日光温室中土壤—空气换热器周围土壤内热湿迁移规律探究
发布时间：2018-09-19 11:34
【摘要】：作为一个人口大国,农业发展始终是社会经济增长的关键点。当前能源危机的加剧,自然灾害频发,环境污染加剧,使节能可靠的日光温室热湿调控系统引起了广泛关注。在此背景下,本文开展了土壤——空气换热器的研究。土壤作为该换热系统蓄放热的最主要载体,其土壤内部的热湿分布将直接影响换热器运行效果。为此,本文探究了日光温室土壤——空气换热系统在运行中,非饱和土壤蓄放热过程的热湿耦合迁移规律。土壤中的温度和湿度场之间相互影响,温度梯度将促使土壤空隙中湿分的移运,湿分的迁移又将带走热量引起温度的变化。针对这一复杂问题,运用Fluent软件建立了,水平换热管周围非饱和土壤中热湿耦合传递的双场驱动模型,将温度梯度、土水势作为湿分迁移的驱动力,模拟非饱和土壤蓄放热过程中温湿度的动态分布。为直观探究土壤蓄热过程中三维的热湿耦合作用,模拟了各向同性的非饱和土壤温湿度动态分布,对温湿度之间相互耦合作用进行了分析;之后建立了日光温室中农耕土壤湿度分层模型,打破了前人对土壤各向同性的认识,并分析了湿度分层对换热运行的影响。最后,将热湿耦合模型与纯导热模型进行对比,指出湿分迁移引起的能量变化。结果表明,温湿度场的分布密切相关。在土壤中温度梯度作用下,土壤中的湿分沿温度梯度反方向迁移,并依次堆积形成湿峰。各向同性土壤中,距管中心距离相同的各点,同时出现湿峰。管内空气温度越高,湿度峰值越明显。在各向异性的分层土壤中,其土壤水平方向的温湿度分布与竖直方向明显不同。土壤含水率的差别导致土壤热物性的改变。在竖直方向上土壤含湿量差别较大,土壤中水分的分布更容易受到初始水分分层的影响,在湿度梯度和温度梯度共同作用下产生明显移运。在自然状态下运行土壤空气换热器,土壤空气换热器温度和湿度影响半径均小于0.5米范围,且在换热过程中,土壤温度的变化有明显的延时效应。随着与换热管距离的增加,温度变化的延时效应越明显。同样也表明,在换热器正常运行中,为避免管间相互影响,则需保证管间距大于1米。与纯导热模型对比,非饱和土壤中湿分迁移引起的热量的转移,使土壤获得更大的蓄放热能力。
[Abstract]:As a populous country, agricultural development has always been the key point of social and economic growth. With the aggravation of energy crisis, frequent natural disasters and environmental pollution, the energy saving and reliable heat and humidity control system in solar greenhouse has attracted wide attention. In this context, the soil-air heat exchanger is studied in this paper. As the main carrier of the heat transfer system, the heat and moisture distribution in the soil will directly affect the operation effect of the heat exchanger. Therefore, in this paper, the heat and moisture coupling migration of unsaturated soil during the operation of soil-air heat transfer system in solar greenhouse is studied. The temperature gradient will promote the movement of moisture fraction in the soil void, and the migration of moisture fraction will take away heat to cause the change of temperature. Aiming at this complex problem, a two-field driving model of thermal-moisture coupling transfer in unsaturated soil around horizontal heat exchanger pipe is established by using Fluent software. The temperature gradient and soil water potential are taken as the driving force of moisture transport. The dynamic distribution of temperature and humidity in unsaturated soil during heat storage and exothermic process was simulated. In order to investigate the three-dimensional thermo-moisture coupling in the process of soil heat storage, the dynamic distribution of the isotropic unsaturated soil temperature and humidity was simulated, and the interaction between temperature and humidity was analyzed. The model of soil moisture stratification in solar greenhouse was established, which broke the previous understanding of soil isotropy, and analyzed the influence of moisture stratification on heat transfer. Finally, the heat and moisture coupling model is compared with the pure heat conduction model, and the energy changes caused by wet fraction migration are pointed out. The results show that the distribution of temperature and humidity field is closely related. Under the action of the temperature gradient in soil, the wet fraction in the soil migrated in the opposite direction of the temperature gradient, and the wet peak was formed in turn. In isotropic soil, there is a wet peak at the same distance from the center of the tube. The higher the air temperature in the pipe, the more obvious the peak humidity. In anisotropic layered soils, the distribution of temperature and humidity in the horizontal direction is obviously different from that in the vertical direction. The difference of soil moisture content leads to the change of soil thermal properties. In the vertical direction, the moisture content of the soil varies greatly, and the distribution of soil moisture is more easily affected by the initial moisture stratification, resulting in obvious migration under the combined action of the moisture gradient and the temperature gradient. The influence radius of temperature and humidity of soil air heat exchanger is less than 0.5 m, and the variation of soil temperature has obvious delay effect in the process of heat transfer. With the increase of the distance from the heat exchanger tube, the delay effect of temperature change becomes more obvious. It is also shown that in order to avoid the interaction between tubes in the normal operation of the heat exchanger, the distance between the tubes should be more than 1 meter. Compared with the pure heat conduction model, the heat transfer caused by wet fraction migration in unsaturated soil makes the soil obtain greater heat storage and exothermic capacity.
【学位授予单位】：太原理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TU83
[Abstract]:As a populous country, agricultural development has always been the key point of social and economic growth. With the aggravation of energy crisis, frequent natural disasters and environmental pollution, the energy saving and reliable heat and humidity control system in solar greenhouse has attracted wide attention. In this context, the soil-air heat exchanger is studied in this paper. As the main carrier of the heat transfer system, the heat and moisture distribution in the soil will directly affect the operation effect of the heat exchanger. Therefore, in this paper, the heat and moisture coupling migration of unsaturated soil during the operation of soil-air heat transfer system in solar greenhouse is studied. The temperature gradient will promote the movement of moisture fraction in the soil void, and the migration of moisture fraction will take away heat to cause the change of temperature. Aiming at this complex problem, a two-field driving model of thermal-moisture coupling transfer in unsaturated soil around horizontal heat exchanger pipe is established by using Fluent software. The temperature gradient and soil water potential are taken as the driving force of moisture transport. The dynamic distribution of temperature and humidity in unsaturated soil during heat storage and exothermic process was simulated. In order to investigate the three-dimensional thermo-moisture coupling in the process of soil heat storage, the dynamic distribution of the isotropic unsaturated soil temperature and humidity was simulated, and the interaction between temperature and humidity was analyzed. The model of soil moisture stratification in solar greenhouse was established, which broke the previous understanding of soil isotropy, and analyzed the influence of moisture stratification on heat transfer. Finally, the heat and moisture coupling model is compared with the pure heat conduction model, and the energy changes caused by wet fraction migration are pointed out. The results show that the distribution of temperature and humidity field is closely related. Under the action of the temperature gradient in soil, the wet fraction in the soil migrated in the opposite direction of the temperature gradient, and the wet peak was formed in turn. In isotropic soil, there is a wet peak at the same distance from the center of the tube. The higher the air temperature in the pipe, the more obvious the peak humidity. In anisotropic layered soils, the distribution of temperature and humidity in the horizontal direction is obviously different from that in the vertical direction. The difference of soil moisture content leads to the change of soil thermal properties. In the vertical direction, the moisture content of the soil varies greatly, and the distribution of soil moisture is more easily affected by the initial moisture stratification, resulting in obvious migration under the combined action of the moisture gradient and the temperature gradient. The influence radius of temperature and humidity of soil air heat exchanger is less than 0.5 m, and the variation of soil temperature has obvious delay effect in the process of heat transfer. With the increase of the distance from the heat exchanger tube, the delay effect of temperature change becomes more obvious. It is also shown that in order to avoid the interaction between tubes in the normal operation of the heat exchanger, the distance between the tubes should be more than 1 meter. Compared with the pure heat conduction model, the heat transfer caused by wet fraction migration in unsaturated soil makes the soil obtain greater heat storage and exothermic capacity.
【学位授予单位】：太原理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TU83
【参考文献】
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