冷藏设备防除冰表面非连续特征设计与试验
2021-06-30陈奕颖任露泉金敬福陈廷坤
陈奕颖,丛 茜,2,任露泉,2,金敬福,陈廷坤,2
·农产品加工工程·
冷藏设备防除冰表面非连续特征设计与试验
陈奕颖1,丛 茜1,2,任露泉1,2,金敬福1,陈廷坤1,2※
(1.吉林大学生物与农业工程学院,长春 130022;2.吉林大学工程仿生教育部重点实验室,长春 130022)
蒸发器、换热器等冷藏设备表面形成覆冰时,影响设备的正常运行及农副产品的储藏品质。为了减小冰黏附对冷藏设备的影响,该研究通过在材料表面非连续涂覆圆形硅橡胶,改变材料单一的表面特性,提出一种提高部件主动防除冰的方法,并探究圆形涂覆硅橡胶的直径、相邻硅橡胶间的圆心距、涂覆占空比及厚度对冰黏附强度降低率的影响,降低表面冰黏附强度。试验结果表明表面具有不同涂覆参数的试样对冰黏附强度具有不同的降低效果,当涂覆直径为3.50 mm、圆心间距6.50 mm、占空比为8.50%及涂覆厚度为0.250 mm时,铝合金试样表面的平均冰黏附强度降低率为46.83%。对试验结果进行方差分析,建立设计因素与评价指标间的数学回归模型,确定对冰黏附强度影响的显著性主次顺序为:圆形涂覆硅橡胶间的圆心距、占空比、涂覆厚度及直径。分析认为通过改变材料表面特征的连续性,使不同位置内的附着水具有不同的相变时间,利用后结冰区域产生的膨胀应力,干扰冰与材料之间界面的稳定性,降低冰黏附强度,提高冷藏设备部件的主动防除冰特性,为进一步开发新型防除冰技术提供参考。
制冷;冻结;相变;防除冰表面;非连续特性;冰黏附强度;模型
0 引 言
随着中国社会经济的快速发展,国民对肉类、水果、蔬菜等农副产品储存及保鲜质量要求愈发严格,进而推动了国内冷库、冷藏车辆等农产品冷链物流的发展,也促进了农产品冷藏保鲜设备的保有量及设备种类的多样化。据统计,2019年中国食品冷链物流总额约为6万亿元,冷藏车保有量为21.47万台,冷库容量达到6.05×107t[1]。为促进农副产品冷链物流的发展,小型冷库等冷藏设备已经连续5 a被列入农业部补贴项目指南[2],中国发展和改革委员会、农业农村部及国务院分别印发了关于农副产品冷链物流的发展规划[3-4],并在2020年中央一号文件里明确提出启动农产品仓储保鲜冷链物流设施建设工程[5-6]。
冷藏保鲜设备作为农副产品冷藏保鲜链的核心部件,其运行的高效性、节能性一直是该行业追求的目标,但设备运行时压缩机、冷凝器、冷风机、蒸发器等关键零部件的表面温度与环境温度之间存在较大温度差异,环境中的水分极易黏附在部件表面,形成霜层和冰层,导致冷风机热交换能力和换热器换热效率降低[7-9],增加空气流通阻力和冷藏设备的运行能耗[10-12],降低冷藏保鲜设备的工作效果,致使蔬菜、水果、肉类等农副产品储存质量降低[13-16],引起社会经济的损失和资源浪费。
针对冷藏保鲜设备部件表面结霜、结冰现象,国内外已形成了超声振荡、热氨、电加热、热气等多种常规方法[10-12,17-18],但这类防除霜/冰方法存在高成本[19]、高能耗[20]、污染环境[21]且易腐蚀橡胶、金属部件等使用缺陷[22-23]。而超疏水表面作为潜在的防除冰方法,表面存在耐久性差[24-25]、微纳结构易损坏[26]、耐磨性差[27]、易脱落[28]等使用缺陷,暂不满足实际工程领域的使用要求。因此,如何提高冷藏设备防除冰能力,仍是制冷行业尚需解决的问题之一。
课题组初期通过利用结冰过程中发生的相变膨胀现象,提出、设计了一种防除冰模型,达到了降低冰黏附强度的目的[29-30],但该防除冰模型需要在冷藏保鲜设备的本体表面加工,具有实际使用弊端。因此本研究改变材料单一的表面特性,利用水的结冰相变膨胀应力,结合回归方法设计试验,建立防除冰表面设计因素影响冰黏附强度的数学模型,并分析其影响效应,为冷藏保鲜行业提供一种新型且主动的防除冰方法。
1 非连续表面防除冰原理
冰在材料表面黏附,与材料之间形成稳定的黏附系统,而材料的表面特性对冰的黏附稳定性具有重要的影响作用。结合课题组初期研究结果[29-30]及结冰膨胀对沟渠、渡槽等建筑物造成的影响[31-32],改变材料单一的导热特性,如图1所示,使材料表面不同位置具有不同的导热性能。由于材料表面具有非连续的导热性,材料表面不同位置附着的水具有不同相变时间,后结冰区域产生的相变膨胀应力干扰先结冰区域与材料之间形成稳定的附着界面,破坏其黏附稳性,进而降低冰在材料表面的黏附强度。
1.基底 2.水 3.冰 4. 膨胀应力 5.具有非连续特性的表面
1.Substrate 2.Water 3.Ice 4.Swelling stress 5.Surface with non-continuous characteristic
图1 非连续特性表面的防除冰模型
Fig.1 Anti-icing model with non-continuous surface characteristic
2 冰黏附强度测试试验设计
2.1 试验条件与材料
试验中采用冷藏保鲜设备中常用的6061铝合金材料作为试样基体(尺寸为60 mm×60 mm×5 mm),导热系数为238 W/(m·K)[33]。RTV-1硅橡胶导热系数为0.17 W/(m·K)[34],其在金属、橡胶等工程材料表面黏结性好,具有耐久性、耐候性、耐温性等优点[34],已在汽车、电力、制冷等领域广泛应用,因此通过模具法在铝合金表面不同位置环形涂覆不同圆形尺寸参数的硅橡胶。通过低温环境模拟箱控制试验温度,在温度为−20 ℃的低温环境下、冻结1 h,用水量为5 mL,运用内径为32 mm的铝杯制备试样表面的覆冰。试验中的用水均为纯净水(购买自北京化学厂),并利用自制冰黏附力测试装置测量冰在试样表面的黏附力,如图2所示。
2.2 试验方法
2.2.1 试验指标
重复10次测试不同试样及光滑铝合金表面的冰黏附强度,通过公式(1)计算不同试样的冰黏附强度降低率。材料表面的冰黏附强度越低,越易于清除材料表面的覆冰,因此试验中以冰黏附强度降低率为评价指标。
式中为冰黏附强度降低率,%;0为光滑试样表面冰黏附力,N;为防除冰试样表面的冰黏附力,N。
2.2.2 试验因素
试验中在铝合金试样表面不同位置圆形涂覆不同参数的硅橡胶,影响表面附着水内部不同黏附区域的结冰顺序,利用结冰相变膨胀干扰冰黏附稳定性,降低表面冰黏附强度。因此,选取圆形涂覆硅橡胶的直径、相邻圆形硅橡胶间的圆心距、硅橡胶涂覆的占空比及涂覆厚度作为试验因素,并根据铝杯内径及试样尺寸的限制,试验中选取3~4 mm的硅橡胶直径、4~9 mm的圆心距、6%~11%的占空比及0.2~0.3 mm的涂覆厚度。其中占空比为硅橡胶涂覆面积与冰黏附面积比值。
2.2.3 试验设计
选取圆形涂覆硅橡胶的直径1、不同圆形硅橡胶间的圆心距2、占空比3及涂覆厚度4作为试验因素,并运用四元二次正交旋转组合[35]设计试验方案,各因素水平的编码如表1所示。
在基底表面粘贴厚度约为0.020~0.025 mm的双向拉伸聚丙烯薄膜(Biaxially Oriented Polypropylene,BOPP),通过粘贴多层BOPP薄膜满足硅橡胶的涂覆厚度,运用激光雕刻机在BOPP层表面加工待涂覆硅橡胶的分布图形。室温条件下自然固化24 h,清除BOPP薄膜表面多余的硅橡胶,移除基底表面粘贴的多层BOPP薄膜,并用酒精擦拭、清除基底表面的杂质。
利用冰黏附力测试装置,根据设定的试验条件及编制的试验方案,利用公式(1)计算不同防除冰试样表面的冰黏附强度降低率,计算每种试样的平均降低率,并作为该试样防除冰效果的评价指标。
3 结果与分析
3.1 回归模型与显著性分析
由表2中的试验测试结果可知:相对于冰在光滑试样表面的黏附力,冰与表面不同位置具有圆形涂覆硅橡胶的铝合金之间形成了低黏附力。防除冰表面设计因素均为0水平时,铝合金试样表面的平均冰黏附强度降低率为46.83%,其中最大冰黏附强度降低率为52.38%,降低后期材料表面覆冰的清除难度和成本。
表2 试验方案与测试结果
式中1为圆形涂覆硅橡胶的直径,mm;2为圆形硅橡胶间的圆心距,mm;3为硅橡胶涂覆的占空比,%;4为涂覆厚度,mm。
表3 冰黏附强度降低率R回归模型的方差分析
注:<0.05为显著,<0.01为极显著。
Note:<0.05 represents significance,<0.01 means extremely significance.
3.2 响应面分析
利用响应面法分析圆形涂覆硅橡胶各尺寸参数对冰黏附强度降低率的影响,固定4因素中的2个因素为0水平,考察其余2个因素对冰黏附强度降低率的影响效应。
如图3a所示,当圆心距2和厚度4分别保持6.50 mm、0.250 mm时,直径1和占空比3与冰黏附强度降低率的影响规律:冰黏附强度降低率总体上随直径1的增加而逐渐降低;随占空比3的增加,冰黏附强度降低率先增加后降低,并且响应面沿3方向的变化速率大于沿1方向的变化速率,说明在试验水平下,涂覆硅橡胶的占空比其涂覆直径对冰黏附强度降低率的影响更大。当圆形硅橡胶涂覆直径1增大时,晚冻结区域面积变大,而产生的相变膨胀力作用面积成平方趋势增加,降低了对冰黏附强度的作用效果,但当占空比3超过8.91%时,冰黏附强度降低率不再降低,甚至升高;占空比3增加时,基底表面硅橡胶的涂覆位置增多,晚冻结区域数量增大,冰黏附强度降低率升高,但当直径1超过一定值时,冰黏附强度降低率先升高后降低。
如图3b所示,当直径1和占空比3分别保持3.50 mm、8.50%时,圆心距2和厚度4与冰黏附强度降低率的影响规律:冰黏附强度降低率总体上随圆心距2的增加而降低;随厚度4的增加,冰黏附强度降低率先增加后降低。响应面沿2方向的变化速率大于沿4方向的变化速率,表明在试验水平下,圆形涂覆硅橡胶间的圆心距2比涂覆厚度4对结冰附着强度降低率的影响显著。当圆心距2减小时,减小了涂覆区域附着水结冰产生相变膨胀力的作用距离,冰黏附强度降低率变大,但当涂覆厚度4超过0.249 mm时,冰黏附强度降低率不再升高,甚至降低;硅橡胶涂覆厚度4增大时,扩大了基底表面附着水内部不同区域的冻结结冰时间,降低了冰在材料表面的黏附强度,当涂覆厚度超过0.249 mm时,未涂覆区域已与基底表面形成了稳定的黏附界面,对冰黏附强度的影响逐渐降低,冰黏附强度降低率增大。
注:响应面试验因素和水平见表1,响应值见表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.
图3 因素交互作用对冰黏附强度降低率的影响
Fig.3 Effects of interactive factors on reduction rate of ice adhesion strength
依此类推,试验中铝合金试样表面涂覆硅橡胶的尺寸参数对冰黏附强度降低率的影响顺序依次为:圆形涂覆硅橡胶间的圆心距2、硅橡胶涂覆的占空比3、硅橡胶涂覆厚度4及其涂覆直径1。
3.3 讨 论
由试验测试结果及响应面分析可见,表面不同位置具有圆形涂覆硅橡胶的铝合金试样降低了其表面的冰黏附强度,并且表面具有不同分布参数、不同尺寸参数的圆形硅橡胶对冰黏附强度具有不同的影响作用。当圆形涂覆硅橡胶的直径为3.50 mm、圆心间距6.50 mm、占空比为8.50%及涂覆厚度为0.250 mm时,铝合金试样表面的平均冰黏附强度降低率为46.83%;试样表面具有直径3.75 mm、圆心间距7.75 mm、占空比9.75%及厚度为0.225 mm的圆形硅橡胶时,试样表面的冰黏附强度降低率为18.74%。
图4为非连续导热性表面对冰黏附稳定性的影响示意图。如图4a所示,试样表面圆形涂覆硅橡胶的直径增大时,基底表面附着水内部受影响区域增大,相变膨胀力的作用面积呈平方趋势增大,降低对已冻结区域覆冰黏附稳定性的影响,因此,材料表面冰黏附强度增大,冰黏附强度降低率减小;反之,硅橡胶涂覆直径减小,冰黏附强度降低率增大。如图4b所示,相邻圆形涂覆硅橡胶间的圆心距增大时,降低了涂覆区域附着水结冰产生的膨胀应力对已冻结区域的影响,有利于冰与未涂覆区域重新形成稳定的黏附界面,减小对基底表面冰黏附强度的降低作用;当圆心距达到5.92 mm时,干扰冰与相邻硅橡胶之间未涂覆区域的黏附稳定性,降低了冰在基底表面的黏附强度。如图4c所示,当占空比增大,圆形硅橡胶在基底表面涂覆位置增多,提高了硅橡胶涂覆区域附着水结冰相变产生的膨胀应力对冰黏附强度的影响,冰黏附强度降低率增大。如图4d所示,硅橡胶涂覆厚度减小,缩减了基底表面附着水内部不同位置之间的相变结冰时间差,导致未涂覆区域的覆冰重新附着于基底表面,减小了基底表面冰黏附强度降低率;涂覆厚度过大时,扩大了材料表面附着水内部不同区域之间的相变结冰时间差,延长硅橡胶表面附着水的过冷状态,但冰与材料表面未涂覆区域已形成稳定的黏附界面,因此降低了对材料表面冰黏附强度的影响效果。
1.冰 2.基底 3.硅橡胶表面附着的水/冰 4.硅橡胶表面附着水相变产生的膨胀应力 5.硅橡胶
1.Ice 2.Substrate 3.Water/Ice adhesion on silicon rubber 4.Swelling stress generated from water adhesion on silicone rubber 5.Silicone rubber
图4 表面非连续导热性对冰黏附稳定性的影响
Fig.4 Effects of discontinuous thermal conductivity of the surface on ice adhesion stability
4 结 论
1)试验表明在材料表面不同位置圆形涂覆硅橡胶改变材料连续的表面特性,使表面不同位置附着水之间存在不同的相变结冰时间,利用后结冰区域产生的相变膨胀应力可明显降低材料表面的冰黏附强度,使基底材料可主动降低冰黏附强度,减小后期表面覆冰清除的难度。
2)以圆形涂覆硅橡胶的直径、圆心间距、占空比及涂覆厚度为设计因素,利用四元二次正交旋转组合设计方法,编制试验方案,并进行数据分析。建立圆形涂覆硅橡胶的直径、分布间距、占空比及涂覆厚度影响铝合金基底表面冰黏附强度降低率的数学模型,并且各因素对冰黏附强度降低率影响的显著性顺序依次为:圆心距、占空比、厚度及直径。当圆形涂覆硅橡胶的直径为3.50 mm、圆心间距6.50 mm、占空比为8.50%及涂覆厚度为0.250 mm时,铝合金试样表面的平均冰黏附强度降低率为46.83%。
通过改变材料表面特征的连续性,利用水冻结过程中产生的相变膨胀应力,提升冷藏设备表面的主动防除冰特性,减小覆冰对冷藏设备关键部件运转效果的影响,并为其他工程领域开发一种主动式防除冰技术提供参考。
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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
国家自然科学基金面上项目(51775234);吉林省科技发展计划项目(20200801049GH);吉林省教育厅科学研究项目(JJKH20211070KJ)。
陈奕颖,博士生,研究方向为低温防冻黏技术。Email:yiyingc20@mails.jlu.edu.cn
陈廷坤,讲师,博士,研究方向为工程仿生学与防冻黏机理。Email:chentk@jlu.edu.cn
10.11975/j.issn.1002-6819.2021.07.032
TB131
A
1002-6819(2021)-07-0261-07
陈奕颖,丛茜,任露泉,等. 冷藏设备防除冰表面非连续特征设计与试验[J]. 农业工程学报,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