Measurement of flow field and wall temperature distribution in a scraped heat exchanger crystallizer
Résumé
During crystallization the control and effective distribution of heat transfer from the solution to the heat exchanger (HE) plays an important role. Inhomogeneity in temperature on the HE-surface limits the production capacity and increases the tendency of an isolating scale layer formation. To avoid scaling, scraper blades on the HE are commonly used. In a typical axi-symmetric geometry the scale layer formation was investigated. We studied the influence of the flow field on the actual heat transfer distribution 1) by directly measuring the surface temperature field of the HE using LCT and 2) by measuring the flow field inside the crystallizer using 3C-PIV. From Liquid Crystal Thermometry, we can clearly see that, except for the centre region, near the inside the heat transfer is considerable better than near the outside. Stereoscopic PIV velocity measurements reveal that the central region of the crystallizer is in solid body rotation; in the outer parts there is a strong secondary flow, where crystallizer solution descends onto the HE-surface, causing a high heat transfer; near the outer wall the solution rises again having a small heat transfer. We conclude that the present design of the stirrer / scraper system in the crystallizer provides sufficient secondary flow to keep the system mixed. However, near the outer wall and near the shaft the heat transfer is considerably lower than average. This is also the zone where during Eutectic Freeze Crystallization experiments the first not-removed ice-scaling layer is observed. This is thus not due to a limited scraping function of the scraper, but due to the HE getting the lowest temperature, hence the highest sub-cooling.
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