Study On Heat Transfer Of Gas - Solid Bed And Wall With Different Flow Patterns

Heat Transfer between high-temperature gas-solid bed and cooling wall are widelyapplied in many occasions. Establishing a heat transfer model covering variousgas-solid flow patterns will be beneficial to improving the application of gas-solid heattransfer theory as well as enhancing heat transfer in relevant process.In this paper, flow behavior and heat transfer mechanism between hot bed andcooling wall in three typical and widely-used flow patterns, which is packed bed,bubbling bed and fast bed, has been systematically analyzed. A bench-scale test rig forsolid phase emissivity measurement is constructed and experimental investigation onthe apparent emissivity of hot particle cluster has been carried out focusing on buildinga theoretical model for calculating emissivity of gas-solid mixtures. An integrated heattransfer model is put forward which is valid for various gas-solid flow pattern based onthe above analysis. Both bench-scale and pilot-scale packed bed heat exchanger hasbeen built, offering abundant heat transfer data for model validation. Field test in a300MW industrial CFB boiler has been conducted, providing experimental data forvalidation in fast bed. Model prediction and experimental validation is carried out,which shows that the model possesses reasonable reliability. Then the prediction of theproposed model is analyzed. The detailed conclusions are as follows:(1) The heat transfer mechanism in three targeted flow patterns are all consist ofparticulate phase convection, gas phase convection and gas-solid comprehensive phaseradiation. Particulate phase convection is mainly achieved by two essential componentsincluding particle-to-wall contact heat transfer and heat conduction through the thermalpenetration layer within particulate phase. Gas convection is related to gas velocity aswell as flow patterns while only plays limited role in the entire heat transfer process.Radiation is determined by the gas component and solid phase emissivity. Heat transfermechanism in these three solid-gas flow is essentially the same, providing theoreticalbasis for building an integrated heat transfer model.(2) Experimental research in this paper shows that the apparent emissivity of thesolid cluster is increasing with the increasing of the solid volume concentration and thedecreasing of the solid temperature while in the test temperature range while solidparticle diameter is not so relevant to the emissivity. An empirical model on particulate emissivity is generalized and can be applied to the prediction of bed emissivity.(3) Based on particulate phase convection, a heat transfer model which is valid forvarious flow patterns is presented in this paper. Experimental validation with literaturedata including packed bed, bubbling bed and fast bed is executed in detail. Particularly,heat transfer data in bubbling bed and part of the data in fast bed is from literatureswhile data in packed bed is from the self-built test rig. Among the range of theconcerned typical gas-solid flow patterns, the predicted results are in good agreementwith the experimental results from experimental studies both in literatures and test inthis study, which proves the reliability of the model.(4) The experimental results from the high-temperature packed bed heat transfertest facility, including the bench scale one and the pilot scale one, shows that themeasured heat transfer coefficient is increasing with the increasing of the particletemperature, the decreasing of the particle residence time and the decreasing of theparticle diameter.(5) Using the model established in this paper, the heat transfer coefficient ispredicted continuously in successive different flow patterns for the range of packed bedto fast bed. The prediction shows that with the increase of gas velocity, heat transfer inbubbling bed is higher than packed bed. After reaching a maximum value, heat transferin fast bed is gradually decreasing with the further increase of gas velocity.