Abstract:

To measure the density of a new endothermic hydrocarbon fuel (EHF) at supercritical pressures, a new densimeter, which is based on the mass conservation equation and cross-correlation method, was designed and constructed. Two K-type thermocouples with the same thermoelectric characteristics were installed in the upstream and downstream of the test tube with distance of 148 mm as the cross-correlation sensor. The delay time was found using the cross-correlation between upstream and downstream signal which were measured by the thermocouples. High sampling rate and correlation coefficient were used to ensure the accuracy and similarity of the signals. So the mean flow velocity could be calculated by the delay time and distance. With the flow velocity and mass flow rate known, the density could be determined based on the mass conservation equation. The n-dodecane and the binary mixture of n-octane and n-heptane with mass ratio of 1∶1 were tested for the system calibration. According to error analysis, the maximum relative deviations were below ±1.2%, and the absolute average relative deviations were within 0.6%. On the basis of calibration experiments, this measurement covers the temperature range of 302.0—529.6 K under pressures from 3.0 MPa to 5.0 MPa. The application of this method provides a new thinking of the density on-line measurement of endothermic hydrocarbon fuel under supercritical pressures.

Key words: hydrocarbons, supercritical fluid, cross-correlation, density, measurement

摘要：

为实现超临界压力下吸热型碳氢燃料密度的准确测量，以相关测量技术为基础，设计搭建了一套密度在线测量系统。该系统在间距148 mm的管道上下游安装两只具有相同热电特性的K型热电偶作为互相关法的传感器，通过对热电偶检测到的随机温度信号进行相关计算，得出该信号经过上下游热电偶的延迟时间，计算得到流体截面的平均流速，在已知质量流量后，根据质量守恒定律进而求得流体密度。采用纯物质十二烷及质量比1∶1正辛烷/正庚烷二元混合物标定测量系统精度，实验结果与文献值的最大误差在±1.2%以内，平均相对误差小于0.6%。在此基础上，对压力p = 3.0、4.0、5.0 MPa，温度T = 302.0～529.6 K，吸热型碳氢燃料密度进行测量，该方法的应用为超临界压力下吸热型碳氢燃料密度的准确测量提供了新思路。

关键词: 碳氢化合物, 超临界流体, 互相关法, 密度, 测量