尚增強(qiáng) 張開(kāi)飛 楊東福 馬質(zhì)璞
摘要:針對(duì)氣吸式排種器在高速氣流場(chǎng)下種子的運(yùn)動(dòng)規(guī)律及受力特性復(fù)雜、難以準(zhǔn)確分析計(jì)算的問(wèn)題,通過(guò)離散元(DEM)與計(jì)算流體力學(xué)(CFD)耦合的方法,對(duì)氣吸式大豆排種器的工作過(guò)程進(jìn)行氣固兩相仿真并對(duì)其種盤(pán)參數(shù)進(jìn)行優(yōu)化。分析充種過(guò)程中種子所受曳力的變化規(guī)律,對(duì)充種各階段臨界點(diǎn)進(jìn)行劃分,定義各階段充種性能評(píng)價(jià)指標(biāo)。以吸附持續(xù)時(shí)間和種子移出阻力作為充種性能評(píng)價(jià)指標(biāo),以托種臺(tái)高度、托種臺(tái)角度、攪種桿厚度為試驗(yàn)因素,進(jìn)行三因素二次旋轉(zhuǎn)正交組合試驗(yàn),并對(duì)試驗(yàn)結(jié)果進(jìn)行響應(yīng)曲面分析和多目標(biāo)尋優(yōu),確定最佳的排種盤(pán)參數(shù)組合:托種臺(tái)高度為2.8mm、托種臺(tái)角度為31.2°、攪種桿厚度為1.4mm。此時(shí),排種器充種性能指標(biāo)為:吸附持續(xù)時(shí)間0.104s,種子移出阻力0.0081N。對(duì)優(yōu)化結(jié)果進(jìn)行臺(tái)架試驗(yàn)驗(yàn)證,與優(yōu)化前的排種盤(pán)在不同作業(yè)速度下進(jìn)行對(duì)比。臺(tái)架試驗(yàn)結(jié)果表明:優(yōu)化后的排種盤(pán)的排種性能有顯著提升,其合格指數(shù)和漏播指數(shù)均優(yōu)于原排種器,滿足大豆精量播種要求。
關(guān)鍵詞:大豆;氣吸式排種器;離散元;計(jì)算流體力學(xué);氣固耦合仿真
中圖分類號(hào):S223 文獻(xiàn)標(biāo)識(shí)碼:A 文章編號(hào):20955553 (2023) 11000108
Simulation optimization of seed plate of air suction soybean seed-metering device
based on DEM-CFD coupling
Shang Zengqiang Zhang Kaifei Yang Dongfu Ma Zhipu
(1. Nanyang Vocational College of Agriculture, Nanyang, 473000, China;
2. College of Mechanical & Electrical Engineering, Henan Agricultural University, Zhengzhou, 450002, China)
Abstract:Aiming at the problem that the movement law and force characteristics of the air suction seed metering device under the high-speed air flow field are very complex and difficult to accurately analyze and calculate, this paper carries out gas-solid two-phase simulation of the working process of the air suction soybean seed metering device and optimizes the parameters of its seed tray through the coupling method of discrete element (DEM) and computational fluid dynamics (CFD). The change law of the traction force on the seeds in the process of seed filling was analyzed, and the critical points of each stage of seed filling were divided, and the evaluation indexes of seed filling performance at each stage were defined. Taking the adsorption duration and seed removal resistance as the evaluation indexes of seed filling performance, and taking the height of the seed supporting table, the angle of the seed supporting table and the thickness of the stirring rod as the experimental factors, a three factor quadratic rotation orthogonal combination test was carried out. The response surface analysis and multi-objective optimization were carried out on the test results, and the best parameter combination of the seed metering plate was determined: the height of the seed supporting table was 2.8 mm, the angle of the seed supporting table was 31.2°, and the thickness of the stirring rod was 1.4 mm. At this time, the performance indicators of seed metering device were as follows: adsorption duration 0.104 s, seed removal resistance 0.008 1 N. The optimized results were verified by bench test, and compared with the optimized seed metering tray under different operating speeds. The bench test results showed that the seed metering performance of the optimized seed metering tray was significantly improved, and its qualification index and missed seeding index were better than the original seed metering device, which met the requirements of soybean precision sowing.
Keywords:soybean; air suction seed-metering device; discrete element; CFD; gas-solid coupling simulation
0引言
排種器作為精密播種的核心部件,其工作性能直接影響播種質(zhì)量[12]。精量排種器按工作原理可以分為機(jī)械式排種器和氣力式排種器兩大類[34]。兩者的核心區(qū)別是分別采用機(jī)械結(jié)構(gòu)和氣流作用完成對(duì)種子的取種、運(yùn)移和投遞。氣吸式排種器是氣力式排種器中應(yīng)用較廣的類型之一,由于該型排種器采用氣流充種,因此對(duì)種子尺寸適應(yīng)性較高,且具有不傷種的優(yōu)點(diǎn),是發(fā)展高速精量播種技術(shù)所采用的主流排種器。但是氣吸式排種器在高速作業(yè)過(guò)程中,由于排種盤(pán)高速旋轉(zhuǎn),種子的充種時(shí)間較短,漏播現(xiàn)象普遍的問(wèn)題難以有效解決。
近年來(lái),隨著計(jì)算機(jī)技術(shù)的發(fā)展,為了詳細(xì)描述顆粒相和氣流相之間的相互作用,結(jié)合計(jì)算流體力學(xué)和離散元法(DEM-CFD)的耦合計(jì)算方法已應(yīng)用于谷物清選[5],流化床顆粒運(yùn)動(dòng)[68],氣力顆粒運(yùn)動(dòng)[911]等領(lǐng)域。針對(duì)氣力式排種器,張開(kāi)興等[12]采用Fluent分析了不同吸孔對(duì)氣室流場(chǎng)的影響,得出了最佳的排種盤(pán)吸孔結(jié)構(gòu);廖宜濤等[13]通過(guò)計(jì)算流體力學(xué)建立了負(fù)壓氣室的流體域模型,分析得出了影響吸種效果的主要因素;Yazgi等[14]通過(guò)建立氣吸式排種器各物理量間的數(shù)學(xué)模型,并通過(guò)高速攝像和響應(yīng)面分析法,確定了種盤(pán)轉(zhuǎn)速、吸孔直徑和真空壓力之間的對(duì)應(yīng)關(guān)系;nal等[15]通過(guò)排種機(jī)理的分析,計(jì)算得出了種盤(pán)的最佳吸孔數(shù)。在這些研究中均采用單一的離散元方法或理論計(jì)算的方式優(yōu)化氣力式排種器的結(jié)構(gòu)參數(shù),存在一定局限性。
本文采用DEM-CFD耦合的方法分析氣吸式大豆排種器的工作過(guò)程,研究充種過(guò)程中各階段影響充種性能的主要因素,分析種子作業(yè)過(guò)程中的運(yùn)動(dòng)規(guī)律和受力特性,開(kāi)展三因素二次旋轉(zhuǎn)正交組合試驗(yàn),優(yōu)化排種盤(pán)主要結(jié)構(gòu)參數(shù),并進(jìn)行臺(tái)架試驗(yàn)驗(yàn)證。
1充種原理與結(jié)構(gòu)設(shè)計(jì)
1.1整體結(jié)構(gòu)與工作原理
氣吸式大豆排種器的整體結(jié)構(gòu)如圖1所示,主要由氣室、卸種機(jī)構(gòu)、毛氈擋板、密封氣墊、排種盤(pán)、清種機(jī)構(gòu)等部件組成。其中排種盤(pán)是核心工作部件,由吸孔、托種臺(tái)、攪種桿和種盤(pán)本體組成,托種臺(tái)和攪種桿均勻分布于吸孔周邊。當(dāng)排種器工作時(shí),種子通過(guò)進(jìn)種口在重力作用下落入種腔室,排種盤(pán)在動(dòng)力軸的作用下作周向旋轉(zhuǎn)運(yùn)動(dòng),氣流在排種盤(pán)吸孔兩側(cè)形成壓差,將種子吸附在吸孔上隨排種盤(pán)轉(zhuǎn)動(dòng),排種盤(pán)上的攪種桿將種腔室內(nèi)的種群離散化,而托種臺(tái)起到輔助充種的作用,直至吸附在吸孔上的種子脫離種群完成充種過(guò)程;多余的種子隨后將在清種刀的作用下回落至種腔室;最終僅存單粒種子到達(dá)氣室末端,此時(shí)氣流被阻斷,負(fù)壓消失,種子將在重力、離心力和卸種機(jī)構(gòu)的外力作用下落入預(yù)先開(kāi)好的種溝中,完成精量播種作業(yè)。
1.2排種盤(pán)關(guān)鍵結(jié)構(gòu)設(shè)計(jì)
排種盤(pán)作為排種器中主要與種子接觸的部分,是影響種子充種性能的關(guān)鍵部件,合理設(shè)計(jì)排種盤(pán)結(jié)構(gòu)可有效提升排種器的作業(yè)質(zhì)量[1617]。
1.2.1托種臺(tái)高度設(shè)計(jì)
托種臺(tái)是排種盤(pán)上的關(guān)鍵結(jié)構(gòu),能夠使處于種腔室內(nèi)充種區(qū)的種子具有一定的初速度,起到輔助托持種子的作用。具有托種臺(tái)設(shè)計(jì)的排種盤(pán),種子在吸孔處的受力分析如圖2所示。
為了計(jì)算托種臺(tái)的合理高度,選取設(shè)計(jì)排種盤(pán)最高速度為16km/h。如圖2所示,當(dāng)種子靠近吸孔時(shí),種子將在氣流曳力的作用下朝吸孔運(yùn)動(dòng),并最終被吸孔捕獲。種子如果沒(méi)有被吸附發(fā)生掉落,將從上一托種臺(tái)邊緣掉落至下一吸孔區(qū)域,此時(shí)滿足式(1)~式(3)。
托種臺(tái)高度增加,吸附種子所需的氣室負(fù)壓減小,托種臺(tái)可發(fā)揮輔助托種的作用。但托種臺(tái)的高度過(guò)大,則對(duì)種子的托持作用越大,將使吸孔穩(wěn)定吸附雙?;蚨嗔7N子,導(dǎo)致重播現(xiàn)象嚴(yán)重。本文試驗(yàn)采用的大豆種子等效直徑均值為6.7mm,選取托種臺(tái)高度為2~5mm,具體參數(shù)還需進(jìn)一步試驗(yàn)分析。
1.2.2托種臺(tái)角度設(shè)計(jì)
2排種盤(pán)輔助充種性能仿真分析
2.1DEM-CFD方法的數(shù)學(xué)建模
2.2種子顆粒與排種器幾何體建模
在氣固耦合仿真中,氣流場(chǎng)和顆粒場(chǎng)分別基于ANSYS Fluent 2021和EDEM 2021軟件。在EDEM中對(duì)排種器整體結(jié)構(gòu)進(jìn)行簡(jiǎn)化,去除無(wú)關(guān)仿真精度的相應(yīng)部件。本文以應(yīng)用較為廣泛的合農(nóng)91種子為試驗(yàn)對(duì)象,其三軸尺寸均值為6.2mm×6.5mm×7.8mm。根據(jù)EDEM官方材料庫(kù)和預(yù)先試驗(yàn),仿真采用的材料力學(xué)特性如表1所示。
在排種器的氣流場(chǎng)建模中,在排種盤(pán)吸孔處需要?jiǎng)澐旨?xì)致的網(wǎng)格尺寸,為了確保耦合仿真精度,大豆種子顆粒模型需通過(guò)粘結(jié)顆粒方法進(jìn)行建立,該方法可以在耦合模擬中準(zhǔn)確計(jì)算顆粒的體積分?jǐn)?shù)和動(dòng)量源項(xiàng)。此外,組成大豆種子粘結(jié)顆粒的子球數(shù)量越多,則顆粒模型與實(shí)際大豆種子形狀越接近,但耦合計(jì)算成本急劇增加,因此綜合考慮耦合精度和計(jì)算成本后,選擇基于102球組成的大豆種子粘結(jié)顆粒模型,如圖4所示,子球與子球之間通過(guò)內(nèi)聚鍵粘結(jié)。
種子—種子、種子—排種盤(pán)的接觸模型采用Hertz-Mindlin接觸模型。在排種器工作過(guò)程中,排種盤(pán)作周向旋轉(zhuǎn)運(yùn)動(dòng),因此采用滑移網(wǎng)格法,將托種臺(tái)與吸孔的體網(wǎng)格區(qū)域?qū)傩栽O(shè)定為動(dòng)態(tài)網(wǎng)格,其余結(jié)構(gòu)設(shè)定為靜態(tài)網(wǎng)格。分別將吸孔與種腔室和氣室的接觸面定義為交界面,以實(shí)現(xiàn)排種盤(pán)旋轉(zhuǎn)過(guò)程中種腔室與氣室之間的數(shù)據(jù)交換,并設(shè)定EDEM中排種盤(pán)的旋轉(zhuǎn)速度與Fluent中吸孔旋轉(zhuǎn)速度一致。
設(shè)置共生成400粒大豆種子粘結(jié)顆粒模型,由于EDEM的時(shí)間步長(zhǎng)遠(yuǎn)小于CFD,因此選擇EDEM和CFD時(shí)間步長(zhǎng)分別為2×10-5s和1×10-4s以獲得良好的計(jì)算收斂性,總模擬時(shí)間為2.0s,每0.002s在EDEM和Fluent中記錄數(shù)據(jù),以提取種子顆粒的運(yùn)動(dòng)信息和受力特性,耦合仿真過(guò)程如圖5所示。
2.3充種過(guò)程仿真分析
在充種區(qū)采用Fluent導(dǎo)出種子與氣流跡線的關(guān)系示意圖,如圖6所示。在EDEM中導(dǎo)出種子顆粒速度與位置示意圖,如圖7所示。由于EDEM與Fluent耦合仿真是雙向的,顆粒在流體中運(yùn)動(dòng),顆粒會(huì)受到氣流的曳力,使種子顆粒能夠在種腔室內(nèi)按既定路線運(yùn)動(dòng),與此同時(shí),氣流會(huì)受到顆粒的阻力(顆粒阻礙氣流流動(dòng)),曳力與阻力大小相等,方向相反。圖6和圖7表明,流場(chǎng)中種子顆粒與氣流跡線的關(guān)系與排種器種子運(yùn)動(dòng)機(jī)理一致,驗(yàn)證了基于DEM-CFD耦合仿真的準(zhǔn)確性。
3仿真優(yōu)化與臺(tái)架試驗(yàn)
3.1仿真試驗(yàn)方案
3.2仿真試驗(yàn)結(jié)果與分析
3.3排種盤(pán)結(jié)構(gòu)參數(shù)優(yōu)化
根據(jù)Matlab的尋優(yōu)算法優(yōu)化求解,可得排種盤(pán)的最佳參數(shù)組合托種臺(tái)高度為2.8mm、托種臺(tái)角度為31.2°、攪種桿厚度為1.4mm,此時(shí)吸附持續(xù)時(shí)間為0.104s,種子移出阻力為0.0081N。
3.4驗(yàn)證試驗(yàn)
為了驗(yàn)證氣固耦合仿真優(yōu)化排種盤(pán)結(jié)構(gòu)參數(shù)的準(zhǔn)確性,對(duì)優(yōu)化后的氣吸式大豆排種器的排種性能進(jìn)行臺(tái)架對(duì)比試驗(yàn),優(yōu)化前排種盤(pán)的結(jié)構(gòu)參數(shù)對(duì)應(yīng)表2中的零水平。試驗(yàn)在河南農(nóng)業(yè)大學(xué)STB-700排種性檢測(cè)試驗(yàn)臺(tái)上進(jìn)行,排種盤(pán)轉(zhuǎn)速選擇20r/min,30r/min,40r/min和50r/min進(jìn)行單因素重復(fù)試驗(yàn),氣室風(fēng)壓固定為-4kPa,誤差為±0.1kPa,每組試驗(yàn)重復(fù)3次,結(jié)果如圖10所示。優(yōu)化前后排種器的合格指數(shù)均隨排種盤(pán)轉(zhuǎn)速的增加呈現(xiàn)先上升后下降的趨勢(shì);重播指數(shù)均隨排種盤(pán)轉(zhuǎn)速的增加呈現(xiàn)下降趨勢(shì);漏播指數(shù)均隨排種盤(pán)轉(zhuǎn)速的增加呈現(xiàn)上升趨勢(shì)。
在較低排種盤(pán)轉(zhuǎn)速下(20r/min),優(yōu)化前的排種器表現(xiàn)出較佳的排種性能,其合格指數(shù)為90.27%,但在較高排種盤(pán)轉(zhuǎn)速下(≥20r/min),氣固耦合仿真優(yōu)化后的排種器在合格指數(shù)上均優(yōu)于原排種器,在排種盤(pán)轉(zhuǎn)速為40r/min時(shí),其合格指數(shù)最大為92.87%,且優(yōu)化后的排種器在各排種盤(pán)轉(zhuǎn)速下,漏播指數(shù)較低,顯著改善了高速作業(yè)下氣吸式排種器漏播現(xiàn)象普遍的問(wèn)題,因此采用DEM-CFD氣固耦合方法優(yōu)化排種器關(guān)鍵結(jié)構(gòu)參數(shù)能夠有效提高排種器的排種性能。
4結(jié)論
1) 建立了氣吸式排種器充種過(guò)程的數(shù)學(xué)模型,分析了吸附階段和跟隨階段影響排種器充種性能的主要因素,確定了排種盤(pán)結(jié)構(gòu)優(yōu)化的目標(biāo)參數(shù)為托種臺(tái)高度、托盤(pán)臺(tái)角度和攪種桿厚度。
2) 通過(guò)DEM-CFD方法對(duì)排種器工作過(guò)程進(jìn)行耦合仿真,分析了氣流曳力對(duì)種子顆粒的作用過(guò)程,定義了氣吸式排種器的充種性能評(píng)價(jià)指標(biāo)為吸附持續(xù)時(shí)間和種子移出阻力,通過(guò)正交仿真試驗(yàn)和多目標(biāo)尋優(yōu)分析,確定了排種盤(pán)的最佳參數(shù)組合托種臺(tái)高度為2.8mm、托種臺(tái)角度為31.2°、攪種桿厚度為1.4mm,此時(shí)吸附持續(xù)時(shí)間為0.104s,種子移出阻力為0.081N。
3) 臺(tái)架對(duì)比試驗(yàn)結(jié)果顯示,優(yōu)化后的排種器在各排種盤(pán)轉(zhuǎn)速下,漏播指數(shù)較低,顯著改善了高速作業(yè)下氣吸式排種器漏播現(xiàn)象普遍的問(wèn)題,此外在較高排種盤(pán)轉(zhuǎn)速下(≥20r/min),優(yōu)化后的排種器在合格指數(shù)上均優(yōu)于原排種器,在排種盤(pán)轉(zhuǎn)速為40r/min時(shí),其合格指數(shù)最大為92.87%,表現(xiàn)出優(yōu)越的作業(yè)性能。
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