X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Received: from cdptpa-omtalb.mail.rr.com ([75.180.132.120] verified) by logan.com (CommuniGate Pro SMTP 5.2c2) with ESMTP id 2471744 for flyrotary@lancaironline.net; Wed, 14 Nov 2007 09:01:28 -0500 Received-SPF: pass receiver=logan.com; client-ip=75.180.132.120; envelope-from=eanderson@carolina.rr.com Received: from edward2 ([24.74.103.61]) by cdptpa-omta02.mail.rr.com with SMTP id <20071114140048.WHAW26419.cdptpa-omta02.mail.rr.com@edward2> for ; Wed, 14 Nov 2007 14:00:48 +0000 Message-ID: <003401c826c7$1165f0e0$2402a8c0@edward2> From: "Ed Anderson" To: "Rotary motors in aircraft" References: Subject: Re: [FlyRotary] Re: Rebutal to the rebutal {:>) Thick vs Thin was : Diffuser Configuration Comparison Date: Wed, 14 Nov 2007 09:03:00 -0500 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_0031_01C8269D.284F8480" X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2900.3138 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.3138 This is a multi-part message in MIME format. ------=_NextPart_000_0031_01C8269D.284F8480 Content-Type: text/plain; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable Such debates certainly helps me to focus on where my understanding = diverges from that of others {:>). =20 Yes, I understand that IF you keep the mass flow the same between thin = and thick radiator (with 1/2 less frontal area and twice as thick) that = the air velocity through the thicker core will increase and the time = spent in the cores will be essentially the same for both - and under = those conditions the detalT would be approximately the same for both = cores. So we are all (I think) in agreement on those points. However, if you put in a thicker radiator, then presumably it would be = because it could provide some benefit over the thin one in some area. I = would certainly have to modify the duct if for no other reason, to have = the duct cross sectional area match the smaller frontal area of the = core. To fully benefit from using the thicker radiator then I would = naturally have a duct matched to achieve those benefits - not for the = thinner radiator. So IF in installing the thicker core, I slow down the mass flow through = the duct by 1/2 (which I can easily do - increasing it might be more = difficult) then the air velocity through the core will be reduced. With = the velocity decreased, the air will take longer to transverse the core. = The longer the air takes to move through the core the more heat it will = absorb, the more heat the air absorbs the greater the DeltaT of the air. = The greater the deltaT of the air the more heat will be removed even = with the slower velocity and mass flow ( the drag would also be reduced, = just had to throw that one in {:>)). =20 Q =3D m cp DeltaT. Clearly shows that heat removal is proportional to = BOTH mass flow AND deltaT (as well as the specific heat but that is = fixed by nature). Yet, for some reason that I still don't understand, = you skeptics seem to fixate on the mass flow factor {:>) and ignore = DeltaT as if it were a factor you can do nothing about. Certainly you = can not cool without some mass flow but then the same is true about the = deltaT factor. As long as the product of the mass flow and the deltaT = equals the heat (Q) you need to remove - then you have achieved your = cooling objective. =20 However, I can see that we have about rung any usefulness out of this = discussion (for the time being {:>)) and I'm certain we are all getting = a bit tired of it not to mention others on the list. Again, appreciate your viewpoints and counter arguments - it has given = me something to sit and think about - I mean it just borders on the = possibility of reality that I might be incorrect with my viewpoint {:>), = so I am going to go do some more research and thinking. =20 Onward to other topics - ones which I refrain from suggesting. Best Regards Ed ----- Original Message -----=20 From: David Leonard=20 To: Rotary motors in aircraft=20 Sent: Tuesday, November 13, 2007 10:39 PM Subject: [FlyRotary] Re: Rebutal to the rebutal {:>) Thick vs Thin was = : Diffuser Configuration Comparison Thanks Al, Saved me the trouble. That last bit about pressure recovery on exit = is a nice touch, and you are right it is important - with either = radiator. I know where this discussion is going, and we have there (nowhere) = before.=20 And I think we all realize that the difference (if there is one) is = too small to be significant in our applications, so everyone just keep = building and install what you can. Ed, your jedi mind trick smoke and mirrors will not work on me. The = mind trick only works on intelligent life forms....=20 I think the disconnect there is that you are envisioning a guy with a = thick radiator, who changes nothing other than the radiator and minimal = duckting changes. But clear your mind, and picture this: Two guys (twins)=20 Two Airplanes identical in every way except the radiator, inlet = ducting and exit ducting. Frontal drag is the same, needed heat = rejection is the same etc. The ducting systems are designed such that the mass airflow is = identical. ie, the thin radiator version has smaller inlet and outlets = but a larger plenum or whatever is necessary so that the airflow through = the two radiators is identical.=20 GIVEN identical airflow, identical rad volume, inlet air temp and = identical coolant temp... the Delta T on average is the SAME. The air, although going slower in the thinner rad, spends exactly the = same amount of time in each rad. (same amount of time for heat transfer = and same average temp rise of the air and same average temp drop of the = coolant)=20 Now, I will admit that heat transfer is more efficient in the higher = velocity thick rad. But it is not squared, or even linear. It is less = than linear. (ie, as velocity increases in both rads heat transfer = becomes less dependent on velocity - they are both turbulent flow and = you cannot achieve a lower temp than the ambient air. At some point the = faster speeds become detrimental due to heating from friction) I submit = that the the speeds of most of our aircraft and relative dimensions of = radiators, both radiators are in turbulent flow and increased efficiency = of heat transfer is nearly negligible.=20 So in the end the difference between these two systems is that the = thin rad has smaller velocities across the rad, smaller pressure drop, = improved pressure recovery, higher exit air velocity, same delta T, same = heat transfer, same exit air temp, smaller inlets and outlets, and = larger volume sucked by ducting. All of which is relatively negligible = in our applications.=20 Thanks for the fun discussion Ed. Anxiously awaiting reason for Delta = T difference between rads but will probably not buy it. (but still = realize there is a very real possibility that I am wrong). Dave Leonard=20 On Nov 13, 2007 8:45 PM, Al Gietzen wrote: Ed; I swore after the last one, it was my last on this subject. But, OK; = one more. This is all a bit like the three blind men describing an = elephant. None are necessarily wrong, just different points of view. Let's look at it from a sort of systems approach: First I calculate = the heat load. Then I determine the mass flow rate of air I need using = a certain delta T; one that I know is 'reasonable' - or even optimum = (radiator size and weight, etc.), based on other analyses which we = haven't/won't go into, but a number between 50 and about 80F is good = based on the OAT and coolant temps we deal with. We already know how to = compute the mass flow rate. 1st addressing your statement regarding the inlet sizing. =20 Assuming that selecting your inlet opening size controls mass flow = is incomplete. It is the total pressure loss for the entire duct (and = core) which combined with the available freestream kinetic energy (due = to velocity) that determines mass flow. I can make changes to any of = the intake, diffuser, core, and outlet and make changes in the mass flow = - so its not just the inlet.=20 Then I compute the required opening area of my ram scoop based on = the velocity of the incoming air; the speed of the airplane at the = design point; generally a fast climb. Area x velocity =3D mass flow = rate. The ram scoop is not necessarily 100% efficient, so add maybe 10% = to the area. Now, yes, that is incomplete; but on first order, unless I = do something wrong that causes the scoop to spill air (like poor = diffuser design or too thick a radiator), then it is complete - that is = the mass flow rate. Only changes I make that cause air to spill around = the scoop is going to change that; so on this macro view I can ignore = the details of the diffuser of which you are more expert than I. Now if I have a well designed diffuser with an area ratio of say = about 4+ I know that the pressure recovery will be greater than the = pressure drop through the rad, so all that air is going through. Right? = Well; here is where we have to start thinking about both the thickness = and the density of the core because the pressure drop through the core = must be less than the amount of pressure recovery =96 and is why I say = that talking thickness without specifying core density and diffuser = ratio is 'incomplete' because they depend on one another. =20 But back to fist order; given the conditions of adequate pressure = recovery the flow rate is fixed. 2nd regarding the deltaT "swap" I made:=20 I do not agree that you need to compare on the basis of sam e mass = flow or deltaT - the only factor that really matters is that the needed = heat be removed. There are a large combinations of mass flow and deltaT = that will remove X amount of heat. True, but: The design point heat load is fixed, the mass flow rate is fixed; = therefore the delta T is fixed (see formula used for computing the the = flow rate in the first place.) Now, given these conditions; we can look at thick vs thin. We can = have a large frontal area, slow velocity through the core, and have = pressure left over for accelerating the air back toward free stream = (along with the heat energy that we pick up which gives velocity by = expansion); or we can make the core thicker up to the limit of the = pressure recovery that we have achieved; and have no remaining pressure = at the core exit. But keep in mind that the have a fixed scoop area for = the speed we designed for; and the frontal area of the rad is the other = area in the diffuser ratio; so making the rad smaller and thicker both = cut into the available pressure recovery. When we reach that limit where = we have used all the available pressure recovery; we have no pressure = left over to accelerate the air back to something closer to that at = which it came in. Because of the effects of the velocity on heat transfer, as well as = pressure drop, there does happen to be a rad pressure drop (thickness) = that results in minimum drag =96 just as there is a corresponding delta = T that results in minimum mass of the core (all other things equal; = i.e., properly designed). Now the big caveat - It is clear here that to take advantage of the = less pressure drop in the thin rad to reduce drag, we have to have an = exit configuration that efficiently re-accelerates the air. If not, or = if we going to release the exit air into the free stream at a negligibly = small velocity, then it's a different ball game. Then from a drag = standpoint there may be little difference, and that using a radiator = thickness (pressure drop) that exhausts the pressure recovery, may be = the way to go for fitting into a confined space. So there ya go; cooling system design in a nutshell; minus all the = magicJ . Al G --=20 David Leonard Turbo Rotary RV-6 N4VY http://N4VY.RotaryRoster.net http://RotaryRoster.net ------=_NextPart_000_0031_01C8269D.284F8480 Content-Type: text/html; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable
 Such debates certainly helps me to focus = on where my=20 understanding diverges from that of others = {:>).  
 
Yes,  I understand that IF you keep the = mass flow the=20 same between thin and thick radiator (with 1/2 less frontal area and = twice as=20 thick) that the air velocity through the thicker core will increase and=20 the  time spent in the cores will be essentially the same for = both=20 - and under those conditions the detalT would be approximately the = same for=20 both cores.  So we are all (I think) in agreement on those=20 points.
 
However, if you put in a thicker radiator, then = presumably=20 it would be because it could provide some benefit over the thin one in = some=20 area.  I would certainly have to modify the duct if for no =  other=20 reason, to have the duct cross sectional area match the smaller frontal = area of=20 the core.  To fully benefit from using the thicker radiator then I = would=20 naturally have a duct matched to achieve those benefits - not for = the=20 thinner radiator.
 
So IF in installing the thicker core, I slow = down the mass=20 flow through the duct by 1/2  (which I can easily do - increasing = it might=20 be more difficult) then the air velocity through the core will be = reduced. =20 With the velocity decreased, the air will take longer to transverse the=20 core.  The longer the air takes to move through the core the more = heat it=20 will absorb, the more heat the air absorbs the greater the DeltaT of the = air.  The greater the deltaT of the air the more heat will be = removed even=20 with the slower velocity and mass flow ( the drag would also be reduced, = just=20 had to throw that one in {:>)). 
 
 Q =3D m cp DeltaT.  Clearly shows = that heat=20 removal is proportional to BOTH mass flow AND deltaT (as well as = the=20 specific heat but that is fixed by nature).   Yet, for some = reason=20 that I still don't understand, you skeptics seem to fixate on the mass = flow=20 factor {:>) and ignore DeltaT as if it were a factor you can do = nothing=20 about.  Certainly you can not cool without some mass flow but then = the same=20 is true about the deltaT factor.  As long as the product of the = mass flow=20 and the deltaT equals the heat (Q) you need to remove - then you have = achieved=20 your cooling objective. 
 
However, I can see that we have about rung any = usefulness=20 out of this discussion (for the time being {:>)) and I'm certain = we are=20 all getting a bit tired of it not to mention others on the = list.
 
Again, appreciate your viewpoints and counter = arguments=20 -   it has given me something to sit and think about - I mean = it just=20 borders on the possibility of reality that I might be incorrect with my=20 viewpoint {:>), so I am going to go do some more research and = thinking. =20
 
 Onward to other topics - ones which I = refrain from=20 suggesting.
 
 
Best Regards
 
Ed
 
 
----- Original Message -----
From:=20 David=20 Leonard
Sent: Tuesday, November 13, = 2007 10:39=20 PM
Subject: [FlyRotary] Re: = Rebutal to the=20 rebutal {:>) Thick vs Thin was : Diffuser Configuration = Comparison

Thanks Al,

Saved me the trouble.  That last = bit=20 about pressure recovery on exit is a nice touch, and you are right it = is=20 important - with either radiator.

I know where this discussion = is=20 going, and we have there (nowhere) before.

And I think we all = realize=20 that the difference (if there is one) is too small to be significant = in our=20 applications, so everyone just keep building and install what you=20 can.

Ed, your jedi mind trick smoke and mirrors will not work = on=20 me.  The mind trick only works on intelligent life forms.... =

I=20 think the disconnect there is that you are envisioning a guy with a = thick=20 radiator, who changes nothing other than the radiator and minimal = duckting=20 changes.  But clear your mind, and picture this:

Two guys = (twins)=20
Two Airplanes identical in every way except the radiator, inlet = ducting=20 and exit ducting.  Frontal drag is the same, needed heat = rejection is the=20 same etc.
The ducting systems are designed such that the mass = airflow is=20 identical.  ie, the thin radiator version has smaller inlet and = outlets=20 but a larger plenum or whatever is necessary so that the airflow = through the=20 two radiators is identical.
GIVEN identical airflow, identical rad = volume,=20 inlet air temp and identical coolant temp... the Delta T on average is = the=20 SAME.
The air, although going slower in the thinner rad, spends = exactly the=20 same amount of time in each rad. (same amount of time for heat = transfer and=20 same average temp rise of the air and same average temp drop of the = coolant)=20

Now, I will admit that heat transfer is more efficient in the = higher=20 velocity thick rad.  But it is not squared, or even linear.  = It is=20 less than linear. (ie, as velocity increases in both rads heat = transfer=20 becomes less dependent on velocity - they are both turbulent flow and = you=20 cannot achieve a lower temp than the ambient air.  At some point = the=20 faster speeds become detrimental due to heating from friction)  I = submit=20 that the the speeds of most of our aircraft and relative dimensions of = radiators, both radiators are in turbulent flow and increased = efficiency of=20 heat transfer is nearly negligible.

So in the end the = difference=20 between these two systems is that the thin rad has smaller velocities = across=20 the rad, smaller pressure drop, improved pressure recovery, higher = exit air=20 velocity, same delta T, same heat transfer, same exit air temp, = smaller inlets=20 and outlets, and larger volume sucked by ducting.  All of which = is=20 relatively negligible in our applications.

Thanks for the fun=20 discussion Ed.  Anxiously awaiting reason for Delta T difference = between=20 rads but will probably not buy it. (but still realize there is a very = real=20 possibility that I am wrong).

Dave Leonard

On Nov 13, 2007 8:45 PM, Al Gietzen <ALVentures@cox.net> = wrote:

Ed;

 

I swore = after the=20 last one, it was my last on this subject. But, OK; one more. This is = all a=20 bit like the three blind men describing an elephant.  None are=20 necessarily wrong, just different points of view.

Let's = look at it=20 from a sort of systems approach: First I = calculate=20 the heat load.  Then I determine the mass flow rate of air I = need using=20 a certain delta T; one that I know is 'reasonable' - or even optimum = (radiator size and weight, etc.), based on other analyses which we=20 haven't/won't go into, but a number between 50 and about 80F is good = based=20 on the OAT and coolant temps we deal with.  We already know how = to=20 compute the mass flow rate.

 1st addressing = your=20 statement regarding the inlet sizing. 

 

Assuming that = selecting your=20 inlet opening size controls mass flow is incomplete.  It = is the=20 total pressure loss for the entire duct (and core) which combined = with the=20 available freestream kinetic energy (due to velocity) that=20  determines mass flow.   I can make changes to any of the = intake,=20 diffuser, core, and outlet and make changes in the mass flow - so = its not=20 just the inlet. 

 

Then I = compute=20 the required opening area of my ram scoop based on the velocity of = the=20 incoming air; the speed of the airplane at the design point; = generally a=20 fast climb. Area x velocity =3D mass flow rate.  The ram scoop = is not=20 necessarily 100% efficient, so add maybe 10% to the area.  Now, = yes,=20 that is incomplete; but on first order, unless I do something wrong = that=20 causes the scoop to spill air (like poor diffuser design or too = thick a=20 radiator), then it is complete - that is the mass flow rate.  = Only=20 changes I make that cause air to spill around the scoop is going to = change=20 that; so on this macro view I can ignore the details of the diffuser = of=20 which you are more expert than I.

 

Now if = I have a=20 well designed diffuser with an area ratio of say about 4+ I know = that the=20 pressure recovery will be greater than the pressure drop through the = rad, so=20 all that air is going through. Right? Well; here is where we have to = start=20 thinking about both the thickness and the density of the core = because the=20 pressure drop through the core must be less than the amount of = pressure=20 recovery =96 and is why I say that talking thickness without = specifying core=20 density and diffuser ratio is 'incomplete' because they depend on = one=20 another. 

 

But = back to fist=20 order; given the conditions of adequate pressure recovery the flow = rate is=20 fixed.

 

2nd regarding the = deltaT "swap"=20 I made: 

I do not agree that = you need to=20 compare on the basis of sam e mass flow or deltaT - the only factor = that=20 really matters is that the needed heat be removed.  There are a = large=20 combinations of mass flow and deltaT that will remove X amount of = heat.=20 True,=20 but:

 

The = design point=20 heat load is fixed, the mass flow rate is fixed; therefore the delta = T is=20 fixed (see formula used for computing the the flow rate in the first = place.)

 

Now, = given these=20 conditions; we can look at thick vs thin.  We can have a large = frontal=20 area, slow velocity through the core, and have pressure left over = for=20 accelerating the air back toward free stream (along with the heat = energy=20 that we pick up which gives velocity by expansion); or we can make = the core=20 thicker up to the limit of the pressure recovery that we have = achieved; and=20 have no remaining pressure at the core exit. But keep in mind that = the have=20 a fixed scoop area for the speed we designed for; and the frontal = area of=20 the rad is the other area in the diffuser ratio; so making the rad = smaller=20 and thicker both cut into the available pressure recovery. When we = reach=20 that limit where we have used all the available pressure recovery; = we have=20 no pressure left over to accelerate the air back to something closer = to that=20 at which it came in.

 

Because = of the=20 effects of the velocity on heat transfer, as well as pressure drop, = there=20 does happen to be a rad pressure drop (thickness) that results in = minimum=20 drag =96 just as there is a corresponding delta T that results in = minimum mass=20 of the core (all other things equal; i.e., properly=20 designed).

 

Now the = big=20 caveat - It is clear here that to take advantage of the less = pressure drop=20 in the thin rad to reduce drag, we have to have an exit = configuration that=20 efficiently re-accelerates the air.  If not, or if we going to = release=20 the exit air into the free stream at a negligibly small velocity, = then it's=20 a different ball game.  Then from a drag standpoint there may = be little=20 difference, and that using a radiator thickness (pressure drop) that = exhausts the pressure recovery, may be the way to go for fitting = into a=20 confined space.

 

So = there ya go;=20 cooling system design in a nutshell; minus all the = magicJ=20 .

 

Al=20 G

 

 

 

 

 

 

 


--
David Leonard

Turbo Rotary RV-6 = N4VY
http://N4VY.RotaryRoster.nethttp://RotaryRoster.net=20 ------=_NextPart_000_0031_01C8269D.284F8480--