基于CAD-DEM-CFD耦合的气吹式排种器数字化设计方法研究
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摘要
本文以一种气吹式大豆精密排种器为研究对象,在对国内外有关气固耦合理论进行深入研究的基础上,将气相按连续介质处理,通过求解控制方程分析气相运动信息,即计算流体力学方法(简称CFD);将大豆种子按离散介质处理,通过牛顿第二定律求解每个颗粒的运动速度和位移,即离散元法(简称DEM),同时考虑气相与固相的双向耦合作用,在此基础上研制出基于CAD-DEM-CFD耦合的气吹式精密排种器二维设计分析软件。采用该软件通过改变排种器的CAD模型、种子模型和仿真参数,来分析不同结构排种器的性能,以期为气吹式排种器的研究和优化设计建立一种新方法。本文的主要工作和结果如下。
1)对3种大豆种子(吉科豆、吉新豆和吉豆)的物理力学参数进行了测试分析,包括大豆种子的含水率、密度、三轴尺寸、动静摩擦系数、刚度系数、弹性模量及碰撞恢复系数,其中静摩擦系数和碰撞恢复系数采用自制仪器测试。
2)设计制作了内部可视的气吹式大豆精密排种器,在PSJ型排种器试验台上进行了台架试验,借助高速摄像技术和爆破分析软件分析了气吹式大豆精密排种器在不加气和加气2种情况下的工作过程与性能,包括排种性能、种子运动速度和轨迹、投种角,由分析可知:
①在不加气时,随着型孔轮转速(13.99 r/min~33.01 r/min)增大,排种单粒率减小、空穴率增大、双粒率无变化规律且值均小于0.6%;离型孔轮较远处的种子速度和位移很小,基本处于原位置自转或静止状态;拖带层种子和投种口种子速度均随型孔轮转速增大而增大;在型孔轮转速一定时,3种大豆种子投种角差别较小,但随转速增大投种角增大;投种后种子的运动轨迹为开口向下的抛物线,且随型孔轮转速增大抛物线开口增大;排3种大豆种子时上述变化规律基本相同;
②加气且吹气口方向与水平夹角成60°时,型孔轮转速一定时,随风压(0.4 kPa~ 2.0 kPa)增大,排种单粒率增大、空穴率减小,而双粒率无变化规律且值均较小,特别是转速26.67 r/min、风压1.2 kPa时,排种性能明显优于不加气时的排种性能;风压一定时,随型孔轮转速增大,排种单粒率减小、空穴率增大、双粒率无变化规律且值均较小;离型孔轮较远处种子的速度和位移很小,基本处于原位置自转或静止状态;拖带层种子速度随型孔轮转速增大而增大,随风压增大而减小;投种口种子速度随型孔轮转速增大而增大,随风压增大也增大;风压一定时,种子投种角随型孔轮转速增大而增大,与不加气时规律相同,转速一定时,投种角随风压增大而减小;投种后种子的运动轨迹为开口向下的抛物线,风压一定时,随型孔轮转速增大抛物线开口增大,型孔轮转速一定时,随风压增大抛物线开口减小;排3种大豆种子时上述变化规律基本相同;
③加气且吹气口方向与水平夹角成90°时,在型孔轮转速为26.67 r/min时,风压增大,排种(吉科豆)单粒率增大、空穴率减小,而双粒率无变化规律,但值均小于等于1.3%,这与倾斜进气口排种规律基本相同;投种后种子运动轨迹的抛物线开口不随风压变化而呈规律性变化,其它与夹角60°时变化规律相同。
3)在深入研究二维结构和非结构网格生成方法的基础上,建立了2种网格生成的改进方法,即TM-魏改进法和基于局部弹簧平衡系统的布点方法,这2种方法可提高网格的贴体性、正交性、疏密性等网格质量。
4)在对DEM、CFD和DEM-CFD耦合方法进行深入研究的基础上,基于同位网格和有限体积法建立了求解气相场的改进算法——SIMPLERC。该算法吸收了SIMPLER算法的优点,克服了SIMPLE算法对压力修正的不理想,同时吸纳了SIMPLEC算法的核心思想,即略去速度修正方程中的∑a nb (u n′b?u′P)项,因而减轻直接由压差项修正速度的负担,使速度场求解速度加快,也避免了SIMPLE算法中“不协调一致”的错误。
5)在对CAD软件进行二次开发的基础上,研制了网格生成模块、气固耦合计算模块,并集成到了课题组研发的二维离散元软件,还采用OpenGL改善了仿真结果的动态显示效果,在此基础上研制出基于CAD-DEM-CFD耦合的气吹式精密排种器二维设计分析软件。通过对该软件的测试和实例验证,包括网格生成模块、气固耦合计算模块等,初步证明了本文开发的二维设计分析软件能实现气体和气固耦合的分析计算。
6)根据试验测得的3种大豆种子的物理力学参数,分别采用圆形和椭圆形2种颗粒模型建立了6种大豆种子的分析模型,采用本文研制的二维设计分析软件分别对不加气和加气情况下的气吹式大豆精密排种器工作过程与性能进行了仿真分析,由仿真分析与试验结果的对比可知:
①不加气时仿真分析,在2种颗粒模型(圆形和椭圆形)和4种转速(13.99 r/min、20.33 r/min、26.67 r/min、33.01 r/min)情况下,排种单粒率均为100%、空穴率和双粒率均为0,与试验结果差别较大,其原因还需深入探讨;
②不加气时仿真分析,离型孔轮较远处种子的速度和位移很小,基本处于原位置静止状态,与试验结果基本相同;在2种颗粒模型和4种转速情况下,拖带层种子和投种口种子(吉科豆)的速度随型孔轮转速增大而增大,与试验结果变化验趋一致;圆形颗粒模型且型孔轮转速为13.99 r/min时,种子(吉科豆)投种角的仿真与试验结果误差为1.01%,种子模型为椭圆形时,投种角仿真与试验结果误差为1.49%,随着型孔轮转速的增大,2种颗粒模型投种角的仿真与试验结果误差均在增大,但仿真与试验结果的变化趋势一致;投种后种子和拖带层种子(吉科豆)运动轨迹与试验结果变化验趋一致;
③加气时且吹气口方向与水平成60°情况下仿真分析,在2种颗粒模型(圆形和椭圆形)、3种转速(13.99 r/min、26.67 r/min、39.35r/min)和3种风压(0.4 kPa、1.2 kPa、2.0 kPa)情况下,排种性能指标均与不加气仿真结果相似,与试验结果差别较大;离型孔轮较远处种子的速度和位移很小,基本处于原位置静止状态,与试验结果基本相同;拖带层种子速度比试验值大、投种口种子速度比试验值小,但总体变化趋势均与试验相同;风压一定时,种子(吉科豆)投种角随型孔轮转速增大而增大,与试验和不加气情况变化趋势相同,转速一定时,随风压增大投种角仿真与试验结果误差增大,但变化趋势相同;投种后种子(吉科豆)运动轨迹为开口向下的抛物线,风压一定时,抛物线开口随型孔轮转速增大而增大,与试验结果变化趋势相同,但较不加气时略小;转速一定时,投种后种子(吉科豆)运动轨迹的抛物线开口随风压增大而略减小,与试验结果变化趋势相同;拖带层种子运动轨迹的仿真与试验虽有误差,但变化趋势基本相同;
④加气时且吹气口方向与水平成90°情况下仿真分析,在2种颗粒模型(圆形和椭圆形)、1种转速(26.67 r/min)和3种风压(0.4 kPa、1.2 kPa、2.0 kPa)情况下,吉科豆的排种性能指标与试验差别较大;离型孔轮较远处种子的速度和位移很小,基本处于原位置静止状态,与其它情况仿真结果及试验结果基本相同;拖带层和投种口种子速度的仿真分析与试验结果变化趋势相同;投种后吉科豆轨迹为开口向下的抛物线,且随风压增大仿真与试验的误差增大,但整体变化趋势相同;拖带层种子运动轨迹的仿真与试验变化趋势相同;风压为0.4 kPa和椭圆颗粒模型时,吉科豆投种角的仿真与试验误差为4.28%,圆形颗粒模型时为8.92%,风压增大时,2种颗粒模型投种角的仿真与试验结果误差均在增大,且椭圆颗粒模型较圆形颗粒模型误差大;
⑤刚度系数和摩擦系数在实测范围内取值时,对仿真结果影响不大。
综上所述可知,虽然仿真结果与试验结果有误差,但除去排种性能指标外,种子运动速度和轨迹、投种角的仿真与试验结果变化趋势一致,由此初步证明了采用DEM-CFD耦合方法和本文研制的二维设计分析软件研究气吹式大豆精密排种器的可行性。
In this paper, a kind of precision soybean metering device is treated as research object, in order to deal with the continuous gas media and analysis the gas motion information through solving equations on the basis of further research in domestic and foreign gas-solid coupling theories, as The Computational Fluid Dynamics (CFD); The soybean seeds are treated as discrete media, through Newton’s second law each granule’velocity and trajectory can be obtained, as The Discrete Element Method (DEM), meanwhile the two way coupling effect of gas-solid is taken into consideration, on this foundation a two-dimensional design and analysis software of gas blown precision soybean metering device is developed based on CAD-DEM-CFD coupling technology. We can use this software to change the metering device’s CAD model, seed model and simulation parameter to analysis the performances of different kinds of metering devices, and we can also compare the simulation results and the test results of the metering device’s experiments, looking forward to establish a novel method to the research and the optimization of air-blowing seed-metering device. The main work and results of the paper are as follows:
1) The physical and mechanical properties of three soybean seeds (GK, GX, G)have been tested, include moisture content, density, triaxial dimension, static and dynamic friction coefficient, stiffness coefficient, elastic modulus and coefficient of restitution, which static friction coefficient and coefficient of restitution are tested by homemade instruments.
2) A visual gas blown precision soybean metering device has been designed and tested in the PSJ-type metering device test bed, the working process and performance of the air-blowing seed-metering device are able to be analyzed in the condition both with and without gas by the use of high speed video camera technology and BLASTER’S MOTION ANALYSIS SOFTWARE, include the seeding performance, speed and trajectory of the seed, dropping angles, the analysis shows:
①In the condition without gas, when the rotational velocities(13.99r/min~33.01r/min) of the wheel increase, the rate of single seed decrease, the rate of empty increased, the double seed rate has no change law and values of the double seed rate are all less than 0.6%, The velocity and displacement of the seed that is far from the wheel is very small, basically at the original location or at rest. The velocity of the seed that is in towing layer and dropping exit increase while the velocity of the wheel increase. When the rotational velocity of the wheel is stable, the dropping angles of the 3 kinds of soybean seeds have little difference, but when the rotational velocities increase, they increase. After dropping the trajectory of the seed is a downwards-opening parabola and when the wheel’s velocities increase, the parabola’s opening increase. It has almost the same discipline when seeding the 3 kinds of soybean seeds.
②When filling gas and the blowing direction has a 60°angle between horizon, if the wheel’s velocity is unchanged, the single seed rate increase, the empty rate decrease, while the double-seed rate has no change law and values of the double-seed rate are small according to the increase of wind pressure(0.4kPa~2.0kPa), especially when the rotation velocity is 26.67r/min and the wind pressure is 1.2kPa, the seeding performance is better than no-gas condition. If the wind pressure is unchanged, the single seed rate increase, the empty rate decrease, while the double-seed rate has no change law and values of the double-seed rate are small according to the increase of wheel’s velocity, the same tendency with no-gas condition. The velocity and displacement of the seed that is far from the wheel is very small, basically at the original location or at rest. The velocity of the seed that is in towing layer increase while the velocity of the wheel increase and decrease while the wind pressure increase. If the wind pressure is unchanged, the dropping angle increase according to the increase of wheel’speed, the same tendency with no-gas condition. If the rotation speed is unchanged, the dropping angles decrease according to the increase of wind pressure. After dropping the trajectory of the seed is a downwards-opening parabola and if the wind pressure is unchanged, the parabola’s opening increase according to the increase of wheel’s velocity, if the wheel’s velocity is unchanged, the parabola’s opening decrease according to the increase of wind pressure. It has almost the same discipline when seeding the 3 kinds of soybean seeds.
③When filling gas and the blowing direction has a 90°angle between horizon, if the wheel has a velocity of 26.67r/min, the seeding(GK) single seed rate increase, the empty rate decrease, while the double-seed rate has no change law and values of the double-seed rate are less than or equal to 1.3%, the result is just in contrast with the way of incline blown seeding. After dropping the trajectory of the seed is parabola which the opening has no regular change according to the change of wind pressure, except this, it has the same discipline with the 60°angle.
3) Based on deep researching of two-dimensional structured and unstructured grid’s generation method, improvements of two grid generation have been established, as the TM-Wei improvement and spring balance system based method, these two methods are able to improve the body-fitted, orthogonal nature and the density of the gird quality.
4) Based on deep researching DEM, CFD and the coupling method of DEM-CFD, an improved algorithm—SIMPLERC has been established according to collocated grid and finite volume method. This algorithm has absorbed the advantages of SIMPLER algorithm and overcome the defective of SIMPLER algorithm in pressure correction, at the same time it has absorbed the core idea of SIMPLER algorithm, as omitting the item in velocity correction equation, so as to reduce the burden directly from rate of pressure, accelerate the speed of solving in velocity field and avoid the‘inconsistent’mistake happened in SIMPLER algorithm too.
5) Based on redevelopment of CAD software, grid generation module, gas-solid coupling calculation module has been developed and integrated into the two-dimensional DEM software developed by the research group. OpenGL has also been used to improve the dynamic display of simulation result. On the foundation of this, CAD-DEM-CFD coupling analysis software has been developed. By instances of the software testing and validation, including grid generation module, gas-solid coupling calculation module etc, it demonstrated the two-dimensional design and analysis software introduced in this paper is able to work out the gas and gas-solid coupling calculation.
6) According to the physical and mechanical parameters of three soybean seeds tested in experiment, six kinds of soybean seed analysis model have been established using round and oval-shaped particle model, the working processes and performances of the gas blown precision soybean metering device both in gas and no-gas condition have been simulated by the software introduced in this paper, comparison between simulation and experimental results shows:
①When the simulation without gas, in the condition of 2 particle models(round and oval-shaped) and 4 rotation speeds(13.99r/min、20.33r/min、26.67r/min、33.01r/min), each of the seeding single seed rate is 100%, each of the empty rate and double seed rate is 0, it has great difference with the test bed result and the reason needs further study.
②When the simulation without gas, the velocity and displacement of the seed which is far from the wheel is very small, basically at the original location or at rest, basically the same with the test. In the condition of 2 particle models and 4 rotation speeds, the velocity of the seed(GK) which is in towing layer and dropping exit increase according to the increase of wheel’s velocity, the same changing tendency with the test. When using the round-shaped particle model and the wheel’s velocity is 13.99r/min, the test error of dropping angle between simulation and test is 1.01%. When using oval-shaped model, the error is 1.49%, increase while the wheel’s velocity increase, the dropping angles error of two seed models increase both, but the changing tendency are the same with test. After dropping, the trajectory of seeds and seeds in towing layer has the same changing tendency with test.
③When filling gas and the blowing direction has a 90°angle between horizon, in the condition of 2 particle models(round and oval-shaped), 3 rotation speeds(13.99r/min、26.67r/min、39.35r/min) and 3 wind pressures(0.4kPa、1.2kPa、2.0kPa), the seeding performances are familiar with the simulation without gas and has a large difference between test results. The velocity and displacement of the seed that is far from the wheel is very small, basically at the original location or at rest, basically the same with the test. The velocity of the seed in towing layer is larger than the test result, the velocity of the seed in dropping exit is smaller than the test result, but the whole changing tendency is the same with the test result. When the wind pressure is unchanged, the dropping angle (GK) increase according to the increase of wheel’s velocity, it has the same changing tendency with test result and no-gas condition. When the rotation speed is unchanged, the dropping angle’s error between simulation and test increase, but they have the same changing tendency. After dropping the trajectory of the seed is a downwards-opening parabola, when the wind pressure is unchanged, the parabola’s opening increase according to the increase of wheel’s velocity, it has the same changing tendency with the test result, but smaller than no-gas condition. When the rotation speed is unchanged, after dropping the trajectory of the seed (GK) is a parabola which its opening decreases according to the increase of wind pressure, it has the same changing tendency with test result. Although the trajectory of seed in towing layer in simulation has error when compared with test result, they have the same changing tendency.
④When filling gas and the blowing direction has a 90°angle between horizon, in the condition of 2 particle models (round and oval-shaped), 3 rotation speeds(13.99r/min、26.67r/min、39.35r/min) and 3 wind pressures(0.4kPa、1.2kPa、2.0kPa), the seeding performance of GK seed are the same with incline blown seeding and no-gas seeding. The velocity and displacement of the seed that is far from the wheel is very small, basically at the original location or at rest, basically the same with other test. The velocity of the seed (GK) that is in towing layer and dropping exit has the same changing result with test result. After dropping the trajectory of the seed (GK) is a parabola which its error between simulation and test increase according to the increase of wind pressure, but they have the same changing tendency. The trajectory of seed in towing layer has the same changing tendency with test result. When the wind pressure is 0.4kPa and using oval-shaped particle model, the dropping angle of GK seed has an error of 4.28% compared with test result, and 8.92% when using round-shaped model, but when the wind pressure increase, the errors of the two seed model both increase, and oval-shaped model has a larger number than round one.
⑤When using the stiffness coefficient and friction coefficient in the range of measured values, it has little effect to the simulation result.
In summary we can see, although there is error between simulation and test result, except the performance of seeding, the velocity, trajectory and seeding angle all have the same changing tendency with test result, so it initially proved that using DEM-CFD coupling method and the software introduced in this paper to research gas blown precision soybean metering device is feasible.
1)对3种大豆种子(吉科豆、吉新豆和吉豆)的物理力学参数进行了测试分析,包括大豆种子的含水率、密度、三轴尺寸、动静摩擦系数、刚度系数、弹性模量及碰撞恢复系数,其中静摩擦系数和碰撞恢复系数采用自制仪器测试。
2)设计制作了内部可视的气吹式大豆精密排种器,在PSJ型排种器试验台上进行了台架试验,借助高速摄像技术和爆破分析软件分析了气吹式大豆精密排种器在不加气和加气2种情况下的工作过程与性能,包括排种性能、种子运动速度和轨迹、投种角,由分析可知:
①在不加气时,随着型孔轮转速(13.99 r/min~33.01 r/min)增大,排种单粒率减小、空穴率增大、双粒率无变化规律且值均小于0.6%;离型孔轮较远处的种子速度和位移很小,基本处于原位置自转或静止状态;拖带层种子和投种口种子速度均随型孔轮转速增大而增大;在型孔轮转速一定时,3种大豆种子投种角差别较小,但随转速增大投种角增大;投种后种子的运动轨迹为开口向下的抛物线,且随型孔轮转速增大抛物线开口增大;排3种大豆种子时上述变化规律基本相同;
②加气且吹气口方向与水平夹角成60°时,型孔轮转速一定时,随风压(0.4 kPa~ 2.0 kPa)增大,排种单粒率增大、空穴率减小,而双粒率无变化规律且值均较小,特别是转速26.67 r/min、风压1.2 kPa时,排种性能明显优于不加气时的排种性能;风压一定时,随型孔轮转速增大,排种单粒率减小、空穴率增大、双粒率无变化规律且值均较小;离型孔轮较远处种子的速度和位移很小,基本处于原位置自转或静止状态;拖带层种子速度随型孔轮转速增大而增大,随风压增大而减小;投种口种子速度随型孔轮转速增大而增大,随风压增大也增大;风压一定时,种子投种角随型孔轮转速增大而增大,与不加气时规律相同,转速一定时,投种角随风压增大而减小;投种后种子的运动轨迹为开口向下的抛物线,风压一定时,随型孔轮转速增大抛物线开口增大,型孔轮转速一定时,随风压增大抛物线开口减小;排3种大豆种子时上述变化规律基本相同;
③加气且吹气口方向与水平夹角成90°时,在型孔轮转速为26.67 r/min时,风压增大,排种(吉科豆)单粒率增大、空穴率减小,而双粒率无变化规律,但值均小于等于1.3%,这与倾斜进气口排种规律基本相同;投种后种子运动轨迹的抛物线开口不随风压变化而呈规律性变化,其它与夹角60°时变化规律相同。
3)在深入研究二维结构和非结构网格生成方法的基础上,建立了2种网格生成的改进方法,即TM-魏改进法和基于局部弹簧平衡系统的布点方法,这2种方法可提高网格的贴体性、正交性、疏密性等网格质量。
4)在对DEM、CFD和DEM-CFD耦合方法进行深入研究的基础上,基于同位网格和有限体积法建立了求解气相场的改进算法——SIMPLERC。该算法吸收了SIMPLER算法的优点,克服了SIMPLE算法对压力修正的不理想,同时吸纳了SIMPLEC算法的核心思想,即略去速度修正方程中的∑a nb (u n′b?u′P)项,因而减轻直接由压差项修正速度的负担,使速度场求解速度加快,也避免了SIMPLE算法中“不协调一致”的错误。
5)在对CAD软件进行二次开发的基础上,研制了网格生成模块、气固耦合计算模块,并集成到了课题组研发的二维离散元软件,还采用OpenGL改善了仿真结果的动态显示效果,在此基础上研制出基于CAD-DEM-CFD耦合的气吹式精密排种器二维设计分析软件。通过对该软件的测试和实例验证,包括网格生成模块、气固耦合计算模块等,初步证明了本文开发的二维设计分析软件能实现气体和气固耦合的分析计算。
6)根据试验测得的3种大豆种子的物理力学参数,分别采用圆形和椭圆形2种颗粒模型建立了6种大豆种子的分析模型,采用本文研制的二维设计分析软件分别对不加气和加气情况下的气吹式大豆精密排种器工作过程与性能进行了仿真分析,由仿真分析与试验结果的对比可知:
①不加气时仿真分析,在2种颗粒模型(圆形和椭圆形)和4种转速(13.99 r/min、20.33 r/min、26.67 r/min、33.01 r/min)情况下,排种单粒率均为100%、空穴率和双粒率均为0,与试验结果差别较大,其原因还需深入探讨;
②不加气时仿真分析,离型孔轮较远处种子的速度和位移很小,基本处于原位置静止状态,与试验结果基本相同;在2种颗粒模型和4种转速情况下,拖带层种子和投种口种子(吉科豆)的速度随型孔轮转速增大而增大,与试验结果变化验趋一致;圆形颗粒模型且型孔轮转速为13.99 r/min时,种子(吉科豆)投种角的仿真与试验结果误差为1.01%,种子模型为椭圆形时,投种角仿真与试验结果误差为1.49%,随着型孔轮转速的增大,2种颗粒模型投种角的仿真与试验结果误差均在增大,但仿真与试验结果的变化趋势一致;投种后种子和拖带层种子(吉科豆)运动轨迹与试验结果变化验趋一致;
③加气时且吹气口方向与水平成60°情况下仿真分析,在2种颗粒模型(圆形和椭圆形)、3种转速(13.99 r/min、26.67 r/min、39.35r/min)和3种风压(0.4 kPa、1.2 kPa、2.0 kPa)情况下,排种性能指标均与不加气仿真结果相似,与试验结果差别较大;离型孔轮较远处种子的速度和位移很小,基本处于原位置静止状态,与试验结果基本相同;拖带层种子速度比试验值大、投种口种子速度比试验值小,但总体变化趋势均与试验相同;风压一定时,种子(吉科豆)投种角随型孔轮转速增大而增大,与试验和不加气情况变化趋势相同,转速一定时,随风压增大投种角仿真与试验结果误差增大,但变化趋势相同;投种后种子(吉科豆)运动轨迹为开口向下的抛物线,风压一定时,抛物线开口随型孔轮转速增大而增大,与试验结果变化趋势相同,但较不加气时略小;转速一定时,投种后种子(吉科豆)运动轨迹的抛物线开口随风压增大而略减小,与试验结果变化趋势相同;拖带层种子运动轨迹的仿真与试验虽有误差,但变化趋势基本相同;
④加气时且吹气口方向与水平成90°情况下仿真分析,在2种颗粒模型(圆形和椭圆形)、1种转速(26.67 r/min)和3种风压(0.4 kPa、1.2 kPa、2.0 kPa)情况下,吉科豆的排种性能指标与试验差别较大;离型孔轮较远处种子的速度和位移很小,基本处于原位置静止状态,与其它情况仿真结果及试验结果基本相同;拖带层和投种口种子速度的仿真分析与试验结果变化趋势相同;投种后吉科豆轨迹为开口向下的抛物线,且随风压增大仿真与试验的误差增大,但整体变化趋势相同;拖带层种子运动轨迹的仿真与试验变化趋势相同;风压为0.4 kPa和椭圆颗粒模型时,吉科豆投种角的仿真与试验误差为4.28%,圆形颗粒模型时为8.92%,风压增大时,2种颗粒模型投种角的仿真与试验结果误差均在增大,且椭圆颗粒模型较圆形颗粒模型误差大;
⑤刚度系数和摩擦系数在实测范围内取值时,对仿真结果影响不大。
综上所述可知,虽然仿真结果与试验结果有误差,但除去排种性能指标外,种子运动速度和轨迹、投种角的仿真与试验结果变化趋势一致,由此初步证明了采用DEM-CFD耦合方法和本文研制的二维设计分析软件研究气吹式大豆精密排种器的可行性。
In this paper, a kind of precision soybean metering device is treated as research object, in order to deal with the continuous gas media and analysis the gas motion information through solving equations on the basis of further research in domestic and foreign gas-solid coupling theories, as The Computational Fluid Dynamics (CFD); The soybean seeds are treated as discrete media, through Newton’s second law each granule’velocity and trajectory can be obtained, as The Discrete Element Method (DEM), meanwhile the two way coupling effect of gas-solid is taken into consideration, on this foundation a two-dimensional design and analysis software of gas blown precision soybean metering device is developed based on CAD-DEM-CFD coupling technology. We can use this software to change the metering device’s CAD model, seed model and simulation parameter to analysis the performances of different kinds of metering devices, and we can also compare the simulation results and the test results of the metering device’s experiments, looking forward to establish a novel method to the research and the optimization of air-blowing seed-metering device. The main work and results of the paper are as follows:
1) The physical and mechanical properties of three soybean seeds (GK, GX, G)have been tested, include moisture content, density, triaxial dimension, static and dynamic friction coefficient, stiffness coefficient, elastic modulus and coefficient of restitution, which static friction coefficient and coefficient of restitution are tested by homemade instruments.
2) A visual gas blown precision soybean metering device has been designed and tested in the PSJ-type metering device test bed, the working process and performance of the air-blowing seed-metering device are able to be analyzed in the condition both with and without gas by the use of high speed video camera technology and BLASTER’S MOTION ANALYSIS SOFTWARE, include the seeding performance, speed and trajectory of the seed, dropping angles, the analysis shows:
①In the condition without gas, when the rotational velocities(13.99r/min~33.01r/min) of the wheel increase, the rate of single seed decrease, the rate of empty increased, the double seed rate has no change law and values of the double seed rate are all less than 0.6%, The velocity and displacement of the seed that is far from the wheel is very small, basically at the original location or at rest. The velocity of the seed that is in towing layer and dropping exit increase while the velocity of the wheel increase. When the rotational velocity of the wheel is stable, the dropping angles of the 3 kinds of soybean seeds have little difference, but when the rotational velocities increase, they increase. After dropping the trajectory of the seed is a downwards-opening parabola and when the wheel’s velocities increase, the parabola’s opening increase. It has almost the same discipline when seeding the 3 kinds of soybean seeds.
②When filling gas and the blowing direction has a 60°angle between horizon, if the wheel’s velocity is unchanged, the single seed rate increase, the empty rate decrease, while the double-seed rate has no change law and values of the double-seed rate are small according to the increase of wind pressure(0.4kPa~2.0kPa), especially when the rotation velocity is 26.67r/min and the wind pressure is 1.2kPa, the seeding performance is better than no-gas condition. If the wind pressure is unchanged, the single seed rate increase, the empty rate decrease, while the double-seed rate has no change law and values of the double-seed rate are small according to the increase of wheel’s velocity, the same tendency with no-gas condition. The velocity and displacement of the seed that is far from the wheel is very small, basically at the original location or at rest. The velocity of the seed that is in towing layer increase while the velocity of the wheel increase and decrease while the wind pressure increase. If the wind pressure is unchanged, the dropping angle increase according to the increase of wheel’speed, the same tendency with no-gas condition. If the rotation speed is unchanged, the dropping angles decrease according to the increase of wind pressure. After dropping the trajectory of the seed is a downwards-opening parabola and if the wind pressure is unchanged, the parabola’s opening increase according to the increase of wheel’s velocity, if the wheel’s velocity is unchanged, the parabola’s opening decrease according to the increase of wind pressure. It has almost the same discipline when seeding the 3 kinds of soybean seeds.
③When filling gas and the blowing direction has a 90°angle between horizon, if the wheel has a velocity of 26.67r/min, the seeding(GK) single seed rate increase, the empty rate decrease, while the double-seed rate has no change law and values of the double-seed rate are less than or equal to 1.3%, the result is just in contrast with the way of incline blown seeding. After dropping the trajectory of the seed is parabola which the opening has no regular change according to the change of wind pressure, except this, it has the same discipline with the 60°angle.
3) Based on deep researching of two-dimensional structured and unstructured grid’s generation method, improvements of two grid generation have been established, as the TM-Wei improvement and spring balance system based method, these two methods are able to improve the body-fitted, orthogonal nature and the density of the gird quality.
4) Based on deep researching DEM, CFD and the coupling method of DEM-CFD, an improved algorithm—SIMPLERC has been established according to collocated grid and finite volume method. This algorithm has absorbed the advantages of SIMPLER algorithm and overcome the defective of SIMPLER algorithm in pressure correction, at the same time it has absorbed the core idea of SIMPLER algorithm, as omitting the item in velocity correction equation, so as to reduce the burden directly from rate of pressure, accelerate the speed of solving in velocity field and avoid the‘inconsistent’mistake happened in SIMPLER algorithm too.
5) Based on redevelopment of CAD software, grid generation module, gas-solid coupling calculation module has been developed and integrated into the two-dimensional DEM software developed by the research group. OpenGL has also been used to improve the dynamic display of simulation result. On the foundation of this, CAD-DEM-CFD coupling analysis software has been developed. By instances of the software testing and validation, including grid generation module, gas-solid coupling calculation module etc, it demonstrated the two-dimensional design and analysis software introduced in this paper is able to work out the gas and gas-solid coupling calculation.
6) According to the physical and mechanical parameters of three soybean seeds tested in experiment, six kinds of soybean seed analysis model have been established using round and oval-shaped particle model, the working processes and performances of the gas blown precision soybean metering device both in gas and no-gas condition have been simulated by the software introduced in this paper, comparison between simulation and experimental results shows:
①When the simulation without gas, in the condition of 2 particle models(round and oval-shaped) and 4 rotation speeds(13.99r/min、20.33r/min、26.67r/min、33.01r/min), each of the seeding single seed rate is 100%, each of the empty rate and double seed rate is 0, it has great difference with the test bed result and the reason needs further study.
②When the simulation without gas, the velocity and displacement of the seed which is far from the wheel is very small, basically at the original location or at rest, basically the same with the test. In the condition of 2 particle models and 4 rotation speeds, the velocity of the seed(GK) which is in towing layer and dropping exit increase according to the increase of wheel’s velocity, the same changing tendency with the test. When using the round-shaped particle model and the wheel’s velocity is 13.99r/min, the test error of dropping angle between simulation and test is 1.01%. When using oval-shaped model, the error is 1.49%, increase while the wheel’s velocity increase, the dropping angles error of two seed models increase both, but the changing tendency are the same with test. After dropping, the trajectory of seeds and seeds in towing layer has the same changing tendency with test.
③When filling gas and the blowing direction has a 90°angle between horizon, in the condition of 2 particle models(round and oval-shaped), 3 rotation speeds(13.99r/min、26.67r/min、39.35r/min) and 3 wind pressures(0.4kPa、1.2kPa、2.0kPa), the seeding performances are familiar with the simulation without gas and has a large difference between test results. The velocity and displacement of the seed that is far from the wheel is very small, basically at the original location or at rest, basically the same with the test. The velocity of the seed in towing layer is larger than the test result, the velocity of the seed in dropping exit is smaller than the test result, but the whole changing tendency is the same with the test result. When the wind pressure is unchanged, the dropping angle (GK) increase according to the increase of wheel’s velocity, it has the same changing tendency with test result and no-gas condition. When the rotation speed is unchanged, the dropping angle’s error between simulation and test increase, but they have the same changing tendency. After dropping the trajectory of the seed is a downwards-opening parabola, when the wind pressure is unchanged, the parabola’s opening increase according to the increase of wheel’s velocity, it has the same changing tendency with the test result, but smaller than no-gas condition. When the rotation speed is unchanged, after dropping the trajectory of the seed (GK) is a parabola which its opening decreases according to the increase of wind pressure, it has the same changing tendency with test result. Although the trajectory of seed in towing layer in simulation has error when compared with test result, they have the same changing tendency.
④When filling gas and the blowing direction has a 90°angle between horizon, in the condition of 2 particle models (round and oval-shaped), 3 rotation speeds(13.99r/min、26.67r/min、39.35r/min) and 3 wind pressures(0.4kPa、1.2kPa、2.0kPa), the seeding performance of GK seed are the same with incline blown seeding and no-gas seeding. The velocity and displacement of the seed that is far from the wheel is very small, basically at the original location or at rest, basically the same with other test. The velocity of the seed (GK) that is in towing layer and dropping exit has the same changing result with test result. After dropping the trajectory of the seed (GK) is a parabola which its error between simulation and test increase according to the increase of wind pressure, but they have the same changing tendency. The trajectory of seed in towing layer has the same changing tendency with test result. When the wind pressure is 0.4kPa and using oval-shaped particle model, the dropping angle of GK seed has an error of 4.28% compared with test result, and 8.92% when using round-shaped model, but when the wind pressure increase, the errors of the two seed model both increase, and oval-shaped model has a larger number than round one.
⑤When using the stiffness coefficient and friction coefficient in the range of measured values, it has little effect to the simulation result.
In summary we can see, although there is error between simulation and test result, except the performance of seeding, the velocity, trajectory and seeding angle all have the same changing tendency with test result, so it initially proved that using DEM-CFD coupling method and the software introduced in this paper to research gas blown precision soybean metering device is feasible.
引文
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