3. Results and discussion3.1. Analysis of orthogonal test Nine groups of numerical simulation results of the orthogonal test were obtained by CFD simulations (Table 1). Bed expansion was selected as the evaluation index to investigate the effects of three structural parameters on device performance and to search for the optimum combination of factor levels. Finally, the structure of the CB FSB was optimized. Table 1 showed information on the range analysis for these simulation results. Intuitive analysis was simple and clear. The importance of factors to performances of devices can be determined and the optimum combination thereof can be chosen. Range is the difference between the maximum and minimum values in different levels under the same factor, which reveals the effects of different levels on the evaluation index. The larger the range, the more sensitive the response of the evaluation index to different levels, and thus the more important is the corresponding factor. Following this analysis,