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# Gearbox casing heat rejection(2)

 murpia (Mechanical) (OP) 5 Jun 09 10:05
 Hi,I'm looking for an approximate value for the heat rejection to ambient air of a typical transmission casing.Watts/surface-area for an aluminium casting in free air would be a good starting point.I need to calculate the size of the oil cooler, but want to subtract the heat rejection of the casing first.Many thanks, Ian
 (2)  tbuelna (Aerospace) 5 Jun 09 23:21
 GregLocock (Automotive) 6 Jun 09 3:23
 Hmm, informative post but orthogonal to the thread I think. According to my thermo book the heat transfer coefficient in air is.0005 to 1 kW m-2 K-1That's right the estimate varies by a factor of 2000!My Heat transfer book is even less helpful.Here's what Ye Olde gearbox booke (practically a scroll) saysH=C*A*dTH in ft lb/min (that'd be hp/33000)dT in fahrenheitA in sq inC is 0.45 for A=0, .33 for A=10000 and .27 for A=20000Well, it is an answer!   CheersGreg LocockSIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
 IceStationZebra (Mechanical) 6 Jun 09 17:28
 I think the wide variance in transfer coefficient is probalby due to clean vs. oxidized surface. My data for room temperature is 840-1540 BTU/hr./sq.ft./°F.ISZ
 Tmoose (Mechanical) 9 Jun 09 12:16
 FWIW - My old "Machine Design" by Deutschman, Michels, and Wilson says a widely used heat transfer coefficient (C) for bearing heat balance calcs is approximated as 2 BTU/(hr)(ft2)( degree F) for still air. If air velocity is 500 ft/min, then 5.9 BTU.......  is suggested.
 tbuelna (Aerospace) 11 Jun 09 0:56
 Sorry if you thought my post was a bit long winded, but the original question posed was more than simply asking for a heat transfer coefficient for aluminum in still air.  Any decent text will give you that.  I just spent a very long time doing exactly this analysis for a large aircraft transmission.  Sizing the oil heat exchanger core was no straightforward task.  It ultimately involves lots of educated assumptions.  And I'm sure the heat exchanger core size will change after both bench testing and flight testing.What Ian stated is that his real task is to size an oil cooler for his transmission.  And he wished to start by subtracting the heat rejected thru the aluminum case from the total heat load that must be rejected thru the cooler core.  All other things being equal, heat transfer thru the case wall is most affected by the deltaT.  To accurately establish the deltaT across the case wall, you must firstly understand how the oil-to-inner-case-wall heat transfer mechanism works, and what factors drive it.  Some of the oil, like that used to cool bearings, will not have much temperature rise and will likely drain directly into the sump without contacting the case wall.  Other oil flows, like those used to cool gear meshes, will experience much greater temperature rise, and will be flung and wetted-out against almost all inner surfaces of the case.  Thus the oil flow heating the case will be at a higher local temperature than the bulk oil flow temps measured in the sump.To make a long story short, accurately sizing a transmission heat exchanger (by analysis) is no simple matter.  And that was the real point I feebly attempted to make with my long-winded post.Best regards,Terry
 GregLocock (Automotive) 11 Jun 09 1:17
 Yes, and it was a very useful post. In fact have a star. That and 3 dollars will get you a cup of coffee. CheersGreg LocockSIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
 tbuelna (Aerospace) 11 Jun 09 2:16
 Three bucks for a cup of coffee?  That works out to over $30/gal. And to think people complain about paying$3/gal for gasoline!In case you're interested regarding the topic of how complicated some gearbox lube system analyses can get, the one I'm currently working on has over 160 separate oil jet orifices in the system.  Thank God for Excel!Best regards,Terry
 patprimmer (Publican) 11 Jun 09 2:34
 Well Starbucks need to show a profit to cover the franchise fees. RegardsPatSee FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &http://eng-tips.com/market.cfmfor site rules

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