The advantage the constant-volume combustion type gas turbine engine

[oatlids]

Qumf,
I do not deal directly with wave rotor engines, but I do have my hand in constant-volume combustion type concepts. Constant-volume combustion is always the goal but is quite difficult to achieve. You will realize this as you move from theoretical to experimental work. I would look more towards isolating whether your ideas on achieving constant volume combustion is feasible. Constant volume combustion is very well-defined.

As far as the wave rotor engine, you may be able to find more research regarding it if you have a subscription to a journal or are affiliated with a university with access to such. AIAA is a common on in our industry. There are quite a few Chinese Universities that do propulsion research and have publication archives as well. The wave rotor engine if you read the descriptions online is more of a "pressure-gain" combustor. Constant-volume combustion is that hard to achieve. Pulse detonations engines are a more common approach to achieving constant-volume combustion.

Notes:
How do you plan on injecting the fuel and air in the rotating chambers? Moreover how do you plan to make it so that it can be considered pre-mixed?
Since you say that the flow is relatively stagnant at the time of combustion, how do you plan on ejecting the gas to the turbines for work extraction? If you were planning on relying pressure expansion to purge gases, the turbines now see a lower pressure than you estimated at time of combustion, does this combustion process still provide a benefit?
Since you don't have flame holding in your design, how do you plan on initiating the combustible mixture?

 

[qumf]

Thank you for the concerns. I try to answer your questions:

“How do you plan on injecting the fuel and air in the rotating chambers? Moreover how do you plan to make it so that it can be considered pre-mixed?”-----Do you know the old type internal combustion engine on gasline? I plan to use same principle to mix fuel to air.It uses carburettor. Of course I need to improve the structure to get much better effection. The fuel will be injected at many points.
"Since you say that the flow is relatively stagnant at the time of combustion, how do you plan on ejecting the gas to the turbines for work extraction? If you were planning on relying pressure expansion to purge gases, the turbines now see a lower pressure than you estimated at time of combustion, does this combustion process still provide a benefit?"----I can not understand the sentences well. I can say, the gas pressure just after combustion and it at the turbine can be much different because there is branch pipe system the two places. There are many times of combustion happen in turn, not at the same time. they support each other to push turbine. I suggest you read my article again.

"Since you don't have flame holding in your design, how do you plan on initiating the combustible mixture?"------because the front door is rotating, and it is thick comparatively, while working orderly, the flame is led from one chamber to the one neighborhood through a path inside the front door. To choose the proper position(occasion) in the front door can get the proper temperature of the flame.(it is not the most hot, but can initiate gas)

 

[qumf]

Continue to discuss the feasibillity,
Now I discuss the the structure and function of exhaust system, that including the branch pipes, general pipe and turbine.

The branch pipe, general pipe,the turbine install behind the chambers.

The function of branch pipes behind the chambers:

because I need energy to drive the compressor(blower) in front of the chambers, the common way is to install a turbine to receive the energy of the exhaust gas from the chambers. So I set a turbine.
When the burnt gas will go through the turbine, we need to keep the speed and pressure stable. It’s good to utilize the kinetic energy efficiently from the gas and is benificial for turbine to bear the load.
for a certain exhaust branch pipe, the gas spout intermittently, so the condition of gas inside a branch pipe is changable. I have to arrange these flows to cooperate to get a comparable stable current.
So I set a group of output branch pipes after the chambers. each branch pipe is corresponding to a chamber. then these branch pipes merge one general pipe. the turbine wheel is installed within the general pipe.
several pairs of chambers spout gas alternately and cooperate, we also can add a space behind branch pipes as buffer storage and to regulate gases before turbine, when the gases encounters the turbine, the condition is relatively stable.
These branch pipes have a certain volume space. when the gas rush out off the chambers, A certain volume of buffer storage can reduce the fluctuation.depending on the branch pipes, at last in the general pipe the flow can become comparable stable.
When the working course turns to exchange tha gas ,(ie,the fresh gas enters the chambers),the speed of gases in the corresponding branch pipes is the highest and the pressure is lowest at the end of the spouting course, the Momentum of the burnt gas cause the negative pressure inside the corresponding chambers, it can suck the fresh gas into chambers afterward.
Without the branch pipes, when gas spouts out, the current will influence the gas flow in the other branch pipes, such as the course of exchange gas . Installing the branch pipes; the course will help each other by the effect of suction.
The quantity, the shape and the size of the branch pipes should be studied carefully, They are relative to the functions I state upper. it is nessesary to do some experiments to while building the engine.
The shape of the branch pipe to genaral pipe should be designed carefully in order that the flows can cooperate well, the energy loss and flow resistance should be as possible as small.

 

[qumf]

I try to calculate the temperature before the turbine:
Pls see below figure 4: the figure show how I find their relationship of the main parameters in different course. (Sometimes I call constant-volume combustion type as closed combustion type)

Normally the pressure after compression is pretty high, if other things do not change, we make the pressure by combustion lower , the efficiency reduce a little, meanwhile the temperature before turbine reduce much,
I make a example: Set the temperature of outside air: 300K, k=1.4, set the pressure after compression/before compression α=7, the pressure after combustion/before combustion β=7.5, thus the temperature turbine 2206K; theory efficiency: η=60.2%； k=1.4, α=7, β=7, the temperature turbine 2100K, η=59.6%; k=1.4, α=7, β=6.5, the temperature turbine 1992K, η=58.9%; k=1.4, α=7, β=6, the temperature turbine1881K, η=58.2%;
Actually changing β is by change the volume proportion of fresh gas in each time or the concentration of fuel a little. In this example, η is reduced by 3%, the temperature before turbine decrease by 400K.

α and β all contribute to η. usually we expect the temperature before turbine not very high because of reliabity and life, thus in order to ensure a certain η, we need to increase α.
High temperature normally means high efficiency,but if it is too high, it will cause a series problem, it will influence the life and reliabity of the engine. Because the restriction, sometimes we have to make concession on efficiency.
This example just reminds the relation. The data to some parameters may not be so precise; Some modulus will change a little from in normal case to a very high temperature. Here I assume them unvaried for convenient study . Anyway it is a good enough reference for us.
The method also is used to calculate the temperature of each work course.

 

[qumf]

I have to compensate a little.
There is a sentence in the upper derivation "I set the pressure before turbine same as it after compressor. You can the same description appear in #9 floor. You can refer to it.

 

[qumf]

For years I always hope to find a way to estimate the time spent for each work course: combustion, spouting out off the chambers, exchange gases by suction (fresh gas and burnt gas) in chambers.
According to the data from some manuals about the internal combustion engine, I know the combustion course need several millisecond to complete. They have similar condition for combustion referred to combustion theory.
Until to the recent days, I find a way to estimate the time spent on later two courses. I study the principle and motivation of the gases in chambers and branch pipes ,I study their change on the gases conditions and interactions ,at last find the reasonable one after many times of hypothesis. That basically is based on Engineering Thermodynamics.
By calculation, each course costs also at the same level of millisecond. Thus the whole work cycle can continue even if accord to the current layout. Of course a few parts' sizes and proportion need to be adjusted a little and some features should be noticed.

 

[qumf]

I always think of how to expand the use of the engine,recently I think over if it can be used as the jet engine on the rocket flying in atmosphere.Because in this field engines usually are used once, the engine neednt those reinforcement parts, the engine will be much lighter.
Compared with ramjet, this jet engine can start at much lower flying speed., because by this type of combustion the ignition is easilier.
The engine has some advatages in the field, the turbine, compressor, and even most vane wheels can be cancaled if the rocket flys in very high speed.
It is possible that the back door of the combustion chamber can be omitted if the fuel can mix into air vey well in front of chambers and burn out very soon insides the chambers. thus the general outlet pipe also can be abandoned. but the branch pipes for outlet must be retained.
of course the shape of chambers and branch pipe will be studied carefully and changed a little.
The front door of the combustion chambers rotates and is driven by the inlet gas through a simple way, which provide kinetic energy while the rocket flys at very high speed. after all the rotation need not much energy.
the whole structure can be simplified a lot ,meanwhile some reinforcement and safety parts are ignored, the whole weight will decrease a lot as long as it is used only once and short time.

 

[qumf]

I have stated the functions of the branch pipes for the output gas,A reader ever mentioned the front door of the chambers is a importent one. Turly!

I summarized the three main functions of the front door. They must be realized.
1, As the passage for the input gas which let the streams of mixed gas enter chambers in turn and orderly.
2, As the wall to form the combustion space with chambers, so it bears the considerable force and heat, but they are basically stable for the front door while working.
It is very important to control the clearance with the end face of chambers. It need a special structure.
3, As the passage for the blame from burnt chambers to refresh chambers to ignite the mixed gas.

So the shape of the front door need to be considerd carefully, to allocate each place/space of the front door to realize the three main functions.

 

[KennethLangr]

Although it won't improve efficiency much, there are also no moving parts in the inlet ducting for the CT inlet pressurization proposed by myperfectworld - using some compressed air to create a venturi effect to "induce" more airflow sounds plausible. Loss of output in warm ambient or at higher elevations is a significant problem. (See Mechanical Engineering Thread) On a costant volume CT (land based power generator,) a motorized fan could also be used - but of course pressure pulsations and potential fan blade failure upstream of the compressor would be a concern. A compressor stall could also result if the combustion process didn't back down fast enough as result of the booster fan or auxiliary "blower?" shutting down unexpectedly.

 

[qumf]

Thank for your reply.

Though there is a little words in your reply which I can’t understand, I try to explain as possible as I can.

If I design the engine with one pair combustion chambers, one pair outlet pipe, the problem you worried will happen probably. But I design it with several pairs of chambers, and several pairs of branch outlet pipes and a general outlet pipe. These aspects will improve a lot. Thus they can cooperate, some certain process must need help from other process in other chambers or other pipes.
For a few issues you mentioned, it will exist no matter what type engine, even if you choose constant-pressure combustion engine.

On the other side, I will set a minimum startup speed. (an rotation speed for the engine). Only within a proper speed, the engine can work orderly.

I will give a sketchy feasibility report about heat and force later though that mainly can be got from internet already.

What does “CT” mean in your reply?

 

[KennethLangr]

Sorry. CT is combustion turbine.

 

[qumf]

Recently I think we can simplify the engine, the compressor will be cancelled, replaced with a fan or a pipe. The inlet gas pressure before combustion will decrease a lot and the total efficiency will be smaller definitely.

After studying the principle and designing the structure carefully, the modified one can suck the air from outsides automatically. Meanwhile the structure after combustion will change to be simple.

Thus the engine is like the current detonation engine to some extent. But it may be easy to realize comparably. I think.

 

[DrClaude]

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