A study of metal/die interfacial heat transfer behavior of vacuum die cast pure copper

High pressure die casting copper is used to produce rotors for induction motors to improve efficiency.Experiments were carried out for a special"step-shape"casting with different step thicknesses.Based on the measured temperature inside the die,the interfacial heat transfer coefficient(IHTC)at the metal/die interface during vacuum die casting was evaluated by solving the inverse problem.The IHTC peak value was 4.5×10^3-11×10^3 W·m^-2·K^-1 under the basic operation condition.The influences of casting pressure,fast shot speed,pouring temperature and initial die surface temperature on the IHTC peak values were investigated.Results show that a greater casting pressure and faster shot speed could only increase the IHTC peak values at the location close to the ingate.An increase of pouring temperature and/or initial die surface temperature significantly increases the IHTC peak values.

Interfacial heat transfer behavior at metal/die in finger-plated casting during high pressure die casting process

Heat transfer at the metal-die interface has a great influence on the solidification process and casting structure. As thin-wall components are extensively produced by high pressure die casting process(HPDC), the B390 alloy finger-plate casting was cast against an H13 steel die on a cold-chamber HPDC machine. The interfacial heat transfer behavior at different positions of the die was carefully studied using an inverse approach based on the temperature measurements inside the die. Furthermore, the filling process and the solidification rate in different finger-plates were also given to explain the distribution of interfacial heat flux(q) and interfacial heat transfer coefficient(h). Measurement results at the side of sprue indicates that qmax and hmax could reach 9.2 MW·m^(-2) and 64.3 kW ·m^(-2)·K^(-1), respectively. The simulation of melt flow in the die reveals that the thinnest(T_1) finger plate could accelerate the melt flow from 50 m·s^(-1) to 110 m·s^(-1). Due to this high velocity, the interfacial heat flux at the end of T_1 could firstly reach a highest value 7.92 MW·m^(-2) among the ends of T_n(n=2,3,4,5). In addition, the q_(max) and h_(max) values of T_2, T_4 and T_5 finger-plates increase with the increasing thickness of the finger plate. Finally, at the rapid decreasing stage of interfacial heat transfer coefficient(h), the decreasing rate of h has an exponential relationship with the increasing rate of solid fraction(f).

Experimental Analysis of Heat Transfer Behavior inside Heat Pipe Integrated with Cooling Plates

This work used experimental methods to study heat transfer behavior inside a heat pipe and found that heat transfer behavior inside the heat pipe was changed due to its integration with cooling plates. This change caused the heat pipe to have copper-like heat transfer behavior. Experimental performances first built a CPU simulator with maximum heat power 300 W in accordance with the ASTM standard as heat source and measured temperature distribution by using infrared thermography and thermocouple thermometer. Observation of heat transfer behavior inside heat pipe influenced by its integration with cooling plates used color schlieren technique. A commercial CPU heat pipe cooler was also used as reference object in this work. Integration of the heat pipe with cooling plates causes the heat pipe to have the copper-like heat transfer behavior. The results indicate that rebuilding the bare heat pipe’s heat transfer behavior is the best solution for improving cooling efficiency of the heat pipe cooler.

Determination of Interfacial Heat Transfer Behavior at the Metal/Shot Sleeve of High Pressure Die Casting Process of AZ91D Alloy

The interfacial heat transfer behavior at the metalJshot sleeve interface in the high pressure die casting （HPDC） process of AZ91D alloy is carefully investigated. Based on the temperature measurements along the shot sleeve, inverse method has been developed to determine the interfacial heat transfer coefficient in the shot sleeve. Under static condition, Interracial heat transfer coefficient （IHTC） peak values are 11.9, 7,3, 8.33kWm-2K-1 at pouring zone （S2）, middle zone （S5）, and end zone （510）, respectively. During the casting process, the IHTC curve displays a second peak of 6.1 kWm-2 K-1 at middle zone during the casting process at a slow speed of 0.3 ms 1 Subsequently, when the high speed started, the IHTC curve reached a second peal〈 of 12.9 kW m-2K-1 at end zone. Furthermore, under different slow casting speeds, both the calculated initial temperature （TIDs） and the maximum temperature （Tsimax） of shot sleeve surface first decrease from 0.1 ms-1 to 0.3 ms-1, but increase again from 0.3 ms-1 to 0.6 ms-1. This result agrees with the experimental results obtained in a series of ＂plate-shape＂ casting experiments under different slow speeds, which reveals that the amount of ESCs decreases to the minimum values at 0.3 m s-1 and increase again with the increasing casting slow speed.

Heat transfer behaviors of some supercritical fluids: A review

Supercritical fluids(e.g.,hydrocarbon fuels,water,carbon dioxide,and organic working medium,etc)have been recognized as working media to improve thermal efficiencies in power cycles and energy conversion,and have been used or selected as the working fluids in engineering fields such as aerospace,nuclear power,solar energy,refrigeration,geothermal energy,chemical technology,and so on.To better understand the interesting characteristic or abnormal behaviors of supercritical fluids,most valuable research works(including experimental results and numerical studies)from domestic and abroad have been documented.As such,this paper presents a comprehensive review on heat transfer behaviors of some supercritical fluids in engineering applications.This review focuses on recently available articles published mainly from 2016 up to the present time.The common problems(i.e.,heat transfer enhancement and heat transfer deterioration particularly for the supercritical hydrocarbon fuels)in the supercritical field are summarized and some perspectives on future prospects are also included.

Evolution of interfacial heat transfer,contact behavior and microstructure during sub-rapid solidification of molten steel with different hydrogen contents

Typical Q235 low-carbon steel samples with different hydrogen contents(0.0004,0.0008,and 0.0013 wt.%)were prepared by adjusting the environment humidity and moisture.The effects of hydrogen on interfacial heat transfer,contact behavior,and microstructure evolution were investigated using a novel droplet solidification technique.The results revealed that when the hydrogen content increases from 0.0004 to 0.0013 wt.%,the maximum heat flux between the molten steel and cooling substrate decreases from 8.01 to 6.19 MW/m^(2),and the total heat removed in the initial 2 s reduces from 10.30 to 8.27 MJ/m^(2).Moreover,the final contact angle between the molten steel and substrate increases from 103.741°to 113.697°,and the number of pores on the droplet bottom surface increases significantly from 21 to 210 with the increase in hydrogen.The surface roughness of the droplet bottom surface increases from 20.902 to 49.181 pm.In addition,the average grain size of the droplet increases from 14.778 to 33.548 pm with the increase in the hydrogen content.The interfacial contact condition becomes worse due to the escape of hydrogen from the steel matrix during the cooling process,which leads to the reduction in the interfacial heat transfer and the increase in the grain size.

Development and application of inverse heat transfer model between liquid metal and shot sleeve in high pressure die casting process under non-shooting condition

To predict the heat transfer behavior of A380 alloy in a shot sleeve, a numerical approach(inverse method) is used and validated by high pressure die casting(HPDC) experiment under non-shooting condition. The maximum difference between the measured and calculated temperature profiles is smaller than 3 °C, which suggests that the inverse method can be used to predict the heat transfer behavior of alloys in a shot sleeve. Furthermore, the results indicate an increase in maximum interfacial heat flux density(q_(max)) and heat transfer coefficient(h_(max)) with an increase in sleeve filling ratio, especially at the pouring zone(S2 zone). In addition, the values of initial temperature(T_(IDS)) and maximum shot sleeve surface temperature(T_(simax)) at the two end zones(S2 and S10) are higher than those at the middle zone(S5). Moreover, in comparison with fluctuations in heat transfer coefficient(h) with time at the two end zones(S2 and S10), 2.4-6.5 kW ·m^(-2)·K^(-1), 3.5-12.5 kW ·m^(-2)·K^(-1), respectively, more fluctuations are found at S5 zone, 2.1-14.7 kW ·m^(-2)·K^(-1). These differences could theoretically explain the formation of the three zones: smooth pouring zone, un-smooth middle zone and smooth zone, with different morphologies in the metal log under the non-shot casting condition. Finally, our calculations also reveal that the values of q_(max) and h_(max) cast at 680 °C are smaller than those cast at 660 °C and at 700 °C.

果香香韵单体在壁式加热卷烟中转移行为分析研究

为量化表征果香单体香原料在壁式加热卷烟中的调香可用性,基于气相色谱质谱联用技术和嗅觉感官阈值测定,分别研究了果香单体香料在壁式加热卷烟中的转移分布、加香贡献度及逐口释放行为。通过研究发现:1)果香单体香原料的加香贡献度D值差距甚大,其中D值最大的γ-辛内酯(12485.0)是苯甲醇(0.1)的10万倍;2)果香单体香料的主流烟气转移率不高,约7%~37%,但高沸点香料损失主要原因是冷凝截留而低沸点香料损失主要原因是抽吸过程中空气散失;3)果香单体香料的逐口释放量随抽吸口序增加有两种趋势,一种呈逐口递减趋势,另一种呈先增后递减趋势。其中庚酸乙酯,辛酸乙酯,癸酸乙酯,乙酰丙酸乙酯和苯乙酸乙酯属递减,肉桂酸乙酯,γ-庚内酯,γ-辛内酯,γ-癸内酯等属先增后减。

黄土地基中能量桩群桩承载变形性状模型试验

基于室内缩尺试验研究了含能量桩群桩在4次冷热循环过程中的桩身热力学特性及桩基承载变形特性变化规律,分析冷热循环对桩土温度、承台沉降、桩顶荷载及桩身摩阻力的影响。试验结果表明:冷热循环过程中能量桩较浅位置处桩身及桩侧土温度始终高于较深处,热循环过程中能量桩的传热效率高于冷循环过程。冷热循环会使承台顶面的工作荷载及群桩中各基桩的桩顶荷载发生往复变化,相应地引起承台发生往复倾斜,热循环引起的承台倾斜稍大于冷循环。热循环引起的能量桩桩身上部摩阻力为负,下部为正,冷循环时则相反。

CMT电弧增材制造铝合金传热传质与熔池行为数值模拟

为研究冷金属过渡(cold metal transition,CMT)电弧增材制造铝合金传热传质与熔池流动特性,基于Fluent软件建立了三维CMT电弧增材制造数值模型.模型中,采用动网格技术模拟焊丝竖直方向上的往复运动,利用流体体积法捕获气/液界面,焓-孔隙率法追踪固/液界面,并施加周期热量输入和阶段电弧力作用来等效电弧放电行为,研究分析了焊道成形传热传质过程与熔池动态行为.结果表明,焊道成形初期,熔池余高和坡度较大,形貌犹如半个球体,成形后期热量积累造成焊道余高后方较前方略小,而后端熔宽较前端略宽;单滴过渡周期内,焊丝机械回抽对熔池表面流动影响最为明显,液桥断裂产生较大反冲作用于熔池;熔池内部则是电磁力作为主导驱动力产生一股顺时针环流,环流随燃弧阶段周期切换而不断加强与减弱,并基本贯穿整个过渡周期,使得熔池内部热对流更加充分.模拟结果与试验结果显示吻合良好.

固体热载体热解中传热行为的Fluent模拟和实验

固体热载体热解(SHC)和外热式普通热解(CP)由于传热方式和热解挥发物经历温度场的不同,其传热行为和热解行为存在较大差异。采用小型密闭固定床反应器,以核桃壳(WS)为原料、石英砂(QS)为固体热载体,在石英砂预热温度800℃、QS与WS质量比9∶1条件下研究了传热行为和热解规律。采用实验和Fluent模拟两种方式研究了固体热载体热解过程的传热行为,并将温度场解耦为热解温度(T_(WS))和挥发物温度(T_(QS-h))。研究结果表明:相比于模拟所得的T_(WS)和T_(QS-h)最大值(490和612℃),实验数据(460和508℃)更小,实验过程存在散热现象。T_(WS)平均值(T_(WS))和T_(QS-h)平均值(T_(QS-h))的温差的实验值和模拟值分别为39和72℃,说明低温挥发物在逸出过程中经过高温石英砂层时发生剧烈的二次反应。相比于CP,SHC热解方式下的焦炭得率和气体得率更高,分别为67.42%和12.51%;油得率和水得率更低,分别为8.69%和11.38%。同时,SHC热解方式下的油中极轻馏分(VLO)、轻馏分(LO)和重馏分(HO)含量显著增多,残留分(CR)含量显著减小,油的品质得到提高。

电弧增材制造传热传质数值模拟技术综述

电弧增材制造过程涉及丝材的送入和熔化,熔融金属向熔池的过渡,熔池中液态金属的对流、凝固和成形。缺陷的形成与电弧增材制造过程中发生的复杂多物理场现象密切相关。因此,需要借助高保真数值模拟技术来深入理解这些物理现象,并将其作为优化工艺条件、制造高质量产品的理论依据。本文综述了电弧增材制造传热传质数值模拟涉及的关键技术,并对未来研究方向进行了展望:首先,介绍了几种典型的热源模型,鉴于电弧增材制造过程中熔池的形成与演变是多种驱动力共同作用的结果,分析了浮力、电磁力、表面张力、电弧压力、电弧剪应力模型对流体流动和熔池表面变形的影响;然后,总结了三种金属过渡模型,包括速度入口填充液态金属、指定位置添加球状质量源项以及直接建立固态金属焊丝;最后,介绍了常用的气液界面跟踪方法。

Seasonal Behavior of Pavement in Geothermal Snow-Melting System with Solar Energy Storage

A two-dimensional unsteady heat transfer model of pavement of geothermal road snow-melting system (GRSS) with solar energy storage is established and numerical simulation is carried out based on annual hourly meteorological data and boundary conditions. Simulated results show that ground surface temperature and heating flux decrease with the increase of buried depth, but increase with the increase of fluid temperature in winter. Heat-extracted amount and efficiency drop with the increase of fluid temperature in summer.Compared with ambient temperature, solar radiation has more direct influence on the heat-extracted flux of pipe walls of GRSS in summer. The relationships among maximum and idling snow-melting load, the rate of snowfall, ambient temperature and wind speed are made clear, which provides necessary references for the design and optimization of a practical road snow-melting system.

Combustion behavior and influence mechanism of CO on iron ore sintering with flue gas recirculation

The properties of circulating gas have a significant effect on sintering with flue gas recirculation,and the influence of CO in sintering process was investigated.The results show that the post-combustion of CO conducts in sinter zone when flue gas passes through the sintering bed,which releases much heat and reduces the consumption of solid fuel.The ratio of coke breeze can be reduced from 5% to 4.7% with 2% CO in circulating flue gas.In addition,with the increase of CO content in circulating flue gas,the combustion efficiency of fuel is improved,and the flame front is increased slightly while still matches with the heat transfer front.These are beneficial to increasing the maximum temperature and prolonging the high temperature duration,especially in the upper layer of sintering bed.As a consequence,the productivity,vertical sintering velocity and quality of sinter are improved.

Thermal Behavior of Externally Driven Spindle: Experimental Study and Modelling

This paper focuses on model development for computer analysis of the thermal behavior of an externally driven spindle. The aim of the developed model is to enable efficient quantitative estimation of the thermal characteristics of the main spindle unit in an early stage of the development process. The presented work includes an experimental validation of the simulation model using a custom-built test rig. Specifically, the effects of the heat generated in the bearings and the heat flux from the bearing to the adjacent spindle system elements are investigated. Simulation and experimental results are compared and demonstrate good accordance. The proposed model is a useful, efficient and validated tool for quantitative simulation of thermal behavior of a main spindle system.

Na_(2)SO_(4)循环流化床蒸发器中的颗粒碰撞行为和传热性能

将流化床换热防垢节能技术和Na_(2)SO_(4)蒸发过程相结合,设计并构建了1套Na_(2)SO_(4)循环流化床蒸发装置。选用质量分数为20%的Na_(2)SO_(4)溶液作为液相工质,采用聚甲醛(POM)和碳化硅(SiC)颗粒作为惰性固体颗粒,考察了颗粒类型、颗粒加入量(1%~3%)、循环流速(0.37~1.78 m·s^(-1))和热通量(7.29~12.14 kW·m^(-2))等操作参数对于颗粒的碰撞行为和蒸发器传热性能的影响。研究结果表明,POM和SiC颗粒的加入均可以强化传热。实验范围内,POM和SiC颗粒的最大增强因子分别为9.5%和13.4%,所对应的操作参数分别为ε=1%,u=1.78 m·s^(-1),q=7.29 kW·m^(-2)和ε=3%,u=0.37 m·s^(-1),q=12.14 kW·m^(-2)。液相和固相碰撞加速度信号的特征频率范围分别为0~1000 Hz和6000~17000 Hz;汽相的产生可以显著增加其他相的碰撞强度。颗粒加入量增加,POM颗粒的标准偏差增大;SiC颗粒的标准偏差在循环流速较低时先减小、后增大,而在循环流速较高时增大。2种颗粒的流动沸腾传热系数受颗粒加入量的影响较小。循环流速增加,2种颗粒的标准偏差和传热系数均增大。热通量增加,2种颗粒的传热系数均明显减小。颗粒加入量较小时,2种颗粒的标准偏差随热通量的变化不明显。颗粒加入量较大时,POM颗粒的标准偏差随着热通量的增加而增大;SiC颗粒的标准偏差随热通量的变化受循环流速的影响。

水平管内低质量流量超临界二氧化碳异常传热行为研究

研究了水平管内低质量流量超临界二氧化碳(S-CO_(2))异常传热行为,采用Fluent软件模拟了水平管内低质量流量条件下S-CO_(2)传热过程,分析了加热和冷却条件的异常传热行为和热流密度对传热影响。结果表明:热边界条件为压力8 MPa、质量流率200 kg/(m^(2)·s)、热值比q/G=0.2 kJ/kg时,S-CO_(2)管内流动冷却过程中上、下壁面温度均沿程降低,在S-CO_(2)主流温度达到拟临界温度时,距离入口551.0 mm处上壁面换热系数出现突变峰值,该处传热强化;S-CO_(2)管内流动加热过程中上壁面温度均先沿程升高,而后下降至395 K后缓慢上升,下壁面温度短暂降温后缓慢升温,距离入口69.5 mm处上壁面传热系数出现谷值,该处传热恶化;热流密度的增大使加热条件下换热恶化程度加剧,但对冷却换热并无明显影响。由此可见,特征截面的热物性分布是导致出现不同换热行为的主要原因。最后,基于低质量流量条件、热物性及浮升力影响,构建了预测超临界强化传热关联式,为超临界流体换热设备的设计与运行优化提供理论指导。

平行微通道内气泡撞壁分裂行为数值模拟

本文基于体积分数法(VOF)模型,对流动沸腾中气泡撞击加热肋面的过程进行了数值模拟,研究了不同流速和初始尺寸下气泡撞击行为对流动沸腾换热特性的影响。通道入口主流速度设为0.1 m/s和0.2 m/s,气泡无因次直径设为db*为1.6和2.0。结果表明,气泡在撞击过程中发生内凹,最终分裂为2个子气泡;在撞击时,气泡与撞击肋面端点处形成了薄液膜层,增强了蒸发效应,但同时使气泡更易破裂,从而造成局部干涸并形成高温;在较高流速下,气泡破裂现象会有所延迟,db*为1.6和2.0时,换热性能分别提高了2.57%和6.60%,较低流速下则分别减弱了25.98%和23.67%。

Experimental investigation of surface roughness effects on flow behavior and heat transfer characteristics for circular microchannels

This paper experimentally investigates the effect of surface roughness on flow and heat transfer characteristics in circular microchannels. All test pieces include 44 identical, parallel circular microchannels with diameters of 0.4 mm and 10 mm in length. The surface roughness of the microchannels is R= 0.86, 0.92, 1.02 lm, and the Reynolds number ranges from 150 to 2800.Results show that the surface roughness of the circular microchannels has remarkable effects on the performance of flow behavior and heat transfer. It is found that the Poiseuille and Nusselt numbers are higher when the relative surface roughness is larger. For flow behavior, the friction factor increases consistently with the increasing Reynolds number, and it is larger than the constant theoretical value for macrochannels. The Reynolds number for the transition from laminar to turbulent flow is about 1500, which is lower than the value for macrochannels. For the heat transfer property, Nusselt number also increases with increasing Reynolds number, and larger roughness contributes to higher Nusselt number.

黏土岩温度-渗流-应力耦合特性试验与本构模型研究进展

高放废物处置库、垃圾填埋场等工程中常常涉及到温度场(T)、渗流场(H)和应力场(M)的耦合作用的问题。从试验和理论模型两个角度综述国内外黏土岩温度-渗流-应力耦合特性的研究进展,主要包括其传热特性、温度影响下的渗流特性、变形、强度、蠕变特性。在此基础上,重点分析了黏土岩水-热迁移模型以及热-力耦合本构模型的适应性。基于上述认识,通过试验研究了比利时Boom clay在温度作用下的强度、渗透性、蠕变性等特征。结果表明:随着温度升高,Boom clay的强度有所降低,渗透性显著增强,蠕变速率明显加快。提出了适用于Boom clay的THM耦合弹塑性损伤模型,计算结果验证了模型能合理反映温度的影响。最后,探讨了黏土岩THM耦合机理研究的不足和今后的研究方向。