土壤温度和水分日变化实验

在露天和温室两种条件下 ,对 2 0 0mm× 5 0mm (高×半径 )的土壤床中温度和水分的日变化规律进行了实验研究。结果表明 :1)土壤温度随着太阳辐射和大气温度的周期性变化而呈现出周期性变化 ,随着深度的增加 ,温度变化的滞后效应越明显 ;2 )夏天的白天大部分时间中 ,土壤浅层的温度高于深层的温度 ;从下午的某个时间至第二天日出 ,这种现象则相反 ;3 )云层夺太阳幅射的阻挡 ,对土壤表层的温度有较大的影响 ,而对较深处影响不大 ;4)由于有遮阳设施 ,温度中土壤的最高温度要比露天情况下低 ;5 )从傍晚开始 ,土壤中的水蒸汽出现冷凝 ,浅层土壤中水蒸汽开始冷凝的时间比深层早 ,露天下土壤中水蒸汽的冷凝现象比温室中更明显     

五道梁地区土壤热状况的讨论

利用五道梁1994-1999年之辐射观测资料及0cm地温资料，结合五道梁附近的较深层的地温观测资料，从土壤温度梯度、土壤热通量、土壤导热率、地表净辐射、季节性冻土的融化深度以及2．0m地温的变化等方面分析了五道梁地区的土壤热状况。通过计算分析，得到(1)该地区土壤温度梯度5至8月份为负，10月份至来年3月份为正；(2)土壤热通量的变化规律与土壤温度梯度一致；(3)土壤导热率呈双峰型，4月份最大，9月份次大；(4)土壤温度梯度与地表净辐射负相关，线性相关系数达-0．95；(5)季节冻融层的融化深度有增大的趋势；2．0m处地温有增大的趋势。     

那曲流域季节冻土区土壤水热动态变化分析

鉴于青藏高原冻土区土壤水热动态变化过程对高寒区气候变化、植被演替及退化等方面的研究意义重大。基于2019年9月10日～2020年8月10日那曲流域4个监测站点分层土壤温度和湿度数据,探讨了冻融过程中土壤水热的变化规律及土壤水分和温度间的互作效应。结果表明,那曲流域季节冻土区在冻结过程期、完全冻结期、融化过程期、完全融化期土壤含水率呈现降低—稳定—升高—波动的变化趋势。冻结过程中,土壤水在温度梯度的作用下开始向冻结锋面运移并补充冻结锋面以下的土壤水分,各层土壤含水率均有所下降;融化过程中季节冻土呈现双向消融特征,各层土壤含水率上升,小唐古拉山、措玛乡、那曲大桥、香茂乡分别在20、35、10、50cm土壤水分高值区。土壤在冻结过程中处于放热状态,表层土壤温度较深层小,而融化过程处于吸热状态,表层土壤温度较深层大。随着土层深度增加,气温对土壤温度的影响越来越小,表层土壤温度变化速率均较深层大。5、10、20、35、50cm土层土壤温度与土壤含水率呈正相关关系,确定性系数R2分别为0.596、0.500、0.499、0.304、0.414。研究结果为及时明晰青藏高原土壤水热动态变化状况提供了一定的理论依据。     

太阳能热泵干燥室气流组织优化研究

为优化太阳能热泵干燥室内气流组织,将计算流体力学(CFD)技术与正交性实验相结合,采用9组正交实验对干燥室(2 400 mm×1 200 mm×2 200 mm)内部气流进行模拟,对比送风孔板的入口风速(2～6 m/s)、孔径大小(6～16 mm)以及送风量(1 000～3 000 m~3/h)对气流组织均匀性的影响程度,对模拟数据进行极差和方差分析,综合分析各因素的影响趋势,进而选取最优工况。结果表明:流速均匀性随入口风速的增加而降低,随孔径的增大而增加,风量的变化对其影响相对较小;温度均匀性随孔径的增大而增加,风量变化对温度均匀性影响较大;确定在入口风速2 m/s,孔径16 mm,送风量为3 000 m~3/h时干燥室气流均匀性最优。     

土壤空气换热器换热特性的模拟研究

文章针对日光温室环境下土壤空气换热器的换热特性进行了研究。首先通过监测土壤空气换热器沿程空气温度的全天变化,分析了试验工况下土壤空气换热器的动态换热过程及系统性能变化规律。研究结果表明,在试验工况下,土壤空气换热器系统的性能系数(COP)可高达24.1。在此基础上,通过建立土壤空气换热器的非稳态换热模型,模拟研究不同的入口风速对土壤空气换热器换热性能的影响。研究结果表明,当换热管入口空气温度相同时,随着入口风速的增加,土壤空气换热器进出口空气温度差逐渐减小,出口处空气温度与土壤温度差值逐渐增大,这意味着土壤空气换热器有效换热长度逐渐变长。在此过程中,土壤空气换热器系统的换热量和COP随着入口空气风速的增加呈现出先增后减的规律。通过模拟结果可知,当入口风速达到5.5 m/s时,土壤空气换热器系统的换热量与COP均达到最大值。     

混合型热源对热泵系统性能影响的分析

通过对太阳能辅助地源热泵系统热泵两循环侧进出口流体温度、流量以及钻孔内各深度土壤温度进行实时监测,评价系统供热性能(COPs)及土壤温度变化规律。2009年冬季系统实际供热运行结果显示,COPs在3.0～4.9,太阳能的辅助作用直接体现在土壤温度的变化上,使用太阳能辅助9d后,各深度土壤温度比不使用时平均高1.15℃,COPs提高0.20～0.25;经实测,太阳能对末端供热的能量贡献占总量的6.83%,土壤占56.30%,电能占36.87%。从两循环侧流体不同流速组成与系统性能的关系中,可知存在高效流量组合使给定系统在其他条件不变时,COPs达到最大,本系统的最优流量组合为(1.83,2.54)m3/h。     

煤粉链条炉不同送粉风速燃烧特性研究

针对节能改造后的煤粉链条炉的燃烧过程,运用数值模拟方法研究了不同送粉风速下炉膛内温度、速度,各组分物质的浓度以及炉膛出口温度的变化特性。模拟结果发现风速为10 m/s,20 m/s时火焰都有所偏移,16 m/s时火焰居中,温度高,25 m/s时火焰温度降低。送粉风速较低时(10 m/s、12 m/s)烟气有冲刷喉口上部左侧壁的趋势,当送粉速度为20 m/s时,炉膛出口可燃气体浓度最小。结果显示送粉风速为16 m/s和20 m/s时燃烧效果较好。     

地埋管换热器周围土壤热湿迁移的实验研究

文章搭建了土壤热湿迁移实验装置,并利用该装置研究了不同进口流体温度、土壤体积含水率条件下,地埋管换热器周围土壤温度场、湿度场的变化特性。分析结果表明:随着进口流体温度逐渐升高,土壤温度、湿度的最大值均逐渐升高,土壤温度、湿度的下降速率逐渐加快;土壤的初始体积含水率越大,热源对土壤温度的作用半径越大,当土壤的初始体积含水率分别为0%,35%时,热源对土壤温度的作用半径分别为280,380 mm;当土壤与地埋管之间的径向距离为0～225 mm时,土壤含水率主要受温度梯度的影响,随着该径向距离逐渐增加,土壤含水率逐渐升高,当土壤与地埋管之间的径向距离为225～380 mm时,土壤含水率会受到温度梯度与含水率梯度的共同作用,使得水分逐渐向热源方向逆向迁移,当土壤与地埋管之间的径向距离为225 mm时,土壤含水率出现峰值,当土壤与地埋管之间的径向距离大于380 mm时,温度梯度对土壤水分迁移的影响逐渐减弱,土壤含水率逐渐降低,当土壤与地埋管之间的径向距离为525 mm时,土壤含水率保持稳定。     

土壤-空气换热系统流动与供冷性能的计算模拟

以土壤热传导方程和K_1—ζ湍流模型为基础,采用温度场叠加法对土壤—空气埋管换热系统进行了三维动态数值模拟,计算了埋管出口温度随时间的动态变化。研究了埋管管长、管径、风速等对换热器出口温度的影响。模拟结果表明：①换热器出口温度随外界气温和土壤表面获得的辐射能的周期性变化而呈周期性变化；②随着管长增加和管径减小,出口温度降低,而且受气温和太阳辐射的影响减小；③随着人口风速减小,出口温度降低且变化幅度减小,供冷性能提高。并进行了实验验证,实测数据与模拟结果之差不超过0.8℃。     

槽式腔体接收器多场耦合热性能模拟研究北大核心CSCD

文章针对一种槽式太阳能利用的倒梯形腔体接收器,采用ANSYS软件对腔体倾角、工质进口温度、环境风速变化造成的接收器热性能影响进行了模拟研究,并采用多场耦合的方式对其进行分析。结果表明:无风时,各管出口温度的变化趋势与腔体倒梯形结构相吻合,且进口温度T_(in)为353 K时的腔体热损失量是303 K时的6.9倍;腔体倾角θ_(cav)为30°,T_(in)为353 K时,中间管受扰流影响程度随风速增大逐渐减小,且风速V_(w)为3.0 m/s时的总热损失量是0.8 m/s时的2.8倍;在腔体变倾角研究中,当T_(in)为353 K,V_(w)为1.5 m/s,θ_(cav)为90°时的热损失是0°时的12倍。     

Analysis of heat transfer and frost layer formation on a cryogenic tank wall exposed to the humid atmospheric air

In this paper heat transfer characteristics and frost layer formation are investigated numerically on the surface of a cryogenic oxidizer tank for a liquid propulsion rocket, where a frost layer could be a significant factor in maintaining oxidizer temperature within a required range. Frost formation is modeled by considering mass diffusion of water vapor in the air into the frost layer and various heat transfer modes such as natural and forced convection, latent heat, solar radiation of short wavelength, and ambient radiation of long wavelength. Computational results are first compared with the available measurements and show favorable agreement on thickness and effective thermal conductivity of the frost layer. In the case of the cryogenic tank, a series of parametric studies is presented in order to examine the effects of important parameters such as temperature and wind speed of ambient air, air humidity, and tank wall temperature on the frost layer formation and the amount of heat transfer into the tank. It is found that the heat transfer by solar radiation is significant and also that heat transfer strongly depends on air humidity, ambient air temperature, and wind speed but not tank wall temperature.     

Solar collectors for the Beirut climate

The performance of solar collector systems is optimized for the Beirut climate with respect to the following parameters: angle of tilt and orientation, plate emissivity and number of glass covers. The Beirut hourly ambient temperature, wind speed, wind direction and the monthly averaged hourly and daily global, beam and diffuse radiation are used in the calculation of the useful harnessed energy, collector's efficiency, storage-tank temperature and other relevant parameters.

The optimal thermal performance of the solar collector in Beirut is obtained for a south-facing collector all the year with a slope angle equal to latitude +15° in winter and latitude −15° in the summer. The use of a selective surface of low emissivity in the collector gives a higher useful energy gain of about 25% in summer and 10% in winter as compared with a nonselective plate surface. The collector's daily efficiency, under optimal conditions, varies from 55 to 65% depending upon the month of the year.     

Simulation of the daily change of soil temperature under different conditions

In this paper, by employing a mathematical model which was established to describe the physical mechanisms of heat and mass transfer in unsaturated porous media, some comparisons between the daily changes of soil temperatures under five different conditions have been conducted. The numerical simulations show that the ambient temperature, solar radiant heat flux, soil initial water content, ambient wind speed, atmospheric relative humidity, and soil porosity exert significant effects on the daily change of soil temperature. The differences between the daily changes of soil temperatures in those cases are also discussed and analyzed, which may provide some useful information for agriculture applications. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(6): 533–544, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10106     

Periodic heat transfer in sunlit moist ground: Evaluation of rates of moisture evaporation

This paper present an analysis of the periodic heat transfer in moist ground exposed to periodic solar radiation and atmospheric temperature. This analysis yields a novel method for the evaluation of daily/hourly moisture evaporation from the ground. It is found that in harsh climates a significant amount of moisture ( 4 lb/ft²/day) is evaporated from sunlit ground near the sea and in windy regions. The rate of moisture evaporation decreases with relative humidity and increases with wind speed.     

Performance of a single-basin solar still

A single-basin solar still having a base area of 1.5 m² (1.5 m × 1.0 m) was designed and fabricated from galvanized steel sheet with an inclined glass cover. The unit was insulated with Staropor. Hourly and daily measurements of the still productivity, temperature of water, glass cover, and ambient air were recorded during both summer and winter seasons. Efficiency of the still was found to be independent of solar radiation, however, an increased diffused radiation lead to slight decrease in its efficiency. An increase in still productivity was observed with the increase in ambient temperature and decrease in wind velocity.     

The Effect of Climatic Parameters on the Heat Transfer Mechanisms in a Solar Distillation Still

This paper deals with a simple and inexpensive solar desalination configuration. The performance of a simple solar still operating under Maltese climatic conditions is analyzed both theoretically and experimentally. The internal and external heat transfer modes of the distillation unit are examined. This paper concludes that the distillation rate in a simple solar distiller increases with ambient temperature and solar radiation, even though the condensation capacity of the glass is reduced. A higher wind speed decreases the evaporation and condensation processes. The energy fractions within the solar still have also been analyzed. The simulations and the experiments conclude that the glass components handle the bulk of the heat transferred in a solar still, namely, radiation, evaporation, and condensation, and thus the distillation efficiency is enhanced by improving the thermal and optical properties of the glass.     

利用太阳能制取海盐传热过程的试验分析

测试了盐田卤水、土壤和环境芭及同期太阳辐照度等数据，计算了卤未与土壤间的导热，卤水以空的辐射换热和卤水对环境的对流换热，并分析了它们对卤水蒸发速率的影响。结果表明，卤水与土壤间采取良好的隔热措施，抑制卤水对土壤的传热，可将卤 水的蒸发速率近１０％。同时进行了试验验证。     

Study on shallow solar pond applications at Tehran

In this study, a theoretical model which is validated experimentally is used to predict the performance of a shallow solar pond in Tehran. The theoretical and experimental results show good agreement. The maximum hourly water temperature of the shallow solar pond is found to lag behind the maximum hourly ambient temperature and solar radiation by 1–2 and 3.5 h, respectively.The maximum monthly daily-average water temperature follows the trend of the monthly daily-average solar radiation but leads the monthly daily-average ambient temperature in one month. The shallow solar pond, with 10-cm water depth, cannot be used as a thermal source in winter but can be used for many thermal applications in summer. With 5-cm water depth, the shallow solar pond can be used as a thermal source for low heat applications in most of the winter but can be used, even for moderate applications, where high temperature up to 95°C is obtained in summer. Using a reflector makes the 10-cm depth shallow solar pond useful for low heat applications and the 5-cm depth useful for moderate heat applications in most of the winter. Using a double cover top glazing is found to have no effect on improving the system performance.     

太阳辐射强度和温度对增强型冷却塔换热性能影响的研究

太阳能增强型自然通风冷却塔(SENDDCT)作为一种新型空气冷却系统,可利用太阳能提高自然通风冷却塔的效率。利用FLUENT软件建立太阳能增强型冷却塔的三维模型,研究太阳辐射强度和环境温度对其性能的影响;同时设计并搭建太阳能增强型冷却塔的实验系统,在实际天气条件下研究太阳辐射强度对其换热性能的影响。模拟结果表明太阳辐射强度一定时,通过换热器的空气流速以及换热率都会随着环境温度升高而降低。太阳辐射强度为500 W时,环境温度由10℃升到40℃,换热率由1.106 kW下降到0.281 kW。实验结果表明有辐射时冷却塔入口处的平均空气温度要比无辐射时的高5℃;实验期间无辐射时的平均换热率为0.682 kW,有辐射时的平均换热率为0.794 kW,即利用太阳能将平均换热率提高了0.112 kW。     

太阳能辅助二氧化碳热泵热水系统实验研究

利用实验的方法,研究了太阳辐照度、外界气温和风速、初始水温、蒸发器出口温度和压力等对太阳能辅助二氧化碳热泵热水系统运行状况和COP的影响。实验结果表明,系统COP随初始水温的升高而增大;太阳辐照度、外界温度和风速对热泵系统性能的影响主要体现在对系统循环水温的影响;在一定范围内,蒸发压力和蒸发温度越高,热泵系统的COP越大。