Central bore cooling
Central bore cooling
(OP)
Water cooling through a central bore hole can be improved by inserting a rod through the bore to reduce the area and increase the water velocity. This rod would effectively provide an annular region through which the water flows to provide cooling to the outer radius of the bore hole.
What parameters would optimise the diameter of the rod in order to improve the cooling rate? My guess is that for the same mass flow rate then the smaller the annular region then the faster the water flow and higher heat transfer coeficcient.
What parameters would optimise the diameter of the rod in order to improve the cooling rate? My guess is that for the same mass flow rate then the smaller the annular region then the faster the water flow and higher heat transfer coeficcient.
corus





RE: Central bore cooling
Going the Big Inch!![[worm] worm](https://www.tipmaster.com/images/worm.gif)
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RE: Central bore cooling
I agree with Big Inch that there will be a trade off at the pump due to the increase in H.P. required, brought on by the increase in pressure drop.
Please correct me if my thinking is wrong, and steer me in the right direction.
I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI
RE: Central bore cooling
The only reason an annular channel with the same mass flow increases thermal transport is if you think the flow is so laminar that the center part of the flow is not contributing to thermal transport.
How big is the bore? Too narrow a bore and the channel resistance will be dictated by the bore surface roughness and you'll need to run at a much higher pressure to get the same mass flow.
TTFN
RE: Central bore cooling
There's still a chance it could work, if Corus is in the laminar flow regime now and (s)he increases velocity enough that he gets into turbulent flow. With turbulent flow, there's a greater heat transfer coefficient available in the film layer next to the pipe wall. Under those conditions Corus can increase overall heat transfer, but that's about the only way I see that it could give any net gain.
Going the Big Inch!![[worm] worm](https://www.tipmaster.com/images/worm.gif)
http://virtualpipeline.spaces.msn.com
RE: Central bore cooling
I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI
RE: Central bore cooling
Going the Big Inch!![[worm] worm](https://www.tipmaster.com/images/worm.gif)
http://virtualpipeline.spaces.msn.com
RE: Central bore cooling
If you actually are looking to quantify the increase in heat transfer that the rod method will produce, there are existing nussult correlations for annular flow that are widely accepted. Also with the rod, if it is mounted eccentrically, I believe there is also another minor augment to the heat transfer coefficient.
RE: Central bore cooling
FYI the original bore size is 50mm dia. with a water flow rate of 60ltr/min.
As Yorkman implies, I was expecting that any increase in water coolant temperature is offset by the reduced temperaure of the bore surface, caused by the increase in heat transfer. Hence the advantage of the system is in reducing the bore surface temperature, with generally no significat increase in exit water temperatures. Thermally I can see no optimum annular size, other than to make it as small as possible. The increase in pressure is something I'd not considered and is worth more thought.
Ta
corus
RE: Central bore cooling
corus
RE: Central bore cooling
You need to use equivalent diameter to take into account the effect of the bore, this is calculated by 4 x flow area / wetted perimeter
Reynolds is calculated by diameter x density x velocity / viscosity
For pressure drop the losses are proportional to 1 / equivalent diameter to the fifth power
hope that helps
RE: Central bore cooling
If you want to do the math, here are the equations:
Nu = h D(hydraulic)/k
where h is the liquid film coefficient, a heat transfer CONDUCTANCE (i.e. bigger h means less heat transfer resistance). k is the thermal conductivity of water.
Nu = 0.023 Re^0.8 Pr ^0.3
D(hydraulic) = cross sectional area / wetted perimeter for your annulus, ie. (D^2-d^2)/(D+d) where D is 50 mm and d is the diameter of the rod
Re = D(hydraulic) V mu/k, where mu and k are fluid properties (i.e. constant for your situation) and V is the velocity of the flow
Pr is a function of fluid properties only (i.e. it's constant for your situation).
From this you could derive the dependence of h on D (50mm) and d (the diameter of the inner rod). All that matters for you, though, is that there is no optimum annulus- the smaller the annulus, the greater the heat transfer at a given mass flow- and the (vastly) greater the pressure drop too.
Practical considerations, such as how much pressure you can reasonably supply to force 60 L/min through this hole, and how much this pressure will increase with scaling, and how much erosion/corrosion you get at high velocities, will determine what the REAL optimum annular space would be.
As to using turbulators, static mixers or enhanced roughness rather than a rod in the ID, I doubt they'd make that much more effective use of available pressure drop for beneficial radial mixing in your situation than would be afforded by a simple rod, since your flow conditions are fully developed turbulent even without the rod in place. They'd make a world of difference if your flow was laminar, though!
RE: Central bore cooling
I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI
RE: Central bore cooling
TTFN