Does a Hairdryer blow harder with heat

[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.