Revisiting the Nature of Turbulent Fluidization

Looking back to the history of the gas-solids fluidization, the term “turbulent fluidization” has been evolved from describing a characteristic phenomenon to defining a distinct flow regime with the increasing understanding of the nature in gas-solid fluidization system. The discussion on the existence of the turbulent fluidization regime started in the 1970s after Kehoe and Davidson (1970) first came up with the concept. The demarcation of the turbulent flow regime was first proposed by Yerushalmi et. al (1978) and then widely studied and accepted that the transition to turbulent fluidized bed (TFB) is signified by the maximum pressure fluctuations. Getting into the 1990s, more studies have been expanded to the general hydrodynamics, as well as on gas-solids mixing and heat/mass transfers. In the industry, however, the application of the turbulent fluidized bed reactors actually pre-dated the inception of its “official name”, recognizing its practical benefits with its high solids holdup, excellent gas-solids contacting, and efficient heat/mass transfers, as early as the 1950s. On the other hand, the academic studies provided more theoretical understanding, that in turn benefited the industry, such as leading to the switch of the Sasol Synthol reactor to turbulent fluidized bed operation. The research and development on the turbulent flow regime and the turbulent fluidized reactors not only manifests the significance of critical thinking but also inspires more young scholars to search for novel variations of fluidized beds for newer applications.

At the 50-year point, it is perhaps a good time to revisit the nature of turbulent fluidization with some possible new interpretations. Different transition velocities to TFB regime based on various measuring techniques and criteria especially developed after 2000 will be reviewed. The flow mechanisms and the interactions between the dilute bubble/void phase and the dense particulate phase will be discussed based on both the time average data such as the overall pressure drops or the solids holdups and the instantaneous signals such as the time series analysis of signals or images. The two-phase flow structure which is well accepted in bubbling flow regime will be examined and extended in the turbulent regime to reveal the link between the conventional flow regimes. More quantitative analysis on the dynamic behavior of both the gas bubbles/voids and particles will be reviewed and the spatial heterogeneity of flow structure will be discussed.