University of Twente Student Theses


Characterisation of Low Flow Valves for Gases

Hebbink, R.H.J. (2018) Characterisation of Low Flow Valves for Gases.

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Abstract:Bronkhorst High-Tech develops and assembles mass flow meters and controllers for low flows. In order to serve requests to control increasingly lower flows, the Piezo valve has been developed. However, it appeared that the valve capacity could not be characterized by the Kv-model as used for the common magnetic valves. Rather than the orifice diameter, as for the magnetic valves, the throughflow height is restrictive for the flow capacity of the Piezo valve. Various correction methods to the Kv-model were tested in earlier studies, but found to be inappropriate. In this study, a new, theoretical, model to predict the gaseous mass flow through the Piezo valve was derived from the Navier-Stokes equations with a slip condition at the boundary. The geometry was simplified to an axisymmetric disk, the flow was assumed steady and isothermal and the gases were assumed to be ideal. By non-dimensionalising the equations, the less important terms were identified and neglected. The resulting equations could be solved algebraically. After that, flow choking was implemented in the model as a critical situation, and two fudge factors were implemented to allow empirical corrections for effects that were not taken into account. The model was tested by conducting measurements on a test Piezo valve. It was found that a height-based correction and a gas-specific correction, that has to be determined in a calibration measurement, were required to correctly approximate the mass flow with the model. With these corrections, the model was capable of predicting the mass flow for valves with a shim height up to and including 25 µm to a relatively high accuracy in non-choking flows, and to an acceptable accuracy for more critical flows. Deviations between the model and the measurement data existed due to the influence of choking, especially in the transition zone. The flow through the valve with a shim height of 50 µm was found to be characterised by a different relation between pressure drop and mass flow, and hence the model was not valid for this height. By introducing safety factors to the model, it was guaranteed that the desired flow can always be reached by a shim height that is selected with the model. This allows BHT to use the model in practice. The safety margin was relatively large for certain situations due to (the transition to) choked flow. Further research is required to explain the empirically determined correction factors, to further validate the model and to expand the validity of the model to (and to optimise it on) choked flow and flows through valves with larger shim heights.
Item Type:Internship Report (Master)
Faculty:ET: Engineering Technology
Programme:Mechanical Engineering MSc (60439)
Keywords:Navier-Stokes, Slip Flow,, Microchannels, Kv-value
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