污水源热泵系统污水侧声空化防除垢与强化换热特性研究

基于污水源热泵系统,建立超声波声空化防除垢与强化换热动态实验台,对污水流速为0.45、0.65、0.84、0.91、1.29和1.74 m/s,换热管内径为15 mm/20 mm,超声波功率为250 W/500 W,声空化作用时间为0～60 min,污垢含水率为35%～95%以及不同的污水黏度和温度等特性参数下开展实验研究,分析计算污水侧换热管的污垢增长特性和防除垢规律以及强化换热效果的实验数据,对比研究特性参数对污水侧换热管运行特性的影响。研究表明:在较大的换热管径或污水流速下,污水侧换热管积垢速率和结垢率均偏低,除垢率和传热系数均偏高;在较大的超声波功率下,除垢率效果更好,传热系数效率显著提高;在较高的污垢含水率下,传热系数降低及其效率提高明显;随声空化作用时间的增加,除垢率和传热系数均显著提高,但在约50 min时趋于稳定,当流速为1.29 m/s时,传热系数效率提高百分比达到53.4%;另外,污水黏度和污水温度对污水侧换热管运行特性也有一定影响。总之,声空化应用在污水源热泵系统污水侧防除垢与强化换热方面是可行的。     

基于TRNSYS的地埋管/换热水箱复合冷热源运行特性分析

针对地下工程水环热泵空调系统,提出采用地埋管与换热水箱并联的方式作为复合冷热源,可同时实现红外伪装和节能要求。为探讨该复合冷热源系统的运行换热特性,基于TRNSYS软件建立地埋管、换热水箱数理模型,首先对流体流速、进口温度等影响因素进行模拟分析,结果表明:在小流速情况下,增大流体流速可明显提高地埋管和换热水箱和换热效果;管内、外换热温差越大,换热器换热效果越好;在相同流速下,螺旋盘管的传热系数DN15DN20DN25DN32。随后针对3种不同平、战负荷比工况,对复合冷热源的流量配比进行优化分析,结果表明:只要流量配比控制适当,复合冷热源可满足战时空调系统的供冷需求。复合冷热源之间存在最佳流量配比,β值越大,最佳流量配比的值越小,且不受环路流量与进口水温变化的影响,当β为1.0、1.5、2.0时,最佳流量配比依次为4/6、3/7、2/8。     

纳米流体在内置扭带外螺纹管内的流动和换热特性

为了研究纳米流体在内置扭带外螺纹管内的流动与传热特性,在Re(雷诺数)为2 000~12 000的范围内,分别对质量分数为0.1%、0.2%、0.3%、0.4%、0.5%和0.6%的Cu、Al、A1_2O3、Fe_2O_3、多壁碳纳米管和石墨纳米流体在内置扭带外螺纹管内的流动与对流换热特性进行了实验研究。实验结果表明:在相同Re下不同纳米流体都存在最佳浓度比0.5%,其中Cu-水纳米流体的换热性能最好但是摩擦阻力较大,石墨的换热性能和摩擦阻力方面的综合性能最好。内置扭带外螺纹管较光管在换热性能方面提高了50.32%,但摩擦阻力系数也相应增加。根据实验数据对热性能系数进行了综合分析,得到了石墨纳米流体内置扭带外螺纹管对流换热以及摩擦阻力系数关联式,其计算值和实验值有较好的吻合度。     

基于渗流有限长线热源地埋管管群换热分析与优化

地下水运动对地埋管换热器具有重要的影响,通过对比分析渗流无限长与有限长线热源模型,指出考虑轴向传热的有限长线热源更为合理。基于温度场的叠加原理,建立渗流有限长线热源的管群换热模型,并通过耦合管内流体流动,构建管群换热的优化模型。采用位置布置优化措施后,18孔梅花布置比16孔正方形布置的面积利用率提高12.5%,承担相同的负荷其系统的COP值提高7%-10%;16孔管群采用负荷分配优化策略后,其管群的土壤温度场不平衡率大大降低,管群平均温度约降低7.6%,COP提高约10%。     

污水源热泵机组夏季运行工况实测与分析

以采用城市原生污水作热泵冷热源的北京悦都大酒店为实验基地,以该项目的热泵机组为研究对象,连续测得了污水进出口温度,机组蒸发器侧、冷凝器侧的进出口温度和压缩机耗电量.根据测试数据,考察了热泵机组在以城市污水为热源时的工作性能,在测试期间机组运行稳定,夏季的平均性能系数(COP)为4.31,为污水源热泵系统的设计和工程应用提供理论参考和工程借鉴.     

纳米流体对U型深埋管换热特性影响的研究

结合西安市某个埋深为2505 m的U型深埋管换热系统,在原位实验验证的基础上建立三维全尺寸数值计算模型,进而模拟分析埋管内热流载体种类对强化埋管换热的效果。埋管内热流载体除常规使用的水外,还选用Al₂O₃/水、CuO/水和SiO₂/水3种纳米流体。通过改变这3种纳米流体的纳米颗粒体积浓度分别为0.1%、0.3%、0.5%及1.0%,分析纳米流体种类及纳米颗粒体积浓度对埋管换热的影响。研究结果表明,就整个埋管的换热而言,纳米流体的强化换热效果较小。因此,通过改变埋管热流载体来达到埋管强化换热的方法并不高效合理。     

纳米流体在车用机油冷却器中的强化换热试验研究

为了探求新型冷却介质--纳米流体的换热效果,制备了不同粒子体积分数的氧化铝有机纳米流体,并在车用机油冷却器中进行了换热性能的试验研究.研究结果表明:添加纳米粒子能够有效提高纳米流体基础液体的换热能力,且换热能力随着粒子体积分数的增加 而增高.在不同温度和温差条件下,粒子体积分数为5%的纳米流体的传热量和换热系数均超过常规冷却介质(水和防冻液).纳米流体的黏度和流动阻力亦随着粒子体积分数增加而增加.当冷、热介质的进口温差不变时,提高冷却介质的进口温度能在明显增强换热能力的同时大幅度降低流动阻力,并且纳米流体换热能力的增幅要高于防冻液和基础液体.     

翅片管汽轮机油冷却器传热性能试验研究

用实验的方法研究了翅片管油冷却器的传热和流动阻力性能。试验管为肋化系数为10.6的高肋管。将实验结果与理论计算值进行比较,并根据威尔逊法及最小二乘法原理,得出了管外流体油和管内流体水的对流换热关系式、流动阻力关系式。结果表明:本试件以内表面计的传热系数达到了1 000~3 000 W/(m2℃),是光滑管的5~6倍,与具有相同壳体直径的光滑管油冷却器相比,节省了三分之二的换热体积。在相同换热器体积下,高肋管的管子布置量约是光滑管的60%左右,因此在相同换热器体积下高肋管的换热容量是光滑管的3~4倍。     

基于线热源理论的垂直U型埋管换热器传热模型的研究

基于经典常热流线热源理论,通过引入叠加原理、阶跃负荷及孔洞热阻思想将其发展为能够适用于变热流埋管换热与地源热泵系统模拟的变热流线热源模型,并与改进的经实验与理论验证的圆柱源理论模型进行了比较与分析。结果表明:所发展的变热流线热源模型能够有效地模拟地下埋管的换热过程,可作为地下垂直U埋管换热过程的计算模型,为地源热泵地下埋管换热器的设计计算及地源热泵系统的模拟提供了一种新的简单而又实用的计算方法。     

细圆管内氧化铜颗粒悬浮液流动与对流换热的实验研究

实验研究了氧化铜纳米悬浮液在内径为0.68mm不锈钢细圆管内迫流动和对流换热。氧化铜纳米颗粒悬浮液的质量分数W为0.02-0.06，分别去离子水和水－纳米颗粒悬浮液的流动，传热特性进行了实验测定。实验结果表明：所研究尺度下，层流向湍流过渡早于常规大尺度流动，悬浮液的压降要大于去离子水的；纳米颗粒的质量分数越大，压力降也越大，悬浮液的对流换热系统随颗粒质量分数的增大而增大。     

Experimental Investigations of Pool Boiling Heat Transfer Characteristics on a Vertical Surface Using CuO Nanoparticles in Distilled Water

Experiments were conducted on pool boiling heat transfer using dilute dispersions of CuO nanoparticles in distilled water at and above atmospheric pressure. Pool boiling characteristics of CuO nanofluid were studied at different pressures and concentrations. Characterization of the heating surface was done both qualitatively and quantitatively by taking the scanning electron microscopy (SEM) images and by subsequent measurement of surface roughness of the heater. SEM images of the heater surface showed nanoparticle deposition on the heater surface, suggesting surface modification. Thorough visualization showed microcavities on the heater surface, which provide an excellent location for nucleation sites enhancing heat transfer. However, these microcavities, once filled up with the suspended nanoparticles, reduced active nucleation sites, deteriorating the boiling heat transfer coefficient. Based on the experimental investigations it was concluded that there is an optimum thickness of nanoparticles coating at which heat flux is maximum and beyond this coating boiling heat transfer coefficient decreases. At higher pressures, boiling heat transfer coefficient and specific excess temperature remained nearly the same. This showed that pressure has negligible or no role to play in boiling heat transfer using nanofluids.     

塑料管单效溴化锂吸收式制冷机系统设计研究

进行溴化锂吸收式制冷机塑料换热装置及制冷机系统的结构设计,塑料换热装置的传热管采用聚四氟乙烯塑料管,传热管布置为阿基米德螺线状盘管,将实验传热系数与理论计算传热系数进行比较。比较结果表明：冷凝器、蒸发器实验传热系数低于理论计算值,吸收器的实验传热系数高于理论计算值。     

重力热管单线内螺纹分布强化换热实验研究

采用实验方法,研究了不同的内螺纹分布和油浴温度等因素对热管换热特性的影响。实验选用的热管材料为紫铜,外径16 mm,壁厚3 mm,长度为200 mm,传热工质为水,充液率为20%。实验结果表明:在同一油浴温度下,内螺纹重力管的启动特性要优于光滑重力热管。对比不同油浴温度下,布置内螺纹能够有效地降低热管的工作温度。实验选型的内螺纹仅布置在蒸发段不会提高热管的换热系数,而在绝热段和冷凝段布置内螺纹则能够使换热系数显著提升,且随油浴温度的增加,换热系数线性增加。     

An experimental heat transfer study for periodically varying-curvature curved-pipe

Experiments were conducted for water flowing through a varying-curvature curved-pipe inside a larger diameter straight pipe to form a double-pipe heat exchanger with water as the working medium. The heat transfer coefficients were obtained using the Wilson plot method. The effects of the Dean, Prandtl, Reynolds number and the curvature ratio on the average heat transfer coefficients and the friction factors are presented. A higher Dean number results in a higher heat transfer rate. It is found that the heat transfer rate may be increased by up to 100%, as compared with a straight pipe, while the friction coefficient increased by less than 40%. Therefore, it is promising to use S-shaped pipes instead of straight pipes for the performance enhancement for a heat exchanger such as a solar collector.     

Role of heat pipes in improving the hydrogen charging rate in a metal hydride storage tank

Metal hydrides can store hydrogen at high volumetric efficiencies. As the process of charging hydrogen into a metal powder to form its hydride is exothermic, the heat released must be removed quickly to maintain a rapid charging rate. An effective heat removal method is to incorporate a heat exchanger such as a heat pipe within the metal hydride bed. In this paper, we describe a two-dimensional numerical study to predict the transient heat and mass transfer in a cylindrical metal hydride tank embedded with one or more heat pipes. Results from a parametric study of hydrogen storage efficiency are presented as a function of storage tank size, water jacket temperature and its convective heat transfer coefficient, and heat pipe radius and its convective heat transfer coefficient. The effect of enhancing the thermal conductivity of the metal hydride by adding aluminum foam is also investigated. The study reveals that the cooling water jacket temperature and the heat pipe's heat transfer coefficient are most influential in determining the heat removal rate. The addition of aluminum foam reduces the filling time as expected. For larger tanks, more than one heat pipe is necessary for rapid charging. It was found that using more heat pipes of smaller radii is better than using fewer heat pipes with larger radii. The optimal distribution of multiple heat pipes was also determined and it is shown that their relative position within the tank scales with the tank size.     

Jet impingement and spray cooling using slurry of nanoencapsulated phase change materials

Polymer encapsulated nano phase change materials (paraffin) in particulate form (nano PCM) are added in water to enhance the heat transfer performance of jet impingement and spray cooling. The nano PCM particles absorb heat when paraffin changes from solid to liquid phase. The encapsulation prevents paraffin leakage and agglomeration. The volume fraction of nanoparticles plays an important role on pressure drop and heat transfer. Slurry with 28% particle volume fraction enhances heat transfer coefficient by 50% and 70% when compared to base solution for jet impingement and spray cooling, respectively. The structural integrity of shell encapsulation has been demonstrated by repeated use in a closed loop.     

Improvements of gravity assisted wickless heat pipes

The performance of conventional gravity assisted heat pipes and modified heat pipes with a separator in the adiabatic section is investigated experimentally. Heat pipes with a three layered wick in the evaporator section, in addition to the separator, are investigated. The performance of the modified heat pipes was compared to a reference gravity assisted heat pipe. Experiments were conducted on heat pipes of three lengths with a common diameter at constant evaporator and condenser lengths. The effect of varying the adiabatic length was, thus, investigated distinctly in normal heat pipes and in modified heat pipes with a separator. Water was employed as the working fluid in all heat pipes. The experimental program included five inclination angles and a heat flux range form 5 to 32 kW/m². The presence of the adiabatic separator caused a marked improvement in all heat pipes tested for all lengths and inclination angles. A pronounced reduction in heat pipe evaporator temperature was obtained, which is accompanied by an improvement in the heat transfer coefficient. A correlation was developed for prediction of the heat transfer coefficient for gravity assisted heat pipes with an adiabatic separator. The correlation took into consideration the effect of the varying adiabatic length. The correlation was in good agreement with the experimental data.     

Numerical study on heat transfer characteristics of double tube heat exchangers with alternating horizontal or vertical oval cross section pipes as inner tubes

In this paper, a numerical study on the flows in parallel and counter flow double tube heat exchangers with the inner tubes being either alternating horizontal or vertical oval cross section pipes or circular pipes is presented. The results include temperature and pressure contours and velocity vectors at several selected cross sections, axial averaged Nusselt number distributions and distributions of overall heat transfer coefficient and heat transfer enhancement factor versus three different parameters. The computation shows that the introduction of the inner alternating oval tube produces axial vortices in both the inner and outer tube flows, and the tube’s heat transfer performance is improved as a result. In general, the counter flow arrangement returns a higher level of overall heat transfer coefficient than the parallel flow arrangement. However, in terms of the magnitude of heat transfer enhancement, the performance of the parallel flow arrangement is slightly better than that of the counter flow.     

Critical Depth of Buried Isothermal Circular Pipes

We address the two-dimensional heat conduction problem due to a periodic array of isothermal pipes buried in a conductive medium. The upper surface of the medium is subjected to convection with a uniform heat transfer coefficient, and the lower surface is insulated. Similar to the concept of critical thickness associated with a slab embedded with isothermal strips, we show that there exists a critical depth such that the heat transfer rate is maximized. As the Biot number tends to infinity, the critical depth approaches zero for a single pipe buried in a semi-infinite medium. For a periodic array of isothermal pipes, there is also a critical Biot number beyond which the critical depth is zero. Furthermore, insulating the pipes reduces the critical depth, and the heat transfer rate does not vary significantly with respect to the depth.     

热管置入式墙体在实际建筑中的传热特性研究

该文在热管置入式墙体稳态传热实验和动态传热模拟的基础上,于天津地区搭建实际环境下热管置入式墙体测试系统。该测试系统基于2个相同结构尺寸房间,在其中一间的南外墙上置入热管,测试期间用电暖器维持室内18 ℃。依据测试数据对实际环境下墙体的动态传热特性进行分析。结果表明,相比于普通墙体,热管置入式墙体的内表面温度提升率为3.4%,蓄热能力提高;热管置入式墙体平均延迟时间为11.12 h,比普通墙体滞后约0.50 h;热管置入式墙体平均衰减倍数为89.37,比普通墙体大;测试期间热管置入式墙体节能率为25.9%。