Return-Path: Received: from [24.25.9.102] (HELO ms-smtp-03-eri0.southeast.rr.com) by logan.com (CommuniGate Pro SMTP 4.1.8) with ESMTP id 2778680 for flyrotary@lancaironline.net; Sun, 30 Nov 2003 22:30:19 -0500 Received: from o7y6b5 (clt78-020.carolina.rr.com [24.93.78.20]) by ms-smtp-03-eri0.southeast.rr.com (8.12.10/8.12.7) with SMTP id hB13UEA4015914 for ; Sun, 30 Nov 2003 22:30:16 -0500 (EST) Message-ID: <002c01c3b7ba$05804bc0$1702a8c0@WorkGroup> From: "Ed Anderson" To: "Rotary motors in aircraft" Subject: Ducts Vs Diffuser - Cooling (a bit long) Date: Sun, 30 Nov 2003 22:20:06 -0500 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_0029_01C3B790.1C026B00" X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2800.1106 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106 X-Virus-Scanned: Symantec AntiVirus Scan Engine This is a multi-part message in MIME format. ------=_NextPart_000_0029_01C3B790.1C026B00 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Ok folks, Been developing a bit more understanding (I hope) of the cooling = science/art/mystery - still a long ways to go, but finally understood a = bit more about ducts and diffusers and why you want to use a diffuser = (where possible) in front of an heat exchanger. What's a duct and what's a diffuser? A duct can be though of as an airpath with no change in its cross = sectional area.=20 A diffuser on the other hand has an airpath that goes from a smaller to = larger cross-sectional area or the area diverges (as they say). _____ /=20 / ____ / _________________ -----------------> Diffuser = ---------------------> Duct _____ __________________ \ \ \_______ Clearly the constant cross section duct is easier to build so why would = you want the complication of making a varying cross-section diffuser. = Note: The terms Duct and Diffuser are sometimes used interchangeably - = so it can get a bit confusing when reading. But, for our purposes a = duct is with unchanging cross-sectional area.=20 Well, it took me a while to understand what all the math meant in real = world terms regarding a diffuser (assuming that I now have it correct). = A subsonic diffuser does one principal thing - it slows down the = velocity of the air mass entering it. So? you say. (when I want all the = cooling airmass I can get) Why should I want to slow down the air? Well, = because its good for your cooling and your airspeed. =20 Yep! that what it ultimately boils down to and why you want a diffuser = rather than just a duct. Don't ask me (yet) how to go about designing the diffuser you need at = this point, but here is what happens in a nut shell. What the diffuser = does is transform the kinetic and pressure energy of the airstream. = With the cross section area diverging (getting larger), the air pressure = increases, the air density increases, the temperature increases (but not = much) and the velocity decreases. The following is my attempt to explain = how this happens. Explanation of Diffuser =20 Picture a tube of air created by drawing a circle of area Ai (your = cooling inlet) through the air at 120 MPH (176 ft/sec) for one second. = That is then the air mass entering your inlet of area Ai for your = Diffuser of area Ae (the large divergent end). The tube of air is 176 ft = long (Li) with an area of Ai which is the same as saying the volume is = Ai*Li or since Li =3DVi (for one second), we can say the volume is = Ai*Vi. This is the volume of air that flows into the diffuser section = with cross-section area of Ae =3D 3*Ai (remember to be a diffuser, the = outlet area has to be larger than the inlet area) but where the = velocity of the airmass has dropped to velocity Ve (where Ve =3D 1/10*Vi = nothing is 100% efficient so there will be some residue velocity left) Some terms Inlet (small area) Diffuser (large = area) Vi (Velocity ft/sec) Ve (velocity = remaining in diffuser (always some)) Li =3D Vi/(1 sec) Le (length of = large area) Ai =3D Area of Tube (ft^2) Ae (area of large = section) pi =3D air density (Lbm/ft^2) pe (air density in = diffuser) Mass =3D (Lbm) Kinetic Energy Ke =3D BTU Consider a air mass flowing at a velocity Vi through the inlet of area = Ai to a diffuser for one second of flow. This volume of air is Ai*Vi, = taking the density (pi -air density 0.0076lbm/ft^3 -not PIE)) of this = air mass, one can calculate the mass flow =3D pi*Ai*Vi. Mass flow = through the system does not change (subsonic velocity assumed) !!!! Now have this mass flow be decelerated to Ve (where Ve=3D1/10*Vi - a = good value for a good diffuser) by a diffuser of area Ae placed at the = end of the inlet. Assume that the area Ae is 3 times larger than the = inlet area Ai or Ae =3D 3*Ai. Consider what this means: 1. Since the mass flow must remain constant through the ducts then = pi*Ai*Vi =3D pe*Ae*Ve. However we know that area Ae is 3 times larger = than Ai, which means the product of pe*Ve must be smaller by a factor of = 1/3 to maintain the mass flow equation balance. In reality, we know = that Ve is also less as the basic function of a diffuser is to = decelerate an airflow. This then means that the density pe must = increase. Actually the density pe, the pressure Pe and the temperature = Te all increase as a results of converting the kinetic energy of the = mass flow into dynamic pressure and increased density within the smaller = volume of the diffuser. 2. It t may seem counterintuitive to have the pressure and density = increase when the area is increased. Most would predict that the = density and pressure would decrease with an increase in area. This is = indeed what would happen in a closed system. However, a diffuser with = air mass flow into and out of is an open system and the airmass flow = continuously brings energy and air mass into the system (diffuser)=20 3. Consider this: You have a volume of air Li Long travelling at speed = Vi (think of a long tube of air) with the kinetic energy of=3D = m*1/2piVi^2. This energy is for the column Li long. Now when the air = enters the increased area Ae, the velocity has decreased to Ve. So we = have a volume =3D Ae*Le. Recall that Ae was =3D 3*Ai however Le is = 1/10*Li (because Length of tube is equal to the velocity divided by time = (1 second in this case), so Le is effectively Ve at 1 sec). Therefore = we have the same amount of kinetic and pressure energy that had = previously occupied an volume of Ai*Li, but now it occupies a lesser = volume of Ae*Le. =20 ( I know, I know my duct is not 17.6 ft long (Le =3D 1/10Li =3D 1/10*176 = ft =3D 17.6ft. But that is for 1 second of flow after diffusion. Even = at Ve=3D1/10Vi, my airflow would actually undergo the continous = diffusion process -the amount of time it takes for the denser airmass to = flow through my diffusers larger area- (approx 6" in length) would be = =3D 0.5ft/17.6 ft/sec =3D .0028 sec. So you don't need a diffuser = length of 1/10 your Vi as the diffuser is continously processing the = airmass as it flow in and out of it (it does it Very,very fast {:>)). So the airmass that now occupies Ae =3D 3*Ai and Le =3D 1/10Li (because = we assumed the diffuser decelerated the Vi to 1/10Vi =3D Ve (and for 1 = sec timing Le =3D Ve) so Volume Ae*Le =3D 3*Ai*1/10*Li =3D 3/10*Ai*Li. = Again the diffuser volume AeLe =3D 3/10AiLi volume. So the volume the 1 second of airmass occupies after being processed by = the diffuser only 3/10ths (in this example) of the original volume it = previously occupied in the 176 ft long tube of air. 4. So with the original column's airmass now in a volume 1/3 the = original size, we can see why the pressure, density and temperature = increases in the diffuser. Its because all the kinetic and pressure = energy of the original airflow (and its air molecules) now occupies a = volume only 3/10's as large as it once did. 5. Therefore in summary, the density, pressure and temperature all = increase and the airflow velocity decrease in a subsonic diffuser (you = can check this in plenty of references). Additional notes of interest: Duct vs Diffuser. So, Ok, fine a = diffuser is magic, but so what? Here is the so-what! Both a duct and diffuser act as a container for the air flow. However, = the duct does not reduce the velocity of the air as does a diffuser = (Note: There is some reduction due to side wall friction and turbulence = in a duct (but that is loss energy and not recoverable) but nowwhere = near the reduction of airflow velocity cause by a diffuser). =20 If the radiator were unshrouded, then while some airflow would continue = to go through the center, much of the air at the outer edges would flow = around the sides of the radiator as the path of least resistance. Both = the duct and diffuser acts as a container. If a radiator were abutted = to the end of duct the air trapped in the duct would tend to flow = through the radiator (at a higher velocity, with more turbulence and = energy loss) than the air out of a diffuser. So both diffuser and duct = prevent this from happening. So "even-stephen" so far as containing the = air flow goes - they both do it.. You could just make a duct to contain the airflow (to prevent it from = spilling around the edges of the radiator) much as a diffuser would do. = However, there is yet another factor that favors the diffuser. Drag is = directly proportion to the frontal area of the radiator. However, drag = is also proportional to the square of the velocity of the air flow = through that radiator area. So higher velocity through the radiator = (say from a duct) causes more drag than just making your radiator = larger . But, you say - if I slow down the air velocity (as a diffuser = does) , I'll impede cooling. Well, not quite. While a diffuser does slow down the air velocity, it does not slow down = the airmass flow which remains constant (less velocity - but more = density and area). Remember it is the air mass quantity not the = velocity that cools. So velocity has slowed but air density has = increased such that the airmass flowing through the radiator is = essentially the same (there is some loss) as originally entered the = inlet. So we get the same air mass for cooling but with much less drag due to = the slower velocity of the air flowing through the radiator after a = diffuser (Remember the mass flow through a diffuser system and radiator = does not change from one end to the other - can't make mass disappear). = Note, this is one reason that if you do have less than perfect sealing = of your diffuser to your radiator that your cooling effectiveness loss = will be much higher than you would think. The air pressure is higher = and the air is denser, both of which contribute to a relative large = airmass loss even though fairly small holes. Less drag would still be true even if we had to made the radiator area = larger to compensate from some of the pressure unrecovered (nothing is = 100% efficient) as the increase in area to compensate does not produce = as much drag effect as a higher velocity air flow would. So by slowing = the air velocity, we get better cooling and less drag than just using a = duct would do for us. So in summary you want a diffuser and not a duct for your cooling system = because you get better cooling and less drag Well, thats what I think I've learned - so those of you with aerodynamic = backgrounds, how about hopping in an correcting me. Now, when (if?_I can just get the design parameters figured out, I will = provide that for you consideration. =20 Still recoverying from being forced stuff with food on Thrusday. . =20 Ed Anderson RV-6A N494BW Rotary Powered Matthews, NC eanderson@carolina.rr.com ------=_NextPart_000_0029_01C3B790.1C026B00 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
Ok folks,
 
    Been developing a = bit more=20 understanding (I hope) of the cooling science/art/mystery - still a = long=20 ways to go, but finally understood a bit more about ducts and diffusers = and=20 why you want to use a diffuser (where possible) in front of an = heat=20 exchanger.
 
What's a duct and what's a=20 diffuser?
 
A duct can be though = of as an=20 airpath with no change in its cross sectional=20 area. 
 A diffuser on = the other hand=20 has an airpath that goes from a smaller to larger = cross-sectional area=20 or the area diverges (as they say).
       =20             = _____
          =   =20    /
          =20    /
   ____ /     &= nbsp;           &n= bsp;           &nb= sp;     =20 _________________
 
----------------->=20 Diffuser           = ;            =  --------------------->=20 Duct
 =20 _____           &n= bsp;           &nb= sp;           &nbs= p;   __________________
       =20     \
        =   =20     \
          &nbs= p;   =20    \_______
 
 Clearly the constant cross = section duct is=20 easier to build so why would you want the complication of making a = varying=20 cross-section diffuser. Note: The terms Duct and Diffuser are sometimes = used=20 interchangeably - so it can get a bit confusing when reading.  = But,=20 for our purposes a duct is with unchanging cross-sectional=20 area. 
 
Well, it took me a while to understand = what all the=20 math meant in real world terms regarding a diffuser (assuming that I now = have it=20 correct). 
 
A subsonic diffuser does = one principal=20 thing - it slows down the velocity of the air mass entering = it. =20 So? you say. (when I want all the cooling airmass I can get) Why should = I want=20 to slow down the air? Well, because its good for your cooling and your=20 airspeed. 
Yep! that what it ultimately boils down = to and why=20 you want a diffuser rather than just a duct.
 
Don't ask me (yet) how to go about = designing the=20 diffuser you need at this point, but here is what happens in a nut = shell. =20 What the diffuser does is transform the kinetic and pressure energy of = the=20 airstream.  With the cross section area diverging (getting = larger),=20 the air pressure increases, the air density increases, the temperature = increases=20 (but not much) and the velocity decreases. The following is my attempt = to=20 explain how this happens.
 
Explanation of=20 Diffuser
 
Picture a tube of air created by = drawing a circle=20 of area Ai (your cooling inlet) through the air at 120 MPH (176 ft/sec) = for one=20 second.  That is then the air mass entering your inlet of area = Ai for=20 your Diffuser of area Ae (the large divergent end). The tube of air is = 176 ft=20 long (Li) with an area of Ai which is the same as saying the volume is = Ai*Li or=20 since Li =3DVi (for one second), we can say the volume is Ai*Vi.  = This is the=20 volume of air that flows into the diffuser section with cross-section = area of Ae=20 =3D 3*Ai (remember to be a diffuser, the outlet area has to be = larger than=20 the inlet area)  but where the velocity of the airmass has dropped = to=20 velocity Ve (where Ve =3D 1/10*Vi nothing is 100% efficient so there = will be some=20 residue velocity left)
 
    =    =20             =    =20             Some=20 terms
 
Inlet (small=20 area)          = ;            =             <= STRONG>Diffuser=20 (large area)
Vi (Velocity ft/sec)   =20             =    =20             Ve (velocity = remaining=20 in diffuser (always some))
Li =3D Vi/(1 sec)   =20             =    =20             =    =20     Le (length of large area)
Ai =3D Area of Tube=20 (ft^2)           =20             Ae (area of = large=20 section)
pi =3D air density=20 (Lbm/ft^2)          &nb= sp;          =20 pe (air density in diffuser)
Mass =3D (Lbm)
Kinetic Energy Ke =3D=20 BTU

Consider a  air mass flowing = at a velocity=20 Vi  through the inlet of area Ai to a diffuser for one second of=20 flow.  This volume of air is Ai*Vi, taking the density (pi -air = density=20 0.0076lbm/ft^3 -not PIE)) of this air mass, one can calculate the mass = flow =3D=20 pi*Ai*Vi.  Mass flow through the system does not = change=20 (subsonic velocity assumed) !!!!
 
Now have this mass flow be decelerated = to Ve (where=20 Ve=3D1/10*Vi - a good value for a good diffuser)  by a diffuser of = area Ae=20 placed at the end of the inlet.  Assume that the area Ae is 3 times = larger=20 than the inlet area Ai or Ae =3D 3*Ai.  Consider what this =20 means:
 
1.  Since the mass flow must = remain constant=20 through the ducts then pi*Ai*Vi =3D pe*Ae*Ve.  However we know that = area Ae=20 is 3 times larger than Ai, which means the product of pe*Ve must be = smaller=20 by a factor of 1/3 to maintain the mass flow equation balance.  In = reality,=20 we know that Ve is also  less as the basic function of a = diffuser  is=20 to decelerate an airflow.  This then means that the density pe must = increase.  Actually the density pe, the pressure Pe and the = temperature Te=20 all increase as a results of converting the kinetic energy of the mass = flow into=20 dynamic pressure and increased density within the smaller volume of the=20 diffuser.
 
2.  It t may seem counterintuitive = to have the=20 pressure and density increase when the area is increased.  Most = would=20 predict that the density and pressure would decrease with an increase in = area.  This is indeed what would happen in a closed system.  = However,=20 a diffuser with air mass flow into and out of is an open system and the = airmass=20 flow continuously brings energy and air mass into the system = (diffuser)=20
 
3. Consider this:  You have a = volume of air Li=20 Long travelling at speed Vi (think of a long tube of air) with the = kinetic=20 energy of=3D  m*1/2piVi^2.  This energy is for the column Li = long. =20 Now when the air enters the increased area Ae, the velocity has = decreased to=20 Ve.  So we have a volume =3D Ae*Le. Recall that Ae was =3D 3*Ai = however Le is=20 1/10*Li (because Length of tube is equal to the velocity divided by time = (1=20 second in this case), so Le is effectively Ve at 1 sec).  Therefore = we have=20 the same amount of kinetic and pressure energy that had previously = occupied an=20 volume of Ai*Li, but now it occupies a lesser volume of=20 Ae*Le.  
 
( I know, I know my duct is = not 17.6 ft=20 long (Le =3D 1/10Li =3D 1/10*176 ft =3D 17.6ft.  But that is for 1 = second of flow=20 after diffusion.  Even at Ve=3D1/10Vi, my airflow would actually = undergo the=20 continous diffusion process -the amount of time it takes for the denser = airmass=20 to flow through my diffusers larger area-  (approx 6" in length) = would be =3D=20 0.5ft/17.6 ft/sec =3D .0028 sec.  So you don't need a = diffuser =20 length of  1/10 your Vi as the diffuser is continously processing = the=20 airmass as it flow in and out of it (it does it Very,very fast=20 {:>)).
 
So the airmass that now occupies Ae =3D = 3*Ai and Le =3D=20 1/10Li (because we assumed the diffuser decelerated the Vi to 1/10Vi =3D = Ve (and=20 for 1 sec timing Le =3D Ve) so Volume Ae*Le =3D 3*Ai*1/10*Li =3D = 3/10*Ai*Li. =20 Again the diffuser volume AeLe =3D 3/10AiLi volume.

So the volume the  1 second of = airmass=20 occupies after being processed by the diffuser only 3/10ths (in this = example)=20  of the original volume it previously occupied in the 176 ft long = tube of=20 air.
 
4.  So with the original column's = airmass now=20 in a volume 1/3 the original size,  we can see why the pressure, = density=20 and temperature increases in the diffuser.  Its because all the = kinetic and=20 pressure energy of the original airflow (and its air molecules) =  now=20 occupies a volume only 3/10's as large as it once did.
 
5.  Therefore in summary, the = density,=20 pressure and temperature all increase and the airflow velocity decrease = in a=20 subsonic diffuser (you can check this in plenty of = references).
 
Additional notes of = interest:  Duct vs=20 Diffuser.  So, Ok, fine a diffuser is magic, but so what? = Here is=20 the so-what!
 
Both a duct and diffuser act as a = container for the=20 air flow.  However, the duct does not reduce the velocity = of the=20 air as does a diffuser (Note: There is some reduction due to = side wall=20 friction and turbulence in a duct (but that is loss energy and not=20 recoverable) but nowwhere near the reduction of airflow velocity = cause by a=20 diffuser).  
 
 If the radiator were unshrouded, = then while=20 some airflow would continue to go through the center, much of the air at = the=20 outer edges would flow around the sides of the radiator as the path of = least=20 resistance.  Both the duct and diffuser acts as a container.  = If a=20 radiator were abutted to the end of duct the air trapped in the duct = would tend=20 to flow through the radiator (at a higher velocity, with more turbulence = and=20 energy loss) than the air out of a diffuser.  So both diffuser = and=20 duct prevent this from happening. So "even-stephen" so far as containing = the air=20 flow goes - they both do it..
 
 You could just make a duct to = contain the=20 airflow (to prevent it from spilling around the edges of the radiator) = much as a=20 diffuser would do.  However, there is yet another = factor that=20 favors the diffuser.  Drag  is directly proportion to = the=20 frontal area of the radiator.  However, drag is also = proportional=20 to the square of the velocity of the air flow through that = radiator=20 area.  So higher velocity through the radiator (say from a duct) = causes=20 more drag  than just making your radiator larger .  But, = you say=20 - if I slow down the air velocity (as a diffuser does) , I'll impede=20 cooling.  Well, not quite.
 
While a diffuser does slow down = the air=20 velocity, it does not slow down the airmass = flow which=20 remains constant (less velocity - but more density and area).  = Remember it is the air mass quantity not the velocity that cools.  = So=20 velocity has slowed but air density has increased such that the airmass = flowing=20 through the radiator is essentially the same (there is some loss) = as=20 originally entered the inlet.
 
So we get the same air mass for = cooling  but with much less drag due to = the=20 slower velocity of the air flowing through the radiator after a diffuser = (Remember the mass flow through a diffuser system and radiator does not = change=20 from one end to the other - can't make mass disappear).  Note, = this is=20 one reason that if you do have less than perfect sealing of your = diffuser to=20 your radiator that your cooling effectiveness loss will be much higher = than you=20 would think.  The air pressure is higher and the air is denser, = both of=20 which contribute to a relative large airmass loss even though fairly = small=20 holes.
 
 Less drag would still be true = even if we had=20 to made the radiator area larger to compensate from some of the pressure = unrecovered (nothing is 100% efficient) as the increase in area to=20 compensate does not produce as much drag effect as a higher velocity air = flow=20 would.  So by slowing the air velocity, we get better cooling and = less drag=20 than just using a duct would do for us.
So in summary you want a = diffuser and not a=20 duct for your cooling system because you get better cooling and less=20 drag
 
Well, thats what I think I've learned - = so those of=20 you with aerodynamic backgrounds, how about hopping in an correcting=20 me.
 
Now, when (if?_I can just get the = design parameters=20 figured out, I will provide that for you consideration.  =
 
Still recoverying from being forced = stuff with food=20 on Thrusday.
 
 
. 
Ed Anderson
RV-6A N494BW Rotary=20 Powered
Matthews, NC
eanderson@carolina.rr.com
------=_NextPart_000_0029_01C3B790.1C026B00--