Return-Path: Sender: "Marvin Kaye" To: lml@lancaironline.net Date: Wed, 20 Oct 2004 13:54:43 -0400 Message-ID: X-Original-Return-Path: Received: from imo-m21.mx.aol.com ([64.12.137.2] verified) by logan.com (CommuniGate Pro SMTP 4.2.5) with ESMTP id 480405 for lml@lancaironline.net; Wed, 20 Oct 2004 12:35:23 -0400 Received-SPF: pass receiver=logan.com; client-ip=64.12.137.2; envelope-from=REHBINC@aol.com Received: from REHBINC@aol.com by imo-m21.mx.aol.com (mail_out_v37_r3.8.) id q.12e.4eb492ce (3858) for ; Wed, 20 Oct 2004 12:34:42 -0400 (EDT) From: REHBINC@aol.com X-Original-Message-ID: <12e.4eb492ce.2ea7eda1@aol.com> X-Original-Date: Wed, 20 Oct 2004 12:34:41 EDT Subject: Re: Plenum Cooling X-Original-To: lml@lancaironline.net MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="-----------------------------1098290081" X-Mailer: 9.0 for Windows sub 5114 -------------------------------1098290081 Content-Type: text/plain; charset="US-ASCII" Content-Transfer-Encoding: 7bit In a message dated 10/20/2004 10:26:50 AM Eastern Standard Time, Rob.Logan@Philips.com writes: > Once the cross sectional area through which a stream of air passes > is increasing at a rate greater than 5 degrees, the flow becomes > turbulent and the Bernoulli principle ceases to apply. had 7 degrees written down here, hmm, gata find the source... this also applys to increasing pressure in a "ram air" induction too :-) Rob, The generally accepted cone diffuser angle for the onset of separation is indeed 7 degrees for fully developed flow. The lowest energy loss is found at about 5 degrees for this condition. Angles exceeding 40 - 60 degrees are actually worse than a step change due to recirculation. Cone angles up to 10 degrees are still quite efficient but the losses increase rapidly as the angle increases. However, in the inlet of your cowl the flow is not fully developed, as this represents the inlet of the "duct". In this region the optimum cone angle is closer to 4 degrees, but the losses are about 50% of what they would be for the fully developed condition. A cone angle of about 15 degrees will have roughly the same efficiency as would 10 degrees in the fully developed condition. Sources are: White, Fluid Mechanics and Panton, Incompressible Flow. The transition to fully developed flow takes place over around 10 diameters, depending upon Reynolds number. The entrance condition data are probably correct for about 3 duct diameters. This exposes a common error in entrance diffuser design. While it is certainly desirable to have a radiused transition to prevent "tripping" the flow, continuing an expanding radius beyond the optimum angle throughout the length of the diffuser is counter productive. If it is necessary to over expand the diffuser, it will be most efficient to generate the majority of the over expansion in the up stream half of the duct. Thus the cone angle should be greatest just inside the entrance and gradually decrease toward the engine in this case. Rob -------------------------------1098290081 Content-Type: text/html; charset="US-ASCII" Content-Transfer-Encoding: quoted-printable
In a message dated 10/20/2004 10:26:50 AM Eastern Standard Time, Rob.Lo= gan@Philips.com writes:
> Once the cross sectional area through whi= ch a stream of air passes
> is increasing at a rate greater than 5 deg= rees, the flow becomes
> turbulent and the Bernoulli principle ceases=20= to apply.

had 7 degrees written down here, hmm, gata find the source.= ..
this also applys to increasing pressure in a "ram air" induction too := -)
Rob,
 
The generally accepted cone diffuser angle for the onset of separation=20= is indeed 7 degrees for fully developed flow. The lowest energy loss&nb= sp;is found at about 5 degrees for this condition. Angles exceeding 40 - 60=20= degrees are actually worse than a step change due to recirculation. Cone ang= les up to 10 degrees are still quite efficient but the losses increase rapid= ly as the angle increases.
 
However, in the inlet of your cowl the flow is not fully developed, as=20= this represents the inlet of the "duct". In this region the optimum cone ang= le is closer to 4 degrees, but the losses are about 50% of what they would b= e for the fully developed condition. A cone angle of about 15 degrees will h= ave roughly the same efficiency as would 10 degrees in the fully developed c= ondition. Sources are: White, Fluid Mechanics and Panton, Incompressible Flo= w.
 
The transition to fully developed flow takes place over around 10 diame= ters, depending upon Reynolds number. The entrance condition data are probab= ly correct for about 3 duct diameters.
 
This exposes a common error in entrance diffuser design. While it is ce= rtainly desirable to have a radiused transition to prevent "tripping" the fl= ow, continuing an expanding radius beyond the optimum angle throug= hout the length of the diffuser is counter productive. If it is necessary to= over expand the diffuser, it will be most efficient to generate the majorit= y of the over expansion in the up stream half of the duct. Thus the cone ang= le should be greatest just inside the entrance and gradually decrease toward= the engine in this case.
 
Rob
-------------------------------1098290081--