X-Virus-Scanned: clean according to Sophos on Logan.com X-SpamCatcher-Score: 10 [X] Return-Path: Received: from ms-smtp-05.southeast.rr.com ([24.25.9.104] verified) by logan.com (CommuniGate Pro SMTP 5.1.7) with ESMTP id 1876032 for flyrotary@lancaironline.net; Thu, 01 Mar 2007 08:23:23 -0500 Received-SPF: pass receiver=logan.com; client-ip=24.25.9.104; envelope-from=eanderson@carolina.rr.com Received: from edward2 (cpe-024-074-103-061.carolina.res.rr.com [24.74.103.61]) by ms-smtp-05.southeast.rr.com (8.13.6/8.13.6) with SMTP id l21DMMT0015298 for ; Thu, 1 Mar 2007 08:22:22 -0500 (EST) Message-ID: <001901c75c04$a6990820$2402a8c0@edward2> From: "Ed Anderson" To: "Rotary motors in aircraft" References: Subject: Re: [FlyRotary] Re: Pinched ducts was : [FlyRotary] Re: cowl openings for water radiators Date: Thu, 1 Mar 2007 08:22:24 -0500 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_0016_01C75BDA.BD71D2E0" X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2900.3028 X-MIMEOLE: Produced By Microsoft MimeOLE V6.00.2900.3028 X-Virus-Scanned: Symantec AntiVirus Scan Engine This is a multi-part message in MIME format. ------=_NextPart_000_0016_01C75BDA.BD71D2E0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable =20 Also: Would it be a reasonable understanding, that the flow will separate = for lack of airspeed? One reason for the expanding duct is to slow down the air to be able = to do some aork in the radiator. True increased pressure may "help" = seperation, but I would think if you keep the pressure lower with = exhaust augmentation you still will see separation once you get the = airspeed below a certain energy level. Like a wing will eventually = stall, not for AOA, but lack of airflow - you need a certain flow to = keep the airstream attached to the airfoil even at 0=BA AOA..... Am I way off?? Thomas J. I don't see anything unreasonsable about your viewpoint, Thomas. = While I think there is little question that less back pressure will = permit more flow through the radiator, I am not certain how it will = affect the separation point. Would less back pressure mean less = recovery pressure in front of the core? If there is no effect on = pressure buildup and maintenance before the core then the pressure = gradient there might remain the same -but the pressure differential = across the core increase. There is also the airmass flow (which = actually does the cooling) factor and even if the pressure build up in = front of the core remains constant the lessen back pressure may permit = more through-flow in the core.=20 There is no question that there are many factors at work here and = many of them conflicting with others - so balance rather than elmination = is the key. One reason there is no cut/dry answer - it all depends on = so many factors. Here is an example from K&W chapter 12 The heat flow coefficient Kp = for turbulent flow in smooth passages (radiator core passages) Kp =3D = 1/2*L/D* 0.326/(Re)^-4 .=20 If Kp is a measure of goodness, then clearly if L increases and D = gets smaller Kp increases. Or if the Reynolds number Re gets smaller Kp = goes up. So what does this mean? It basically shows that for the heat = transfer to be large, the Reynolds number should be low (I.e. the = airflow through the core should be slow), the core should be deep(large = L) and the hole's hydraulic diameter (D) should be small.=20 This makes sense as the thicker the core the more heat transfer = (although the further into the core the less efficient the heat = transfer), the holes should be smaller (area exposed area - with large = holes some of the cooling air in the center will simply not have as much = contact with the hot metal of the core) and the air velocity should be = slow (dwell time adds to heat transferred to the unit volume of air per = unit time). =20 However, if you make the core too thick or the holes too small or slow = the air too much - then your KP factor may be high - but your over all = cooling will suck because you have too little mass flow through a too = restrictive core. This is just one example of where optimizing on one = set of factors can play havoc with the overall system function. One = way of looking at it is that you have to suboptimize a lot of factors in = order to get an optimum system {:>)=20 My 0.02 Ed ------=_NextPart_000_0016_01C75BDA.BD71D2E0 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
 
 
Also:
Would it be a reasonable = understanding, that the=20 flow will separate for lack of airspeed?
One reason for the expanding duct is = to slow down=20 the air to be able to do some aork in the radiator. True increased = pressure=20 may "help" seperation, but I would think if you keep the pressure = lower with=20 exhaust augmentation you still will see separation once you get the = airspeed=20 below a certain energy level. Like a wing will eventually stall, not = for AOA,=20 but lack of airflow - you need a certain flow to keep the airstream = attached=20 to the airfoil even at 0=BA AOA.....
 
Am I way off??
 
Thomas J.

 I don't see anything unreasonsable = about your=20 viewpoint, Thomas.  While I think there is little question that = less back=20 pressure will permit more flow through the radiator, I am not certain = how it=20 will affect the separation point.  Would less back pressure mean = less=20 recovery pressure in front of the core?  If there is no effect on = pressure buildup and maintenance before the core then the pressure = gradient=20 there might remain the same -but the pressure differential = across=20 the core increase.  There  is also = the airmass flow=20 (which actually does the cooling) factor and even if the pressure = build up in=20 front of the core  remains constant the lessen back pressure may = permit=20 more through-flow in the core. 
 
 There is no question that there are many = factors=20 at work here and many of them conflicting with others - so balance = rather than=20 elmination is the key.  One reason there is no cut/dry answer - = it all=20 depends on so many factors.
 
Here is an example from K&W chapter = 12  The=20 heat flow coefficient Kp for turbulent flow in smooth passages = (radiator=20 core passages)   Kp =3D 1/2*L/D* 0.326/(Re)^-4 =20 . 
 
  If Kp is a measure of goodness, then = clearly if L=20 increases and D gets smaller Kp increases.  Or if the Reynolds = number Re=20 gets smaller Kp goes up.  So what does this mean?  It = basically=20 shows that for the heat transfer to be large, the Reynolds number = should be=20 low (I.e. the airflow through the core should be slow), the core = should be=20 deep(large L) and the hole's hydraulic diameter (D) should be=20 small. 
 
 This makes sense as the thicker the core = the more=20 heat transfer (although the further into the core the less efficient = the heat=20 transfer), the holes should be smaller (area exposed area - with = large=20 holes some of the cooling air in the center will simply not have as = much=20 contact with the hot metal of the core)  and the air velocity = should be=20 slow (dwell time adds to heat transferred to the unit volume of air = per unit=20 time). 
 
However, if you make the core too thick or the = holes too=20 small or slow the air too much -  then your KP factor may be high = - but=20 your over all cooling will suck because you have too little mass flow = through=20 a too restrictive core.  This is just one example of where = optimizing on=20 one set of factors can play havoc with the overall system=20 function.   One way of looking at it is that you have to = suboptimize=20 a lot of factors in order to get an optimum system {:>) =
 
My 0.02
 
Ed
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