|
Trevor:
A few clarifications of terminology. As you mention, a centrifugal pump creates positive pressure on the outlet side and negative pressure on the inlet side. It is also adding kinetic energy to the fluid by accelerating it. As the fluid circulates through the system, friction dissipates the positive pressure induced by the pump.
The problem occurs when the fluid reaches the inlet of the pump. Theoretically, all of the positive pressure induced by the pump will have ben dissipated by the flow at that point. If the negative pressure induced by the pump inlet causes the absolute pressure of the fluid to fall below the vapor pressure of the fluid at that point, it boils. This pressure drop at the inlet is high, because the fluid is being accelerated from a low velocity before the pump inlet to a high velocity at the pump discharge. The higher the pump speed, the greater this velocity difference, and the greater the pressure drop through the pump inlet. That is the net positive suction head problem. The warmer the fluid, the higher the vapor pressure, the bigger the problem. It is similar to propellor cavitation, except that a propellor is not in a housing (a volute). Because of this housing the cavitation is mostly limited to the throat of the pump.
Increasing the pressure drop near the pump outlet is effective, because it increases the pressure drop through the system, thereby decreasing the fluid flow rate at any specific impellor speed, thus, decreasing the velocity at the pump inlet. The pressure drop at the pump inlet is thus decreased, increasing the absolute pressure of the fluid at that point and avoiding boiling.
Actually, the solution to our problem might be a Griswold flow control valve. This is a needle valve that is held open against the flow by a spring. Greater pressure on the upstream side pushes the needle into the orifice against the spring pressure, reducing the open area. They are designed to maintain nearly constant flow over a wide range of upstream pressure. We use them all the time in the HVAC industry to facilitate hydronic system balancing. I will look into this.
__________________
____________________________________________
95 LS-i Red, 31,xxx; bone stock for now; Daily Driver
94 LS-i Emerald Pearl, 106,xxx,; 246 whp; Tomyx snorkus and HKS Cold air intake; PWR aluminum radiator, silicone hoses; Inline thermostat; enhanced coolant routing; external power steering and oil coolers; Phenolic intake manifold spacers; 2004 WRX 5 speed transmission; ACT Clutch Kit, Heavy Duty Pressure Plate, Lightweight flywheel, performance disc; Group N motor mounts; ‘07 WRX 4-pot front calipers, cryo-treated slotted Tribeca rotors; Hawk HPS ferro-carbon pads; Frozenrotor rear slotted rotors; SS brake lines, Axxis Ultimate pads; Rota Torque 17x8 wheels; 245/40-17 Bridgestone RE01-R's; Koni inserts with Ground Control coilovers, Eibach springs; K-Mac camber/caster adjustable strut mounts; Urethane swaybar bushings; Bontrager rear sway bar; Urethane differential bushing; Custom Whiteline adjustable rear lateral links; Outlaw Engineering forged underdrive pulley; custom grind Web intake and exhaust cams (11 mm lift, 250° duration); solid lifters; CP custom aluminum forged 11 to 1 pistons, Brian Crower coated SS intake & exhaust valves; Brian Crower upgraded springs w/ titanium retainers; NGK sparkplugs; RallyBob (Bob Legere) ported and polished cylinder heads; Eagle H-beam rods; ACL Bearings; Cometic Head gaskets; ARP head studs & fasteners; Hydra Nemesis EMS; Wideband O2 sensor; 740cc Injectors; Walbro 255lph fuel pump; Upgraded WRX starter; Equal length SS headers (3 into 1); dual Magnaflow cat converters; 2 into 1 into 2 SS exhaust with Bullet muffler; OT Fiberglass hood; Oil pressure gauge; Programmable shift light,
2017 Subaru Forester XT, metallic dark gray, 29,xxx
2005 Porsche 911 Turbo S Cabrio, 24,xxx
2006 Subaru Outback LL Bean, 166,xxx sold
92 LSL Dark Teal, Smallcar Shift Kit - sold
|