| Microelectromechanical(MEMS)technology originated in the early 1960 s and refers to the coupling technology based on micron-sized mechanical structures with circuit structures.With the research progresses,increasing number of MEMS devices have been developed,among which the thermal micro flow sensor is one of the classic MEMS products.The thermal micro flow sensor has the advantages of lower power consumption,higher accuracy,more reliable performance,and the possibility of mass production.Accordingly,MEMS micro flowmeter has a wide range of applications,such as Internet of Things,automotive manufacturing,drug delivery and other fields,and it has a vast development prospect.The existing thermal micro flow sensor made by 0.18μm CMOS MEMS process achieves the flow measurement.Although its sensitivity is relatively high at low flow rates,its structure has not been further optimized.Therefore,a numerical simulation-based optimization of the micro flow meter is necessary in order to enhance its performance as well as minimize costs.Firstly,the background and significance of this research about MEMS sensors were discussed,various types of MEMS thermal micro flow meters and micro processing technologies were introduced briefly as well.The development of MEMS micro flow meters were summarized broadly.Besides,the research content and innovations of this paper were described.Secondly,basic theories concerning fluids and heat transfer were presented,including verification of the continuity assumption at microscale,methods of transferring heat energy,and conservation laws of numerical heat transfer.Also,the circuits that need to be applied in thermal micro flow meters were illustrated.A 0.18μm CMOS MEMS single-layer thermal micro flow meter was investigated and a doublelayer thermal micrometer was proposed.The computational and mesh models were established using ANSYS simulation software,and the parameter settings were completed.The comparison of simulation results with experimental ones indicates the reliability of models and the feasibility of the double-layer thermal micro flow sensor.Next,improvement measures were proposed for the single-layer thermal micro flowmeter from two perspectives: the thermal sensing part and the micro flow channel structure,respectively.The results demonstrate that the measurement range is widened,the sensitivity is improved,and the robustness is enhanced when the space between detectors and the heater is 4.6 μm and supported by the bridge structure,and an obstacle is added 2 μm from the front of the upstream detector.For the microchannel,it can be found that widening the length to 520 μm or 800 μm is also beneficial to the sensitivity and accuracy by varying the length of the small cavity of the MEMS wafer.In addition,a multi-range sensor that could effectively reduce power consumption was raised based on the asymmetry of the heater wire.To enlarge the application scenarios,nitrogen was replaced with argon and carbon dioxide,and the results show that different gases have the same response to the thermal micro flow meter.Then,a parametric study of the double-layer thermal micro flow meter was carried out,i.e.,the preferential parameters of the structural parameters were determined according to the computational domain model.The velocity field and temperature field distributions in the flow channel of the double-layer micro flow meter were carefully analyzed to further reveal that the flow separation phenomenon was the limiting factor for obtaining improved sensor performance.Taking accuracy and sensitivity into account,the switching threshold of the two flow channels was determined to be 63m/s,and the measure range of the sensor was 0-145m/s.In addition,the distributions of viscous dissipation near the thermal sensing part were given quantitatively,revealing the cause of sensor failure.Finally,the research of this paper was reviewed,the innovations of the study were listed,the shortcomings of the work were noted,and an outlook on the future development of thermal micro flow meters was outlined. |
