陳奕穎,叢 茜,2,任露泉,2,金敬福,陳廷坤,2
·農產品加工工程·
冷藏設備防除冰表面非連續特征設計與試驗
陳奕穎1,叢 茜1,2,任露泉1,2,金敬福1,陳廷坤1,2※
(1.吉林大學生物與農業工程學院,長春 130022;2.吉林大學工程仿生教育部重點實驗室,長春 130022)
蒸發器、換熱器等冷藏設備表面形成覆冰時,影響設備的正常運行及農副產品的儲藏品質。為了減小冰黏附對冷藏設備的影響,該研究通過在材料表面非連續涂覆圓形硅橡膠,改變材料單一的表面特性,提出一種提高部件主動防除冰的方法,并探究圓形涂覆硅橡膠的直徑、相鄰硅橡膠間的圓心距、涂覆占空比及厚度對冰黏附強度降低率的影響,降低表面冰黏附強度。試驗結果表明表面具有不同涂覆參數的試樣對冰黏附強度具有不同的降低效果,當涂覆直徑為3.50 mm、圓心間距6.50 mm、占空比為8.50%及涂覆厚度為0.250 mm時,鋁合金試樣表面的平均冰黏附強度降低率為46.83%。對試驗結果進行方差分析,建立設計因素與評價指標間的數學回歸模型,確定對冰黏附強度影響的顯著性主次順序為:圓形涂覆硅橡膠間的圓心距、占空比、涂覆厚度及直徑。分析認為通過改變材料表面特征的連續性,使不同位置內的附著水具有不同的相變時間,利用后結冰區域產生的膨脹應力,干擾冰與材料之間界面的穩定性,降低冰黏附強度,提高冷藏設備部件的主動防除冰特性,為進一步開發新型防除冰技術提供參考。
制冷;凍結;相變;防除冰表面;非連續特性;冰黏附強度;模型
隨著中國社會經濟的快速發展,國民對肉類、水果、蔬菜等農副產品儲存及保鮮質量要求愈發嚴格,進而推動了國內冷庫、冷藏車輛等農產品冷鏈物流的發展,也促進了農產品冷藏保鮮設備的保有量及設備種類的多樣化。據統計,2019年中國食品冷鏈物流總額約為6萬億元,冷藏車保有量為21.47萬臺,冷庫容量達到6.05×107t[1]。為促進農副產品冷鏈物流的發展,小型冷庫等冷藏設備已經連續5 a被列入農業部補貼項目指南[2],中國發展和改革委員會、農業農村部及國務院分別印發了關于農副產品冷鏈物流的發展規劃[3-4],并在2020年中央一號文件里明確提出啟動農產品倉儲保鮮冷鏈物流設施建設工程[5-6]。
冷藏保鮮設備作為農副產品冷藏保鮮鏈的核心部件,其運行的高效性、節能性一直是該行業追求的目標,但設備運行時壓縮機、冷凝器、冷風機、蒸發器等關鍵零部件的表面溫度與環境溫度之間存在較大溫度差異,環境中的水分極易黏附在部件表面,形成霜層和冰層,導致冷風機熱交換能力和換熱器換熱效率降低[7-9],增加空氣流通阻力和冷藏設備的運行能耗[10-12],降低冷藏保鮮設備的工作效果,致使蔬菜、水果、肉類等農副產品儲存質量降低[13-16],引起社會經濟的損失和資源浪費。
針對冷藏保鮮設備部件表面結霜、結冰現象,國內外已形成了超聲振蕩、熱氨、電加熱、熱氣等多種常規方法[10-12,17-18],但這類防除霜/冰方法存在高成本[19]、高能耗[20]、污染環境[21]且易腐蝕橡膠、金屬部件等使用缺陷[22-23]。而超疏水表面作為潛在的防除冰方法,表面存在耐久性差[24-25]、微納結構易損壞[26]、耐磨性差[27]、易脫落[28]等使用缺陷,暫不滿足實際工程領域的使用要求。因此,如何提高冷藏設備防除冰能力,仍是制冷行業尚需解決的問題之一。
課題組初期通過利用結冰過程中發生的相變膨脹現象,提出、設計了一種防除冰模型,達到了降低冰黏附強度的目的[29-30],但該防除冰模型需要在冷藏保鮮設備的本體表面加工,具有實際使用弊端。因此本研究改變材料單一的表面特性,利用水的結冰相變膨脹應力,結合回歸方法設計試驗,建立防除冰表面設計因素影響冰黏附強度的數學模型,并分析其影響效應,為冷藏保鮮行業提供一種新型且主動的防除冰方法。
冰在材料表面黏附,與材料之間形成穩定的黏附系統,而材料的表面特性對冰的黏附穩定性具有重要的影響作用。結合課題組初期研究結果[29-30]及結冰膨脹對溝渠、渡槽等建筑物造成的影響[31-32],改變材料單一的導熱特性,如圖1所示,使材料表面不同位置具有不同的導熱性能。由于材料表面具有非連續的導熱性,材料表面不同位置附著的水具有不同相變時間,后結冰區域產生的相變膨脹應力干擾先結冰區域與材料之間形成穩定的附著界面,破壞其黏附穩性,進而降低冰在材料表面的黏附強度。
1.基底 2.水 3.冰 4. 膨脹應力 5.具有非連續特性的表面
1.Substrate 2.Water 3.Ice 4.Swelling stress 5.Surface with non-continuous characteristic
圖1 非連續特性表面的防除冰模型
Fig.1 Anti-icing model with non-continuous surface characteristic
試驗中采用冷藏保鮮設備中常用的6061鋁合金材料作為試樣基體(尺寸為60 mm×60 mm×5 mm),導熱系數為238 W/(m·K)[33]。RTV-1硅橡膠導熱系數為0.17 W/(m·K)[34],其在金屬、橡膠等工程材料表面黏結性好,具有耐久性、耐候性、耐溫性等優點[34],已在汽車、電力、制冷等領域廣泛應用,因此通過模具法在鋁合金表面不同位置環形涂覆不同圓形尺寸參數的硅橡膠。通過低溫環境模擬箱控制試驗溫度,在溫度為?20 ℃的低溫環境下、凍結1 h,用水量為5 mL,運用內徑為32 mm的鋁杯制備試樣表面的覆冰。試驗中的用水均為純凈水(購買自北京化學廠),并利用自制冰黏附力測試裝置測量冰在試樣表面的黏附力,如圖2所示。
2.2.1 試驗指標
重復10次測試不同試樣及光滑鋁合金表面的冰黏附強度,通過公式(1)計算不同試樣的冰黏附強度降低率。材料表面的冰黏附強度越低,越易于清除材料表面的覆冰,因此試驗中以冰黏附強度降低率為評價指標。
式中為冰黏附強度降低率,%;0為光滑試樣表面冰黏附力,N;為防除冰試樣表面的冰黏附力,N。
2.2.2 試驗因素
試驗中在鋁合金試樣表面不同位置圓形涂覆不同參數的硅橡膠,影響表面附著水內部不同黏附區域的結冰順序,利用結冰相變膨脹干擾冰黏附穩定性,降低表面冰黏附強度。因此,選取圓形涂覆硅橡膠的直徑、相鄰圓形硅橡膠間的圓心距、硅橡膠涂覆的占空比及涂覆厚度作為試驗因素,并根據鋁杯內徑及試樣尺寸的限制,試驗中選取3~4 mm的硅橡膠直徑、4~9 mm的圓心距、6%~11%的占空比及0.2~0.3 mm的涂覆厚度。其中占空比為硅橡膠涂覆面積與冰黏附面積比值。
2.2.3 試驗設計
選取圓形涂覆硅橡膠的直徑1、不同圓形硅橡膠間的圓心距2、占空比3及涂覆厚度4作為試驗因素,并運用四元二次正交旋轉組合[35]設計試驗方案,各因素水平的編碼如表1所示。
在基底表面粘貼厚度約為0.020~0.025 mm的雙向拉伸聚丙烯薄膜(Biaxially Oriented Polypropylene,BOPP),通過粘貼多層BOPP薄膜滿足硅橡膠的涂覆厚度,運用激光雕刻機在BOPP層表面加工待涂覆硅橡膠的分布圖形。室溫條件下自然固化24 h,清除BOPP薄膜表面多余的硅橡膠,移除基底表面粘貼的多層BOPP薄膜,并用酒精擦拭、清除基底表面的雜質。
利用冰黏附力測試裝置,根據設定的試驗條件及編制的試驗方案,利用公式(1)計算不同防除冰試樣表面的冰黏附強度降低率,計算每種試樣的平均降低率,并作為該試樣防除冰效果的評價指標。
由表2中的試驗測試結果可知:相對于冰在光滑試樣表面的黏附力,冰與表面不同位置具有圓形涂覆硅橡膠的鋁合金之間形成了低黏附力。防除冰表面設計因素均為0水平時,鋁合金試樣表面的平均冰黏附強度降低率為46.83%,其中最大冰黏附強度降低率為52.38%,降低后期材料表面覆冰的清除難度和成本。

表2 試驗方案與測試結果

式中1為圓形涂覆硅橡膠的直徑,mm;2為圓形硅橡膠間的圓心距,mm;3為硅橡膠涂覆的占空比,%;4為涂覆厚度,mm。

表3 冰黏附強度降低率R回歸模型的方差分析
注:<0.05為顯著,<0.01為極顯著。
Note:<0.05 represents significance,<0.01 means extremely significance.
利用響應面法分析圓形涂覆硅橡膠各尺寸參數對冰黏附強度降低率的影響,固定4因素中的2個因素為0水平,考察其余2個因素對冰黏附強度降低率的影響效應。
如圖3a所示,當圓心距2和厚度4分別保持6.50 mm、0.250 mm時,直徑1和占空比3與冰黏附強度降低率的影響規律:冰黏附強度降低率總體上隨直徑1的增加而逐漸降低;隨占空比3的增加,冰黏附強度降低率先增加后降低,并且響應面沿3方向的變化速率大于沿1方向的變化速率,說明在試驗水平下,涂覆硅橡膠的占空比其涂覆直徑對冰黏附強度降低率的影響更大。當圓形硅橡膠涂覆直徑1增大時,晚凍結區域面積變大,而產生的相變膨脹力作用面積成平方趨勢增加,降低了對冰黏附強度的作用效果,但當占空比3超過8.91%時,冰黏附強度降低率不再降低,甚至升高;占空比3增加時,基底表面硅橡膠的涂覆位置增多,晚凍結區域數量增大,冰黏附強度降低率升高,但當直徑1超過一定值時,冰黏附強度降低率先升高后降低。
如圖3b所示,當直徑1和占空比3分別保持3.50 mm、8.50%時,圓心距2和厚度4與冰黏附強度降低率的影響規律:冰黏附強度降低率總體上隨圓心距2的增加而降低;隨厚度4的增加,冰黏附強度降低率先增加后降低。響應面沿2方向的變化速率大于沿4方向的變化速率,表明在試驗水平下,圓形涂覆硅橡膠間的圓心距2比涂覆厚度4對結冰附著強度降低率的影響顯著。當圓心距2減小時,減小了涂覆區域附著水結冰產生相變膨脹力的作用距離,冰黏附強度降低率變大,但當涂覆厚度4超過0.249 mm時,冰黏附強度降低率不再升高,甚至降低;硅橡膠涂覆厚度4增大時,擴大了基底表面附著水內部不同區域的凍結結冰時間,降低了冰在材料表面的黏附強度,當涂覆厚度超過0.249 mm時,未涂覆區域已與基底表面形成了穩定的黏附界面,對冰黏附強度的影響逐漸降低,冰黏附強度降低率增大。
注:響應面試驗因素和水平見表1,響應值見表2,下同。
Note: Factors levels of response surface test are shown in Table 1, and response values are shown in Table 2. Same as below.
圖3 因素交互作用對冰黏附強度降低率的影響
Fig.3 Effects of interactive factors on reduction rate of ice adhesion strength
依此類推,試驗中鋁合金試樣表面涂覆硅橡膠的尺寸參數對冰黏附強度降低率的影響順序依次為:圓形涂覆硅橡膠間的圓心距2、硅橡膠涂覆的占空比3、硅橡膠涂覆厚度4及其涂覆直徑1。
由試驗測試結果及響應面分析可見,表面不同位置具有圓形涂覆硅橡膠的鋁合金試樣降低了其表面的冰黏附強度,并且表面具有不同分布參數、不同尺寸參數的圓形硅橡膠對冰黏附強度具有不同的影響作用。當圓形涂覆硅橡膠的直徑為3.50 mm、圓心間距6.50 mm、占空比為8.50%及涂覆厚度為0.250 mm時,鋁合金試樣表面的平均冰黏附強度降低率為46.83%;試樣表面具有直徑3.75 mm、圓心間距7.75 mm、占空比9.75%及厚度為0.225 mm的圓形硅橡膠時,試樣表面的冰黏附強度降低率為18.74%。
圖4為非連續導熱性表面對冰黏附穩定性的影響示意圖。如圖4a所示,試樣表面圓形涂覆硅橡膠的直徑增大時,基底表面附著水內部受影響區域增大,相變膨脹力的作用面積呈平方趨勢增大,降低對已凍結區域覆冰黏附穩定性的影響,因此,材料表面冰黏附強度增大,冰黏附強度降低率減小;反之,硅橡膠涂覆直徑減小,冰黏附強度降低率增大。如圖4b所示,相鄰圓形涂覆硅橡膠間的圓心距增大時,降低了涂覆區域附著水結冰產生的膨脹應力對已凍結區域的影響,有利于冰與未涂覆區域重新形成穩定的黏附界面,減小對基底表面冰黏附強度的降低作用;當圓心距達到5.92 mm時,干擾冰與相鄰硅橡膠之間未涂覆區域的黏附穩定性,降低了冰在基底表面的黏附強度。如圖4c所示,當占空比增大,圓形硅橡膠在基底表面涂覆位置增多,提高了硅橡膠涂覆區域附著水結冰相變產生的膨脹應力對冰黏附強度的影響,冰黏附強度降低率增大。如圖4d所示,硅橡膠涂覆厚度減小,縮減了基底表面附著水內部不同位置之間的相變結冰時間差,導致未涂覆區域的覆冰重新附著于基底表面,減小了基底表面冰黏附強度降低率;涂覆厚度過大時,擴大了材料表面附著水內部不同區域之間的相變結冰時間差,延長硅橡膠表面附著水的過冷狀態,但冰與材料表面未涂覆區域已形成穩定的黏附界面,因此降低了對材料表面冰黏附強度的影響效果。
1.冰 2.基底 3.硅橡膠表面附著的水/冰 4.硅橡膠表面附著水相變產生的膨脹應力 5.硅橡膠
1.Ice 2.Substrate 3.Water/Ice adhesion on silicon rubber 4.Swelling stress generated from water adhesion on silicone rubber 5.Silicone rubber
圖4 表面非連續導熱性對冰黏附穩定性的影響
Fig.4 Effects of discontinuous thermal conductivity of the surface on ice adhesion stability
1)試驗表明在材料表面不同位置圓形涂覆硅橡膠改變材料連續的表面特性,使表面不同位置附著水之間存在不同的相變結冰時間,利用后結冰區域產生的相變膨脹應力可明顯降低材料表面的冰黏附強度,使基底材料可主動降低冰黏附強度,減小后期表面覆冰清除的難度。
2)以圓形涂覆硅橡膠的直徑、圓心間距、占空比及涂覆厚度為設計因素,利用四元二次正交旋轉組合設計方法,編制試驗方案,并進行數據分析。建立圓形涂覆硅橡膠的直徑、分布間距、占空比及涂覆厚度影響鋁合金基底表面冰黏附強度降低率的數學模型,并且各因素對冰黏附強度降低率影響的顯著性順序依次為:圓心距、占空比、厚度及直徑。當圓形涂覆硅橡膠的直徑為3.50 mm、圓心間距6.50 mm、占空比為8.50%及涂覆厚度為0.250 mm時,鋁合金試樣表面的平均冰黏附強度降低率為46.83%。
通過改變材料表面特征的連續性,利用水凍結過程中產生的相變膨脹應力,提升冷藏設備表面的主動防除冰特性,減小覆冰對冷藏設備關鍵部件運轉效果的影響,并為其他工程領域開發一種主動式防除冰技術提供參考。

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Design and experiment of the non-continuous anti-icing surface for refrigeration equipment
Chen Yiying1, Cong Qian1,2, Ren Luquan1,2, Jin Jingfu1, Chen Tingkun1,2※
(1. College of Biological and Agricultural Engineering, Jilin University, Changchun 130022, China;2. Key Laboratory of Bionic Engineering, Ministry of Education, Changchun 130022, China)
Cold storage facility is widely expected to promote the development of agricultural and sideline products for better food quality in the evolving demand of a market. Meanwhile, a large amount of ice normally accumulates on the surface of cold storage equipment, such as the evaporator and heat exchanger, further deteriorating the operational performance of equipment and storage quality of products. Most anti/de-icing approaches have been developed to remove the accreted ice, including mechanical, heating, or chemical ways. These conventional anti-icing methods have caused high cost and energy consumption, even environmental pollution. Alternatively, a super hydrophobic surface presents the most potential anti-icing, but the durability and mechanical properties have been limited in the engineering field. A discontinuous circular coating of silicone rubber can be utilized to tailor the surface property of materials. However, there is a different phase transition time of attached water at different material surfaces, where the active anti-icing power is from the swelling force further to determine the ice adhesion strength. In this study, a feasible anti-icing model was proposed to change the continuity of substrate surface in a refrigerator. Silicone rubber with low thermal conductivity was coated at different locations on the substrate surface to modify the thermal conductivity of the substrate. The dimensional parameters of circular coated silicone rubber were determined to obtain the reduction rate of ice adhesion strength, such as the diameter, the center distance between the adjacent silicone rubbers, the duty cycle, and the thickness of coated silicone rubber. Meanwhile, an orthogonal rotation combination was used to design the experimental scheme. In addition, a self-developed device was utilized to measure the ice adhesion strength, further to calculate its reduction rate. The experimental results showed that the samples with different coating parameters on the surface had different reduction effects on ice adhesion strength. Furthermore, the non-continuous coating of silicone rubber on the sample surface significantly reduced the ice adhesion strength. Specifically, the reduction rate of ice adhesion strength on the aluminum alloy reached 52.38%, when the size of coated silicone rubber was 3.50 mm in diameter, 6.50 mm in center spacing, 8.50% in duty cycle, and 0.250 mm in coating thickness. The average reduction rate of ice adhesion strength on the aluminum alloy could reach 46.83%. And the maximum ice adhesion strength reduction rate was 52.38%. The variance analysis and Response Surface Method (RSM) were used to analyze the experimental data, and thus the mathematical regression models were established between the dimensional factors and the evaluation index. The significant influence on ice adhesion strength was determined in a descending order: center distance, duty ratio, thickness, and diameter of the coated silicone rubber. Phase change times depended mainly on the variation in continuous surface characteristics of the material at the various positions of attached water. In the post-icing area, the swelling stress rapidly generated to break the interfacial stability between the ice and coating, indicating an obvious reduction of ice adhesion strength. Therefore, a significant increase was achieved in the active anti-icing characteristics of the material for the further development of new anti-icing technology. This finding can provide new ideas for the subsequent research, particularly the effects of wettability and morphology of material surface on the ice adhesion strength. The anti/de-icing can also be expected to serve some engineering fields, including refrigeration, high-speed railway trains, and aircraft.
refrigeration; freezing; phase change; anti-icing surface; non-continuous characteristic; ice adhesion strength; models
2020-12-14
2021-03-13
國家自然科學基金面上項目(51775234);吉林省科技發展計劃項目(20200801049GH);吉林省教育廳科學研究項目(JJKH20211070KJ)。
陳奕穎,博士生,研究方向為低溫防凍黏技術。Email:yiyingc20@mails.jlu.edu.cn
陳廷坤,講師,博士,研究方向為工程仿生學與防凍黏機理。Email:chentk@jlu.edu.cn
10.11975/j.issn.1002-6819.2021.07.032
TB131
A
1002-6819(2021)-07-0261-07
陳奕穎,叢茜,任露泉,等. 冷藏設備防除冰表面非連續特征設計與試驗[J]. 農業工程學報,2021,37(7):261-267. doi:10.11975/j.issn.1002-6819.2021.07.032 http://www.tcsae.org
Chen Yiying, Cong Qian, Ren Luquan, et al. Design and experiment of the non-continuous anti-icing surface for refrigeration equipment[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2021, 37(7): 261-267. (in Chinese with English abstract) doi:10.11975/j.issn.1002-6819.2021.07.032 http://www.tcsae.org