Does a Hairdryer blow harder with heat

[Jedi_Sawyer]

klimatos- I think you are wrong. That is the basic definition of temperature, a measure of energy? When it comes to gases, the kinetic energy, 3/2 MV^2, is all the sensible heat you can measure, so for a unit volume if the air feels warmer it means the kinetic energy is higher, period.

 

[Jedi_Sawyer]

So that takes care of the motor speed change. The heater must increase pressure and volume, in some relative proportion but I still don't see why, if a bigger volume of air is leaving the tube, why that doesn't correspond to greater velocity and, hence, more momentum.
* Jet engines work the same way - only the effect is more extreme. In that case, the added energy (chemical burning) definitely produces more thrust than the compressor would, on its own.
I think we are definitely getting somewhere with this now.

I think we should be talking about air molecules leaving the tube/per second, and the velocity of those molecules. So we are talking about the momentum of the hairdryer's exhaust. If we increase the back pressure against the fan, as we assume we are doing when the heater is on, that means the fan is not pushing as much air through the hair dryer as it did when the heater is off. The air that is pushed through the heater is heated up meaning, the kinetic energy of that air was increased so it has a higher velocity. So the exhaust of the hairdryer is (less air) x (traveling faster) = momentum = pressure you can feel = approximately the same momentum as you had before you heated the air.

I am surprised myself how the two effects seem to cancel each other out for momentum from a hairdryer.

 

[sophiecentaur]

Except that the load / speed characteristic of the motor would affect the actual change in mass flow of the air.
You have two engines involved. The Fan drives the air and then the heater is a further energy source so you have a heat engine - it's like a very feeble jet engine in more or less every respect.

I think that the hair dryer result they got is only a special case and it 'just happens' that there was no detectable difference. That could be confusing the wider issue.

 

[256bits]

This post is getting to the heart of the question of this thread. The air damming effect basically means that turning the heat on in the hair dryer causes the pressure to build up in front of the fan and causes the air mass transfer to decrease for the fan. That is the heater is blocking the airflow and the air that is being reflected back at the fan is more energetic when the heater element is on, so it will cause more momentum from the fan to be canceled out.

I wanted to explain why heating the blowing air was not creating more momentum for the hairdryers heated air output which is what I experimentally got when I tested for it, see http://api.viglink.com/api/click?fo...=P_ZcpC2Tlk8&jsonp=vglnk_jsonp_13906829808369

Your comment that momentum should increase also was what I thought when I was trying to do another much more ambitious experiment, and was surprised when it does not seem to be true.

No. I never said that momentum increases as a whole.

Since we seem on the same page.
The air stream is moving to the right, let's say. We then add heat. The volume increases. The exit pressure still is at atmospheric and the pressure from is the fan is also the same. So the stream expands in both directions, towards the exit and towards the fan. So while it has gained momentum towards the exit, gaining velocity, but it has lost momentum towards the fan loosing velocity. The expansion towards the fan gives a slight rise in pressure on the fan side, which becomes what you are calling the the damming effect. The mass flow should then decrease slightly. The lower mass flow together with the higher exit velocity will give the same exit momentum as previously with the heat off.

That is my desciption in layman terms what is happening, except I am not sure sure about the last statement of the exit momentum being the same with heat on and with heat off. I would have thought it would have increased slightly.

 

[nsaspook]

I was in the car today and I came to that conclusion too. It makes sense and accounts for the change of tone as the fan does more work against the back pressure of the heated air. *
I think we can forget about 'advanced' dryers with fancy temperature control as this effect happened with ancient two speed / two heat models.

I just think you have to careful about what's really happening in systems designed to limit or maintain the exit temperature for human comfort when changing fan speeds. Even ancient car two speed / two heat models have air-blending systems adjusting ducting and mixing that could the change back pressure independent of fan speed settings while limiting temperature. The physics of an independent fan and ducted heater system without designed limits is really the subject being discussed (very nicely by others than me) here.

The hairdryer is a good example of engineering using basic physics to increase the heating power:
http://www.engr.mun.ca/~molgaard/courses/1000/dryerlcth.htm

 

[256bits]

Except that the load / speed characteristic of the motor would affect the actual change in mass flow of the air.
You have two engines involved. The Fan drives the air and then the heater is a further energy source so you have a heat engine - it's like a very feeble jet engine in more or less every respect.

I think that the hair dryer result they got is only a special case and it 'just happens' that there was no detectable difference. That could be confusing the wider issue.

A feeble jet engine is what I was tying to point out in post #6.
I expected there to be some extra thrust, but perhaps the compression of the air from the fan is just not enough, since only the friction from the interior walls and parts would be responsible for any back pressure.

 

[klimatos]

klimatos- I think you are wrong. That is the basic definition of temperature, a measure of energy? When it comes to gases, the kinetic energy, 3/2 MV^2, is all the sensible heat you can measure, so for a unit volume if the air feels warmer it means the kinetic energy is higher, period.

Air temperatures are a measure of the mean kinetic energy of translation per molecule. The kinetic energy per molecule is higher, as I said in my post. However, the total kinetic energy per unit of volume is not.

At 25°C, one cubic meter of dry air will have 2.43 x 10²⁵ molecules and the mean molecular kinetic energy along those molecules true paths will be 6.17 x 10^-21 joules. This give a total kinetic energy of translation per cubic meter of 149 kilojoules. AT 65°C, one cubic meter of dry air will have 2.14 x 10²⁵ molecules and the mean molecular kinetic energy along those molecules true paths will be 7.00 x 10^-21 joules. This gives a total kinetic energy of translation per cubic meter of 149 kilojoules. There is no significant change per unit of volume.

However, the original one cubic meter now occupies 1.13 cubic meters after heating. The total kinetic energy of translation along those molecules' true paths is now 170 kilojoules. This is the same mass, but a greater volume. Hence, the mean kinetic energy per unit of mass increases, but the mean kinetic energy per unit volume stays the same.

Not really intuitive, but true nonetheless.

 

[sophiecentaur]

Air temperatures are a measure of the mean kinetic energy of translation per molecule. The kinetic energy per molecule is higher, as I said in my post. However, the total kinetic energy per unit of volume is not.

I think the word 'random' needs to be in there, to avoid the possibility of confusing internal energy with bulk translational energy. The KE of a moving mass of gas (or anything) is nothing to do with its temperature.
Imo, this is an example of a problem where introducing molecules only serves to complicate matters without resolving anything. The gas laws work very well without people needing to hop in and out of the microscopic world so answer questions.

 

[klimatos]

I think the word 'random' needs to be in there, to avoid the possibility of confusing internal energy with bulk translational energy. The KE of a moving mass of gas (or anything) is nothing to do with its temperature.
Imo, this is an example of a problem where introducing molecules only serves to complicate matters without resolving anything. The gas laws work very well without people needing to hop in and out of the microscopic world so answer questions.

Since I am not a physicist, your comment sent me hopping to my bookshelves to make sure I had not committed some grievous error in my statement. The first book I grabbed put my mind at ease. On page 344 of Haliday and Resnick’s (1970) Fundamentals of Physics is the statement: “Similarly, the temperature of a gas may be related to the average kinetic energy of translation of the molecules.”

A few lines earlier, the authors state: “For any system, the macroscopic and the microscopic quantities must be related because they are simply different ways of describing the same situation. In particular, we should be able to express the former in terms of the latter.”

I admit to being not a little disappointed in your final comment, since I am a great admirer of your posts in the various PFs. Are you suggesting that kinetic gas theory and statistical mechanics are intrinsically less desirable scientific perspectives for the study of gases than classical physics? Or, that they should not be used to answer PF questions or describe relevant phenomena? It certainly seems that way to me. I hope I am mistaken.

 

[Jedi_Sawyer]

Klimatos and to a lesser extent Sophiecentaur I think that you guys are both wrong.

Klimatos, Go back to the books and find out thattemperature for a fixed volume and mass of gas is proportional to pressure, which by the kinetic theory of gas, pressure is proportional to the average kinetic energy of the gas molecules. Then deduce that a higher temperature means more kinetic energy.

Sophiecentaur, when you talk about internal energy of atoms, (the vibrational, rotational, and translational) having nothing to do with bulk translational energy, at least for gas, you are wrong. Basically a moving fan blade is a variation of a rocket, you are deflecting air molecules more in the through direction than in other directions, that is having the effect of giving the gas molecules a preferred direction for in, due to suction, and out sides of the fan. For a hotter incoming gas they will be less dense, so fewer collisions with the blades, more gas with the wrong directional orientation will get through, net effect is less suction and blowing.

 

[sophiecentaur]

Klimatos and to a lesser extent Sophiecentaur I think that you guys are both wrong.

Klimatos, Go back to the books and find out thattemperature for a fixed volume and mass of gas is proportional to pressure, which by the kinetic theory of gas, pressure is proportional to the average kinetic energy of the gas molecules. Then deduce that a higher temperature means more kinetic energy.

Sophiecentaur, when you talk about internal energy of atoms, (the vibrational, rotational, and translational) having nothing to do with bulk translational energy, at least for gas, you are wrong. Basically a moving fan blade is a variation of a rocket, you are deflecting air molecules more in the through direction than in other directions, that is having the effect of giving the gas molecules a preferred direction for in, due to suction, and out sides of the fan. For a hotter incoming gas they will be less dense, so fewer collisions with the blades, more gas with the wrong directional orientation will get through, net effect is less suction and blowing.

If you take that to an extreme, you are implying that a capsule of gas, traveling at 1000m/2 is 'hotter' than when it is 'stationary'. In this question, we are considering the bulk movement of gas as well as its temperature. That was my point.

So what keeps the the volume constant in this experiment? It isn't in a sealed cylinder - far from it. You need PV=nRT for this one.
Also, the fan comes before the heater so what is that supposed to mean?

 

[sophiecentaur]

I admit to being not a little disappointed in your final comment, since I am a great admirer of your posts in the various PFs. Are you suggesting that kinetic gas theory and statistical mechanics are intrinsically less desirable scientific perspectives for the study of gases than classical physics? Or, that they should not be used to answer PF questions or describe relevant phenomena? It certainly seems that way to me. I hope I am mistaken.

All I meant was that, having gone to the trouble of deriving valid gas laws, using microscopic considerations, those laws are well suited to a problem like this. I have similar views on problems involving Young's Modulus and Electrical Circuits. Why is there a need to go 'back down the chain', here?

 

[Jedi_Sawyer]

This thread, (originally), is all about adding internal energy, or heat, and it's effect on kinetic energy, or pressure of airflow from a hairdryer. I thought you had already agreed that the heat of air, internal energy, did make a difference to the air flow from a fan, so why change your mind about that now.

Also Sophiecentaur why are you not agreeing with me? Kilmatos definitely said that a higher temperature did not make any change in kinetic energy for a fixed volume of the higher temperature gas and that is plain wrong, It seems from reading the posts , that you are reluctant to tell him why and how he was wrong like I did.

 

[klimatos]

Klimatos and to a lesser extent Sophiecentaur I think that you guys are both wrong.

Klimatos, Go back to the books and find out thattemperature for a fixed volume and mass of gas is proportional to pressure, which by the kinetic theory of gas, pressure is proportional to the average kinetic energy of the gas molecules. Then deduce that a higher temperature means more kinetic energy.

I find your post condescending. You say, “go back to the books”. As I said in my post (did you read it?), I did go back to the books. Haliday and Resnick’s “Fundamentals of Physics” was the source of my quotes. Is it your opinion that these two physicists were wrong in their understanding of temperature?

You go on to say that “pressure is proportional to the average kinetic energy of the gas molecules”. This is true as far as it goes, but it is woefully short of going far enough! Pressure is also proportional to the mean molecular number density (n), as in P= nk_BT.

You continue with the statement that “higher temperature means more kinetic energy”. Again, your understanding is incomplete. This is only true on a per molecule basis. It is perfectly possible for a mass of less dense air to have both a lower (or higher) temperature and a higher (or lower) total kinetic energy of translation. The total kinetic energy of translation per unit volume is a product of the mean molecular number density and the mean molecular kinetic energy of translation.

If I add a bucket of cold water to a tub full of warm water, I will have more kinetic energy in the tub, but the water will have a lower temperature. The total kinetic energy (as its name implies) of a mass of air or a mass of anything else is a total, its temperature is a mean. You seem to be confusing totals with means.

I don’t see the relevance of your reference to a “fixed volume and mass of gas”. My work has dealt primarily with the free atmosphere (unconstrained by laboratory containers or any sort of barrier to natural processes). There, we commonly deal with parcels of air with indefinite volumes and masses. This doesn't stop us from taking their temperatures.

 

[klimatos]

All I meant was that, having gone to the trouble of deriving valid gas laws, using microscopic considerations, those laws are well suited to a problem like this. I have similar views on problems involving Young's Modulus and Electrical Circuits. Why is there a need to go 'back down the chain', here?

Okay. As a simple statement of personal preference, I can't fault you. I have to say, however, that in teaching atmospheric sciences to college students (most of whom had had no college-level physics courses) I found that the kinetic gas/statistical mechanics approach was far easier for them to understand than the fluid mechanics and classical thermodynamics approach.

This was especially true for phenomenological descriptions of common atmospheric parameters. Using molecules made more sense to them than using compressible fluids. (Most of them had no real math background, either.) Of course, this could be my fault. It made more sense to me, as well.

Keep in mind that no gas law that requires conditions of equilibrium to exist to be valid is of much use in dealing with non-equilibrium weather phenomena.

 

[sophiecentaur]

All through this thread, people are ignoring the difference between the KE of internal energy and the KE of bulk movement. Calling on what the textbooks say is pointless because they do not consider , at that level, flowing air; they are just defining temperature in a simple case. Does anyone disagree with the following paragraph? Stick with the steps and point out where you disagree, if you do.

Forget the fan. Take a cylinder, closed at one end, with a heating element in it; make it a well distributed element so that it is intimately in contact with a lot of the air and the effect will be more noticeable. When the element is turned on, the temperature will rise (internal energy), the pressure will rise and air will move out of the end - the volume will increase. That moving air has bulk KE. If you want molecules the I can say that more will be moving out of than into the cylinder You do not have constant pressure and you do not have constant volume, in the early stages. Work is done on the ambient air and in accelerating the air that leaves the cylinder so the temperature of the air will not increase as much as in a sealed cylinder (important point). At some stage, the pressure in the cylinder will balance AP and the air will no longer expand. But work will have been done on the air in the atmosphere and the exiting air will have had bulk KE. Now repeat the experiment but admit a steady stream of air into the closed end - you will need higher than AP and work will need to be done by a pump. We can then reach a steady state, in which the bulk KE of the exiting air has contributions due to the Work done by the input pump and the behaviour of the air, according to the gas laws as it is heated by the element.

The only experimental evidence that's been quoted is that hairdryer video. I question whether it is measuring what we actually want to know. What do we mean by 'harder'? Surely that refers to the feeling on your head, hair and face, which have a much smaller area than the banner.
I have been motivated to do an experiment of my own., using a fan heater and a light, fabric bag suspended in front by two strings. The fan heater has a tangential fan, which doesn't change note when the elements are switched in. The 'sound' is different, with a more 'rushing' sound when the air is being heated.
These are the results:

Mean deflection from rest position with fan alone: 50mm
Mean deflection from rest position with fan + 1kW element: 60mm
Mean deflection from rest position with fan + 2kW element: 70mm

The bag is about 120mm X 60mm and held held horizontally on two light strings, about 400mm long, 140mm from the centre of the fan
There is a fair amount of turbulence and the bag would move within about 10mm limits but the trend is quite conclusive: more than 30% increase in deflection with the heater on full power!
Two different experiments with two different outcomes. I think that my experiment goes along with my theoretical scenario, which applies to the situation near the nozzle. The 'banner' experiment is dealing with a different, more complex situation and involves the circulation of air, surrounding the heater and the pressure over a much larger area. Also, the air speeds and exit temperatures are very different in the two experiments. You could say that the banner experiment was not sensitive enough to show the effect.I suggest you all go away and try it with anything you can lay your hands on. It took literally 10 minutes for me to gather the 'equipment', to tie the string to the bag and to hang it from a kitchen chair. Along with the measurements, just 'feel' the effect. It's hard to deny its existence - it's literally In Your Face!

 

[256bits]

Brayton cycle

This picture is the ideal cycle for a gas turbine:

and a representation of the closed cycle.

The hair blower dryer would use only the compressor and the heat input.

1-2 is an isentropic compression (constant entropy), where the air is compressed to a higher pressure and higher temperature.

2-3 is a constant pressure heat addition, with a rise in remperature.

for 1-2,compression, h = enthalpy, rate flow
------------------------------------------
- W(in) = h(out)-h(in)
or,
-W(in) = Cp( Tout-Tin ) = Cp( T2 - T1 )

for 2-3, heat addition
--------------------
Qin = h(out) - h(in)
or
Q(in) = Cp( Tout - Tin ) = Cp( T3 - T2 )

We do not have a turbine, nor a heat rejection reservoir( heat is rejected to ambient) so both of these processes can be neglected.
-------------------------------------------------------------------------------
What is usually neglected in the ideal Brayton cycle is other forms of energy input and transfers such as potential, kinetic, chemical.

For a container control volume, with air moving in(state 1) and out(state 2) and having heat or work added to it,

From the first Law,
dE = ∂Q -∂W

and do not neglect the change in kinetic energy of the fluid due to its velocity,
we end up with,
q$_{1-2}$ - W$_{1-2}$ = u$_{2}$ - u$_{1}$ + ΔKE

If we separate the flow work from shaft work,
( ie flow work is PV of the fluid and we have it at the entrance and the exit )
q$_{1-2}$ - W$_{shaft,1-2}$ = h$_{2}$ - h$_{1}$ + ΔKE

or,
q$_{1-2}$ - W$_{shaft,1-2}$ = C$_{p}$( T$_{2}$ - T$_{2}$ ) + ΔKE

Obviously shaft work = 0 for the heating container.

The change in kinetic energy of the gas,
ΔKE = c$_{2}$ $^{2}$/2 - c$_{1}$ $^{2}$/2

If anyone wishes to find out the mass flow rate for an hair dryer and plug in some numbers...

 

[Jedi_Sawyer]

I ran the suggested experiment and you can see it at, http://www.youtube.com/watch?v=WBUQWMCULLw&feature=c4-overview&list=UUxQL4_6qKdL8Ou_g4Q3lt6w

 

[cjl]

I find your post condescending. You say, “go back to the books”. As I said in my post (did you read it?), I did go back to the books. Haliday and Resnick’s “Fundamentals of Physics” was the source of my quotes. Is it your opinion that these two physicists were wrong in their understanding of temperature?

I won't judge whether or not they are correct, but that definition is certainly oversimplified. Temperature for a flowing gas is based on the so-called "thermal velocity" or "thermal motion" of the gas particles, which is defined as the remaining velocity after the bulk flow is subtracted from the velocity of each particle. The bulk flow does not contribute to the temperature of the gas, though it does add to its overall energy content.

 

[sophiecentaur]

I ran the suggested experiment and you can see it at, http://www.youtube.com/watch?v=WBUQWMCULLw&feature=c4-overview&list=UUxQL4_6qKdL8Ou_g4Q3lt6w

Great stuff. You tried it.
That's amazingly different from my result, which repeatably showed a real difference in the deflection of my suspended bag. The only practical difference between my method and yours was that I used a very light weight sensor (just a few grams), covering only a small portion of the heater outlet, and the suspension was deflected by about 7° and 10°, with heater off and on, respectively.

This is much more complicated than it seemed at first, I think. My measuring device was very much a 'probe', not altering the total air flow much. The previous 'banner' experiment involved intercepting the whole of the air flow and your method was an intermediate condition. Did you actually measure the deflection from vertical and the length of the suspension? Your system was much more stable than mine, which was very twitchy, in general - although there was no doubt about the proportional trend for the two cases. I will try it with a heavier load on the string and I could suggest you try it with a lighter one.
I have an anemometer on my boat and will bring it back with me next time I go over there and actually measure the air speeds. That should give a definitive measurement.
Meanwhile, people can be arguing further about the theory.

 

[sophiecentaur]

@256bits
Bottom line; after that burst of turbine theory. What is you conclusion and what should we expect to see? So far, Jedi and I have had different results - at least, we can say that I detected a difference and he didn't. I guess you'll say "it depends" (?)

 

[256bits]

@256bits
Bottom line; after that burst of turbine theory. What is you conclusion and what should we expect to see? So far, Jedi and I have had different results - at least, we can say that I detected a difference and he didn't. I guess you'll say "it depends" (?)

Originally, I was thinking that with the same mass flowrate with or without heat, an addition of heat, since energy is being added to the system, would have more produced more deflection, as the output velocity would have had to increase, due to PV=nRT expansion.

Seeing the first video, leads me to question whether either the mass flow does remain the same, or the experiment is not a perfect representation of achieving stagnation pressure.

The simplest things...are confounding!
Who says physics isn't fun.

Your anemometer will be a good touch.

 

[sophiecentaur]

Originally, I was thinking that with the same mass flowrate with or without heat, an addition of heat, since energy is being added to the system, would have more produced more deflection, as the output velocity would have had to increase, due to PV=nRT expansion.

Seeing the first video, leads me to question whether either the mass flow does remain the same, or the experiment is not a perfect representation of achieving stagnation pressure.

The simplest things...are confounding!
Who says physics isn't fun.

Your anemometer will be a good touch.

Something that hasn't yet been discussed is the turbulence from the fan. My heater has a 'hamster wheel' style fan, which pushes air out along a letterbox slot. This may produce a less turbulent flow than a propellor type of fan, which could have a significant effect on what happens to the air on either side of the heater - stirring it up and messing up the ideal conditions.

I shall do my best to remember the anemometer, next week end. I have set an alarm on my iPhone but you can never be sure that my brain will actually respond properly. haha "Stagnation pressure" could apply there as well.

 

[246ohms]

It seems all agree that the heater increases the tempearature and hence its volume which directly means it needs to go faster to exit the dryer nozzle.
The pressure drop created by an increase in velocity will be both from friction losses and exit losses from the nozzle.

To counter the increase in pressure drop across the outlet of the dryer the fan either needs to work harder to maintain the same mass flow rate or it will follow its fan curve and lower its mass throughput. As it has a nominal constant input power it will move on its fan curve.

So the curve is the next problem - axial type fan, squirrel cage - both forward or backward leaning blades, centrifugal style again with forward or backward leaning blades.

Each of these have a very different fan curve and some may be designed to sit on one side or the other of a maximum in the curve so pressure changes on the outlet have very different consequences on the fan speed and flowrate.

Just the fan characteristics complicate any test as no two apperatus are the same.

So where to now.
To do a nice study we need to stabilise on a 'type' of dryer.
Then we need to model the inlet characteristics for flowrate changes, the fan curve and power consumption changes due to flow/pressure changes, the outlet pressure drops - friction and exit losses. Thats a lot of code but could be done.

The key point here is the fan curve. It can easily explain the vacuum cleaner test as they normally have forward curved centrifugal compreesors which are quite different from a dryer.

 

[256bits]

It seems all agree that the heater increases the tempearature and hence its volume which directly means it needs to go faster to exit the dryer nozzle.
The pressure drop created by an increase in velocity will be both from friction losses and exit losses from the nozzle.
QUOTE]

And an increase in viscosity of the air with the higher temperature!
Less missing pieces of the puzzle.!

 

[NascentOxygen]

My present hairdrier has, in addition to the usual pair of multiposition slide switches, a press-and-hold button located at the trigger position. Depressing this cuts power to the heating element without any change in the set speed, and I observe no discernible change in the motor's tone or throughput.

However, watching in the mirror as I depressed the trigger, it was unequivocal that the hair was being significantly more strongly parted (i.e., pushed aside) when this trigger button was depressed, meaning when no heating was occurring.

This is a surprise finding.

The hairdrier is rated 1600W, and I never use it on its hottest setting. The emergent air bast at that setting is more suited to paint stripping.

 

[sophiecentaur]

Curiouser and curiouser. You'll all have to go to the boat chandler's and buy yourselves an explorer wind meter (only about £35). We are clearly dealing with several extra variables here and the air speed right across the output and input apertures must be relevant. Now where did I put my smoke machine?

 

[Jedi_Sawyer]

Involved in all of this is conservation of energy/momentum which is what I was originally experimenting for. I think I know what is going on in all of this, adding heat is expanding gas in all directions so its net effect is zero for adding momentum. I am in the process of building the next generation model and hopefully will have more to talk about in a week or two.

Nascent, I have a similar hair dryer and I do notice a change in pitch and a corresponding brightening of bath room lights when I push the button to turn heat off. Can not say that I noticed any change in hair blowing but with hair like mine I probably would not have noticed.

Go Seahawks, even though I wish Sherman was a Bronco, as he has tempted Karma, maybe other guys Karma will cancel that out, sort of like heat added to airflow.

 

[246ohms]

And an increase in viscosity of the air with the higher temperature!
Less missing pieces of the puzzle.!

True, the density, viscosity and temperature will change so the Reynolds Number will follow and the friction factor will also change. Probably could be solved in a CFD program as they allow an input section to be assigned a mass flow then any section can be a fan followed by heating and a ambient far field exhaust. That would really give a nice set of results!

 

[sophiecentaur]

Involved in all of this is conservation of energy/momentum which is what I was originally experimenting for. I think I know what is going on in all of this, adding heat is expanding gas in all directions so its net effect is zero for adding momentum. I am in the process of building the next generation model and hopefully will have more to talk about in a week or two.

Nascent, I have a similar hair dryer and I do notice a change in pitch and a corresponding brightening of bath room lights when I push the button to turn heat off. Can not say that I noticed any change in hair blowing but with hair like mine I probably would not have noticed.

Go Seahawks, even though I wish Sherman was a Bronco, as he has tempted Karma, maybe other guys Karma will cancel that out, sort of like heat added to airflow.

That is clearly wrong or a gun cartridge wouldn't work, would it? Momentum conservation doesn't rule out an imbalanced flow of air if the fan stops it flowing backwards. Conservation laws must be invoked carefully if you want to get a valid conclusion.

If you are doing experiments with a supply that actually changes volts with a tiny 1.5kW load then I suggest you need to move to somewhere with a better supply - like a good old UK ring main system. There are enough unknowns in this issue without introducing additional cable resistances.