Return-Path: Received: from mailout1.pacific.net.au ([61.8.0.84] verified) by logan.com (CommuniGate Pro SMTP 4.3c1) with ESMTP id 744850 for flyrotary@lancaironline.net; Fri, 18 Feb 2005 16:03:25 -0500 Received-SPF: none receiver=logan.com; client-ip=61.8.0.84; envelope-from=peon@pacific.net.au Received: from mailproxy1.pacific.net.au (mailproxy1.pacific.net.au [61.8.0.86]) by mailout1.pacific.net.au (8.12.3/8.12.3/Debian-7.1) with ESMTP id j1IL2cA6028952 for ; Sat, 19 Feb 2005 08:02:38 +1100 Received: from ar1 (ppp28D9.dyn.pacific.net.au [61.8.40.217]) by mailproxy1.pacific.net.au (8.12.3/8.12.3/Debian-7.1) with SMTP id j1IL2YS4025524 for ; Sat, 19 Feb 2005 08:02:35 +1100 Message-ID: <006a01c515fc$a7698720$d928083d@ar1> From: "Leon" To: "Rotary motors in aircraft" References: Subject: For Todd "Vena Contracta" was Re: Returnless Fuel System - Fuel Cooler Date: Sat, 19 Feb 2005 07:58:52 +1100 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_0067_01C51658.DA166100" X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2800.1409 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1409 This is a multi-part message in MIME format. ------=_NextPart_000_0067_01C51658.DA166100 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable RE: [FlyRotary] Returnless Fuel System - Fuel CoolerHi Todd, Over the years, I've done a lot of carburettor tuning. IDA Webers were = my favourite device until digital EFI came along. Now from practical experience, I know beyond all reasonable doubt that = the size of the hole in any Weber jet controls the flow capacity of = either fuel or air. The bigger the hole, bigger the flow, and vice = versa. I also know from my engineering studies that I can calculate the = actual flow rate, given the viscosity of the fluid, the shape of the = jet, and the pressure drop across the jet. (I don't bother "number = running" because I have a box full of different sized jets and just keep = changing them until I get the correct Air/Fuel ratio out the pipe. All = else being equal, double the diameter, increase the flow rate 4 times. So I just can't see what the size of an attached pipe has to do with the = flow rate through a "vena contracta" ("vena constricta"??). You also = say "but much of this pressure is recovered downstream" Hmmmmmm, ... I = say "so what"??. We aren't interested in pressure drops or pressure = recovery, ... it's flow rates that are the SOLE issue, and the capacity = of an open flow 1/4 inch line to get rid of the fuel dumped by the = regulator. =20 Somehow the sizes have got all muddled up (better give Wally the muddle = headed Wombat the message to move on!!). If the 5/16" supply is = restricted by a 3/16" "vena constricta", then the flow rate is = controlled by the 3/16" constriction, the pressure drop across it, and = the viscosity of the fuel. Doesn't matter if it's flowing into a 12" = sewer pipe or a 5/16" fuel line, the flow limiter is the smallest cross = sectional diameter and the pressure drop across it. Conversely, the = fuel return line is open flow without any back pressure except ambient = barometric. =20 My contention, backed up by LOTS of practical observation is that the = 1/4' open flow return line will flow more than the 3/16" restriction = will allow under the controlled pressure of the regulator. So please = feel free to correct me if I'm wrong in my analysis. Cheers mate, Leon =20 ----- Original Message -----=20 From: Bartrim, Todd=20 To: Rotary motors in aircraft=20 Sent: Saturday, February 19, 2005 6:12 AM Subject: [FlyRotary] Re: Returnless Fuel System - Fuel Cooler Hi Bob;=20 SNIP>> I should point out that while it has been mentioned that = since the outlet of the pumps is often less than the ID of the fuel line = this is your limiting factor in flow. This isn't really the case. A = small restriction such as this creates a pressure drop through the "vena = contracta", but much of this pressure is recovered downstream. If anyone = actually cares about this I can pass along the formula for calculation = of the permanent pressure drop through an orifice. This means that a = 1/4" line with a 3/16" restriction at some point will not have the same = lower flow as a 3/16" line, but will still be slightly less than a 1/4" = line with no restriction. ------=_NextPart_000_0067_01C51658.DA166100 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable RE: [FlyRotary] Returnless Fuel System - Fuel = Cooler
Hi Todd,
 
Over the years,  I've done a lot = of=20 carburettor tuning.  IDA Webers were my favourite device until = digital EFI=20 came along.
 
Now from practical experience,  I = know beyond=20 all reasonable doubt that the size of the hole in any = Weber jet=20 controls the flow capacity of either fuel or air.  The bigger = the=20 hole,  bigger the flow,  and vice versa.  I also know = from my=20 engineering studies that I can calculate the actual flow rate,  = given the=20 viscosity of the fluid,  the shape of the jet,  and the = pressure drop=20 across the jet.  (I don't bother "number running" because I have a = box full=20 of different sized jets and just keep changing them until I get the = correct=20 Air/Fuel ratio out the pipe.  All else being equal,  double = the=20 diameter,  increase the flow rate 4 times.
 
So I just can't see what the size = of an=20 attached pipe has to do with the flow rate through a  "vena = contracta"=20 ("vena constricta"??).  You also say "but much of this pressure is=20 recovered downstream"  Hmmmmmm, ... I say "so what"??.  We = aren't=20 interested in pressure drops or pressure recovery, ... it's flow = rates that=20 are the SOLE issue,  and the capacity of an open flow 1/4 = inch=20 line to get rid of the fuel dumped by the regulator. 
 
Somehow the sizes have got all muddled = up (better=20 give Wally the muddle headed Wombat the message to move on!!).  If = the=20 5/16" supply is restricted by a 3/16" "vena = constricta",   then=20 the flow rate is controlled by the 3/16" constriction,  the = pressure drop=20 across it,  and the viscosity of the fuel. Doesn't matter if = it's=20 flowing into a 12" sewer pipe or a 5/16" fuel line,  the flow = limiter is=20 the smallest cross sectional diameter and the pressure drop across it.=20 Conversely,  the fuel return line is open flow without any back = pressure=20 except ambient barometric.   
 
My contention, backed up by LOTS of = practical=20 observation is that the 1/4' open flow return line will flow more than = the 3/16"=20 restriction will allow under the controlled pressure of the = regulator.  So=20 please feel free to correct me if I'm wrong in my analysis.
 
Cheers mate,
 
Leon 
----- Original Message -----
From:=20 Bartrim, Todd
Sent: Saturday, February 19, = 2005 6:12=20 AM
Subject: [FlyRotary] Re: = Returnless Fuel=20 System - Fuel Cooler

Hi = Bob;

   SNIP>>

         I should point out that while it has been = mentioned that=20 since the outlet of the pumps is often less than the ID of the fuel = line this=20 is your limiting factor in flow. This isn't really the case. A small=20 restriction such as this creates a pressure drop through the "vena = contracta",=20 but much of this pressure is recovered downstream. If anyone actually = cares=20 about this I can pass along the formula for calculation of the = permanent=20 pressure drop through an orifice. This means that a 1/4" line with a = 3/16"=20 restriction at some point will not have the same lower flow as a 3/16" = line,=20 but will still be slightly less than a 1/4" line with no=20 restriction.

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