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The success of quenching process during industrial heat treatment mainly depends on the heat transfer characteristics of the quenching medium. In the case of quenching, the scope for redesigning the system or operational parameters for enhancing the heat transfer is very much limited and the emphasis should be on designing quench media with enhanced heat transfer characteristics. Recent studies on nanofluids have shown that these fluids offer improved wetting and heat transfer characteristics. Further water-based nanofluids are environment friendly as compared to mineral oil quench media. These potential advantages have led to the development of nanofluid-based quench media for heat treatment practices. In this article, thermo-physical properties, wetting and boiling heat transfer characteristics of nanofluids are reviewed and discussed. The unique thermal and heat transfer characteristics of nanofluids would be extremely useful for exploiting them as quench media for industrial heat treatment. 相似文献
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In high-viscosity polymer solutions and blends, temperature variations during the quench process of the thermally-induced phase separation (TIPS) influence the dynamics and thermodynamics of phase separation. Hence, this study aims to investigate the impact of temperature variations on the morphology formation during the TIPS process. First, the influence of temporal temperature variations on phase separation is investigated by coupling a transient heat conduction model and the Cahn–Hilliard equation, and the results are compared with the isothermal phase separation process. Next, the morphology formation during phase separation is inspected by applying quench from two opposite sides of the sample to the same and different temperatures through coupling the Fourier heat transfer equation and the Cahn–Hilliard equation. The influence of the enthalpy of demixing on the morphology formation and the competition between the heat and mass transfer is also evaluated. It is confirmed that temporal variations of temperature alone have a significant impact on the morphology formation during the TIPS process. In addition, quenching the system to the same and different temperatures both leads to anisotropic morphology formation, which is affected by the quench rate, quench temperature, solution viscosity, and enthalpy of demixing. Upon applying different quench temperatures from opposite sides, two different types of morphologies and droplet sizes were formed as a result of the difference in the cooling rates between the two sides. Employing the enthalpy of demixing during phase separation induced a shallow quench effect on the deep quench side due to the fact that the heat moved toward the lowest temperature in the system, which led to the formation of a distinctive structure. 相似文献
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在许多强吸热化学反应的化工过程中,常常需要对反应流体流出反应器时进行快速急冷来避免副反应或逆反应发生,以期最终获得可观的目标产物。在本实验室前期开展的热等离子体裂解二氧化碳实验研究中,采取在高温反应器出口加装收缩喷管将裂解气高速导入夹套水冷管的方法,实现了对高温裂解气的快速急冷,显著地避免了裂解气中CO与O的逆反应,获得了意想不到的CO2高转化率。本文利用计算流体力学软件模拟这一过程,以期揭示这种新的冷却方法导致极快速冷却的机制。模拟结果表明,加装收缩喷嘴确实可以期待对高温射流产生107 K·s-1量级的温降速率。深入分析表明,仅仅靠气体动力学效应不能完全解释如此快速的冷却速率。从喷管高速喷出的黏性流体在夹套水冷管内形成高速涡流,这种涡流一方面增强了主流体对周围气体的卷吸,另一方面加强了被卷吸流体在被卷入之前与夹套水冷管壁面的强制换热过程,是导致快速急冷的主要机制。 相似文献
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Zhiliang Zhou Fan Song Yingfeng Shao Songhe Meng Chiping Jiang Jia Li 《Journal of the European Ceramic Society》2012,32(12):3029-3034
In this paper, we determined the surface heat transfer coefficient of Al2O3 ceramics quenched from different initial temperatures into a water bath at room temperature. By using the multipoint temperature measurement technique and the inverse heat conduction method, this coefficient was measured as function of surface temperature of the ceramics during the water quench. The obtained results indicate that the surface heat transfer coefficient largely depends not only on the initial quenching temperature and their evolution in quenching media but also on the sizes of tested specimens. In addition, brief discussion was completed on the rationality of the traditionally used approach, which considers the surface heat transfer coefficient as a joint constant of materials and quenching media, in previous studies on heat transfer and thermal stresses. 相似文献
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T. OZYENER K. SATYAMURTHY CHARLES EUGENE KNIGHT JITENDRA P. SINGH D. P. H. HASSELMAN G. ZIEGLER 《Journal of the American Ceramic Society》1983,66(1):53-58
A study was conducted of the effect of a spatial variation in heat transfer coefficient during forced convection and the effect of temperature dependence of the heat transfer coefficient during free convection on the magnitude of thermal stress encountered during the thermal shock testing of brittle ceramics by the quenching method. For specimens with circular geometry, the results obtained by the finite element method indicate that, depending on the value of the Biot number, a factor of three spatial variation in heat transfer coefficient changes the tensile thermal stresses on cooling by a maximum of about 17% over the value for spatially uniform heat transfer. On heating, the corresponding tensile thermal stresses are lower by a maximum of about 6%. In free convection for a heat transfer coefficient, h , proportional to Δ Ti1/4 , where Δ Ti is the instantaneous temperature difference between the specimen and quenching medium, the stresses are appreciably less than those for Δ Ti -independent heat transfer for the same initial value of h at the onset of the thermal quench. The validity of the dependence of h on Δ Ti was established experimentally. The relevance of this result to the interpretation of thermal quenching studies is discussed. 相似文献
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直角坐标系下的辐射传递方程可以看作一类特殊的对流扩散方程,且具有强对流特性。采用数值方法求解该问题需要特别的稳定技术,否则计算结果往往会出现非物理振荡现象。本文发展了基于流向迎风彼得罗夫-伽辽金(SUPG)格式的谱元法来求解多维半透明介质内的辐射传递。采用3个算例对SUPG谱元法求解多维半透明介质内辐射传递的性能进行了检验。结果表明,与伽辽金谱元法相比,SUPG谱元法有效地消除了解的非物理振荡现象,同时与解析解及文献中的结果相比较,SUPG谱元法对于求解多维半透明介质内的辐射传递有着很好的精度。 相似文献
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Eli Ruckenstein 《Chemical engineering science》1982,37(10):1505-1511
This paper emphasizes the generality of a simple algebraic procedure suggested in 1962 by the author to predict the rate of heat or mass transfer in complex situations. It consists in replacing the terms in the convective diffusion equation by expressions involving scaling quantities multiplied by constants. Thus the convective-diffusion equation is replaced by an algebraic equation for the mass transfer coefficient containing a number of constants. The constants are determined from the solutions available for some simpler limiting cases. To illustrate the procedure a variety of examples are given which include: penetration theory for diffusion with chemical reaction, laminar flow along a plate accompanied by a chemical reaction, diffusion with chemical reaction in a turbulent liquid, mass transfer from a drop to a continuous phase, unsteady heat transfer to a liquid in steady laminar flow along a plate and free convection superimposed on forced convection. Numerous other complex situations have been or can be treated by interpolation between some simpler limiting cases. 相似文献
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准确预测直流蒸汽发生器流动沸腾及蒸干对其设计、安全可靠运行极其重要。通过对B&W公司直流蒸汽发生器进行合理简化,引入两流体三流场数学模型及壁面热通量分区模型,分别进行基于常热通量和耦合传热的蒸汽发生器流动沸腾数值模拟。结果表明:蒸干发生时传热性能急剧下降,常热通量边界下壁温升高的幅度相当大(约300 K·m-1),而耦合传热边界下壁温飞升幅度约为25 K·m-1,与实际情形相一致;两种热边界中预热区会发生过冷沸腾,壁面处传热由液相对流换热、淬火换热和蒸发换热3部分构成,核态沸腾区蒸发换热为主要换热方式,同时伴随着液相对流换热和淬火换热,蒸干发生时淬火换热和蒸发换热全部降到0,在蒸干后传热区域换热方式为气相对流换热。 相似文献
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This paper describes the modelling of a hydrogen production process by direct splitting of water in a dissociation reactor in which the high temperature reaction occurs, followed by quenching with gaseous jets to recover the hydrogen produced. Proposed models, dealing with the dissociation reactor and principally with the two designed quenching devices, take into account the various physicochemical phenomena (heat transfer, complex chemical reaction, gas mixing). The quenching rate (up to 106 K/s) and theoretical gas composition at the outlet of the overall reactor can be calculated in a fairly good agreement with experimental results. 相似文献
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基于纵向涡强化换热理论提出了新型的强化换热管--弯曲内肋强化换热管。运用数值计算的方法,在Re=500~40000研究了新型强化换热管结构参数肋宽a、导程p、肋深e和Re数对Nu、f及换热性能评价指标PEC的影响,并拟合出换热管Nu、f的量纲为1关联式。结果表明:由于内肋的作用,在换热管的近壁面区域能够有效诱导产生沿纵向的涡旋结构,破坏壁面边界层,实现强化换热;导程p和肋深e对综合换热性能影响较大,肋宽a对综合换热性能影响较小,在Re=500~40000范围内,新型换热管较优的结构参数为p=10mm、e=0.6mm、a=1.8mm;与普通平滑圆管相比,当Re<2300,其换热性能Nu可达到普通圆管的2.9~7.0倍,沿程阻力系数f约为普通圆管的1.4~3.6倍;当500< Re <2300,其综合换热评价指标PEC可以达到2.6~4.6;当2300 <Re <40000时,其PEC可以达到1.2~2.3。 相似文献
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以空气为介质,以纵向流动方式冲刷光滑管与花瓣状翅片管进行管壳式换热器壳程的传热强化与流阻性能实验研究。实验结果表明,与文(一)的横向冲刷情况相比,在相同功耗下,Re=2×104时,按实际换热面积计算,横向冲刷花瓣状翅片管比光滑管提高给热系数135.8%,纵向冲刷花瓣管仅与光滑管的给热系数相当。文中通过设计实例的比较得到横向冲刷是花瓣管空冷器设计方案的较佳选择。 相似文献
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Treating Methanol‐to‐Olefin Quench Water by Minihydrocyclone Clarification and Steam Stripper Purification
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In the chemical industry, methanol‐to‐olefin (MTO) technology is a novel process for producing ethylene and propylene from naphtha thermal cracking. The process of recovering MTO quench water by minihydrocyclone and steam stripping treatment was successfully applied in industrial plants. The fine catalyst in the quench water is removed by the two‐stage minihydrocyclone separation. The method and equipment for this system present various advantages: the quench water can be recycled in the cooling system and prevents heat loss in heat transfer systems; and the stripping tower can be blocked by the catalyst. Maintenance activities are reduced and a stable operation cycle is extended. The proposed treatment process improves the economic efficiency of the MTO device. 相似文献
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This paper describes a new way to control temperatures of heterogeneous exothermic reactions such as heterogeneous catalytic reaction and polymerization by using encapsulated nanoparticles of phase change materials as thermally functional additives. Silica-encapsulated indium nanoparticles and silica encapsulated paraffin nanoparticles are used to absorb heat released in catalytic reaction and to mitigate gel effect of polymerization, respectively. The local hot spots that are induced by non-homogenous catalyst packing, reactant concentration fluctuation, and abrupt change of polymerization rate lead to solid to liquid phase change of nanoparticle cores so as to avoid thermal runaway by converting energies from exothermic reactions to latent heat of fusion. By quenching local hot spots at initial stage, reaction rates do not rise significantly because the thermal energy produced in reaction is isothermally removed. Nanoparticles of phase change materials will open a new dimension for thermal management of exothermic reactions to quench local hot spots, prevent thermal runaway of reaction, and change product distribution. 相似文献