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Rice grains breakage during hulling in a rubber roll huller influences the desired final rice quality and thus its economic value. Clarifying the grains breakage mechanism in the hulling process is the key to reducing the breakage ratio and improving quality. The breakage mechanism was clarified by quasi-static compression, three-point bending, compression-shear and hulling tests. The results show that rigid compression and three-point bending breakage cannot be used to characterise grains breakage mechanism in the hulling process, and flexible compression cannot break grains. The breakage mechanism involved is flexible compression; the application of shear load rotates grains resulting in the action of bending and shear and this finally produces bending tension and shear failure. Shear force and pressure increase with the increase of shear direction, resulting in increasing breakage ratio. • Rice grain breakage is not caused by excessive pressure in the hulling process. • Shearing force is closely related to the grain breakage in the hulling process. • Grain breakage is caused by bending tension and shear breakage. • Limiting the rotation of grains can reduce breakage ratio in the hulling process. [ABSTRACT FROM AUTHOR]

Aerodynamic separation of rice husks from brown rice grains after paddy husking is one of the important processes in rice processing. So far, the realisation of complete separation of rice husks from brown rice is still a challenging task. In this study, computational fluid dynamics (CFD) and discrete element method (DEM) were used to simulate the process, and the simulation results were verified by experiment. Based on this, the process of the separation of rice husks from the mixture was clarified. The mechanisms of brown rice loss and the rice husk retention were explored by analysing the collision process of the husks and rice in the separation duct. The interaction between mixture particles and airflow was explored. The results show that feeding of the mixture particles led to the decrease in the airflow velocity; however, it did not disturb the airflow stability or produce obvious turbulence. Adjustment of airflow velocity significantly influences the separation; nonetheless, incomplete separation of rice husks and brown rice still remains. The mechanism of brown rice loss and rice husk retention in the separation process occurred simultaneously to some extent in the separation region. As a result, brown rice and rice husk entered the opposite regions and collided with each other in their opposite region, respectively. • The separation process of mixture particles was studied by DEM-CFD simulation method. • Feeding mixture doesn't disturb the airflow stability and produce turbulence. • Adjusting the airflow can not solve the problem of incomplete separation. • Brown rice losing is caused by rice husk collision. • The rice husk remaining is caused by brown rice coercion and collision. [ABSTRACT FROM AUTHOR]

The orientation of paddy grains during hulling is one of the main factors that determines the one-time hulling ratio and breakage ratio during processing with rubber rollers. Understanding the adjustment mechanism of paddy grain orientation angle in feeding process is the key to controlling the orientation angle, but the mechanisms involved remain poorly understood. The feeding process using an inclined plate feeder was simulated using the discrete element method (DEM). The effect of the inclination angle of the long feeding plate was analysed and the requirements for an adjustment mechanism were clarified. The results showed that the uniformity of inclination angle of paddy grains decreased with the increasing inclination angle for the long feeding plate. The results showed that the regular movements of paddy grains were controlled by shear action of paddy grain flow. The depth of the grain layer determines the distribution of paddy grain orientation angle. This study could provide guidance for the design and optimisation of feeding systems and help understand the adjustment mechanism requirements for orientating non-spherical grains in grain flow • Adjustment mechanisms of orientation angle of paddy grain examined numerically. • Paddy grains rotate around one of their vertices. • Regular rotational motion is caused by the shear action of the grain flow. • Number of flow layer determines the change of orientation angle. • Regular rotational movements can align orientation angle to flow direction. [ABSTRACT FROM AUTHOR]

The low ratio of one-time hulling is one of the main factors restricting the development of rice processing. Understanding the hulling mechanism and the effect of feeding direction of paddy grain on the hulling performance is the key to increasing the one-time hulling ratio of rubber roll hullers, but the mechanism remains poorly understood. In this work, the effects of the feeding direction on rotation movements were explored using the discrete element method (DEM), observing the microstructure of paddy grain, and conducting the static shearing experiments. The effect of feeding direction on the hulling ratio and hulling pattern were investigated. The results showed that rotation angle increases as feeding direction increases and the axis of rotation of the grain changes from the short to long axis. This difference in rotation leads to differences in the shearing effect of the rubber rolls on the apexes of the grain. From the microstructure of the grain it can be seen that the grain core is wrapped by the lemma and palea under a process of bonding and interlocking. The experimental results show that the process of hulling destroys bonding and interlocking by a shearing action. Therefore, hulling ratio changes with changes in feeding direction. The results were confirmed by hulling experiments using a rubber roll huller and it was verified that feeding direction has a significant effect on the breakage ratio. • The mechanism of hulling is to release the bond and the interlocking between lemma. • Different feeding direction affects stresses and rotation of paddy in hulling process. • Hulling performance is related to the feeding direction of paddy grain. • The vertical feeding of paddy can effectively increase hulling performance. [ABSTRACT FROM AUTHOR]

Moderate removal of the bran layer plays an important role in reducing broken rice and nutrient loss during rice milling. However, the moderate removal of bran layer remains a challenge due to limited understanding of the mechanisms of rice bran removal. The mechanism of bran layer removal was studied by analysing the motion characteristics of rice grains on a rice sieve and the tissue structure and mechanical properties of the bran layer from different parts of rice grain. The results showed that the removal order of bran layer is firstly, lateral, followed by ventral and finally dorsal. In the milling chamber the grains on the sieve are almost always in contact with the rice sieve by the lateral, resulting in the lateral bran being removed the first during milling. The dorsal bran layer has a thicker tissue structure and stronger mechanical properties, which leads to the dorsal bran layer being removed after the ventral bran layer. The mechanism of bran layer removal reduces the efficiency and quality of milling. These findings should helpful for the process optimisation of moderate milling and for the design of rice mills. • The bran layers are removed in a certain order in friction rice mill. • The removal efficiency of bran layer decreases with the increase of milling time. • The primary damage occurs almost only on the lateral of the rice grain. • Rice mills and rice properties together determine the bran layer removal. [ABSTRACT FROM AUTHOR]

The continuous feeding rice into a rice mill is one of the main factors affecting milling quality. Understanding the conveying mechanism of rice in the connected chamber is the key to control the feeding, however the internal mechanisms involved remain poorly understood. In this work, the discrete element method was used to simulate rice movement in a rice mill and the validity of the simulation verified by experiments. The influence of rotational speed ratio i on rice conveying effect in connected chamber was analysed. A mathematical model involving the relationship between mill parameters and conveying uniformity was established. The simulations showed that particle groups will enter continuous limited state when particles enter the entrance of milling chamber. In particular, when i was 0.5 and 0.75, the flux near the shaft is even <0. With the increase of i , the rice conveying efficiency increased, but the milling time decreased. When i was <1.25, the amount fed into the milling chamber gradually increased but then suddenly decreased when i was >1.25. The ability of the particle group breaking through the restriction of convex rib increased with i. Thus, the phenomenon of uneven conveying can occur in the conveying process and the mechanism involved is convection. The degree of axial mixing is closely related to the sweep volume difference between screw conveyor and convex rib. This study helped clarify the rice conveying mechanism and provided guidance for the design of rice mills and similar equipment. [Display omitted] • Rice will enter a continuous limited state when they enter milling chamber entrance. • Conveying efficiency increases but milling time decreases with the increase of i. • The axial movement of rice particles is mainly inhibited by convex ribs. • Rice axial mixing is decided by swept volume difference between screw and rib. [ABSTRACT FROM AUTHOR]

Rice hulling is the process of separating rice husks from the grain. Impact hulling is the norm for paddy grain processing because it produces higher quality hulled brown rice. However, high breakage fractions and low one-time hulling percentages can directly affect the quality of subsequent rice processing and restrict the development of paddy grain processing. Understanding the mechanism of paddy grain impact hulling is key to improving the performance of centrifugal rice hullers. In this paper, a single-seed impact test and the discrete element method (DEM) were used to simulate the paddy grain impact hulling. The effects of impact speed and angle on hulling percentage, breakage percentage and hulling pattern were investigated. The results showed that the effectiveness of paddy grain hulling after impact was due to the relative motion between rice husk and its kernel. The combined effect of impact and friction produces a relative motion between the husk and the kernel and increasing the relative motion improves hulling. Optimal impact parameters were obtained using a weighting method. The results of the study could improve the hulling performance of centrifugal rice hulling providing the basis improving the design. • Impact hulling relies on the relative motion between rice husk and brown rice. • The effect of impact velocity and angle on hulling performance was investigated. • The combined effect of impact and friction is key to the relative motion. • The combined weighted scoring method was used to evaluate the hulling performance. • Impact velocity of 35–40 m s−1, impact angle of 60°, are the optimum. [ABSTRACT FROM AUTHOR]

Broken rice can gather in the core of a cylinder during rice grain separation because it is wrapped by whole rice and cannot contact the indents causing low separation efficiency. To improve contact with the indented cylinder by broken rice the actions of a baffle were simulated using the discrete element method. By analysing rice movement in the mixing area under the action of baffle, the mechanism of reducing the segregation of grains, and therefore lack of contact with the indents, was clarified. The simulations showed that effects of blade length on separation characteristics were more obvious, compared with that of the rotational speed of the baffle. Shorter blade lengths and lower rotational speeds were more favourable for separation. Optimal parameters of the baffle were obtained after a comprehensive evaluation of the separation efficiency and loss rate was determined by the intercriteria correlation (CRITIC) method. This study provides a foundation for the design of the baffle and improving separation efficiency of the indented cylinder rice separators. • Separation of indented cylinder with baffle was simulated by discrete element method. • The motion behaviors of rice with or without a baffle were investigated. • Effect of blade length and rotation speed on separation characteristics was analyzed. • The CRITIC method was used to evaluate separation ability comprehensively. • Blade length of 0.54 h and rotational speed of 0.5 v are the optimum. [ABSTRACT FROM AUTHOR]

The uniformity of rice milling is of vital significance in the rice processing industry. Unfortunately, the reasons for non-uniform milling of grains in friction rice mills still remain poorly understood. In this paper, the motion of rice in the mill is numerically simulated based on the discrete element method. The removal mechanism for the bran layer in the friction mill is discussed and verified by experiment. A method of quantifying milling uniformity was introduced. The results showed that for successful milling, the intact bran layer must experience abrasion caused by sieve-rice contact then be completely removed by rice–rice friction. Non-uniform milling occurs due to the existence of "milling blind zones", where the bran layer cannot be removed. Milling uniformity is closely related to the ability of rice to move from blind zones to the outer ring of the mill. The use of alternating rate to quantify this ability is proposed. The higher alternating rate the better milling uniformity. Alternation rate is affected by the tumbling movement of rice in the milling cavity. The research helps clarify the milling process of rice and provides guidance for the design of rice friction mills. [Display omitted] • Removal of bran layer mainly depends on the abrasion effect by sieve. • Non-uniform milling is due to existence of milling blind zones. • Higher alternating rate between zones is beneficial for milling uniformity. • Stronger tumbling ability of rice grains can lead to higher alternating rates. [ABSTRACT FROM AUTHOR]

The effect of the rice movement and the forces acting on rice grains during abrasion in a mill is unclear limiting the development of rice milling machines. The aim of this work is to provide a numerical methodology for predicting the abrasion rate of rice to provide the basis for the design and use of rice mills and improving milling quality and efficiency. Here, the milling process was investigated experimentally in a laboratory using a vertical mill to identify the dominant abrasion mechanism and to the analyse of particle size distributions. In order to evaluate the abrasion rate, the effect of rotational speed of the roller shaft on the normal force and the sliding distance between particles and particle-geometry was examined using the discrete element method. Abrasion rate calculated according to the Archard abrasion equation from the simulation results and this agreed well with the experiment results thereby validating the feasibility of numerical-experimental approach. The relationship between rotational speed and abrasion rate was established based on polynomial fitting. • Abrasion is the dominate mechanism in the vertical mill under the tested condition. • Effect of rotational speed on rice movement and force acting on rice was discussed. • Abrasion rate of rice was predicted based on the numerical-analytical approach. [ABSTRACT FROM AUTHOR]

The discharging process is ubiquitous in the production of food and pharmaceuticals. Usually, discharging devices have relatively simple structures. But unlike the outflow of liquids, due to the transitions in granular flow it is considered a discontinuous process and there is not yet a complete theoretical framework that describes granular outflow from an orifice. In this work, the transition of granular kinematic states near to the outlet is examined experimentally. The transition boundary of a kinematic discontinuity was not found in the study, but there was a transition boundary of velocity profile including velocity gradient transition and motion direction transition near to the outlet. Furthermore, two methods to determinate the transition boundary of velocity profile were developed. This work further clarifies the mechanism and position of flow transition near the outlet during the discharge of grains. This can not only help to solve problems in the discharging process, but also help to enhance our understanding of complex flow properties of granular solids confined by a bottleneck. Image 1 • General rule of kinematics during silo discharging examined experimentally. • New insights for the kinematics transition of grain outflow proposed. • Two methods developed to identify regions of flow transition. • Transition region presents the parabolic arch with height similar to the outlet radius. [ABSTRACT FROM AUTHOR]