冷表面结/融霜行为及超声波去除表面融霜液滴试验研究

【作者】 朱琳；

【导师】 赵孝保；

【作者基本信息】 南京师范大学 ， 制冷及低温工程， 2017， 硕士

【摘要】 结霜现象广泛存在于制冷与低温、风力发电、航空航天等领域,霜层的存在严重影响制冷空调系统运行效率以及航天飞行器的飞行安全。传统的热力除霜方式在除霜结束后依然有残留液滴的存在,这些液滴会对再次结霜产生巨大影响。液滴在再次结霜初期冻结,在冷表面上形成致密的冻结液滴基底,增加霜层与冷表面间的粘结力,对除霜产生不利影响。因此,探求简便高效的脱除液滴的方法,在结霜前期营造无液环境,对抑制霜层生长具有重要的科学意义和工程应用价值。目前,对于融霜后残留液滴的脱除主要采用传统的加热蒸发方式,虽然可以成功脱除,但同时也带来了脱除时间长、能耗高、温度波动大等弊端。近年来,超声波以其频率高、波长短、能量集中的特点,广泛运用于暖通空调及制冷领域,如雾化除湿、工业除尘、换热器除垢等等。因此,本文将超声波引入冷表面抑、除霜领域,应用声学、热力学、相变动力学、界面学等相关理论,全面深入地研究了水平冷表面霜层生长过程,并利用反复融结霜过程,分析了融霜液滴的生长分布规律,提出融霜液滴对再结霜过程的不利影响。对外加超声波脱除表面液滴的可行性进行了探索性研究。概括起来,本论文主要研究内容及结论如下:(1)设计并搭建了水平冷表面霜层生长的微观可视化试验平台,仔细观察并描述了水平冷表面霜层生长的整个过程,根据霜层生长过程中的特点和霜晶演化规律,将结霜过程分为了冷凝液滴形成阶段、冷凝液滴冻结及初始霜晶形成阶段、霜晶生长阶段、霜层充分生长阶段四个阶段。并对霜层生长过程各阶段划分标准以及各阶段主要生长特征进行了描述。(2)对结霜前期冷表面水蒸气凝结成核进行了理论分析,提出水蒸气凝结成核的驱动力为相变驱动力,并根据相变动力学和界面学等理论推导了相变驱动力关系式和临界成核半径关系式,根据吸附理论推导了冷表面成核中心密度关系式。分析表明:冷表面温度越低,空气含湿量越大,相变驱动力也就越大,水蒸气越容易在冷表面凝结。过冷度越大,接触角越小,冷表面凝结的液滴数量越多。(3)利用第二章中的水平冷表面霜层生长的微观可视化试验平台,通过反复融结霜过程提出了热力融霜后融霜液滴残留现象。分析了融霜液滴的生长分布规律,提出融霜液滴粒径分布可以近似得看作自由度为6的卡方分布。探索了融霜液滴覆盖率在不同冷表面温度和不同结霜时间下随结霜次数的变化规律。得出结论:当液滴平均粒径在0.1mm以下时,影响液滴覆盖率的主导因素为霜沉积量;当液滴平均粒径在0.1mm～1.0mm范围内时,影响液滴覆盖率的主导因素为液滴聚合作用;当液滴平均粒径在1.0mm以上时,影响液滴覆盖率的主导因素为霜沉积量。试验同时表明:融霜液滴的存在给再结霜过程带来了严重的不利影响。首先,融霜液滴在冻结后促使霜晶提前生长;其次,冻结的融霜液滴成为了霜层基底,增加了霜层与冷表面之间的粘结力,不利于霜层的脱除;另外,融霜液滴的存在使霜层突破了相同结霜工况下的稳定生长高度,霜层最大高度超过稳定生长高度2倍以上,霜层整体密度和霜沉积量都大幅增加,使结霜状况恶化,增加了除霜的困难性。(4)为探求快速有效脱除融霜液滴的方法,进行了超声波与加热蒸发脱除液滴的对比试验,试验验证了超声波脱除水平表面液滴的可行性,并通过对比凸显了超声波在脱除液滴的过程中时间短、能耗低、热稳定性好的优势。超声波脱除液滴所用时间至多是加热法的1/60,能耗仅为加热法的1/100左右。超声波脱除液滴铝板最大温升仅仅为5.7℃,并且在3s内铝板恢复至原有温度,而加热蒸发脱除液滴铝板最大温升为177.6℃,为超声法的31.2倍,并且铝板温度波动在25min后未恢复至原有温度。试验表明:超声波脱除液滴的作用机制并非是超声波的热效应,而主要应该是空穴效应和机械效应。从对不同超声功率脱除不同体积液滴的试验中可以得出结论:当超声功率低于54W时,液滴脱除概率明显下降。当超声功率大于54W时,液滴脱除概率接近100%,而再提升超声功率,超声波脱除液滴的时间越短,但节省时间不明显,能耗基本不变,脱除效果也无明显提升。更多还原

【Abstract】 Frost deposition is a common phenomenon in refrigeration and cryogenics,wind power generation, aerospace and other fields. The existence of the frost layer seriously influences the operational efficiency of refrigeration and air conditioning systems and the safety of aerospace aircraft flight. The residual water droplets will exist after traditional heat defrosting,these droplets will have a huge impact on the next frosting cycle. Melt water droplets will still remain on the surface and refreeze in the next frosting cycle, forming thick substrate on the cold surface. Frozen water droplets will increase the cohesive force between frost layer and cold surface, brought in negative influence on defrosting. Hence, searching for a effective method of removing the water droplets and building aneroid environment before frost to inhibit the growth of frost layer have important scientific significance and engineering application value.At present, the traditional mothed to remove the water droplets after defrosting is heatingevaporation. Although it can successfully remove the water droplets,it also brought some corrupt practice such as long time, high energy consumption and huge temperature fluctuations.In recent years, ultrasonic has been wild used in heating ventilation air conditioning and refrigeration domains such as atomization dehumidification, industrial dust removal,descaling heat exchanger,due to its good characteristics of high frequency, short wave length and energy concentration. Therefore, this article introduces ultrasonic in frost resisting and defrosting field. Using acoustics, thermodynamics, kinetics of phase transformation, interface theory and so on, this article studied the growth process of frost layer on cold surface deeply.Meanwhile, the growth and distribution regularities of melt water droplets are analyzed in virtue of the melt and frost cycles. The negative impact of water droplets on next frosting process was put forward. The possibility of ultrasonic vibration on the release of the melt water droplets from cold surface was investigated. In summary, the main research contents and conclusions in this paper are as follows:(1) Microscopic visualization experimental platform of frost layer growth on horizontal cold surface was designed and build. The whole process of the frost layer growth on cold surface was observed and described carefully. The frosting process was divided into four stages according to the characteristics during frost layer growth process and the evolution rules of frost crystals. These four stages are condensate droplets formation stage, condensate droplets freeze and formation of initial frost crystals stage, frost crystal growth stage, frost layer sufficient growth stage.(2) The theory of water vapor on the surface condensation into cold nucleation in the early period of frosting was analyzed. It was put forward that the driving force of water vapor condenses into nuclear is phase transformation driving force. Transformation driving force formula and the critical nucleation radius formula were deduced according to the theory of phase transformation kinetics and interface theory. Analysis shows that the cold surface temperature lower, the air moisture content greater, the phase transformation driving force will be greater. The water vapor will be easier to condensate on the cold surface.(3) Taking advantage of microscopic visualization experimental platform of frost layer growth on horizontal cold surface, melt-frost-melt cycle process was carried out. Then the water droplets residual phenomenon after heat defrosting was put forward. Regularities of growth and distribution of the melt water droplets was analyzed. The size distribution of melt water droplets can be regard as chi-square distribution of degrees of freedom as six. The coverage rate of melt water droplets changing with times of frosting under different cold surface temperature and different frost time was explored. Here comes to a conclusion that:when the average particle size of the droplets are under 0.1 mm, the dominant factor affecting droplets coverage rate is frost deposit amount. When the average particle size of the droplets is between 0.1mm to 1.0mm, the dominant factor affecting droplets coverage rate is droplet polymerization. When the average particle size of the droplets is over 1.0mm, the dominant factor affecting droplets coverage rate is frost deposit amount.The experiment also proves that: the exist of melt water droplets have brought serious adverse effects on the process of frosting again. First of all, defrosting droplet after frozen prompted the frost crystal growth started ahead of time. Meanwhile, The frozen water droplets became into frost layer basal which increased the cohesive force between frost layer and cold surface. It certainly fight against the removal of the frost layer. What’s more, the frost layer broke the stable growth height duo to melt water droplets. The maximum height of frost layer can exceed stable growth height for more than 2 times in this experiment.(4) In order to searching for quick and efficient method of removal water droplets, The contrast experiment of removing water droplets by ultrasonic and heating evaporation was conducted. Experiments verify the feasibility of removing the water droplets by ultrasonic.Ultrasonic show many excellent qualities during the process of removing the water droplets,such as short removal time, low energy consumption and good thermal stability. The water droplets removal time by ultrasonic is no more than 1/60 of removal time by heating method.Its’ energy consumption is only about 1/100 of heating method. During the droplets removal process by ultrasonic,the maximum temperature rise of the aluminium plate is just 5.7 ℃.In addition, the temperature rise can recover within 3 minutes. However, the maximum temperature rise of the aluminium plate is 177.6 ℃ under heating. The numerical value is 31.2 times of ultrasonic vibration. What’s worse, the aluminum plate temperature fluctuations didn’t die down even after 25 minutes later.Experiment also showed that: the mechanism of removing water droplets by ultrasonic is not heating effect. It can be cavitation effect and mechanical effect. From the experiment of removing different size water droplets by different power ultrasonic, we can conclude that:when the ultrasonic power under 54W, the droplets removal probability decreases obviously.When the ultrasonic power upon 54W, the droplets removal probability reach close to 100%.Improving the ultrasonic power, the removal time cann’t be cut down obviously. The energy consumption is also essentially unchanged. The removal manifestation doesn’t increase distinctly as well.更多还原