Following a recent investigation into chemi-ionization and chemiluminescence during gradual aeration of small, laminar methane flames, we proposed that partial oxidation products, or syngas constituents, formed in the pre-flame zone well below the luminous region, were responsible for the observed effects. We therefore map temperature, CO, and H2 for geometries and conditions relevant to burners in domestic boiler systems, to assess the potential hazard of CO release into the ambient atmosphere, should any partial quenching occur. CO concentrations peaks of 5.5 volume % are recorded in the core surrounding the axis. Appreciable CO concentrations are also found in the absence of added air. Experiments on various burner port geometries and temperatures suggest that this is not due to air entrainment at the flame base but to diffusion from zones closer to the flame. Next, quenching surfaces such as grids, perforated plates and flame trap matrices of different metals are progressively lowered into the flame. To avoid flow line distortion, suction aspirates the quenched products. The highest emission rate occurs with the quenching plane some 4mm above the burner; further lowering of the quenching surface causes flame extinction. The maximum CO release is close to converting 10% of the CH4 feed, with some variation with quenching material. Expressing this potential release in terms of, e.g. boiler power, predicts a potentially serious hazard. Results of numerical simulations adequately parallel the experimental sampling profiles and provide insights into local concentrations, as well as the spatially resolved CO flux, which is calculated for a parabolic inlet flow profile. Integration across the stream implies, on the basis of the simulation, a possible tripling of the experimental CO release, were quenching simply to release the local gas composition into the atmosphere. Comparison with experiment suggests some chemical interaction with the quenching surface.

Saturation currents and chemiluminescence, especially at the CH∗ and C2∗ wavelengths, are measured for a range of small, laminar methane flames during progressive addition of air, with the principal objective of distinguishing between pure diffusion flames, premixed flames of compositions falling between the upper and lower flammability limits, and the broad range of aerated flames lying in between these regimes. Flame areas defined by the loci of maximum luminosity and by schlieren contours were recorded, so that saturation current densities, CH∗ and C2∗ emission per unit flame area, as well as burning velocities could be deduced. For admixtures of less than 70 vol.%, air appears to act, surprisingly, as an inert diluent as regards saturation currents, so that saturation currents are essentially proportional to fuel flow alone. Much the same applies to chemiluminescence. However, schlieren contours, which were recorded both to provide a basis for burning velocity measurements and to explore density changes in the reactants, indicated the presence of a burner – stabilised propagating reaction zone ahead of the luminous flame surface starting at around 50 vol.% and possibly even at lower air admixtures. This evidence of a steep change in refractive index is indicative of a premixed reaction zone involving the added oxygen, which however generates no chemi-ionization and emits no light. Even photographing the flame by radiation emitted at the CH∗ and C2∗ wavelengths shows no sign of its existence. Its burning velocity is about 10 cm/s, when stabilized by the surrounding diffusion flame. The most plausible rationale for these observations is the formation of syngas by the partial oxidation of methane. The subsequent burning of CO and H2 is known to occur without chemi-ionization or appreciable light emission.

The use of field-induced drag exercised on corona-generated ions is optimised for inducting the air needed for stoichiometric combustion in compact, simple, lightweight, robust and self-contained burners with no moving parts. These burners operate with a wide variety fuels without sooting, at intensities sufficient to release power in the region of several kW. Gaseous and liquid fuels are injected and the flame stabilized immediately above an earthed ring electrode using various designs of capillary feeds to wicks of ceramic wool which also act as porous plug burners and bluff body flame stabilizers. The several regimes of flame stabilization at various flow velocities are discussed. Blowers involving multi-staging and the addition of field-induced swirl are explored but the favored ion blower design is the simplest, smallest and most economical. At about 7 ml in volume, it can deliver some 1.2 L/s of air, corresponding to a thermal output of 4–5 kW for stoichiometric hydrocarbon mixtures. The fraction of a Watt needed for the ion-driven air induction is so negligible a proportion of the thermal output that, if necessary, it could be generated thermoelectrically at the burner mouth. Short, premixed, highly turbulent flames are stabilized for CnH2n+2 with n=1, 3, 6, 10 and with meths (denatured ethanol), for the smallest blower dimensions, operating at an electrical input of about 0.4 W. The largest thermal power outputs for a given wick area are achieved with those fuels that are least prone to sooting. The burner is designed to operate also in the absence of natural convection; i.e., in microgravity environments.

Air induction and rotation about the axis of a vertical tube is generated by an assembly of corona discharges between pinpoints and earthed electrodes, which induces a swirling ionic wind. The mechanism is elucidated and the geometrical configuration of the electric field lines of force is optimised by studying the deposition of charged particles on the earth electrodes, by numerical modelling of a simplified geometry and ultimately by maximising the measured tangential velocities. Upward convective flows of up to a litre per second are provided by an additional ionic wind pump at the base of the tube. With assemblies of up to three layers of six points each, tangential velocities of up to 3.3 m/s (≈900 rpm) are attained at the periphery, as recorded by small Pitot tubes. The concept, developed particularly for microgravity environments, appears suitable for adding a substantial centrifugal contribution to the operation of electrostatic precipitators and as a basis for further progress on electrical field-controlled burners.

Gas flows of modest velocities are generated when an organized ion flux in an electric field initiates an ion-driven wind of neutral molecules. When a needle in ambient air is electrically charged to a potential sufficient to produce a corona discharge near its tip, such a gas flow can be utilized downstream of a ring-shaped or other permeable earthed electrode. In view of the potential practical applications of such devices, as they represent blowers with no moving parts, we investigate methodologies for increasing their flow velocities, both theoretically and by experiments. The parameters evaluated include divergence of electric field lines, avoidance of regions of high curvature on the second electrode, control of atmospheric humidity, and the use of linear arrays of stages, terminating in a converging nozzle. We find that the behavior of ionic wind generators is analogous to that of fans and our modifications have more than doubled the maximum previously reported ion-driven wind velocities.

When a strong electric field is generated between a sharp object at high voltage and a grounded electrode in a gas medium, a corona is formed near the tip of the sharp object and, as a result, the gas medium is set in motion. The current study reports on the flow behavior of a single-stage ion wind generator with and without a nozzle attached. Ion winds have rarely been measured quantitatively because traditional velocimeters (e.g., pitot tubes) have difficulty detecting their low flow speeds. In addition, seed-based measurement techniques (e.g., particle image velocimetry) are complicated by the possibility of electrical charging of the flow seed. In the present study, therefore, we obtain the velocity profile at the exit plane of an enclosed pin-to-ring ion wind generator using two techniques to help validate our results: (1) particle image velocimetry downstream of the charge containing region and (2) hot wire anemometry. The velocity profiles indicate consistently that a pin-to-ring ion wind generator produces a velocity field with a deficit at its core. The velocities are sensitive to electrode alignment and, consequently, producing symmetric profiles is difficult. The results also show that only a slight increase in velocity can be achieved by placing a converging nozzle downstream of the electrodes.

Light refraction by small diffusion flames in microgravity is analyzed, following previous observations, of the tendency of their shadows to disappear as zero gravity is approached. The theory of density profiles for spherically symmetrical flames is investigated both for the final steady state and as a function of time after the onset of zero gravity. The effects of the spreading of gradients are considered in terms of their consequences for schlieren and shadow recording. Using the drag on flame ions by electric fields to simulate microgravity by opposing natural convection, a highly magnifying schlieren-, shadow-, and deflection-mapping system is used to compare these theoretical results with optical records of small natural gas flames. There is good agreement between the two. We conclude that, while schlieren and shadow marking may not always vanish completely in zero gravity, depending on the sensitivity of the optical system, they deteriorate so much that classical deflection methods based on steep refractive index gradients are not very; suitable for the study of small diffusion flames in microgravity. As regards other optical methods, however, the cancellation of natural convection by acting on flame ions by electric fields does appear to offer the opportunity of carrying out, for example, planar laser-induced fluorescence (PLIF) studies on the bench top instead of having to use parabolic flights or drop towers.