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Radiator size calculation


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This is an offshoot of t'Other Project.   How small can a radiator be - or else how big should it be?

I searched for a way of calculating what will be required.   Several highly academic papers were found, with complex maths in them, but I did find a 'recipe' that was simpler.   Unfortunately it's in cubic inches!  It says allow 2cu-in  per  1 cu-in of engine capacity.   Then add or subtract 0.1 - 0.3 cu-ins/engine capacity for various factors.    I could only allow that it's a V8, wil have a cross-flow rad and fully shrouded rad fan(s), all of which add up to subtraction of 0.5cu-in/Cu-in capacity from the total.   

That recipe delivers a rad of 320cu-ins.

Compare that with the rad I've modelled for t'Project which will have a matrix of 480cu-ins - more than enough, surely!

BUT,

then compare that with the original RV8 rad, which has 792 cu-ins of matrix!   Is the 'model-rad' going to be big enough?

So, compare with the Vitesse rad, at 294cu-ins.   Apply that proportionally from 2.5L, where it's enough,  to 3.5L and you get 412cu-ins, a bit smaller than the model rad, and bigger than the recipe-rad.

So why is the OE RV8 rad so enormous?   Am I missing something?

John

 

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It seem a little odd to me to rate radiators by volume. Surely surface area would be a better measure.

A 420 ci length of 1/4 inch tube would have vastly greater cooling ability than a 420 ci square tank.

Is rad volume an accepted way of comparing radiators?

Ed

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maybe there they just want to use an existing item rathe than commission a new design and maybe they want to have a certain volume of water in the cooling systems.

One of the K series engines problems was the very small water volume in the system so if you lost any water the engine would over heat and the head gasket go.
But agree that I would have thought surface area was the most important factor..
Mike

 

Edited by mpbarrett
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Good point, Nick!

AFAIK it's from a saloon.    Aircon, and other ancilliaries might add to the cooling requirement, I suppose?

ed_h,   I have to disagree!    Heat is lost from the surface of an object, so surface area is critical.    The surface area of a 1/4in tube would 0.25 x Pi x length.    And the smaller a tube, the larger is its surface area, relative to a widebore tube. 

The volume of such a tube (ignoring wall thickness) would 0.125^2x Pi x length, so that this tube would be 420/(0.125^2xPi) long = 8556ins.   Nearly 250 yards long  and completely impractical, even if coiled up.    It's surface area will be 0.25 x Pi x 8556 = 6720 square ins

   A radiator is designed to provide a large surface area in a small volume, but multiple tube and fins in the matrix.        I can't calculate the surface area in a matrix, too complex, but lots and lots of small diameter tubes, so larger surface  area, augmented by the fins attached.   And if 320cu-ins (THREE twenty, ed)  of matrix is to be equivalent in volume to such a long tube, ithe slab would only need to be 0.05ins thick (320/6720).       So the surface area of any car rad that will be at least 2 inches deep will be vastly more than the tube alone.

 

If I can find a commercial rad that will fit, I'll save a lot of money!     I'm no Nik Blackhurst, I'm not about to start hacking rad matrices to bits and rewelding them (even he had enough probs with leaks!) but Project Binky used several mostly unmodifed rads, stacked one in front of the other for its various cooling needs!    Any one know of a source that lists dimensions?

JOhn

 

 

 

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What material are you basing on John?

To select a material for a heat exchanger (which is what a radiator is) you need to consider several factors.

1) Heat transfer rate - obviously different for differing metals/alloys.

2) Type of flow - in the case of a radiator you only need to really consider the water side here, however the more turbulent the flow the more impingement and erosion you suffer (and you want the flow to be as turbulent as possible, as this promotes heat loss)

3) Strength of material - can it withstand vibration, corrosion, erosion etc

4) Wall thickness - pipes and tanks, obviously the softer the material the thicker it must be to retain strength, however to promote heat transfer you want it as thin as you can get away with!

If you have a radiator made out of stainless steel, and one made from copper, they will have wildly different heat transfer rates. Comes down to cost & space really, if you have the space to fit it cheaper materials are better. If space is at a premium, more exotic materials may have to be looked at. Titanium seems to be a popular choice, as it has very good strength and erosion/corrosion characteristics, meaning you can make wall thicknesses less, however it is harder to manufacture and therefore more expensive.

Also to be considered, what type of flow does your radiator have? Some simply have open tanks top and bottom and coolant flows in, through and out. This is inefficient, but adequate for a standard car for example. Others have baffles in the tanks, forcing the flow up and down the radiator following a path, they may even have multiple rows which can provide a "mixed" flow, this is much more efficient.

A mixed flow radiator may be far smaller than a basic radiator, with correspondingly smaller volume, yet outperform it in terms of heat transfer.

Engine type - what the engine is made from affects how much heat it radiates, a nice lightweight 2L modern engine will itself shed heat, whereas a triumph 2L will not shed as much, obviously this plays into how good the cooling system has to be, the former requiring a far smaller system.

Oil Cooler? If you have one, this again will reduce the load on the cooling system.

Phil

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6 hours ago, JohnD said:

 

ed_h,   I have to disagree!    Heat is lost from the surface of an object, so surface area is critical.    The surface area of a 1/4in tube would 0.25 x Pi x length.    And the smaller a tube, the larger is its surface area, relative to a widebore tube. 

 

 

 

 

Yes that was exactly my point, John.  Surface area would be much more indicative of cooling capacity than matrix volume. 

If we can assume a relationship between a cubic inch of matrix and its surface area, then volume could be a proxy for cooling ability, but i didn't know if that was a thing.

Ed

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 Phil,

Ah, a man with an academic education!!

To be honest I hadn't considered the material.   Copper obviously has the highest conductivity ( below silver, while diamond is way ahead, but that's a bit academic too).     Aluminium has less than half of that, but is a lot cheaper, very popular with radiator  makers, and successful if properly made, also strong, light and non-corroding, so that it wins!

Type of flow, cross flow is said to be better, and if the tank on one end is closed in the middle, the coolant will pass through twice, improving efficiency.   But what is "Mixed flow"?    

Turbulence - That is a function of velocity, and as a radiator will divide the flow to pass dowm mnay small tubes, the velocity in the smaller tube will be lower, unless I elect to fit unfeasibly large hoses to and from the engine.    So unless radiator makers fit turbulators inside the matrix tubes, I fear that they will flow laminar.    Flow will probably be by electric pump  and probably with a 115L/m capacity.   I will do the calculations to determine the smallest hose that would allow laminar flow  at that velocity, but the bigger the better, for coolant capacity as well, so 50mm?

Oil cooler, not unless I must!

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This is an interesting subject to me.

I wondered why newer cars than my Dolomite have better heaters
and on investigating found that some actually have smaller matrix overall dimensions.
Examining the matrix gave me an answer, the tubes used are huge compared to the Dolomite
and also have what resembles an archemedes screw inside to force the coolant to the tube walls.
In other words the surface area is greatly increased.

I have managed to fit a VW matrix into a Dolomite heater unit and am hoping for a good result.

 

 

Another factor I am sure is coolant flow rate.
For my Dolomite I have a Stewart pump which is 200 litres/minute.
I have no idea what a slant four engine coolant flow rate might be, but
am thinking it is nowhere close to 200l/minute!

 

 

Ian.

 

 

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"the tubes used are huge compared to the Dolomite and also have what resembles an archemedes screw inside to force the coolant to the tube walls."

So you CAN fit a turbulator inside a rad tube!   That may be how they allowed a wider tube, as that will have a smaller surface area per volume content.

JOhn

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10 hours ago, JohnD said:

"the tubes used are huge compared to the Dolomite and also have what resembles an archemedes screw inside to force the coolant to the tube walls."

So you CAN fit a turbulator inside a rad tube!   That may be how they allowed a wider tube, as that will have a smaller surface area per volume content.

JOhn

The turbulator's job is to increase dwell time of the liquid, thereby improving the heat transfer.

Mixed flow - a mixture of parallel or counterflow and crossflow. A simple radiator, single row, is crossflow. Even baffled, it stays crossflow but increases dwell time in the heat exchanger.

However, if you make it a two (or more) row radiator, with baffles to separate the rows, you not only have crossflow but also either contra or parallel flow. Contra (or counter) is preferred over parallel as it far more efficient. However, since it combines two flow types it is known as mixed flow.

The best option would be to have baffles to force the water up and down on each row, with each row working in series with each other, however obviously this is also the hardest to design/manufacture.

Phil

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Thnak you, Phil!

Just so that I'm clear, is this 'mixed flow'?      It's how I'd like to have the rad.

Differences Between Downflow and Crossflow Radiators | Radiators, Cross flow,  Basic

And when you refer to a two (or more) row rad, does that mean the number of tubes in the depth of the matrix?   If I'm going to get a bespoke one, then I need to know whereof etc.

But has anyone a source that gives the overall dimensions of commmercialy available rads?

John

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No, that is a two pass crossflow. To be honest, unless you are designing some seriously high output engine, that is likely more than enough!

And yes, when I describe two (or more) row, it is the number of tubes in the depth I am referring to.

If you know where your engine has come from, is it worth contacting someone who makes bespoke radiators, giving them your details/usage etc and seeing what they suggest?

Phil

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I think there is also the element of luck, which should not be underestimated.

I can't remember the exact details, but a mate of mine is an ex drag racer (mainly down Madera Drive, used to be tuesday eves with lookouts posted to watch for the plod but they were also regulars at Santa Pod)

His main competitor/friend had a Ford Pop, fitted with a tuned Mustang engine. They tried numerous radiators, all struggled and overheated. In the end they tried a little rad, out of an escort IIRC. And that worked, despite being much smaller.

No maths was ever involved, this was all raw ghetto engineering building fast drag cars. As in seriously quick, they eve pinched superchargers off commercial 2 stroke diesels and strapped them on V8 engines, usually not ending well....

As an aside, I still think the 1/4 mile is probably the best test of how "powerful" a car really is. All a bit brutal, but a much better test that 0-60.

 

EDIT.

Just thinking, engine size is not a good indicator of heat output. Surely engine power is better? 

Plus a story from my own car. My spitfire is a 2 litre ford unit, making about 180bhp. It ran a small golf GTi radiator, came with my old spitfire, and dated from about 1980. But cooled the car fine. Temp gauge would kick up a tad if you stopped after a run, but settle when the fan came on. On that basis, I fitted the same sized rad, but a new one, to my Toledo when I fitted a TR7 engine making less than 100bhp. However, in that car, the faster you drove, the higher the temperature. In other words, the rad was too small. Solved by fitting a (40%) larger Passat radiator, now all is well. 

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I think what you are showing there Clive is that airflow through the rad is a critical part of the equation with the same rad performing worse in the Toledo installation due to less air passing through it, especially at speed.  This can be offset by increasing the area.

On the 2009 10CR we spent time travelling in convoy with DollyRoo which had suffered radiator damage due to the viscous fan coupling failing and could only hold water if unpressurised . It was discovered that running with the bonnet propped open a couple of inches at the back made a huge difference and was probably the factor that kept the thing alive.

Small rad properly ducted could out-perform a bigger one just thrown in.

ISTR that the rule of thumb is that the rad needs to be able to dissipate 30% of the max bhp.

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Cooling was Silverback's Achilles Heel!       

The rear radiator was so that I could reduce to the minimum the amount of air flowing under the car, as none came in at the front of the engine bay, and it could have an airdam.    The rear rad was ENORMOUS!   Fullwidth of the back, about two feet high and ventilated by two enormous truck fans each 300mm wide.   It was fine on the road but clearly was near its limit and would overheat on the track.     And having 2.5" hoses through the bulkhead and along the floor to the rad was always a hazard.    The hot end came off once and sprayed my legs with boiling water, another time the cold end end soaked but didn't scald a passenger.    After that, I gave up and put the front rad back in.    The Gods intended radiators to be at the front!

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When I started racing the GT6, out here in the Mojave, I suffered many overheats.  The solutions have been documented on these pages before but maybe not the tool?  I bought a Magnahelic differential presuure gauge and hung it in the cockpit with plastic tubes before and behind the rad.  I was initially amazed at how low the Delta was until I started shrouding and baffling the air.  Biggest single improvement was of course side boards sealing the front of the rad to the bonnet.  That was further dramatically improved by closing the space between the frame rails ahead of the rad.

I could go on and on with details of the improvements but the Point is I pursued a Delta P more than water or materials

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14 minutes ago, GT6Steve said:

but the Point is I pursued a Delta P more than water or materials

Yes!  Which is also I think the underlying message in the last few posts, though less clearly expressed.  Air flow through the rad is king!

John,

Am I right in thinking that this is for the GT40 replica?  Are you planning a single front mounted rad or side-pods?

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Single front, Nick.

You and Steve know what I learnt later.   Air intake was two panels in the rear wings, into the open rear space of the car.    Later attempts to duct the air into the rad helped but not enough and I should have thought that out initially!     A lesson I'm thinking on now!

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On 12/10/2020 at 2:46 PM, GT6Steve said:

When I started racing the GT6, out here in the Mojave, I suffered many overheats.  The solutions have been documented on these pages before but maybe not the tool?  I bought a Magnahelic differential presuure gauge and hung it in the cockpit with plastic tubes before and behind the rad.  I was initially amazed at how low the Delta was until I started shrouding and baffling the air.  Biggest single improvement was of course side boards sealing the front of the rad to the bonnet.  That was further dramatically improved by closing the space between the frame rails ahead of the rad.

I could go on and on with details of the improvements but the Point is I pursued a Delta P more than water or materials

Delta P and hence air mass flow are critical. Formula 1 're-discovered' the experience of the aeronautical industry back in the day (and incidentally James Watt and his steam condenser back in 1769) that getting the air in is only a quarter of the story. The other three quarters is getting the air out.

In essence the best radiator installations suck :biggrin:

And presumably was a significant contributor to Silverback at least once the beast was traveling 'at pace'.

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As I mentioned, two enormous truck fans sucking air through and out of the tailgate.

img_2402.jpg.75f786a24cbc9f64025d9cd23afe0f2b.jpg

The left air intake - there was a twin on the other side - is seen between the rear wheel arch extension (under"back") and the Moss decal.   They were intake grids from the Toyota MR2 Mk1. which only had one per car!

Later modified to ensure no shrouding to

1226452534_Insurancepics2006014.thumb.jpg.7d1532b54fcbae50707bd4cad0294a2b.jpg

The size of the radiator is better seen here

1262962973_Insurancepics2006015.thumb.jpg.3425650a8c3e5d0ebb6771020aa7b2fe.jpg

And STILL it would overheat on track!

Mayb the afterburner didn't help

S.thumb.jpg.e3aaaf75cc69ad15f81565ad851d39f6.jpg

Edited by JohnD
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Trying to apply exact science to this is really really difficult.

There are so many influences.  Radiator material and the number of rows (depth) are only the beginning.

Water flow is key too, but not that critical, as remember there are 2 heat transfers.  One is taking heat from the engine, the other is putting heat out through the radiator.
Measuring delta T across the radiator gives a good indication of the effectiveness of these two.  Industry standard is to look for something like 4 to 8 deg.C at worst load conditions (you'd need to measure with the thermostat fully open).
...then what is worst load?  Sitting in traffic (no air flow across the core); pulling up hill in low gear (low air flow, low engine speed so coolant circ. down, and low fan effect -if mechanical / fixed fan driven by engine), or .....?

A good measure would be to pick a rad. sized to a similar horsepower engine, but remember modern car rads. have air conditioning condensors in front of them and sometimes charge air coolers.  This is good for us, as it will give us extra margin (ambient air temp onto radiator is lower for us without these things).

Also as mentioned above we need the forward motion of the car to push as much air as possible through the core (so side shields and baffles).
These are also essential for the sitting in traffic situation as they will help prevent recirculation, where the hot air runs round and round the fan / radiator core.
.....and don't forget the fan.  Modern cars have cowling over the whole radiator core ducted to the fan.  This is to minimise recirculation, and to drive air over the whole core  so maximising the effectivness.

How much does the fan sit in the cowling (it makes a big difference, and it matters if the fan is 'pulling' or 'pushing')?
It's amazing the uncowled fans we fit to our cars work - but they do.  They could be much better though!

Oh, and fin design has a big effect too.  Some crimping of the fins is poor (cheap), so the radiator can't reject the heat.

 

Sorry, no answers, but ideally cowl the fan in (it's on my list of jobs using a Peugeot 206 fan - that sits in front of the rad. with a nice big cowl).  Seal everthing up, so cold air in, hot air out.  And I'd advise picking something readily available for a radiator (taken from existing car).  The aluminmum ones are tempting and I think there are some good ones, but some crap too....

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