屏蔽式总温热电偶的稳态误差分析及改型设计

针对航空发动机排气温度高精度测试的需求,采用流热耦合的数值模拟方法研究了某型屏蔽式热电偶在马赫数为0.2、0.3、0.4、0.5、0.6五种工况下的速度误差、导热误差、辐射误差的大小和变化趋势,以及偶丝长径比、屏蔽罩进出口面积比对屏蔽式热电偶稳态误差的影响规律。结果表明:在马赫数小于0.3工况下,辐射误差占总误差的比例最高；而在马赫数为0.6工况下,导热误差成为影响测温精度的最主要因素；合理地增加偶丝长径比能显著地降低导热误差,减小进出口面积比值会使速度误差增大辐射误差减少；因此,存在最佳的进出口面积比使得总稳态误差最小。最后选取了偶丝长径比为12,屏蔽罩进出口面积比为6对原屏蔽式热电偶进行了改型设计,使该屏蔽式热电偶的总稳态误差降低50%以上。 

Steady state error estimation and modification of a shielded thermocouple

For high accuracy measurement of aero-engine exhaust temperature, a conjugate heat transfer (CHT) numerical simulation method was employed to estimate different sources of steady state errors and their variation characteristics of a typical shielded thermocouple working at different Mach number condition namely 0.2,0.3,0.4,0.5,0.6. The results showed that under the Mach number less than 0.3 working condition, the radiation error took the highest proportion of total error, while at the 0.6 Mach number, the conduction error was the main factor affecting the accuracy of measurement. To improve the measurement accuracy of the shielded thermocouple, the influences of the length to diameter ratio of the wire and inlet to outlet area ratio of the shield on the conduction error and radiation error were separately studied. The results showed that the conduction error can be significantly reduced by reasonably increasing the length to diameter ratio of the wire, and as the decrease of inlet to outlet area ratio, the velocity error increased while radiation error decreased there existed a best value of inlet to outlet area ratio to minimize the overall value of the steady state error. Finally, the length to diameter ratio of the wire equal to 12 and the inlet to outlet area ratio equal to 6 were selected to modify the primary shielded thermocouple. After modification, the overall value of the steady state error was reduced by more than 50% in these 5 working conditions, thus the measurement accuracy of the shielded thermocouple was improved.