In many applications in process engineering and biotechnology, chemical reactions between a liquid and a gas take place in a reactor type called bubble column during the rise of bubbles through the liquid. The two-phase flow in bubble columns has received increasing interest since some decades, since its properties influence widely the performance of the bubble column. For the case of a Newtonian liquid as the continuous phase, deep insight into the transport processes between bubbles and liquid has been achieved. Biotechnological processes in bubble columns, however, in many cases apply viscoelastic liquids as the continuous phase. In the field of disperse two-phase flows with gas bubbles rising in a viscoelastic liquid, many open questions still exist to date. One open issue with big importance for, e.g., the residence time of the bubbles in the liquid is the dynamic behaviour of the bubbles during the rise. This is markedly influenced by the shape the bubbles assume during the rising motion. Furthermore, when a certain critical bubble size is exceeded, the so-called bubble rise velocity jump discontinuity occurs. The physical reason for this discontinuity is unexplained to date. The present project is designed to provide knowledge about the rise behaviour of bubbles in viscoelastic liquids and to explain the reason for the jump discontinuity. For this purpose, the liquid is rheologically characterised in an elongational flow. Since elongational stresses in the liquid around the rising bubbles, despite their importance for the bubble motion, were not taken into account in investigations on the rise velocity jump discontinuity as yet, the present approach is promising. Very important for bubbly two-phase flows is also the intensity level of velocity fluctuations produced by the rise of the bubbles in the viscoelastic liquid. The proposed project addresses this field by LDA-based measurements in the liquid to quantify the turbulent velocity fluctuations caused by bubbles both below and above the rise velocity discontinuity. The rheological behaviour of the liquid is characterised by a Deborah number, which is formed with the terminal rise velocity of the bubble and its radius. Once this number exceeds a threshold value, normal stress relaxation begins to play a role (strong flow). The overall aim of the project is to provide basic knowledge about bubbly two-phase flows with viscoelastic liquids.
|Effective start/end date||1/10/04 → 30/09/07|