Abstract

Objective As an important inorganic non-metallic powder material， ultrafine calcium carbonate is widely used in plastics， rubber， coatings， and paper making， etc. Due to its extremely small particle size and adhesive nature， there are difficulties in its conveying process leading to lower conveying efficiency and increased equipment wear. Therefore， it requires improved design and operation of conveying equipment. Screw conveyor， as an efficient and widely used solid material conveying equipment， is characterized by its simple structure， low maintenance costs， and ability to convey materials in horizontal， inclined， or vertical directions. With the advancement of computer simulation technology， the discrete element method （DEM） technique has become an important tool to study the particle flow and interactions in screw conveyor conveying process. Current research on CFD-DEM coupling method for screw conveyors typically focuses on operational parameters for large particle materials. However， they overlook investigations into the role of geometric friction coefficients in the ultrafine powder conveying. In order to improve the efficiency of screw conveyors and reduce power consumption and wear， the study was conducted to examine the particle flow state， outlet mass flow rate， conveyor power consumption， and wear distribution of ultrafine calcium carbonate in a horizontal screw conveyor with variable diameter and pitch. This study provided theoretical and technical support for effective transport of ultrafine powders， as well as new perspectives and methodological foundation for engineering applications and scientific research in related fields.

Methods In this study， we adopted the CFD-DEM coupling method， using FLUENT， a fluid dynamics software， and EDEM， a discrete element software， to explore the gas-solid two-phase flow characteristics of ultrafine calcium carbonate powder during horizontal conveying with variable diameter and pitch of the spiral. It also explored the effects of different operational and geometric parameters on the conveying process. In CFD-DEM gas-solid coupling， the gas phase was treated as a continuous phase， governed by continuity and momentum conservation equations. RNG k-ε model was used in the turbulence model， which was more suited for the complex turbulent flow inside the screw conveyor. The particles were regarded as a discrete phase， and their motion was described by the Newtonian kinetic equations. The Archard wear model was used to simulate the wear process on the spiral blade surfaces correlating the amount of material worn on the surface of the spiral blade to the friction work exerted by the frictional action of the particles on that surface. The simulation model of the horizontal screw conveyor with variable diameter and pitch was constructed in this paper and its specific structure was shown in Fig. 1 and 2. A tetrahedral unstructured mesh with high adaptability was chosen to be applied for meshing in this study， as shown in Fig. 3. Parameters of the simulation experiment were detailed in Tab. 1.

Results Simulated mass flow rates at different rotational speeds closely aligned with experimental results， showing a small margin of error. This high level of accuracy confirmed the model's reliability for simulating the screw conveying of ultrafine calcium carbonate. In the horizontal screw conveyor with variable diameter and pitch， particles uniformly flowed towards the outlet along the screw shaft， demonstrating stable conveying performance. The coefficient of friction between particles and geometry significantly influenced particle behavior in various conveying sections. However， optimal conveying relied not solely on higher or lower friction coefficients but instead on finding an appropriate balance. Variations in peak mass flow rates were linked to particle axial velocity， which was influenced by friction coefficients. Excessive friction impeded particle flow， while insufficient friction reduced particle forward momentum， leading to equipment clogging and reduced mass flow rates. Power consumption analysis at a constant feed rate of 2 kg/s revealed decreasing consumption with increasing rotational speed， with the impact of the friction coefficient diminishing at higher speeds. Wear simulation results indicated that higher rotational speeds reduced material residence time on blade surfaces， thus reducing wear interactions. Conversely， increased friction coefficients significantly elevated wear rates due to heightened sliding or rolling resistance between material and blade surfaces. This abrasive interaction intensified wear rates under conditions of higher friction.

Conclusion In a screw conveyor， materials moved axially and in a circular flow within each section. The coefficient of friction between the particles and the geometry had a significant effect on the particle flow state. Changes in the friction coefficient did not necessarily result in increased axial velocity of the particles or the mass flow rate of the conveyor. Instead， there existed a locally optimal combination of parameters that could be achieved. As the discharge speed and friction coefficient increased， the power consumption of the conveyor also rose. An appropriate increase in rotational speed could reduce power consumption. However， the influence of the friction coefficient on power consumption was more pronounced at high feed speeds and low rotational speeds， compared to low feed speeds and high rotational speeds. The more serious wear area concentrated at the edges of the spiral shaft and spiral blades near the lower feed opening. For any given rotational speed， higher coefficient of friction between the particles and the geometry with greater feed speed resulted in more severe wear of the spiral blades and spiral shaft. Conversely， higher rotational speeds of the spiral shaft led to lower overall wear rates. The study provides theoretical and technical support for efficient conveying of ultrafine powders， and offers new perspectives and methodological basis for its engineering applications and scientific research in related fields.

Keywords：ultrafine calcium carbonate； Computational fluid dynamics； Discrete element method； Screw conveyor； Particle flow