X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Received: from fed1rmmtao104.cox.net ([68.230.241.42] verified) by logan.com (CommuniGate Pro SMTP 5.2c2) with ESMTP id 2470935 for flyrotary@lancaironline.net; Tue, 13 Nov 2007 20:45:41 -0500 Received-SPF: none receiver=logan.com; client-ip=68.230.241.42; envelope-from=alventures@cox.net Received: from fed1rmimpo02.cox.net ([70.169.32.72]) by fed1rmmtao104.cox.net (InterMail vM.7.08.02.01 201-2186-121-102-20070209) with ESMTP id <20071114014503.CEEK26427.fed1rmmtao104.cox.net@fed1rmimpo02.cox.net> for ; Tue, 13 Nov 2007 20:45:03 -0500 Received: from BigAl ([72.192.143.193]) by fed1rmimpo02.cox.net with bizsmtp id CRl31Y0084AaN600000000; Tue, 13 Nov 2007 20:45:03 -0500 From: "Al Gietzen" To: "'Rotary motors in aircraft'" Subject: RE: [FlyRotary] Re: Rebutal to the rebutal {:>) Thick vs Thin was : Diffuser Configuration Comparison Date: Tue, 13 Nov 2007 17:45:21 -0800 Message-ID: <000e01c82660$0488a8c0$6401a8c0@BigAl> MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_000F_01C8261C.F66568C0" X-Priority: 3 (Normal) X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook, Build 10.0.6626 Importance: Normal In-Reply-To: X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.3198 This is a multi-part message in MIME format. ------=_NextPart_000_000F_01C8261C.F66568C0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: quoted-printable Ed; =20 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 =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 =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. =20 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 - and is why I say that talking thickness without specifying core density and diffuser ratio is 'incomplete' = because they depend on one another. =20 =20 But back to fist order; given the conditions of adequate pressure = recovery the flow rate is fixed. =20 2nd regarding the deltaT "swap" I made:=20 I do not agree that you need to compare on the basis of same 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: =20 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.) =20 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. =20 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 - just as there is a corresponding delta T that results = in minimum mass of the core (all other things equal; i.e., properly = designed). =20 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. =20 So there ya go; cooling system design in a nutshell; minus all the = magic:-). =20 Al G =20 =20 =20 =20 =20 =20 =20 ------=_NextPart_000_000F_01C8261C.F66568C0 Content-Type: text/html; charset="us-ascii" Content-Transfer-Encoding: quoted-printable

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. 

 

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. 

 

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 – and is = why I say that talking thickness without specifying core density and diffuser = ratio is ‘incomplete’ because they depend on one another. 

 

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

 

2nd regarding the deltaT "swap" I made: 

I do not agree that you need to compare on the basis = of same 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 – 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

 

 

 

 

 

 

 

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