Physics of a jacuzzi spa

Factual Questions
1

Relax. Feel the soothing warmth. Let yourself go as all your cares dissolve away into the bubbling froth.

Now then, you should have the proper mindset for the OP.

Ready? Good.

I have recently moved into a home equipped with a whirlpool spa. Man it’s nice. Even with the chilly Northwest September weather, a 97[sup]o[/sup] spa is pure heaven. But I digress.

The spa affords a user the option of introducing air into the water returning through the jets. This is accomplished by means of a passive system (simply open the valve) which presumably works by the Venturi principle.

Anyway, I consistently notice that the return jets eject their stream more forcefully when air is incorporated than when it is not. To confirm this observation I performed the following experiment:

My back was held vertically, perpendicular to and in close juxtaposition with a group of water return jets. The group consisted to two parallel rows of jets, arranged vertically, with four jets in each row. I observed the force with which I was bathed in blissfully warm water with the venturi valve closed (ie. no air in the return stream) and subsequently, with the valve opened. The experiment was repeated 30-40X to confirm the observation (No, that wasn’t why. Hell, I did it because it felt so damn good!).

What I observed was that indeed the water did exit the jets more forcefully when air was introduced (as evidenced by my having to push harder against the water stream to maintain my position when the air valve was opened). Why is this the case?

NB: As stated above, the air introduction system is completely passive. There is no “air pump” which one might imagine would increase the return pressure. Also, the spa’s water pump does not run any faster when the passive valve is opened.

2

The water pump is moving a certain flowrate of water (volume/time). Introducing air bubbles in the flow downstream of the pump displaces some of the water, so the flow has to speed up to move the same volume of water in the same time. From the perspective of the water, adding air is equivalent to constricting the diameter of the pipe.

3

micco, thanks. I had considered something along those lines. But what confuses me is shouldn’t this place an added load on the pump? After all, to accelerate the stream, the energy must come from somewhere. It’s certainly not coming fron the air.

There is no detectable change in the sound of the pump when the air valve is opened. To me that reduces the likelihood of a mechanism that implies an increased load on the pump, but perhaps the pump is not straining enough to produce a perceptible change in its tone?

4

Adding air to the stream would probably increase the pressure on the output of the pump, but the pump should probably be well below it’s rated change in pressure for the given flowrate, especially since this is a design condition, not something exceptional.