| With the requirement for measuring a very small mass flow-rate increasing in various areas such as semiconductor,medicine,chemical industry,environmental area and so on,sonic nozzle is widely used as the transfer standard for other types of gas flow meters,because of no moving parts,high accuracy,good repeatability and flow unaffected by the downstream conditions.In recent years,sonic nozzle with throat diameter less than 1 mm have been widely used,but its geometry is difficult to meet the structural requirements of the standard sonic nozzle in the international standard ISO 9300:2005.The empirical formula for the flow characteristics of the sonic nozzle in the standard is not applicable.It is necessary to accurately calibrate the micro-sonic nozzle by the higher standard facility to ensure the accuracy and consistency of the quantity.At present,there are few primary gas flow standard facilities involving small flow in China.The pVTt gas flow standard facility in National Institute of Metrology(NIM)which is the highest legal measurement institution of China is the national primary gas flow standard.It is responsible for the quantity traceability of over 80%sonic nozzles in China,and the minimum flow is 1 m~3/h.Due to the low machining precision of the micro sonic nozzle and the lack of the small gas flow facility with high accuracy,the experimental research on the flow characteristics of the micro sonic nozzle has not made great progress.In order to meet the demand for the quantity traceability of small gas flow in the fields of biomedicine and environmental monitoring in recent years,it is urgent for China to build the primary gas flow facility for the small flow range.The main task of this paper is to improve the capacity of the 100 L pVTt gas flow facility established by National Institute of Metrology for the small flow range,and to carry out an experimental study on the flow characteristics of the micro sonic nozzle.The main research results of the paper are as follows:(1)The 100 L pVTt standard gas flow facility was improved,which can realize the flow-rate as low as 0.016 m~3/h within the pressure range of(0.1~1)MPa.The effects of the inventory mass and leakage on the performance of the facility was re-evaluated.The measurement uncertainty of the 100 L pVTt facility in two flow ranges of(0.016~0.1)m~3/h and(0.1~5)m~3/h was analyzed.With the support of international comparison,the uncertainty and the measurement capability of the facility was verified.The expanded uncertainty of the discharge coefficient of the sonic nozzle is 0.10%(k=2)when the flow-rate is larger than 0.1m~3/h,while it is 0.15%(k=2)for smaller flow-rate.(2)Based on the 100 L pVTt gas flow standard device as the experimental platform,the experimental measurements were carried out for 6 sonic nozzles of ISO type with throat diameter range from 0.2 mm to 1 mm and 3 sonic nozzles of non-ISO type with throat diameter range from 0.2 mm to 0.4 mm with the stagnation pressures range from 0.1 MPa to1 MPa.The experimental results show that the maximum variation of the discharge coefficient for the same one nozzle of the ISO type can be 5.07%with the stagnation pressures range from 0.1 MPa to 1 MPa.The smaller the throat diameter of the sonic nozzle is,the more significant the influence on discharge coefficient with stagnation pressure is.Under the same conditions,compared with the ISO type sonic nozzle,the discharge coefficient of the non-ISO type sonic nozzle changes greatly.(3)The numerical simulation was used to analyze the influence of the shape of diffuser section on the flow characteristics of the micro sonic nozzle.It was found that the diffuser angle and the length of the diffuser section almost has no effect on the discharge coefficient for the sonic nozzle with throat diameter larger than 1 mm.The smaller the throat diameter is,the more significant the influence of the diffusion angle and the length of the diffuser section on the discharge coefficient is. |
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