Influence of packed honeycomb ceramic on heat extraction rate of packed bed embedded heat exchanger and heat transfer modes in heat transfer process

Under the condition that the transient oxidation heat extraction process of coal mine ventilation air methane (VAM) is equivalent to a series of steady state process, the steady state heat extraction experiment platform is built. The influence of the honeycomb ceramic packed in heat extraction zone and its two-side space on heat extraction rate and heat transfer modes is investigated. The experimental results show that the honeycomb ceramic packed in heat extraction zone two-side space can always strengthen heat extract ion of heat exchanger by increasing gas physical flow velocity in bed and radiation heat exchanging area and disturbing heat exchanger leeward side flow field. The contradictory dual characteristic of the influence of the honeycomb ceramic packed in heat extraction zone on heat exchanger heat extraction rate determines that the honeycomb ceramic has no great influence on heat extraction rate and doesn't always strengthen heat exchanger heat extraction. Contribution of heat transfer modes on packed bed embedded heat exchanger heat extraction is investigated using the method of coating heat exchanger outer surface silver; the experimental result shows that 55% contribution of packed bed embedded heat exchanger heat extraction rate is from radiation when gas mass flow rate is 0.15 kg·s^− 1·m^− 2 and its temperature is 1113 k; with the gas temperature being increased further, radiation will become the main way of packed bed embedded heat exchanger heat extraction.     

蜂窝状陶瓷蓄热式热交换器的设计计算

依据填充球蓄热体传热的数学解析方法,经传热分析和类比,导出蜂窝状陶瓷蓄热器的设计计算方法。经实验验证表明,类推得到的蜂窝状陶瓷蓄热器的设计计算方法具有较高的精度,可以很好地满足工程应用的需要,为蜂窝状陶瓷蓄热器的设计计算提供了参考依据。     

Transient solution of temperature field of conjugate laminar forced convection heat transfer in functionally graded hollow cylinder

In this study, the numerical analysis of conjugate heat transfer of laminar flow in a functionally graded hollow cylinder (FGHC) made of metal/ceramic for a two‐dimensional fluid and wall conduction subject to Newton boundary condition is considered. The fluid and FGHC energy equations are coupled through the continuity of temperature and heat flux at the inner wall‐fluid interface while the outer surface is subject to convective heat transfer. The continuity, momentum, and energy equations of the fluid are discretized using the finite volume approach. The effects of fluid and functionally graded material parameters, such as volume fraction index, volume composition, time history, wall‐to‐fluid thermal diffusivity ratio, wall‐to‐fluid thermal conductivity ratio, Biot number, Peclet number, and Prandtl number are investigated on the temperature field in the FGHC. The result shows that on account of the inhomogeneity of the material property, the volume fraction index has a significant effect on the other parameters and the temperature variation along the thickness. The lower the volume fraction index, the higher the inner wall (metal side) temperature, and the temperature gradient along the thickness. However, except for the variation in the wall‐to‐fluid thermal conductivity ratio, the lower the volumetric fraction, the lower the outer wall (ceramic side) temperature distribution.     

不同纵向涡发生器流动传热的数值模拟

利用三维数值模拟的方法对带有3种异形纵向涡发生器的H型翅片椭圆管换热器的空气侧流动传热特性进行研究。基于H型翅片椭圆管束,讨论了在不同雷诺数下,纵向涡发生器的摆放位置、摆放攻角和形状对空气侧流动传热的影响。研究表明:纵向涡发生器能够将高能量的流体引向流速较低的壁面区域,使冷热流体之间的混合加剧,增强流体的湍流动能,进而达到强化传热的效果;与无纵向涡发生器的管束相比,带纵向涡发生器管束的传热效果有明显的提高;当纵向涡发生器后置时,换热器的传热效果最优;在雷诺数相同,攻角为30°时,流体的传热性能和阻力特性均达到最优;相同攻角摆放时,椭圆角矩形发生器的传热性能和阻力因子均优于其他两种形式的发生器。研究结果为烟气余热回收系统换热器传热性能强化提供理论依据。     

换热管内旋向自交叉转子强化传热性能研究

利用综合传热性能实验台,研究对比旋向自交叉转子、同向转子的换热器和光管的传热性能。结果发现:通过Webb性能比较方法,在同样面积和同等泵功率情况下,同向转子比光管换热器的换热量最大可提高为6.99%,而旋向自交叉转子比光管的换热量最大可提高8.11%,并计算推导了旋向自交叉转子与同向转子、光管之间的准则关联式;同时研究了换热量与转子外径的关系,结果发现换热量随转子外径变大而增多。     

扭曲椭圆管换热器壳程强化传热的数值研究

基于扭曲椭圆管的换热器是一种新型的新风系统换热器,针对扭曲椭圆管及其应用特点,设计了两种不同结构参数的新风系统换热器。应用FLUENT软件,在夏季工况下对两种不同结构参数的新风系统换热器壳程进行模拟分析,并通过与实验数据的对比,验证计算模型的可靠性。结果显示在相同体积流量下,随着壳程开孔面积的增大,对流换热系数h不断减小,压降Δp不断减小,综合性能系数h/Δp^1/3变化不明显;随着螺距的减小,对流换热系数h不断增大,压降Δp不断增大,综合性能系数h/Δp^1/3也不断增大;流场分析显示,扭曲椭圆管换热器壳程流道内,呈现出明显沿着扭曲椭圆管壁面的螺旋流,使得空气在流道内充分扰动,增强换热效果。     

多管型套管式换热器传热与流阻性能试验研究

多管型套管式换热器是在大尺寸外管的内部布置多根内管所构成的换热设备 ,与单根内管的套管式换热器相比 ,流量大幅增加 ,选用螺纹内管和管间折流板可以强化传热。对于多管型套管式换热器的传热性能试验 ,采用修正威尔逊法进行试验计算 ,得出了两种多管型套管式换热器的传热与流动阻力性能试验结果。     

几种带冷却结构的双层管换热及流动对比

对相同质量流量下的光管、双层光管、带冷却结构（肋、扰流柱、凹坑、螺旋通道）的双层管等不同结构的管流动进行了流固耦合三维数值模拟,获取了固体壁温的分布特征;对各结构下,外层壁冷热侧温差、冷气温升、流动特性及综合换热效率进行了研究分析。研究结果表明：相同质量流量下,带螺旋通道双层管的外层壁冷热侧温差最小、综合换热效率最高;凹坑结构双层管与双层光管的流动及换热特性相似,流阻较小但换热效果也较差;扰流柱和肋结构双层管的流动换热特性相近,其温度分布均匀性、换热量介于双层光管和螺旋通道双层管之间,其流阻大且综合换热效率低。     

两种用于HCPV/T的水冷换热器传热特性的对比实验研究

本文对两种适用于高倍聚光发电供热（HCPV/T）系统的多槽道和微通道水冷换热器进行了实验研究。利用模拟热源模拟了HCPV/T系统中光伏电池工作时的热流密度,分别研究了流量、壁面温度和输入电压对两种换热器传热特性的影响,并利用传热学理论对两种换热器的特点进行分析,获得了两种换热器努赛尔数Nu与雷诺数Re的拟合经验公式。实验结果表明,微通道换热器在低流量下有较强的换热能力,但在高流量下,换热能力无法随流量增大继续提高;多槽道换热器在低流量下换热能力不佳,但在高流量下仍可随流量增大继续提高。     

套管式中深层地埋管换热器传热建模及取热分析

为了研究无干扰换热条件下,中深层地热能的实际取热性能,文章通过数值模拟方法模拟计算了套管式中深层地埋管换热器的名义取热量。模拟结果表明,套管式中深层地埋管换热器的名义取热量随着钻孔深度、大地热流、循环水流量、当地大气年平均温度的增加而增加。套管式中深层地埋管换热器周围土层的地质条件分布也影响着中深层地埋管换热器的名义取热量,具体表现为浅层土层的导热系数越小,中深层地埋管换热器的名义取热量越大;深层土层的导热系数越大,中深层地埋管换热器的名义取热量也越大。通过调整地埋管换热器的相关参数,并选择合适的地埋管埋设地点等优化措施,可使套管式中深层地埋管换热器达到可观的名义取热量。     

Analysis of coiled-tube heat exchangers to improve heat transfer rate with spirally corrugated wall

Steady heat transfer enhancement has been studied in helically coiled-tube heat exchangers. The outer side of the wall of the heat exchanger contains a helical corrugation which makes a helical rib on the inner side of the tube wall to induce additional swirling motion of fluid particles. Numerical calculations have been carried out to examine different geometrical parameters and the impact of flow and thermal boundary conditions for the heat transfer rate in laminar and transitional flow regimes. Calculated results have been compared to existing empirical formulas and experimental tests to investigate the validity of the numerical results in case of common helical tube heat exchanger and additionally results of the numerical computation of corrugated straight tubes for laminar and transition flow have been validated with experimental tests available in the literature. Comparison of the flow and temperature fields in case of common helical tube and the coil with spirally corrugated wall configuration are discussed. Heat exchanger coils with helically corrugated wall configuration show 80–100% increase for the inner side heat transfer rate due to the additionally developed swirling motion while the relative pressure drop is 10–600% larger compared to the common helically coiled heat exchangers. New empirical correlation has been proposed for the fully developed inner side heat transfer prediction in case of helically corrugated wall configuration.     

Numerical studies on flow and heat transfer in membrane helical-coil heat exchanger and membrane serpentine-tube heat exchanger

The flow and heat transfer characteristics of synthesis gas (syngas) in membrane helical-coil heat exchanger and membrane serpentine-tube heat exchanger under different operating pressures, inlet velocities and pitches are investigated numerically. The three-dimensional governing equations for mass, momentum and heat transfer are solved using a control volume finite difference method. The realizable k-ε model is adopted to simulate the turbulent flow and heat transfer in heat exchangers. There flows syngas in the channels consisting of the membrane helical coils or membrane serpentine tubes, where the operating pressure varies from 0.5 to 3.0 MPa. The numerically obtained heat transfer coefficients for heat exchangers are in good agreement with experimental values. The results show that the syngas tangential flow in the channel consisting of membrane helical coils is significant to the heat transfer enhancement to lead to the higher average heat transfer coefficient of membrane helical-coil heat exchanger compared to membrane serpentine-tube heat exchanger. The syngas tangential velocity in the membrane helical-coil heat exchanger increases along the axial direction, and it is independent of the gas pressure, increasing with the axial velocity and axial pitch rise and decreasing with the radial pitch rise.     

Numerical study on the effects of design parameters on the heat transfer performance of coaxial deep borehole heat exchanger

Deep borehole heat exchanger (DBHE) is attracting attention intensively owing to much more geothermal extraction, higher efficiency for heat pumps, and lesser land demand compared with shallow borehole heat exchanger. DBHE is usually dipped into several thousand meters in the subsurface, having a complicated heat transfer with surrounding rock–soil. However, the heat transfer characteristics below surface under different conditions are rarely studied. In this study, a numerical model considering the comprehensive effects of geothermal gradients and heat loss from inner pipe was proposed. The model was validated with experimental data and Beier analytical solution. Based on the model, the effects of primary design parameters on the heat transfer performance below surface along the pipe were investigated. The results indicate that temperature at pipe bottom increases with inlet flow rate decreasing, while the heat load cannot be extracted fully to the surface because of the heat loss of inner pipe. When the inlet flow rates decrease from 41.39 to 4.52 m³/h, the heat loss ratio increases from 25.5% to 63.7%. It is an effective way of insulating inner pipe to reduce heat loss under low inlet flow rates. Increasing the velocity in inner pipe by lessening the inner pipe diameter can also decline the heat loss well. While by this way, the increasing pumping power resulting from the higher velocity in inner pipe has to be considered. This study is significant to effective optimization of DBHE and energy conservation of buildings.     

多孔介质中高温气体非稳态渗流传热数值计算

针对水平导管中填充颗粒物料层内的高温气体参流传热现象，考虑渗流与传热的相互作用并采用局部非平衡假设建立多孔介质中的瞬态渗流传热物理数学模型。研究不同情况下填充物料中的渗流速度和气固温度分布。计算结果表明，高温热气体对水平导管中移动颗粒料层的热渗透主要发生在渗流入口端区域，随着渗流时间延长，热渗透深度沿导管推进。增大入口渗流速度以及减小出料速度，将导致物料温度沿导管慢速下降，热渗透深度扩大，热渗透作用区域内的物料温度水平提高。在热渗透作用区域，孔隙率对流场和温度场有很大的影响。研究对于高温反应器的颗粒输运和给料器的设计与运行有一定的参考作用。     

椭圆热管传热与阻力性能分析

为解决圆形热管换热器换热效率低、积灰和腐蚀严重的问题,应用椭圆管束进行了改造。通过实例分析计算,效果显著：在相同折合速度下,椭圆热管换热器的传热系数明显高于圆形热管换热器;在相同总压降下,椭圆热管换热器的传热系数也比圆形热管换热器高得多;无论在烟气侧,还是在空气侧,同一速度下椭圆热管换热器的流阻损失明显下降．下降幅度达80％左右。提出了进一步改善积灰和腐蚀的措施。     

纵向涡发生器强化传热管的实验研究

介绍了一种高效强化传热管,沿传热管内壁轴向均匀排布三列成对的纵向涡发生器,在常壁温条件下进行加热空气在管内流动的冷却实验,研究强化传热管的传热和阻力特性。结果表明,在过渡流区管内置纵向涡发生器的强化传热大大增强,Nu增大为光管的2.02.3倍,阻力损失也相应有所增加,提出一种比较优化的发生器的形状设计,探讨了传热和阻力随设置间距变化的规律。     

Experimental investigation of the heat exchange to a compartmentalized plug flow

A two-phase (air–water), tubular counterflow heat exchanger was developed for industrial unitary process applications to realize an active heat transfer control. A controlled train of air plugs was injected in a stratified smooth water flow to yield a compartmentalized, continuous two-phase flow whose stability was carefully monitored. Related Nusselt numbers were evaluated for single- and two-phase flows, at the same flow regimes. This comparison shows how the traveling, controlled air plug “turbulators” allowed for a large heat transfer rate increase, for all observed heating rates and phase velocity ratios. The analysis includes a comparison with the available literature correlations, and the efficiency of the adopted heat transfer control is finally inferred while a range of flow regimes is identified which optimizes the heat exchange.     

Comparison of performance of heat regenerators: Relation between heat transfer efficiency and pressure drop

Heat regenerators transfer heat from one gas to another, with an intermediate storage in solids. The heat transfer surface for gas flow application should provide at the same time high surface area and low friction factor. Three geometries of heat transfer surface, monolith, stack of woven screens and bed of spheres, have been compared. Their performance was evaluated from the pressure drop of the heat regenerator working at a given heat transfer efficiency. The comparison was performed using numerical simulation and published measurements of heat transfer and flow friction characteristics. By adjusting the length and the period of the exchanger, it is possible to obtain the same heat transfer efficiency with the three geometries. Beds of spheres give very short and compact heat regenerators, working at high pressure drop. At the opposite, monoliths form long regenerators working at low pressure drop. Stacks of woven screens cover a wide range of performance: low porosity woven screens give high heat transfer efficiency and high pressure drop, while high porosity woven screens offer performance similar to that of the monoliths. Copyright © 2001 John Wiley & Sons, Ltd.     

新型蜂窝板式换热器实验与模拟分析

板式换热器在工业生产中广泛应用,采用数值模拟和实验相结合的方法对新型蜂窝板式换热器进行研究。结果表明:数值模拟和实验结果在合理的误差范围内,验证了数值模拟的可靠性。新型板式换热器内部蜂窝结构附近的速度场出现规律的周期性变化,流体湍流强度增加,Nu和阻力系数f随焊孔直径和蜂窝直径的增加而增大,通过对蜂窝板在3 000≤Re≤25 000的数据结果进行关联,得到了蜂窝板流动换热的准则关系式,为蜂窝板式换热器的优化提供了理论依据,并为工业生产提供参考。     

不同冷气/燃气温比下高温涡轮叶片旋流冷却流固耦合特性研究

采用流固耦合方法对燃气轮机高温涡轮叶片旋流冷却结构进行数值模拟分析。探究了不同冷气/燃气温度比条件下旋流冷却的流动与传热特性、叶片前缘区域固体温度、热应力以及热应变分布。研究表明：在进气腔入口雷诺数固定的条件下，随着温度比升高，冷气密度降低，冷气流速逐渐提升，同时湍动能升高，靶面努塞尔数逐渐升高；当温度比较低时冷气的流速较低、单位时间冷气带走的热量较少，当温度比较高时冷气温度较高、单位质量冷气所能吸收的热量有限，靶面处热流密度先升高后降低。受靶面热流密度分布影响，随着温度比升高，叶片前缘固体的温度、热应力以及热应变先降低后升高。