Temperature measurement of aqueous ammonium chloride solution during solidification process using laser-induced fluorescence.

摘要

A wide variety of products are now made of metal alloys. The superior properties of these alloys permit products to be used in very demanding environments where other materials would fail. The major quality issues for these alloys are melt-related defects, such as segregation defects and freckles. Buoyancy-induced fluid flow during solidification, which is primarily responsible for most forms of defects (chimneys), is not directly controllable. If buoyancy-induced fluid flow is left uncontrolled, natural convection will contribute to nonuniform distributions of alloy constituents and grain structure in castings. Materials with acceptable defects can be produced only by trial-and-error and their acceptability is determined by costly inspection. The quantitative measurement of the temperature field during the solidification process will enhance our understanding of the natural convection phenomena, and provide benchmark temperature data valuable to other analytical and numerical studies.; Aqueous ammonium chloride is a popular low temperature analog to metal alloys because of its transparent nature. A two-color laser induced fluorescence (LIF) technique was developed to measure instantaneous whole field temperature distribution above the mushy zone during uni-directional solidification of aqueous ammonium chloride that was cooled from below. The temperature resolution was improved and doubled by using two temperature sensitive dyes having opposite signs in their sensitivity. The fact that the intensity of one dye increased and the other decreased with decreasing temperature, resulted in a higher temperature sensitivity (4% K-1) in the two-color LIF two-dye approach compared to all previous implementations of the two dye approach (maximum of 1.7% K-1). This was an important outcome of this study.; In this study, in order to understand some of the observed flow features during the solidification processes of the ammonium chloride solution, two different optical diagnostic techniques (PIV and MTV) were used to obtain the velocity fields in these regions.; The temperature mapping relied on the temperature dependence of fluorescence intensity, estimated from the intensity ratio of two temperature sensitive dyes. By using this technique, temperature fields of some of the complex flow features, such as plumes and wisps, were obtained that had not been available before.; Along with the fundamental study based on temperature measurements, a novel technique was also found to enforce the chimney formation at desired locations in the mushy zone during the solidification process of ammonium chloride. This was done by placing metallic nucleators in particular arrangements on the bottom cooling plate. The effects of the arrangement and size of the nucleators on the plume structure and solidification process was investigated. Results showed that it is possible to obtain a relatively large area free of chimneys in the final product of the solidification process by placing the nucleators in certain arrangements. The fact that the flow was calm and the temperature was uniform laterally above this area after t=60 min suggested that the solidification process was taking place uniformly at mush-melt interface, resulting in a better quality of the final product.