INVESTIGATION ON CRYSTALLIZATION AT HEAT TRANSFERRING WALLS USING HOLOGRAPHIC-INTERFEROMETRY

The holographic interferometry was used for investigations on temperat ure and concentration boundary layers during the crystallization of sa lts from aqueous solutions. Results are discussed for aqueous solution s of sodium sulfate and potassium nitrate. The Nusselt numbers derived from temperature profiles coincide very good with calculated results from heat transfer relations. However, no significant changes of conce ntration in the vicinity of the surface could be found. Investigations with a dissolving crystallization layer show a distinctive diffusion boundary layer. The measured results led to Sherwood numbers 24% highe r than published. On the other hand no diffusion boundary layer could be detected for progressive crystallization. This means that the cryst al layer grows is reaction controlled. Microscopical pictures of the c rystal layer surface show a strongly rugged surface with steps of abou t 60 mum height. These coarse structures disturb the formation of a bo undary layer and prevent the light beam which passes through the test channel. Therefore an existant diffusion boundary layer may be covered up by the crystal layer.