CAJ | 학술논문

为了能较精确地预测大颗粒低填充率外热式回转窑内壁与颗粒间的传热系数，该文在对外热式回转窑传热机理分析的基础上，提出了回转窑内壁与物料颗粒间换热的基本构成为：窑内壁与气体介质间的导热、气体介质与颗粒间的对流以及窑内壁与颗粒间的辐射换热。在此基础上，建立了预测窑内壁与颗粒间总传热系数的数学模型，并以6 mm直径的氧化铝球为物料，在5%填充率下，分别在回转窑转速1，2，3 r/min，窑体壁面温度在220～420℃的11种不同壁温条件下测量了窑体内壁与颗粒间的传热系数，对模型进行验证。试验结果表明，模型预测的误差小于10%，可满足工程计算的精度要求；该文所采用的试验物料及试验条件下，在窑壁温度高于220℃时，辐射传热系数所占的比例高于75%，是总传热系数的主要构成部分。研究结果可为外热式回转窑传热机理的深入研究提供参考。
위료능교정학지예측대과립저전충솔외열식회전요내벽여과립간적전열계수，해문재대외열식회전요전열궤리분석적기출상，제출료회전요내벽여물료과립간환열적기본구성위：요내벽여기체개질간적도열、기체개질여과립간적대류이급요내벽여과립간적복사환열。재차기출상，건립료예측요내벽여과립간총전열계수적수학모형，병이6 mm직경적양화려구위물료，재5%전충솔하，분별재회전요전속1，2，3 r/min，요체벽면온도재220～420℃적11충불동벽온조건하측량료요체내벽여과립간적전열계수，대모형진행험증。시험결과표명，모형예측적오차소우10%，가만족공정계산적정도요구；해문소채용적시험물료급시험조건하，재요벽온도고우220℃시，복사전열계수소점적비례고우75%，시총전열계수적주요구성부분。연구결과가위외열식회전요전열궤리적심입연구제공삼고。
Heat transfer coefficient is one of the most crucial parameters in thermal calculation and design for an externally heated rotary kiln. Suitably designed kiln dimensions, structure and operating parameters rely on the accuracy of the employed heat transfer coefficient. For an externally heated kiln, heat transfers from an outside source to inside particles through a wall. Generally, the filling ratio in an externally heated rotary kiln is low. So, the heat transfer mechanism for large particles with a low filling ratio in an externally heated rotary kiln is quite different from that in an internally heated rotary kiln, whose filling ratio is usually more than 15 percent. Despite the existence of some achievements in particles motion behavior and heat transfer mechanisms in an internally heated rotary kiln, so far, there is no reliable heat transfer model to describe the heat transfer process between the kiln’s surface and particles in an externally heated rotary kiln with low filling large particles. As a result, the main approach of heat transfer coefficient determination is still an experimental test. On the basis of heat transfer mechanism analysis, this paper regards the heat transfer process between the kiln’s surface and large particles as consisting of heat conduction between the kiln’s surface and gas film, heat convection between the gas film and particles, and heat radiation between the kiln’s surface and particles. Finally, a mathematical model is created for the prediction of the heat transfer coefficient between the kiln’s surface and large particles. To validate the developed model, a series of experimental tests are performed. Alumina spherical grains with a diameter of 6 mm are used as testing particles. When the filling ratio is 5 percent, the heat transfer coefficients are measured in the range of 220℃-420℃ at 20℃ surface temperature intervals, corresponding to the rotary speeds of 1r/min, 2r/min, and 3r/min, respectively. The tests find that the heat transfer coefficient only slightly increases with rotary speed increase. However, the coefficient increases intensely when the kiln’s surface temperature increases. Comparisons of the experimental results and predictions show that the maximum relative error (emax) is about 9.8 percent, and the average error (eave) is 5.86 percent. According to the engineering designheat transfer coefficient model experience, the model is able to well match the engineering requirement that it can refer to thermal calculation. The results also show that, for the testing material in this paper, the fraction of radiation heat transferred from kiln’s surface to particles is more than 75 percent of the total heat transfer when the surface temperature is higher than 220℃. If the surface temperature is beyond 320℃, a more intense increasing percentage of cure will appear. The error analysis shows that the prediction values are all larger than testing results, which could be caused by the assumptions of both particle distribution and radiation heat transfer between the kiln’s surface and particles. To obtain a more accurate heat transfer coefficient model for large particles with low filling ratio in an externally heated rotary kiln, it is necessary to carry out further investigate into the performance of the motion behavior of particles. The achievement in this paper is helpful for further investigation of heat transfer mechanism in an externally heated rotary kiln.