Study On The Electric Heating And Thermal Radiation Properties Of The Carbon Heating Element

Carbon fiber and carbon composite have excellent thermal radiation property, and the carbon heating element used it as radiation source has favorable wavelength matching ability and heating efficiency. In order to obtain high quality carbon heating element, it is important to carry out application study on the electric heating and thermal radiation property of self developed carbon fiber and carbon composite heating elements. The researches on the correlation between preparation process and electric heating properties of element, as well as the change law of thermal radiation property with phase structure and preparation process of carbon fiber and carbon composite can provide help for the preparation of high performance carbon radiation material. In this work, the failure problem analysis was carried out based on self developed carbon fiber and carbon composite heating elements, with the purpose of process optimization, the twisting and winding equipment for ply yarn carbon fiber heating core and the high temperature holding equipment for carbon composite heating core were developed, on the basis of preparing high quality carbon heating element, the related electric heating properties and mechanism analysis of which were measured, then, the change law of thermal radiation properties of carbon fiber and carbon composite was systematically and deeply studied, at last, with the view of engineering application, the numerical simulation of radiative heat flux density distribution of carbon composite heating element was carried out.According to the failure cause summary of abandoned elements, it is determined the twisting and winding process of carbon fiber ply yarn and high temperature holding process of carbon composite were key steps during the preparation of carbon heating element. On the basis of which, the twisting and winding equipment for ply yarn carbon fiber heating core was manufactured and the process calculation model was established, meanwhile, the high temperature holding equipment for carbon composite heating core was developed and the removal process of holding heating core was optimized. The performance tests show that the power deviation of carbon composite heating elements after process optimization can be controlled in the range of 5W, the uniformity of surface temperature was better, the service life can reach 12000h, the resistance variability rate was less, safety performance at high voltage load was better, the distribution of thermal radiation energy was stronger in the range of 2～20μm.The correlation between preparation process and related electric heating properties of carbon heating elements (relationship between power density and temperature, resistance temperature characteristic and thermal inertia) were systematically studied, the results show that the sizing agent on the surface of carbon fiber electric heating body can be removed by means of the instant high temperature method in nitrogen, the ply yarn carbon fiber heating element with supporting core from which has relatively higher stable working temperature under the same power density, the resistance variability rate was lower, and the thermal inertia was smaller. In addition, the existence of supporting core goes against the improvement of electric heating property of element, by removing supporting core, using carbon fiber ply yarn and combining with the reasonable holding process, the carbon composite heating element without supporting core can have relatively higher stable working temperature under the same power density, lower resistance variability rate and smaller thermal inertia, and all the electric heating properties of which is better than that of the carbon felt element.The heat transfer differential equation of carbon heating element was established, and the mathematical expressions for stable temperature rise and instant temperature increasing time can be obtained from solving above transfer equation. The analysis shows that the thermal radiation properties of carbon heating core were key factors, which affected the relationship between power density and temperature and the thermal inertia, the stable working temperature can be improved and the instant temperature increasing time can be reduced by increasing total emissivity of carbon heating core. The relationship between resistance variability rate of element and the characteristics of ends' connections, the initial resistivity of carbon material, the analysis shows that the smaller the initial resistivity of carbon material, the more the overlapping contact of filament, the higher the contact pressure in the ends, and the more the contact point, the less the resistance variability rate of element.The normal spectral emissivity and normal total emissivity of carbon fiber heating core with different fibrous type, high temperature graphitization and surface desizing treatment were measured by using automatic spectrum radiation testing system, and the phase structure analysis was carried out by using XRD and LRS. The change relation between phase structure and thermal radiation property of carbon fiber shows that the turbostratic graphite structure of T700 carbon fiber was relatively more regular than T300, the enhancement of carriers radiation mechanism and graphite crystallite lattice vibration modes made the normal spectral emissivity of T700 carbon fiber in the ranges of 2500～5000nm and 5000～6500nm relatively higher than T300 carbon fiber, the decrease of disturbance structure such as amorphous carbon weakened corresponding vibration modes caused by distortion structures and impurities and defects, which made the normal spectral emissivity in the ranges of 6500～13000nm lower, the wavelength dependence of spectral emissivity for T700 carbon fiber was obvious, and the normal total emissivity was relatively lower. After being graphitized, the degree of regular structure of T300 carbon fiber and domestic carbon felt were improved, the enhancement of carriers radiation mechanism and graphite crystallite lattice vibration modes made the normal spectral emissivity in the ranges of 2500～5000nm and 5000～6500nm obviously higher, the radiation mechanism caused by distortion structures and impurities and defects were weakened, the spectral emissivity in the range of 6500～13000nm decreased, spectral emissivity demonstrated obvious wavelength dependence, and the total emissivity decreased. Compared with the instant high temperature method in nitrogen, the instant high temperature method in oxygen and nitric acid liquid phase treatment caused different etching degree of carbon phase structures, the carriers radiation mechanism was weakened, which caused the decrease of spectral emissivity in the range of 2500～5000nm, the structure of the turbostratic graphite was relatively stable, the degree of etching during treatment was relatively lower, which made the spectral emissivity in the range of 5000～6500nm relatively stable, but the disturbance structure such as amorphous carbon had comparatively poor regularity, the level of etching was greater, the corresponding radiation mechanism was weakened, which caused the decrease of spectral emissivity in the range of 6500～13000nm, the wavelength dependence of spectral emissivity was improved, and the total emissivity was reduced. Under the combined action of carrier radiation mechanism and the graphite lattice vibration radiation mechanism together with the radiation mechanism caused by distortion structures and defects, the carbon fiber showed a poor wavelength dependence of normal spectral emissivity, which had a greybody characteristic.The normal spectral emissivity and normal total emissivity of carbon composites were measured, and the surface morphology of samples were characterized by scanning electron microscope (SEM), the change relation between normal spectral emissivity and normal total emissivity of carbon composites and fiber distribution, carbon composition, impregnation densification process and surface morphology was investigated. The results show that the extent of loose structure can be changed by different fiber distribution, the normal spectral emissivity and total emissivity of carbon fiber perform by adopting ply yarn process and carbon felt were higher than carbon cloth perform, and the normal spectral emissivity and total emissivity of porous C/C performs reinforced by short-cut fiber were also relatively superior to that reinforced by carbon fiber cloth. The carbon composition have great effect on the change of thermal radiation property, the proportion of disturbance and distortion structure in resin carbon matrix was relatively larger than fiber carbon, the thermal radiation property of resin carbon was better than that of fiber carbon because of more distortion lattice vibration modes and local vibration modes caused by impurities and defects, this is the reason why thermal radiation properties of C/C performs reinforced by carbon fiber cloth and short-cut fiber were superior to that of its corresponding fiber performs. With the increase of impregnation densification time, the normal spectral emissivity and total emissivity of carbon composites firstly decreased and then increased, the difference of total emissivity between two samples gradually reduced, the increase of resin carbon content was favorable to improve emissivity. After being polished, the surface roughness of carbon composites decreased significantly, the normal spectral emissivity and total emissivity of samples obviously decreased .The mathematical models of radiative heat flux density distribution for single carbon composite heating element and multiple parallel-arranged carbon composite heating elements were established, the numerical calculation and visual analysis of which were carried out by software of MATLAB, the results show that the radiative heat flux density of single element increased with the increase of emissivity of carbon composite, the non-uniformity of heat flux density also increased. The radiative heat flux density of single element increased with the increase of emissivity of quartz, also the non-uniformity of heat flux density also increased. With the increase of outside radius of quartz tube, the radiative heat flux density of single element firstly increased, under the setting initial condition, the heat flux density decreased rapidly when the outside radius of quartz tube was equal to 0.45cm, the non-uniformity of heat flux density slowly increased with the increase of outside radius of quartz tube firstly, and then had little change. The radiative heat flux density of single element improved with input power, the non-uniformity of heat flux density firstly decreased rapidly and then mildly. The radiative heat flux density distribution of multiple elements was affected by the distance between axes of element and the heated surface, the distance between axes of two adjacent elements and the input power distribution of different elements. Under the setting initial condition, when the distance between axes of element and heated surface h was equal to 12cm, the distance between axes of two adjacent elements s was equal to 15cm, the power distribution of three elements was 1000W-600W-1000W, the radiative heat flux density field presented flat arched distribution, and the relative uniform radiative heat flux density field can be achieved by alternate power distribution of multiple parallel-arranged elements.