Thermal Conductivity Of Fiber Bragg Grating Temperature Sensitive Sensor Research

Thermal conductivity is an essential parameter of the thermophysical property of material, which reflects the power of conducting heat along/through the material.In the thermotechnical industry, thermal conductivity is the quantitative basis to ensure whether the material can satisfy the thermal process or not, and it's also a key parameter, in designing thermal system. The material thermal conductivity becomes an essential way to know the material thermophysical property. The reason that the thermal conductivity of the material are effected by its components, moisture content, temperature, etc.makes the laboratory measurement method the only way to measure the thermal conductivity. But the traditional laboratory measurement method requires fieldwork sampling which usually causes damage to the integrity of the material, thus the on-site measurement can not be carried on. By inserting or burying probe needle in the material, the hot probe method of the hot-wire model at present can measure the average thermal conductivity of along hot-wire of the material but not the partial thermal conductivity.In this article, the fiber Bragg grating sensor for measuring the materials thermal conductivity by using the steady-state method is developed,and the fiber Bragg grating is attached at the surface of the positive temperature coefficient thermistor of heating element with an cylinder-shaped aluminous shell.When the heating element is heated in the material to reach thermal balance and form a steady temperature distribution, which leads to the steady wavelength shifts of the fiber Bragg grating that attached at the surface of the heating element, to achieve thermal conductivity of the partial measurement. The thermal expansion of aluminum shell is larger than the fiber Bragg grating, which causes the additional wavelength shifts of the fiber Bragg grating, increasing the temperature sensitivity of the sensor. The main completed work as follows:1. According to the structure of the sensor, the mathematical relationships between Fiber Bragg grating wavelength shifts and the materials thermal conductivity is established.2. According to the material choice and size design of the sensor, the characteristics of the sensor were calculated. When the diameter of the heating element takes 10.57 mm, working voltage takes 100 V, working internal resistance takes 4040.161Ω.the sensor buried in sandy soil,the initial temperature of sandy soil is 10℃,the thermal conductivity is 0.83 W/m·K. Heating to reach thermal balance, the theory temperature sensitive coefficient of the sensor is 23.6 X 10^-6℃^-1,wavelength shifts is 1665 pm.3. Through the water-bath method,thermal sensitization of temperature sensor by attaching at the metal surface of the heating element is verified, and sensitive coefficient of experiment is18.32X 10^-6℃^-1.hen the center wavelength of Bragg grating is1550 nm, and its theory temperature sensitive coefficient is 6.0X10^-6℃^-1,the temperature sensitive coefficient of this experiment is increased 3.04 times.Each item of static performance indices of sensor as follows:nonlinearity error:1.24%FS;repeatability error:3.47%FS.4.The temperature-rise test of the different material indicates that the variance of the stability wavelengths values of the sensor were significant differences. The maximum difference was 564 pm and 20.4℃about the stability wavelengths values and the temperature values.