Computation of synthetic surface heat transfer coefficient of 7B50 ultra-high-strength aluminum alloy during spray quenching

According to inverse heat transfer theory, the evolutions of synthetic surface heat transfer coefficient（SSHTC） of the quenching surface of 7B50 alloy during water-spray quenching were simulated by the Pro CAST software based on accurate cooling curves measured by the modified Jominy specimen and temperature-dependent thermo-physical properties of 7 B50 alloy calculated using the JMat Pro software. Results show that the average cooling rate at 6 mm from the quenching surface and 420-230 ℃（quench sensitive temperature range） is 45.78℃/s. The peak-value of the SSHTC is 69 kW/（m^2·K） obtained at spray quenching for 0.4 s and the corresponding temperature of the quenching surface is 160 ℃. In the initial stage of spray quenching, the phenomenon called ＂temperature plateau＂ appears on the cooling curve of the quenching surface. The temperature range of this plateau is 160-170℃ with the duration about 3 s. During the temperature plateau, heat transfer mechanism of the quenching surface transforms from nucleate boiling regime to single-phase convective regime.

Spatial interfacial heat transfer and surface characteristics during gravity casting of A356 alloy

As one of the key boundary conditions during casting solidification process, the interfacial heat transfer coefficient (IHTC) affects the temperature variation and distribution. Based on the improved nonlinear estimation method (NEM), thermal measurements near both bottom and lateral metal-mold interfaces throughout A356 gravity casting process were carried out and applied to solving the inverse heat conduction problem (IHCP). Finite element method (FEM) is employed for modeling transient thermal fields implementing a developed NEM interface program to quantify transient IHTCs. It is found that IHTCs at the lateral interface become stable after the volumetric shrinkage of casting while those of the bottom interface reach the steady period once a surface layer has solidified. The stable value of bottom IHTCs is 750 W/(m^2·℃), which is approximately 3 times that at the lateral interface. Further analysis of the interplay between spatial IHTCs and observed surface morphology reveals that spatial heat transfer across casting-mold interfaces is the direct result of different interface evolution during solidification process.

Heat Transfer in a Liquid-Solid Circulating Fluidized Bed Reactor with Low Surface Tension Media

Heat transfer characteristics between the immersed heater and the bed content were studied in the riser of a liquid-solid circulating fluidized bed, whose diameter and height were 0.102 m (ID) and 2.5 m, respectively. Effects of liquid velocity, particle size, surface tension of liquid phase and solid circulation rate on the overall heat transfer coefficient were examined. The heat transfer coefficient increased with increasing particle size or solid circulation rate due to the higher potential of particles to contact with the heater surface and promote turbulence near the heater surface. The value of heat transfer coefficient increased gradually with increase in the surface tension of liquid phase, due to the slight increase of solid holdup. The heat transfer coefficient increased with the liquid velocity even in the higher range, due to the solid circulation prevented the decrease in solid holdup, in contrast to that in the conventional liquid-solid fluidized beds. The values of heat transfer coefficient were well correlated in terms of dimensionless groups as well as operating variables.

Time Dependent Surface Heat Transfer in Light Weight Aggregate Cement Based Materials

Surface temperature changes of building materials affect the calculation of heat flow and thus the energy use in heating and cooling. The surface heat transfer coefficient , limiting the heat flow between material surface and ambient air is normally taken as a constant. In this study we propose a time-dependent function . We estimate from unidirectional heat flow experiments with transient and steady-state conditions. Using temperature measurements and the conservation of energy at the surface including convective and irradiative boundary conditions, the value of was obtained both using Finite Difference and Taylor Polynomials methods. Numerical solutions of temperature distribution as function of time were improved with the obtained -functions compared to with constant . There were no clear difference between on different materials, and the final values observed were in the order of magnitude expected from the literature.

Numerical Modeling of Non-Similar Mixed Convection Heat Transfer over a Stretching Surface with Slip Conditions

In this paper, the heat transfer effect on the steady boundary layer flow of a Casson fluid past a stretching surface in the presence of slip conditions was analyzed. The stretching surface is maintained at a constant temperature. The boundary layer conservation equations, which are parabolic in nature, are normalized into non-similar form and then solved numerically with the well-tested, efficient, implicit, stable Keller-box finite difference scheme. The resulting equations are solved numerically by using the Kellerbox finite-difference method, and the expressions for velocity and temperature are obtained. They satisfy all imposed initial and boundary conditions and reduce to some well-known solutions for non-Newtonian fluids. Numerical results for velocity, temperature, skin friction and Nusselt number are shown in various graphs and discussed for embedded flow parameters. It is found that both velocity and temperature decrease with an increase of the Casson fluid parameter.

Calculation and experimental study on heating temperature field of super-high strength aluminum alloy thick plate

Stepped heating treatment has been applied to aluminum alloy thick plate to improve the mechanical performance and corrosion resistance.Accurate temperature control of the plate is the difficulty in engineering application.The heating process,the calculation of surface heat transfer coefficient and the accurate temperature control method were studied based on measured heating temperature for the large-size thick plate.The results show that,the temperature difference between the surface and center of the thick plate is small.Based on the temperature uniformity,the surface heat transfer coefficient was calculated,and it is constant below300°C,but grows greatly over300°C.Consequently,a lumped parameter method(LPM)was developed to predict the plate temperature.A stepped solution treatment was designed by using LPM,and verified by finite element method(FEM)and experiments.Temperature curves calculated by LPM and FEM agree well with the experimental data,and the LPM is more convenient in engineering application.

漫反射与非漫反射表面间的辐射传递系数快速计算

针对蒙特卡洛方法计算目标表面间辐射传递系数耗时过长的问题,提出了一种计算漫反射灰体与非漫反射灰体表面间辐射传递系数的快速方法。首先,利用蒙特卡洛方法在漫反射表面生成发射光束集并进行射线追踪,计算漫反射表面对目标表面的辐射传递系数;然后,在非漫反射表面生成发射光束并进行射线追踪,如果射线在多次反射过程中击中任意漫反射表面,则利用反射光束的漫反射特性,直接调用漫反射表面的辐射传递系数对反射能量进行分配,并停止射线追踪。对正六面体模型内表面(两个漫反射面元、四个镜反射面元)的辐射传递系数进行仿真,结果表明,当表面反射率ρ=0.4时,镜反射表面的辐射传递系数计算时间仅为蒙特卡洛方法的1/3;ρ=0.8时,相应的计算时间占比低于1/10,并且保持良好的计算精度。理论推导并分析了影响该方法计算速度的模型参数,结果表明:表面反射率越高,系统中漫反射表面占比越大,快速方法中单束光线的平均追踪次数较蒙特卡洛方法越少,非漫反射表面的辐射传递系数计算优势越明显。

地热储层岩体粗糙裂隙的热流耦合效应研究

为了解决地热开采涉及复杂的多物理场耦合问题,提高开采效率,本文针对开采过程中的渗流-传热问题以离散元软件3DEC构建岩体粗糙裂隙热-流耦合数值模型。考虑不同三维形貌特征的岩体裂隙,模拟水力开度为19.17μm在不同流速时的水-岩温度变化规律。结果表明:由于裂隙形貌的阻滞作用,粗糙面出水口温度下降较慢,出水口温度有所上升,模型达到稳态所需的时间随流速和对流换热系数的增大而减小。裂隙形貌对流体和岩体温度分布均有影响,粗糙裂隙面的冷锋形态和裂隙面的形貌密切相关。光滑裂隙热突破快于粗糙裂隙,增加裂隙面的粗糙度有助于延长热突破时间。粗糙裂隙面相对于光滑裂隙面的总热量提取率略有提升,流速和对流换热系数的增加显著提高总热量提取率。通过本文研究可以为地热能系统的设计提供重要参数和指导,能够提高地热能开发利用效率。

预制内外复合保温模壳拼装墙体热工性能研究

为探究预制内外复合保温模壳拼装墙体结构中的卡板连接件对墙体热工性能的影响,通过热工理论计算、试验研究、ANSYS有限元热分析模拟软件3种方法,对比分析了无连接件、不同数量连接件以及双面抹灰不同条件下墙体的传热系数。结果表明:在无连接件条件下,实际墙体热工试验测试值与热工理论计算值的误差较小;连接件数量越多,热桥效应越明显,墙体的热工性能越差;采用有限元模拟软件对墙体进行热工设计结果较精确,可作为设计依据;研究结果可为模壳拼装墙体在超低能耗建筑的应用及能耗分析提供借鉴。

Internal heat transfer coefficients in microporous media with rarefaction effects

The internal heat transfer of different gases in microporous media was investigated experimentally and numerically.The experimental test section had a sintered bronze porous media with average particle diameters from 11 μm to 225 μm.The Knudsen numbers at the average inlet and outlet pressures of each test section varied from 0.0006 to 0.13 with porosities from 0.16 to 0.38.The particle-to-fluid heat transfer coefficients of air,CO 2 and helium in the microporous media were determined experimentally.The results show that the Nusselt numbers for the internal heat transfer in the microporous media decrease with decreasing the particle diameter,d p,and increasing Knudsen number for the same Reynolds number.For Kn>0.01,the rarefaction affects the internal heat transfer in the microporous media.A Nusselt number correlation was developed that includes the influence of rarefaction.The computational fluid dynamics(CFD) numerical simulation was carried out to do the pore scale simulation of internal heat transfer in the microporous media considering the rarefaction effect.Pore scale three-dimensional numerical simulations were also used to predict the particle-to-fluid heat transfer coefficients.The numerical results without slip-flow and temperature jump effects for Kn<0.01 corresponded well with the experimental data.The numerical results with slip-flow and temperature jump effects for 0.01<Kn<0.13 are lower than the numerical results without rarefaction effects,but closer to the experimental data.The numerical results with rarefaction effects can accurately simulate the unsteady heat transfer in the microporous media.

The Heat Transfer Coefficients of the Heating Surface of 300 MWe CFB Boiler

A study of the heat transfer about the heating surface of three commercial 300 MWe CFB boilers was conducted in this work. The heat transfer coefficients of the platen heating surface, the external heat exchanger (EHE) and cyclone separator were calculated according to the relative operation data at different boiler loads. Moreover, the heat transfer coefficient of the waterwall was calculated by heat balance of the hot circuit of the CFB boiler. With the boiler capacity increasing, the heat transfer coefficients of these heating surface increases, and the heat transfer coefficient of the water wall is higher than that of the platen heating surface. The heat transfer coefficient of the EHE is the highest in high boiler load, the heat transfer coefficient of the cyclone separator is the lowest. Because the fired coal is different from the design coal in No.1 boiler, the ash content of the fired coal is much lower than that of the design coal. The heat transfer coefficients which calculated with the operation data are lower than the previous design value and that is the reason why the bed temperature is rather high during the boiler operation in No.1 boiler.

柴油喷雾撞击高温壁面的瞬态传热特性

通过瞬态测温技术对定容燃烧室中柴油喷雾撞击高温壁面的瞬态传热特性进行了研究.结果表明:初始壁面温度从250℃升高到550℃,壁面上传热模式由膜态蒸发转化为核态沸腾,进而转化为过渡沸腾,这种转化使表面热流密度和换热系数先增大后减小,且分别在440℃和400℃达到最大值.增大喷射压力、减小撞壁距离和环境温度有助于提高表面热流密度和换热系数.初始表面温度的升高增加了喷射压力和撞壁距离对传热特性的影响,但降低了环境温度的作用.以毕渥数和傅里叶数表示的量纲为1表面热流密度曲线形态高度相似,且揭示了喷雾撞击高温壁面过程中的内部传热机制.此外,建立了一种准确性高、适用壁面温度范围广的预测最大表面热流密度的量纲为1模型.

EXPERIMENTAL ANALYSIS OF THE TURBULENT TRANSFER COEFFICIENT FOR SENSIBLE HEAT IN THE SURFACE LAYER OVER THE QINGHAI-XIZANG PLATEAU

This study deals with the turbulent structure in the surface layer over the Qinghai-Xizang Plateau.Using gradient transfer and heat balance methods we have determined the nondimensional coefficient 1/(?)_m(?)h in the expression of turbulent transfer coefficient for sensible heat (K_h).It is found that the results are in good agreement with the 1/(?)_m(?)_h obtained by Pruitt,et al.The K_h at a height of 1m under cloudy and cloudless conditions is calculated.Finally,the ratio of K_h to momentum turbulent coefficient over the plateau is compared with those over plains.

基于改进集中热容法的TC17钛合金淬火表面换热系数测算

表面换热系数是TC17钛合金淬火数值分析研究的重要边界条件,其准确性影响工件淬火温度与应力场的分布精度。基于深埋热电偶动态实测的温降曲线,采用改进后的集中热容法,计算求解TC17钛合金淬火表面换热系数,并通过数值模拟验证了结果的可靠性。结果表明,TC17钛合金水浴淬火过程中会经历3个主要阶段:蒸汽膜阶段、核沸腾阶段及对流冷却阶段。表面换热系数在蒸汽膜阶段、核沸腾阶段会快速增大,随后在对流冷却阶段会逐渐减小,在第50 s达到峰值1299 W/(m^(2)·℃),此时淬火面温度为355℃。将换热系数代入数值模型中进行温度场计算验证,计算结果与实测温降曲线吻合较好,平均相对误差仅为1.4%。

高压涡轮导叶吸力面异型孔的对比研究

本文研究了高压涡轮导叶吸力面4种类型气膜孔在4组吹风比下的流动和换热特征,采用了数值模拟的方法获得了吸力面研究区域中4种气膜孔的绝热气膜冷却效率η、换热系数比hr/h0和净热通量减少量(NHFR),且分别进行对比分析。结果显示,簸箕孔因同时具有展向扩张角(β)增大冷气展向覆盖面积和流向扩张角(δ)提高射流贴壁性的优势,呈现出最大的绝热气膜冷却效率η和净热通量减少量(NHFR),但同时其增强了换热,使簸箕孔比其它3种气膜孔拥有最大的净热通量减少量(NHFR)的优势相较于其拥有最大的气膜冷效η的优势有所降低。而水滴孔相较于圆柱孔的优势不明显,故展向扩张角对气膜孔流动和换热特征的影响比流向扩张角大。

板式气冷器中超临界CO_(2)侧表面换热系数研究

为了探究板式气冷器中主导超临界CO_(2)侧表面换热系数h最值出现的因素及最值点的中移对平均换热系数大小的影响,采用基于MATLAB的换热器分布参数分析方法,研究了超临界CO_(2)侧表面换热系数h沿气冷器板片上的分布规律,并针对h最值出现的主导因素、改变板片尺寸和流量使最值点移动对平均换热系数的影响进行研究。结果表明,伪临界温度对板式气冷器中h最值的出现更具有决定性;一定压力下的C_(p)最大值会随着压力的增加不断增大,但伪临界温度逐渐降低;减小板片尺寸或者增大质量流量,可以使得平均换热系数增加,对应的换热量也会增大,而且由质量流量增加所带来的换热量增加幅度在3种工况下分别为12.72%,12.79%,11.75%,同时压降也会增加;当h最大值保持在6170 W/(m^(2)·K)或5120 W/(m^(2)·K)附近时,最值点中移不会使平均换热系数增加明显,而且对应的换热量也会减小。

真空绝热板内保温板缝漏热的测试与解决措施

真空绝热板是一种具有低导热系数和高防火性能的保温材料,因此在严寒和寒冷地区受到越来越多的关注。由于真空绝热板的尺寸一般是固定的,其用于建筑墙体需要拼接且容易产生热桥效应,通过实验研究几组真空绝热板发现板缝处的传热系数相较于板中心较高。当采用双真空绝热板与单层真空绝热板+聚苯乙烯泡沫板的保温墙体时,板缝处的传热系数明显降低且两种组合方式都可以满足75%标准的墙体节能要求,同时,文章还提出了减小真空绝热板产生热桥效应的外墙构造做法。

R245fa在丁胞型扁管内流动沸腾传热和压降特性的实验研究

实验研究了丁胞型扁管内R245fa的流动沸腾传热和压降特性,重点分析了干度和热流密度的影响。实验结果表明:随着干度的增大,表面传热系数呈先增大后减小的趋势,并且表面传热系数最大位置处对应的干度受热流密度的影响。当热流密度为2、5、10 kW/m2时,表面传热系数分别在干度为0.35、0.40、0.45时达到最大值,随后逐渐降低。增大热流密度对摩擦压降的影响很小,但有利于提升表面传热系数。分析表明,表面传热系数随着干度的增大而降低是由于核态沸腾受到抑制所致。对比R245fa和R134a的实验结果发现,干度和热流密度对表面传热系数的影响不同。此外,在相同工况下,R245fa和R134a的表面传热系数比值在0.64~1.31之间。

集中生活热水系统循环流量有关问题研究

梳理了不同版本建筑给水排水设计标准对生活热水管道热损失计算的有关规定,以及相关设计手册、标准图集等对生活热水管道热损失计算的方法,参考国家标准《工业设备及管道绝热工程设计规范》(GB50264-2013)的有关规定,对生活热水管道热损失的计算方法进行研究,并就有关问题提出了看法。

表面粗糙度对混凝土对流换热影响的试验研究

对流换热系数是混凝土重要的热工参数之一。目前关于混凝土结构的建筑节能、结构温度荷载及温度效应等温度相关的研究和工程应用中,在对流换热系数的选取上存在较大差异,且未考虑材料表面状况的影响,这直接影响了分析结果的准确性和可靠性。文中基于牛顿冷却定律和热平衡原理,设计和制备了一套混凝土表面对流换热系数的试验测定系统,以参数定量可控的方法开展不同粗糙度混凝土表面对流换热系数的试验研究,进一步改进和完善混凝土对流换热系数的定量计算方法。结果表明:所设计的测试系统和方法能很好地在构件尺度下测定混凝土表面的对流换热系数;表面粗糙度显著影响混凝土表面的对流换热,在10 m/s风速范围内,不同粗糙度的混凝土表面对流换热系数相差40.1%~77.4%;换热系数随表面粗糙度的增加而增大,且相比于自然对流,受迫对流受表面粗糙度的影响更大。基于试验数据的回归分析,得出了4种典型粗糙度混凝土表面对流换热系数的计算公式,同时给出了耦合风速和表面粗糙度影响的混凝土对流换热系数计算公式,并就工程中如何考虑混凝土表面粗糙度进行热交换系数取值给出了建议。