Measurement of the critical properties of endothermic hydrocarbon fuels

Y. Liu1, M. He1 and Y. Zhang1

1Xi'an Jiaotong University, P.R. China

Keywords: flow method
property: critical properties
material: endothermic hydrocarbon fuels

The critical temperature and the critical pressure of aviation fuels are important parameters in evaluating its combustibility. Fuels at supercritical states have advantages for combustion such as high density, strong heat absorption ability, good dissolve capacity and certain cage effect etc. So the experimental critical properties data of endothermic fuels and high density fuels are necessary for the design of hypersonic engine. Aviation fuels are mostly complex mixtures which consist of linear hydrocarbons, alkenes, arenes and unsaturated hydrocarbons etc. The aviation fuels have poor thermal stability, and thermal cracking or thermalpolymerization would arose easily under high temperatures. At present, the experimental methods for measuring the critical parameters of thermally unstable substances include sealed ampule method[1], pulse-heating method[2,3] and flow methods[4,5]. The sealed ampule method was developed from the static method for thermally stable substances. Adopted this method, the critical temperature and critical density can be measured at the same time. But it has an unsatisfactory long heating time about several minutes, and the critical pressure can not be measured due to the limitation of the experiment principle, meanwhile, the critical temperature and critical density are obtained by extrapolation, which maybe lead to large deviation. The pulse-heating method has a residence time of (0.01 to 1.0) ms, so it could significantly reduce the influence of thermal cracking. But the method relies on superheating and several corrections and some extrapolation are required to obtain the critical temperature and the critical pressure. The flow methods have a satisfactory heating time about ten seconds, and were proven to be suitable for measuring the critical temperature and the critical pressure of thermally unstable substances. At present, large numbers of experimental data of organic compounds obtained by the flow method were reported. The literatures show that the measuring uncertainties of the critical temperatures and critical pressures were mainly within ±10K and ±2%. In this paper, according to the experiment principle of flow method, a critical properties measurement apparatus was established which consists of the experimental cell, temperature and pressure control system and data acquisition system of temperature, pressure and images. The temperature was controlled by a Simaden FP23 temperature controller; the pressure was controlled by a double constant flow pump plunger coupling with a matching back pressure valve; the temperature and the pressure were measured by a Fluke 5608-12 PRT and a Rosemount pressure transmitter, and were collected by a Keithley 2002 digital multimeter. In order to observing and recording in real time, a DALSA 1M120 high speed camera was applied to record the images of the critical opalescence. The measurement can be carried out up to 10 MPa for pressures and 773 K for temperature. The total uncertainty for pressure and temperature is estimated to be not greater than ±1.25kPa and ±0.08K, respectively. N-pentane and n-hexane were used as standard reagents to test the reliability of the apparatus. The results showed that the apparatus satisfied the design requirements. Then the critical properties of several endothermic hydrocarbon fuels, TX-1 and TX-2 etc. (the dominant sectors are alkanes), were measured.

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