/radiators are probably the most challenging task.

There is one indication - NONE OF THEM DO

66000ft for the boeing condor and 80000ft for the Grob Strato 2C

Both well below 100,000 ft, note that the F-15 reached 103,000 ft
and for the last 10,000 ft it was reliant on pure momentum


The max altitude at which a turbo/supercharger system can deliver
sea level pressure air is called the critical altitude. Typically for
late piston engines this was around 25,000 ft and they required several
hundred hp to do this. You seem to think that you can do better and
take it up to 100,000 ft

It can be done. Key is
1 Highly efficient compressor
2 Effective Intercooling.

3) Coupled with an active imagination and a disregard of physical laws

Keith


.

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