OH Radical Emission Research Articles

Ground based instrument for observing near IR nightglow inhomogeneities at zenith and throughout the sky

An instrument for the study of the dynamics of the upper terrestrial atmosphere by optical method has been built. It is an automatic high sensitivity imaging system able to record the spatial variation of the nightglow intensity in the near IR. The low intensity and contrast of the structures led to a system combining a TV imager and a photo camera. This instrument will be routinely operated at Dumont d'Urville (Antarctica). We present below the pictures of spatial structures of the OH radical emission observed at zenith. The emission originates from a region 85 km above the surface. These pictures were obtained at the Observatoire de Haute-Provence (France) for the verification of the instrument performances.

Heterogeneous-homogeneous decomposition of HNO3

Thermal decomposition of HNO3 and HNO3−CO mixtures has been studied at 473–673 K. It has been found that heterogeneous decomposition is the only reaction path and no emission of surface OH radicals into gas phase takes place below 573 K. Surface intermediates do not interact with CO.

Burning characteristics of premixed sprays and gas-liquid coburning mixtures

A premixed spray burner was carefully designed for the observation of stable turbulent flames propagating in a droplet cloud suspended in a stream of air or for a gaseous fuel-air mixture. Various methods were examined to determine the most suitable reference surface of a spray flame, corresponding to its burning velocity. Difficulties arose in finding the reference flame surface, which governs the burning velocity of sprays, because the spray flames were usually thick and unsteady. It was found that the surface on which the OH radical emission had a peak intensity was more suitable as a reference flame surface than the surfaces determined by either direct or schlieren photography. The qualitative features of our burning velocity data for kerosene sprays agree with those of other investigators; that is, we found a linear dependence on the fuel-to-air ratio and an inverse proportionality to the Sauter mean diameter. Concerning coburning, there exists an optimum kerosene-to-propane mass flow ratio and an optimum mean droplet diameter that maximizes the burning velocity. Instantaneous flame images in OH and C 2 radical bands seem to suggest that group combustion occurs in a spray flame.

Sound emission from a turbulent flame

The objective of this paper is to obtain knowledge of the relationship between combustion sound and the heat release rate in turbulent flames. Using an ion probe and a light emission detector, the correlation between the field sound pressure and the local fluctuating intensity of ion current or OH radical light emission in a turbulent Bunsen burner flame was studied. Based on the observations, the following three mechanisms of sound emission from the flame were proposed: (1) “local combustion roar” generated locally in the flame due to the combustion of turbulent eddies and the resultant thermal expansion. (2) “flame tip combustion roar” caused by the fluctuation of heat release rate at the flame tip which is enhanced by the propagated disturbance originated in the upstream flame element and (3) “jet sound” due to the turbulence at the jet boundaries. The turbulent combustion in a flame element emits a “local combustion roar” and generates a flame disturbance simultaneously which propagates and enhances the “flame tip combustion roar”. This investigation has suggested that both the light emission of OH radicals and the ion current correspond well to the local heat release rate.

Burning velocities of premixed sprays and their coburning characteristics.

Various methods were examined for determination of the burning velocities of liquid fuel sprays. Difficulties existed in finding the reference flame surface which governed the burning velocity of sprays since spray flames were usually thick and rather unsteady. The surface where the OH radical emission had a peak intensity was more suitable as a reference flame surface than the surfaces determined by schlieren or direct photography. The burning velocities of premixed sprays of kerosene were determined using a carefully-designed premixed spray burner. The coburning characteristic of premixed sprays of kerosene were also determined for the case of propane-kerosene spray-air mixture combustion.

Soot and pah distributions in oil spray flames, inferred by elastic and inelastic laser light scattering

The space distribution of droplets, soot particles and fluorescing PAH species have been determined for vertical unconfined swirled oil spray flames, by analyzing light scattering, and laser-excited fluorescence coefficients, and radially inverted emission of OH radicals. The angular patterns of the scattering coefficient on vertical and horizontal polarization planes have been measured in the flame and compared with theoretical predictions for transparent and absorbing large spheres and soot particles and with the measured patterns on isothermal sprays. The fuel droplets were mainly concentrated in the initial part of the flame around the axisand the maximum concentration of the fluorescing species were detected on the periphery of this zone, which is externally bounded by a layer of intense OH emission. Soot was produced in a small toroidal zone near the burner outlet and more massively in a high temperature recirculation region downstream. The influence of swirl strength on the combustion process has been studied by means of the axial scattering and the radial fluorescence profiles. Vaporization was delayed along the centerline and the off-axis maximum concentration of the fluorescing PAH increased up to an intermediate swirl level. These effects were attributed to the outward displacement of the swirled air jet, which thus reduced the mixing rate between fuel and air in the central part of the flame. Further increase of the swirl level reduced the PAH concentration because the recirculation of hot gases accelerated the vaporization and combustion processes.