A Novel Dual-mode Micro Thermal Flow Sensor With Conjugated Heat Transfer Simulation

Micro-Electro-Mechanical System?MEMS?,a leading technology in 21 th century,is a system which realizes electro-mechanical convert function in micron-size.Micro flow sensor is one popular kind of MEMS,compared with tradition one,it has superior advantages like low-cost,fast-respond,good-accuracy and wild applications in such as automobile,aerospace,biology,clinical diagnosis and so on.Micro thermal flow sensors have been investigated extensively due to their simple structure and reliable performance,wherein calorimetric is preferred since it can detect flow direction and at low flow rate it has high sensitivity.However,at large flow rate,it behaves saturated and even invalid.This work proposes a novel dual-mode thermal sensor which can flexible switch between hot wire mode and calorimetric mode hence extends measurement range with high sensitivity.The paper using 2D,compressible,property variable conjugated heat transfer?coupled fluid-solid heat transfer?simulation numerical presents flow pattern in the micro flow channel of the chip,which is widely ranged from 0.01 SCCM to 160 SCCM.In addition,its influences on the temperature filed distribution of the chip are also studied.At low flow rate,thermal diffusion domains,temperature perturation caused by convection can be deemed as linear,hence temperature difference between upstream and downstream detectors is linear.When flow rate increses,boundary layer around solid domain is destroyed and more heat is transported downstream.Due to the nature of convection,temperature difference tends to flat.If flow rate increases further,heat dissipation imposes great effect on detectors,especially on lead edge.Detectors rise temperature due to drastic heat generated by friction and Temperature difference drops with increased flow rate,which means invalid detectors.In addition,flow pattern in channel becomes complicated at large flow rate,for which 1D model preditction with uniform velocity inlet assumption fails.Also,this work gives some dimensionless numbers about critical detectors response,which provide reference for future optimal design according to principle of similitude.The power response of heater keeps good sensitivity at large flow rate.It is fitted by modified King's law with fine correlation.For compressible flow,if additional compression term M²_mi/Re_mii is considered,correlation may be better.The experimental chip of the sensor system is preceded by InvenSense 0.18?m CMOS MEMS technology.After simple package and circuit conection,the chip is able to convert flow into electrical signal that then is read by microcontroller?Arduino Yun?.Calibration is conducted using 2 commercial flow sensors as reference.Later,a critical flow rate,by which either calorimetric or hot wire mode works,is determined according to higher sensitivity.And different functions are used to fit 2 modes.The results compared between present work and 2reference sensors show that the design has good precision with relative error less than±2%.What is more,measurement range expands by nearly 3 times.The maximum flow velocity is73m/s calculated at position across heater.Additionaly,the microcontroller of the protype system is integrated with WIFI module,which enables data acquired uploading to cloud server.It is promising IOT flow monitor in HVAC system of future energy-efficient building.