Rad air control flaps

[Fspp369]

I forgot to say that I have since ground off the mounting lugs on the fan bodies that were present for the original fitment because they catch on everything inc my hands.... having shovels for hands that is!
P

[MarekH]

Without those flaps fans will suck air into tbose holes, not through the radiator.

They must be fitted.

That's only going to be true at low road speeds.

If it were universally true, then you could just have a plain sealed box with no need for flaps at all.

The true purpose of the flaps is more likely that at high road speeds, only a limited amount of air can get through past the fan blades and so the excess airflow can get through by simply pushing the flaps open. Cars with aircon will have an evaporator matrix ahead of the radiator and both fans running all of the time, even when the car is going fast. Without flaps, the front of the car is going to act more like a solid wall so limiting the air getting through the aircon matrix and car radiator.

kind regards
Marek

[jagwit]

My view is that the purpose of those flaps is to close the cowling when the fans are running (just like a closed box) at low vehicle speeds. If they also open at high speeds with fans running, then sure, more air is being pushed through the radiator than what the fans can suck through. Do we know this for a fact??

But at high speeds - when the fans are NOT running - they are meant to be pushed open with ram air to allow the maximum amount of air flow through the radiator, TO LIMIT the amount of fan use to a minimum, preferrably none.

Bottom line: They must all be fitted.

So who is going to do a video to prove things either way????

[PeterCrespin]

You should fit a vertical divider to S2 & S3 fan shrouds. The day one fan fails or runs low speed only (if two-speed) the remaining fan will preferentially suck air from the adjacent half across the back of the matrix instead of through it. A vane almost touching the matrix prevents this.

With a downflow S3 rad it’s a nuisance in traffic. If it’s the right fan on an S2 it can be a bit more significant.

[jagwit]

You should fit a vertical divider to S2 & S3 fan shrouds.

I recon that is a VERY good suggestion Peter. If one fan somehow displaces less air than the other, surely the lack of a divider would also render the weaker fan's contribution even less efficient? Eg, if the good fan flows "10" units of air and the weaker one flows "8", having them suck on the same space, could that not render the weaker one to flow 6 rather than 8 ?

IIRC, the fan shroud of the XJS has exactly such a divider between the mechanical fan and the smaller electrical fan, surely for this very reason??

[Fspp369]

Mmm?
Surely if one fan fails then that’s sides flaps will tend to remain closed, as their resting position is closed. Any air drawn through the rad by the working fan will be coming from the rad and not through the cowl from the rear, and thus any air going backwards will come via the matrix and hence cool the rad, indeed if the central divide is fitted then one side of the matrix will be cooler than the other, leading perhaps to a horizontal stratification within the rad with possibly detrimental effects.
If you need a definitive answer then talk to a thermodynamics expert working on radiator design. I know several...NOT ! Any other theories are simply that, informed speculation. The idea of watching rad flap action whilst driving and concurrently monitoring thermal patterns is an interesting topic in itself ...made for Marek I think

[MarekH]

A central partition sounds like a very good idea.

This is also the logic behind the fitting of the radiator foam over the shroud:- this forces air through the radiator as there is no easy path around instead.

I shan't investigate further as I have used up my flow sensors to monitor the water flow through the radiator and bypass tubes on the v12 and am not too bothered to make more available. We already know the limiting case for cooling capacity is when rpm and airflow are both at a minimum, exacerbated only if you are too close to the car in front in stop/go traffic or crawling steep uphill.

In any case Philip and I are saying the same thing; one from the point of view of slow speed and the other from the point of view of faster road speeds.

kind regards
Marek

[Series1 Stu]

indeed if the central divide is fitted then one side of the matrix will be cooler than the other, leading perhaps to a horizontal stratification within the rad with possibly detrimental effects.

That would only be true if there was no coolant flow in the radiator. When the thermostats are open and the flow is correct, there would be no real hot or cold spots. Just reduced cooling efficiency.

I think all the answers you need have already been provided by Peter and Marek, although I can arrange for some expert advice from a thermodynamicist if you would like.

Regards

[Fspp369]

A good thermodynamics appraisal would be interesting.
Does the rad have any lateral flow, given that the tubes run vertically?
If you treat each tube as a separate entity, then given an even air flow, all tubes would be the same temp but if the air flow is asymmetric then differentials would exist.
I still see no advantage to a central partition, BTW neither did Jaguar apparently.
Peter.

[PeterCrespin]

There is no advantage if both fans are functioning. Nor would there be an advantage if the fans were the modern multi-blade type with a circumferential ring preventing spill-over and acting as a built-in circular cowl over the blade swept area.

However the Jag twin fans are quite far from the matrix in an angled box, not flat. This lets two marginally larger fans fit in a tighter space, like valves in a hemi or pent-roof chamber. That’s great while everything works, but if one dies the other one is already angled towards the dud one’s hole in the cowling, ready to steal air...

It’s not a binary yes/no scenario but clearly some air would be drawn across the back of the adjacent matrix with such a twin cowl design and a separator would prevent that. It’s a harder sell, of course, to recommend a mod which will likely never be required to kick in...