低压条件下的等离子射流特性分析

低压等离子喷涂由于具有特殊的射流特性，以及可以沉积组织均匀的特殊结构涂层而备受关注.涂层的形成受到等离子射流特性的影响，比如焓值、温度、速度等.研究中利用热焓探针技术在环境压力为3 kPa的条件下，测量了Ar-H₂等离子体不同轴向位置射流中心的焓值和压力，并且进一步计算了射流的温度和速度，以及表征等离子体对粉体加热能力的努森数.结果表明，等离子射流在距离喷嘴出口12.5 mm处的温度为11 000 K；400 mm处降为7 000 K；等离子射流速度在喷嘴出口处25 mm左右达到最大值，约为2 000 m/s；喷嘴外部等离子射流的努森数处于过渡区，对粉体的加热能力较低.

Characterization of plasma jet at low pressure

Low pressure plasma spraying has been attracted attention due to special properties of the expanded plasma jet which may probably deposit specific and unique structure coatings. The formation of the coatings is affected by plasma generation and working conditions. The operating parameters such as arc power, plasma gas flow rate, and chamber pressure have influences on enthalpy, temperature and velocity of plasma jet. In this paper, an enthalpy probe was applied to measure the enthalpy of argon-hydrogen plasma jet generated at chamber pressure of 3 kPa. Attempts were also made to measure the pressure of plasma jet by U-tube fixed on the enthalpy probe. The temperature and velocity of plasma jet were calculated by measuring the enthalpy and pressure at different axial distances of the plasma jet. The results showed that the temperature of plasma jet reaches to 11 000 K at 25 mm from nozzle exit, and then dropped slowly to 7 000 K at 400 mm. The velocity of plasma jet reaches the maximum value of 2 024 m/s from the nozzle exit about 25 mm. The Knudsen number of the plasma jet is about 1.0, so the heating capacity of the plasma jet is significantly reduced.