| MEMS is one kind of high-tech device or intelligent system with a size of a few millimeters or less,and its structure is generally of the order of micrometers and nanometers.In recent decades,micro-flowmeters based on micro-electro-mechanical systems have been studied deeply due to the advantages,such as small size,high reliability,low power consumption etc.A lot of researches have been devoted to improve the performance of the flowmeter,which is expected to be reasonable in price,reliable in performance and high accuracy in measurement.Up to now,various types of flowmeters have been proposed and employed in many fields,such as aviation,clinical surgery,construction industry and automobile manufacturing etc.In addition,compared with other flowmeters,thermal micro-flowmeters have a higher market share because of their simple structure and reliable performance.In order to furthermore research the thermal sensing micro-flowmeter,the internal flow and temperature distribution of the thermal sensing micro-flowmeter have been analyzed.Moreover,the methods that can further improve sensitivities and expand the measuring range also have been explored by numerical simulation.At first,the domestic and foreign researches and related technologies of microflowmeters have been listed,from which it can be found that the sensitivity of the micro-flowmeter has been improved by degrees,the response has been further accelerated,the range has been continuously expanded,and the fields of application have also been gradually expanded in recent years.Secondly,the applicability of continuity hypothesis for fluids in microchannels has been analyzed.Besides,the three conservation equations and other related equations of fluids have been listed.What’s more,the working principle and circuit principle of thermal sensing micro-flowmeters have also been briefly introduced.Thirdly,the geometric model of the thermal sensing micro-flowmeter has been designed and the related works such as modeling,grid division,solution setting,simulation have also been conducted.In addition,according to the different conditions,a large number of numerical simulations have been carried out,and the post-processing module have been used for data analysis,from which some conclusions have been drawn.For example,numerical simulation has been used to study the internal conditions of the fluid channel under the inlet boundary conditions of 0～165 SCCM(standard-state cubic centimeter minute),from which it’s found that the influence of thermal diffusion is the greatest in the velocity range of 0～2 SCCM.Moreover,an increasingly large flow vortex has also been found at the sudden expansion of the channel by the increased flow velocity,which will reduce the local heat transfer efficiency and destroy the linear response of temperature difference to the flow velocity.Besides,the results also show that viscous dissipation appears and gradually affects the linear response when the flow velocity exceeds 75 SCCM.Furthermore,a series of attempts have been conducted by changing the design of the model,which can be used to improve the performance of the thermal sensing microflowmeter.Specifically,the temperature of different heat sources has been analyzed in order to verify the applicability of the heat source temperature.Besides,the study of a double bridge model has been investigated.The simulation results show that the heat and the detector would vibrate with the rapid flow of the fluid in high flow condition,and compared with the single bridge model,the structural stability of the double bridge model is significantly higher.Moreover,the research of changing the distance between heats and detectors has also been explored,which is speculated that with the narrowing of the distance between heats and detectors,the range of the micro-flowmeter will be reduced,and when the distance between heats and detectors is less than a certain value,the micro-flowmeter will lose its effect.In addition,the influence of adding an obstacle in front of the upstream detector has been examined,from which it’s found that the upstream temperature is higher than that of the model without the obstacle when the flow velocity in the range of 0～75 SCCM.Finally,the research content of this paper has been briefly summarized,and the problems that may exist in the research process have also been analyzed.Besides,the future development of micro-flow sensor has been prospected,too. |
