直流条件下空气与SF_6喷口电弧换热机理分析

Analysis of the Energy Transport Processes in DC Air and SF_6 Nozzle Arcs

空气作为来源丰富的自然气体其绝缘和电流开断性能得到了很多研究。近几年来作为Fluoroketone的稀释气体在中压开关中进行了SF_6替代试验。大量结果表明基于空气的气吹断路器故障电流开断能力远远小于SF_6断路器,但长期以来一直未能回答为什么空气电弧的时间常数远大于SF_6电弧的时间常数、其开断性能远远弱于后者的问题。文中基于可靠的实验数据对二维轴对称微分湍流电弧模型中的湍流参数首先进行了标定,并在此基础上详细分析了空气电弧的能量交换机理。通过对比研究发现小电流空气喷口电弧径向和轴向散热相当,而在SF_6电弧中径向散热占绝对主导地位。两者散热机理的差异与径向温度分布(弧柱半径大小)有直接关系,而径向温度分布由物性参数ρCp通过湍流换热控制。因此,文中的研究结果为找出瞬态电弧中(开断过程)气体的不同开断能力的原因指明了方向。

As a naturally existing gas of abundant supply, the insulation and interruption capability of air has been extensively investigated. Research has also recently been done to use air as a dilution gas of Fluoroketone for SFs free interruption in medium voltage switchgear. Much evidence has shown that air gas-blast circuit breakers have far inferior interruption capability than SFs breakers. However, no clear explanation has so far been given regarding why an air switching arc has much larger time constant than an SF6 arc, and thus exhibiting far inferior interruption capability than the latter. Based on reliable experimental measurements, the present work first calibrated the turbulence parameter in a two-dimensional axisymmetrical turbulent arc model which was then used for computational investigation of the air arc behaviour and analysis of the dominant energy transport processes. Through comparative study, it has been found that, for air nozzle arc at low current, the radial and axial cooling mechanisms play almost equal roles, while for SF6 arc the radial cooling mechanism is dominant. The difference in cooling mechanisms for air and SF6 arcs is directly related to the difference in radial temperature profiles which is controlled by the material property, ρCp, through turbulence enhanced thermal conduction. The present work therefore provides clear direction on the explanation of the differing interruption performance of different SF6 replacement gases under transient discharge conditions.