铝电解槽用石墨化阴极炭材料热物性测试研究.pdf

Determination and Study on Thermo physical Properties of Graphitized Cathode Carbon Material for Aluminum Reduction Cell1 Chang hong WANG Jie min ZHOU School of Energy and Power Engineering School of Energy and Power Engineering Central South University Central South University Chang sha Hunan PRC 410083 Chang sha Hunan PRC 410083 windyploy jmzhou Abstract In this research a transient experimental system was designed to determine the thermo physical properties of the graphitized cathode material for aluminum reduction cell by employing modern electrical facilities and data acquisition apparatus according to the experimental modeling and the characteristic of graphitized cathode carbon The periodic heat flow was recognized by the two thermocouples as a sinus temperature variation with decreasing amplitude and increasing phase lag when the heat wave propagated through the specimen The thermal diffusivity can be calculated directly by processing experimental data and then the thermal conductivity can be obtained with relation to thermal diffusivity if the specific heat and the density are known The method and the instrument are applicable in the temperature 20 300 The research on the method and the measuring system will pave the way to obtain the feasible thermo physical properties data Key words Periodic heat flow method Thermal conductivity Thermal diffusivity Graphitized cathode Aluminum reduction cell 1 Introduction At present cathode carbon material for Aluminum reduction cell is mainly semi graphitic carbon which uses anthracite coal and graphite fragment as raw material in China Compared with graphitized carbon block the resistivities of heat shock and sodium corrosion penetration of semi graphitic carbon block are bad the thermal conductivity is low and the electric resistivity is high Those bad lining materials make them difficult in further developing aluminum electrolytic technology of China 1 2 The application of graphitized cathode carbon material is presently unprecedented in China In order to understand more about the thermo physical properties of graphitized cathode carbon material and to improve its quality a classical transient thermo physical properties measurement method called as periodic heat flow method has been developed in this research The conductivity and the diffusivity of several kinds of graphitized cathode block and semi graphitic cathode block samples for industrial test have been determined and calculated Compared with the apparatus using a laser as a periodic heat source this experimental test facility is cheap and feasible 3 Its convenient operating system and reliable determining results give a possibility to establish a national data bank of thermo physical properties of carbon material for aluminum reduction cell 2 Principle and analysis 2 1 Experimental principles Based upon the assumption of this experiment that the specimen could be considered as a semi infinite cylinder and the heat conduction process in it is of one axial dimension the thermal diffusivity of the specimen could be formulated by measuring the decreasing amplitude or the increasing phase lag between the two different positions as the heat wave propagated through the specimen in its axial direction 4 When the one end of the specimen is exposed to a periodic sinus heat flow and the other end is kept the constant temperature the thermal diffusivity can be calculated by using the theoretical formula of periodic heat flow method 5 The thermal conductivity can be got with relation to thermal diffusivity if the specific heat and the density are known The two methods which are the amplitude decay and the phase lag are combined to improve the classical method in this research Figure 1 showed the transient temperature curves of the two test positions The test position of test1 is closer to the bottom chip heater Figure 1 Transient temperature curves of the test positions 2 2 Theoretical analysis There were two common methods to determine the thermal diffusivity when the experimental test system was designed based on the principle of periodic heat flow method They were respectively the amplitude decay and the phase lag which were both deduced in an axial one dimension The values of the thermal diffusivity calculated from the above two methods may be differ from each other due to the inevitable radical heat transfer between the cylindrical specimen and the surroundings A new formula where the radical heat transfer was considered could be obtained based on the research of Professor M Lamvik The values of the thermal diffusivity could be taken into account by a correction factor 5 6 The improved formulas of the two classical methods are aa F q L a 2 2 ln2 1 and ll F L a 2 2 2 2 Where are respectively the thermal diffusivities of the amplitude decay and the phase lag method m a a l a 2s 1 is angular frequency of temperature wave s 1 L is range interval of two test positions m is phase lag of two temperature waves rad is amplitude ratio of attenuation q A2A1 are the correction factors of They can be calculated according to the following equations a F l F a a l a 3 5 02 1 a F and 4 5 02 1 l F Where BiFo 1 is heat loss factor hrBi 2Crh r C and are respectively radius density specific heat capacity and surface coefficient of heat transfer m kg m 3 Jkg 1 1 Wm 2 1 2 raPFo is thermal diffusivity P is period of sinus temperature wave s a Equation 2 and equation 3 showed us that la FF1 when heat loss factor 0 The relationships between and meet the following equations according to equation 1 equation 4 5 7 a F l F 1 la FF and 2 qln F F l a The corrected value is evaluated by aaF aa or llF aa The relationship of thermal conductivity with thermal diffusivity is Ca 5 Where is thermal diffusivity ma 2s 1 is thermal conductivity Wm 1 1 is density kg m 3 C is specific heat capacity Jkg 1 1 3 Experimental systems 3 1 Instrument Figure 2 showed the schematic plan of major experimental structure The specimens were cylindrical ingots with 30 150mm 30 160mm 25 130mm and 25 125mm 10mm test position1 and 25mm test position2 from bottom end of the cylinders respectively radical measuring holes were grilled into axis in which thermocouples were inserted during the measurement The thermocouples were type K with 0 5mm external diameters and the measuring region was between the two thermocouples The specimen is mounted in a concentric position of a vertical tubular furnace where the temperature is controlled to keep a given value initially The cooling water system connected with the top of the specimen is made as an independent temperature controller to meet the boundary condition at the top The bottom of the specimen connects with a small electrical chip heater Additionally an electric tube furnace made as an auxiliary side heater is applied to reduce the radical heat dissipation Figure 3 showed scheme of test system The regulated power supply insured that all facilities work in a steady condition The function generator was used as a signal source to create a normal sinus signal and its continuously adjustable output voltage and frequency could meet the different test conditions The sinus signal from the function generator magnified by a power magnifier directly connected with the bottom electric heater That whether the magnified signal was distorted was detected by an oscilloscope DX200 temperature recorder combined with the thermocouple of type K was used to check and display the temperature signal 3 2 Measuring process When t 0 the bottom surface of the specimen was exposed to a periodic sinus heat flow from the chip heater about 200s per period The periodic heat flow was recognized by DX200 temperature recorder and thermocouple as a harmonic temperature variation over time with deceasing amplitude and increasing phase lag when the temperature wave axially propagated through the cylindrical specimen A periodic curve was intercept to calculate the thermal diffusivity when the temperature wave got reach to a quasi stable state The top of the specimen was kept at a given initial temperature by the circulating water system The heat transfer process could coincide with the experimental model 4 Results and discussion 4 1 Data processing Figure 4 and Figure 5 respectively showed the temperature rise graph of test positions and the temperature time graph at a quasi stable state The determined temperature curves coherent with the heat transfer demonstrated by the experimental principle The test temperature can be adjusted by controlling the electric power of the bottom heater The asymmetry of the temperature curves at quasi stable state is analyzed to obtain the amplitudes and the phases of fundamental wave through using harmonic analysis method The analyzed periodic curves are intercepted to calculate the thermal diffusivity then to determine the thermal conductivity according to equation 5 1 circulating water system 2 temperature recorder 3 oscilloscope 4 regulated power supply 5 fuction generator 6 voltage meter 7 amperemeter 8 sinus electric heater 9 test position 10 water circulating pump 11 supporting frame 12 heat insulating material 13 specimen 14 rubber tube Fig 3 Schematic plan of test system 1 coolinig water system 2 insulation material 3 electric tube furnace 4 specimen 5 thermocouple 6 sinus electric chip heater Fig 2 Schematic plan of major structure test2 test1 T t h m s Fig 4 Temperature rise curves of test positions 00 00 00 01 00 00 02 00 00 03 00 00 200 150 100 50 test2 test1 t h m s T a Tmax 40 5 b Tmax 206 test2 T t h m s test1 T test2 test1 t h m s Fig 5 Temperature time curves at quasi stable state a Tmax 40 5 b Tmax 206 4 2 Error analysis In general the factors influencing the experimental result are artificial error computational error and systematic error When the radical measuring holes are grilled onto axis and the thermocouples are placed at the test positions the radical heat dissipation may increase The theoretical formula to calculate the thermal diffusivity is deduced by using the arithmetic product of coefficient of heat transfer and temperature difference between the specimen and the surroundings to express the radical heat dissipation 8 The hypothesis of a linear relationship between the temperature difference and the radical heat dissipation is only tenable at a low temperature difference During the testing process it is partly subjective when to record the data at the quasi stable state The other potential factors which result in the bias errors are the temperature fluctuation on the top surface and the radical dissipation from the sidewall caused by the limit of precision and sensitivity of the circulating water system and the heat insulated property of the heat insulating materials 4 3 The results of determination In the research the graphitic carbon materials with different graphite percentage composition and the graphitizing carbon materials are chosen as the measuring objects in order to understand the thermo physical properties of graphitizing carbon materials for aluminum reduction cell According to the principle of least squares procedure the results of determination are optimized and treated by using single variable searching method 9 Figure 6 and Figure 7 respectively show the temperature thermal diffusivity graph and temperature thermal conductivity graph in the temperature 20 300 The thermal diffusivity and the thermal conductivity decrease with increasing of the testing temperature The thermal diffusivity of the graphitic carbon materials increases when the graphite percentage composition is higher in the same condition The thermal conductivities of the graphitized carbon materials fluctuate slightly which are higher than those of the graphitic carbon materials in the temperature 20 300 Table 1 shows the calculated results and the reference value from the corresponding documents 10 11 Tab 1 The result of determination subject Experimental value Wm 1 1 Reference value Wm 1 1 Relative error Sample1 29 6 34 5 30 38 9 2 Sample2 32 8 44 2 35 45 6 3 Sample3 96 4 108 6 100 120 9 5 Sample4 94 2 105 4 100 120 12 2 m Wm 1K 1 T Fig 7 Temperature thermal conductivity graph of sample 10 5a 2s 1 T Fig 6 Temperature thermal diffusivity graph of specimens The average absolute relative error between the calculating values and the reference values is 9 5 which is mainly resulted by the radical heat dissipation and the temperature fluctuation on the top surface 5 Conclusions The corrections to the two classical solutions of periodic heat flow method are approved to be correct which make the theoretical analysis bias error Several types of samples including graphitizing and graphitic carbon materials are determined The average absolute relative error is no more than 9 5 in the temperature range of 20 300 The method for determining thermal diffusivity and thermal conductivity of graphitizing carbon materials is feasible and practical Acknowledgements The research is sponsored by the Natural Science Foundation of China under Grant No 50376076 and also by the Special Foundation for Doctorate Discipline of China under Grant No 20010533009 References 1 Qiu H P Research on the relationship between thermal conductivity and micro c