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Old 11-25-15, 08:22 PM   #1811
AC_Hacker
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Len,

Here's a nifty graph regarding turbulence & thermal transfer:


...and here is the text to go with:

Quote:
Heat Transfer Enhancement
Convective heat transfer coefficient increases drastically when the flow becomes turbulent, due to effective mixing of different fluid layers in the flow. This behaviour is shown in following figure.So it is a common practice among designers to covert laminar flows into turbulent by introducing suitable vortex generators in the flow.

Also another especially juicy bit:


...and the text to go with it:

Quote:
Drag reduction
Coefficient of drag around a body reduces by a huge amount when flow changes from laminar to turbulent.This phenomenon is shown in following figure.This is the reason why golf ball has got lot of dimples on it.This irregularities on surface of the ball will help in transforming laminar flow into turbulent and reduces drag, with low drag ball can travel more distance.
The take away from this is that there is a small 'sweet spot' after turbulence is achieved, when there is a dramatic drop in fluid friction.

The big news is, at this sweet spot, there is both a 'drastic' increase in thermal transfer, and a 'huge amount' of fluid friction reduction.

Well worth working toward.

Fluid velocities beyond this sweet spot result in increasing fluid friction by the square of fluid velocity.

( above information from: What is Turbulence ? ~ Learn Engineering )

Best,

-AC

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Old 11-25-15, 08:53 PM   #1812
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AC,

Thanks a ton for those two pics. I noted much earlier in this thread the discussions on the "turbulent sweet spot" & tabled it as I didn't quite wrap my brain about how one could get a little bit of the best of both worlds. I was planning on doing some more research on it, but was striking out in the google department.

It seems one could do a real world test with some of the new variable speed pumps by monitoring GPM & instantaneous power. Slowly bring the speed up and map the GPM vs Power. At that spot where the drag coefficient takes a dive, we should see a corresponding dip in the power required. It of course should be somewhere around the theoretical GPM to put us at that laminar/turbulent boundary. That may be a fairly wide range as I have seen Reynolds numbers anywhere from 2000-4000 as the boundary.

Given a good solid consistent EWT and measure the LWT and we could also plot the heat rejection at the same time & learn something about how sensitive heat transfer is around that boundary. I think that the increased transfer from the water to the tube due to the turbulence may be overshadowed by the extremely slow transfer from the plastic to/through the dirt. As you have mentioned & I like the analogy...molasses slow transfer into the dirt.


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Old 11-26-15, 06:10 AM   #1813
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Len, - here is an easy way to tell if you have turbulent flow. Set up a rig with the flow rate and tube length you are using. Turn the water on. Put an inexpensive stethoscope on the pipe and listen - no noise laminar, noise (whooshing) then means turbulent. You only need 20-30 feet of pipe. We used this in engineering classes where we would look at clear plastic tubes with an air/water mix coming in (bubbles allow you to visualize turbulence). Fascinating how at about 10-20 diameters downstream, the fluid transitions from turbulent to laminar and the noise (via stethoscope) goes away.

But back to your point on energy use. I fully appreciate the importance of lowering a pump's electrical needs. Here I pay a lot for my pump and dump open loop.

Steve
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Old 11-26-15, 07:50 AM   #1814
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Just did a couple quick calculations for Reynolds number (predictor of turbulent conditions) in a pipe of 1.5 inch internal diameter at 15 gallons per minute of water flow. I believe these are the numbers Len spoke of in his supply and return pipes. The dynamic and kinematic viscosity of water are essentially the same at 20 C (1.002 vs 1.004).

The velocity of a flowing fluid in such a diameter pipe (3.8 cm) is 0.83 m/sec.

Using Rn = [Velocity (m/sec) x pipe diameter (m) x density (kg/m3)]/dynamic viscosity (kg/m.s)

Rn = (0.83 x .038 x 1000)/1.004

Rn = 31.41

This is WAY below the critical Reynolds number for turbulence (typically must be 2000 - 4000).

Since Len's loop field has five parallel loops, the velocity of the water is 1/5th of the above and the Reynolds number goes even lower.

QED


Steve

ps here is a quick read for the above that I have used teaching graduate classes (biomedical engineering). It has great examples of Darcey's, Hagen-Pouseuille, Hazen-Williams, Bernoulli and a great Moody plot. Really worth reading.


http://udel.edu/~inamdar/EGTE215/Laminar_turbulent.pdf
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Old 11-26-15, 08:10 AM   #1815
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Just found a GREAT flow rate calculator that calculates diameter, velocity and flow rate. Put in two and it calculates the other in more units than I could imagine. Had to check my calculations for water velocity in Len's situation.

Here it is:

FLOW RATE CALCULATOR


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Old 11-26-15, 12:56 PM   #1816
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Quote:
Originally Posted by stevehull View Post
Here I pay a lot for my pump and dump open loop.

Steve
*OFF TOPIC- EXCUSE MY RUDENESS FOR ONE MOMENT PLEASE*
Do you have picture/details of this unit and set-up?
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Old 11-26-15, 03:08 PM   #1817
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Details on my "pump & dump" (open-loop) geothermal situations.

Water sources are two 250 ft deep wells in multi-strata sand/shale/clay layers. Water table typically about 30-40 feet from surface. Ground water temp ~ 60 F. Water of minimal hardness and with low iron.

Two 1.5 HP 240 V submersible pumps each about 150 feet down - each can pump 30-35 GPM. One is a back - up the other is actively used. The pumps feed into two 60 gallon bladder air tanks in parallel. This is so that if one fails, the other tank will prevent short cycling (early/premature death) of the submersible pump. Pump cut-off/on at 40/60 PSI. Bladder tanks set 5 psi below that and large bladder tank capacity markedly decreases pump cycle frequency.

Cheap kWhr electricity ($0.08 in summer, $0.07 in winter). About double the number of cooling degree to heating degree days so the lower winter kWhr rate helps.

At least 10 kW of PV to go onto shop next spring/summer. Wind is plentiful here but it can't compete with $1.25/watt for PV (self install). Putting up a 10 kW wind turbine on a 100 ft tower is NOT a self install issue.

Local utility does month to month net metering paying about 1/2 of retail back when you supply surplus to grid.

Three WaterFurnace water to air heat pumps (two sorta new and one very old). The two new (8 and 4 years old) units are true two stage with variable ECM motors. One is a 24K BTU (stage 2) 18K BTU (stage 1) for a guest house (1600 ft sq) with COP of 5 and EER of 35. The old farm house has a 36 KBTU/24K BTU (stage 2/1) for about 2900 sq ft (COP 5, EER 35). The old 1 ton unit is a resurrected unit I patched together that heats/cools shop (COP 3, EER 25). All water lines from geo units go to livestock waterers that supply ~ 100 head. Output of waterers then drain passively to ponds that are then used for irrigating pastures. Have warm water in winter and cool water in summer 24x7 for livestock. Ponds are back up for that.

Prior WaterFurnace open loop installed in 1991 in prior home (2 ton and 3 ton) and savings put at least two kids through college. No repairs to either unit in almost 25 years (exception air filters).

The issue I am contending with now is to continue to use pump and dump or to put in some vertical or horizontal loops. About 25% of my heating/cooling cost is well pump operation. It is expensive to pump up water to 40/60 psi, much less to a lower pressure. That is why I am considering putting the geo units on a 20/30 PSI setting (or lower) and using a water psi booster pump for those few occasions when I need high pressure in house (showers, for example).

Horizontal loops are expensive here due to caliche rock ~ 3 feet down. But plenty of acreage to do this on. Have dug many a trench with an excavator crawler - hard to go 8 feet down through 2 feet of caliche. Ponds are not deep enough for loops in them. Vertical loops can be done, but thick caliche rock (very hard) requires substantial drilling rig ($$).

Tax credits, farm depreciation and rebates from the local electric coop made geo incredibly affordable. Now looking at using the standby well for a DX system - wonder where I heard of that . . . .


Steve
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Old 11-26-15, 05:00 PM   #1818
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Steve, thanks! Looks like we are swapping ideas, I want pump and dump, you want DX. Don't get me wrong I love DX, but pump and dump seems better when I have a pond and a greenhouse to build my system in. Any other comments and questions I'll reserve for my 4 ton pond thread.
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Old 11-26-15, 06:41 PM   #1819
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Steve,

Thanks for running those numbers. I'll need to double check the quick online calculator I used as they aren't matching. I trust the equation more than the online calculator & I'll run my own to compare.

One thing to note though is that my parallel loop diameters are .75" vs the 1.25" supply/return, so while the flowrate is 1/5 or 3gpm, the velocity is more than that due to the smaller diameter pipe. I'll add the reynolds calculation equation into my spreadsheet so I can check the number for each section.

Also on your pump and dump, if you have a supply and injection well, you could just pump from one to the other directly & save the pumping power. However, you would of course lose the use of the water for stock watering, ponds, ect so that may not be an option.

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Old 11-27-15, 06:38 AM   #1820
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Just re did calculations for Reynolds number (predictor of turbulent conditions) in each of the loop fields Len spoke of above. Here 0.75 inch internal diameter at 3 gallons per minute of water flow.

The velocity of a flowing fluid in such a diameter pipe (0.75 inch; 0.0195 M) is 0.66 m/sec.

Using Rn = [Velocity (m/sec) x pipe diameter (m) x density (kg/m3)]/dynamic viscosity (kg/m.s)

Rn = (0.66 x .0195 x 1000)/1.004

Rn = 12.82

This is also way below the critical Reynolds number for turbulence (typically must be 2000 - 4000).

To get turbulence (Rn = 2,000) the flow rate of 3 GPM would have to be 156 times higher - or about 468 GPM - clearly impossible in a pipe of this diameter.

The important bottom line here is that straight pipes lead to laminar flow conditions with the exception of inlet, abrupt turn and exit conditions.

I have toyed with running HDPE pipe through a machine that creates dimples inside the pipe. That would minimize the laminar boundary flow conditions and should increase heat flow markedly.

BTW, I guess I may be wrong on turbulence in the supply and return pipes! As a proper engineering disclosure report would have it, the calculations suggest a distinct laminar flow situation, but physical measurements in the pipe field would confirm. However, the mechanical energy to supply this exchange bed with water flow also suggests laminar conditions.

Steve

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