Research On Flow Mechanism And Measurement Characteristics Of Ultra-fine Critical Flow Nozzles

The critical flow nozzle is widely used in the fields of aerospace,petrochemical,biomedical and other fields as an international universal flowmeter.In recent years,the development of industrial technology has put forward higher requirements for the performance of critical flow nozzles.And the critical flow nozzles at the ultra-miniature scale have been further studied.However,it is difficult to process critical flow nozzles that meet the ISO9300 standard at ultra-miniature scales.In order to balance the contradiction between the processing difficulty and accuracy requirements of sonic nozzles at a small scale,three new types of critical flow nozzles are discussed.Based on the aerodynamics basis,the flow mechanism of a standard critical flow nozzle at an ultra-miniature scale is analyzed.With reference to the latest research results,the internal flow field of the critical flow nozzle is analyzed,and the influence of the internal flow state of the nozzle on the discharge coefficient is discussed in depth.In the range of 0.03 mm to 0.12 mm throat diameter,three new types of nozzles,such as constriction nozzle,diffusion nozzle and conical inlet Venturi nozzle,are designed.According to the structure of the critical flow nozzle,six geometric structural factors are set according to the division of the constriction section,the diffusion section and the throat.The fluid mechanics method and numerical simulation are used to discuss the influence of geometric size on the flow characteristics of critical flow nozzles with different structures.Through numerical simulation experiments,it is found that the throat diameter is the primary factor affecting the flow characteristics of the three new nozzles.The larger the throat diameter,the better the flow characteristics.The length of the part affects the discharge coefficient of the shrink nozzle and the diffusion nozzle.The smaller the throat length,the better the discharge coefficient;the diffusion angle affects the discharge coefficient and back pressure ratio of the conical inlet Venturi nozzle,but when the diffusion angle is larger than 5 °,the discharge coefficient is almost unchanged.Compare 3 new types of nozzles with standard critical flow nozzles,considering the flow characteristics and processing difficulty,the diffusion nozzle is a better critical flow nozzle.In order to verify the credibility of the simulation experiment,five groups of shrinkage(diffusion)nozzles were processed.And a super-miniature critical flow nozzle test platform was established to measure the mass flow rate and discharge coefficient of the nozzles.Results show that the error between the experimental results and the simulation results is within 2% except for a set of nozzles on the smallest scale,which proves that the simulation results are credible.