INFLUENCE OF SURFACE ROUGHNESS ON THE FLOWRATE BEHAVIOUR OF SMALL CRITICAL VENTURI NOZZLES
G. Wendt, E. von Lavante
Abstract:
The paper describes the results of experiments carried out with a set of 50 critical Venturi nozzles with diameters between 0.15 mm and 10 mm with special emphasis on small nozzles with diameters of less than 1 mm. All nozzles were shaped in accordance with the current ISO Standard 9300, only the quality of surface finish having average roughness heights between 0.1 mm and 1 mm differed from the requirements of this standard. The present investigations contain the determination of common relations used to characterize critical flow through Venturi nozzles and their dependence on surface quality. The paper also offers photographs of different inner nozzle surfaces and the results of mechanical throat scans of larger nozzles as well as the results of computational simulations of flow characteristics through a nozzle with comparable imitated surface perturbation. All results of the investigations come to the same conclusion: No systematic or significant difference between the flowrate behaviour of small critical Venturi nozzles could be found when the surface roughness was changed between 0.1 mm and 1 mm (this range of surface roughness corresponds to the usual manufacturing surface qualities, from common lathe finish to well polished surface). Therefore, critical nozzles can be used without changes in their flowrate behaviour even in cases where the requirements of the valid standards can not be strictly met.
Keywords:
critical Venturi nozzle, ISO standard 9300, discharge coefficient, critical pressure ratio, nozzle surface, CFD
