Which way does air want to flow?

[family wagon]

My bus has about 20 feet center-to-center between the front and rear axles. I'm designing in a compartment for a generator in the skirt just behind the left front wheel, and planning to reserve some space just behind that for air conditioning gear -- compressor and condenser, primarily. Both the generator and the condenser require air flow to cool them off.

So the question is this: if I want to push air through this area while the bus is driving, say at highway speed, does the air "prefer" to go in a particular direction?

For the condenser I'm thinking about an automotive condenser take-out complete with its "puller" electric fans arranged just as it would be in a transaxle car: condenser facing to the outside, fans facing inboard pulling air in. (I don't know yet whether this is enough condenser surface area.) I suppose the hot air would discharge out the bottom. But are there high- or low-pressure zones that develop because the vehicle is in motion? Obviously if I try to fight physics with fans I'll probably lose.

The only point of reference I could find is the engine cooling on the same bus. It's a rear engine and has the intercooler and radiator mounted to the side of the bus. I infer in that place air must be pulled in through the sidewall and discharged out the bottom around the engine. This is based on noting the intercooler is closest to the outside of the bus and the guess that air should go through the intercooler before going through the radiator. It makes sense; I'd expect a natural low pressure/suction to be created behind the vehicle. I just don't know what natural pressure difference to expect from beside to underneath.

[Kubla]

You want to use fans to pull air from the outside of the bus and have it exhaust under the bus, if you pull it from under the bus, the dirt in it will plug up everything in short order

[Elliot Naess]

Originally Posted by family wagon

... ... But are there high- or low-pressure zones that develop because the vehicle is in motion?... ...

Yes! And they are strong forces indeed.
You probably ought to research this quite thoroughly. I imagine there is plenty info online, but it may take a lot to time to find it.

I know a little bit: On the sides and roof close to the front, there is low pressure. This is from sort'of a venturi effect, caused by all the air hitting the front of the bus and being forced outward at high speed. Or maybe a sort'of back eddy effect.

You can confirm this yourself: Open the driver's window and toss some scraps of tissue paper around. They will blow OUT the window.

In Millicent we have a problem with exhaust fumes in the front of the bus. The air that is "sucked" out of the front area must of course be replaced, and that air comes from the rear, where exhaust fumes are drawn in.
Millicent has a wall with a door across the middle, separating the "residential" and "cargo" areas. There is a one inch gap under that door. We have learned to stuff an old towel under that door, to reduce exhaust fumes up front.

How does all this exhaust get in? Well, that's my fault. The "toy hauler" tailgate is not exactly airtight. So I need to work on that too.

(Mind you, this exhaust problem is not enough for me to even notice. But some of our passengers are more sensitive.)

Now...: Last year I cut a 12 inch hole and installed an electric fan in the wall between the bathroom and the cargo area. This is specifically for camping in a hot desert, when the bathroom can become awful hot.
Guess what.... On the road, airflow from the cargo area into the bathroom spins that fan backward like gangbusters. The bathroom door is almost difficult to close against that air flow!
So I have a ton of research and experimenting to do on this.

[Robin97396]

Would it be logical to assume that skirt mounted radiators would draw internal heat out just from the vacuum created by the movement of the bus?

[cadillackid]

Originally Posted by Robin97396

Would it be logical to assume that skirt mounted radiators would draw internal heat out just from the vacuum created by the movement of the bus?

I had one skirt mounted condenser on the last bus for the air-conditioner... sometimes in perfect conditions I could get a decent airflow without the fans running.. but as soon as a car or truck passed me or I got in a cross-wind, or 2 lane road with oncoming traffic.. the fans were always running to keep the coils cool...

I never hit a situation where the airflow was so positively pressurized that the fans couldnt overcome it...

-Christopher

[Robin97396]

It just seems that since we're jamming these bus shapes through the air at 60 miles per hour there should be some way to channel the air to provide that same cooling effect efficiently without fans, unless we're stopped.

I like that triple fan setup you chose. That looks very much like something you might see in a meat locker. I want one.

[cadillackid]

Originally Posted by Robin97396

It just seems that since we're jamming these bus shapes through the air at 60 miles per hour there should be some way to channel the air to provide that same cooling effect efficiently without fans, unless we're stopped.

I like that triple fan setup you chose. That looks very much like something you might see in a meat locker. I want one.

the problem is if you start channeling air up from the road you are also channeling dirt and debris up from the road too... dirty condenser coils are the biggest killer of air-conditioners... mobile and fixed... window A/C and camper A/C included..

ive never done a study but i liken changing airflow under or around a bus to somewhat like it would be pacing a windmill on top of a bus for electricity.. you are likely to change the airflow to some extent and possibly use more energy than otherwise for the air to just flow under and beside the bus..

is there any gains or losses? minimal either way... unless I really wanted to play physics.. which I dont need to when I can just turn on 1,2, or 3 fans..

that triple fan unit i chose is the Outdoor portion (in a meat locker they run similar units as the indoor cooling coils).. so that triple unit will mount under my bus and not penetrate the skirt.. its lightweight too so its easy to mount... its a bit oversized for the application, however because of lack of ram-airflow under the bus I can effectively control the capacity of that coil to match the conditions needed for cooling.. I'll control each fan individually based on head pressure.. on those hot days while im stuck in traffic im guessing all 3 fans will be on... on those cold rainy days where the only reason the A/C is engaged is for pre-chilling the defroster air, likely only one fan will be used.. cycling on and off as needed to maintain proper pressures... in my case im using lack of airflow under the bus to my advantage...

that coil was part of a complete setup I bought from ProAir LLC.. probably $$$ to most on here but for bus A/C it was Very reasonable at $2700..

to have a system like it installed at a shop you would be looking at 7k and up.. for a large bus.. ie 12 row+ youd likely need 2 and are looking at 11k+..

engine-based A/C is NOT cheap for a bus... alas why many choose to install a generator and some rooftop camper units.. you can kill 2 birds with one stone.. gaining 110 volt service and A/C..

for what im doing with my bus engine driven A/C makes the most sense... for most it doesnt..
-Christopher

[Kubla]

Cadillackid's post is why I bought the second bus with factory air, the cost of adding engine powered AC to a bus that does not have it

[cadillackid]

Originally Posted by Kubla

Cadillackid's post is why I bought the second bus with factory air, the cost of adding engine powered AC to a bus that does not have it

take care of that factory air.. keep the outdoor coils CLean and make sure the outdoor fans all work or you will wipe out the compressor(s).. and when that happens its really tough to ever get the system reliable again..

large bus systems are TOUGH to flush out if you "black-death" a compressor.. which selttec / sanden love to do when they are run at too high of pressure (but not high enough to trip the safety switch)...

also keep the refrigerant fully charged.. dont run it low.. oil return can become a problem is a large system like a bus... when the refrigerant level leaks down a bit, the system tends to pull more oil away from the compressor and alot of it ends up in the condenser(if skirt mounted) or worse yet stuck in the low pressure side (evaporator) if the condenser is roof-mount...

take care of it and it will run trouble-free for a very long time...

oh and dont accidentilly leave the driver slider open and get the controls wet.. Transir and Thermoking controls are notorious for not handling even small amounts of water on them very well..

-Christopher

[family wagon]

You all have given me much to think about. I had hoped for an easy answer, but alas..

I wish I could just go hit the road and do a test. I generally like doing that. But as my bus is currently wearing only its floor and roof (no skin inside or out on the walls) its airflow behaviors would be entirely unrepresentative of real life!

cadillackid, you mentioned having a skirt-mounted condenser that sometimes got good airflow even with the fans turned off. Where was it located, and which direction did the air flow?

[Elliot Naess]

Another phenomenon to keep in mind is "boundary layer". Boundary, as in the border between two things.
The air that is IMMEDIATELY next to the side of the bus travels with the bus. Then, perhaps one millimeter from the bus the air travels at a little less speed than the bus, and so on. This is why air intakes are often on top of a pedestal -- to get away from the boundary layer and get full ram effect.

Another way is the NASA or NACA (or some such) duct, which is recessed in the vehicle, so the boundary layer follows the bottom of the recess.

Some land speed record cars have the air intake sticking straight out ahead of the vehicle, so it encounters undisturbed air for the full ram effect.

This is a complex issue, but I think we can learn quite a bit from a few hours of Googling.

[cadillackid]

Originally Posted by family wagon

You all have given me much to think about. I had hoped for an easy answer, but alas..

I wish I could just go hit the road and do a test. I generally like doing that. But as my bus is currently wearing only its floor and roof (no skin inside or out on the walls) its airflow behaviors would be entirely unrepresentative of real life!

cadillackid, you mentioned having a skirt-mounted condenser that sometimes got good airflow even with the fans turned off. Where was it located, and which direction did the air flow?

the skirt mounted condenser was located on the bus skirt, left side near the bottom.. right under the 1st and 2nd passenger windows...

im not sure whuch way the air flowed when the fans were turned off.. I only knew the coils stayed cool during that time.. when the unit was running with its fans, the air was sucked in from the grill in the skirt and discharged underneath the bus.

-Christopher

[Elliot Naess]

A classic way to learn about air flow on a moving vehicle is to tape pieces of yarn or some such to the vehicle and photograph it as it drives by. Short pieces of "flag tape" might work well. This stuff is about one inch wide, in bright colors, and costs practically nothing. Any hardware store should have it. It is used by land surveyors and such.

You can also mount the camera on the vehicle.

[turf]

My bus came with a diesel generator hanging in a box in the underbelly.

the motor is reversed from what you'd think normal air flow is.

from the front tire, first is the battery box, then the generator head, the diesel motor, and last in the box is the radiator. just after the the radiator not shown in the picture is the exhaust from the generator, then the rear tire.




hope that helps

[Tango]

Hmmm...have to wonder about that arrangement. Could it be that the airflow around the other items creates a draw through the rad at the rear? Or possibly slows the airflow down for more effectiveness?

Many air-cooled aircraft engines and old 2-stroke formula race bikes used what is called a "still air box" to aid in extracting heat. At speed, the air is not in contact with the cylinder heads long enough to take away adequate BTU's, so they enclosed the heads in a box that had a larger opening at the rear than the front. By slowing down the flow of air, it removed more heat. Maybe something like that effect?

[Robin97396]

Maybe they just didn't want the radiator against the battery box?

[Kubla]

did not want the front tire throwing rocks into the generator radiator

[Elliot Naess]

The concept of air or water moving too fast to pick up heat has always puzzled me. I am NOT convinced this phenomenon exists -- UNLESS it has to do with the boundary layer I mentioned earlier. Heat transfer, I expect, primarily takes place in the boundary layer. Possibly, the thickness of the boundary layer matters. I wold love to learn more about this!

[Robin97396]

Like the experiment where they put golf ball dimples all over a car and found that it had less wind resistance.

[cadillackid]

Originally Posted by Elliot Naess

The concept of air or water moving too fast to pick up heat has always puzzled me. I am NOT convinced this phenomenon exists -- UNLESS it has to do with the boundary layer I mentioned earlier. Heat transfer, I expect, primarily takes place in the boundary layer. Possibly, the thickness of the boundary layer matters. I wold love to learn more about this!

my own personal OPINION on that is it is due to humidity.. If I raise the temperature of the water in the air that it continues to change states more and more to a gas it is going to transfer a LOT more heat than if the air simply goes through and no state change occurs..
-Christopher