Study Of Flow And Heat Transfer And Fouling Preventing And Removing In Circulating Fluidized Bed Heat Exchanger

The vapor-liquid-solid circulating fluidized bed heat exchanger can effectively enhance heat transfer, prevent and remove the foulings, so that it has wide application prospect. In this paper, further researches were carried out for the flow, enhancing heat transfer and the effect of preventing and removing the foulings in the vapor-liquid-solid natural circulation fluidized bed heat exchanger experimentally, theoretically and by using numerical simulation.The main contents of this paper are as follows:Firstly, the development and research state for the circulating fluidized bed heat exchanger were summarized. The brief introductions were given about the present research conditions for enhancing heat transfer, the effects of preventing and removing the foulings, as well as measuring means and numerical simulation method for multi-phase flow heat transfer. The train of thought for this paper was also given.Secondly, a set of new type vapor-liquid-solid circulating fluidized bed heat exchanger with glass transparent plated film heating was established. Its visual parts include both flow-up bed and flow-down bed. The CCD image measurement and dealing system were used; softwares for image dealing were developed. The researches were carried out about both the flow and distribution of the solid particles in this circulating fluidized bed heat exchanger, and the heat transfer as well as flow pattern for liquid-solid two-phase flow and vapor-liquid-solid three-phase flow. The research results show the changes of the concentration and velocity of the solid particles and the effect for heat transfer with heat flux and the adding amount of solid particles.Thirdly, the mathematical and physical considerations for the multi-phase flow were introduced, and the numerical calculation method was briefly analyzed. Also the research idea for numerical simulation in this paper was given.Fourthly, according to the hydrodynamics and computational heat transfer, the flow and heat transfer model was set up. In this model, the interaction between solid particles and fluid was taken into account. Here, the conception of two-phase turbulent Prandtl number was given for the first time. The calculation results of the model show that the addition of solid particles makes the fluid turbulent flow enhance, the heat transfer enhance and the wall temperature decrease. The deviation between the simulated values and the experimental data for the heat transfer coefficient is 9.6%.Finally, the technique of vapor-liquid-solid three-phase flow was applied in TCM concentration. The feasibility of this technique applied in the new field was proved by small-scale test and pilot-plant test experiment with Gengnianan as the work medium. The research results show that in the laboratory experiment, the heat transfer coefficient of the vapor-liquid-solid three-phase flows is 1.5～2.0 times that of the liquid-solid two-phase flow for approximately Gengnianan ethanol extraction. And in the pilot-plant test, the heat transfer coefficient increases by 1.5 fold; the output increases by 1.63 fold. The three-phase flow evaporator operated stably for 15 days with no fouling on wall. While the evaporator without solid particles fouled when it operated for 4 to 12 days. For Gengnianan water extraction, the output increases by 1.34 fold.