Mailing List flyrotary@lancaironline.net Message #64117
From: Ed Anderson eanderson@carolina.rr.com <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Oil
Date: Thu, 28 Jun 2018 10:33:51 -0400
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Hi Finn
 
Good to hear from you.l
 
You are right, there are lots of factors involved in each one of the terms of the heat transfer equation that are not reflected in that basic equation.
However, there is not a whole lot you can do about Cp.  One thing I’ve found out is that pure water is the best (ready avaliable) mass for coolant flow and of course its Cp = 1.   So for good or bad that parameter is pretty much fixed for whatever fluid you want to chose.  Given that - then I look at the other two factors,
Mass flow and Delta T.  While a large Delta T improves heat transfer, there is a limit governed by how hot you permit your engine to get vs how cool you can get your coolant.  So while you can improve it by turbulanting the liquid, inserting internal fins or wire turbulators, larger radiators, etc, again there is a limit.
 
Mass flow on the other hand is pretty straight forward, the more you have the more you cool.  While it appears that if you increase you mass flow by three times you reduce the transfer efficient by 10% you still come out way ahead by increasing mass flow.  The down side as I see if of increasing mass flow is that it takes power away from you drive train.  That drain means to get the same power to your prop,  you now need to burn more fuel, which in turn produces more heat, which requires removal, which ---------  Well, you get the idea.
 
So it more about optimizing what is practical to optimize and accepting there is no final, magic solution {Open-mouthed smile)  Nature is funny that way.
 
Ed
 
Sent: Tuesday, June 26, 2018 1:52 PM
Subject: [FlyRotary] Re: Oil
 
Hi Ed, good to see you on the list.

Unincumbered by engineering education when it comes to flows, restrictions and heat transfer I'm free to ponder the subject.

I keep thinking about a garden hose laying in the sun for a while. Initially you get hot water, but after a while it gets cold again when you let it run. So I think a number of factors hide in the "cp" factor below.

How many of the water/oil molecules get in contact with the walls (engine and cooler) must be a factor. Thus the turbulators in the Mazda oil cooler. Time must also be a factor, at least in terms of letting each molecule move to the wall.

So even if increased speed of flow may not hurt, it seems counter intuitive to make water/oil flow so fast that heat transfer does not take place.

There must be some kind of bell curve where you have a fluid speed for optimum heat transfer. Actually that curve must be a composite of conditions in the engine and of conditions in the cooler. An additional factor for the cooler is how well the heat is transferred from cooler to air. A cold wall should make heat transfer from fluid to air go that faster?

So what kind of instrumentation (measuring points) would enable us to optimize speed of fluid flow?

Finn


On 6/24/2018 4:36 PM, Ed Anderson eanderson@carolina.rr.com wrote:
You said it well, Ernest
 
What you want is heat removal from the engine.  Slowing flow down through a radiator will indeed show a larger Delta T from intake to outlet because the coolant(in the radiator) is exposed to cooling air longer.  That has led many to believe erroneously that slow flow = more heat removal.  I once argued an hour with old man Lou Ross about this issue and when I told him that the obvious conclusion was that if slowed water cooled better, then stopped water would cool best – there was silence on the other end of the phone and then Lou hung up and never spoke to me again.
 
Part of the myth also stems the results when attempts were made to improve flow rate by speeding up the coolant pump expecting better cooling.  When worst cooling occurred, it was concluded erroneously that the faster flow resulted in worst cooling. In most if not all of those instances, the poor cooling resulted from less flow – the faster water pump was actually cavitating and therefore actually pumping less coolant than at slower rotating speeds where cavitation did not occur.  But, it all fed into the myth.
 
 
Heat duty (Q): Heat duty is defined as the product of mass flow
rate specific heat capacity and the temperature difference
between inlet and outlet fluid temperatures
 
Q = m*cp* DT
 
A rule of thumb regarding heat removal and flow rate is:
 
The heat transfer coefficient decreases by ˜10% with a threefold increase in the mass flow rate
 
 
 
So a 10% decrease in transfer resulting from  three times the mass flow shows that increased mass flow (in of itself) will result in increased heat removal even though the heat transfer rate may lessen slightly.
 
At some point you get pressure losses and other factors - not to mention the greatly increased power required to get the large increase in mass flow - makes it impractical to infinitely increase flow rate.  Once you get the flow good enough to cool your engine under whatever the conditions you are operating within, it makes little sense to waste power to get more flow
 
However, we want best heat removal from the engine.  Heat is removed via mass flow of the liquid – no mass flow = no cooling.  So the more mass flow(within reason) - the more heat is removed from the engine and provided we can get rid of a certain amount of that heat though the radiator the more cooling of the engine occurs. So it’s a system, the cooler the oil returned to the engine the better heat transfer, the more mass flow from engine to radiator the more cooling, the cooler the air flowing across the radiator the better the heat rejection, etc.  All factors contribute and you can not focus on just one factor, the optimum cooling results from the optimization of all the major variables involved for a particular situation.
 
Just my 0.02  Back to my cave
 
Ed

 

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