Can sound-waves affect thermal dissipation/rate of heat transfered?

[David_]

Hello.

I wonder if sound-waves which is variations in pressure in our atmosphere could in any way(theoretically or in reality) affect the rate at heat would be transferred from a object into the surrounding air?

We frequently use heat sinks in electronics and when the heatsink itself isn't enough we ether increase the size of the heatsink or apply airflow with a fan(or through the design of an enclosure), and it sound to me that if we would adjust the pressure of the air surrounding the heatsink that might affect the heat transfer?

I wish I had one of those really fancy thermal cameras, some day perhaps.

Regards

 

[CataM]

Yes, it affects theoretically.

A bit of Heat Transfer theory.

In convection heat transfer, which is what you are talking about, the heat is transferred from the hot body to the air with the following formula:

Flux [W]=h·S·(T_hot-T_cold)

S=contact surface; h=convection heat transfer coefficient which depends on the way objects are placed, their shape etc..(depends on many things).

(By the way, Flux=h·S·(T_hot-T_cold) =(1/ R_th)·(T_hot-T_cold)

Rth is the thermal resistance which is seen in datasheets and in electronics we replace Flux by Power Dissipated as heat)

So, the important parameter I am going to focus on is "h".

In natural convection, there are some empiric numbers which related are used to find out "h".

Here are those numbers in natural convection:



So, as you can see, depending on the fluids involved and lots of factors, you can calculate those empiric numbers (Grashof,Prandtl) and with the specific case i.e. your "sink" and the fluid involved (air) you can calculate Nusselt's number and with it, the "h".

In those numbers, one parameter involved is the density which varies with pressure. If you change pressure => change density => change Grashof => change Nuttselt => change "h" => change Flux.

I gave you the example with the density, but the specific heat, viscosity also varies with pressure.

With forced convection happens the same, just in this case Grashof's number is replaced by Reynold's number.

EDIT:

We frequently use heat sinks in electronics and when the heatsink itself isn't enough we ether increase the size of the heatsink or apply airflow with a fan(or through the design of an enclosure),

Heat sink => results in increase of "S" => increase of Flux

Apply airflow (Forced Convection) =>increase of Reynold's number (because it is dirrect proportional to the speed of the fluid involved) => increase of Nuttselt => increases "h" => increases Flux

 

[zorro]

If my understanding is right, there are two subjects in the original post:
1)

...sound-waves which is variations in pressure in our atmosphere could in any way(theoretically or in reality) affect the rate at heat would be transferred from a object into the surrounding air?

2)

... if we would adjust the pressure of the air surrounding the heatsink that might affect the heat transfer?

CataM answered to 2).

About 1), consider that sound consist of relatively small changes in local pressure. There is not a mechanism that allows hot air to carry off heat. Waves propagate but the air has just minimal vibrations.
So it doesn't affect heat transfer, at least in a noticeable way.

 

[analogkid]

While the compression half of a cycle of a sound wave does increase the air pressure which does increase heat flow, the following rarefaction half cycle decreases pressure and heat flow. By definition the average value of the air pressure of an audio wave is zero, so there is no net increase in heat flow. IOW, yelling at an amplifier does not make it run cooler.

ak

 

[BradtheRad]

Not that I know anything about the subject... I believe ultrasonic welding sends intense sound waves at plastic pieces, to melt the joint momentarily, welding it. That's heating, not cooling.

As for the cooling question, a powerful piezo fan could conceivably vibrate the air around overheated electronic parts, cooling them. (This is a different effect than the usual one-way air movement of a piezo fan.) Sort of like sound waves, or variations in pressure. It would be extremely loud and disruptive sound waves, I think. Not practical.

 

[analogkid]

Ultrasonic welding does not create heat through increased convection. It does it through contact and friction. You know how rubbing your hands together worms them up? That's ultrasonic welding.

ak

 

[c_mitra]

There are three modes of heat transfer: conduction, convection and radiation. Heat loss by radiation depends only on the surface finish and the absolute temp difference. So we can forget about it in this specific case.

Using a fan helps in convection. That is simply a cooler medium flowing over a hotter surface. Sound waves are different from convection.

In case of a sound wave, the air physically does not move. The vibrations are longitudinal and the compressions and rarefactions are adiabatic - can they couple to a hot surface and help in energy transfer?

The example of ultrasonic waves were brought up in an earlier post. No, I do not think the example of ultrasonic welding is appropriate. But how about ultrasonic cleaning?

The compression and rarefaction stages are not perfect and there is always some dissipation. It is perfectly possible that some heat is extracted from the surface during the cycle - this is irreversible because the heat taken out during one step cannot be returned back to the surface fully.