Elsevier

Advanced Powder Technology

Volume 31, Issue 9, September 2020, Pages 3974-3992
Advanced Powder Technology

Original Research Paper
CFD-DEM study of the effects of solid properties and aeration conditions on heat transfer in fluidized bed

https://doi.org/10.1016/j.apt.2020.08.002Get rights and content

Highlights

  • A CFD-DEM study of gas-solid heat transfer process in fluidized bed is presented.

  • The effects of particle size, density and heat capacity are investigated.

  • Two aeration settings are considered (same gas velocity or fluidization number).

  • Heating rate and temperature uniformity in the bed are quantitatively evaluated.

Abstract

The solid properties are of significant influence on the thermal performance of the fluidized bed. In order to provide valuable information for the application of this equipment, a numerical study is carried to clarify the effects of solid properties on the heat transfer characteristics in a lab-scale fluidized bed by means of the CFD-DEM method. Specially, two aeration conditions, i.e. the same superficial velocity and the same fluidization number, are considered. The results show that the violent convective mechanism at bed bottom plays a significant role in the heating of the bed material. The entrainment of rising bubbles and hence solid mixing are the key factors to get better temperature uniformity of the bed during the heating process. With the decrease of particle density and size, the internal circulation of solid phase is strengthened under the same superficial velocity, while slightly weakened under the same fluidization number. Obvious resemblance can be captured between solid mixing and temperature uniformity, and the enhanced solid mixing usually leads to homogeneous temperature distribution of the bed. It can be found that the heating rate decreases with increasing solid density regardless of aeration setup. In addition, it is positively related to particle diameter under the same fluidization number, while keep unchanged under the same superficial velocity. Furthermore, enhanced solid mixing and better temperature uniformity can be captured with increasing solid heat capacity, which confirms that gas temperature shows considerable effect on gas-solid flow.

Introduction

As the main operation mode of fluidized bed, gas fluidization can be encountered in numerous industrial fields such as coal combustion [1], [2], material drying [3], [4], coating [5], [6] and granulation [7]. These processes are generally accompanied by intense interphase momentum and heat exchange, which ultimately affect the efficiency of the reactor. For the drying process of wet material, the drying rate is primarily determined by the heat exchange between gas phase and particle phase. During the gas-phase polymerization process, heat generated by the bed material is absorbed by low temperature gas, and the heat transfer between bubbles and emulsion phase is of great importance [8], [9], [10]. Moreover, the bed hydrodynamic characteristics such as solid circulation also produces some impacts on the overall performance of reactor. In the past few decades, there have been a large number of research on hydrodynamics and thermal performance of fluidized bed [11], [12], [13].

To obtain better bulk performance in reactor, the selection of bed material is very concerned. Due to the difference in size, density and hence fluidization characteristics, the particles can be divided into Geldart A, B, C and D particles [14]. As summarized in previous study [15], [16], [17], the bed exhibits different gas-solid flow patterns under different bed material compositions. One typical example is that for Geldart A particles, specific expanded bed structure can be captured before superficial velocity reaches the minimum bubbling velocity. Moreover, the differences in the heat transfer performance are also reported from published literatures [18], [19], [20]. Basically, for the sake of providing guidance for the selection of bed materials in above applications, profound understanding of bed hydrodynamics and interphase heat exchange under different bed material properties is of highly significance.

Efforts have been made to experimentally investigate the effect of solid properties on heat transfer characteristics. Temperature probes, which are extended into or on the walls of the beds, are used to record the bed temperature at certain positions. On this basis, many empirical correlations between heat transfer coefficient (HTC) and solid properties are carried out [11], [13]. The overall thermal performance of fluidized bed is well reflected by these macroscopic correlations. To further explore the mechanisms at a particle scale, the tracer particle method has been developed to investigate the heat transfer characteristics of individual particles in bed [21], [22], [23], [24]. Based on this method, the instantaneous temperature response of tracer spheres with different diameters was effectively measured by Collier et al. [22]. Considering the complexity of dense gas-solid system, obtaining the temperature field information in the system is still a challenging task. Even so, some researchers have successfully developed the non-invasive technology to measure the temperature in bed [25], [26], [27], [28]. A series of experiments had been carried by Patil et al. [26] to measure the distribution of solid volume fraction, velocity and temperature in fluidized bed. And the time evolution of bed temperature under two particle size is presented. These results provide comprehensive information and further enhance people’s understanding of interphase heat exchange mechanism. However, most of the bed geometry in these non-invasive measurement experiments are limited to quasi two-dimensional beds, the heat dissipation on the front and back walls is also non-ignorable.

With the development of mathematical modeling and the enhancement of parallel computing capability, numerical simulations have led to possibilities of acquiring comprehensive hydrodynamics and heat transfer information in dense particulate system. Measurement error caused by wall heat loss can be easily avoided by numerical simulations, which is difficult to achieve in visual experiments. Among numerous methods to simulate the coupled dense granular flow and heat transfer, the computational fluid dynamics coupled with discrete element method (CFD-DEM) based on Eulerian-Lagrangian framework has attracted more and more attention of researchers. After Tsuji et al. [29] firstly modelled a two-dimensional fluidized bed with CFD-DEM model, more and more researchers are devoted to investigate the heat transfer process in dense gas-solid system, which includes gas-solid phase heat transfer [19], [30], [31], [32], [33], bubble-emulsion phase heat transfer [34], [35], surfaces-bed material heat transfer [36], [37], [38], etc. The effects of solid properties on gas-solid heat transfer characteristics has been reported in published literatures [19], [33], [39], [40]. Patil et al. [40] conducted a CFD-DEM simulation of fluidized bed with hot gas injection, and the bed thermal behaviour under different particle size is discussed. The results show that the bed material with smaller particle size exhibits better uniformity under the same inlet gas mass flux. Moreover, a tracer particle analysis is also presented to reveal the effect of particle size on temperature evolution at a particle scale. Zhou et al. [33] investigated the effect of particle thermal conductivity on particle HTC, where they found that the conduction path gradually dominates with increasing solid thermal conductivity, while the contribution of convection path seems keep unchanged. A comprehensive study of the solid collisional parameters effect had been carried out by Hu et al. [39], and the results confirm that the bed hydrodynamics and heat transfer does show an obvious relevance with these collisional parameters. Furthermore, Hou et al. [19] explored the relationship between particle size and the bulk heating rate of the bed in the range of 0.02 mm < dp < 3 mm. However, the bed dimensions change proportionally with particle size to keep the total number of particles consistent, which can only provide a limited reference.

Besides the solid properties involved in above literatures (e.g. diameter, thermal conductivity and collisional parameters), solid density also shows great impact on bed hydrodynamics. It can be inferred that the bed heat transfer characteristics may exhibit some differences under different material densities. In addition, most of the above study were carried out under single aeration condition. As pointed out by Luo et al. [41], the results may show contrary trend under different aeration setup. In general, the investigation on the effects of solid properties on heat transfer characteristics is still limited, and the correlation between solid properties and some critical indexes (e.g. temperature uniformity and heating rate) is not clear.

In this paper, the major work is as follows. The governing equations, physical models and metrics involved in current simulation are first briefly introduced. Subsequently, a typical case is simulated to analyze the bed hydrodynamics and heat transfer characteristics in fluidized bed. Instantaneous flow pattern, time average statistics and solid temperature evolution are discussed qualitatively and quantitatively. On this basis, the effects of solid properties are further analyzed, including particle density, diameter and heat capacity. Two different aeration modes (i.e. the same superficial velocity or the same fluidization number) are considered. The bed hydrodynamics, time-averaged temperature, bulk heating rate and temperature uniformity under different solid properties are discussed.

Section snippets

Governing equations

The current simulation is carried out in the 3D Eulerian-Lagrangian framework and so-called CFD-DEM method. The motion and heat transfer of continuous phase (i.e. gas phase in this paper) are described by local averaged continuity equation, Navier-Stokes equations and energy equation. The gas turbulence is also taken into account and described by a standard k-ε turbulence model [42]. The particles in bed are considered as the discrete phase, where the trajectory of each individual particle is

Model validation

Validation test is a key step before the model is applied to the formal study. Actually, the CFD-DEM principles have been well validated by comparison with published experimental data, and successfully used in our previous study. Among them, the validation work of cold CFD-DEM model can be found at Ren et al. [48] and Xu et al. [50]. Because the interest of current work lies in the heat transfer process in dense gas-solid system, the validation of heat transfer model is introduced here.

A series

Heat transfer characteristics in a typical case

Firstly, the gas-solid flow and heat transfer characteristics in a typical case are analyzed. The initial packed bed material is composed of coal tar pitch spheres, have a density of 1200 kg/m3 and a diameter of 1.2 mm. The superficial velocity is 1.6 m/s. The specific heat capacity of coal tar pitch is about 1600 J/(kg∙K). For the simulation condition in this paper, the whole heat transfer process lasts for hundreds of seconds, which takes several months for CFD-DEM model to complete the case.

Conclusions

In this work, the gas-solid heat transfer process in a laboratory scale fluidized bed is numerically studied by means of the CFD-DEM method. The key hydrodynamics and thermal behaviour of the fluidized bed can be captured by a typical case analysis. In order to provide valuable information for the application of this equipment, the instantaneous flow pattern, time-averaged temperature distribution, bulk heating rate and temperature uniformity under different solid properties (particle density,

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work was financially supported by the National Natural Science Foundation of China [project number: 51876037].

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