CFD Works on Occurrence of Cavitation at the Downstream of Orifice

One of wide used flowmeter is an orifice flowmeter, which is installed inside pipes as figure 1. Flowrate is converted into by the pressure difference between the upstream and downstream of orifice. Definition of cavitation is formation of gas bubbles from a flowing liquid in a region where the pressure of the liquid falls below its vapor pressure. Cavitation occurs when a liquid is subjected to rapid changes of pressure by something like orifice, pump suction and discharge or sudden area contraction causing the formation of gas or vapor bubbles in the lower pressure regions of the liquid. During cavitation, vapor bubbles make iterative growth and collapse process damaging pipe and components. Figure 2 shows general 4 step process of cavitation. There are some codes about installation of orifice for accurate measurement. ASME PTC code is representative. This code requires maintaining the upstream length of orifice plate for assuring that inlet velocity is developed fully. In this context, PTC code published at 1974 was revised at 2004. And PTC code published at 2004 present more conservative requirement. It means criteria on fully developed flow was changed. If flows are not developed fully, the inlet velocity may be distorted and it leads the change of local static and total pressure. Therefore, unexpected results could be occurred such as cavitation, measurement distortion and etc. In this study, unexpected cavitation by distorted velocity profiles was investigated using commercial CFD code, CFX 13.0