Page 1 of 3

A new cooling discussion

Posted: Wed May 29, 2019 10:28 am
by TrevorG
I don't know if anyone has done this before, so I thought I'd put my musings here.

Further to the discussions 're scimitar cooling problems resulting from airflow problems, and as an ex airflow designer (albeit nearly fifty years ago), I am musing that maybe the whole cooling airflow is upside down. To get the air flowing over an object there should be a reasonably straight path from entry to exit...a bit like a jet engine.

The problem seems to be the air comes in low and is directed through the radiator up over the engine and then swirls around trying to find an exit, causing buffering.

I'm musing that removing the spare wheel, moving the radiator up or increasing height and cutting slats in the plastic front grill, even closing off some or all of the under grill would create a ram air effect over the engine and down the tunnel and out.

Not that my scimitar has ever had problems, but has it been tried?

A new cooling discussion

Posted: Wed May 29, 2019 10:54 am
by rebel alliance
Almost certainly....now the thing is as far as i know the origional se5 did not suffer from overheating for one very good reason....which i reinstated on my 5a

A new cooling discussion

Posted: Wed May 29, 2019 12:49 pm
by scimjim
With a radiator, you don’t necessarily need direct airflow across the fins as you can end up with inefficient heat transfer at high speeds. It’s much more to do with pressure changes than direction, particularly at low forward speeds. IIRC the ideal is a divergent duct with entry and exit areas about 1/3 the area of the rad.

Here’s a good example
A1C58A4F-0CFA-4EE7-B055-B33C88E7DDFD.jpeg
A1C58A4F-0CFA-4EE7-B055-B33C88E7DDFD.jpeg (246.57 KiB) Viewed 1210 times

A new cooling discussion

Posted: Wed May 29, 2019 12:54 pm
by Old and Slow
IMO the best way to get airflow through a radiator is to create low pressure behind it, as per the oil cooler on the Supermarine Spitfire. Trying to push high pressure through from the front isn't straightforward as air will spill out in the path of least resistance, which might be over the bonnet or under the car.
Of course, how to get a low pressure area behind the rad is another question; I've no idea how to do it, apart from sticking a big fan there!
Not sure I'm being helpful here; I'll get my coat...

A new cooling discussion

Posted: Wed May 29, 2019 1:08 pm
by rebel alliance
Back to my last comment the se5 had a very big fan behind the radiator 😊

A new cooling discussion

Posted: Wed May 29, 2019 1:13 pm
by scimjim
If you mean the mechanical fan, it’s good for low speed/stationary but won’t do much at speed. The only way out for engine bay hot air in most cars is down the transmission tunnel (the rear of the divergent duct) - the Scimitar is no different.

A new cooling discussion

Posted: Wed May 29, 2019 3:17 pm
by TrevorG
I think the se5 is better at directing air flow than the se6. I agree that pressure of air is also a great way to give good airflow...I refer back to a jet engine again. It's probably why the se6 can suffer as air is coming from low down and being fed upwards, nowhere to go then equals higher pressure under the bonnet, hence why some put in louvres. Yes, the air will go down the tunnel, but maybe having it directed there would probably improve things.

A new cooling discussion

Posted: Wed May 29, 2019 3:45 pm
by scimjim
I don’t think the jet engine/gas turbine analogy is the best one - air isn’t being mechanically compressed or burnt and you’re not trying to recover energy or produce thrust at the end of the process :D not all gas turbines use straight through design either.

General aerodynamics/thermodynamics apply, but you have two main scenarios to consider. At high forward speeds, you need to slow the airflow down to allow heat transfer, but at low forward speeds you need to ensure the airflow doesn’t stop altogether.

Bonnet vents will only help when stationary if there’s no pressure differential (so it’s simply exiting from the uppermost point) that’s why we have a fan, to create that pressure differential.

A new cooling discussion

Posted: Wed May 29, 2019 4:00 pm
by rkclodhopper
Adding louvres may not be helping in the way people think, a high pressure area exists in front of the windscreen (the heater intake is placed there to make use of it) so a bonnet louvre fitted too far back may be in that same zone and letting air into the engine bay, it will be helping under bonnet temps by adding cooler air but not by letting hot air out.
The positioning of the 5 and 6 cooling air intake low down under the nose is so placed to be in another high pressure zone that forms just there.
I would love to see examples of our cars in a wind tunnel, I suspect Reliant never did.

A new cooling discussion

Posted: Wed May 29, 2019 5:23 pm
by AndyKing
I experimented on another car, with a similarly lowdown intake to the radiator, that struggled to cool the engine at speed, after I'd removed the viscous fan it had.

I used a differential pressure gauge (magnehelic), which has two connections, to compare the pressure in front of the radiator with that from behind, using a couple of lengths of screen washer pipe fixed at right angles to the radiator.

This proved that at 60 mph there was no airflow through the radiator at all. Only when I overtook a truck did it produce any airflow and even then it was flowing forward.....

A few experiments later, with first sealing off the intake around the radiator and I had enough airflow to cool the engine ok. The big change however was adding a small chin spoiler (just a bent piece of ally) under the air intake. This increased the airflow massively.
I don't think this increased the airflow in to the radiator particularly but more lowered the pressure behind it.

A new cooling discussion

Posted: Wed May 29, 2019 6:11 pm
by peter freeman
I think the best solution as to put a couple of vents at the rear of the front wings as per Jenson Interceptors

A new cooling discussion

Posted: Wed May 29, 2019 8:49 pm
by Old and Slow
peter freeman wrote:
Wed May 29, 2019 6:11 pm
couple of vents at the rear of the front wings
I hope so, the Sabre Six 660XYB has small louvres at the rear of the front wings - as the fan is currently the original Reliant design - it sits in front of the rad and tries to push air through, but with an inch gap between fan and rad so most air spills round the sides - i'd like to think the louvres provide low pressure.

A new cooling discussion

Posted: Wed May 29, 2019 10:07 pm
by cannonball
Years ago I was heading down country and after a drive of just over a hundred miles a cheeky Mini nipped past me as I exited a roundabout, stuff that I thought so I stuck it in third and floored it. The next thing the catch of the front hinged bonnet popped at around 70mph and up it came. As I slowed down the bonnet descended so I put the foot down and the bonnet rose to the occasion yet again, the faster I went the higher it would go. As I had another hundred miles to go I pulled in and slammed it shut.
Maybe air vents at the back edge of the bonnet to let all that hot air out is the way to go.

A new cooling discussion

Posted: Wed May 29, 2019 10:13 pm
by peter freeman
Vents at the rear of the bonnet do not work as that's a high pressure air area - which is why the air intake for the heater in the Coupe's and 5's is situated there - Don Pither made me wise to that years ago.

A new cooling discussion

Posted: Wed May 29, 2019 10:31 pm
by Dave 6726
Would the higher pressure at rear-bonnet vents then mean more outside (cold) air pushing in to force hot air along the tunnel at high speed, but allowing hot air out at low speed?