This paper describes an UV laser diagnostic system by means of which laser spectroscopic experiments were performed under microgravity conditions in a ground-based drop tower for the first time. A tunable, narrow bandwidth exchner laser is positioned at the top of the drop tower. The laser beam enters a falling drop capsule containing a specially adapted burner or combustion chamber. By the use of laser induced fluorescence spectroscopy measurements of 2dimensiona1 concentration and temperature profiles can be performed. Solutions of selected experimental problems such as laser beam collimation over a distance of more than 120 m, compensation of capsule drift, signal detection and data acquisition (250 frames/s, 4.7 s measuring period), are discussed in detail. First measurements of laser induced predissociation fluorescence (LIPF) of OH radicals in a methanol flame under microgravity conditions are presented.
2INTRODUCTIONInvestigation of combustion processes under buoyancy-free conditions give important insight into the proceeding physical and physico-chemical reactions, since the overlapping effect of buoyant convection is eliminated [1]. As a representative for many important technical problems the study of evaporation, self ignition and burning of single droplets can be mentioned, where under normal gravity conditions convectional effects prevent detailed investigation of the complicate interacting two-phase processes. Under strongly reduced gravity conditions these processes can be experimentally investigated in detail [2-5] and compared with numerical simulations [6,7].Measurements under reduced gravity conditions can be conducted in orbiting spacecrafts, in parabolic flights and in ground based drop towers [1]. While experiments on board of space shuttles or orbital space stations allow long experimental times they are extremely expensive and quite unflexible compared to ground based facilities. The drop tower in Bremen offers experimental times of 4.7 seconds, which are sufficiently long for various combustion experiments. Residual acceleration levels are below 1O g [8].Up to now the measurement techniques applied to these experiments were restricted to photography, videofilming, interferometric devices [2-5] and various intrusive probing techniques. The reasons are strongly restricted limits of payload mass, volume, power consumption and mechanical strength. In this paper an advanced experimental set-up is presented, which for the first time allows the application of laser spectroscopic measurement techniques such as two-dimensional laser induced fluorescence [9-121 to strongly reduced gravity combustion experiments at drop tower Bremen. Since this laser system might have wide applications for experiments under microgravity in different fields of combustion research and fluid mechanics, details of the applicated experimental method and its technical solution as well as first experimental results are presented here. 102 ISP1E Vol. 2506 0-8194-1864-1/95/$6.00 Downloaded From: http://proceedings.spiedigita...
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