Thrust from Air Discharge

Alright, and for convenience, suppose that everything proceeds at 25 degrees C.

Energy in moving from pressure Pa to Pb (ie, 20 whatsits to 1 whatsit, as it is a ratio) is about 2·5*Ln(Pa/Pb) KiloJoules per mole of gas. The average molar weight of a mix of N2 and O2 is easily determined from atomic weights, and somewhere there is a conversion from power = Joules/second = const*Horsepower. Thus, you require a flow of a certain number of moles of air per second to make available that rate of energy flow, and, given the diameter of the inlet pipe and the pressure of 20atm, the air velocity could be determined.

This is based on the full conversion of the available energy to useful work (the length of the moment arm is irrelevant), but you'll have to start doing better than Hero's design to get more than 50% efficiency.

More seriously, for a cyrogenic energy storage project involving liquid air, temperature and phase change will have to be considered.

Hello Nicky:

Thanks for the enlightenment. I must be missing something but it seems to me that a "Hero's engine" should be a pretty efficient device for turning thrust into torque; no turbulence loss upon entry and exit as with a turbine; no significant difference whether flow is laminar or turbulent, and less situation of wind resistance from the back of the turbine blade. Anything you wish to add as to most efficient way to turn compressed air into torque would be appreciated. It is necessary to have high enough surface speed that ice and frost are sheared through centrifugal force which rules out "vane type".

Could you explain the shortcomings of the "hero's engine" as a compressed air motor for me?

I believe I am aware of the other factors "involving liquid air, temperature, and phase change"

bill michaels

It's not just a matter of turning thrust into torque; but rather utilizing as much as possible of the energy contained in the compressed air. If the molecules of air leave the engine with high velocity, they still have significant energy.

If anyone had developed a device more efficient than the turbine, that device would be used in aircraft...

Not to say it can't be done, but billions of dollars have been spent trying...

The nozzle size would be 0.00314" inside diameter, surrounded by a coil of aluminum tubing that is 0.087 inside diameter, wall thickness .005". Liquid heated to a temperature of 137 degrees C. would have to pumped through the coil, to keep the nozzle from freezing, since the expansion of air at the nozzle tip would bring the temperature of the nozzle down to -51.6 degree C., and soon after, the nozzle accumulate moisture, due to the draw of surround air rushing in after the expulsion, and soon after the nozzle would freeze and eventually freeze the water and clog up the nozzle opening.

Guest:

Thank you very much for your calculations.

I am not entirely convinced that heat is necessary to prevent frost buildup. The compressed air presumably contains no moisture, original moisture having been taken out in the liquefaction process, and as the nozzle is spinning at a rate at which the centrifugal force exceeds the shear strength of ice, I am hoping that it will not experience a layer of ice over one molecule in thickness which I plan to exclude from calculations. When shut down a damper ring will prevent outside air from depositing frost.

Don't we need a "@feet per minute" figure to conclude that a .00314 nozzle size will generate 5 horsepower?

Obviously I am no "rocket scientist".

bill michaels

You have a number of problems here:

The DeLavan nozzle is intended to break a stream of liquid into very tiny particles that spread out for rapid combustion. You don't want your air molecules to spread out - That just wastes energy.

There is no such thing as 'tangential to the axis'. An axis is a single straight line. The term 'tangential' refers to a circle.

It took a lot of energy to liquefy that air. Don't waste that energy on anything akin to Hero's engine. There is a reason why jets use turbines!

Sir:

Thank you for your comments. I meant to say DeLaval, and I stand corrected on the wording of the "tangenital to the axis" blunder.

Two questions, if you will, #1, Would a greater amount of thrust be produced by discharging air at 20 atmospheres and 25 degrees Centigrade through a DeLaval nozzle than if a cylindrical short pipe was used?

What are the reasons why a "Hero's" engine is assumed to be quite inefficient? Isn't this classed as a reaction turbine? Other than a piston motor with variable cutoff or a compound motor what would you recommend for a motor? Vane type seems out because of speed and lubrication problems at low temperatures. Not trying to be argumentative, I just don't know.

thanks, bill michaels

Well, to continue with Hero's engine, I'm thinking of a pelton wheel in reverse. These for an incompressible fluid such as water can be quite efficient (more than the bladed turbines operating on the same flow and pressure, and only Pelton wheels can deal with very high pressures) so long as turbulence is not created, which is difficult. Anyway, imagine a hollow disc with 20atm gas inside, and around its periphery are a balanced set of expansion nozzles in the style of rocket engines arranged of course tangentially to the edge of the disc. Gas will emerge from such nozzles at some velocity, caused by the pressure difference being converted to kinetic energy. Now arrange the operating conditions so that the disc will spin at such a rate that its circumference is moving at that same speed (in the opposite direction!) so that (ideally, in the limit), the emerging gas having expanded arrives in the outside world at zero velocity and just sits there with no kinetic energy. Thus, all the energy of the compressed gas will have been converted...

But, the gas velocity will be supersonic. The disc will be spinning rather swiftly, and the gas inside it will be affected, and by centrifugal force too. And there will be temperature changes.

This is why gas turbines have multiple rings of blades (with gas flow parallel to the axis), across each set of which there is only a small pressure drop so that the gas speeds remain low so that the optimum turbine speed remains low. No violent transitions means low turbulence and good efficiency. Handbooks for engineers have endless details on design and performance.

Sounds that it could be awfully loud.

(talking seriously with tongue in cheek)

well..seeing as no one else wants to say it.. I will. Nikola Tesla invented a method of turning fluid pressure or flow into mechanical rotation. He called it the Bladeless Turbine. Your best link is the group of people (TEBA) who have been working diligently to create commercial applications. Its fascinating if nothing else.

Chris

Hi Chris:

I have studied the "Tesla Turbine" and don't see much efficiency in the smaller diameters. My design uses "bladeless turbine" principles to generate a flow of air between the surface of a number of hollow plates for the purpose of evaporating the liquid air. I believe that it is agreed that a bladeless turbine can be used as a compressor by spinning it.

The big problem with former liquid air or liquid nitrogen motors has been frost buildup, which I plan to control by spinning the surfaces of the heat exchangers fast enough that the frost and ice are displaced by centrifugal force.

Speaking of facinating things, know anything about the "Bourke Engine"? This was a crude twocycle device which through controlled detonation would demonstrate efficiencies comparable to the best of the big commercial diesels. He spent years demonstrating that it would spin an airplane propeller of a given rating at a given speed and burn a certain amount of fuel doing it. "Experts" refused to look at it as it was clearly impossible. It is not quite as remarkable today as modern car engines are now pretty effecient and clean burning, but it probably would make an excellent generator engine for a hybrid automobile.

ayyon

hmm. not experienced in these areas very much.. but I do remember one tidbit fact... (I was looking into supercentrifugation) If you take quartz glass, and form it, and anneal it perfectly (reflow the surface and remove all casting/forming stresses), it can withstand up to 800,000 psi.

This would be a great application, because the annealed surface would be close to frictionless. Even touching with fingers puts microscratches on the surface, and creates a seed where cracks can start at high force levels. but done right, with no scratches, the high centrifugation, low friction surface, amorphous material, might be a solution. also, it is non reactive in most cases to chemicals, oxygen, etc. If you are well funded, then it might be worth a test or two.

Only shortcomings are from vibration and the potential for glass to flow with time and force. With the right recipe, design and reinforcement.. maybe it could work. (I used to have a small 1960's era book on glass, and I believe the source was Dow-Corning, as it was also discussing the 200" Palomar mirror and the development of pyrex, etc)

As for Tesla, I believe the trick was in high rpm and disk spacing. Lower means closer disk spacing and higher rpm's, which of course runs into material strength problems. (30000 rpm on a 10 inch disk is pushing it.. but then he didn't have aerospace materials available now)

can you solve the problem of pumps and evaporation by having a large piston and cylinder, and the withdrawal of the piston creates a very low pressure, thereby causing the boiling point to be exceeded, and evaporation resuts? The drawing of the piston acts replaces the pump by sucking the fluid in for part of the stroke, and then after the intake valve closes, a vacuum pump for the rest of the [long] stroke. maybe i'm off base there.. not knowing the applicaiton..

Chris

Hello Chris:

My criteria is to design a motor to run on liquid air as cheap to construct, as long lasting, as environmentally favorable, and as efficient as practical. Initially these could be expected to operate rickshaws, golf carts, forklifts, and the like.

From my perspective, piston or even vane types are out of consideration because of the solidification of lubricants, and the undesirability of having traces of them in the exhaust air. Present design (untested) consists of a 1 1/4 inch shaft bored and closed at one end of the bore through which liquid air is introduced. A number of hollow copper washers somewhat resembling brake rotors are fixed to this shaft with a "standpipe" conducting the liquid air to the extreme outside of these "washers". This "standpipe would act like a pump through the centrifugal force acting on the liquid air inside, and aided by a pressire cap on the liquid air tank, be expected to keep the space partially filled.

These copper disks are narrowly spaced and "warmant" air because of the boundary layer principle will flow between them transferring heat through the surface of the hollow disks causing evaporation of the liquid air confined inside them. A tube would connect the thrust nozzles and the part toward the center of the hollow disk which contained already evaporated air segregated by centrifugal force.

One moving part; unless you count the governor mechanism. A thin layer of teflon or similar material would drastically lower the amount of force required to shear the ice and frost. The devil is in the details, but I believe I have a solution to all I have anticipated.

Sorry I can't explain things very well.

I had not considered glass; and certainly will give some thought to your "piston" idea. Something to think about in the future. As to funding, I have none currently. The state of Indiana used to have a TECHNICAL ASSISTANCE PROGRAM which previously helped us, but the program was canceled. Here in the USA. lobbys have sprung up for the lithium battery industry, the hydrogen fuel cell industry, and the ethanol industry, and if you don't fall into one of those categories you are not considered "legitimate".

bill michaels

Bill,

cool application.. glad to hear about it.

I'll keep thinking about it. in the meantime.. there was this article where somebody got money for a liquid air vehicle.

Chris.

Those guys failed bigtime, and had to return the remainder of their grant money. If they had copied one of the experimental cars of a century ago they would probably have been considered successful. Dr. "Mitty" Plummer (University of West Texas) built a successful one using all "off the shelf" components on a VW chassis. His heat exchangers were baseboard heaters and didn't take up much room at all or weigh much. Consider the very large temperature differential and high velocity air flow involved here, and the fact that we are in the 5 hp range.

ayyon

The trick might be to mix it with liquid alcohol and combust it. the Heat to convert the liquid air to gaseous air (and liquid alchol to gaseous alcohol) has to come from somewhere, and to get it from the environment may well be too much to ask of a system that is to run in a vehicle. it would be a rolling heat exchanger. Alcohol is clean burning, mixes with air liquid air, or gaseous air, and can be produced in third world countries from biomass.

chris

Thanks again for your ideas. I guess if I had alcohol, I would be thinking along the lines of a "Tesla Turbine" spun with the hot gases of a ramjet or pulsejet burning the alcohol. (too much precision and expensive materials in a turboprop) A five HP Briggs&Stratton lawnmower engine will already do a fairly good job of turning alcohol into torque, and doesn't seem to cost much.

I am aware of the fact that condensing steam cars require huge heat exchanger surfaces, and according to my calculations was pleasantly surprised at how small an area of even ordinary copper in a pretty brisk air stream would transfer enough BTUs to evaporate enough liquid air to produce five horsepower.

Years ago, I calculated how it would be possible to modify a Corvair van with a huge compressor and an even bigger heat exchanger "super heat pump style" and utilizing the heat thus harvested from the atmosphere to operate a steam engine. While I concluded that it was possible, I also concluded that the initial additional cost would be much greater than any potential fuel saving.

thanks again for your ideas.

I took these screenshots from the following book.. you can download it here

you can also get the deja vu reader here

Chris.