besserheimerpha
New member
</font><font color="blue" class="small">( Thank you besserheimerpha, for a great explanation. Hope you can answer a couple of questions for me.
First, just out of idle curiosity, what kind of fluid velocity are we talking about that will reduce the pressure of the fluid enough to boil at normal coolant temperatures? )</font>
Well, in a particular test I ran, cavitation started with a fluid velocity of ~77 fps, and had become damaging around ~150 fps. Cavitation starts small with just a few bubbles, and the very low percentage of bubbles that contact the pipe/valve internals keeps damage to a minimum. The real problem occurs when cavitation is present throughout the entire cross section of the flow, when lots of bubbles are making contact with the metal parts. That's when the "gravel in the pipe" starts, and yeah it turns everything to swiss cheese really fast.
</font><font color="blue" class="small">( Second, is there any kind of nucleation effect that will trigger vapor bubbles to collapse, like there can be to form them in the first place? By that I mean, will a rough spot in the engine casting or pipe have any effect on the rate of bubble collapse as the bubble-laden coolant stream passes over it? )</font>
I would assume yes, but it is so difficult to predict cavitation especially on such a small scale. For our valve applications, we just test every product. Sometimes a seemingly insignificant change to the product can change the flow enough to really screw things up. As the fluid flows over small imperfections in the pipe/valve, it could cause enough change in pressure to form or collapse the bubbles. I should note, however, that flashing (bubbles form but don't collapse) is also bad for the fluid system. The ultimate goal is to not have any bubbles form. When the bubbles appear, the fluid expands as it goes from liquid to gas, and to maintain a constant mass flow rate the velocity has to skyrocket. Anyway, if the fluid is flashing there isn't much you can due to prevent it from cavitating other than keep the velocity high or vent to a tank or atmosphere. Eventually the roughness on the inside of the pipe will slow the fluid down enough that it would cavitate.
Cavitation is also very dependant on the fluid properties. In our lab we have water at 60°F. The addition of coolant alone would make our data not apply to this situation.
First, just out of idle curiosity, what kind of fluid velocity are we talking about that will reduce the pressure of the fluid enough to boil at normal coolant temperatures? )</font>
Well, in a particular test I ran, cavitation started with a fluid velocity of ~77 fps, and had become damaging around ~150 fps. Cavitation starts small with just a few bubbles, and the very low percentage of bubbles that contact the pipe/valve internals keeps damage to a minimum. The real problem occurs when cavitation is present throughout the entire cross section of the flow, when lots of bubbles are making contact with the metal parts. That's when the "gravel in the pipe" starts, and yeah it turns everything to swiss cheese really fast.
</font><font color="blue" class="small">( Second, is there any kind of nucleation effect that will trigger vapor bubbles to collapse, like there can be to form them in the first place? By that I mean, will a rough spot in the engine casting or pipe have any effect on the rate of bubble collapse as the bubble-laden coolant stream passes over it? )</font>
I would assume yes, but it is so difficult to predict cavitation especially on such a small scale. For our valve applications, we just test every product. Sometimes a seemingly insignificant change to the product can change the flow enough to really screw things up. As the fluid flows over small imperfections in the pipe/valve, it could cause enough change in pressure to form or collapse the bubbles. I should note, however, that flashing (bubbles form but don't collapse) is also bad for the fluid system. The ultimate goal is to not have any bubbles form. When the bubbles appear, the fluid expands as it goes from liquid to gas, and to maintain a constant mass flow rate the velocity has to skyrocket. Anyway, if the fluid is flashing there isn't much you can due to prevent it from cavitating other than keep the velocity high or vent to a tank or atmosphere. Eventually the roughness on the inside of the pipe will slow the fluid down enough that it would cavitate.
Cavitation is also very dependant on the fluid properties. In our lab we have water at 60°F. The addition of coolant alone would make our data not apply to this situation.
