余隙率对内置扭带换热管传热性能影响的数值研究

为了研究管内插入扭带对换热的影响,建立了余隙率分别为0.1,0.15,0.2,0.25,0.3的内置扭带换热管流体流动的三维物理模型,采用RNGk-ε湍流模型对这几种余隙率的扭带换热管的流动和传热特性进行了数值模拟,分析了插入不同余隙率扭带换热管的换热效果和流动阻力。结果表明,Nu数和摩擦阻力系数均随着余隙率的增大而减小,在低Re数的时候内置扭带有明显的经济性,特别是余隙率为0.1的扭带换热管综合性能最高,强化传热效果最为明显。     

横纹槽管内插断续扭带复合强化传热的实验研究

本研究以导热油为工质,在层流和过渡流(Re7 000)范围内研究横纹槽管内插三种不同规格的断续扭带和同扭率(Y=4.13)连续扭带的流动与强化换热特性。将实验数据进行回归分析得到了阻力系数和Nu的实验关联式,为复合强化传热的计算提供了理论依据。研究表明:横纹槽管内插扭带的综合换热性能优于内插相同扭带的光管,横纹槽管内插长度为66 mm的断续扭带的综合换热性能要优于同条件下内插连续扭带的光管。实验结果可为换热器的改造和新型换热器的设计提供理论依据。     

外切圆弧波壁管半径比对传热及流动特性的影响

为了研究波壁管外切圆弧半径比i对管内流体换热及流阻特性的影响,分别建立了i=0.1、0.5、1.0、5.0、10.0和相同尺寸参数的正弦波壁管几何模型,分析了管内流场和温度场,并用性能评价因子PEC对不同波壁管的综合换热性能进行了评价。结果表明:i越小,速度场与温度梯度场间的协同程度越高,换热能力越强;Nu随i的增大而减小,降幅随Re的增大而减小;0.1i1.0时摩擦阻力系数与i成正比,1.0i10.0时摩擦阻力系数与i成反比,当i=1.0时摩擦阻力系数出现最大值; PEC随Re增加而增大,圆弧波壁管的PEC高于正弦波壁管。     

热管换热器内部流动与换热的数值模拟

单螺旋翅片热管管束的翅片结构中心对称处存在流体流动及换热不足,因此,将单螺旋翅片结构改为新型双螺旋翅片结构,并利用Fluent软件模拟分析改进后流场与温度场的变化情况。结果表明:在Re=500~6 500时,与未加翅片相比,单螺旋翅片热管换热增强33%~51%,摩擦阻力系数增加6%~24%;双螺旋翅片热管换热增强69%~84%,摩擦阻力系数增加19%~48%。且双螺旋结构的综合性能明显优于单螺旋结构的综合性能,且在Re=2 000时性能表现最优。根据场协同原理可知,双螺旋翅片结构对流体的强烈扰动可促使流体的速度矢量与温度梯度矢量协同程度更好。综合比较得知双螺旋翅片热管更有利于强化换热。     

半圆形微通道内纳米流体流动与传热特性

为了研究流体流经半圆形微通道的传热与流动特性,对去离子水、Cu-水纳米流体及Al-水纳米流体在21个当量直径为612μm的平行半圆形微通道热沉(微型散热片)中的流动与对流换热特性进行了实验研究。研究发现:与截面为矩形的常规形状相比,半圆形微通道也具有很好的换热效果,与去离子水相比,添加Al和Cu纳米颗粒的纳米流体压降损失增大。当纳米流体的质量浓度为0.5%时,在微通道换热器中的纳米流体效应由于粘度过大等原因发生了恶化,并且这种恶化在高流速下也出现了。根据实验数据得到了半圆形微通道内低浓度纳米流体的层流对流换热以及摩擦阻力系数关联式,对热性能系数的分布曲线进行了综合分析,研究结果对于集成高效芯片散热系统设计具有重要意义。     

双向扭曲管传热与流阻特性及其多目标优化

以水为工质,对双向扭曲管湍流状态下传热与流阻特性进行数值计算和准确性实验验证,分析了其强化换热机理及截面尺寸a与导程S对其传热流阻的影响,并在其基础上对双向扭曲管结构参数进行多目标优化以及努塞尔数和阻力系数的关系式拟合。结果表明:相同工况下,双向扭曲管的综合换热性能优于光管和扭曲管,在Re小于20 000时具有较好的强化效果,且高出扭曲管3.92%;流体旋转流动的方向周期性改变,进一步强化了对流传热;当a增加到一定程度,管内的湍流程度不再增加;相同Re下,η、Nu和f对导程S的变化反应较灵敏。在Re=2 300～20 000、介质为水时,拟合关系式具有较高的准确性,且通过优化得到了结构的最优组合。     

自转清洗扭带管对流传热强化机理的实验研究

具有传热强化功能的自转螺旋扭带清洗防垢技术发展较快。应用激光测速仪LDV(Laser Doppler Velocimeter)实验研究自转清洗扭带管内流体的湍流特性。结果表明：在自转扭带竹带动下,管内流体的流动结构发生了反常态的变化。在近管壁环形区域内流体的轴向分速度明显比管中心区域的高,轴向湍流度比无白转扭带时大;切向分速度随半径的增大而增大,并且存在很大的径向湍流度。这些结果初步说明了自转螺旋扭带管对流传热强化的机理是：管内由扭带带动形成的强制旋流和轴向平行流叠加而形成的螺旋流动,以及近管壁环形区域内流速的增大,不仅加强了边界层流体的扰动以及边界层流体与主流流体的混合,并且使边界层厚度减簿,从而才使管内的对流传热得以强化。本文试验研究的结果为自转螺旋扭带管内对流传热强化机理的深入理论研究提供实验基础。     

Cu-H_2O纳米流体流动与强化换热的数值模拟研究

模拟纳米流体在三维管道中的流动和强化传热过程,运用数值计算方法研究纳米流体的流动特性和传热机理,探究不同纳米颗粒体积分数和不同纳米颗粒大小在不同雷诺数(Re)下对纳米流体的流动和传热特性的影响。基于DPM模型对纳米流体在圆管中的对流换热进行了数值模拟研究,研究结果表明,在一定范围内,每增加0.5%的体积分数,纳米流体的传热性能平均增强7.82%。随着纳米颗粒的减小,纳米流体的传热系数不断增加。     

内置异形等间距扭带的管内传热与流动特性对比

数值研究了空气在内置异形等间距扭带管内的传热与流阻特性,对模拟结果进行了实验验证,然后分析了结构对空气传热与流阻特性的影响;进一步通过对局部流场、温度场和绝对涡量的分析,揭示了其强化传热机制。结果表明:模型与实验值吻合较好,最大相对偏差约为8%,在大多数Re(雷诺数)范围内,异形等间距扭带相对传统等间距扭带提高了空气传热综合性能,在Re 10 000~20 000范围内,最大传热综合性能约为1.08,交错布置的等间距扭带对传热综合性能的提升没有其它等间距扭带明显。等间距扭带能够强化空气传热主要在于通过诱导二次流,加强了近壁区流体与主流区流体的置换程度,均化了流场,并且能够以自旋流的形式在下游维持较长的距离。     

矩形微通道中流体流动阻力和换热特性实验研究

以去离子水为流体工质,对其在矩形微尺度通道中的流动阻力和传热特性进行了实验研究。通过测量流量、进出口压力和温度等参数,获得了流体流过微通道时的摩擦阻力系数、对流换热过程中的热流通量和N u等。微尺度通道中流体流动的摩擦阻力系数较常规尺度通道中的摩擦阻力系数小,仅是常规尺度通道中摩擦阻力系数的20%~30%;且流动状态由层流向湍流转捩的临界R e也远小于常规尺度通道的。微尺度通道中对流换热的N u与常规尺度通道的显著不同。流量较小时,N u较常规尺度通道中充分发展段的小;随着水流量的增加,微通道的N u迅速增加,并很快超过常规尺度通道的N u,表现出微尺度效应。热流通量对微尺度通道中对流换热N u存在影响,其影响规律在不同流速条件下呈不同趋势,流速较小时,N u基本保持不变;而在流速较大时,N u随热流通量增加而呈增加趋势。     

两种产自云南的生物质燃烧性质研究

用热天平对产自我国云南省的两种生物质－烟梗和木屑的热解和燃烧性质进行了研究,比较了燃烧特点,利用ＩＣＰ分析了灰成分,并用热台显微镜测定了灰熔点,讨论了它们的灰污特性。结果表明：木屑的着火性能比烟梗的差一些,但木屑的燃烧速度比烟梗的高,燃尽性能比烟梗的好;两者均具有很强的灰污趋向,但烟梗由于有较高的灰含量和钾含量,其实际灰污性比木屑的强;两者形成灰污的机理也不一样。     

In situ photodeposition of metalloid Ni2P co-catalyst on Mn0.5Cd0.5S for enhanced photocatalytic H2 evolution with visible light

Developing a suitable and low-cost co-catalyst is highly desired for promoting photocatalytic water splitting of H₂ production. Herein, we adopted a simple in situ photodeposition strategy by coupling Mn_0.5Cd_0.5S with non-noble co-catalyst Ni₂P to construct Ni₂P/Mn_0.5Cd_0.5S composites and achieved obviously improved H₂ amount (31.83 mmol/h/g) in visible-light region, which is nearly 2.8 times than that of pure Mn_0.5Cd_0.5S. Such photocatalytic performance is in a relatively superior position among Mn_xCd_1?xS-based photocatalysts. The apparent quantum efficiency of Ni₂P/Mn_0.5Cd_0.5S-7 composites reaches 32% at 420 nm. Through optical and photoelectrochemical measurements, a possible mechanism was proposed, it was found that the interface between metalloid Ni₂P and Mn_0.5Cd_0.5S contacts closely, which facilitates transfer and separation of charge carriers, thus promotes the reduction of H⁺ to H₂. This study provides a new design of cut-price, high-efficiency photocatalyst for H₂ evolution.     

LPG与汽油燃料微粒排放的时效特性研究

介绍了应用液化石油气(LPG)和汽油燃料时，微粒排放时效特性的对比研究。试验在一台四冲程、水冷125mL、点燃式单缸电喷发动机上进行。试验结果表明：对于两种燃料，随着沉降时间的增加，微粒排放的粒数不但大幅度下降，其粒度分布规律也发生了明显的变化；LPG和汽油燃料排出的小粒径部分微粒在空气中存在时间短，在排出后15min左右即可大部分挥发或凝聚形成大粒径粒子，而挥发消失的颗粒物比率LPG大于汽油；80％以上的大粒径部分(≥O．237um)粒子是在沉降过程中形成的；在测试条件下，汽油燃料微粒排放的大粒径部分粒子比LPG燃料容易沉降或挥发，随着沉降时间的延长，LPG与汽油微粒排放物的粒度分布特性趋于同一水平。     

Thermal and emission characteristics in a tangentially fired boiler model furnace

Tangentially fired furnaces are vortex‐combustion units and are intended for use as low NO formation furnaces. NO characteristics inside these furnaces depend on many parameters. The present study investigates numerically the problem of pollution in a real furnace of a 1699 MW tangentially fired boiler having 32 burners. NO formation contour maps in a tangentially fired furnace are presented. The study covered different combustion air temperature values, different fuel–air ratios and different cases of tripped burners. Available experimental measurements were used for validating the calculation procedure. The details of the temperature and NO fields were obtained from the solution of the conservation equations of mass, momentum and energy and transport equations for scalar variables in addition to the equations of the turbulence model. The equations governing the NO formation were solved to calculate the NO distribution. The simulation provided more insight on the correlation between the total NO concentration and the maximum furnace temperature and furnace average temperature. The results have shown that the furnace average temperature and NO concentration decrease as the excess air factor increases for a given air mass flow rate. As the combustion air temperature increases, furnace temperature increases and the thermal NO concentration increases sharply. The results show that the temperature distributions are significantly distorted by tripping any of the burners. The results show that tripping one or two burners either adjacent or opposite or tripping four results in regions of high temperature gases close to the walls. Heat absorptions in super‐heater and economizer are greatly influenced by combustion air temperature and excess air factor and are slightly influenced by burner tripping. Copyright © 2009 John Wiley & Sons, Ltd.     

Study on the heat transfer and flow characteristics in a spiral-coil tube

In the present study, numerical and experimental results of the heat transfer and flow characteristics of the horizontal spiral-coil tube are investigated. The spiral-coil tube is fabricated by bending a 8.00 mm diameter straight copper tube into a spiral-coil of five turns. The innermost and outermost diameters of the spiral-coil are 270.00 and 406.00 mm, respectively. Hot and cold water are used as working fluids. The k-ε standard two-equation turbulence model is applied to simulate the turbulent flow and heat transfer characteristics. The main governing equations are solved by a finite volume method with an unstructured nonuniform grid system. Experiments are performed to obtain the heat transfer and flow characteristics for verifying the numerical results. Reasonable agreement is obtained from the comparison between the results from the experiment and those obtained from the model. In addition, the Nusselt number and pressure drop per unit length obtained from the spiral-coil tube are 1.49 and 1.50 times higher than those from the straight tube, respectively.     

辽宁省场次暴雨时空分布特征研究

基于辽宁省基础水文数据库,选取资料完整可靠的481处雨量站作为代表站,筛选出1965～2014年间最大点降水量超过200mm的超标准暴雨124场,采用线性倾向估计、M-K检验、小波分析、克里金插值等技术,系统分析了辽宁省场次暴雨时空分布特征。结果表明,辽宁省超标准暴雨年发生次数在0～6次/年之间,年际变化基本持平,并呈现出3～5、13～15年2个主要周期;年内尺度上,超标准暴雨集中发生于7～8月,且主峰降水多集中于24h之内;空间分布上,暴雨中心多位于东南部地区,出现频次由东南部向西北部递减。暴雨中心位于辽西地区的场次暴雨多发生于7月;暴雨中心位于辽东地区的场次暴雨在7、8月均有发生,且位于东南部地区的暴雨多发生于8月。     

锅炉单相受热面动_静态特性通用计算方法

在流体微远追踪计算方法的基础上,提出了锅炉单相受热面热力过程动,静态特性的通用计算方法。利用该算法模型既可以进行锅炉单相受热面的动态过程分析,又能够完成锅炉单相受热面热力特性的静态校核计算,并能十分方便地确定出锅炉单相受热面热力参数的动,静态分布情况。     

An experimental study on performance and emission characteristics of a hydrogen fuelled spark ignition engine

In the present paper, the performance and emission characteristics of a conventional four cylinder spark ignition (SI) engine operated on hydrogen and gasoline are investigated experimentally. The compressed hydrogen at 20  MPa has been introduced to the engine adopted to operate on gaseous hydrogen by external mixing. Two regulators have been used to drop the pressure first to 300 kPa, then to atmospheric pressure. The variations of torque, power, brake thermal efficiency, brake mean effective pressure, exhaust gas temperature, and emissions of _NOx${\mathrm{NO}}_x$, CO, _CO2${\mathrm{CO}}_2$, HC, and _O2${\mathrm{O}}_2$ versus engine speed are compared for a carbureted SI engine operating on gasoline and hydrogen. Energy analysis also has studied for comparison purpose. The test results have been demonstrated that power loss occurs at low speed hydrogen operation whereas high speed characteristics compete well with gasoline operation. Fast burning characteristics of hydrogen have permitted high speed engine operation. Less heat loss has occurred for hydrogen than gasoline. _NOx${\mathrm{NO}}_x$ emission of hydrogen fuelled engine is about 10 times lower than gasoline fuelled engine. Finally, both first and second law efficiencies have improved with hydrogen fuelled engine compared to gasoline engine. It has been proved that hydrogen is a very good candidate as an engine fuel. The obtained data are also very useful for operational changes needed to optimize the hydrogen fueled SI engine design.     

EGR对可变压缩比SI发动机动力性和经济性的影响

基于一台可变压缩比SI单缸发动机,在不同EGR率(0~20%)、不同压缩比(8∶1~11∶1)条件下,对发动机动力性和经济性进行对比分析研究。结果表明:发动机的有效热效率、IMEP和功率在EGR率较小时变化均不明显,但随着EGR率继续增大,三者都迅速下降。而增大压缩比会使有效热效率、IMEP和功率在不同程度上都有所升高。同时,在全负荷或定负荷工况下,有效燃油消耗率总体上随EGR率的增大而升高,但在EGR率较小时变化比较平缓。随着压缩比增大发动机有效热效率越高,燃油利用率越高,有效燃油消耗率逐渐降低。     

多孔泡沫金属散热模型数值模拟与分析

多孔泡沫金属在紧凑型换热器领域具有广阔的应用前景。本文对具有随机结构的多孔介质中的流动和传热进行了耦合分析。采用有限元法研究了不同孔隙率(30%、50%、70%和90%)泡沫多孔材料的传热特性和阻力特性。利用 作为性能评价指标,即h越大,Δp越小,换热性能越好。通过改变孔隙度和入口流速从而分析整体的换热性能。结果表明,对于换热特性,泡沫金属的对流换热在高度方向上存在明显的温度梯度,边界层沿流动方向可以很好地发展,在高流速下具有更好地换热能力。随着孔隙率的增大,流体扰动变少从而局部速度将会减小,因此换热量也会降低。对于阻力特性,随着孔隙率的减小,内部压力显著增大,压力损失也显著变大。孔隙率为70%时,多孔泡沫金属具有更高的均匀性,同时通过实验验证了模型具有一定的有效性。