制冷用水平管降膜蒸发器换热特性数值模拟

水平管降膜蒸发器相比满液式具有换热效率高、制冷剂充注量少等优点。文中采用分布参数法建立水平管降膜蒸发换热模型,应用MATLAB软件,对换热管在理想条件下的换热特性进行了数值模拟。研究了光管及Turbo-BII管在管外无干斑发生换热条件下,管外降膜蒸发换热系数、管内对流换热系数、总换热系数、热流密度、降膜蒸发因子、以及换热管单元换热量沿管长方向分布规律,并根据计算结果分析了管间距对换热的影响。该研究为应用于制冷空调领域水平管降膜蒸发器的设计提供理论指导,促进其在制冷空调领域的推广应用。     

表面润湿性对水平管降液膜流型的影响

降液膜的流动形态对水平管降膜蒸发器的强化传热过程具有十分重要的意义。本文构造了三种不同润湿性的表面,超亲水,亲水以及疏水,通过可视化实验和机理分析,研究了表面润湿性对降液膜管间流动的作用机理。结果表明,直列与错列柱状流的出现受到表面润湿性的影响。在亲水与超亲水表面只存在错列柱状流,而在疏水表面只存在直列柱状流。同时,错列柱状流的表面存在接触角上限,临界接触角为67°,直列柱状流的表面存在接触角下限,临界接触角为106°。     

双离子束溅射中红外SiO2薄膜热稳定性研究

通过双离子束溅射方法在蓝宝石、硅衬底上制备了单层SiO2薄膜,分析了SiO2薄膜残余应力、表面形貌、微观结构以及光学性能(可见-近红外0.4~1.2 μm和中红外3~5 μm波段)在400 ℃~1 000 ℃温度范围内的演化规律.研究结果表明:在400 ℃附近,SiO2薄膜残余应力存在局部极小值;SiO2薄膜光学性能的演化与膜层表面质量、内部残余应力及微观结构变化密切相关;经1 000 ℃高温处理后,蓝宝石窗口表面SiO2薄膜红外透射性能仍能保持很好的稳定性,且膜层表面没有出现显著的气泡、开裂等损伤形貌.该研究结果可为恶劣环境下光学窗口头罩表面薄膜系统的设计提供指导.     

双离子束溅射中红外SiO_2薄膜热稳定性研究（英文）

通过双离子束溅射方法在蓝宝石、硅衬底上制备了单层SiO_2薄膜,分析了SiO_2薄膜残余应力、表面形貌、微观结构以及光学性能(可见-近红外0.4~1.2μm和中红外3~5μm波段)在400℃~1 000℃温度范围内的演化规律.研究结果表明:在400℃附近,SiO_2薄膜残余应力存在局部极小值;SiO_2薄膜光学性能的演化与膜层表面质量、内部残余应力及微观结构变化密切相关;经1 000℃高温处理后,蓝宝石窗口表面SiO_2薄膜红外透射性能仍能保持很好的稳定性,且膜层表面没有出现显著的气泡、开裂等损伤形貌.该研究结果可为恶劣环境下光学窗口头罩表面薄膜系统的设计提供指导.     

单液滴撞击倾斜液膜飞溅过程的耦合Level Set-VOF模拟

采用耦合水平集--体积分数法(CLSVOF)对液滴撞击倾斜表面液膜后液膜的形态演化及飞溅过程进行数值模拟, 并对液滴撞击液膜过程中形成的空气卷吸现象进行研究并探讨了撞击角对此的影响, 分析了液滴撞击后液体内部的压力和速度分布, 对液滴撞击倾斜表面液膜的飞溅过程进行讨论, 并与实验结果进行了对比, 验证了CLSVOF方法研究液滴撞击倾斜液膜的可行性. 结果表明, 液滴撞击倾斜液膜时前后两部分飞溅现象产生的机理不同, 前半部分飞溅是由于压差引起的颈部射流, 而后半部分则是由液膜径向流动产生的飞溅现象. 随着撞击角的增大, 空气卷吸气泡数量减少.     

液滴撞击超亲水表面冷液膜的研究

液滴撞击过程因具有较强的传热传质性能被广泛应用于工业领域中。本文利用高速摄影机和红外热像仪,研究了液滴撞击超亲水表面冷液膜的水力学特性和温度分布,探讨了撞击We数和液膜温度对撞击过程中水力学特性以及液膜温度分布的影响.总结了液滴在超亲水表面的液膜上的水力学特征的变化规律。实验结果表明,液滴铺展速度和最大铺展直径随撞击We数的增大而增大。同时,液膜温度会影响液滴撞击薄液膜后的水力学特性.在低We数下液滴撞击低温薄液膜后液膜的温度呈高低相间的环状分布,随着撞击We的增大,该环状温度分布消失。这对要求精确喷雾控温的工业过程起到了十分重要的意义.     

水平管降膜蒸发器传热特性研究

建立了水平管降膜蒸发和管内凝结传热实验台,通过对实验结果的归纳,获得了水平管降膜蒸发器总传热系数随喷淋密度、蒸发温度、传热温差和蒸汽入口流速的变化规律,以及管间距对传热特性的影响。结果表明,总传热系数随喷淋密度、蒸发温度的增大而增大,随传热温差的增大而减小,而蒸汽流速对传热系数影响较小;在本文研究的管间距范围内,当管间距为46.7 mm时,总传热系数最高。     

高功率激光装置光传输管道污染规律及对光学表面损伤性能的影响

研究高功率激光装置光传输管道内部洁净度变化规律,分析其对内部重要光学元件光学性能的影响规律,提出污染控制措施。对光传输管道内部的气溶胶进行采样,并利用空气品质分析仪及扫描电镜对其进行分析,得到光传输管道内部洁净度变化规律和污染源;采用内部放置透射膜元件的方法,研究洁净度等级水平对透射膜的微观结构和透射率的影响,并利用"1-on-1"的测试方式进行透射膜元件的损伤阈值测试。研究结果表明:光传输管道内部的洁净度在激光辐照后迅速上升至万级水平,透射膜元件在此环境下其透过率严重下降,下降幅度为2.5%,且表面微观形貌发生变化。光学透射薄膜表面损伤阈值随表面污染水平呈现线性下降规律,最大下降幅度约为10%。污染监测和成分分析结果表明管道内部灰尘及杂散光或者鬼光束辐照金属产生的等离子体是管道内污染的主要源头,在此基础上提出了正压密封保持的技术手段确保内部光学表面洁净度水平,延长使用寿命。     

水平三维肋管管外凝结换热实验与分析(I实验研究)

本文对不同饱和蒸汽温度下R11在水平Thermoexcel-C管的管外凝结换热性能进行了实验研究。实验结果表明:随着饱和蒸汽温度的提高,C管的凝结换热系数下降。C管凝结换热强化的主要机理在于孤立三维齿结构增大了表面张力减薄凝结表面液膜厚度的作用,而C管凝液淹没区小于相同肋间距的低肋管,且在淹没区内的凝结换热大于低肋管。     

凹槽基底上含不溶性活性剂液膜的流动特性

针对凹槽基底上含不溶性活性剂液膜的流动过程,采用润滑理论建立液膜厚度和浓度演化模型,通过数值模拟得到液膜的流动特性及相关参数的影响规律.研究表明:含不溶性活性剂液膜在凹槽基底上流动时,重力和活性剂浓度梯度引起的Marangoni力对液膜的流动起促进作用,表面活性剂通过引起表层液体流动进而牵引内部液体运动,但其作用力相对重力较弱,重力起主导作用;与基底尺寸有关的粘性力则起阻碍作用;提高邦德数G和减小毛细力数C具有减弱液膜变形的作用;增大凹槽高度或减小凹槽斜度,均使Marangoni力增加,促使液膜变形加大.     

Large eddy simulation of the low temperature ignition and combustion processes on spray flame with the linear eddy model

Large eddy simulation coupled with the linear eddy model (LEM) is employed for the simulation of n-heptane spray flames to investigate the low temperature ignition and combustion process in a constant-volume combustion vessel under diesel-engine relevant conditions. Parametric studies are performed to give a comprehensive understanding of the ignition processes. The non-reacting case is firstly carried out to validate the present model by comparing the predicted results with the experimental data from the Engine Combustion Network (ECN). Good agreements are observed in terms of liquid and vapour penetration length, as well as the mixture fraction distributions at different times and different axial locations. For the reacting cases, the flame index was introduced to distinguish between the premixed and non-premixed combustion. A reaction region (RR) parameter is used to investigate the ignition and combustion characteristics, and to distinguish the different combustion stages. Results show that the two-stage combustion process can be identified in spray flames, and different ignition positions in the mixture fraction versus RR space are well described at low and high initial ambient temperatures. At an initial condition of 850 K, the first-stage ignition is initiated at the fuel-lean region, followed by the reactions in fuel-rich regions. Then high-temperature reaction occurs mainly at the places with mixture concentration around stoichiometric mixture fraction. While at an initial temperature of 1000 K, the first-stage ignition occurs at the fuel-rich region first, then it moves towards fuel-richer region. Afterwards, the high-temperature reactions move back to the stoichiometric mixture fraction region. For all of the initial temperatures considered, high-temperature ignition kernels are initiated at the regions richer than stoichiometric mixture fraction. By increasing the initial ambient temperature, the high-temperature ignition kernels move towards richer mixture regions. And after the spray flames gets quasi-steady, most heat is released at the stoichiometric mixture fraction regions. In addition, combustion mode analysis based on key intermediate species illustrates three-mode combustion processes in diesel spray flames.     

Simultaneous measurement of particle size, velocity, andtemperature in thermal plasmas

The in-flight measurement of particle parameters (size, velocity, temperature, and local number density) can prove insight into the plasma processing of solid materials. A measurement technique for simultaneously obtaining the size, velocity, and temperature of particles entrained in high-temperature flow fields is described. Particle size and velocity are obtained from a combination laser-particle-sizing system and laser Doppler velocimeter. The particle temperature is determined by a two-color pyrometry technique and the data rate is a measure of relative particle number density. Typical measured temperatures and velocities for the 5-100 μm particles used in plasma spraying are 1600-3500 K and 100-300 m/s, respectively. Since particle size, velocity, and temperature are measured simultaneously, cold particles (<1600 K) are identified and their relative number density can be quantified. Data from two plasma spray systems, a metal one (Ni-Al) and a metal oxide one (Al₂O₃), are presented and their application to understanding the plasma spray-coating process is illustrated     

Nonlinear properties of multiphase high-temperature superconductors of the Bi–Sr–Ca–Cu–O system in the temperature range of the superconducting transition

The nonlinear characteristics of high-temperature superconductors of the Bi–Sr–Ca–Cu–O system have been experimentally investigated in the temperature range of the superconducting transition under the influence of a harmonic alternating magnetic field. The effect of the generation of odd harmonics in the signal of response to a harmonic alternating magnetic field for multiphase high-temperature superconductors containing regions with different values of the critical temperature in their bulk has been observed for the first time. The mechanism of harmonic generation in a superconductor in the resistive state, which is associated with the switch effect, i.e., with the redistribution of eddy current density between the local regions of the superconductor, has been considered.     

Numerical simulation and laser-based imaging of mixture formation, ignition, and soot formation in a diesel spray

Laser-based imaging of fuel vapor distribution, ignition, and soot formation in diesel sprays was carried out in a high-pressure, high-temperature spray chamber under conditions that correspond to temperature and pressure in a diesel engine. Rayleigh scattering and laser-induced incandescence are used to image fuel density and soot volume fraction. The experimental results provide data for comparison with numerical simulations. An interactive cross-sectionally averaged spray model based on Eulerian transport equations was used for the simulation of the spray, and the turbulence-chemistry interaction was modeled with the representative interactive flamelet (RIF) concept. The flamelet calculation is coupled to the Kiva3V computational fluid dynamics (CFD) code using the scalar dissipation rate and pressure as an input to the RIF-code. The flamelet code computes the instationary flamelet profiles for every time step. These profiles were integrated over mixture fraction space using a prescribed β-PDF to obtain mean values, which are passed back to the CFD-code. Thereby, the temperature and the relevant species in each CFD-cell were obtained. The fuel distribution, the average ignition delay as well as the location of ignition are well predicted by the simulation. Furthermore, simulations show that the experimentally observed injection-to-injection variations in ignition delay are due to temperature inhomogeneities. Experimental and simulated spatial soot and fuel vapor density distributions are compared during and after second stage ignition.     

Effect of methane on pilot-fuel auto-ignition in dual-fuel engines

The ignition behavior of n-dodecane micro-pilot spray in a lean-premixed methane/air charge was investigated in an optically accessible Rapid Compression-Expansion Machine at dual-fuel engine-like pressure/temperature conditions. The pilot fuel was admitted using a coaxial single-hole 100?µm injector mounted on the cylinder periphery. Optical diagnostics include combined high-speed CH₂O-PLIF (10?kHz) and Schlieren (80?kHz) imaging for detection of the first-stage ignition, and simultaneous high-speed OH* chemiluminescence (40?kHz) imaging for high-temperature ignition. The aim of this study is to enhance the fundamental understanding of the interaction of methane with the auto-ignition process of short pilot-fuel injections. Addition of methane into the air charge considerably prolongs ignition delay of the pilot spray with an increasing effect at lower temperatures and with higher methane/air equivalence ratios. The temporal separation of the first CH₂O detection and high-temperature ignition was found almost constant regardless of methane content. This was interpreted as methane mostly deferring the cool-flame reactivity. In order to understand the underlying mechanisms of this interaction, experimental investigations were complemented with 1D-flamelet simulations using detailed chemistry, confirming the chemical influence of methane deferring the reactivity in the pilot-fuel lean mixtures. This shifts the onset of first-stage reactivity towards the fuel-richer conditions. Consequently, the onset of the turbulent cool-flame is delayed, leading to an overall increased high-temperature ignition delay. Overall, the study reveals a complex interplay between entrainment, low T and high T chemistry and micro-mixing for dual-fuel auto-ignition processes for which the governing processes were identified.     

Electric field involved transport at elevated temperature in nanocrystalline silicon carbide nitride (nc-SiCN) thin films for harsh environment applications

The present study represents a systematic temperature dependent charge transport and dielectric properties of nanocrystalline silicon carbide nitride (nc-SiCN) thin films grown on Pt/Ti/SiO₂/Si substrate. A large negative temperature coefficient of resistance (TCR) ranging from 6200 to 2300 ppmK^-1 in the temperature range 300–773 K, suggests that the nc-SiCN thin films could be useful for futuristic thermal-based sensors. The current density vs. electric field (J-E) characteristics was measured at different temperatures (300–673 K). Detailed J-E analysis revealed an ohmic conduction at the low applied electric field (<65 kV/cm) within the entire temperature range. However, at high electric field (>65 kV/cm), space charge limited conduction (SCLC) mechanism was found to be dominating in low measurement temperature (300–473K), whereas, a transition from SCLC mechanism to Poole-Frenkel mechanism was observed with further increment in the temperature beyond 473 K. The temperature invariant dielectric tunability (n_r ∼10%) and low zero electric field leakage current density (J ∼10⁻⁷A/cm²) at 673 K temperature, demonstrates the feasibility of nc-SiCN thin films for tunable device applications in the high-temperature and harsh environment.     

Theoretical and Experimental Investigations of the Suitability of Low‐Current Z‐Pinch Plasma as an Absorption Medium for Laser Radiation

In this study the electron density of z‐pinch plasmas driven at relatively low currents (ca. 2‐5 kA) was determined using only emission spectroscopy. The suitability of a hollow‐cathode‐triggered z‐pinch plasma as an absorption medium for laser radiation was investigated. The temporal and spatial behaviors of electron temperature and density profiles were estimated using magnetohydrodynamic (MHD) simulations to evaluate the experimental results. Temperature measurements were performed according to the Boltzmann plot method in the visible spectrum range, using the fact that, in low‐current z‐pinch plasma, a local thermodynamic equilibrium prevails for states at high principal quantum numbers (partial local thermodynamic equilibrium). In this case, the Saha equation can be used to determine the electron density. The results demonstrate that this method of determining the electron temperature and density of z‐pinch plasmas is only applicable during the pinching phase. However, in this case the experimentally determined values are in fairly good agreement with the values determined using the MHD model. A user‐oriented 1‐D radiation MHD code was used to simulate the dynamic evolution of the plasma. The experimentally determined maximum electron temperature of approximately 12 eV is in fairly good agreement with the simulated value. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Interpreting the influence of fuel spray impact on mixture preparation for HCCI combustion with port-fuel injection

This paper addresses the influence of fuel spray impact on fuel/air mixture for combustion in port-fuel injection engines. The experiments include time resolved measurements of surface temperature synchronized with PDA measurements of droplet dynamics at impact and were conducted to quantify the effects of interactions between successive injections on the mixture preparation for combustion in homogeneous charge compression ignition (HCCI) engines. Analysis shows that, during engine warm up, the heat transfer over the entire valve surface occurs within the vaporization-nucleate-boiling regime and the local instantaneous surface temperature correlates with the dynamics of droplets impacting at the same point. A functional relation is found for the heat transfer coefficient, which also describes other experiments reported in the literature. Similarity does not hold after the engine warms up because heat transfer and droplet vaporization at the surface are dominated by multiple interactions between droplets arisen from diverse heat transfer regimes. However, results evidence the existence of a critical surface temperature which sets a transition between overall heat transfer regimes dominated by local nucleate boiling at lower temperatures and by local intermittent transition regimes at higher temperatures. The heat transfer within the overall nucleate boiling regime is shown to be due to a thin film boiling mechanism leading to breakdown of the liquid-film at a nearly constant surface temperature, regardless of injection frequency or any other spray conditions. While at low frequencies this regime is not limited neither by the delivery of liquid to the surface, nor by the removal of vapour from the surface, at higher frequencies it is triggered by enhanced vaporization induced by piercing and mixing the liquid film. The results further evidence the important role of spray impingement for mixture preparation as required for HCCI.     

Crisis phenomena in falling liquid films at periodic heat loads

Experimental results on development of crisis phenomena in a falling film of cryogenic liquid at alternating pulse heat release are presented. Experimental data on local temperature evolution along a heat releasing surface are obtained. It is shown that amplitude of heater temperature pulsations depends significantly on the heat flux density and coordinates along the liquid film flow. New experimental data on the critical heat fluxes corresponding to formation of stable dry spots and drying crisis are presented depending on duration of heat release pulses. It is shown that parameters of the forming metastable regular structures and critical drying parameters of the heat-releasing surface are determined by dynamics of the movable boundaries of wetting during self-organization of a system.     

Efficient simulation of one-dimensional quantum many-body systems

We present a numerical method to simulate the time evolution, according to a generic Hamiltonian made of local interactions, of quantum spin chains and systems alike. The efficiency of the scheme depends on the amount of entanglement involved in the simulated evolution. Numerical analysis indicates that this method can be used, for instance, to efficiently compute time-dependent properties of low-energy dynamics in sufficiently regular but otherwise arbitrary one-dimensional quantum many-body systems. As by-products, we describe two alternatives to the density matrix renormalization group method.