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Experiments were performed under low gravity condition in order to investigate the surface reaction model. A spherical platinum of 1.5 mm in diameter was used, and the surface temperature was measured by thermocouple. The mole fraction of nitrogen of the mixture was 0.91. As the effect of natural convection can be neglected under low gravity condition, the governing equations of mass, energy and species concentrations are formulated in one-dimensional polar coordinates. Axial symmetry, zero azimuthal velocity and no gradient along the axis are assumed. The low gravity condition of the present experiment was realized by the parabolic flight. The relative hydrogen concentration of the mixture was varied. If the relative hydrogen concentration is larger than 0.23, the numerical results simulated the experimental results successfully. However, at relative hydrogen concentration less than 0.23, the numerical results did not explain the experimental results well.

The hot surface ignition of methane air mixtures in microgravity has been studied experimentally and numerically. Experiments on the ignition of the mixtures with electrically heated nickel wires and platinum wires in microgravity have been performed. Numerical calculations, including the catalytic reaction rate for platinum, have been performed to understand the experimental results obtained in microgravity. The ignition delays and ignition temperatures for a wide range of equivalence ratios were investigated. Experimental results show that the ignition temperatures with platinum wires have a maximum near the stoichiometric mixture ratio, while those with nickel wires increase as the equivalence ratio increases. Ignition temperatures with platinum wires are higher than those with nickel wires. Numerical results show that reactants next to platinum wires are consumed by the catalytic reaction. Therefore, a higher temperature is required to ignite mixtures with platinum wires. The catalytic inhibition of hot surface ignition is simulated successfully by the numerical model.

In order to investigate the effects of equivalence ratio and total pressure on hot catalytic surface ignition, experimental and numerical studies were conducted. A platinum sphere of 1.5 mm diameter was used as the catalyst. Premixed gases contained hydrogen, oxygen and nitrogen. The mole fraction of nitrogen was 75% in all cases. Surface temperature of the platinum catalyst was measured by a thermocouple. A photodiode was used to detect the ignition. In this paper, ignition temperature was defined as the output of the thermocouple at which ignition occurred. When total pressure decreased, ignition temperature increased and the effect of equivalence ratio changed. This seems to be caused by the difference in diffusion characteristics of dominant species.

Hot surface ignition of methane-air mixtures has been experimentally studied in normal gravity and microgravity. The primary aim of this research is to explain the effects of natural convection and catalytic reaction on the hot surface ignition in a closed vessel. The hot surfaces used are platinum wire for catalyst and nickel wire for non-catalyst. In order to define the initial condition and make the analysis simple, the following control unit was developed; which heats the wire to the setting temperature in a very short time, and maintains the wire temperature constant until ignition. From experimental results, ignition temperatures with platinum wire are higher than those with nickel wire by the catalytic inhibition of ignition due to the reactant depletion on the surface of the wire. Ignition temperatures with platinum wire are highest near the stoichiometric mixture ratio and decrease with equivalence ratio depart from stoichiometric mixture ratio, while those with nickel wire increase with equivalence ratio. Ignition temperatures in normal gravity are higher than those in microgravity. Natural convection supplies reactants to the platinum wire surface, and therefore reactants are consumed because of promoting catalytic reaction in normal gravity.

A surface reaction occurs on a catalyst surface when the catalyst sample is heated up to a certain temperature in a reactive mixture. After the surface reaction occurs, a steady state is observed without energy supply because the heat produced by the surface reaction is balanced with the heat loss caused by convection, conduction and radiation. The heat release of the surface reaction can be evaluated by the surface temperature of the catalyst sample at the steady state.This paper treats the effects of the equivalence ratio (ER) and the dilution ratio (DR) of the reactive mixture on the surface reaction. Hydrogen and oxygen are used as reactive mixture and nitrogen as inert gas. The height, width and length of the combustion chamber are 76mm, 140mm and 140mm, respectively. A spherical Pt catalyst sample of 1.5mm in diameter is sustained in the chamber with two wires of 0.1 mm in diameter. The chamber is filled with a hydrogen-oxygen-nitrogen mixture at the atmospheric pressure and the room temperature. The surface temperature is measured with a thermocouple. As the results, the heat produced by the surface reaction decreases with increasing dilution ratio, and hydrogen is adsorbed more easily compared with oxygen.