Article pubs.acs.org/jced
Speed-of-Sound Measurements in Compressed Nitrogen and Dry Air Karsten Meier* and Stephan Kabelac Institut für Thermodynamik, Helmut-Schmidt-Universität/Universität der Bundeswehr Hamburg, Holstenhofweg 85, D-22043 Hamburg, Germany ABSTRACT: This paper reports accurate measurements of the speed of sound in compressed pure nitrogen and a synthetic ternary mixture of nitrogen, oxygen, and argon with a composition close to that of natural dry air. The data have been measured by a double-path-length pulse-echo technique and cover the temperature ranges between 275 and 400 K for nitrogen and between 240 and 420 K for the dry air mixture with pressures up to 100 MPa. The expanded uncertainties (at the 0.95 confidence level) amount to 2.1 mK for temperature, 50 ppm for pressure, 80 ppm for speed of sound in nitrogen, and 110 ppm for speed of sound in the dry air mixture.
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INTRODUCTION
In order to examine the capability of our pulse-echo instrument, which was primarily designed for liquid samples, for measurements in compressed gases, the speed of sound in compressed nitrogen was measured before the air measurements. Nitrogen is well-suited for this purpose because it is the main component of air and the speed of sound in nitrogen is well known. Costa Gomes and Trusler14 measured the speed of sound in nitrogen with a spherical resonator on the four isotherms 250, 275, 300, and 350 K at pressures between 0.1 and 30 MPa with an uncertainty of less than 0.01%. This very accurate data set enables a comparison of our pulse-echo technique with the spherical resonator method in the pressure range where both methods overlap. Besides, Gedanitz et al.15,16 published very accurate data for the speed of sound in nitrogen at the same four temperatures at pressures between 20 and 30 MPa, which were measured with a pulse-echo technique similar to ours. Furthermore, with the Helmholtz energy formulation for nitrogen of Span et al.,17 a very accurate basis for comparisons is available.
Air is the gas mixture of the earth’s atmosphere, which mainly consists of nitrogen, oxygen, and argon. As such, it is of vital biological, meteorological, and industrial importance. Therefore, the precise knowledge of the thermodynamic properties of air is of interest to scientists in many fields of research as well as to engineers in a variety of industrial sectors. For developing Helmholtz energy formulations, which describe the thermodynamic properties of fluids over wide temperature and pressure ranges, among data for other properties, accurate speed-of-sound data are required. The speed of sound in air has been the subject of many investigations since the early days of acoustics. A comprehensive overview of the speed of sound in air is given in the monograph of Zuckerwar.1 In many early works, experimental results for the speed of sound at ambient pressure or low pressures, for example, refs 2−9, were reported. An overview of experimental data published between 1900 and 1941 was given by Hardy et al.6 However, surprisingly little experimental data for the speed of sound in air at high pressures can be found in the literature. Van Itterbeek and de Rop10 published speed-of-sound data for gaseous air in the temperature range between 230 and 313 K at pressures up to 2 MPa. Younglove and Frederick11 provided a comprehensive speed-of-sound data set in liquid air between 90 and 130 K and in gaseous and supercritical air between 110 and 300 K with pressures up to 14 MPa. Ewing and Goodwin12 measured the speed of sound in gaseous air at the supercritical temperature 255 K with pressures up to 6.9 MPa. Recently, De Stefani and Trusler13 measured the speed of sound in air with a spherical resonator between 300 and 450 K at pressures up to 16.3 MPa. At pressures above 16.3 MPa, the speed of sound in dry air has not yet been measured. It is the aim of this work to fill this gap by providing speed-of-sound data in compressed supercritical air at high pressures up to 100 MPa. © 2016 American Chemical Society
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EXPERIMENTAL SECTION
Our speed-of-sound instrument employs a double-path-length pulse-echo technique with an operating frequency of 8 MHz and was described in detail in refs 18 and 19. It was operated in this work in the same way as in previous measurement campaigns.19−22 The expanded uncertainties (at the 0.95 confidence level) for the temperature and pressure measurements are 2.1 mK and 50 ppm, respectively. A detailed summary of the uncertainty budgets is given in Table 1. Before the measurements in nitrogen, the acoustic path length in the sensor and the thermal expansion coefficient of the sensor Received: August 11, 2016 Accepted: October 12, 2016 Published: October 26, 2016 3941
DOI: 10.1021/acs.jced.6b00720 J. Chem. Eng. Data 2016, 61, 3941−3951
Journal of Chemical & Engineering Data
Article
Table 1. Summary of Uncertainty Budgets source of uncertainty
expanded uncertainty (confidence level =0.95) nitrogen
SPRT calibration calibration of reference resistor ASL F18 bridge temperature variation in pressure vessel total calibration of pressure balance differential pressure indicator hydrostatic pressure correction ambient pressure measurement total calibration fluid time difference temperature measurement pressure measurement correction of ΔL to ambient pressurea diffraction correctiona uncertainty of reference data agreement with reference data total acoustic path length pressure dependence of acoustic path lengtha time difference diffraction correctiona dispersion correctiona sample compositiona total
Temperature Measurement 2.0 mK 0.1 mK 0.1 mK 0.5 mK 2.1 mK Pressure Measurement 45 × 10−6·p 5 × 10−6·p 2 × 10−6·p 7 Pa 46 × 10−6·p Determination of Acoustic Path Length water 5 × 10−6·ΔL 10 × 10−6·ΔL
