X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Received: from cdptpa-omtalb.mail.rr.com ([75.180.132.121] verified) by logan.com (CommuniGate Pro SMTP 5.2c2) with ESMTP id 2470167 for flyrotary@lancaironline.net; Tue, 13 Nov 2007 13:04:43 -0500 Received-SPF: pass receiver=logan.com; client-ip=75.180.132.121; envelope-from=eanderson@carolina.rr.com Received: from edward2 ([24.74.103.61]) by cdptpa-omta01.mail.rr.com with SMTP id <20071113180404.XFKA6760.cdptpa-omta01.mail.rr.com@edward2> for ; Tue, 13 Nov 2007 18:04:04 +0000 Message-ID: <001301c8261f$e1ca1ca0$2402a8c0@edward2> From: "Ed Anderson" To: "Rotary motors in aircraft" References: Subject: Rebutal to the rebutal {:>) Thick vs Thin was : Diffuser Configuration Comparison Date: Tue, 13 Nov 2007 13:06:14 -0500 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_0010_01C825F5.F8B62140" 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_0010_01C825F5.F8B62140 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Hi Dave, Sure had me going for a spell, however, I got out the equations and = believe I can point out a different view point. If I understood you correctly, your basic assertion is that the same = mass flow is required for both thin and thick radiators and since the = thicker radiator has a smaller frontal area it must therefore have a = higher velocity air flow to generate the same mass flow to remove the = same heat. Furthermore the higher velocity also translates into more = drag (even with the reduced frontal area due to the drag being = proportional to the square of the velocity) - but all the above is not = necessarily true. In fact I found a NACA study where they looked at the effects of using = thicker radiators and I have worked out the equations on a spreadsheet = which I believe sheds some concrete facts on the old thin Vs Thick = debate - but, it is complex and I'll wait a bit before springing it = {:>). =20 However back to your contention that both radiators the thin and the = thick required the same mass flow to remove the same amount of heat - it = just isn't so and here is why. =20 First, we have two radiators one is 1" thick and 1 square ft in frontal = area, the second one is 1/2 square feet of frontal area and twice (or = more) as thick. Now turning to our trusty equation for heat rejection = and mass flow. Q =3D m*Cp*DeltaT is the basic equation that tells us how much heat we = remove for a mass flow "m", a specific heat (air =3D 0.24) and = temperature increase in the medium (air) or DeltaT. =20 Taking a specific example of say - 5000 Btu/min (which is about the = amount of heat an NA 13B generates at 175 HP that needs to be rejected = by the coolant). We know the Cp so that leaves the DeltaT and that is = what makes the difference. We have to assume a DeltaT, lets say 50F = (yes, it could easily be different but bear with me) then we have m =3D 5000/(0.24)*(50)/60 =3D 6.94 lbm/sec of mass flow . and lets = say we have a 1 square foot radiator to get rid of that heat. Then the = velocity requires V1 =3D m/(p1A1) =3D 6.94 lbm/min/(.0765*1) =3D 90 = ft/sec =3D 61.36 mph through the 1 square foot radiator. Perhaps a bit = higher than desirable but that's what we get. Now if I understood you correctly your point is that the same mass = flow is also required for the smaller radiator (1/2 sq ft) to remove the = same amount of heat and therefore since frontal area is 1/2 the size, = the velocity must be double that of the larger radiator to get the same = mass flow and remove the same quantity of heat. But, it just isn't = necessarily so. Taking the same conditions as before, except this time I use a DeltaT of = 100F (hey! its permitted as I'm using a different core here{:>) see = further discussion on effects of thickness on DeltaT). Now we have m = =3D 5000/(0.24)*100/60 =3D 3.47 lbm/sec of mass flow is required. That = is 1/2 of the mass flow required with a DeltaT of 50F. Therefore even with 1/2 the frontal area, I can use the same air = velocity as before and remove the same amount of heat with 1/2 the mass = flow and with LESS drag because my frontal area is now 1/2 that of the = thinner larger radiator and the velocity is the same. Now you can say I = cheated by having a different radiator, but that is certainly what you = would do - as that is what we are discussing are the relative merits of = thinner vs thicker for our application. But, If you reduce the frontal area of the radiator, then you must = increase the thickness (or add more fins, turbulators, etc) to increase = its Heat transfer coefficient to continue to reject sufficient heat to = the air flow. Therefore, The air temperature coming out of a thicker = radiator is going to be higher than a thin radiator. The reason is both = radiators are flowing at the same velocity (remember I did used the = same velocity for both radiators), and since the velocity of the flow is = the same for both radiators, the air spends more time (twice, three, = four times depending on the thickness) in the thicker core of the = smaller radiator. The longer duration of the air in the thicker core = causes it to be absorb more heat and be raised to a higher temperature = than the thinner radiator, therefore the higher deltaT (for the same = velocity air). This probably did not/and will not convince you of the merits of the = thicker vs thinner and besides I know your reservations about my = deductive reasoning {:>). So I am working on understanding fully the = Naca study I found that addresses the effect of thickness on required = mass flow and heat rejection. I believe it would be considered a fairly = credible source and will hopefully enable all to reach their own = conclusion. I think its going to blow the socks off this thick vs thin = debate - but, then I've been wrong before {:>) Boy, this is fun!!! Sure keeps the old brain working (hopefully). Anyhow, Dave, I respectively disagree with your assertion (see above) = {:>) Best Regards Ed =20 ----- Original Message -----=20 From: "Ernest Christley" To: "Rotary motors in aircraft" Sent: Tuesday, November 13, 2007 9:19 AM Subject: [FlyRotary] Re: Thick vs Thin was : Diffuser Configuration = Comparison > David Leonard wrote: >> Why is it going slower? BECAUSE YOU HAVE DESIGNED YOUR THIN RADIATOR = SYSTEM >> DUCTS SUCH THAT AN EQUAL AMOUNT OF AIR PASSES THROUGH AN EQUAL VOLUME = OF >> RADIATOR AS WOULD OCCUR ON A THICK RADIATOR SYSTEM. (This is the big = if... >> system design... but bear with me). ie, equal amount of air, equal = volume >> of radiator - in the thin radiator system the air will be flowing = more >> slowly. >> =20 >=20 > I agree with your concept, Dave, but I think you underestimate the=20 > difficulty of fitting a large faced radiator into the physical=20 > constraints of the area available in a small airplane. I worked on=20 > trying to use a large, 1" thick radiator for a while, and this was in = a=20 > delta planform. I had comparitively HUGE amounts of volume to work=20 > with. I eventually gave up, as there was just no reasonable way to = get=20 > a duct built around it that would slow the air down. As you increase=20 > the face area, you increase the size of the duct necessary to expand = the=20 > air without separation. The best radiator and duct ever created will = be=20 > useless if we have to leave it on the ground because it doesn't fit in = > the airplane. >=20 > I think the flow chart for sizing a radiator for our needs should = follow=20 > something like this: >=20 > 1) Mark out a space for the largest volume that you can fit a radiator = > and its associated ducting into. Insure that routing for the hoses = will=20 > be convenient, and the ducting can be made something resembling = efficient. >=20 > 2) Visit one of the websites like frigidair.com and find a radiator = that=20 > meets the dimensional specs you came up with. Or contact Jerry and = have=20 > him make you one of that size. >=20 > 3) If the core volume is less than 700 cubic inches, add another. >=20 > 4) Go fly. If it is to cool (<160F), choke off the inlet a little. = If=20 > it is to hot (>200F), fiddle with the ducting. >=20 > -- > Homepage: http://www.flyrotary.com/ > Archive and UnSub: = http://mail.lancaironline.net:81/lists/flyrotary/List.html ------=_NextPart_000_0010_01C825F5.F8B62140 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
Hi Dave,
 
Sure had me going for a spell, however, I got = out the=20 equations and believe I can point out a different view = point.
 
If I understood you correctly, your basic = assertion is=20 that  the same mass flow is required for both thin and thick = radiators and=20 since the thicker radiator has a smaller frontal area  it = must=20 therefore have a higher velocity air flow to generate the same mass = flow to=20 remove the same  heat.  Furthermore the higher velocity also=20 translates into more drag (even with the reduced frontal area due to the = drag=20 being proportional to the square of the velocity) - but all the = above is=20 not necessarily true.
 
  In fact I found a NACA study where they = looked at=20 the effects of using thicker radiators and I have worked out the = equations on a=20 spreadsheet which I believe sheds some concrete facts on the old thin Vs = Thick=20 debate - but, it is complex and I'll wait a bit before springing it=20 {:>). 
 
However  back to your contention that both = radiators=20 the thin and the thick required the same mass flow to remove the same = amount of=20 heat - it just isn't so and here is why. 
 
First, we have two radiators one is 1" thick and = 1 square=20 ft in frontal area, the second one is 1/2 square feet of frontal area = and twice=20 (or more) as thick.  Now turning to our trusty equation for heat = rejection=20 and mass flow.
 
Q =3D m*Cp*DeltaT is the basic equation that = tells us how=20 much heat we remove for a mass flow "m", a specific heat (air =3D 0.24) = and=20 temperature increase in the medium (air) or DeltaT. 
 
Taking a specific example of say - 5000 Btu/min = (which is=20 about the amount of heat an NA 13B generates at 175 HP that needs to be = rejected=20 by the coolant).  We know the Cp so that leaves the DeltaT and that = is what=20 makes the difference.  We have to assume a DeltaT, lets say 50F = (yes, it=20 could easily be different but bear with me)  then we = have
 
m =3D 5000/(0.24)*(50)/60  =3D 6.94 =  lbm/sec=20 of mass flow  . and lets say we have a 1 square foot = radiator to=20 get rid of that heat.  Then the velocity requires V1 =3D m/(p1A1) = =3D 6.94=20 lbm/min/(.0765*1) =3D 90 ft/sec =3D 61.36 mph through the 1 square foot=20 radiator.  Perhaps a bit higher than desirable but that's what we=20 get.
 
  Now if I understood you correctly your = point is=20 that  the same mass flow is also required for the smaller = radiator=20 (1/2 sq ft) to remove the same amount of heat and therefore since = frontal area=20 is 1/2 the size,  the velocity must be double that of the larger = radiator=20 to get the same mass flow and remove the same quantity of heat.  = But, it=20 just isn't necessarily so.
 
Taking the same conditions as before, except = this time I=20 use a DeltaT of 100F (hey! its permitted as I'm using a different core=20 here{:>) see further discussion on effects of thickness on = DeltaT).  Now=20 we have m =3D 5000/(0.24)*100/60 =3D 3.47 lbm/sec of mass flow is = required. =20 That is 1/2 of the mass flow required with a DeltaT of 50F.
 
Therefore even with 1/2 the frontal area, I can = use the=20 same air velocity as before and remove the same amount of heat with 1/2 = the mass=20 flow and with LESS drag because my frontal area is now 1/2 that of the = thinner=20 larger radiator and the velocity is the same.  Now you can say I = cheated by=20 having a different radiator, but that is certainly what you would do - = as that=20 is what we are discussing are the relative merits of thinner vs thicker = for our=20 application.
 
But,  If you reduce the frontal area of the = radiator,=20  then you must increase the thickness (or add more fins, = turbulators, etc)=20 to increase its Heat transfer coefficient to continue to reject = sufficient=20  heat to the air flow.  Therefore, The air temperature coming = out of a=20 thicker radiator is going to be higher than a thin radiator.  The = reason is=20 both radiators are flowing at the same velocity (remember I did = used=20  the same velocity for both radiators), and since the velocity of = the flow=20 is the same for both radiators, the air spends more time (twice, three, = four=20 times depending on the thickness) in the thicker core of the = smaller=20 radiator.  The longer duration of the air in the thicker core = causes it to=20 be absorb more heat and be raised to a higher temperature than the = thinner=20 radiator, therefore the higher deltaT (for the same velocity = air).
 
This probably did not/and will not convince you = of the=20 merits of the thicker vs thinner and besides I know your reservations = about my=20 deductive reasoning {:>).  So I am working on understanding = fully the=20 Naca study I found that addresses the effect of thickness on required = mass flow=20 and heat rejection.  I believe it would be considered a fairly = credible=20 source and will hopefully enable all to reach their own = conclusion.  I=20 think its going to blow the socks off this thick vs thin debate - but, = then I've=20 been wrong before {:>)
 
Boy, this is fun!!!  Sure keeps the old = brain working=20 (hopefully).
 
Anyhow, Dave, I respectively disagree with your = assertion =20 (see above) {:>)
 
Best Regards
 
Ed
 
 
 
 
 
 
----- Original Message -----
From: "Ernest Christley" <echristley@nc.rr.com>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Tuesday, November 13, 2007 9:19 = AM
Subject: [FlyRotary] Re: Thick vs Thin was : = Diffuser=20 Configuration Comparison

> David = Leonard=20 wrote:
>> Why is it going slower?  BECAUSE YOU HAVE = DESIGNED YOUR=20 THIN RADIATOR SYSTEM
>> DUCTS SUCH THAT AN EQUAL AMOUNT OF AIR = PASSES=20 THROUGH AN EQUAL VOLUME OF
>> RADIATOR AS WOULD OCCUR ON A = THICK=20 RADIATOR SYSTEM.  (This is the big if...
>> system = design... but=20 bear with me).  ie, equal amount of air, equal volume
>> = of=20 radiator - in the thin radiator system the air will be flowing = more
>>=20 slowly.
>>  
>
> I agree with your = concept,=20 Dave, but I think you underestimate the
> difficulty of fitting a = large=20 faced radiator into the physical
> constraints of the area = available in a=20 small airplane.  I worked on
> trying to use a large, 1" = thick=20 radiator for a while, and this was in a
> delta planform.  I = had=20 comparitively HUGE amounts of volume to work
> with.  I = eventually=20 gave up, as there was just no reasonable way to get
> a duct = built around=20 it that would slow the air down.  As you increase
> the face = area,=20 you increase the size of the duct necessary to expand the
> air = without=20 separation.  The best radiator and duct ever created will be =
>=20 useless if we have to leave it on the ground because it doesn't fit in =
>=20 the airplane.
>
> I think the flow chart for sizing a = radiator for=20 our needs should follow
> something like this:
>
> = 1) Mark=20 out a space for the largest volume that you can fit a radiator
> = and its=20 associated ducting into.  Insure that routing for the hoses will =
>=20 be convenient, and the ducting can be made something resembling=20 efficient.
>
> 2) Visit one of the websites like = frigidair.com and=20 find a radiator that
> meets the dimensional specs you came up=20 with.  Or contact Jerry and have
> him make you one of that=20 size.
>
> 3)  If the core volume is less than 700 = cubic=20 inches, add another.
>
> 4) Go fly.  If it is to cool=20 (<160F), choke off the inlet a little.  If
> it is to hot = (>200F), fiddle with the ducting.
>
> --
> = Homepage: =20 http://www.flyrotary.com/
> Archive=20 and UnSub:   http://mail.lancaironline.net:81/lists/flyrotary/List.html ------=_NextPart_000_0010_01C825F5.F8B62140--