自然对流条件下冷表面结霜的实验研究

本文利用微观可视化观测方法,实验对比研究了裸铝(接触角78°)、疏水（接触角141°）和亲水(接触角26°)三种特性表面结霜过程。结果表明,当相对湿度为26%,表面温度为﹣10 ℃时,相比于裸铝表面,疏水表面可以延迟结霜29 min;当温度降至﹣18 ℃,疏水表面仍具有较好的抑霜效果。1 h后,疏水表面上的霜层高度仅为裸铝表面上霜层高度的68%。当表面温度为﹣26 ℃时,3种不同接触角的表面上初始霜晶的形态差异较大,疏水表面倾向于形成更加稠密的沿水平方向生长的枝状霜晶并向片状转变,而亲水表面则倾向于形成沿竖直方向生长的稀疏的针状霜晶,随着空气中水蒸气向霜晶扩散凝华,针状霜晶从顶部向底部逐渐向片状霜晶转变。     

透明超疏水表面的制备与抑霜性能研究

本文介绍了一种透明超疏水表面的制备方法,并在冷表面温度(-3-5℃).相对湿度(20%-60%)下研究了其抑霜性 能。结果表明:用纳米二氧化硅和聚二甲基硅氧烷制备的透明超疏水玻璃表面的最大接触角可达到154.1,透光性能良好;在自 然对流条件下竖直放置的超疏水玻璃表面有良好的抑霜效果,形成的霜层先是呈麦穗状,后期呈无规则分叉状,这与普通玻璃表 面上先是花簇状,后期是霜晶呈六边形的霜层形成鲜明对照;接触角随纳米二氧化硅质量呈倒U型曲线关系,最佳质量为0.067 g/cm2,修饰玻璃表面可维持700s表面不结霜,与普通玻璃表面结需出现时间相比题长了66.7%;发现随着湿度增大,雷晶枝晶 生长越明显,超疏水表面上霜层霜晶生长的方向性越明显。     

自然对流下强吸水表面上结霜特性的研究

在自行研制的强吸水表面上进行了结霜实验研究,测得了其上的霜表面温度随时间的变化情况,并与普通金属表面上的进行了对比.进行了不同实验条件下吸水表面上霜厚的测量,与普通金属表面进行了对比,研究了涂层厚度对结霜的影响.结果表明,这种涂层在较易结霜的实验条件下可使初始霜晶出现时间延迟555分钟左右,在低于冰点的一定范围内,初始霜晶出现可延迟3小时以上;并且霜厚减少可达40%以上,霜层疏松,在外力的作用下很容易除去.涂层越厚,吸水能力越强,抑霜作用也就越明显.     

水平冷面上霜晶生长规律的实验研究

对水平铜冷面上的结霜过程进行了显微实验观察，实验结果表明：结霜过程基本上都经历了水珠生成、长大、冻结、初始霜晶生成、长大以及霜层成长等过程。根据霜晶的外观形状特点将初始霜晶分成了四大类，讨论了初始霜晶形状随冷面温度和空气相对湿度的变化规律，指出冷面温度是影响霜晶形状的主要因素，而空气的相对湿度对霜晶形状也有一定影响。     

疏水表面改性在换热器抑霜上的实验研究

对分体式空调室外换热器翅片管表面进行了疏水改性处理并搭建了可视化的结霜测试平台,在干、湿球2℃/1℃环境工况下测试了改性后和常规换热器翅片表面结霜、除霜及融霜过程的性能。实验表明疏水纳米涂层翅片表现出一定的抑霜效果:其液滴冻结时间延后、霜层薄且疏松、除霜周期延长、化霜时间缩短,但抑霜效果会在结霜后期减弱且融霜后翅片上存在残留液滴。而对于未处理的亲水裸铝翅片:其霜层冻结快、霜层较为致密、结霜程度较严重,但融霜后排液效果好。     

疏水性对竖直冷表面上自然对流结霜特性的影响

本文采用控制表面氧化法制备超疏水表面(153. 2°),并对自然对流条件下竖直放置的超疏水表面与裸铜表面进行可视化结霜实验,观察并对比实验初期有液核与无液核生成成霜时疏水性对结霜过程的影响,研究了疏水性对结霜的影响随冷表面温度(-50～-30℃)、空气相对湿度(30%～70%)的变化规律。结果表明,有液核成霜时,超疏水表面具有显著的抑霜效果;无液核成霜时,疏水表面不再具有抑霜效果,反而超疏水表面霜晶生长更为密实;疏水性对无液核成霜过程的影响随空气相对湿度的增大、冷表面温度的降低而减弱;从云物理学与核化理论角度分析了无液核生成时超疏水表面霜晶分布更密的原因,发现实验制备的超疏水表面上凹坑与CuO晶体颗粒为凝华核化提供了有利位置。     

冻干机用捕水器结霜特性研究中存在的问题

真空冷冻干燥过程中，从物料中升华出来的水分在真空泵的抽吸作用下，不断地在捕水器（也即冷阱）表面结霜，随着霜在捕水器表面不断凝结，致使其捕水效率不断降低，对真空系统形成不利的影响。但目前，对于低温、低压下的结霜机理研究极少，本文介绍了国内外有关低温冷壁面结霜的研究现状，并分析研究了真空冷冻干燥系统中冷壁面结霜过程中存在的问题。     

水平阵列方微柱表面自然对流的结霜特性研究

本文制作了微机械加工的阵列方微柱金属表面,实验研究了其在环境温度Tatm=24℃,相对湿度RH=17%,不同冷表面温度(Tw=-5.2、-10.1、-15.2℃)及不同试件尺寸时,自然对流条件下的结霜特性,分析了表面温度和持续时间对霜层厚度和霜重量的影响,提出了阵列方微柱表面的抗结霜机理,并通过仿真模拟进行了验证。结果表明:当冷表面温度为-10℃时,阵列方微柱表面的霜层质量比平表面降低了约32%;自然对流使微柱之间产生了空气涡旋,水蒸气会随着空气涡旋流动而无法在凹槽内停留,霜仅形成在微柱顶部,凹槽内不结霜,从而显著减少了有效结霜面积;当微柱间距L过大时,凹槽内会形成冷凝液滴并结霜。     

结霜工况下空温式翅片管气化器传热试验研究

为深入掌握低温液体在空温式翅片管汽化器内的气化情况以及其与翅片管表面霜层生长的相互影响规律,以液氮为介质进行了低温液体在空温式翅片管气化器内的气化试验。通过热电偶和刻度带分别对翅片管上不同位置的温度和霜层厚度进行了测量,并分析了翅片管表面霜层的生长规律及翅片管内低温液体的流动特性。结果表明:气化器表面结霜过程受冷表面温度影响较大,冷表面温度越低,结霜速率越大,霜层越厚。结霜工况下的气化器工作状态分预冷和稳态两种工作状态。预冷工作状态低温液体进入气化器后迅速气化,其过程包含气液两相和单气相两个换热段。稳态工作状态低温液体在气化器内气化经历单液相、气液两相、单气相三个换热段,单液相段翅片管表面结霜最为严重,单气相段翅片管表面无霜晶形成。因此认为,可通过分状态分段设计空温式翅片管气化器从而减弱结霜对翅片管传热的影响,提高气化器换热效率。     

基于分形论的深冷翅片管气化器结霜数值模拟

利用分形理论DLA模型,建立翅片管气化器深冷表面霜层生长模型,通过控制程序循环次数模拟了深冷表面霜层生长不同阶段。结合实验观测到的深冷表面霜层生长过程,从分形维数和霜层密度两个方面验证数值模拟的合理性。结果表明,结霜初期随着初始霜枝晶上不断长出分枝晶,霜层密度随厚度的增大而减小,且深冷表面上霜层分形维数较一般冷表面分形维数大,说明深冷表面霜的生长更加复杂,充满空间的能力更大。这对进一步理解空温式深冷翅片管气化器表面结霜机理,探索有效除霜方法,提高气化器换热特性有重要意义。     

The effect of stationary and sweeping frequency AC electric fields on frost crystal removal on a cold plate

The effect of stationary and sweeping frequency AC electric fields on frost crystals growth and frost control/removal on a cold plate was studied for the first time in this paper. The main results of this study showed that the presence of AC electric fields can greatly affect both the frost crystals growth pattern and mass accumulation on cold surfaces. The ice surface electrical properties and basic electrostatics were used to explain the main findings in this paper. Up to 46% frost reduction was obtained when the electric field frequency spanned 370 Hz to 7.5 kHz while the applied voltage was 14.5 kV. Two different sets of environmental conditions were tested, which showed that the plate temperature placed an important effect on frost crystals growth under electric fields. An optimum application time of the AC electric fields was found based on least frost mass accumulation on the cold plate.     

霜层生长初期冰晶体分布状况实验研究

研究霜层的结构对于理解结霜现象有着重要的意义。利用自行研制的图像放大及采集系统，对霜层生长初期冰晶体的形态进行显微观测，得到了不同生长条件下冰晶体的图像。随后采用数字图像处理方法，将原图像转换为二值图。通过对图像的分析，发现霜层的生长初期冰晶体的分布与充分生长的霜层有所不同，此时霜层靠近冷表面固含率最大；随着高度的增长，固含率以近似线性的方式减小。实验还发现，对霜层生长初期影响最大的两个因素是冷表面的温度和空气的相对湿度。随着冷表面温度的降低，霜层的高度明显增长，冰晶体沉积量增加，而平均密度的变化则不明显；随着空气相对湿度的增加，霜层的高度、平均密度以及冰晶体总的沉积量都有所增加。空气温度对霜层生长的影响不明显。     

Modeling of frosting behavior on a cold plate

This paper proposes a mathematical model to predict the frost properties and heat and mass transfer within the frost layer formed on a cold plate. Laminar flow equations for moist air and empirical correlations for local frost properties are employed to predict the frost layer growth. Correlations for local frost density and effective thermal conductivity of the frost layer, derived from various experimental data, are expressed as a function of the various frosting parameters: the Reynolds number, frost surface temperature, absolute humidity and temperature of the moist air, cooling plate temperature, and frost density. The numerical results are compared with experimental data to validate the proposed model, and those agree well with the experimental data within a maximum error of 10%. Heat and mass transfer coefficients obtained from the numerical analyses are also presented. The results show that the model for the frost growth using the correlation of the heat transfer coefficient without considering the air flow has a limitation in its application.     

A lattice Boltzmann model for solidification of water droplet on cold flat plate

Since the solidification of water droplet is the initial and essential process in the whole process of frosting, a model is developed by the lattice Boltzmann method (LBM) that applies the velocity and temperature distribution functions to investigate the solidification process of water droplet on cold flat plate. The thermal transport and liquid–solid phase transition in the present model are both based on the pseudo-potential model combined with the enthalpy formation. By this LB model, the solidification process is simulated in form of temperature and solid phase variations in water droplet on cold flat plate, and the shape of solid phase in freezing can also be predicted. In addition, we apply the present LB model to preliminarily study the frost formation process. Numerical results agree well with our experimental data.     

Dimensionless correlations of frost properties on a cold plate

This study proposes dimensionless correlations for predicting the properties of frost formed on a cold plate. Frosting experiments are carried out to obtain the correlations with various environmental parameters including the air temperature, air velocity, absolute humidity, and cooling plate temperature. The thickness, density, surface temperature, effective thermal conductivity, average heat and mass transfer coefficients of the frost layer are correlated as functions of the Reynolds number, Fourier number, absolute humidity, and dimensionless temperature by using a dimensional analysis. The correlations proposed in this study agree well with the experimental data within a maximum error of 10%, and can be used to predict the average frost properties in the following ranges: the air temperature of 5–15 °C, air velocity of 1.0–2.5 m s⁻¹, absolute humidity of 0.00322–0.00847 kg kg_a⁻¹, and cooling plate temperature of −35–−15 °C.     

Modeling of frost formation over parallel cold platesModélisation de la formation de givre sur les plaques froides parallèles

This paper numerically evaluates some of the parameters involved in modeling the process of frost formation on flat cold surfaces subject to the flow of humid air. The model employs one-dimensional transient formulation based upon the local volume averaging technique. The modeling process was validated by comparison with available experimental data. Numerical experiments were realized to determine the best initial values of the diffusivity, initial radius and geometry of the ice crystals. This model was applied to the known case of flow of humid air over a single flat cold plate to predict the frost temperature, density and thickness distribution along the flow direction and also the void fraction. The results were compared with available results in the literature. The model was then extended to solve the case of flow of humid air between two parallel cold plates for which there are no available results.     

空气源热泵结霜机理及除霜/抑霜技术研究进展

结霜导致蒸发器的热阻增加,传热系数降低,系统COP减小,制约了空气源热泵的推广应用。本文在表面结霜机理研究现状的基础上,总结了影响结霜的各种因素相应除霜/抑霜技术,综述了逆循环、热气旁通和电加热三种常用除霜方法的研究进展,概括了改变空气参数、表面温度和换热器结构的抑霜效果,以及表面改性抑霜技术的研究现状。指出霜导热系数模型的局限性及除霜/抑霜技术存在的问题,提出今后应结合多种措施着重探索对水蒸气凝结、冷凝水冻结、霜层回融和塌陷等阶段均有较强抑制作用的抑霜技术。     

The influence of electro-freezing on ice formation on high-voltage DC transmission lines

Supercooled droplets ranging from 15 to 38 μm diameter are accumulated on an aluminium conductor of 1 cm in diameter installed in the working section of an open wind tunnel. Density and adhesive strength of the ice are measured as a function of the applied field strength at the surface of the conductor. All experiments are conducted in a cold chamber under constant atmospheric conditions.The results obtained with ice formed from droplets of 20 μm mean diameter show that DC negative and positive electric fields above ?10 and +15 kV/cm respectively decrease considerably the density, and consequently the adhesive strength of ice. Alternating fields show weaker effects upon ice density when compared with DC fields.The effect of the electric field upon ice density decreases with increasing size of the supercooled droplets. Alternating fields have practically no effect upon the density of ice formed from 38 μm droplets. An increase in the conductivity of water by the addition of a small quantity of NaCl results in an increase in ice density even in the absence of electric field.Ice accretions under DC electric fields contain many heavily branched ice trees. These ice branches present a converging structure under a positive field and a diverging one under a negative field. From the similarity of the shapes of ice branches and corona streamers, it appears that supercooled droplets freeze along the corona streamers due to the corona wind, which is caused by the evacuation of ionised gas particles.