Can convergent nozzles convert heat into motion?

In summary, a convergent and/or convergent-divergent nozzle can convert internal heat into forward motion, resulting in an increase in dynamic pressure. However, this increase is limited to a certain temperature and the nozzle cannot convert heat into velocity beyond the speed of light.
  • #71
I've been thinking about the initial question:
pranj5 said:
In case of Nitrogen at 4 barA pressure and 27°C, if a turbine is used to release the Nitrogen at 1 barA with a flowrate of 1 kg/sec; then the output is around 95 kW. Now, if the pressurised Nitrogen is released through a convergent or c/d nozzle shaped structure before the turbine, it's velocity will be higher than the previous case. Does that means effective rise in the pressure? If yes, then how much?

The work of a turbine is the difference between inlet and outlet enthalpy, ##w_t = h_{out} - h_{in}##.

If you put a nozzle before the turbine inlet, it won't change anything for the for the turbine, because for a nozzle ##h_{out} = h_{in}##. So the same enthalpy will be available at the turbine inlet.

Enthalpy can be calculated based on total temperature, i.e. ##h = C_p T_0 = C_p \left(T + \frac{v^2}{2C_p}\right)##. Thus, for a nozzle, whatever increase in velocity you get, it will be at the expense of a temperature decrease. Enthalpy wise, you gain nothing, you loose nothing and the turbine sees the same thing.

The only reason to put a nozzle at the inlet of a turbine would be to adjust the flow conditions such that the turbine is doing its job as efficiently as possible.
 
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  • #72
jack action said:
Enthalpy can be calculated based on total temperature, i.e. h=CpT0=Cp(T+v22Cp)h = C_p T_0 = C_p \left(T + \frac{v^2}{2C_p}\right). Thus, for a nozzle, whatever increase in velocity you get, it will be at the expense of a temperature decrease.
That's exactly I want to know. Enthalpy gain is simply impossible because that's against 1st law of thermodynamics. But, by converting internal heat into motion, we can convert a little more of the gross enthalpy into power IMO.
No nozzle (actually nothing) can change the gross enthalpy of a fluid stream (in fact anything) without some kind of energy input or extraction. That's violation of 1st law of thermodynamics. But, what matters here is the question of exergy. Motion can be more useful in extracting energy than internal heat alone.
 
  • #73
pranj5 said:
But, by converting internal heat into motion, we can convert a little more of the gross enthalpy into power IMO.
You just said that you agree that enthalpy is the same, why do you insist it will be converted to something else?

Power is the mass flow rate times the enthalpy (##\dot{m}h##). The nozzle doesn't change the mass flow rate nor the enthalpy, thus potential power cannot be affected.

Like I said earlier, any machine is designed to perform based on certain inlet & outlet conditions. In a case of turbine, it works best with a high velocity flow and a low pressure pressure differential. But if you replace your turbine by a piston engine, it will be better to have a low velocity flow and high pressure differential. Both will produce the same power under correct flow conditions, but the torque-RPM output will also be different. A gearbox can be used on the shaft to adapt the torque and RPM to what you need. In any case the power is conserved (minus some minor friction losses).
 
  • #74
jack action said:
You just said that you agree that enthalpy is the same, why do you insist it will be converted to something else?
I want to mean that the gross amount of energy stored in the fluid will be the same while the form may change. I have repeatedly mentioned 1st law of thermodynamics.
jack action said:
Power is the mass flow rate times the enthalpy (˙mhm˙h\dot{m}h). The nozzle doesn't change the mass flow rate nor the enthalpy, thus potential power cannot be affected.
What you want to mean is the gross amount of power embedded in the flow, point is how much can be converted into useful power.
jack action said:
Like I said earlier, any machine is designed to perform based on certain inlet & outlet conditions. In a case of turbine, it works best with a high velocity flow and a low pressure pressure differential. But if you replace your turbine by a piston engine, it will be better to have a low velocity flow and high pressure differential. Both will produce the same power under correct flow conditions, but the torque-RPM output will also be different. A gearbox can be used on the shaft to adapt the torque and RPM to what you need. In any case the power is conserved (minus some minor friction losses).
There is no doubt about that because that will violate 1st law of thermodynamics. Again, question is how much of the embedded power can be converted into useful power.
 
  • #75
pranj5 said:
Again, question is how much of the embedded power can be converted into useful power.
Again, it depends on the design of the machine itself. It must be adapted to the given flow to produce as much power as possible. The capacity of a machine to produce useful work is called isentropic efficiency. No flow is better than the others. No flow has the potential of producing more work than another. They all have the same potential.
 
  • #76
The nozzle itself here is a part of the machine design.
 
  • #77
pranj5 said:
The nozzle itself here is a part of the machine design.
It doesn't change anything to my statement.

For example, if you add a nozzle to accelerate the flow feeding a piston engine, you will most likely see a decrease in performance. Even with a turbine, you will see a drop in efficiency if the velocity gets too high.

You cannot state that an increase in velocity necessarily correspond to an increase in isentropic efficiency.
 
  • #78
Just give a look at the http://twisterbv.com/products-services/twister-supersonic-separator/how-it-works/ below that works well with supersonic input.
fig11.jpg

In short, such machinery is available that can perform with supersonic velocity.
 
  • #79
From what I can tell, the device you have pictured has absolutely nothing to do with the questions you've been asking. For example:
  • It does not feature a turbine.
  • It's inlet is subsonic; it only generates supersonic flow downstream of its vortex generator using a Laval nozzle.
  • The primary purpose is not power generation, but instead the use of supersonic expansion to cause a temperature drop that allows water and hydrocarbons to condense and be centrifugally separated from the gas flow.
This really is not germane to the previous pages of discussion.
 
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  • #80
pranj5 said:
Just give a look at the http://twisterbv.com/products-services/twister-supersonic-separator/how-it-works/ below that works well with supersonic input.
fig11.jpg

In short, such machinery is available that can perform with supersonic velocity.

I am not sure why you think that this machine represents a nozzle that converts heat into motion and enables air to move from a low pressure zone to a higher one. The input to the system had both a higher temp AND pressure than the outlet.
 
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  • #81
RogueOne said:
I am not sure why you think that this machine represents a nozzle that converts heat into motion and enables air to move from a low pressure zone to a higher one. The input to the system had both a higher temp AND pressure than the outlet.
I am not. It's just an example that there is machinery that can work well with supersonic flow.
350px-Turbines_impulse_v_reaction.png

The above graph of the impulse turbine shows that it's the pressure, that will be converted into velocity first and then this velocity will produce power. Now, anybody can calculate that whenever the pressure difference is 3 bar and above, the flow coming out will be supersonic. And I am sure that impulse turbines in use at present are working with much more than the mentioned pressure difference.
 
  • #82
upload_2017-2-16_20-48-7.png

The photo given above is from a home made experiment. I have made a simple homemade device where I have put a turbine like structure inside a tube and placed a convergent nozzle like structure at the inlet and placed the structure before a table fan. Thanks to one of my friend who temporarily handed me over the infrared camera to take pictures. I have take and few pictures with varying speed, but every time I have found that the air coming out of the structure is colder than the inlet like the picture given above. Though the slower the input speed, the lesser is the difference.
Now, what can be proved by it. At least I can say that the nozzle like structure have converted internal heat of the input flow into velocity and that has been converted into power. That's why the exhaust is colder.
 
  • #83
I have no idea what I'm looking at.
 
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  • #84
pranj5 said:
View attachment 113309
The photo given above is from a home made experiment. I have made a simple homemade device where I have put a turbine like structure inside a tube and placed a convergent nozzle like structure at the inlet and placed the structure before a table fan. Thanks to one of my friend who temporarily handed me over the infrared camera to take pictures. I have take and few pictures with varying speed, but every time I have found that the air coming out of the structure is colder than the inlet like the picture given above. Though the slower the input speed, the lesser is the difference.
Now, what can be proved by it. At least I can say that the nozzle like structure have converted internal heat of the input flow into velocity and that has been converted into power. That's why the exhaust is colder.
This is tough to interpret, but it seems you might be circling back to what you were told in post 2: during expansion out of a nozzle, internal thermal energy is converted to work. You were claiming the opposite: that during compression, you could convert thermal energy to work. That is still wrong.
 
  • #85
pranj5 said:
I am not. It's just an example that there is machinery that can work well with supersonic flow.

Except that isn't what you showed. That image had a series of vanes that interacted with subsonic flow, then expanded the flow to supersonic speed in order to utilize the resulting temperature drop. At no point was any energy extraction occurring. In fact, in general, supersonic flow is a huge negative when it comes to flows interacting with machinery since it must necessarily involve shock waves, which increase entropy, thereby decreasing the available pool of energy for extraction.

pranj5 said:
350px-Turbines_impulse_v_reaction.png

The above graph of the impulse turbine shows that it's the pressure, that will be converted into velocity first and then this velocity will produce power.

You didn't source the image so I can't comment on exactly what it is showing, but it isn't showing what you claim it is. This (1) is not dealing with compressible flows, (2) is not dealing with supersonic flows, (3) is not quantitative and therefore not a definitive source on exactly what the pressure and velocity are doing simultaneously with respect to energy extraction, and (4) appears to be simply showing some sort of comparison between these two methods of spinning a turbine, not any of the things you are trying to extract from it.

pranj5 said:
Now, anybody can calculate that whenever the pressure difference is 3 bar and above, the flow coming out will be supersonic.

Really? Show me how to prove this. Show it using just raw variables if possible, but if you really prefer to use actual values, I choose a reservoir pressure ##p_0 = 10\mathrm{\; bar}## and a downstream pressure of ##p = 7\mathrm{\; bar}##.
 
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  • #86
boneh3ad said:
I have no idea what I'm looking at.
An infrared photograph of the device during its work.
 
  • #87
russ_watters said:
This is tough to interpret, but it seems you might be circling back to what you were told in post 2: during expansion out of a nozzle, internal thermal energy is converted to work. You were claiming the opposite: that during compression, you could convert thermal energy to work. That is still wrong.
The turbine like structure is inside the tube and the nozzle like structure is at the inlet. Therefore, if the turbine doing some work here, it's out of the nozzle. How you are interpreting something real is beyond my knowledge.
Can you tell me which factor is reducing the temperature of the input air?
 
  • #88
pranj5 said:
An infrared photograph of the device during its work.

But you've made no indication of scale or where the walls or devices are located or pressures or anything. The photo is essentially useless without more information.
 
  • #89
pranj5 said:
The turbine like structure is inside the tube and the nozzle like structure is at the inlet.
What device from which post are you referring to? If you are referring to Post #82, the only thing I know for sure about the setup (a regular photo would help...) is you powered the device with a table fan, which means it is way, way below the velocity required for the effects of pressurization to manifest.

Also, air is transparent to the infrared, so you can't take pictures of it with an infrared camera. So there's that too...
 
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  • #90
boneh3ad said:
But you've made no indication of scale or where the walls or devices are located or pressures or anything. The photo is essentially useless without more information.
It's a homemade experiment and laboratory like perfection can't be expected.
russ_watters said:
Also, air is transparent to the infrared, so you can't take pictures of it with an infrared camera. So there's that too...
As far as I know, we can study the effect of temperature to air by adjusting the infrared. Actually, I can't say much about that as my friend is the expert. He has adjusted the camera to detect whether the air coming out is colder or not. But I can say that the picture isn't something made with animation. It's real!
 
  • #91
pranj5 said:
It's a homemade experiment and laboratory like perfection can't be expected.

Take your photo, open up MS paint or whatever image editor you prefer, and draw on it where objects are located. Annotate it. Don't get snippy with us when we tell you that what you have posted doesn't make any sense as it is presented. That sort of attitude is not any way to get people to help.

pranj5 said:
As far as I know, we can study the effect of temperature to air by adjusting the infrared. Actually, I can't say much about that as my friend is the expert. He has adjusted the camera to detect whether the air coming out is colder or not. But I can say that the picture isn't something made with animation. It's real!

A couple points here:
  1. You shouldn't interpret results when you don't understand how those results were obtained. That's a fundamentally flawed method of experimentation. I'd suggest you go back and try to understand exactly what it is you are measuring.
  2. You really need to explain your experimental setup better if you want us to be able to help you with that. You still haven't clarified what you did or how you were even able to see the air through the tube. At any rate, my suspicion is that you are actually measuring the temperature of a surface somewhere that may be in contact with the air, so given enough time, it should tell you the temperature of the air at that location.
pranj5 said:
Now, anybody can calculate that whenever the pressure difference is 3 bar and above, the flow coming out will be supersonic.

I still would like you to address my earlier question about this, please. Show me why this is true. Prove it to me.
 
<h2>1. How does a convergent nozzle convert heat into motion?</h2><p>A convergent nozzle works by using the heat from a combustion reaction to expand a gas, typically air, and accelerate it through the nozzle. This acceleration creates a force that propels the nozzle and any attached object in the opposite direction.</p><h2>2. What is the principle behind the conversion of heat into motion in a convergent nozzle?</h2><p>The principle behind the conversion of heat into motion in a convergent nozzle is known as the conservation of energy. The heat energy from the combustion reaction is converted into kinetic energy, which is the energy of motion.</p><h2>3. Can any type of heat source be used to power a convergent nozzle?</h2><p>Yes, convergent nozzles can be powered by various heat sources such as chemical reactions, electricity, or even solar energy. However, the efficiency of the conversion may vary depending on the type of heat source used.</p><h2>4. Are there any limitations to the conversion of heat into motion in a convergent nozzle?</h2><p>One limitation of using a convergent nozzle to convert heat into motion is the maximum temperature that the nozzle can withstand. If the temperature exceeds this limit, the nozzle may become damaged or fail to function properly.</p><h2>5. Are there any practical applications of using convergent nozzles to convert heat into motion?</h2><p>Yes, there are many practical applications of using convergent nozzles to convert heat into motion. Some examples include rocket engines, jet engines, and gas turbines, which all use convergent nozzles to convert heat into motion for propulsion. Convergent nozzles are also used in industrial processes, such as in steam turbines, to generate electricity.</p>

1. How does a convergent nozzle convert heat into motion?

A convergent nozzle works by using the heat from a combustion reaction to expand a gas, typically air, and accelerate it through the nozzle. This acceleration creates a force that propels the nozzle and any attached object in the opposite direction.

2. What is the principle behind the conversion of heat into motion in a convergent nozzle?

The principle behind the conversion of heat into motion in a convergent nozzle is known as the conservation of energy. The heat energy from the combustion reaction is converted into kinetic energy, which is the energy of motion.

3. Can any type of heat source be used to power a convergent nozzle?

Yes, convergent nozzles can be powered by various heat sources such as chemical reactions, electricity, or even solar energy. However, the efficiency of the conversion may vary depending on the type of heat source used.

4. Are there any limitations to the conversion of heat into motion in a convergent nozzle?

One limitation of using a convergent nozzle to convert heat into motion is the maximum temperature that the nozzle can withstand. If the temperature exceeds this limit, the nozzle may become damaged or fail to function properly.

5. Are there any practical applications of using convergent nozzles to convert heat into motion?

Yes, there are many practical applications of using convergent nozzles to convert heat into motion. Some examples include rocket engines, jet engines, and gas turbines, which all use convergent nozzles to convert heat into motion for propulsion. Convergent nozzles are also used in industrial processes, such as in steam turbines, to generate electricity.

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