The push toward the miniaturization of electromechanical devices and the resulting need for micro-power generation (milliwatts to watts) with low-weight, long-life devices has led to the recent development of the field of micro-scale combustion. Since batteries have low specific energy (~200 kJ/kg) and liquid hydrocarbon fuels have a very high specific energy (~50000 kJ/kg), a miniaturized power-generating device, even with a relatively inefficient conversion of hydrocarbon fuels to power, would result in increased lifetime and/or reduced weight of an electronic or mechanical system that currently requires batteries for power. Energy conversion from chemical energy to electrical energy without any moving parts can be achieved by a thermophotovoltaic (TPV) system. The TPV system requires a radiation source which is provided by a micro-combustor. Because of the high surface area to volume ratio for micro-combustor, there is high heat loss (proportional to area) compared to heat generation (proportional to volume). Thus the quenching and flammability problems are more critical in a micro-scale combustor. Hence innovative schemes are required to improve the performance of micro-combustion. In the current study, a micro-scale counter flow combustor with heat recirculation is adapted to improve the flame stability in combustion modeled for possible application to a TPV system. The micro-combustor consists of two annular tubes with an inner tube of diameter 3 mm and 30 mm long and an outer tube of 4.2 mm diameter and 30 mm long. The inner tube is supplied with a cold premixed combustible mixture, ignited and burnt. The hot produced gases are then allowed to flow through outer tube which supplies heat to inner tube via convection and conduction. The hot outer tube radiates heat to the TPV system. Methane is selected as the fuel. The model parameters include the following: diameter d , inlet velocity u , equivalence ratio φ and heat recirculation efficiency η between the hot outer flow and cold inner flow. The predicted performance results are as followings: the lean flammability limit increased from 7.69% to 7.86% and the quenching diameter decreased from 1.3 mm to 0.9 mm when heat recirculation was employed. The overall energy conversion efficiency of current configuration is about 2.56.

abstract: Micro/meso combustion has several advantages over regular combustion in terms of scale, efficiency, enhanced heat and mass transfer, quick startup and shutdown, fuel utilization and carbon footprint. This study aims to analyze the effect of temperature on critical sooting equivalence ratio and precursor formation in a micro-flow reactor. The effect of temperature on the critical sooting equivalence ratio of propane/air mixture at atmospheric pressure with temperatures ranging from 750-1250°C was investigated using a micro-flow reactor with a controlled temperature profile of diameter 2.3mm, equivalence ratios of 1-13 and inlet flow rates of 10 and 100sccm. The effect of inert gas dilution was studied by adding 90sccm of nitrogen to 10sccm of propane/air to make a total flow rate of 100sccm. The gas species were collected at the end of the reactor using a gas chromatograph for further analysis. Soot was indicated by visually examining the reactor before and after combustion for traces of soot particles on the inside of the reactor. At 1000-1250°C carbon deposition/soot formation was observed inside the reactor at critical sooting equivalence ratios. At 750-950°C, no soot formation was observed despite operating at much higher equivalence ratio, i.e., up to 100. Adding nitrogen resulted in an increase in the critical sooting equivalence ratio. The wall temperature profiles were obtained with the help of a K-type thermocouple, to get an idea of the difference between the wall temperature provided with the resistive heater and the wall temperature with combustion inside the reactor. The temperature profiles were very similar in the case of 10sccm but markedly different in the other two cases for all the temperatures. These results indicate a trend that is not well-known or understood for sooting flames, i.e., decreasing temperature decreases soot formation. The reactor capability to examine the effect of temperature on the critical sooting equivalence ratio at different flow rates was successfully demonstrated. / Dissertation/Thesis / Masters Thesis Aerospace Engineering 2019

Les travaux de cette thèse ont porté sur la conception, la réalisation et la caractérisation de matrices de micro-propulseurs à propergol solide intégrés sur silicium. Ces structures sont dédiées la stabilisation de drone miniature et pouvant aussi être utilisées pour la propulsion des Micro/Nano-Satellites. Les travaux se sont effectués dans le cadre d'un projet financé par la Direction Générale pour l'Armement (DGA) en collaboration entre le LAAS-CNRS et la société PROTAC du groupe THALES. Le principe de fonctionnement d'un micropropulseur repose sur l'initiation thermique d'un matériau pyrotechnique de type propergol introduit dans la cavité des micropropulseurs. Une fois soumis à une polarisation de type courant, une résistance micro-usinée sur une membrane diélectrique très fine chauffe le propergol par effet Joule jusqu'à initié de l'auto-combustion. Les gaz générés vont traverser la micro-tuyère et fournir la poussée. Après avoir évalué les besoins en propulsion pour la stabilisation d'un drone miniature en vol, nous avons opté pour la micropropulsion à propergol solide qui présente de nombreux avantages pour l'application visée : c'est une technologie simple, nécessitant peu de puissance de fonctionnement (quelque 100mW) et qui est adaptable facilement au besoin de la mission. Les forces générées sont réglables entre quelques 100µN jusqu'au N en modifiant seulement - pour un type utilisé de propergol - la dimension du col de tuyère. Au cours de ce manuscrit de thèse, nous présenterons tout d'abord les spécifications de la DGA qui ont guidées nos conceptions, nous présenterons ensuite la technologie de fabrication et d'assemblage mis en œuvre au sein de la centrale technologique du LAAS. Et en fin, les résultats de caractérisation qui valident le fonctionnement et la gamme de poussée accessible par cette technologie seront donnés.

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