Even after an initial flush/bleed, it is common for Heat Exchangers and filter containers to accumulate entrained air. The resulting condition is called "air bound" and for heat exchangers, not all of the available surface area is utilized and the heat xfer goes down proportionately. In industrial applications, it is very common for HExs to implement a trickle vent at the top of one of their headers. As for our applications, try to have at least a little slope with flow through the cooler bottom->up. You're gonna get anecdotal "evidence" here to the contrary. That's because our components tend to have (sometimes significant) margin. Best to keep margin as margin. Up front consideration of the aforementioned can save you some frustrating rework later. My $0.02I was wondering if air can get trapped in the oil cooler when it was mounted on its sides/flat when filled and remain trapped. Logically thinking about it, this is not only possible but probable but I was wondering if anyone has better info.
Viscosity has nothing to do with determining flow path. Delta P rules and gases are compressible. If there's no associated device pressure drop budget, u is really only used to verify turbulent flow in the HEx tubes via the Reynolds number. The header is quite the opposite story (reference the attached link).Take the example where the cooler is mounted horizontally, with air going vertically through it: I suspect that there would be very little air trapped.
If oil were very, very (glacially!) slowly pumped through an empty cooler at essentially atmospheric pressure, one can see that there would be air trapped above the ports. However, oil is pumped quite vigorously through the cooler, and it also at around 5 atmospheres of pressure. The oil will take the path of least resistance, which means it would prefer to flow where there is only air, since the viscosity of air is extremely low compared to oil, thereby pushing the majority of the air through.
No Sir. The headers are designed for very low velocity/to be manifolds. There are no turbulators applied in the fin pack passes; thus, no way to adjust back pressure through losses. The header design is really the only way to achieve such. Air can get trapped there and can accumulate in these high points. If on the inlet side, you’ll get dead tubes and a proportionally lower heat xferIn any pressurized fluid system, air gets pushed out quickly in one slug, or gets mixed into the fluid and then liberated once it exits the pressurized system (such as being dumped into the oil sump). it simply can't remain "trapped".
I do not have personal experience based on testing to debate whether air can remain trapped in an oil cooler with both of the fitting ports oriented towards the bottom, but I do know that there are certificated aircraft that have a special fitting with a riser tube installed on the outlet port of the cooler to assure that the cooler has to fully fill with oil before any starts flowing out the exit side.No Sir. The headers are designed for very low velocity/to be manifolds. There are no turbulators applied in the fin pack passes; thus, no way to adjust back pressure through losses. The header design is really the only way to achieve such. Air can get trapped there and can accumulate in these high points. If on the inlet side, you’ll get dead tubes and a proportionally lower heat xfer
The cooler designer may have added margin in these areas where area above the inlet/may not have been considered in the design calcs. This adds weight and cost.
Ultimately, if the cooler OEM has installation requirements or even recommendations, best to adhere to them.
Fluid dynamics is one of my areas of expertise.No Sir. The headers are designed for very low velocity/to be manifolds. There are no turbulators applied in the fin pack passes; thus, no way to adjust back pressure through losses. The header design is really the only way to achieve such. Air can get trapped there and can accumulate in these high points. If on the inlet side, you’ll get dead tubes and a proportionally lower heat xfer
The cooler designer may have added margin in these areas where area above the inlet/may not have been considered in the design calcs. This adds weight and cost.
Ultimately, if the cooler OEM has installation requirements or even recommendations, best to adhere to them.
Fluid dynamics is one of my areas of expertise.
The situation you describe can happen with a non-pressurized system. Where both the air and the oil will exist at the same pressure, and the oil will flow past the trapped air. Because it lacks pressure to expand into all the available space.
Once you pressurize the system, the air gets entrained in the oil as the oil expands to fill the voids. It's the main reason that oil coolers are on the pressure side of the pump and not the suction side.
please read again. Ullage (and residence time) was referring to the design limitations of the oil sump -> why there is air entrained in the oil -> why it can accumulate in high points/low velocity points in the system -> why cooler orientation can be restricted based upon the OEM’s design considerstions. You’re previous stated reasons for HEx placement in a fluid process are far off as already noted. You’ve truly brought zero benefit to this discussion.The OEM's design considerations are maximum heat rejection, minimum size/weight, and cost per unit. I'm one of the people who works with the OEM application engineers to size and source things like this. There would be zero ullage area in a heat exchanger like this, it's not a reservoir like an oil tank. Ullage would defeat the transfer of heat from oil to air.
I will tell the fine professors at ERAU who use their Cray supercomputer to analyze my data that they are idiots, as determined by some posters on the Van's forum.please read again. Ullage (and residence time) was referring to the design limitations of the oil sump -> why there is air entrained in the oil -> why it can accumulate in high points/low velocity points in the system -> why cooler orientation can be restricted based upon the OEM’s design considerstions. You’re previous stated reasons for HEx placement in a fluid process are far off as already noted. You’ve truly brought zero benefit to this discussion.
those are low pressure systems, operating at 15 psi or less. Not up to 110 psi, as in the case of Lycoming oil systems.It seems common sense to me that air will get trapped. I'm just an industrial construction worker, but I've spent enough time around process systems, heat exchangers, and engineers to recognize Freemasm is correct. The academic arguments to the contrary are just that, academic.
The famous chevy LS v8's have a spot in the heads with a vent/ crossover line to prevent air from being trapped there. If not vented properly when filling the coolant system, that air will allow the head to overheat and warp. BMW's convoluted cooling systems are notorious for air pockets and problems. Just a couple examples of air being trapped in pressurized fluid systems. I've dealt with these problems in both cases.
Like I mentioned in my previous post, this subject is way outside of my knowledge base, and in those cases I always defer to others that work within those disciplines, but one thing I have learned is that even if you have the most advanced computing capabilities and designers, after you have a result from all of the high tech. design and analysis you do actual physical testing to prove that the result is valid.I will tell the fine professors at ERAU who use their Cray supercomputer to analyze my data that they are idiots, as determined by some posters on the Van's forum.
They can contact the Van's Forum to get the latest info on computational fluid dynamics research, and stop spending so much time with their expensive supercomputer and the associated professors who don't know anything. I guess the manifolds we made from plexiglass for visualization of the fluid flow through the system are also junk.