Mailing List flyrotary@lancaironline.net Message #36710
From: Ed Anderson <eanderson@carolina.rr.com>
Subject: Re: [FlyRotary] Re: oil coolers
Date: Wed, 2 May 2007 10:57:03 -0400
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
> Question from the peanut gallery.  I don't plan to use a oil/water
exchanger, but aren't they counterproductive?

The efficiency of the cooling relies on deltaT.  The best situation is having the hottest coolant interfacing with the air, and it has always been my understanding that the oil exits the engine hotter than the water. Will an oil/water exchanger reduce the overall efficiency of the system by allowing the exit air, on average, to leave with less BTUs?


Ernest, I believe like most other things a lot depends on constraints and needs.  If constrained by space and location that makes it impossible/difficult to place and duct sufficient air  a separate oil cooler, then the oil/coolant exchanger offers an alternative.  While you are correct , there is greater delta T if you dump your oil heat directly to the air -remember that delta T is only one part of the equation.  There is the mass flow and the Cp of the cooling medium.  In this case the heat carrying capacity of water is  higher than air. So, the specific heat of water is higher and its mass is of course much higher than air.  Therefore while the delta T between oil and coolant is less, the heat transfer can be greater because of the much higher mass flow of the coolant transporting heat away from the metal interface with the oil.  Also remember that while your delta T between Oil and coolant is less once the heat is transferred to the coolant -  the delta T between the now hotter coolant and air increases thereby offsetting some of the lost efficiency.

So you trade off the benefit of more freedom in installing the oil/coolant heat exchanger for perhaps a small lose in efficiency (not even certain about that) for larger radiator cores to shed the additional heat coupled from the oil.
But, I'll stick with separate oil cooler and radiators.

Ed
Ed Anderson
Rv-6A N494BW Rotary Powered
Matthews, NC
eanderson@carolina.rr.com
http://members.cox.net/rogersda/rotary/configs.htm#N494BW
http://www.dmack.net/mazda/index.html


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