Bruce...well put...but we are quibbling over my late night lack of vocabulary. By what rate of advance...I mean...excatly how many degrees of advance are you advanced to while running at constant speed? Because you will be advanced and you may run hot from that if advanced too far.
So...its not a power pulley right? cause that would be smaller than stock. But then...a 12% larger pulley...would drop the rpm slightly...correct? Maybe you are running the fan slightly slower at speed...than the cooling system requires? Its something to explore.
Cavitation. The term is for fluid.....I realize that. Denotes shearing of gas from liquid. The effect in a fan from turbulence, is much the same in what it does to flow...and exists just the same. All fans have stall speeds.
Mharney...the last part of your post is what I was getting at. You do not have to be sonic in fan blade velocity, to create turbulence in a fan, great enough to cause partial blockage or resistance. You said it better than I.
Baja wes. Yes...your fan calculations are common in most HVAC books. One thing you are missing is that those do not measure the "stacking" of all elements involved in any given fan installation.
yes Rpm and output are linear....or can be...but only in unducted form and with no restrictions...ie: head pressure, or supply duct being smaller in cross section than the max rpm flow rate of the fan...requires. The calculations show none of that.
Use that fan to start pushing air into the cooling manifold of a VW engine...or an AC duct with a large volume of air ahead of the fan....and numerous constricted duct openings (or any other number of examples)....and you will reach a point where the resistance on the output side of the fan...creates turbulence at the tips of the vanes.
Centrifugal fans...like these..cannot tolerate that. Flow is laminar at the tips. Air in these fans is not pushed by vanes..it is flung from the tips due to centirfugal force.
The design velocity at the vane tips, the area of the vanes, the shape of the vanes, the design HP allowed, and the design rpm....are also largely dictated by the type of centrifugal (or radial ...whatever you want to call them) fan in use. There are 3 basic types....forward canted, backward canted, and one that can only be described as composite with supercritical helical blades....and can only be properly used in a duct.
It is that "flinging" movement, noted above, that creates velocity....that creates the laminar flow at the vane tips....that continues the process by creating a depression on the leading edge and underside of the vane...drawing in air. The design, texture and angle of the fan floor base also contributes greatly.
Turbulence from head pressure...which builds as rpm rises....can cause a breakdown of laminar flow at the vane tips. It appears almost as "slippage" and many people have described it that way. Which is why I term it cavitation (though it is not). Its effects are very similar. Efficiency of the fan at that point DROPS...though the rpm may continue to rise. The math doesn't quite describe that (although there are formulas to describe the effect...when calculating head pressure and frictional loss).
The only way to solve it...is to vent head pressure. This is a totally different effect than what happens when a turbine goes "sonic". In that scenario, heat and vapor expansion create localized "sonic booms" or violent charge expansion, that may actually reverse flow on a turbine with catastrophic results.
Just as a thought, the belt slippage you note...is not prevalent in type 4's like I drive. They are crank driven. In the stock system...by my experience, limiting turbulence begins around 4700 rpm. I find reductions of about 20% by 5200 rpm (which was all I could get out of it
). It was measured as reduction of velocicty at a preset drop rate of 3% by a hot wire type anemometer. That simply means that I pre-set it to squeel when the rate started going negative instead of positive.
Though I appreciate and undertand the formulas you provided....I propose that they alone, are not enough to explain all the aspects of the application we are speaking of. Fan volume cannot remain linear with rpm in a restricted system. Sooner or later something has to give, In a type 1...as you note...its the belt. In a type 4, it turbulence...loss of cooling air...and increased braking action by load, on the engine.
As I have just started to discover in the last year and a half, a smoke cartridge and a camera can greatly distort the results that the math swears we should get. I havn't started playing with VW fans this way...but its been enlightening on the manifold. It has been enlightening in the past...in HVAC centrifugal fan systems...where I first used smoke to define fan stall due to excessive head pressure. It was happening where the math swears it shouldn't.
Just a friendly note please....don't poke at peoples education too hard. You might be stunned at what you find in this forum. My poor typing may not allude to either one of my degrees, my years of experience in process controls and systems or my position on the process engineering staff in a semiconductor facility. Aside from all of that we can only be familiar with what we use. The rest of it we eventually lose through dis-use. Ray
).