Reynolds number at vehicle water radiator 2

[JalalManuchehrian]

Hi
I have a question about water radiator design for vehicle. How much can increase Reynolds number? What happen if it reaches to 10000? When we use largeer Reynolds number then reduce weight of radiator and this is very important for manufacturer.
Thanks

 

[MikeHalloran]

You would do that, how?

By increasing the flow rate?
By making the passages smaller?

Wouldn't that increase the pressure drop?

Is it even possible to make a tube flatter than they do now?

Wouldn't it clog more easily?



Mike Halloran
Pembroke Pines, FL, USA

 

[reidh]

Are you asking how much can you increase the Re Number at the radiator inlet? Like Mike said, there are many ways to do this. Increasing the flow rate and a smaller passage will increase your erosion rate. You can always make the tubes less square, but packaging will quickly become an issue.

-Reidh

 

[IRstuff]

But, are you talking about the coolant or the air? Isn't the limiting factor the air and not the coolant?

TTFN

FAQ731-376

 

[MikeHalloran]

It's a race between bug parts clogging the microlouvers on the fins, and precipitates clogging the tubes.



Mike Halloran
Pembroke Pines, FL, USA

 

[JalalManuchehrian]

Thanks for your fast replies.
I try to reduce the weight by increase number of coolant pass. Something likes U or even N shape.
When the motor works at idle state the flow of coolant is about 1/3 and coolant side is limitation and Reynolds number is less than 2000. I want to change the flow from one top to bottom to U or even N shape. At this new shape the Reynolds number increase to 3800 at idle state (lowest coolant flow rate) but at highest coolant flow rate the Reynolds number reach to near 10000.
Low flow rate is 20 liters per minute and high flow rate is about 60 Lpm

 

[MikeHalloran]

Adding a pass requires adding at least one tank, and the weight and complexity.

For the weight of even putting a baffle inside a tank, you could probably have another row of tubes.

... which might allow you to subtract a little from the overall height and width of the radiator, which would give you a weight reduction, mostly by making the frame smaller.





Mike Halloran
Pembroke Pines, FL, USA

 

[JalalManuchehrian]

Dear Mike
I want to add only one or two baffles at one tube row and not more, then core width will be the same.

 

[reidh]

IRStuff,

In general you are correct. The slope of the heat rejection- coolant mass flow graph is just about 0 for most reasonable coolant flows (obviously ramps up from 0 flow rate).

-Reidh

 

[daubery]

The airside heat transfer dominates the radiator performance. The equivalent heat-transfer coefficient on the airside is around 150-250 W/m2.K.


The coolant generally runs a Reynolds Number greater than 2350 (transition to turbulence). The equivalent heat-transfer coefficient on the coolant side is anywhere from say 2000 to 20,000 W/m2.K, depending on the coolant flowrate & tube turbulence mechanism eg. dimples, turbulator etc.

So, speeding up coolant velocity often does very little to increase the overall radiator heat-transfer performance. Try speeding up your airside flow & see what effect it has.

Best of success.

Des Aubery...
(adTherm Technology)
(

http://www.adthermtech.com)

Best regards,

Des Aubery...
(adTherm Technology -

http://www.adtherm.com

- info@adtherm.com)

 

[RossABQ]

Des, thanks for a very valuable answer. On various automotive forums I frequent, there is always a debate on the use of thermostats; that is, whether removing them is a good thing to cure overheating. The people advocating use of thermostats frequently state that running without thermostats will reduce cooling because the water "moves thru the radiator too fast to cool down". Personally I advocate use of thermostats, but I have not bought into the argument about water flowing too fast to cool down. I think the overall issue is that there is insufficient airflow or fouling in either case. Please comment!

 

[patprimmer]

If it moves through the radiator to fast to cool down, it also moves through the engine to fast to heat up.

I most definitely do not promote removing properly working thermostats, but removing it cannot cause overheating, and removing it TEMPORARILY may indicate a faulty thermostat.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Tmoose]

Even a fairly recent copy of the Moroso catalog perpetuates the "too fast to cool" theory. Since any type of heat transfer improves with greater temp difference, pausing to let coolant "warm up" has to degrade heat flow, and thus cooling. Combine that with the stagnant liquid boundary layer stuck to the hot cylinder head through which only water's crappy thermal conductivity can work, and greater speed will again only help by scrubbing the boundary layer thinner. Look at the cooling passages in modern high performance engines. Thin for high velocity over hot surfaces because the coolant in the middle of a large channel is just going along for the ride.

Or, eat that ice cream cone while driving with the window open and see whether it warms up and melts quicker than in still air.

Well then, are the folks who believe they have had even good running engines overheat when running completely 'stat-less liars or fools?
Probably not. The race shops that advocate thermostat removal add a restrictor plate at the same location. This would have the effect of creating pressure drop as the coolant leaves the block, which does 2 good things. First it reduces pump flow a bit, and that improves the relative pump inlet NPSH condition making cavitation less likely. Second, it raises the pressure in the block a bit (which is already over the pressure set by the pressure cap), reducing the chance of local boiling at hot spots in the cylinder heads, which WOULD clobber the engine cooling in exactly the areas that need it the most.

 

[RossABQ]

Thanks for the discussion, I completely agree. The typical level of understanding on many of the public forums makes any technical discussion a waste of time, but I may have a shot at it nect time this comes. Pat's simple, rational explanation about too fast to pick up heat in the engine has a definite shot of being understood.