Section A - Cooling System, continued.....

Subject: Re: u s radiators Date: Sun, 30 Aug 1998 17:52:30 EDT From: TigerCoupe@aol.com To: Tigers@autox.team.net

To all professors, rocket scientists, fluid dynamics engineers, etc. on the list,
Uh-oh. He we go.
Maybe you guys could discuss this radiator thing on another channel and then get back to us stupid Tiger owners later with a summary of your conclusions.
Dick Barker
Subject: Re: u s radiators Date: Sun, 30 Aug 1998 17:01:51 -0700 From: Bob Palmer To: :@ames.ucsd.edu

Steve,
I probably should have included you, as the resident "rocket scientist" on this post. Especially since I saw your message to Dick.
Best regards,
Bob
Joe, Brian,
I'm taking the TigerCoupe's plea seriously and keeping this just between us "professors, rocket scientists, fluid dynamics engineers, etc."
First, your question Brian. I'm considering all other things equal; same volume, area, tube sizes, materials, etc. The only thing that changes is whether the three sections flow in parallel or in series. Use the analogy of three resistors; we can hook them in parallel or in series. Hence, the 9X difference. Other, second order effects such as bends, etc. I've neglected, but are probably negligible in practice. The main point here is that if you baffle a given radiator to make it a three-pass, then there is a tremendous (ca. 9X) increase in the flow resistance.
At 03:30 AM 8/31/98 -0400, you wrote: >Bob, > >Just curious, but where does the 9 times flow resistance come from? > >Friction loss in pipe is directly proportional to the length of pipe (the >additional bends would accounted for by adding equivalent lengths for them) >and is inversly proportional to the 4.87 power of the diameter. > >Joe > >-----Original Message----- >From: Bob Palmer >To: Armand & Lorie Ritchie ; tigers@autox.team.net > >Date: Sunday, August 30, 1998 2:57 PM >Subject: Re: u s radiators > >
Brian,
Some good thinking here which takes the "three-pass" concept a lot further than anything I've actually seen done with automotive radiators. All multiple pass radiators I know of simply block off sections of the end tanks to force a serpentine flow path. The coolant gets progressively cooler from top to bottom. Stacking the passes from back to front would, as you point out, be more efficient. Maybe you could model this design and do a heat flow analysis and see just how much better it would be. My guess is not much, but maybe this idea has some merit for the $$ is no object crowd (NASCAR, F1, etc.)
As I said above, Brian, I think you need to keep all other parameters constant to evaluate the virtue of the multiple pass idea. For example, " So, as the number of passes increases so does the surface area and thus heat removal ability." this is spurious thinking since you are arbitrarily adding surface area in your example which you could just as easily do with the single pass idea. I'm sure you see my point.
SOOO, we've spared the other Listers the agony of this highly technical discussion. Anyone want to bring our conclusions back to the group?
Thanks to both of you for your interest; I think it's fun too.
Bob
>>Armand, Lorie, et Listers, >> >Hi Gang, >I don't get to write in very often as I don't (as yet) have a Tiger, but in >the area of heat transfer, fluid mechanics, and heat exchangers I can >contribute! > > >The comment on the triple pass radiator that "I have never heard an >explanation" prompted me to write in. While I haven't actually seen one of >these rascals the comment that "it works very well but may require >excessive water pump pressure" and "a two-pass... leaves the outlet and >inlet on the same side" leads me to believe that the design concept would >be similar to "stacking" multiple single pass radiators and plumbing them >in serial with the hot fluid coming in the back most radiator and the >coolest fluid leaving out the front radiator. > > >The amount of heat transfered from a given heat exchanger (such as a >radiator) is proportional to the area available for contact between the hot >fluid and the cooling media. So, as the number of passes increases so does >the surface area and thus heat removal ability. In addition to surface >area, the heat transfered is also a function of the temperature difference >between the hot and cold fluids at the inlet and exit. This is not a simple >average temperature (as they are changing from the inlet to outlet) but a >"log-mean" temperature difference...suffice to say that a bigger >temperature difference will lead to increased heat transfer. Therefore the >design mandates that (hopefully) the hot fluid enters at the back most pass >where the air is now hot but the temperature differece remains large and >the coolest fluid leaves at the front where the air is the coolest thus, >again, giving the biggest temperature difference. The last variable in this >calculation is the velocity of the two fluids (in our case air and cooling >fluid). Higher velocity of each leads to increased heat transfer by several >mechanisms that I will spare you from hearing about (Lecture # 37!). > > >Hope the above rattle is helpful. If you have questions let me know. I love >this stuff! > > >Brian Farkas >Tiger owner someday! and >Assistant Professor, Process Engineering >NC State University > >Hi Joe and the gang. Me again... >Regarding the 9 times and Joe's statement that: > > >">>Friction loss in pipe is directly proportional to the length of pipe (the >>>additional bends would accounted for by adding equivalent lengths for them) >>>and is inversly proportional to the 4.87 power of the diameter." > > >That is only true for laminar flow and I would bet (hope) that we have >turbulent flow in our radiators. The presence of turbulant flow makes >things a little more complex. As to the 9x factor...who knows until you >measure it. > > >Brian > >>I couldn't access this site, but with regard to a triple-pass radiator, >>this is certainly not a new concept. I first saw this type of radiator in >>the mid '70s, and they probably weren't new even then. I have never heard >>an explanation I believed about why double or triple pass would be better. >>I had a long conversation with Ron Davis (aluminum racing radiator builder) >>about the multiple pass concept and he believes that two is better than >>one, but three is too much. This I am sure of, a three pass radiator has >>NINE TIMES ( that's 9X) the flow resistance of a single pass radiator. If >>you try using a garden hose to run water through a three-pass, you'll >>quickly see what I mean. I ended up getting a three-pass from Ron, but am >>not convinced I did the right thing; would have preferred a two-pass, but >>that leaves the outlet and inlet on the same side so it requires some >>modifications to the hoses and water pump. At the very least, a three-pass >>needs a good water pump that will provide enough pressure to ensure good >>flow, especially at low rpms. I got my water pump from Sunbeam Specialties >>and it seems to do a good job. In fact, I drilled a 1/8" hole in the >>thermostat and even this small hole allows enough flow to keep it running >>well below the thermostat regulation temp (195 F) most of the time. >> >>Bob >>
Robert L. Palmer Dept. of AMES, Univ. of Calif., San Diego rpalmer@ames.ucsd.edu rpalmer@cts.com

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