A microgravity combustion experiment was performed on a partially pre-vaporized droplet array in November 2009, by the flight of TEXUS 46 sounding rocket, which was launched from the Esrange launch site in northern Sweden. The flight experiment was performed as a collaborative mission of JAXA and ESA. The Droplet Array Combustion Unit (DCU) was developed by JAXA and finally integrated as Japanese Combustion Module (JCM) to be installed into the TEXUS rocket. In the flight experiment, flame spread behavior of n-decane droplet arrays was observed with different degree of pre-vaporization. Also, collection of combustion gas samples during the flight was performed for gas composition analysis on the ground. In this paper, technical achievements of the DCU as well as preliminary scientific results are reported.

Lean, partially premixed, swirl-stabilized spray flames are discussed for novel combustion concepts to allow a further abatement of the nitrogen oxides (NOx) emissions. The relevant knowledge for those concepts is based on combustion research of manageable and observable droplet regimes.This paper initially presents the experiment setup for the combustion of a linear n-decane droplet array under microgravity conditions. After pointing out all relevant aspects of the associated exhaust gas sampling and analysis processes, the final focus is set on the results of the exhaust gas production. A wide range of the degree of droplet pre-vaporization is investigated. The PHOENIX experiment ("Investigation of Partial Pre-vaporization Effects in High Temperature on Evolution of Droplet Array Combustion and Nitrogen Oxides Formation") on the TEXUS-46 sounding rocket mission was conducted against this background. Three successful combustion runs could be performed during this mission. Scientific quality and consistency of the results are high. The results on CO2 and CO are in line with combustion theory, and their absolute values are of the correct order of magnitude but not yet corrected for secondary effects. The NOx emissions decrease with an increase of pre-vaporization rate psi. However, a straightforward portability of the derived NOx production characteristics to technical applications needs to be assessed carefully on the basis of the prevailing droplet burning regime.

Preparation for Japan - Europe cooperative microgravity combustion experiment by using TEXUS sounding rocket, which would be launched in the spring of 2009, is on-going in JAXA. The TEXUS experiment is established based on cooperation between Japanese droplet combustion research working group and European CPS (Combustion Properties of Partially Premixed Spray Systems) research team. In this cooperative project, JAXA prepares experimental plan of the flight experiment and develops experimental apparatus while ESA provides flight opportunity of the TEXUS rocket. In the flight experiment, flame spread and combustion behaviors of partially pre-vaporized n-decane droplet array would be observed 4 times with different degree of pre-vaporization during 6 minutes microgravity duration. Also, combustion gas samples of the array would be collected for subsequent gas composition analysis on the ground.

Flame spread of a linear fuel droplet array has been investigated by many researchers [1-4]. It is expected that such investigations provide fundamental knowledge for better understanding of flame propagation mechanism of fuel spray, since flame spread of a linear fuel droplet array would be the simplest configuration to study flame spread mechanism among fuel droplets. Kikuchi et al. have also studied flame spread mechanism of a linear n-decane droplet array by numerical simulations as well as microgravity experiments [5,6], in cooperation with experimental study by Mikami et al.[4]. In our past research, fundamental flame spread mechanism of a linear fuel droplet array, such as effects of droplet interval S and ambient temperature T on flame spread phenomena, was investigated for the droplet array without pre-vaporization [5] and with pre-vaporization [6]. Experimental results with pre-vaporization showed occurrence of characteristic shape on OH radical emission at spreading flame front as well as increase of flame spread rate Vf with increase in degree of pre-vaporization. Also, numerical results indicated the experimentally observed characteristic shape on OH radical emission to be triple flame structure. In the previous study with pre-vaporization of the droplets [6], gas layer of fuel vapor-air mixture was assumed to be formed around the droplets with fuel concentration gradient. Considering real spray combustion, however, fuel vapor-air mixture would exist as the ambience, in addition to the gas layer with fuel concentration gradient in the vicinity of the droplets. Therefore, effects of fuel vapor-air mixture on flame spread of a linear ndecane droplet array are numerically investigated in this study. Also, comparison of the numerical results with related experimental results in microgravity by Suganuma et al. [7] will be shown.