Air flow balancing - demisters

[stephen low]

Needing some suggestions lads.

I am persevering with the installation of a heater to provide hot air to the footwells and to demist the windscreen.

I've made a fibreglass air plenum that will be stuck to the underside of the dash to bring the air to the two demist vent points. The incoming air point is via a tube at the centre of this plenum and angled towards the drivers side.

Trialled this roughly yesterday and I get huge air flow through the drivers demist vent but none on the passenger side.

Any suggestions as to how I can better balance release of air through both vents.

Do just have a couple of slots at the moment but am going to fit up the Snakebite vents more for visual finish.

Happy to still have more air coming out in front of the driver but do want to get some flow on the passenger side for some effective demisting capability.

Await some suggestions.

Cheers

[guye]

Any fresh pics Steve? I do recall the plenum I think but couldn't be shagged searching the threads

Probably the easiest thing is a baffle. Does the inlet pipe to the plenum enter in a place you could insert a small baffle to a) divert the flow and/or b) restrict the flow to the drivers side?

[stephen low]

Bit like you Guy, I've no doubt early photos but easier to upload these fresh ones.

So the overall plenum.

Right side is the drivers side and as described the air inlet to the plenum is pointing towards the drivers side.



And a photo showing the profile, the driver's end nearest in the photo.




Moulded the fibreglass over half a pvc pipe I had curved to the underside of the dash.

Once I am happy with air flow, I plan to sikaflex the plenum in place to the under-dash.

Have thought of partly introducing a curved baffle that redirects air to the passenger side at the inlet tube.

Just means some fibre glassing once I develop a template that works.

Any other suggestions??

[Aussie Mike]

I can see the problem. The feed in pipe is pointing straight at the drivers side. How about putting a partition in the plenum just after the feed in pipe that fills the bottom 1/2 of the channel that would force some of the air to do a u turn up the other side of the channel and even out the balance. Try cutting a piece of ice cream container and taping it in place to work out the right height.

Cheers

[stephen low]

Mike

I had to go the angled inlet to minimise size and simplify hose routing and did suspect it would cause this issue but needed to live test it to actually find out.

Am trying to understand and picture your description, it certainly sounds right. Can you draw a picture to expand the thinking.

I suspect what I am picturing is not quite what you are suggesting - lol

And thanks for the idea of using plastic initially that should speed up and simplify the testing.

Cheers

[Aussie Mike]

I can understand the angled inlet, space is tight under there.

I'll try to draw you a pic of what I'm thinking.

Cheers

[David Hodgson]

Slowy,

I think what Mike is suggesting is a "dam wall" depicted in red on the images below.

Rather than restricting one end or trying to turn air with a baffle, I think something more positive like splitting the whole device in two right from the start of the inlet tube(green line). You could even curve the plenum end of the divider to help turn the air to the passenger side.

[stephen low]

Thanks David

Will have to have a play with some of these ideas.

Mike

Any suggestions still welcome mate for any different spin of thought.

Cheers

Steve

[Aussie Mike]

David got what i was thinking exactly. David's idea with the splitter is a good one.

[stephen low]

Ok thanks

Will let you know how it goes then.

[stephen low]

So a few tests using cut up strips of plastic off an ice cream lid and grey tape to locate.

Essentially split the intake tube and then folded the plastic back towards the passenger side to force air to arc back to the passenger vent.

First go and I didn't fully segregate between the flows.

Though this caused some flow transference it wasn't terribly significant and the drivers side was still dominant.

So fully sealed between the flows and got fairly balanced airflow.

Have started making a fibreglass splitter off some metal I bent to suit the set up, as shown.



Will see if the polyester goes off in this cold though it should given I upped the catalyst amount.

Will build up the splitter once hardened to fit the duct and fully seal the flows.

I can always open up some holes to help favour the drivers side airflow if that becomes my need.

Thanks for the suggestions guys.

Cheers

[renovationinnov]

slowly
what about moving the feed pipe towards the passage side more so it feeds from that side anyway then it might do what you need
just a thought
rob

[stephen low]

Rob

I suspect it might slightly improve flows but given the angled inlet I'd expect the scenario to be replicated unless the inlet was at or before the passenger side vent location.

This change of inlet could be achieved but I still suspect I'd need to tune the flow as I am already starting to do. So I'll continue the current course of action

But I'll remember your suggestion if I need to go to option two.

Cheers

[glenview289]

By placing your inflow feed in the middle and angling it toward the drivers side you are getting a venturi effect which will direct flow in the desired direction, but will actually draw air from behind the inflow point. Any of the suggestions to bring the flow into the whole plenum from either end will eliminate this condition.

Bill

[stephen low]

Thanks Bill

The fan is on the (left) passenger side so if the air inlet to the plenum was before the passenger vent, is this likely to cause preferential flow out of the passenger vent as the first and closest point of release out of the plenum?

It would be easiest to fit the inlet to the end of the plenum before the passenger vent point so it is an attractive option.

Your thoughts?

Steve

[glenview289]

A couple of thoughts. You could move the inflow to the end like you mentioned, but would probably need some way regulate the flow on the passenger side to move more air over to the drivers side which is more important. Or you might put a second inflow hose before the passenger side (i.e. end), move your damper between the two sides to create two separate zones and connect the two hoses with a "Y" before the fan. I see the second idea as the easiest to do. Keep us posted with your progress.

Bill

[dave295]

Restricting the airflow on the drivers side with a pipe or tubing inside your manifold might help.
Where Dave H. drew the red line in the above, block the manifold with a piece of cardboard. Knock a circle out of it, say the diam. of a toilet roll tube. Do the same at the other end so it resembles a dumbbell or a H shape. Varying the pipe length would hopefully vary your pressure and be adjustable.
Or a pipe running directly to your vent inside the manifold and sealed around the middle of the manifold (give or take) from pipe to edge of manifold might help.

Dave

[stiffy]

Slowly,

I have a similar problem with my Daytona, space is the main

Restrictor.

I spent a heap of time making vents to equalise the airflow to no real

Gain. It is what it is.........

At least the driver gets the coolest and hottest air in my case........


Stiffy

[stephen low]

Thanks Stiff for the reality check.

My test did improve (balance flows) sufficiently to keep me happy so I will continue at it. I can always re-open air access to favour the drivers side even more later on if I think it necessary.

[sambo]

Slowy, how about making two smaller plenums with 25mm tubes on the end and attaching those to the bottom of each vent. Then run a length of 25mm ID flexible hose down to a Y-piece like the one below.



This one has a 38mm inlet and will flow a little better.

Y Piece 25mm 25mm 38mm Tail | eBay