Influence Of Wall Roughness On The Flow Character Of Critical Sonic Nozzle

Sonic nozzle, also known as the critical Venturi nozzle, can be used for directmeasurement of gas flow and transfer standard of other measurement devices. Due tothe simple structure, easy maintenance, stable flow character and wide working range,it is widely used in numerous fields including petrochemical industry, aerospace,energy conservation and environment protection.With the further research of sonic nozzle in various national institutes of metrology,to improve the accuracy of the measurement, the research of discharge coefficient asan important parameter of sonic nozzle are deepening. Due to the advantage of sonicnozzle on small flow, the research of discharge coefficient is quite active. Meanwhile,sonic nozzle is increasingly applied under ultrahigh pressure conditions. Thus, thethroat Reynolds number can exceed107, the influence of wall roughness on dischargecoefficient should be studied systematically. According to the request of surfaceroughness to sonic nozzles by ISO9300, sonic nozzles can be divided into ‘normallymachined’ and ‘accurately machined’ and the corresponding the discharge coefficientequations are given. However, ISO9300does not refers to the influence of wallroughness on the discharge coefficient, and the two equations intersects with eachother at low Reynolds number. Up to now, the research of wall roughness on thedischarge coefficient of sonic nozzle is almost a gap, with the further research of sonicnozzle, the influence of roughness should be studied systematically. Thus, in thispaper, sonic nozzle models with throat diameter ranging from0.5mm to100mm arebuilt. Standard k-epsilon model and unequilibrium wall functions are used to resolvethe core turbulence area and near-wall region respectively. The validity of the modelsare confirmed by experiments and ISO equations. The influence of roughness onnozzle flow field is analyzed from the velocity distribution, density distribution of thenear wall region and the boundary thickness. By numerical calculation of differentmodels with Reynolds number ranging from105to1010using single one ideal gas, itis proved that the discharge coefficient of sonic nozzle is influenced by the relativeroughness and the regulations are analyzed theoretically. Since the boundary layer isinfluenced by multi-dimensional flow, and the multi-dimensional flow is affected bythe gas isentropic exponent and nozzle curvature, so the influence of wall roughness on gas with different isentropic exponent and nozzle curvature are analyzed. Toresearch the physical property of real gas, an intelligent algorithm basing on stepregression is proposed which can be applied to calculate the empirical function for thereal gas physical property and the real gas isentropic exponent are obtained whichindicate the influence of wall roughness on real gas is related to the real gas isentropicexponent and nozzle curvature.