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Syphon or Siphon - but the dictionary is wrong anyway. 6

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As I recall, at last some trees have check valves that, once water makes it up some part of the way, the valves close then it can get a bit farther.

So, syphons don't work in a vacuum then, heh?
How does NASA move fuel from tanks to pump suction before its sent on to the combustion chambers under pressurized flow.

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"Being GREEN isn't easy" ..Kermit

 
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Spacecraft with large liquid fuel tanks use small control jets to accelerate the craft enough to move the fuel to one end of the tank before starting the main engine.
 
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This thread is a little lost in semantics. water is moved by pressure difference - it is pushed not pulled. water tension is not relevant. In a siphon a negative pressure is created on the downstream leg by the weight of water dropping down the column. We could equally well create a suction by pulling a plunger down the downstream column. (we would then call it a vacuum pump). The motive force is atmospheric pressure acting on the water surface which pushes the water through the siphon. Atmospheric pressure is 32 ft head of water and hence this is the maximum head on a siphon - regardless of how long the downstream leg is.

Water rise in tress is by capillary action (tension) and there is no limit to the rise. If the capilary attraction between the water and the capillary is greater than the weight of water in te capillary/unit length then there is no limit, hence 400 ft high redwoods with no non-return valves. The motive force is evapotranspiration - a very small tension caused by a vacuum at the leaves causes water to rise the full height of the tree.
 
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BRIS - I can't see how there can be a capillary action per unit length. If the capillary is full, how does the capillary action "know" to pull up and not down? I can only see capillary action occurring at the top surface.

Also, if water can only move by being pushed and not by being pulled (which I agree with) what is the relevance of "evapotranspiration - a very small tension caused by a vacuum at the leaves". I can see that evaporation is necessary to create the vapor liquid interface that is at the heart of capillary action (in my understanding) but how does this "very small tension" result in water being pushed/drawn up 400 ft?

I don't mean to attack you with these question. I have little/no knowledge of botany and I am trying to understand how water rises to great heights in a tree. I have Googled and read many web pages, but botanists seem to have as little understanding of hydraulics as I do of botany. They all talk of the water being pulled up the tree. At this stage the multi-pump pipeline model, combined with BigInch's natural non-return valves, seems the most plausible to me.

Katmar Software
Engineering & Risk Analysis Software
 
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Nobody remembers the water and mercury experiments from H.S. Physics.

If the minuscus curves up, force and movement due to surface tension is up; water. If the minuscus is down, force and movement due to surface tension is down; mercury.


Apparently the limit for trees is 427 ft (130 m),

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"Being GREEN isn't easy" ..Kermit

 
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Bris, consider two vessels at different levels connected with a flexible hose and containing water at constant (say overflowing) levels. You will agree that water will flow from the higher to the lower container by gravity.

Now, push up the middle of the flexible conduit to a height above the water level on the upper container, and you"ll see that flow continues... possibly slightly reduced by a small increase in friction. You have a siphon operation.

QED: it is gravity that forces the flow, not atmospheric pressure, as Katmar rightly said.
 
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BASIC FLUID STATICS REVIEW

Forgetting surface and any other liquid tension forces for the moment, total pressure of a fluid column is caused by an acceleration on the mass of the fluid (accelaration + gravitational accelerations, where gravity exists) plus any other absolute pressure acting on its surface. Total pressure is not caused by acceleration alone.

Flow in a pipe is caused by an absolute force difference between the ends of the fluid column - frictional force; (Divide by x-sectional area to get pressure). Whether the pressure at either end of the fluid column is caused by acceleration acting on a fluid column's mass (when there is accelerations to account for), or the absolute pressures at each end of the column (if they are not perfect vacuums), AND/OR the sum of BOTH, is irrelevant. It is not one or the other, it is the sum of both - friction that drive any movement of the column, When the sum of both - friction due to flow is = 0, movement is occuring. When the sum of both (and there is no friction) = 0, no movement is occuring.

The attached spreadsheet clear shows how and when flow occurs by pressure imbalance caused by the combination of acceleration and absolute pressures at the end of the pipe, whether there is a siphon involved or not. Change the value of the height of PIPE TK2, and/or the ABSOLUTE PRESSURE IN TANK 4, and the direction of flow is calculated and shown under tank 2. If the height of PIPE TK2 is less than the fluid elevation in Tank 1, you could say a siphon is occuring, otherwise you'd probably just say its "gravity flow", although gravity flow technically only exists (usually in open channels) where the pressure on ALL the surface of a fluid is CONSTANT. That being the case, the only mechanism for driving flow is acceleraton, or gravity if you will, or wind driven waves shoaling up on the beach, etc., but that is to remain outside of my discussion today.

Please be a little careful when changing other input values. There are a few error checks, but this spreadsheet isn't "idot proff". It would be better if you don't use negative numbers without thinking of the etended implications and please try to keep the tanks on the side of the lines as drawn.

OK, now back to water tension as a mechanism for driving flow when gravity and other accelerations do not exist and the temperature is kept below the fluid's bubble point at any given pressure.

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"Being GREEN isn't easy" ..Kermit

 
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25362 (Chemical)

Atmospheric pressure is a result of gravity - gravity is pulling the water down the downstream leg which leaves a partial vacuum behind, Atmospheric pressure on the upstream pushes water into the Vacuum. i,e across a datum line there is an imbalance of pressure between atmospheric on the inlet and a reduced pressure on the outlet. The water column on the outlet is acting as a plunger pulled down by gravity. Atmospheric pressure on the inlet is pushing the water into the negative zone. I have designed many air regulated siphons that operate with an air/vapour phase n the outlet.
 
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If you do not have gravity, the longer leg will not flow(drain). Therefore it's gravity.
Wait. If you do not have atmospheric pressure, the shorter leg will not flow(fill). Therefore it's pressure.

Like Biginch is saying, the siphon depends on both pressure and gravity.

25362, if you try your exercise with a flexible open channel, your argument fails. The same would be said of a partially full pipe. The need for a full pipe is so a seal occurs and intake pressure can push.

Ted
 
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Hytools

Yes atmospheric pressure pushes the water over the hill that is why the dictionary is correct. The water then falls down the other side by gravity.

in an air regulated siphon we bleed air into the crest to control the flow rate.
 
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Tank you hydtools!.

Atmospheric pressure IS the result of gravitational acceleration on the earth's atmosphere. Gas pressure in tank 1 in my spreadsheet is not affected by gravity, as it is outside any gravitational field of influence. It is not an atmosperic pressure. The gas pressure in tank1, as is in any tank not affected by gravity in outer space, is the result of the kinetic energy of the gas alone [∑] 1/2 mv^2. If it was affected by gravity, the gas pressure at the bottom of the tank would be 14.7 + [ρ]_gas * g * height of the tank.

BRIS, you well may have designed a siphon that works with some pressure lower than atmospheric pressure at the gooseneck, which would draw in air from an open valve and you may have reguated water flow into the gooseneck riser by effectively increasing the pressure at the gooseneck by allowing that higher atmospheric pressure to leak in through the valve placed there (thereby reducing the pressure drop in the riser) and lowering its flowrate, but you have not designed one where the pressure at the gooseneck was at or below the vapor pressure of the water.

**********************
"Being GREEN isn't easy" ..Kermit

 
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To hydtools, the conduit is full (primed, as I said in a previous posting), otherwise, the siphon may be broken.

BRIS: The reduced pressure at the (full) constant-diameter conduit summit in a siphon is the result of a mechanical-energy balance, as in any other pipe, expressed by the well-known Bernoulli equation for incompressible fluids w/o friction. Keeping the kinetic energy constant, if you gain height, you lose pressure, and viceversa. In short, no energy accumulation.

For a given flowrate in a horizontal pipe (z[sub]1[/sub] = z[sub]2[/sub]) of increasing diameters you lose velocity head (V[sub]1[/sub]>V[sub]2[/sub]) and gain pressure head. Which shows that the flow advances against increasing pressure heads.

Using dimensions of length or height for pressure head, velocity head and elevations (z):

p[sub]1[/sub]/[ρ]g + V[sub]1[/sub][sup]2[/sup]/2g + z[sub]1[/sub] = p[sub]2[/sub]/[ρ]g + V[sub]2[/sub][sup]2[/sup]/2g + z[sub]2[/sub]​

In case the flowrate and the diameter are constant (V[sub]1[/sub] = V[sub]2[/sub]), an increase in elevation z[sub]2[/sub]>z[sub]1[/sub], the pressure will go down p[sub]2[/sub]> p[sub]1[/sub] to balance mechanical energies.

Please notice the effect of changes of the density [ρ] on the balance.
 
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I should have written p[sub]2[/sub] < p[sub]1[/sub].
 
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Learned something about space fuel systems.
Some are as compositepro said; A submerged pump is in the fuel storage tank and stores some fluid on shutdown used for firing up and accquiring an initial acceleration later on. But it does seem to be more common to use only the pressure of the fuel tank to reach the pump. No mention of liquid tension being considered.


**********************
"Being GREEN isn't easy" ..Kermit

 
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On the subject of liquids under tension (metastability) and the ascent of sap in trees, I recommend reading Pablo G. Debenedetti's Metastable Liquids Concepts and principles, Princeton University Press.
 
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Take a class - fill it with water - place a thin card on top and turn it upside down, the water remains in the glass - is this atmospheric pressure or gravity.?

Take a siphon close a valve on the outlet - the siphon remains primed - is this atmospheric pressure or gravity.?

Water flows down a pipe by gravity it surmounts a siphon crest by atmospheric pressure pushing it over.

Flow through a siphon is dependent on atmospheric pressure and the maximum height of the rising limb would be less at altitude.

Gravity affects everything without gravity we would not be able to hold the water in the pond in the first place.

The dictionary is still correct !!
 
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