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nemaste · 06-22-2002, 09:45 AM

i think the rad exhaust would help cool the rest of the case down. i seriously doubt the small deltaT across the radiator is worth considering in light of other much hotter components inside (what are we talkning about with a BIX? <1C difference in air temp?). the large amount of flow from that rad fan is probably enough to keep the case very near ambient anyways, so shunting that flow out & adding the noise of a dedicated case flow fan would not be necessary.

if you try & force air to move some place (like ducting half of the rad exhaust instead to the PSU of letting it exit into free air & find it's own way to the exhaust fans), you create more back-pressure on the fans. pressure gradients suck for fans, so avoid them period. to avoid do them, you want to make the mass flow of air exiting equal the mass flow entering the case, otherwise you DO in fact build up small amounts of pressure that can greatly affect your fans.

although the pressure will never grow large enough to create an explosion, it will make the fan blades operate at an off design angle of attack, which can make fans completely ineffective(dcrainman, you do know how this explosion occurs right? strong shock waves forming in our outlets end up choking the flow. on rockets, an overpressurized combusiton chamber can cause shock waves to choke outlet. this is the cause of around 90% of our space program's "aborted launches" in the "early days"). as long as you take some measures to cancel net flows in the case, you'll be better off. in other words, estimate the friction through the rads (or do trial & error experiments), filters, & such, & make your inlet CFM = outlet CFM. this flow balancing eliminates back pressure. even if you don't consider friction, you should at least use the fan CFM specs to design for 0 net flow:

+CFM of inlet fans - CFM of exhaust fans = 0 CFM

if it is above zero, your inlets operate with back pressure & exhausts with foreward pressure. if it's below 0, your inlets operate in foreward pressure & exhausts with back-pressure. my post in the "fan math" thread has a diagram of what happens to fans that operate under fore&back pressure.
Off Design Rotor Blades

06-22-2002, 09:45 AM	#19
nemaste Cooling Neophyte Join Date: May 2002 Location: CA Posts: 78	i think the rad exhaust would help cool the rest of the case down. i seriously doubt the small deltaT across the radiator is worth considering in light of other much hotter components inside (what are we talkning about with a BIX? <1C difference in air temp?). the large amount of flow from that rad fan is probably enough to keep the case very near ambient anyways, so shunting that flow out & adding the noise of a dedicated case flow fan would not be necessary. if you try & force air to move some place (like ducting half of the rad exhaust instead to the PSU of letting it exit into free air & find it's own way to the exhaust fans), you create more back-pressure on the fans. pressure gradients suck for fans, so avoid them period. to avoid do them, you want to make the mass flow of air exiting equal the mass flow entering the case, otherwise you DO in fact build up small amounts of pressure that can greatly affect your fans. although the pressure will never grow large enough to create an explosion, it will make the fan blades operate at an off design angle of attack, which can make fans completely ineffective(dcrainman, you do know how this explosion occurs right? strong shock waves forming in our outlets end up choking the flow. on rockets, an overpressurized combusiton chamber can cause shock waves to choke outlet. this is the cause of around 90% of our space program's "aborted launches" in the "early days"). as long as you take some measures to cancel net flows in the case, you'll be better off. in other words, estimate the friction through the rads (or do trial & error experiments), filters, & such, & make your inlet CFM = outlet CFM. this flow balancing eliminates back pressure. even if you don't consider friction, you should at least use the fan CFM specs to design for 0 net flow: +CFM of inlet fans - CFM of exhaust fans = 0 CFM if it is above zero, your inlets operate with back pressure & exhausts with foreward pressure. if it's below 0, your inlets operate in foreward pressure & exhausts with back-pressure. my post in the "fan math" thread has a diagram of what happens to fans that operate under fore&back pressure. Off Design Rotor Blades __________________ StrongShockModz Last edited by nemaste; 06-22-2002 at 09:55 AM.