Prof Michael B Ewing

Research Overview

Our group studies the thermodynamic properties of fluids and fluid mixtures using a variety of techniques.

A. Thermophysical Properties of Gases

Acoustic methods based on spherical resonators are used to study the thermophysical properties of gases. The techniques are applied over wide ranges of temperature and pressure to obtain speeds of sound of extremely high accuracy (about 1 ppm) that yield heat capacities and second virial coefficients to better than 0.1 per cent: transport properties such as bulk viscosities and vibrational relaxation times are also obtained. The systems studied include pure gases, theoretically tractable mixtures, and industrial fluids for applications to sonic nozzles at high pressures. More recently, annular resonators have been developed so that energy transfer can be studied at very low frequencies.

The same resonators are used at microwave frequencies to determine the dimensions of the resonator and the dielectric constant and total polarizability of the gas. Quadrupole moments of simple gases have been derived from microwave measurements with a high-performance cylindrical resonator. The acoustic and dielectric virial coefficients are related to intermolecular potentials through theoretical and computational studies. An important extension to this work combines microwave and acoustic measurements with the same sphere to give the ratio of the speed of sound to the speed of light and, hence, thermodynamic temperatures of metrological quality.

B. Vapour pressures and phase diagrams

Vapour pressures of pure compounds are determined by comparative ebulliometry from 150 Pa to the critical region. The technique avoids direct measurement of pressure but places considerable demand on thermometry. The derived vapour pressures are of unprecedented accuracy (fractionally 20 ppm or better) and are effectively limited by the accuracy of the data for water which is used as a reference fluid. This work supplements studies of phase diagrams of binary and ternary mixtures such as those formed from water, methanol, carbon dioxide and simple hydrocarbons at high pressure. Particular emphasis is given to those mixtures that have three fluid phases in equilibria.

Selected Publications

  1. M.B. Ewing and J.C.S. Ochoa, “Vapour pressures of n-hexane determined by comparative ebulliometry”, J. Chem. Thermodyn., (2006), 38, 283-288.
  2. M.B. Ewing and J.C.S. Ochoa, “Vapour pressures of n-pentane determined by comparative ebulliometry”, J. Chem. Thermodyn. (2006), 38, 289-295.
  3. M.B. Ewing and J.C.S. Ochoa, “Vapor pressures of n-heptane determined by comparative ebulliometry”, J.Chem.Eng. Data, (2005), 50, 1543-1547.
  4. M.B. Ewing and J.C.S. Ochoa, “Vapor pressures of acetonitrile determined by comparative ebulliometry”, J. Chem. Eng. Data, (2004), 49, 486-491.
  5. M.B. Ewing and J.C.S. Ochoa, “The vapour pressures of n-octane determined using comparative ebulliometry”, Fluid Phase Equilibria, (2003), 210, 277-285.
  6. M.B. Ewing and D.D. Royal, “Relative permittivities and dielectric virial coefficients of nitrogen at T = 283.401K and T = 303.409K determined using a cylindrical microwave cavity resonator”, J. Chem. Thermodyn. (2002), 34, 1985-1999.
  7. M.B. Ewing and D.D. Royal, “A highly stable cylindrical microwave cavity resonator for the measurement of the relative permittivities of gases”, J. Chem. Thermodyn. (2002), 34, 1073-1088.
  8. M.B. Ewing and D.D. Royal, “Relative permittivities and dielectric virial coefficients of nitrogen at T = 300 K”, J. Chem. Thermodyn. (2002), 34, 1089-1106.