In order to try to understand these effects a flow channel was devised by clamping a thin plate with a flow channel
cut in it between two plates of quartz glass which were sandwiched between two steel plates. A viewing region
was cut in the steel plates to enable the flow to be observed. The channel was 0,3mm wide and 3,0mm long, cut in a plate of 0,3mm thickness. The arrangement is shown in Figure 1.
A thin brass plate is clamped between two quartz glass windows by steel plates back and front. A viewing window is cut in each of the steel plates so that the flow channel is visible. Fuel is supplied via a hole through the front steel plate and the front window. The fluid flows through the model orifice in the brass plate between the windows and the flow can be observed using a camera. The images are illuminated using a stroboscope to enable the bubbles to be seen.
Figure 2 shows still images of the flows measured at an inlet pressure pin of 100bar and back pressures pout of 10, 5 and 0 bar. The flow is from left to right with the fluid flowing from the top and bending around the corner into the channel.
In the top picture, the inlet pressure is 100bar with a back pressure of 10bar. At the entrance to the orifice the flow turns the corner and a cloud of cavitation bubbles can be seen which dissolve further down the channel.
As the back pressure decreases and the flow velocity increases, the cavitation cloud fills the flow channel. At the exit, the cloud can be seen to expand into the volume behind the channel and the bubbles start to collapse in the middle of the flow stream.
At a back pressure of 0bar, the channel is filled with cavitation bubbles and at the exit there are significantly more bubbles as the back pressure is not only close to the vapour pressure of the fluid but there is also more energy in the flow.
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