Maintaining a constant vacuum level in a milk extraction system, independent of the amount of air consumption, is essential for ensuring high milk quality as well as the good health of the cows. The system valve is one of the controls that ensures this. Recently, engineers at ÅF-Kontroll used FLUENT to analyze the control valve in a DeLaval milk extraction system. The objectives of this CFD study were to gain a better understanding of the features of the flow through the valve, and to reshape some of the valve's components in order to improve its operating characteristics.

The control valve in its original design

In principle, the control valve is similar to a needle valve, where a cone on a rod restricts the flow from ambient pressure through a pipe into the vacuum system. A force balance controls the position of the cone. The cone is pulled down by the low pressure in the throat and pulled up by a vacuum chamber situated above the cone rod. Close to the point of milk extraction, a sensor is attached to the vacuum system, which adjusts the pressure in the chamber.

The analysis involved computations at several operating points, and the results were compared with measurements to see if the aerodynamic force prediction by FLUENT was in agreement with measurements. It was, and the decision was made to analyze a set of valves with modified internal geometry.

An intense preliminary design phase was initiated to evaluate a large number of prototypes. The merits of each prototype were assessed using CFD computations and experimental measurements. The computations provided the following infor- mation: qualitative insight into the physics of the flow at different operation points, visualization of the compression/expansion patterns in the throat region, and estimation of the forces acting on the cone as a function of the flow through the valve. The experimental measurements were set up to obtain pressure loss data, verify the computational results, and detect possible flow instabilities.

Distribution of Mach number in the throat region of the control valve

ÅF-Kontroll has a tradition of creating parametric models for all of their analysis work. In this design process, the engineers took advantage of the journal file concept in GAMBIT. Since the model was to be analyzed with many different positions of the cone, a parameter was created for the vertical gap between the cone and the seat. A 2D hybrid mesh with quadrilateral and triangular elements was automatically created in seconds for each operation point. As the design work proceeded, more parameters were introduced to control the internal geometry of the valve.

The axisymmetric, segregated solver was used for the turbulent simulation. An input file was created with instructions to FLUENT to solve the flow for all operating points in sequence. This allowed the computational portion of the design process to be highly automated. The flow in the vicinity of the throat was of great interest, since it is directly related to the pressure acting on the cone. Quadratic cells were used in this region, and the parameter-based resolution was carefully tailored to capture shock waves and separation zones at all operating points. Several turbulence models were tested to determine the sensitivity to turbulence intensity.

Total pressure of the flow through the control valve in the middle of its operating range

The results from this investigation provided the engineers with a deeper knowledge of the flow through the control valve. This led to development of a control valve with improved operating characteristics.

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