王小萌，吳文福，尹 君，張忠杰，吳子丹※，姚 渠

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玉米糧堆霉變發(fā)熱過程中的溫濕度場變化規(guī)律研究

王小萌1,2，吳文福1，尹 君2，張忠杰2，吳子丹1※，姚 渠2

（1. 吉林大學(xué)生物與農(nóng)業(yè)工程學(xué)院，長春 130022；2. 國家糧食和物資儲(chǔ)備局科學(xué)研究院，北京 100037）

為模擬儲(chǔ)糧糧堆局部含水率偏高引起的霉變發(fā)熱現(xiàn)象，進(jìn)而研究此現(xiàn)象中溫、濕度場的變化規(guī)律，該文在試驗(yàn)倉內(nèi)濕基含水率14.0%的玉米糧堆中心加入濕基含水率18.2%的玉米，在30 ℃室內(nèi)儲(chǔ)藏40 d。試驗(yàn)糧堆由于霉變引起自發(fā)熱。試驗(yàn)過程中，通過計(jì)算玉米糧堆中垂面內(nèi)高溫區(qū)和高濕區(qū)的面積變化，從而揭示玉米糧堆霉變發(fā)熱過程中溫、濕度場的變化規(guī)律。試驗(yàn)結(jié)果表明，糧堆中垂面高濕區(qū)面積緩慢擴(kuò)大，高溫區(qū)面積開始擴(kuò)大緩慢，但在與周圍糧溫最高溫差升至3.7 ℃后，面積擴(kuò)大速率加快，且高溫區(qū)與高濕區(qū)面積的當(dāng)量半徑與溫度差D成正比，此正比關(guān)系經(jīng)過了糧庫淺圓倉的驗(yàn)證。這為進(jìn)一步定量分析糧食倉儲(chǔ)過程中的高溫區(qū)和高濕區(qū)擴(kuò)散提供了依據(jù)。

作物；溫度；濕度傳感器；玉米；溫度場；相對濕度場；霉變發(fā)熱

0 引 言

中國玉米種植范圍廣，產(chǎn)量高，是重要的糧食及經(jīng)濟(jì)作物之一。在儲(chǔ)藏過程中玉米糧堆霉變發(fā)熱一直是比較普遍的問題。

對于儲(chǔ)糧糧堆發(fā)熱的起因，國內(nèi)外已經(jīng)做了大量的研究。微生物作用，害蟲大量滋生，糧食呼吸作用，雜質(zhì)聚集等均可以引發(fā)糧堆發(fā)熱[1]。其中，蟲[2]、霉[3]是引起糧堆發(fā)熱的重要因素[4]，但是如果昆蟲密度不高，僅依靠昆蟲自身呼吸作用引發(fā)糧堆發(fā)熱的幾率比高水分糧食霉變引發(fā)糧堆發(fā)熱的幾率低[5-6]。儲(chǔ)糧真菌呼吸作用是糧堆發(fā)熱的主要熱源，且蟲害一般會(huì)發(fā)生在霉變發(fā)熱之后[7]。

糧食儲(chǔ)藏過程中影響儲(chǔ)糧的主要霉菌是曲霉和青霉[8]，而影響霉菌生長的重要因素是溫度和水分[9]，因此，儲(chǔ)糧霉變發(fā)熱過程中，溫度、水分與霉變的關(guān)系是需要密切關(guān)注的問題。針對這個(gè)問題，國內(nèi)外學(xué)者做了大量的研究。Koehler[10]研究了含水率對玉米霉變的影響。Jian等[11]研究了不同溫度和含水率下蟲害和微生物的演替規(guī)律。Huang等[12]研究了不同溫度和含水率的玉米糧堆CO2產(chǎn)生速率。張航等[13-16]分別研究了中國小麥、玉米、水稻和大豆霉變與溫度和水分的關(guān)系。

在儲(chǔ)糧霉變發(fā)熱過程中，王小萌等[17]對霉變分布與糧堆溫度場和濕度場之間的關(guān)系進(jìn)行了分析。Wallace等[18]研究了發(fā)熱糧堆的水分遷移，并發(fā)現(xiàn)糧堆發(fā)熱會(huì)引起發(fā)熱點(diǎn)上層糧堆含水率增加，下層含水率降低[19]。張中濤[20]利用有限元法研究了小麥呼吸作用對糧堆溫度場、水分場的影響。除此之外，國內(nèi)外學(xué)者也對儲(chǔ)糧發(fā)熱點(diǎn)的溫度變化進(jìn)行了研究。Zhang等[21]研究了2種高水分小麥的發(fā)熱過程，并建立了發(fā)熱量與時(shí)間的指數(shù)模型。Wilson[22]建立了糧堆霉變發(fā)熱過程中熱產(chǎn)生速率和CO2產(chǎn)生速率的方程，但是方程不適用于低水分糧食。Jia等[23-24]分別利用有限元和離散元建立了糧堆內(nèi)存在發(fā)熱點(diǎn)時(shí)的溫度分布模型。但是，目前對糧堆霉變發(fā)熱過程中，溫、濕度場在空間上的變化規(guī)律研究甚少。

本文利用儲(chǔ)糧霉變自發(fā)熱引發(fā)糧堆內(nèi)溫度傳遞和水分遷移。通過計(jì)算玉米糧堆內(nèi)高溫區(qū)和高濕區(qū)的面積，研究糧堆霉變發(fā)熱過程中溫、濕度場在空間上的變化規(guī)律，以期對糧食倉儲(chǔ)過程中的糧情調(diào)控做出指導(dǎo)。

1 材料與方法

1.1 試驗(yàn)樣品

本試驗(yàn)采用的“先玉”玉米樣品初始平均濕基含水率為23%（下文提及的含水率均為濕基含水率），質(zhì)量密度為698 g/L，雜質(zhì)質(zhì)量分?jǐn)?shù)為1.0%，樣品初始真菌孢子數(shù)<104個(gè)/g，無儲(chǔ)藏真菌。

1.2 試劑及儀器

多參數(shù)糧情檢測系統(tǒng)[17]：中國深圳市東大恒豐科技有限公司；DHG—9140型電熱恒溫鼓風(fēng)干燥箱：上海精宏實(shí)驗(yàn)設(shè)備有限公司；NIKON E100顯微鏡：日本尼康公司。去離子水作為試驗(yàn)用水。

1.3 試驗(yàn)方法

1.3.1 試驗(yàn)倉裝置

為模擬淺圓倉中糧堆局部含水率偏高引起的局部霉變發(fā)熱現(xiàn)象，依據(jù)相似性設(shè)計(jì)原理和淺圓倉倉型，選用圓形的試驗(yàn)倉[17](圖1)，并進(jìn)行改進(jìn)，去除倉底的保溫隔熱材料，將倉底中心位置開1個(gè)內(nèi)直徑為0.08 m的通氣孔，目的是有利于試驗(yàn)倉內(nèi)外氣體進(jìn)行交換，確保試驗(yàn)倉內(nèi)氧氣供應(yīng)充足。

1.3.2 樣品處理

將平均含水率為23%的玉米進(jìn)行除雜、清理，玉米樣品鋪成厚度為0.02 m的薄層，在室外通風(fēng)環(huán)境下，將含水率降低。玉米入倉儲(chǔ)藏安全水分為14%左右[25]。儲(chǔ)糧實(shí)踐發(fā)現(xiàn)由于儲(chǔ)糧水分分布不均、雜質(zhì)聚集、水分遷移等因素的影響，玉米糧堆局部含水率可增至16～18%。糧堆內(nèi)的熱傳遞和水分遷移會(huì)受到糧堆高度和跨度等的影響[26]，鑒于試驗(yàn)倉底面直徑和高度較小，為便于觀察糧堆霉變發(fā)熱后的溫濕度遷移現(xiàn)象，因此試驗(yàn)用糧的高、低含水率可分別設(shè)為18%左右和14%左右。

1.3.3 儲(chǔ)藏模擬設(shè)置

試驗(yàn)倉內(nèi)玉米糧堆總高度為0.60 m，高水分玉米（18.2%）和低水分玉米（14.0%）的分布如圖1a所示，2種含水率的糧堆體積比為1∶4，將試驗(yàn)倉儲(chǔ)藏在溫度為30 ℃的室內(nèi)，以期模擬儲(chǔ)糧過程中由于糧堆局部含水率偏高引起的糧堆霉變發(fā)熱現(xiàn)象。2種含水率的玉米的初始糧溫為22 ℃，初始平均相對濕度分別為67%和90%。

1.4 測量指標(biāo)及方法

1.4.1 危害真菌孢子

參考儲(chǔ)糧真菌檢測標(biāo)準(zhǔn)[27]，檢測樣品的真菌孢子數(shù)。

1.4.2 溫濕度

利用溫濕度傳感器實(shí)時(shí)監(jiān)測玉米糧堆內(nèi)溫濕度變化。為了充分重現(xiàn)溫濕度場在2種含水率玉米中的分布，在借鑒文獻(xiàn)[17]溫濕度傳感器布置的基礎(chǔ)上，增加含水率14.0%玉米中的傳感器個(gè)數(shù)。糧堆內(nèi)溫濕度傳感器布置如圖1。

1.4.3 含水率

參考ASAE標(biāo)準(zhǔn)[28]，稱取15.0 g玉米籽粒樣品，放在干燥鋁盒內(nèi)，在103 ℃烘箱內(nèi)干燥72 h，檢測樣品的含水率。

1.5 數(shù)據(jù)處理

1.5.1 溫濕度場的建立

選取糧堆中垂面溫濕度傳感器檢測到的溫度和相對濕度數(shù)據(jù)，再運(yùn)用溫濕度擬合算法得到糧堆內(nèi)各點(diǎn)的溫度和相對濕度，通過Matlab 模擬軟件重現(xiàn)糧堆的溫度場和濕度場，溫濕度場擬合算法如式（1）～（2）[29]。

由數(shù)值計(jì)算方法可知，已知個(gè)離散數(shù)據(jù)(x,(x))=1,2,3,…，在[,]上滿足x?[,]，且(x)=(x)，則

注：“●”表示糧堆溫度和相對濕度的監(jiān)測位置，O點(diǎn)為含水率18.2%的玉米糧堆中心。

1.5.2 當(dāng)量半徑的計(jì)算

利用Matlab軟件對糧堆中垂面的溫、濕度場云圖進(jìn)行高溫區(qū)和高濕區(qū)面積求解，主要步驟是：導(dǎo)入云圖圖片，灰度處理，二值化處理，計(jì)算高溫區(qū)和高濕區(qū)面積。本文中，將高溫區(qū)和高濕區(qū)的面積按照圓形處理，計(jì)算出圓的半徑，作為高溫區(qū)和高濕區(qū)的當(dāng)量半徑。

2 結(jié)果與分析

2.1 溫度和相對濕度變化

圖2和圖3分別是玉米糧堆儲(chǔ)藏0、20和40 d時(shí)糧堆中垂面的溫度場和相對濕度場云圖。由圖2可知，儲(chǔ)藏溫度為30 ℃，平均含水率為18.2%的玉米糧堆中由于微生物作用逐漸出現(xiàn)了發(fā)熱點(diǎn)。微生物迅速生長，發(fā)熱點(diǎn)面積不斷擴(kuò)大，發(fā)熱點(diǎn)最高溫度和周圍糧堆溫度的溫差不斷升高，最大溫差達(dá)到8 ℃。研究表明，糧堆內(nèi)的微氣流運(yùn)動(dòng)是引起糧堆內(nèi)溫度傳遞和水分遷移的主要因素[30]。從圖3可以看出，糧堆內(nèi)微氣流運(yùn)動(dòng)引起糧堆內(nèi)水分向糧堆上層遷移。試驗(yàn)結(jié)束后，取樣檢測真菌孢子數(shù)，點(diǎn)、點(diǎn)、點(diǎn)、點(diǎn)、點(diǎn)的真菌孢子數(shù)分別為6.3′106，6.9′106，2.1′106，5.4′106，5.7′106個(gè)/g，優(yōu)勢真菌為亮白曲霉和黃曲霉。

注：O、A、B,、C、D在溫度場云圖中是共用的,下同。

圖3 玉米糧堆儲(chǔ)藏0、20和40 d時(shí)中垂面相對濕度場云圖

2.2 高溫區(qū)與高濕區(qū)面積變化

糧堆內(nèi)發(fā)熱點(diǎn)周圍平均糧溫為30 ℃，因此本文中的高溫區(qū)選取玉米糧堆中垂面溫度高于30 ℃的區(qū)域。亮白曲霉和黃曲霉是引起本文糧堆發(fā)熱的主要霉菌，而相對濕度高于75%時(shí)易于亮白曲霉和黃曲霉生長，因此，本文中的高濕區(qū)選取玉米糧堆中垂面相對濕度高于75%的區(qū)域。圖4是試驗(yàn)過程中，玉米糧堆中垂面、、、、5點(diǎn)的溫度變化圖。圖5是玉米糧堆中垂面的高溫區(qū)和高濕區(qū)面積隨時(shí)間的變化圖。由圖5可知，在試驗(yàn)的40 d內(nèi)，高濕區(qū)面積增加緩慢；高溫區(qū)面積在試驗(yàn)第25 d后迅速增加，結(jié)合圖4分析，可發(fā)現(xiàn)試驗(yàn)第25 d后糧溫升高加快，導(dǎo)致高溫區(qū)面積迅速擴(kuò)大，這主要是因?yàn)辄S曲霉和亮白曲霉大面積生長，產(chǎn)生大量的熱。

圖4 玉米糧堆中垂面不同點(diǎn)溫度變化

參考當(dāng)量直徑的定義，本文中將高溫區(qū)和高濕區(qū)的面積按照圓形處理，計(jì)算出圓的半徑，作為高溫區(qū)和高濕區(qū)的當(dāng)量半徑。溫度差是糧堆內(nèi)濕熱轉(zhuǎn)移的動(dòng)力源[31-32]。因此，建立玉米糧堆高溫區(qū)和高濕區(qū)當(dāng)量半徑與溫度差D的關(guān)系（圖6）。由圖6可知，高濕區(qū)面積的當(dāng)量半徑與溫度差成正比；當(dāng)溫度差高于3.7 ℃即高溫區(qū)最高溫度高于33.7 ℃后，發(fā)熱區(qū)開始快速向周圍擴(kuò)散，這與糧堆的導(dǎo)熱系數(shù)小有很大關(guān)系，糧堆導(dǎo)熱系數(shù)小，導(dǎo)致糧堆霉變產(chǎn)生的熱量難以快速傳遞出糧堆，很容易聚集在糧堆內(nèi)；高溫區(qū)面積的當(dāng)量半徑與溫度差成線性正相關(guān)，且D/D=0.058 (2=0.99)。儲(chǔ)糧霉變發(fā)熱過程中，發(fā)熱區(qū)域的擴(kuò)散速率會(huì)受到熱傳導(dǎo)和熱對流的共同作用，由于試驗(yàn)倉體積小，高度低，熱對流現(xiàn)象微弱，因此糧堆內(nèi)的熱量傳遞和水分遷移主要受到熱傳導(dǎo)的影響。下面用實(shí)倉對高溫區(qū)當(dāng)量半徑與溫度差的關(guān)系進(jìn)行驗(yàn)證。

圖5 高溫區(qū)和高濕區(qū)面積隨儲(chǔ)藏時(shí)間的變化

圖6 高溫區(qū)和高濕區(qū)面積的當(dāng)量半徑與溫度差的關(guān)系

2.3 實(shí)倉驗(yàn)證

以中國某糧庫的淺圓倉糧溫?cái)?shù)據(jù)驗(yàn)證上述正比關(guān)系。此淺圓倉為鋼屋蓋，倉內(nèi)直徑為25.0 m，裝糧高度為15.6 m，雜質(zhì)質(zhì)量分?jǐn)?shù)為1.5%，糧堆質(zhì)量密度為698 g/L，儲(chǔ)量為5 756.9 t。所儲(chǔ)玉米的平均含水率為13.8%，糧堆局部偏高水分區(qū)域的平均含水量為14.8%。糧堆內(nèi)局部含水率偏高，導(dǎo)致出現(xiàn)發(fā)熱點(diǎn)。通過計(jì)算糧堆中垂面高溫區(qū)的面積變化，得到高溫區(qū)當(dāng)量半徑與溫度差的關(guān)系，如圖7。

圖7 淺圓倉中垂面高溫區(qū)的當(dāng)量半徑與溫度差的關(guān)系

在之前研究[17]中，利用外加熱源引起小麥糧堆內(nèi)的溫濕度遷移，試驗(yàn)過程中沒有出現(xiàn)由于糧堆霉變引發(fā)的發(fā)熱點(diǎn)，以此作者研究了儲(chǔ)糧真菌分布和溫濕度的關(guān)系。此文中，利用高水分玉米自身霉變出現(xiàn)發(fā)熱點(diǎn)，模擬儲(chǔ)糧糧堆局部含水率偏高引起的糧堆霉變發(fā)熱，發(fā)現(xiàn)糧堆高溫區(qū)當(dāng)量半徑與溫度差的正比例關(guān)系，鑒于此文中沒有找到高溫區(qū)當(dāng)量半徑與溫度差的具體定量關(guān)系，還需進(jìn)行更深入的研究。

3 結(jié) 論

模擬試驗(yàn)倉中，由18.2%和14.0% 2種含水率的玉米組成的糧堆在30 ℃恒溫室內(nèi)儲(chǔ)藏，4 d后，亮白曲霉和黃曲霉大量生長引發(fā)糧堆出現(xiàn)明顯發(fā)熱點(diǎn)。

玉米糧堆霉變發(fā)熱過程中，主要受熱傳導(dǎo)影響，高溫區(qū)與高濕區(qū)面積不斷擴(kuò)大。在高溫區(qū)與周圍糧溫最大溫差從3.7升高至8 ℃的過程中，高溫區(qū)和高濕區(qū)面積的當(dāng)量半徑與溫度差呈正相關(guān)。該相關(guān)性也經(jīng)實(shí)倉數(shù)據(jù)得到了驗(yàn)證。

試驗(yàn)倉和淺圓倉高溫區(qū)與周圍糧溫的溫度差均升高3.7 ℃后，高溫區(qū)面積才開始迅速擴(kuò)大，但是淺圓倉熱對流強(qiáng)烈，導(dǎo)致淺圓倉高溫區(qū)擴(kuò)散速率遠(yuǎn)大于試驗(yàn)倉中高溫區(qū)擴(kuò)散速率。

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Research on temperature and humidity field change during corn bulk microbiological heating

Wang Xiaomeng1,2, Wu Wenfu1, Yin Jun2, Zhang Zhongjie2, Wu Zidan1※, Yao Qu2

(1.130022,; 2.100037,)

Corn is widely planted in China with high yield. It is a major source of foods and materials for industrial processing.Microbiological heating occurs easily during corn bulk storage, which seriously affects grain safety. The problem has also attracted much attention. Stored grain is mostly infected byand. Fungal growth in maize is facilitated by hot and humid conditions. Warm pockets initiated by microorganisms are mainly induced by pockets of wet grain because microorganisms need larger than 0.65 water activity to multiply and develop. It has been reported by many researchers that the relationship between temperature, moisture and fungi growth. Moisture diffusion and migration from a hotspot to its surrounding was recorded by some researchers. They also found that as a hotspot develops, the grain moisture at the top of the hotspot increases while the grain moisture below decreases. Since it is difficult to estimate the production rate of heat without using mathematical models, there are a few models developed to understand the development of hotspot. These models was used to calculate the heat production rate of stored grain. However, there is little research on the quantitative change of temperature and humidity field in space. The objective of this study was to develop a method to measure changes of high temperature and high humidity zones in space during grain microbiological heating. To explore quantitative variation of temperature and relative humidity fields in space in corn heating, different moisture corn (14.0% and 18.2%, w.b.) was stored in a simulated silo in 40 d at 30 ℃ non-airtight. The simulated silo was a cylindrical iron silo with 0.54 m in internal diameter, 0.70 m in height and 0.01 m in thickness, respectively. And its inner wall was provided with insulation layer (0.02 m thickness). Two air pipes (0.08 m internal diameter) on the top and bottom of the silo were applied to exchange the gas inside and outside the silo and ensure adequate oxygen supply. In the experiment, the high moisture corn (cylinder, diameter, 0.30 m; height, 0.30 m; 18.2%, w.b.) in the silo was surrounded by low moisture corn (14.0%, w.b.). After 4 days storage, the growth ofandcaused a hot spot appears in corn bulk. In the paper, high temperature areas were temperature higher than 30 ℃, and high humidity areas were relative humidity higher than 75% due toandincreasing greatly. During the storage, temperature and relative humidity cloud maps of the min-vertical plane were drawn. These cloud maps indicate that areas of high temperature and high humidity expanded under heat conduction. Areas of high temperature zone and high humidity zone were calculated. Then, treated these areas as circles and calculated equivalent radii () of high temperature zone and high humidity zone. Besides temperature difference (D) were equal to the highest temperature in high temperature zone minus 30 ℃. DuringDincreasing from 3.7 to 8 ℃, equivalent radii () had a significant linear correlation with temperature difference (D). However, no noticeable change was observed whenDranged from 0 to 3.7 ℃. The corn temperature data of a squat silo during microbiological heating proved the linear relationship between the equivalent radius of high temperature area and temperature difference. But the diffusion rate of heating area in squat silo was higher than the simulated silo due to height and span of grain bulk. Height and span of grain bulk in squat siloincreased heat convection which was weak in simulated silo. This study lays a foundation for the further quantitative research on the prediction of microbiological heating in grain storage.

crops; temperature; humidity sensors; corn; temperature field; relative humidity field; microbiological heating

王小萌，吳文福，尹 君，張忠杰，吳子丹，姚 渠.玉米糧堆霉變發(fā)熱過程中的溫濕度場變化規(guī)律研究[J]. 農(nóng)業(yè)工程學(xué)報(bào)，2019，35(3)：268－273. doi：10.11975/j.issn.1002-6819.2019.03.033 http://www.tcsae.org

Wang Xiaomeng, Wu Wenfu, Yin Jun, Zhang Zhongjie, Wu Zidan, Yao Qu.Research on temperature and humidity field change during corn bulk microbiological heating[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2019, 35(3): 268－273. (in Chinese with English abstract) doi：10.11975/j.issn.1002-6819.2019.03.033 http://www.tcsae.org

2018-12-29

2019-01-16

2015年糧食公益性行業(yè)科研專項(xiàng)項(xiàng)目（201513001）

王小萌，博士生，研究方向?yàn)榧Z食信息化與自動(dòng)化。 Email：wxmhappy99@163.com。

吳子丹，教授，研究方向?yàn)榧Z食儲(chǔ)藏與運(yùn)輸。Email：wuzidan@263.net。

10.11975/j.issn.1002-6819.2019.03.033

S379

A

1002-6819(2019)-03-0268-06