Research on the aerodynamic characteristics of multifunction straw stalk crusher with the kneading condition

Abstract

There are problems of significant vibration and high noise when multifunction straw stalk crushers are operating. Aerodynamic noise is the main component of noise of multifunction straw stalk crushers. To explore the effects of structural configuration and operational parameters on the aerodynamic noise of multifunction straw stalk crushers. The CFD-DEM coupling method is used to numerically simulate the air-straw coupled flow field inside the multifunction straw stalk crusher to obtain the aerodynamic characteristics. On this basis, the aerodynamic noise of the multifunction straw stalk crusher is predicted, and its influencing factors are analyzed. The results show that compared with the idle condition, the resistance coefficient of the discharge pipe under the load condition is increased by 69.81%, and the eddy current viscosity gradient of the discharge pipe (2.33e⁻⁴ Pa·s) is significantly higher than that of the feed tank (4.55e⁻⁵ Pa·s), resulting in a significant increase in noise at the outlet. Compared with the idle condition, the total sound pressure level of the aerodynamic noise at the outlet under the load condition rises from 91.08 dB(A) to 106.29 dB(A), and the total sound pressure level of the aerodynamic noise at the inlet under the load condition rises from 84.75 dB(A) to 95.27 dB(A). The rotor speed, the number of hammers, the discharge angle, the length of the feed tank, and the number of cutting knives are the primary factors that significantly affect the aerodynamic noise of the straw stalk crusher. The research can provide a reference for the low-noise optimization design of straw stalk crushers.

Keywords

Multifunction straw stalk crusher CFD-DEM coupled flow field aerodynamic noise sensitivity analysis

Introduction

The multifunctional straw stalk crusher integrates various functions such as kneading and crushing crop straw and can process straw into filaments or fragments. The structure of the multifunction straw stalk crusher is shown in Figure 1. The rotary impact system comprises three core components: cutting knives, hammer frame plates, and hammers. The lower shell assembly features interchangeable modular attachments, including either tooth plates or sieve mesh. During the operation of the multifunction straw stalk crusher under the condition of kneading, crop straw or other materials are fed into the crusher through the feeding trough and cut into segments by the cutting knives. Due to the impact of the hammers and the rub of the tooth plates, the segmented materials are crushed into filamentous fragments and expelled from the machine through the discharge pipe.

Figure 1.

Schematic of the multifunctional straw stalk crusher. 1. Feed tank 2. Tooth plates 3. Cutting knives 4. Hammers 5. Throwing plate 6. Sieve mesh 7. Upper discharge pipe 8. Upper shell 9. Hammer frame plate 10. Lower shell 11. Lower discharge pipe 12. Frame.

In addition to the rotational noise, the primary noise components of the straw stalk crusher are the pulsation and turbulence of fluid surface pressure generated throughout the airflow from the inlet to the outlet. In addition, the composition of the materials to be processed also affects the noise of the straw stalk crusher.¹ The experiment shows that the noise level of the straw stalk crusher reaches 110 dB(A) at the rated operational speed,² which is significantly higher than the Chinese national standard normative limit.³ Long-term exposure in high noisy environment will cause harm to the operator’s physical and mental health.

The numerical simulation method is the most prevalent and efficient technique for evaluating the aerodynamic noise of a straw stalk crusher. The procedure is typically as follows: initially, the unsteady force on the rotor surface is calculated by computational fluid dynamics (CFD), and then the unsteady force is integrated into the Ffowcs Williams-Hawkings (FW-H) equation as a sound source term. The far-field radiation sound pressure of the straw stalk crusher can be obtained. Li⁴ used a combination of simulation and experimental methods to investigate the aerodynamic noise of the straw crusher and its influencing factors. The results indicate that the crusher’s noise primarily consists of aerodynamic noise, which will increase further under load conditions. Lun and Zhang et al.^5,6 used the CFD-DEM coupling method and the acoustic analogy theory to predict the noise of the straw crusher. In addition, exploring the aerodynamic noise of a straw kneading machine using experimental methods is also useful and precise. Researchers like Zhang, Cao, and Owusu et al.^7–9 used experiments to identify the noise sources of the straw kneading machine and analogous equipment.

This study explores the aerodynamic characteristics of the multifunctional straw stalk crusher under kneading conditions. The CFD method was used to numerically simulate the flow field of straw stalk crusher, and the accuracy of the numerical simulation was validated through comparison with experimental data. Utilising the coupling methodology of CFD and the Discrete Element Method (DEM), the effect of straw crushing on the flow field under kneading condition of the straw stalk crusher was examined, and the aerodynamic properties of the flow field were analysed. The acoustic simulation technique was used to model and numerically predict the aerodynamic noise of the straw stalk crusher. And the aerodynamic noise test were employed to verify the results of the numerical predictions. The straw stalk crusher’s noise sensitivity was computed, along with its factors and degree of influence on the machine’s acoustic noise. The research method and results provide reference for low noise design of straw processing machinery.

Analysis of gas-solid mixed flow field of straw stalk crusher

Flow field model

In the kneading condition, the casing inner wall needs to be equipped with a tooth plate and the lower discharge port needs to be closed. The length, width and height of the shell of the straw stalk crusher are 1250, 580 and 950 mm respectively; The radius and width of the crushing chamber are 220 and 247 mm; The rotating radius of the rotor is 181 mm; The rotor speed is 2800 r/min; The number of hammers on the rotor is 24; The productivity of the straw stalk crusher is 850 kg/h; The global mesh of the flow field model was set to a maximum size of 25 mm, with the mesh at the inlet, outlet, and rotor hammer refined, resulting in a total of 2,619,803 meshes, as depicted in Figure 2.

Figure 2.

Flow field model.

The boundary conditions were set according to the actual operational parameters of the straw stalk crusher. The gravity field, with a magnitude of 9.81 m/s² and directed negatively along the y-axis, was established in the computational domain. The types of inlet and outlet are set as pressure inlet and pressure outlet, respectively. The rotor speed was set to 2800 r/min. Following 645 iterations, the continuity curve falls below 1 × 10⁻³, indicating convergence of the calculation findings.

Model verification

Airflow testing verifies the accuracy of the CFD model of the straw stalk crusher. Four measurement points were established at the outlet of the straw stalk crusher, and the simulation results were validated by comparing the air velocity at each measurement point during idle operation of the straw stalk crusher. The detailed testing procedure has been reported, aligning with Ref. 10. Table 1 presents a comparison between simulation results with experiment data.

Table 1.

Comparison of simulation and experiment results of air velocity at outlet¹⁰.

Monitoring point of outlet	The measured average (m·s⁻¹)	Value of simulation (m·s⁻¹)	Relative error (%)
Point 1	10.05	10.86	8.1
Point 2	4.02	4.24	5.5
Point 3	10.58	10.60	0.2
Point 4	2.05	2.09	2.0

It can be seen from Table 1 that the simulation results were basically consistent with the test air velocity, the established CFD model of the straw stalk crusher could accurately reflect the real flow field inside the straw stalk crusher, and the calculated results are reliable.

Gas-solid mixed flow field model of straw stalk crusher

The flow field of air-straw mixing was numerically computed in a straw stalk crusher under load conditions using the CFD-DEM coupling approach. The internal flow field of the crusher and the straw were simulated respectively using Ansys Fluent 2020R1 and EDEM 2020.^11,12 The calculation process was shown in Figure 3.

Figure 3.

CFD-DEM coupling calculation flow.

The DEM model of the straw stalk crusher was consistent with the CFD mesh model, in which the shell material was set as Q235, the rotor material as 45 steel, and the hammer material as 65Mn. To simulate the kneading process of straw in the straw stalk crusher, particles arranged in a cylindrical shape were used to represent the straw segment, and the Bonding model was used to connect the particles. The established straw DEM model was shown in Figure 4. The material properties of the DEM model were shown in Table 2. The parameters of Bonding model were shown in Table 3.

Figure 4.

Discrete element model of corn straw.

Table 2.

DEM model material parameter.

Materials	Poisson’s ratio	Shear modulus/Pa	Density/(kg·m⁻³)
45 steel	0.31	7.94 × 10¹⁰	7850
65Mn steel	0.35	7.9 × 10¹⁰	8864
Q235 steel	0.30	7.9 × 10¹⁰	7865
Straw skin	0.30	2.86 × 10⁸	780
Straw pulp	0.10	1.82 × 10⁷	410

Table 3.

Bonding parameters.

Type	Parameter
Type	K_bn/(N/m³)	K_bt/(N/m³)	σ_b/Pa	τ_b/Pa	R_b/mm
Core and core	4.59 × 107	2.66 × 106	3.00 × 106	1.00 × 106	1.5
Skin and skin	3.69 × 108	7.72 × 107	5.20 × 106	3.84 × 106	2
Skin and core	2.94 × 108	5.00 × 107	5.00 × 106	5.00 × 106	2

The rotating region was used as the slip mesh model when calculating the coupled flow field. The turbulence model was set as the large eddy simulation model. The number of steps in the transient calculation was 2000, and the time length of each step was set to 5.95 × 10⁻⁵ s (10 times the DEM calculation step). To simulate the feeding process of the material, a plane was set up at the inlet as a particle factory, and the particle generation rate was set to 0.29 kg/s according to the feeding amount measured in the test. The Hertz-Mindlin non-slip contact model was used to simulate the contact behaviour between particles, ignoring the plastic deformation and contact retention times generated by the collision between the straw and shell.¹³

Analysis of aerodynamic characteristics of flow field of straw stalk crusher

The aerodynamic characteristics of the flow field in the straw stalk crusher were analyzed in terms of drag coefficient, turbulent viscosity and velocity vector nebulogram as shown in Figures 5, 6, and 8.

Figure 5.

Resistance coefficient between discharge tube and feed tank of the straw stalk crusher.

Figure 6.

Swirl viscosity cloud image of the casing of the straw stalk crusher.

Following the addition of the material, the resistance coefficient at the discharge pipe significantly increases, as illustrated in Figure 5(a). The average resistance coefficient of the discharge pipe during load conditions rises from 1.06 to 1.80 relative to the idle condition. Figure 5(b)) illustrates that subsequent to the addition of the material, the resistance coefficient at the feed tank also exhibits a significant rise. The average resistance coefficient of the feed tank during load situations increases from 1.03 to 1.61 in comparison to the idle condition. The test revealed that the small particles adhering to the discharge pipe’s wall, following material fragmentation, resulted in increased roughness of the inner wall and a corresponding rise in the resistance coefficient.² In contrast to the idle condition, the augmented amplitude of the resistance coefficient at the discharge port and feed tank varied under load conditions, resulting in a disparity in the amplitude of the noise rise.

Figure 6 illustrates that the eddy viscosity contour at the discharge pipe, either under load or idle conditions, was significantly higher than that at the feed tank. The eddy viscosity gradient at the discharge pipe during load conditions (1.017 × 10⁻³ Pa·s) was markedly higher than that during idle conditions (1.250 × 10⁻³ Pa·s). A bigger disparity in eddy viscosity leads to more adverse steady dissipation of eddy viscosity, resulting in the easy formation of high noise at the discharge pipe. The disparity in eddy viscosity at the feed tank was 5.078 × 10⁻⁴ under idle conditions, while the disparity in eddy viscosity at the load feed tank was 5.533 × 10⁻³. The gradient of eddy viscosity at the feed tank increases following the addition of materials, albeit the shift is not pronounced.

In order to further explore the flow field of the straw stalk crusher, cross-section A (Figure 7) inside the straw stalk crusher was cut for analysis, and the results were shown in Figure 8.

Figure 7.

Selecting an internal section.

Figure 8.

Flow velocity vector diagram of the straw stalk crusher.

Figure 8(a)) illustrates that, under idle conditions, the airflow within the chamber generated a vortex and backflow phenomenon at the upper discharge pipe, attributed to a guide plate on the right side of the discharge pipe, leading to a significant velocity disparity between the left and right sides of the discharge port. Figure 8(b)) illustrates that following the addition of materials, the interference caused leads to the bifurcation of a single vortex at the upper discharge pipe into two vortices, so generating more irregular turbulence and expanding the turbulence area. The research indicates that the vortex adjacent to the discharge pipe wall will exert force on the wall, leading to significant pressure pulsations that generate noise.¹⁴

Aerodynamic noise analysis of straw stalk crusher

Acoustic boundary element model and acoustic meshing

The indirect boundary element method was used to predict the aerodynamic noise of the straw stalk crusher.^15–17 The mesh of the acoustic boundary element model was set to 15 mm; the inner wall of the shell was selected as the acoustic boundary of the model, and the mesh type was mixed. The final constructed mesh contained 10,027 units and 9587 nodes, and the mesh division results were shown in Figure 9.

Figure 9.

Acoustic boundary element model.

Analysis and verification of aerodynamic noise calculation results of straw stalk crusher

Discrete noise is the main component of the aerodynamic noise of the straw stalk crusher.¹⁰ Figure 10 illustrates a comparison of the aerodynamic noise spectrum of the straw stalk crusher in both loaded and idle states. Figure 11 depicts the cloud image of the numerical calculation results of the sound pressure level for the first six harmonic frequencies under load conditions, selected for comparison and analysis due to the analogous far-field noise distribution of the straw stalk crusher in both working conditions.

Figure 10.

Comparison diagram of aerodynamic noise spectrum.

Figure 11.

The sound pressure level nephogram of the straw stalk crusher.

Figure 10 illustrates that the spectrum curves of aerodynamic noise at the inlet and outlet during load conditions exceed those observed under idle conditions. The feeding of corn stalks increases the sound pressure level at both the inlet and outlet of the straw stalk crusher. Additionally, the frequency of discrete noise following the material’s addition exhibits a discernible lag, attributable to the time delay induced by the reaction force of the material upon impact with the hammer after it is introduced.

The overall sound pressure level of noise under idle and load conditions was computed using the formula for sound pressure level superposition (1); the findings were presented in Table 4.

L_{p} = 10 \lg \sum_{i = 1}^{n} 10^{0.1 L_{p i}}

(1)

where, L_p is the total sound pressure level (dB); L_pi is the i sound pressure level (dB).

Table 4.

Comparison between idle and load total sound pressure levels.

	Outlet	Inlet
Idle condition/dB(A)	91.08	84.75
Load condition/dB(A)	106.29	95.27
Difference value/dB(A)	15.21	10.52

Table 4 shows that the straw stalk crusher’s total aerodynamic noise increases with the addition of materials. The total sound pressure level at the output increases, rising from 91.08 dB(A) to 106.29 dB(A). The increase in aerodynamic noise was considerably associated with the air resistance coefficient and viscosity, as demonstrated by the rise in the total sound pressure level at the inlet from 84.75 dB(A) to 95.27 dB(A).

Noise monitoring points (the red points at the outlet and the inlet in Figure 11) were established in accordance with the national standard “Sound Pressure Method for Measuring Sound Power Level and Sound Energy Level of Noise Sources”. As shown in Figure 11, the noise frequency band sound pressure level of the inlet and outlet of the straw stalk crusher developed significantly with the addition of straws. The outlet’s maximum sound pressure level was 98.86 dB(A), and the frequency that corresponds to it was sextupling the rotor passing frequency. The aerodynamic noise radiation primarily radiates outward in the form of ripples that resemble fans. The inlet’s maximum sound pressure level was 82.36 dB(A), and the frequency that corresponds to it was quadruple the rotor passing frequency. While the first three orders radiate outward in a fan-shaped ripple, the acoustic wave at the inlet shows an uneven radiation state after the third order. This is because a noise signal with a lower frequency has a longer wave and is more stable in space, but a noise signal with a significantly higher frequency (>500 Hz) is more prone to interference while propagating.¹⁸ Furthermore, there are particles in the feed tank that have an effect on the radiation from noise propagation.

Result verification

The accuracy of the aerodynamic noise prediction model was verified by field experiments. The test instrument and experimental process have been reported in Ref. 2. The test site was shown in Figure 12, and the final results were shown in Figure 13.

Figure 12.

Test site diagram².

Figure 13.

Spectrum comparison between measured and simulated condition.

Figure 13 illustrates that the variation trend of the sound pressure spectrum curves at the two measurement stations was fundamentally consistent with the numerical calculations. The outlet measuring site exhibited the maximum sound pressure level at roughly 1235 Hz, whereas the inlet measuring point displayed a concentration at approximately 800 Hz. The disparity in maximum sound pressure levels at the entrance and outflow was 3.41 dB(A) and 4.40 dB(A), respectively. This resulted from simplifying the model of the corn stalk segment during the simulation and the inevitable personal error. The numerical results were demonstrably accurate. The numerical computation findings were precise.

Table 5 presents a total of the A-weighted sound pressure level calculated via the superposition formula (1) in relation for the effective sound pressure levels depicted in the spectrum graphs of both the numerical computation and the experimental test.

Table 5.

Comparison of total sound pressure levels under simulation and test load conditions.

	Outlet	Inlet
Test value/dB(A)	106.29	95.27
Simulated value/dB(A)	108.35	93.39
Difference value/dB(A)	2.06	1.88

Table 5 shows that the total calculated sound pressure level of the simulation and the total observed sound pressure level at the outlet differ by 2.06 dB(A) and 1.88 dB(A) at the inlet, respectively. The prediction model was obviously accurate, and the computational results regarding the pneumatic noise of the straw stalk crusher, considering the process of crushing corn stalks, are reliable.

Sensitivity analysis of aerodynamic characteristics parameters of the straw stalk crusher

Sensitivity analysis theory

Rotor speed, hammer thickness, hammer number, and other structural and operational parameters were the variables in the sensitivity analysis of the straw stalk crusher. The aerodynamic noise index function was the total sound pressure level at the measuring point at the outlet and inlet of the straw stalk crusher. With the change of each parameter, the sound pressure level at the two measuring points will also change, and the corresponding mathematical expression is as follows:

E V = h_{x} (x_{1}, x_{2}, \dots, x_{n})

(2)

S P L_{o u t} = h_{o u t x} (x_{1}, x_{2}, \dots, x_{n})

(3)

S P L_{i n} = h_{i n x} (x_{1}, x_{2}, \dots, x_{n})

(4)

where: EV is eddy current viscosity, Pa·s; SPL_out is the sound pressure level of outlet, dB(A); SPL_in is the sound pressure level of inlet, dB(A); h_outx and h_inx represent functions of outlet and inlet parameters respectively; x₁, x₂,···x_n is represents the structural or operational parameters that affect EV, SPL_out, and SPL_int.

The corresponding sensitivity parameter could be obtained by calculating the partial derivative of the sound pressure level of the measuring point at the outlet and inlet of the straw stalk crusher concerning each parameter; the calculation expression was as follows:

E V_{i} = \frac{\partial E V}{\partial x_{i}} = \frac{\partial h (x_{1}, x_{2}, \dots, x_{n})}{\partial x_{i}}

(5)

S_{o u t i} = \frac{\partial S P L_{o u t}}{\partial x_{i}} = \frac{\partial h_{o u t x} (x_{1}, x_{2}, \dots, x_{n})}{\partial x_{i}}

(6)

S_{i n i} = \frac{\partial S P L_{i n}}{\partial x_{i}} = \frac{\partial h_{i n x} (x_{1}, x_{2}, \dots, x_{n})}{\partial x_{i}}

(7)

where: EV_i is the sensitivity of each parameter to eddy current viscosity; S_outi is the sensitivity of each parameter to the sound pressure level of the outlet measuring point. S_ini is the sensitivity of each parameter to the sound pressure level of the inlet measuring point; x₁, x₂,···x_n is represents the structural or operational parameters that affect EV_i, SPL_outi, and SPL_inti.

Assuming that the change in outlet noise sound pressure level with each parameter can be thought of as a linear relationship, the fitting equation for analyzing the parameter sensitivity of pneumatic noise sound pressure level is as follows:

{\overset{⌢}{S P L}}_{i} = a + b_{i} x_{i}

(8)

where:

{\hat{f}}_{i}

is the regression value, and a、b_i are the regression coefficients.

Subtracting the test value from the regression value yields the residual value. To obtain the optimal fitting effect, the residual is squared and then summed, yielding the following residual expression:

S S_{e} = {\sum_{i = 1}^{n} e_{i}^{2} = \sum_{i = 1}^{n} (S P L_{i} - {\overset{⌢}{S P L}}_{i})}^{2} = {\sum_{i = 1}^{n} [S P L_{i} - (a + b x_{i})]}^{2}

(9)

When the residual is the smallest, the fitting effect is the best. Calculate the values of a and b by deflecting a and b to zero on both sides of the equation (9). The least square approach is used to calculate the formula (9), and the solution can be obtained:

{\begin{cases} \frac{\partial (S S_{e})}{\partial a} = - 2 \sum_{i = 1}^{n} (S P L_{i} - a - b x_{i}) = 0 \\ \frac{\partial (S S_{e})}{\partial b} = - 2 \sum_{i = 1}^{n} (S P L_{i} - a - b x_{i}) x_{i} = 0 \end{cases}

(10)

Equation (11) is obtained from the above equation:

{\begin{cases} n a + b \sum_{i = 1}^{n} x_{i} = \sum_{i = 1}^{n} S P L_{i} \\ a \sum_{i = 1}^{n} x_{i} + b \sum_{i = 1}^{n} x_{i}^{2} = \sum_{i = 1}^{n} x_{i} S P L_{i} \end{cases}

(11)

The expression of the regression coefficient is obtained by solving equation (11):

a = \bar{S P L} - b \bar{x}

(12)

b = \frac{n \sum_{i = 1}^{n} x_{i} S P L_{i} - (\sum_{i = 1}^{n} x_{i}) (\sum_{i = 1}^{n} S P L_{i})}{n \sum_{i = 1}^{n} x_{i}^{2} - {(\sum_{i = 1}^{n} x_{i})}^{2}} = \frac{\sum_{i = 1}^{n} x_{i} S P L_{i} - n \bar{x S P L}}{\sum_{i = 1}^{n} x_{i}^{2} - n {\bar{x}}^{2}}

(13)

In the above formula,

\bar{x}

、

\bar{S P L}

are the arithmetic average of simulation results.

Noise sensitivity analysis of the outlet of the straw stalk crusher

Research shows that rotor speed, hammer thickness, number of hammers, number of hammer groups, length of the discharge tube, discharge angle, number of cutting knives, and length of the feed tank are the primary structural and operational parameters that substantially influence the flow field characteristics of the straw stalk crusher.^19,20 Table 6 presents the spectrum of values for these parameters.

Table 6.

Selected values of design variables of the straw stalk crusher.

Design variable	Parameter meaning	Value
N	Rotor speed (r·min⁻¹)	2500	2800	3100
HH	Hammer thickness (mm)		4	5
HN	Number of hammers (pcs)		18	30
PL	Length of the discharge tube (mm)		150	250
PD	Discharge angle (°)		25	45
HS	Number of hammer groups (group)		8	10
GN	Number of cutting knives (pcs)		3	4
FTL	Length of the feed tank (mm)		480	580

The aerodynamic noise was computed using the established calculation method, based on the various factors specified in Table 5, to assess the sensitivity of the noise to each parameter at the outlet measurement point of the straw stalk crusher. The results of the calculations are presented in Table 7. The speed unit was established as k·r·min⁻¹ (1000 r·min⁻¹).

Table 7.

The noise of the discharge port varies with the parameters of the straw stalk crusher.

Parameter meaning	Value	Outlet noise/dB(A)
Rotor speed	2.5	87.32
	2.8	91.27
	3.1	95.56
Hammer thickness	4	92.70
Hammer thickness	5	91.25
Number of hammers	18	92.33
Number of hammers	30	88.98
Length of the discharge tube	150	90.36
Length of the discharge tube	250	92.15
Discharge angle	25	89.85
Discharge angle	45	92.36
Number of hammer groups	8	91.56
Number of hammer groups	10	91.89
Number of cutting knives	3	91.30
Number of cutting knives	4	91.46
Length of the feed tank	480	91.10
Length of the feed tank	580	91.56

The original values of the above parameters are taken as the arithmetic average value and combined with the numerical calculation results of aerodynamic noise in Table 7; the arithmetic average values were obtained: $\bar{x} = 2800, \bar{S P L} = 91.38$ .

The calculation results were brought into the equations (12) and (13), and the solution was: a = 38.07, b = 0.019, then the relationship between aerodynamic noise and speed change was as follows:

S P L_{N} = 13.89 x_{N} + 52.488

(14)

The linear expressions of aerodynamic noise sensitivity of other parameters were obtained according to the least square method, as follows:

{\begin{cases} S P L_{H H} = - 0.01 x_{H H} + 91.78 \\ S P L_{H N} = - 0.279 x_{H N} + 97.556 \\ S P L_{P L} = 0.0179 x_{P L} + 87.68 \\ S P L_{P D} = 0.1255 x_{P D} + 86.934 \\ S P L_{H S} = 0.165 x_{H S} + 90.25 \\ S P L_{G N} = 0.11 x_{G N} + 91.01 \\ S P L_{F T L} = 0.005 x_{F T L} + 88.66 \end{cases}

(15)

By sorting out equations (14) to (15), the functional relationship between the aerodynamic noise at the measuring point of the outlet of the straw stalk crusher and each design parameter were as follows:

\begin{array}{l} S P L_{o u t} = 13.89 x_{N} - 0.01 x_{H H} - 0.279 x_{H N} + 0.0179 x_{P L} \\ + 0.1255 x_{P D} + 0.165 x_{H S} + 0.11 x_{G N} + 0.005 x_{F T L} \\ + 47.83 \end{array}

(16)

According to the above formula, the sensitivity of each parameter change of the straw stalk crusher to the sound pressure level of the aerodynamic noise at the measuring point of the outlet could be obtained, as shown in Table 8.

Table 8.

Sensitivity of each parameter of the straw stalk crusher to the noise pressure level of the outlet.

Parameter meaning	Sensitivity
Rotor speed	13.89 dB/k·r·min⁻¹
Hammer thickness	−0.01 dB/mm
Number of hammers	−0.279 dB/pcs
Length of the discharge tube	0.0179 dB/mm
Discharge angle	0.12556 dB/°
Number of hammer groups	0.165 dB/group
Number of cutting knives	0.11 dB/pcs
Length of the feed tank	0.005 dB/mm

The larger absolute value of sensitivity indicates higher sensitivity. Table 7 illustrates that hammer thickness (HH) and hammer number (HN) exhibited a negative correlation with outlet noise, whereas other parameters demonstrated a positive correlation. The order of sensitivity from large to small was: rotor speed N > number of hammers HN > number of hammer groups HS > discharge angle PD > number of cutting knives GN > length of the discharge tube PL > hammer thickness HH > length of the discharge tube FTL.

Noise sensitivity analysis of the inlet of the straw stalk crusher

The sensitivity value of aerodynamic noise for each parameter at the straw stalk crusher inlet was calculated the same way as at the outlet. Table 9 displays each parameter’s aerodynamic noise response at the inlet.

Table 9.

Aerodynamic noise corresponding to each parameter at the inlet.

Parameter meaning	Value	Inlet noise/dB(A)
Rotor speed	2.5	81.05
	2.8	85.04
	3.1	89.89
Hammer thickness	4	83.20
Hammer thickness	5	82.07
Number of hammers	18	83.53
Number of hammers	30	87.97
Length of the discharge tube	150	84.96
Length of the discharge tube	250	85.59
Discharge angle	25	84.87
Discharge angle	45	85.45
Number of hammer groups	8	85.48
Number of hammer groups	10	86.01
Number of cutting knives	3	85.56
Number of cutting knives	4	86.13
Length of the feed tank	480	87.85
Length of the feed tank	580	83.26

The linear relationships between each parameter and the aerodynamic noise at the inlet measuring point were expressed as follows:

{\begin{cases} S P L_{N} = 14.578 x_{N} + 44.512 \\ S P L_{H H} = - 1.505 x_{H H} + 89.46 \\ S P L_{H N} = 0.373 . x_{H N} + 76.558 \\ S P L_{P L} = 0.0063 x_{P L} + 83.94 \\ S P L_{P D} = 0.029 x_{P D} + 84.12 \\ S P L_{H S} = 0.243 x_{H S} + 83.566 \\ S P L_{G N} = 0.53 x_{G N} + 83.99 \\ S P L_{F T L} = - 0.045 x_{F T L} + 109.88 \end{cases}

(17)

The functional relationship between the aerodynamic noise at the measuring point of the inlet of the straw stalk crusher and each design parameter was as follows:

\begin{array}{l} S P L_{i n} = 14.578 x_{N} - 1.505 x_{H H} + 0.373 x_{H N} + 0.0063 x_{P L} \\ + 0.029 x_{P D} + 0.243 x_{H S} + 0.53 x_{G N} \\ - 0.045 x_{F T L} + 26.32 \end{array}

(18)

According to the above formula, the sensitivity of each parameter change of the straw stalk crusher to the aerodynamic noise pressure level of the inlet measuring point could be obtained, as shown in Table 10.

Table 10.

Sensitivity of each parameter of the straw stalk crusher to the noise pressure level of the inlet.

Parameter meaning	Sensitivity
Rotor speed	14.578 dB/k·r·min⁻¹
Hammer thickness	−1.505 dB/mm
Number of hammers	0.373 dB/pcs
Length of the discharge tube	0.0063 dB/mm
Discharge angle	0.029 dB/°
Number of hammer groups	0.243 dB/group
Number of cutting knives	0.53 dB/pcs
Length of the feed tank	−2.25 dB/mm

Table 10 illustrates that hammer thickness (HH) and feed tank length (FTL) exhibited a negative correlation with the aerodynamic noise of the entrance, whereas the other parameters demonstrated a positive correlation. The hierarchy of sensitivity, arranged from greatest to least, is as follows: rotor speed (N) > feed tank length (FTL) > hammer thickness (HH) > number of cutting knives (GN) > hammer number (HN) > hammer group number (HS) > discharge angle (PD) > discharge tube length (PL).

In conclusion, rotor speed, number of hammers, discharge angle, length of the discharge tube, number of cutting knives were the principal parameters that significantly influenced aerodynamic characteristics and aerodynamic noise. These factors may serve as additional optimization design variables.

Conclusion

(1) In contrast to the idle condition, the resistance coefficient of the discharge pipe increases significantly under load conditions, resulting in a lesser increase in aerodynamic noise of the discharge pipe compared to that of the feed tank. The viscosity gradient of eddy currents in the pipe during kneading conditions is higher than that observed during idle settings. A greater disparity in eddy viscosity diminishes the efficacy of steady eddy current dissipation, resulting in increased noise generation at the discharge pipe.

(2) The sound pressure level within the frequency band at both the inlet and outlet of the straw stalk crusher increases significantly with the feeding of materials. The peak sound pressure level at the outlet is 98.86 dB(A), whereas the peak sound pressure level at the inlet is 82.36 dB(A). In comparison to the idle condition, the maximum sound pressure level at the outlet and inlet has risen by 9.49 dB(A) and 2.98 dB(A), respectively. The sound pressure level of aerodynamic noise during kneading conditions surpasses the Chinese national standard of 90 dB(A), necessitating further optimization to mitigate the aerodynamic noise of the straw stalk crusher.

(3) The primary factors that substantially influence aerodynamic properties and aerodynamic noise include rotor speed, hammer quantity, discharge angle, feed tank length, and cutting knife count.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: Collaborative Innovation Achievement Project of “Double First - Class” Disciplines in Hei long jiang Province (LJGXCG2024-P25) and Basic scientific research funding project of colleges and universities in Heilongjiang Province (145309319).

ORCID iD

Yu Zhao

Data Availability Statement

The authors confirm that the data supporting the findings of this study are available within the article.

References

Gao

Zhai

Lan

, et al. Research on mechanical noise of hammer straw processing. Feed Industry 2024; 45(8): 43–49, (in Chinese).

Zhou

Zhai

Zhao

, et al. Noise measurement and analysis of straw kneading machine. J Inn Mong Univ Technol (Nat Sci Ed) 2024; 43(3): 244–249, (in Chinese).

Ministry of Agriculture and Rural Affairs of the People's Republic of China . Technical specification for quality evaluation of straw kneading machine NY/T 509-2015. Beijing: Industry Standard, 2015. (in Chinese).

. Numerical prediction and experimental verification of aerodynamic noise from straw threshing machines. Hohhot, China: Inner Mongolia University of Technology, 2020. (in Chinese).

Xuejian

Can

Zhiping

, et al. Prediction of vibration radiation noise from shell of straw crushing machine. Noise Vib Worldw 2021; 52(9): 271–284.

Zhang

Zhai

Lun

, et al. Test analysis and numerical simulation of noise from forage crushers. Appl Acoust 2022; 196: 108873.

Zhang

, et al. Noise source identification of crusher based on acoustic array test and acoustic boundary element calculation. Acta Agric Univ Jiangxiensis 2022; 44(2): 486–494, (in Chinese).

Cao

Zhang

, et al. Study on the aerodynamic noise of the hammer mill. Feed Industry 2018; 39(17): 7–12, (in Chinese).

Owusu

Karageorgos

Greet

, et al. Predicting mill feed grind characteristics through acoustic measurements. Miner Eng 2021; 171: 107099.

10.

Shi

Zhai

Zhao

, et al. Numerical simulation and optimization of internal flow field in multifunctional forage kneading machine. J NE Agric Univ 2022; 53(11): 60–67, (in Chinese).

11.

Zhou

Chen

, et al. Fully coupled CFD-DEM model for hydraulic transport of dense particles and its application in inclined pipe. J Tsinghua Univ(Sci Technol) 2024; 64(11): 1987–1996.

12.

Zhao

Zhai

Mou

, et al. Fatigue life prediction and reliability analysis of the forage crusher rotor. J Mech Sci Technol 2022; 36(4): 1771–1781.

13.

Zhao

. Test and analysis of multiple coupling acoustic characteristics of forage crusher. Hohhot, China: Inner Mongolia University of Technology, 2021. (in Chinese).

14.

Liu

Xiao

Zhang

, et al. Study on the vibro-acoustic characteristics of pantograph flat roof of high-speed train body under aerodynamic excitation. J Mech Eng 2025, [2025-07-17]. https://doi.org/10.kns.cnki.net/kcms/detail/11.2187.TH.20250311.1510.018.html

15.

Tanas

Szczepaniak

Kromulski

, et al. Modal analysis and acoustic noise characterization of a grain crusher. Ann Agric Environ Med 2018; 25(3): 433–436.

16.

Zhou

Zhang

Ren

, et al. Improvement design of volute profile for centrifugal fan. Fluid Mach 2025; 53(01): 51–58+67, (in Chinese).

17.

. Influence of dimension and structure of volute tongue on performance and operation noise of centrifugal fans. Noise Vib Control 2025; 45(01): 84–88, (in Chinese).

18.

Zhang

. Fundamentals of aeroacoustics. Peking, China: Defense Industry Press, 2012. (in Chinese).

19.

Cao

Yang

Jiao

, et al. Noise sources identification and noise reduction methods of hammer mill. Trans Chin Soc Agric Eng 2018; 34(7): 59–65, (in Chinese).

20.

ZhiPing

ZhuWei

Long

, et al. Numerical prediction of aerodynamic noise from impeller blowers of straw threshing machines. Noise Vib Worldw 2020; 51(1-2): 21–32.