Pro/Forums - View Single Post

WAJ_UK · 01-02-2004, 11:17 AM

sorry I meant d as the diameter of the jet. 4d is 4*jet diameter and 5d is 5* jet diameter sorry it wasn't clear.

If you increase the distance from the nozzle exit to the impingement surface then there will be increased turbulance caused by the shear between the water jet and the surrounding water. However, the velocity of the jet is inversely proportional to z/d (Where z is the height of the jet exit and d is the jet diameter). This means that the further the nozzle exit is from the base plate the more the water jet is slowed by the interaction with the surrounding water.

The best height is where there is the perfect balance between the jet turbulance and the jet velocity. This is about z/d=5 in most of the experimental data I have come across.

I think the cascade is about z/d=4 probably due to the fact that the water surrounding the impingement jet is not stagnent due to the fact that it is coming back up out of the cup. This will slow the jet down more so it has to exit the nozzle closer to the impingement surface for the optimum heat transfer coefficient.

The graphs of experimental data I have come across show that the heat transfer coefficient drops off quite quickly either side of the optimum z/d value.

Hope this helps

01-02-2004, 11:17 AM	#4
WAJ_UK Cooling Savant Join Date: Oct 2003 Location: Sussex Posts: 109	sorry I meant d as the diameter of the jet. 4d is 4jet diameter and 5d is 5 jet diameter sorry it wasn't clear. If you increase the distance from the nozzle exit to the impingement surface then there will be increased turbulance caused by the shear between the water jet and the surrounding water. However, the velocity of the jet is inversely proportional to z/d (Where z is the height of the jet exit and d is the jet diameter). This means that the further the nozzle exit is from the base plate the more the water jet is slowed by the interaction with the surrounding water. The best height is where there is the perfect balance between the jet turbulance and the jet velocity. This is about z/d=5 in most of the experimental data I have come across. I think the cascade is about z/d=4 probably due to the fact that the water surrounding the impingement jet is not stagnent due to the fact that it is coming back up out of the cup. This will slow the jet down more so it has to exit the nozzle closer to the impingement surface for the optimum heat transfer coefficient. The graphs of experimental data I have come across show that the heat transfer coefficient drops off quite quickly either side of the optimum z/d value. Hope this helps Last edited by WAJ_UK; 01-02-2004 at 11:23 AM.