Effect of entrailed air?

[Curmudgeon]

I am a relative newcomer to hot tubs and this forum and I am curious about the effect of introducing air into the water jets.

I expected to avoid the air option since I am familiar with the danger of aerosols introducing pathogens into one's body (e.g., Legionnaires Disease, etc.). However, I find that entrained air enhances the force of the water jet significantly. I do turn on the air and enjoy the effect.

My question is, how does the introduction of air enhance the impact of water jets?

Thanks,
Curmudgeon

350 gal Infinity using BBB

 

[JoshU]

One thing to consider with air blowers is that they are blowing outside air into the spa or hot tub. If you have your hot tub heated 98 but the air temperature outside is 62 you will feel the cold air being introduced through the jet.

Most portable spas and hot tubs no longer use air blowers. Another thing is consider is that many air blowers are still very noisy. The Polaris QT would be one to look at for reduced noise.

 

[mas985]

My question is, how does the introduction of air enhance the impact of water jets?

Since water is much heavier/denser than air, the air itself does not produce that much force on the body, most of that comes from the water flow. However, what air does is break up the water stream so instead of a continuous stream of water with constant force, there are alternating pulsations of air and water which gives the spa jets much of the message feeling. When air hits you there is little force in that area while when the water hits you in the same spot there is a much greater force. So the more air you add, the more pulsation you will feel.

 

[Curmudgeon]

Effect of Entrained Air (again)

I have some thoughts about a possible answer to my earlier question. Unfortunately I misspelled entrained in the subject of my original question which may explain the lack of responses.

.............................

"I am curious about the effect of introducing air into the water jets.

"I expected to avoid the air option since I am familiar with the danger of aerosols introducing pathogens into one's body (e.g., Legionnaires Disease, etc.). However, I find that entrained air enhances the force of the water jet significantly. I do turn on the air and enjoy the effect.

"My question is, how does the introduction of air enhance the impact of water jets?"

...............................

The force I feel from the water jets seems greatly increased when I open the air valves on my hot tub.

I assume the air is introduced into the water stream by an aspirator (Venturi effect) since I do not have an air pump. When the air valve is open the density of the fluid (air/water mixture) flowing through the jets is lower than the water only. The addition of air makes the volume of this fluid higher than the water alone.

I am thinking that this increased volume (and decreased density) of the post asperator fluid results in an increased flow rate from the jets since the volume of water being pumped into the asperator is unchanged. The mass of the fluid is virtually unchanged so I would expect little effect on the flow from the water pumps. I would also expect additional friction would be insignificant at these flow rates.

The kinetic energy of the water emerging from the jets can be described by the classical equation:

E_k = 1/2 M x V²

On adding air to the fluid moving toward the jet the density of a given volume of fluid is lower, hence the mass is lower which by itself reduces the kinetic energy. However, since the kinetic energy increases by the square of the velocity (flow rate), that factor significantly increases the kinetic energy of the water/air emerging from the jets.

Thus the increased flow rate is the reason the air entrained flow feels more forceful. The flow of the water/air mixture is probably more turbulent, which may also contribute to the enhanced massage feeling.

Any thoughts?

Thanks

Curmudgeon

350 gal Infinity using BBB

 

[JasonLion]

Adding air doesn't change the total kinetic energy in any significant way. If the air is pumped in under pressure there will be a tiny increase in energy, if the air is draw in by Venturi action there will be a tiny decrease in energy. In neither case will the change in total kinetic energy be over one percent.

The answer mas985 gave above is correct. Human senses tend to filter out constant inputs. Variable inputs are much easier to notice. The pulsing makes it feel stronger because you actually notice it happening instead of mostly ignoring it. This difference takes place at a low level in your sensory nerves, long before it accessible to consciousness.