Research Of Thermal Flow Sensing Technology Based On Fiber Bragg Grating

Flow measurement is an important part of modern measurement technology,the thermal mass flowmeter is a common type among the many flow measurement instruments.Traditional thermal mass flowmeter generally uses electrical signals for measurement,so it is susceptible to electromagnetic interference and is not suitable for petrochemical and other flammable and explosive industries.Currently,in the field of sensing,fiber optical grating has the advantages of high sensitivity,high resistance to electromagnetic interference,and small size.Combining the fiber optic grating sensing technology with the traditional thermal flowmeter,using optical signals instead of electrical signals for sensing,can effectively improve the performance of the traditional thermal flowmeter and make it to the direction of miniaturization,multi-functional and all-optical sensing.Therefore,the thermal flow sensing technology based on fiber grating is of great research significance.This paper conducts a systematic analysis and summary of fiber grating thermal flow sensing technology,and carries out all-optical flow measurement and high-sensitivity sensing research,the main works are as follows:(1)Focusing on the findings about the prevailing fiber grating thermal flow sensing technology both domestic and international,four key aspects of this sensing technology are summarized and proposed: fiber optic heating technology,sensing structure design,temperature compensation,and packaging and sensitivity enhancement.In addition,the principle of optical fiber grating thermal flow sensing technology is elaborated,the calculation equation between flow rate and the center wavelength of optical fiber Bragg grating and its derivation process are given,and numerical simulation is carried out to analyze the effect of heat exchange size and heating power of the sensor on the sensitivity of flow sensing.(2)In the aspect of all-optical flow measurement research,a hot-wire flowmeter based on fiber Bragg grating is proposed,which achieves self-heating by using cobalt-doped high-attenuation fiber.The study performed sensor fabrication,temperature sensing characteristics measurement,and flow rate measurement experiments.The results show that the fiber Bragg grating has a temperature sensitivity of 10.3 pm/°C based on the cobalt-doped fiber with an attenuation coefficient of 8.9 d B/cm(～1480 nm),and the flowmeter can realize highly repeatable measurements in the range of 400 L/h to 3700 L/h,flow rate and the central wavelength of the fiber Bragg grating follow a specific non-linear function,the average sensitivity of the flowmeter is about 6.3 L/h/pm.(3)In the aspect of high-sensitivity sensing research,an extrinsic-phase-shift fiber Bragg grating(EPS-FBG)structure is proposed and a hot-wire flowmeter is designed based on this structure.The EPS-FBG is obtained by cutting a common FBG to form a micron gap and filling it with epoxy resin polymer,and the reflection spectrum shows a phase shift peak with a very narrow bandwidth,the flow measurement is realized by monitoring the wavelength change of the phase shift peak.Experiments on temperature sensing characteristics show that the EPS-FBG has a high-temperature sensitivity of 28.3 pm/?C,and the linearity of temperature is 0.996.Flow measurements were performed in the range of 300 L/h to 4000 L/h,the results show that: the relationship between flow rate and phase shift peak of the flowmeter follows a specific non-linear function with compliance of more than 0.998,at a heating power of 33 W,the average sensitivity of the flowmeter was measured to be 12 L/h/pm,and at lower flow rates the sensitivity was up to 1 pm/L/h.The proposed EPS-FBG provides an innovative idea for micro-flow measurement and high-sensitivity measurement.