Harold Y. Wachman

Formula for Thermal Accommodation Coefficients

A simple formula in closed form is proposed for the thermal accomodation coefficient from which the accommodation coefficient of a monatomic gas—solid system may be calculated from certain basic parameters of the system. The formula α(T)=1−exp(−T0/T)+α(∞) tanh{[(MT)1/2/α(∞)](a/λ)} exp(−T0/T) is obtained from consideration of certain aspects of Goodmans lattice theory of accommodation and from conclusions regarding certain properties of available experimental data. Adequate agreement is obtained of the formula with both the lattice theory and experimental data. Certain, corrections are applied to the lattice theory, and the gas—surface‐potential interaction parameters relevant to the lattice theory are revised.

Effect of gravity on laminar premixed gas combustion I: Flammability limits and burning velocities

Abstract Fuel-lean flammability limits and burning velocities in a closed vessel were measured for methaneair mixtures burning at earth gravity (one-g) and zero-gravity (zero-g) at initial pressures of 50–1500 Torr. The zero-g flammability limit was found to be between the one-g upward and one-g downward flammability limits. For sublimit mixtures burning at zero-g, an extinguishment phenomenon unlike any found at one-g was observed. For fast burning mixtures (Su > 15 cm/s), burning velocities were identical at one-g and zero-g. For slower burning but still flammable mixtures, only the zero-g observations could be interpreted to obtain burning velocity data because at one-g natural convection caused severe flame front distortion. Zero-g burning velocities for these mixtures were in good agreement with existing models of laminar flame propagation and with extrapolation of current and prior one-g data. The main conclusions are that the one-g upward flammability limit occurs at a mixture which has a burning velocity which is so low that flame propagation is impractical, that the one-g downward flammability limit is related to the inability of the flame front to propagate downward against buoyant forces, and that near-limit flame propagation at zero-g is mostly independent of the experimental apparatus. Because of the unusual nature of the extinguishment process for sublimit mixtures burning at zero-g, further experiments are required to determine the cause of the zero-g flammability limit.

Measurements of relative concentrations and velocities of small clusters (n⩽40) in expanding water vapor flows

The concentrations of water clusters (2⩽n⩽40) relative to the monomer concentration in expanding water vapor flows have been determined over a range of source pressures and temperatures by mass spectrometry. The corresponding directed flow velocities and thermal velocity distributions of the monomers and clusters of each size have been determined by mass‐filtered time‐of‐flight (velocity) spectrometry. Relative concentration generally decreases exponentially with n. Translational thermal equilibrium ∼65 K prevails for the monomers and clusters of n≲5 under all source conditions. Translational temperatures of larger clusters generally increase with increasing n. The results suggest a kinetic process in which surviving smaller clusters are formed in three‐body collisions and larger ones in two‐body collisions.

Thermal Accommodation Coefficients of Helium on Tungsten and Hydrogen on Hydrogen‐Covered Tungsten at 325°, 403°, and 473°K

Measurements have been made of accommodation coefficients of helium and hydrogen from 324° to 473°K on tungsten under conditions designed to minimize amounts of adsorbable impurities from the experimental system.The helium accommodation coefficient values, lying between 0.0194 and 0.0228, compare favorably with measurements at lower temperatures, obtained by Thomas et al., which are probably the best available. The data are also correlated with results of theories on accommodation.Values of hydrogen accommodation coefficients have been associated with the population of atomic hydrogen on the surface estimated from the adsorption studies of Hickmott. To a first approximation, hydrogen accommodation coefficients are found to be independent of coverage at the high coverages of these experiments and inversely proportional to hydrogen temperature.

Scattering of monoenergetic argon beams from heated platinum: Out-of-plane time-of-flight measurements

Abstract Time-of-flight measurements were made in and out of the plane of incidence, with initially monoenergetic argon beams (speed 5.75 × 104 cm/sec.; incident angle 55° from target normal) before and after reflection from heated platinum. The data were used to compute low order moments of velocity distribution : relative number flux, momentum and energy flux. Flux ratios yield mean speed, mean energy and speed distribution as functions of angle of reflection. The data are consistent with the hypotheses that the tangentical component of momentum of the incident beam is nearly preserved on collision and that dispersion out of the plane of incidence is caused by surface roughness.