Heat transfer coefficients in finned-coil stirred-tank systems

Steady-state and transient heat transfer coefficients were determined for a finned-coil, stirred-tank system using flat-blade turbine impellers in water. Fin spacing, flowrate, hot water temperature and agitator speed were varied for Re between 20 000 - 110 000 for the agitated side. Individual transfer coefficients were calculated from the experimental overall coefficients. The correlation developed for predicting external transfer coefficients in agitated systems with finned-tube coils was tested against transient data.     

超临界CO2在水平螺旋管内的冷却换热特性

采用数值模拟和实验结合的方法研究超临界CO₂在水平冷却螺旋管内的换热特性。研究了质量流量、热通量及压力对传热系数的影响,并比较了直管和螺旋管在加热和冷却条件下的换热特性。结果表明,与直管相比,螺旋管对换热的强化作用随着质量流量的减小和压力的增加而增加。在相同热通量条件下,螺旋管的冷却换热的传热系数比加热的传热系数高,且由于浮升力的影响,加热条件下传热系数存在较大的振荡。     

Bubble size and mass transfer coefficients in dual-impeller agitated reactors

The study relates to the mass transfer and the bubble size in a non standard vessel equipped with various dual-impeller combinations. The effects of the rotational speed, gas flow rate, impeller type and diameter are investigated. The volumetric mass transfer coefficient k_La and the bubble size d_bs were studied. The liquid side mass transfer coefficient k_L and the volumetric interfacial area a were estimated separately. A comparison has been made with some existing correlations.     

Solid-liquid mass transfer coefficients in gas-solid-liquid agitated vessels

The research on mass transfer coefficients in solid-liquid agitated systems has received substantial attention in the past, due both to the interest in fundamental aspects of mass transfer between particles and turbulent fluids and to the importance of practical applications. In contrast, little information is available on solid-liquid mass transfer when a third gaseous phase is also dispersed into the system, in spite of the importance of the applications of gas-solid-liquid agitated systems. In this work a suitable dissolution technique was used to measure the solid-liquid mass transfer coefficient in gas-solid-liquid vessels stirred by either radial or axial impellers. The mechanical power dissipated by the stirrers at various agitation speeds and gas flow rates was also measured by means of a new technique. The mass transfer data obtained were found to be well correlated to the 0.25 power of the specific power dissipation, indicating that the Kolmogorov's theory of mass transfer applies to these systems, while no clear influence of the gas hold-up was ascertained.     

Particle-to-fluid heat transfer coefficients in miniporous media

The heat transfer between the solid particles and the fluid in a miniporous media was studied experimentally and numerically. The experimental test section was a sintered bronze porous media with an average particle diameter of 0.2 mm. Particle-to-fluid heat transfer coefficients in the miniporous media were determined experimentally using a transient single-blow technique. A lumped capacitance method (method 1) was used to calculate the particle-to-fluid heat transfer coefficients from the experimental data. The particle-to-fluid heat transfer coefficients were also calculated from a one-dimensional numerical analysis of the experimental data (method 2) which has been used by many researchers. The experimental results for h_sf using the two methods agreed very well. Therefore, method 1 is also deemed acceptable and much simpler than method 2. Three-dimensional numerical simulations using the CFD code FLUENT was also used to predict the particle-to-fluid heat transfer coefficients and to provide details of the fluid flow inside the miniporous media.     

非均匀热流边界条件下螺旋管内流动传热的场协同分析

建立了螺旋管内流动换热的物理数学模型,对均匀和非均匀热流边界条件下螺旋管内湍流传热进行了数值模拟。结果表明：当对螺旋管表面施加相同的加热功率时,均匀热流边界条件下湍流传热系数高于非均匀热流边界条件下的湍流传热系数,且均匀热流边界条件下螺旋管内的场协同角低于非均匀热流边界条件;非均匀热流边界条件时,在相同的De下,曲率较小的螺旋管传热系数大,且曲率较小的螺旋管内场协同角较小;同时,随着管径的增大,螺旋管内的传热系数也随之下降,但螺旋管内的场协同角随之增大。     

非均匀受热条件下螺旋管内流动沸腾换热特性

目前对螺旋管在其管外表面均匀受热,管内两相流动换热的研究已十分丰富;但是在其管外表面非均匀受热条件下,管内两相流动沸腾换热特性的研究鲜有报道。为了解决螺旋管在实际运用中遇到的非均匀受热问题、得到其换热特性,本文采用了实验的方法研究了卧式螺旋管周向非均匀受热条件下管内流动沸腾换热特性。其中实验工况范围为系统压力P=0.7~1.0MPa,质量流速G=181~364kg/(m²·s),质量干度χ=0.07~0.69。实验考察了螺旋管管外壁在两种非均匀受热条件下管内的两相流动沸腾换热系数与热流密度、质量流速、质量干度的关系,并与管周向均匀受热工况进行了比较。结果表明,在螺旋管外壁面“外半周绝热、内半周受热”情况下管内流动沸腾换热系数值最大,而管外壁面“内半周绝热、外半周受热”情况下最小。     

黑液蒸发器传热系数的实时计算

以四效混流蒸发器为背景，从系统的静态模型出发，提出了一种采用优化拟合的原理来计算蒸发器传热系数的方法。该方法可根据过程控制机采集的现场工作数据，实时地计算出蒸发器的传热系数。该算法适合在集散型系统中监控级机上运行，用于实时监控蒸发器的工作状况。     

固--固换热器传热特性研究

在自制的固—固套管式换热器中,以催化裂化催化剂为原料,对系统的循环特性、传热系数进行了实验研究。分析了床层密度、提升风量、床层温度、催化剂粒径等因素对传热过程的影响。实验结果表明,随着提升风量增加,表观气速增大,床层密度下降,传热系数下降;提高床层温度,减小催化剂的粒径,传热系数增加。     

Experimental and CFD Heat Transfer Studies of Al2Oa-Water Nanofluid in a Coiled Agitated Vessel Equipped with Propeller

This paper presents the heat transfer characteristics of A1203-water nanofluid in a coiled agitated vessel with propeller agitator. The experimental study was conducted using 0.10%, 0.20% and 0.30% volume concentra tion of A1203-water nanofluids. The results showed considerable enhancement of convective heat transfer using the nanofluids. The empirical correlations developed for Nusselt number in terms of Reynolds number, Prandtl number, viscosity ratio and volume concentration fit with the experimental data within ±10%. The heat transfer characteris tics were also simulated using computational fluid dynamics using FLUENT software with the standard ke model and multiple reference frame were adopted. The computational fluid dynamics （CFD） predicted Nusselt number agrees well with the experimental value and the discrepancy is found to be less than ＋8%.     

Gas-side mass transfer coefficients in a falling film microreactor

Gas–liquid mass transfer in a falling film microreactor (FFMR) with 29 microchannels (0.6 mm width each) was investigated. CO₂ was absorbed from a CO₂/N₂ gaseous mixture into a NaOH aqueous solution and the liquid-side mass transfer coefficient and the gas-side mass transfer coefficient were measured. The influence of gas concentration on the value of gas-side mass transfer coefficient has been discussed.     

Heat transfer by radiation and convection in fire testing

Fire safety engineers, especially those experts writing fire test standards, often have different understandings of the concepts of heat transfer from a fire to a surface by radiation and convection. The concepts are therefore often not very well defined, in e.g. ISO and CEN standards, leading in many cases to confusions, misunderstandings and considerable errors. This paper is an effort, in a very simple way, to make the issue a little clearer, particularly for use in international standardization work. ISO/TC92 and the International FORUM of Fire Research Directors are currently endorsing very important work on procedures to calibrate heat flux meters. The two entities, heat transfer by radiation and convection, may be treated independently. The presentation below is deliberately very short and basic avoiding many phenomena that are not immediately needed in most fire standardization work. Recommendations are reached on how to define and measure heat transfer in fire testing. Copyright © 2004 John Wiley & Sons, Ltd.     

酯化反应釜夹套的传热计算

通过计算液相热媒夹套和气相热媒冷凝夹套的给热系数 ,进行定量的比较。指出应用气相热媒冷凝夹套的优点 ,并对它的设计进行了讨论     

间歇釜传热时间和传热面积的计算

通过对间歇釜传热过程的分析,推导出间歇釜传热时间和传热面积的计算公式。附有说明计算方法的实例。     

烧结床层的热质分析

基于烧结生产的复杂物理化学过程,建立了烧结床层传热、传质和流动的二维非稳态数学模型,考虑了孔隙率、物料颗粒当量直径等床层结构影响参数的变化,并对气固传热系数进行了修正。通过数值计算,获得了烧结床层的温度场、结构变化和烟气的流场、温度场、浓度场等。烟气出口温度、床层总压降与生产实测值吻合较好,验证了数学模型的正确性。进一步分析了燃料配比、风量和给料温度等操作参数对烧结过程的影响。研究结果表明：燃烧带的厚度、最高温度随着烧结过程的进行而逐渐增加。床层孔隙率、颗粒当量直径的变化主要发生在燃烧带的熔融、冷凝阶段。料层压损最大的是燃烧熔融层,其次是混合料带,最小的是烧结矿层。增加焦粉含量、提高烧结混合料的初温,有利于提高成矿质量;风量过大时,会造成成矿质量下降、生产成本提高。     

圆形自由射流冲击曲面的换热特性

采用直接表面温度测量的方法对水喷射高温曲面的传热过程进行实验研究.通过实验得到了驻点区及附壁射流区的对流传热系数的分布情况,并且系统地研究了射流出口速度、喷嘴至加热面的间距等参数对对流传热系数的影响.结果表明,圆形射流冲击曲面的局部传热系数沿传热面随X/D的增大而逐渐减小,驻点处的传热系数最大并且随射流速度的增大而增大,但是当射流速度增大到一定值时,驻点的对流传热系数的增大比较缓慢.在射流速度较低的情况下,喷距对局部传热系数的影响较为显著.     

橡胶热传导性能的测定

介绍橡胶热传导性能测定的基本原理及导热系数,热扩散系数和比热容的测量方法。     

多头螺旋管式换热器换热与压降计算

从实际工程设计出发,对多头螺旋管式换热器的设计进行了研究,提出了多头螺旋管束受热面结构的设计方法,推荐了螺旋管内外的传热系数和压降的计算关系式。并对200MW蒸汽发生器进行了实例设计计算。     

颗粒与液相间的湍流涡旋裂变传质模型

湍流在宏观上处于远程混沌无序状态, 而在介微观上处于近程有序状态。从分析湍流场中局 域中涡旋的串级结构出发,通过对N-S方程的涡旋输运形式进行求解, 导出一个具有分形意 义的涡旋群运动表达式,并得到涡旋掠过颗粒表面花费的时间, 然后同Higbie的传质渗透模型相联系,以涡旋的分布函数为权求其数学期望,得到搅拌湍流场中的传质系数表达式,结果与实验数据吻合较好。