Pneumatic temperature measuring thermocouple probe error analysis and solution
1 pneumatic probe temperature measuring principle is shown in figure 1
is double throat pneumatic temperature measuring probe structure diagram. Import and export pressure probe by making errands for measuring gas through cross section 1 into the probe and the first throat section 2 achieve harmony state. Gas by 2 & ndash; 3 section of high temperature gas flow through water cooling cavity for cooling the gas temperature thermocouple to 3 section within the allowable range. Gas in harmony again plug state, through the second throat section 4 by through two throat gas mass flow rate before and after the principle of equal and double throat achieves harmony condition to measure the gas always ask T 0.
assumes that air flow through the before and after the two throat is to assume that the airflow is balance, such as entropy, unsteady one-dimensional homogeneous flow. Nozzle flow equation of one dimensional isentropic flow are as follows:
when the throat revealed huiyong, plug, by the pneumatic function can turn formula ( 1) Can be simplified as:
type of w for the gas mass flow rate, P for airflow total pressure, P for air static pressure, A cross-sectional area of nozzle throat, T for the total airflow temperature, & gamma; For air specific heat ratio, R is the gas constant. Define discharge coefficient C is the ratio of the actual flow am and ideal flow im. Under the condition of steady flow, the probe before and after the two current limiting section flow equal, is:
Angle of 0 4 for the working principle of the probe in figure 1 section.
the acquired before the upper nozzle temperature:
when gas specific heat ratio ignore the effects of ( Gas specific heat ratio of fixed value & gamma; = 1. 33) 。
C to probe coefficient at room temperature ( 0 3T T = ） By the formula ( 6) Calibration.
2 probe temperature measurement error and the reason analysis
( 1) Thermocouple measurement error. Working temperature measuring probe according to the formula ( 6) , probe thermocouple in cross section as shown in figure 2. After cooling airflow total temperature T 3 influence on the accuracy of the measurement of probe measurement accuracy. Pneumatic probe for the jacket cooling structure, limited to the size, type probe thermocouple had to adopt the bare, produced by high temperature gas and water cooled wall temperature difference will lead to radiation error. To probe the strut internal air velocity is higher, the local position to achieve sound velocity, and bare thermocouple for high-speed airflow only certain stagnation effect, is not completely adiabatic stagnation. So the thermocouple will introduce a certain speed affect the measurement error.
( 2) The influence of the gas specific heat ratio. Formula of probe ( 6) Is take gas specific heat ratio is constant, namely the probe in the measurement of gas temperature, the specific heat of the throat 2, 4 cross section than the common value of 1 for gas projects. 33. But in actual probe in the work, because after a probe gas with water cooling, throat section 2 and 4 after cooling water cooling temperature difference is bigger, and gas composition ratio as the air-fuel ratio will also change, gas specific heat ratio is not constant.
（ 3) The first throat probe the influence of thermal deformation. When temperature measuring probe, probe head in high temperature gas flow, due to the probe head received high temperature air impact, hard to avoid the first throat area will change. A formula ( 6) Is the first throat area change to probe the introduction of measurement error.
3 probe temperature measurement error correction
( 1) Correction of thermocouple. In the general hot accidentally error generally use for radiation heat shields, cover for velocity error using stagnation and other measures to avoid or decrease the error. However with the probe due to structure size of the above methods can not, only numerical correction. Heat transfer analysis was carried out on the thermocouple node is known by its heat transfer, there are three ways of hot gas flow node and convective heat transfer, the radiation heat transfer and flow channel wall, and the protection of the porcelain tube thermal conductivity. In the case of thermocouple right to use and install the thermal error is negligible. Put gas temperature T g, thermocouple temperature measurement value for T 3, T w, probe the strut in wall temperature of high temperature gas mainly by way of convective heat transfer heat transfer to the thermocouple, ignore the heat conduction and radiation heat transfer in a gas thermocouples, its heat transfer is:
among them, & alpha; Is the gas convection heat transfer coefficient of thermocouple; A thermocouple is heat transfer surface area.
thermocouple and probe the strut wall surface heat transfer, mainly by radiation.
among them, & epsilon; N for thermocouple radiation rate; σ For the boltzmann constant.
when the thermocouple temperature measuring steady-state Q a = Q, R, namely:
radiation from the thermocouple temperature measurement error is: