Formation of a uniform velocity profile in the section of a profiled channel of reference installation
摘要
We study the velocity profiles in the gravity and pressure channels of flow switches of the reference installations in the field of measuring the liquid flow rate and the amount of liquid. The disadvantages of the design of reference installations are described. If a reference installation operates for a long time, then, due to its imperfect structure, it is impossible to get an almost uniform profile of the flow velocity in sections of the profiled channel of pressure nozzles. It is demonstrated that the procedure of mounting of various straightening devices in the profiled channels of pressure nozzles of the flow diverters of reference installations does not lead to the creation of an almost uniform profile of the flow velocity. An approach to the profiling of the gravity channel of a flow diverter capable of getting an almost uniform profile of the flow velocity is proposed. We experimentally justify the hydraulic efficiency of the geometry of flow-through part of the profiled channel of the flow switch of the ÉU‑3 reference installation from the composition of the GÉT 63-2019 State Primary Special Standard of the Units of Mass and Volume of Liquid in a Flow and of the Mass and Volume Flow Rates of Liquid. The influence of the shape of flow velocity profile in a section of the profiled channel of flow switch and of the stability of the main hydro- and thermodynamic parameters of the water flow (flow rate, absolute pressure, and temperature) in a pressure pipeline on the metrological characteristics of the reference installation is substantiated. Experimental studies of the velocity profile of a free water flow in a section of the profiled channel are performed with the help of high-speed meters of the total pressure and Pitot tubes. We obtain almost uniform velocity profiles of the water flow in a section of the profiled channel with free water flow within the investigated range of Reynolds numbers Re = (35–300)·103. We also perform the comparison of the velocity profiles of water flow in sections of the profiled channel of the ÉU‑3 reference installation and of the flat pressure nozzle of the PTB reference installation. The presented results confirm the long-term stability of the given mass water flow rate. Equations for finding the mass of water in the transient processes are deduced with regard for the variations of thickness of the water flow and the flow rate from zero up to given values. The proposed approach and the accumulated results prove to be useful for the manufacturers of reference installations and can be used for the design, creation, and updating of the profiled channels of flow switches of the reference installations.