Research on the Influence of Wind Speed Parameter Matching on the Operation Quality of an Upper Suction-Type Rice Mill

Abstract

The rice bran is mixed with broken rice. In traditional horizontal rice mill working, the upper suction-type rice mill, equipped with air duct, is designed to separate rice bran from broken rice. The upper suction-type rice mill working principle, rice particles movement in the air duct and the air duct structure are analyzed. The upper suction-type rice mill model is established with the CFD-DEM coupling method. The rice particles’ movement behavior in milling is investigated. The separation test of rice bran and broken rice is conducted with prototype. Simulation reliability is verified and optimal working parameters are determined. The simulation results show larger air speed results in stronger airflow. The rice bran movement is obvious, and the mix rate is lower. With increasing air speed, the rice grains distribution turns to be evenly, grains move and roll more frequently in the milling chamber’s vertical direction. The accumulation at the bottom of the milling chamber is alleviated, and the broken rice rate is reduced. The separation test is conducted at the Hubei Wufeng Grain Machinery Co., Ltd. (Wuhan, China). The air speed (v_o) is 6–8 m/s and the jet air speed (v_p) is 10–20 m/s. The results show that when the air speed is 6 m/s and the jet air speed is 20 m/s, the separation rate is 95.4%, the mix rate is 2.4% and the broken rice rate is 5.4%, indicating that the rice mill operation performance is good. The study can provide a reference basis for optimizing working parameters of upper suction-type rice mills.

1. Introduction

Rice is one of the most important cereal crops related to China’s national economy and people’s livelihood. Furthermore, it is the staple food for more than half of the world’s population [1,2]. Brown rice is de-hulled rice grain, which consists of bran, embryo, and endosperm layers [3,4]. The germ and bran layers on brown rice’s surface are removed in sand roller rice mills with a whitening roller’s high-speed rotation to achieve white rice milling [5]. The rice bran and broken rice are produced during rice milling. A lower suction-type method is used to collect the rice bran and broken rice mixture in traditional horizontal rice mills [6]. The collected mixture needs to be sorted again due to the different uses of rice bran and broken rice. High investment costs and energy consumption are generated with the lower suction-type method [7]. A large amount of rice is deposited at the bottom of a milling chamber with the lower suction-type method, resulting in uneven rice polishing and an increased broken rice rate [8]. The mixing of rice bran and broken rice is not conducive to the development of the rice processing by-product industry. The separation of rice bran and broken rice can be achieved with the upper suction-type rice mill during polishing. The phenomenon of material accumulation at the bottom of the milling chamber can be alleviated with the upper suction air method [9,10].

In previous studies, most of the work focused on researching the broken rice separation process and its characteristics in rice milling. Kim et al. researched the broken rice separation characteristics through broken rice separation experiments using a laboratory indented cylinder separator [11]. Esgici et al. studied the effect of cylinder rotational speed on grain losses and broken germination through rice husking tests [12]. The coupling of computational fluid dynamics (CFD) and discrete element method (DEM) has been successfully applied to various fields [13,14,15]. Chen et al. investigated the aerodynamic separation mechanism of rice husks from brown rice following paddy hulling using coupled CFD-DEM [16]. Yuan et al. researched the impact of airflow velocity, aperture and cylinder sieve deflectors on threshed rice separation in an airflow cylinder sieve device using CFD-DEM [17]. However, few studies have focused on the movement behavior of rice particles in milling. The influence of air speed and jet speed on the separation of rice bran and broken rice remains unclear.

In this study, the upper suction-type rice mill working principle, rice particle movement laws in the air duct and the air duct’s structure are analyzed. The upper suction-type rice mill model is established using the CFD-DEM coupling method, and the movement behavior of rice particles in milling is investigated. The separation test of rice bran and broken rice is conducted with a prototype, simulation reliability is verified, and optimal working parameters are determined.

2. Materials and Methods

2.1. Working Principle of Upper Suction-Type Rice Mill

The upper suction-type rice mill is shown in Figure 1. The milling device consists of a screw conveyor, whitening roller, pulling roller and rice screen. The air duct is composed of a middle air cover, an upper air inlet and a lower air inlet. First, the brown rice is transported to the rice mill with a conveyor device, then it is sent to the milling chamber by a screw device for whitening. The high-speed air flow from the blower enters the milling room through the hollow spindle and whitening roller air groove, and rice bran and rice flow into air duct through the rice sieve during rice milling. The rice bran and broken rice are separated by upward airflow. Rice bran is discharged through the upper air inlet, and broken rice is discharged through the lower air inlet. The rice is discharged by the pulling roller after milling, indicating that the separation of rice bran and broken rice is complete.

2.2. Dynamic Analysis of Rice Particles in Air Duct

The analysis of particle motion force is essential for particle separation. The particle force analysis in the air duct is shown in Figure 2. The particle is subjected to gravity G, airflow drag F_d and buoyance F_b in the air duct. The force analysis of the rice bran and broken rice in the air duct is shown in Figure 2, where buoyance F_b is much smaller than airflow drag F_d. The separation of rice bran and broken rice is achieved with a relative difference between the airflow drag F_d and gravity G between different particles.

The force analysis of the rice bran and broken rice in the air duct is shown in Figure 2. The air volume and air duct of the separation equipment is designed based on the suspension speed between different particles. From Figure 2, the suspended particle force equilibrium equation can be described as Equation (1) [18]:

$$F_b+F_d=G.$$

(1)

According to Equation (1), the particle airflow drag can be described as Equation (2) [19]:

$$F_d={\displaystyle \frac{\pi d_s^2}{8}}C\rho \left|v-v_p\right|(v-v_p),$$

(2)

where d_s is the particle equivalent of the ball diameter (m), C is the particle resistance coefficient, ρ is air density (kg/m³), v is the airflow speed vector (m/s) and v_p is the particle speed vector (m/s). The particle resistance coefficient C has a piecewise function of the particle Reynolds number Re_p, shown in Equation (3) [20]:

$$C=\left\{\begin{array}{l}{\displaystyle \frac{24}{R{e}_p}},R{e}_p<1\\ {\displaystyle \frac{24}{R{e}_p}}\left(1+0.15R{e}_p^{0.687}\right),1\le R{e}_p<1000\\ 0.44,1000\le R{e}_p\end{array}\right..$$

(3)

The particle Reynolds number Re_p can be calculated as Equation (4) [21]:

$$R{e}_p={\displaystyle \frac{\rho \left|v-v_p\right|d_s}{\mu }},$$

(4)

where μ is the aerodynamic viscosity coefficient (Pa·s).

When a particle is suspended, the particle speed v = 0. The particle suspension speed v₀ calculation formula can be obtained by combining Equations (1) and (2), as shown in Equation (5) [22].

$$v_0={\left({\displaystyle \frac{4g}{3}}\cdot {\displaystyle \frac{d_s\left({\rho }_p-\rho \right)\left|v-v_p\right|}{C\rho }}\right)}^{0.5}.$$

(5)

The average equivalent ball diameter of the broken rice particle d_s = 1.2 mm is obtained with measuring samples. The particle resistance coefficient C is obtained based on Equations (3) and (4), and the suspension speed v_o = 6.4 m/s is obtained after being brought into Equation (5). The average equivalent ball diameter of the rice bran particle is 0.22 mm, and the suspension speed is 1.6 m/s. The rice bran particle suspension speed is much smaller than that of the broken rice particle.

2.3. Analysis of Air Duct Structure and Working Parameters

The air duct is shown in Figure 3. In order to ensure the same air speed in the air duct and the compatible air duct structure, the air duct is designed as a polygon structure. The rice bran is discharged from the upper air inlet, and the DEE′D′ section’s distance is short. The rice bran is separated from the broken rice in the lower air inlet, and the CBB′C′ section’s distance is long.

The BCDE and B′C′D′E sections are the main separation areas for the rice bran and the broken rice. In order to ensure effective separation of the rice bran and broken rice, the airflow speed v_b at the air duct’s narrowest part should be lower than the broken rice’s suspension speed. The airflow speed v_b at the widest part should be greater than the rice bran’s suspension speed. The average airflow speed v_o and v_i at the EF and AB sections should be greater than the suspension speed of the rice bran and the broken rice, respectively. The gray areas in Figure 3 represent the airflow’s intersection.

The high-speed airflow is sprayed evenly into the BCDE and B′C′D′E′ section due to the whitening roller’s rotation. It is obtained by the volume conservation principle, as shown in Equation (6) [23].

$$v_o{H}_{o}T=Q_P+v_i{H}_{i}T,$$

(6)

where v_o is the upper air inlet flow rate (m/s), H_o is the upper air inlet width (m), T is the axial length of the discharge section in the whitening roller, Q_p is the jet air volume (m³/s), v_i is the lower air outlet’s flow rate (m/s) and H_i is the lower outlet’s width (m).

The difference in airflow at the air duct’s narrowest and widest parts caused by the whitening roller’s rotation is ignored. Constraint relations exist for the air duct design, as shown in Equation (7) [24].

$$\left\{\begin{array}{l}{l}_b{v}_{b}T\approx l_a{v}_{a}T\approx v_o{H}_{o}T\\ l_b<v_s\\ l_a>v_k\end{array}\right.,$$

(7)

where v_b is the air speed at the air duct’s narrowest part (m/s), l_b is the width of the air duct’s narrowest part (m), v_a is the air speed at air duct’s widest part (m/s), l_a is the width of the air duct’s widest part (m), v_s is the broken rice’s suspension speed (m/s) and v_k is the rice bran’s suspension speed (m/s).

The rice sieve’s length in the whitening roller is 600 mm and the whitening roller’s diameter is 270 mm. To ensure compatible air duct structure, the air duct’s structure should be met with constraint relations, as seen in Equation (7). The air duct’s size is determined using the following calculation: BB′ = 160 mm, DD′ = 380 mm, EE′ = 200 mm, FF′ = 120 mm, AB = 80 mm, DC = 220 mm, CB vertical distance is 150 mm, ED vertical distance is 70 mm and FE vertical distance is 150 mm. The upper air inlet’s flow rate is 0.6–1.2 m³/s and the total flow rate of groove in the rice mill is 0.25–0.6 m³/s.

2.4. Separation Experiment

The separation experiment is conducted at the Hubei Wufeng Grain Machinery Co., Ltd. The rice mill is shown in Figure 4. The air speed and jet air speed are selected based on simulation results, in which the air speed is 6–8 m/s and the jet air speed is 10–20 m/s. The separation experiment results are shown in

Table 1. Separation experiment results.

Working Parameters		Evaluation Factors
vo (m/s)	vp (m/s)	Separation Rate (%)	Mix Rate (%)	Broken Rice Rate (%)
6	10	90.1	2.2	6.3
6	20	95.4	2.4	5.4
8	10	94.3	4	5.2
8	20	93.3	7.6	5

. When the air speed is 6 m/s, the jet air speed is 20 m/s, the separation rate is 95.4%, the mix rate is 2.4%, the broken rice rate is 5.4% and rice mill operation performance is at its best. When the air speed v_o ≥ 8 m/s, the jet air speed v_p ≥ 10 m/s, the separation rate decreases and the mix rate increases.

2.5. Model Development

The upper suction-type rice mill simulation model is shown in Figure 5. To simplify the simulation model, the axial length of the whitening roller is shortened to 300 mm. The remaining dimensions are unchanged. The air duct’s mesh size is 5 mm. The milling chamber and rice sieve’s mesh size are 1 mm.

The Hertz–Mindlin (no slip) model is set as the particle contact model in EDEM. Most brown rice particles are multi-sphere models. The breakable particles are bonded with the Bonding model. Rice bran and broken rice are set as single-sphere particles. The material parameters are shown in

Table 2. Material parameters.

Material	Poisson Ratio	Shear Modulus/Pa	Density/(kg/m3)
Rice grain	0.4	$1.1\times {10}^7$	1500
Broken rice	0.4	$1.1\times {10}^7$	1500
Rice bran	0.4	$1.1\times {10}^7$	1500
Steel	0.3	$8\times {10}^{10}$	7850

, and the model contact parameters are shown in

Table 3. Particle contact parameters.

Contact Relationship	Collision Recovery Coefficient	Static Friction Coefficient	Rolling Friction Coefficient
Rice–rice	0.50	0.55	0.15
Broken rice–broken rice	0.40	0.60	0.19
Rice bran–rice bran	0.20	0.70	0.30
Rice–steel	0.55	0.58	0.12
Broken rice–steel	0.40	0.64	0.17
Rice bran–steel	0.10	0.73	0.23
Broken rice–rice bran	0.20	0.35	0.30

and

Table 4. Bonding model contact parameters.

Normal Stiffness per Unit Area (N/m3)	Tangential Stiffness per Unit Area (N/m3)	Critical Normal Stress (Pa)	Critical Shear Stress (Pa)	Bond Radius (mm)	Contact Radius (mm)
4.43 × 1012	6.13 × 1011	7.92 × 107	2.55 × 107	0.3	0.4

. The k-ε turbulence model is used to simulate the internal airflow in CFD, and the sliding grid method is used to simulate the influence of the whitening roller’s rotation with airflow. The CFD–DEM coupling model is set as the Eulerian model due to the interaction between the particles and the airflow.

2.6. Boundary Condition and Evaluation Factor

The air speed v₀ is set as 6–8 m/s, the jet air speed v_p is set as 10–20 m/s and the whitening roller speed is set as 800 r/min. The ratio of rice multi-sphere rigid particle to rice breakable particle is 35:1 for simplifying the calculation.

For evaluating the rice mill’s operation performance, the internal flow field, the separation rate of rice bran and broken rice (separation rate), the mix rate of rice bran and broken rice (mix rate), the particle mass distribution in the milling chamber and the broken rice rate are set as the evaluation factors. These evaluation factors are analyzed after the test. The separation rate can be described as Equation (8) [25].

$$s={\displaystyle \frac{m_s}{m_s+m_x}},$$

(8)

where s is the separation rate (%), m_s is the rice bran mass collected with the upper air inlet (kg) and m_x is the rice bran mass collected with the lower air inlet (kg).

The mix rate can be described as Equation (9) [26].

$$m={\displaystyle \frac{m_z}{m_s+m_z}},$$

(9)

where m is the mix rate (%), m_z is the broken rice mass collected with the upper air inlet (kg).

The broken rice rate in the simulation can be described as Equation (10) [27].

$$b={\displaystyle \frac{35m_b}{k\left(m_c+m_b\right)}},$$

(10)

where b is the broken rice rate (%), m_b is the broken rice’s mass (kg), k is the whitening roller’s shortening ratio in simulation is 3.3 and m_c is whole rice mass (kg).

3. Results and Discussion

3.1. Flow Field Characteristics in the Milling Chamber

The flow field characteristics significantly influence the separation of the rice bran and the broken rice. The simulation is conducted under an air speed (v_o) of 6–8 m/s and jet air speed (v_p) of 10–20 m/s. The flow field characteristics under working condition are shown in Figure 6. The cloud diagram shows when the air speed (v_o) is 6 m/s, jet air speed (v_p) is 10 m/s and the airflow speed on both sides of the air duct is 3–5.5 m/s. When the air speed is 6 m/s, jet air speed is 20 m/s, the airflow speed on both sides of the air duct is 1.5–5 m/s. When a low-speed zone with local airflow speed lower than 1 m/s exits in the inner wall of the air duct, rice bran accumulation happens. When the air speed is 8 m/s, the jet air speed is 10 m/s, the airflow speed on both sides of the air duct is 4–11 m/s and the airflow speed of the jet air groove is larger. When the air speed is 8 m/s, the jet air speed is 20 m/s and the airflow speed on both sides of the air duct is 1–14 m/s.

As seen in Figure 6, the airflow in the air duct is concentrated, so it is beneficial to discharge the rice bran. The speed distribution in the air duct is uniform, and there is no stagnation zone. These results match the research results presented by [6]. To separate the rice bran and the broken rice, the duct airflow speed should be between the suspended speed of the broken rice and the rice bran. The duct airflow speed is 1.6–6.4 m/s and the airflow direction is upward. When the air speed is 6–8 m/s, the jet air speed is 10 m/s, the airflow is uniform and the flow field distribution characteristics are suitable for the separation of the rice bran and the broken rice.

3.2. Particle Movement and Distribution Characteristics in the Milling Chamber

The particle movement and the distribution characteristics are influencing factors in the separation of rice bran and broken rice. The simulation is conducted under an air speed of 6–8 m/s, and the jet air speed is 10–20 m/s. The movement tracks of the rice bran and the broken rice under working conditions are shown in Figure 7. At the beginning, the rice bran is deposited in the milling chamber, then discharged through a rice sieve under high-speed airflow, after which it is moved to the air duct. The broken rice particles are mainly distributed in the lower area of the air duct, and no broken rice particles are discharged from the upper air inlet.

The results show that when the air speed is 6 m/s and the jet air speed is 10 m/s, the rice bran moves upward and a small amount of rice bran is moved at the lower air inlet. No rice bran is discharged from the lower air inlet, only broken rice. When the air speed is 6 m/s, the jet air speed is 20 m/s, the rice bran moves downward and part of the rice bran is discharged from the lower air inlet. When the air speed is 8 m/s, the jet air speed is 10 m/s, the rice bran moves upward and part of the broken rice is discharged from the upper air inlet due to the upward airflow. The particle movement trend is the same when the air speed is 8 m/s and the jet air speed is 20 m/s, and when the air speed is 6 m/s and the jet air speed is 20 m/s.

The particle mass distribution in the milling chamber is shown in Figure 8. The rice bran particles move upward, no rice bran is discharged from lower air inlet, the separation phenomenon is obvious, and there is no rice bran accumulation.

The results show that when the air speed is 8 m/s, the jet air speed is 20 m/s and the air speed is 8 m/s, the jet air speed is 10 m/s and the particle mass is distributed evenly in the milling chamber’s right vertical direction. The particle mass distribution, when the air speed is 6 m/s and the jet air speed is 10 m/s, is slightly worse. When the air speed is 6–8 m/s, the larger v_o is, the more evenly the particles are distributed in the vertical direction of the right milling chamber. When the air speed is 6–8 m/s, the mass deposited at rice sieve’s bottom is smaller.

3.3. Separation Rate, Mix Rate and Broken Rice Rate

For the rice mill’s design, separation rate, mix rate and broken rice rate are important evaluation factors. The simulation is conducted under an air speed of 6–8 m/s, and the jet air speed is 10–20 m/s. The separation rate, mix rate and broken rice rate under working conditions are shown in

Table 5. Simulation results.

Working Parameters		Evaluation Factors
vo (m/s)	vp (m/s)	Separation Rate (%)	Mix Rate (%)	Broken Rice Rate (%)
6	10	95.9	2	5.9
6	20	88.2	2.3	6.9
8	10	96.8	3.2	5.5
8	20	90.1	4.6	7.2

. When the air speed is 6 m/s, the jet air speed is 10 m/s, the separation rate is 95.9%, the mix rate is 2% and the broken rice rate is 5.9%. When the air speed is 6 m/s, the jet air speed is 20 m/s, the separation rate is 88.2%, the mix rate is 2.3% and the broken rice rate is 6.9%. When the air speed is 8 m/s, the jet air speed is 10 m/s, the separation rate is 96.8%, the mix rate is 3.2% and the broken rice rate is 5.5%. When the air speed is 8 m/s, the jet air speed is 20 m/s, the separation rate is 90.1%, the mix rate is 4.6% and the broken rice rate is 7.2%.

Compared with the separation experiment’s results, when the air speed is 6 m/s, the jet air speed is 10 m/s and the air speed v_o = 8 m/s, the jet air speed is 10 m/s, the separation rate is increased, the mix rate and the broken rice rate are reduced. When the air speed is 6 m/s, the jet air speed is 20 m/s and air speed is 8 m/s, the jet air speed is 20 m/s and the separation rate of the experimental results is higher than that of the simulation results. The results show that the separation rate and the mix rate increase when the upper air inlet’s air speed increases. The larger the jet air speed, the weaker the airflow movement in the air duct. The influence of airflow on the particles in the milling chamber is greater, resulting in a decreasing separation rate and an increasing broken rice rate. The simulation results are consistent with the experiment results when comparing the evaluation factors in Table 1 and Table 5. When the air speed is 6 m/s, the jet air speed is 10 m/s and the air speed is 8 m/s, the jet air speed is 10 m/s and the rice milling performance is better.

4. Conclusions

This study analyzed the upper suction-type rice mill’s working principle and looked at the movement laws of rice particles in the air duct, as well as the air duct’s structure. In the paper, the upper suction-type rice mill model is established using the CFD–DEM coupling method, and the rice particles’ movement behavior in milling is investigated. Combining the CFD–DEM simulation and experiment, the effects of the air speed and the jet air speed on the rice mill’s operation performance are highlighted. The separation test of rice bran and broken rice is conducted with a prototype, the simulation reliability is verified and the optimal working parameters are determined. The simulation results show that when the air speed (v_o) is 6–10 m/s and the airflow speed in air duct is 3–5.5 m/s, it is suitable for the separation of rice bran and broken rice. The larger the air speed, the stronger the airflow; the movement of the rice bran becomes more obvious and mix rate is lower. As the air speed increases, the rice rolls and shifts more frequently in the vertical direction within the milling chamber. The accumulation at the bottom of the milling chamber is alleviated, and the broken rice rate is reduced. The separation test’s results show that when the air speed is 6 m/s and the jet air speed is 20 m/s, the separation rate is 95.4%, the mix rate is 2.4% and broken rice rate is 5.4%, indicating that the rice mill’s operation performance is good.