Real gas choked flow conditions at low reduced-temperatures

Real gas choked mass flux is calculated for a frictionless stream expanding isentropically until it reaches the speed of sound and without phase changes. The other parameters associated with the choked state are the pressure, density, temperature ratios, and the speed of sound. Departure of the choked mass flux from the ideal gas model is discussed first in absolute terms and then in relative terms, using the Principle of Corresponding States, for gases with boiling points in the low temperature range. Reduced-stagnation pressures are examined up to values of 30 for hydrogen, neon, nitrogen, argon, methane, krypton, xenon, and R-14 and up to 100 for ⁴He. The corresponding reduced-stagnation temperatures go down to 1.4 and in some cases down to 1.2 for nitrogen and argon. Also discussed are the limiting values of stagnation parameters for which no phase change occurs in the choked state. Compared to the ideal gas, the mass flux may almost double and the critical pressure ratio may decrease by an order of magnitude. The relevance of results is discussed qualitatively and quantitatively for Joule-Thomson cryocooling.     

Corresponding states correlation for maximum heat flux in two-phase closed thermosyphon

A method is presented for the prediction of performance limit in two-phase closed thermosyphon based on the thermodynamical corresponding states principle. Molecular weight, critical temperature, critical pressure and acentric factor are needed instead of thermodynamic and transport properties of fluid at a given temperature. Developed corresponding states correlation for maximum heat flux is found to be in fairly good agreement with the 436 experimental data for 12 nonpolar and polar fluids with fill ratio greater than 0.4 in vertical two-phase closed thermosyphons with an inner diameter of 3–34 mm and the ratio of the evaporator length to its diameter with values of up to 325.     

Thermo-fluid dynamics of several film boiling modes in He II in the pressure range between atmospheric pressure and saturated vapor pressure

Four film boiling modes including the silent film boiling and the noisy film boiling were discriminated experimentally. Each mode was classified through visual observation and transient pressure and temperature measurements near the heater. It was found that in subcooled He II there were two film boiling modes, which are the strongly subcooled and weakly subcooled film boiling modes. The variation of boiling state between these two modes could be visually observed well by use of a transparent heater. All mode of film boiling is clearly mapped in diagrams as a function of pressure, temperature and heat flux. It is elucidated that the existence of He I layer influences the development of the vapor layer.     

R134a喷雾冷却系统换热性能研究

本文建立了以R134a为冷却工质的封闭式喷雾冷却系统,研究了工质过冷度、质量流量和热流密度对喷雾冷却系统换热性能的影响。其中,工质过冷度由喷嘴入口前的过冷段控制,质量流量通过变频齿轮泵调节,热流密度通过改变加热电源电压和电流控制。实验结果表明,在热流密度和质量流量保持不变时,改变过冷度对热源表面温度和换热系数的影响并不明显;在热流密度和过冷度保持不变的条件下,系统存在一个临界质量流量值,在质量流量达到临界值之前,热源表面温度随质量流量的增大而降低,当质量流量高于临界值时,热源表面温度随质量流量的增大而升高;当质量流量和过冷度保持不变时,存在一个热流密度使液滴的蒸发量等于补充量,在此热流密度下热源表面系数能达到最大。     

Ablation behavior of ZrB2–SiC ultra high temperature ceramics under simulated atmospheric re-entry conditions

ZrB₂–20vol%SiC ultra high temperature ceramic (UHTC) was prepared by hot-pressing. Ablation tests of the flat-face models were conducted under ground simulated atmospheric re-entry conditions using arc-jet testing with heat fluxes of 1.7 MW/m² and 5.4 MW/m² under subsonic conditions, respectively. There was little weight or configuration change after ablation at a heat flux of 1.7 MW/m². However, ZrB₂–SiC composite underwent severe ablation and whose surface temperatures exceeded 2300 °C at a heat flux of 5.4 MW/m². Sharp-shape leading edge models were ablated under supersonic conditions with the stagnation pressure and Mach number of 1.2 atm and 2.7 M, respectively, and sharp-shaped leading edge C/SiC models were also ablated under the same condition for comparison. ZrB₂–SiC composite exhibited an excellent thermal-oxidative and configurational stability in the simulated re-entry environment compared with C/SiC material. Results indicate that ZrB₂–SiC ultra high temperature ceramics are the potential candidates for leading edges. The temperature limit for UHTC is controlled by the softening and degradation of the formed oxide scale.     

An experimental study on the comparative assessment of hydraulic bulge test analysis methods

An in-depth understanding and full characterization of mechanical behavior for sheet materials are required since it is critical to establish the highly reliable material models over a broad range of strain levels for accurate modeling and analysis of sheet material deformation processes such as stamping, hydroforming, deep drawing, etc. Hydraulic bulge testing of sheet materials has been known to provide flow stress properties at higher strain levels compared to commonly used tensile tests mainly due to the fact the tested specimens are strained under biaxial loading conditions. However, analysis of the hydraulic bulge test data has not been standardized yet as there have been numerous approaches developed and adopted throughout the years. In this study, different approaches for the analysis of hydraulic bulge were compared with experimental results to determine the best combination in obtaining accurate flow curves models at room and elevated temperature conditions for different lightweight materials of interest for several industrial applications (AA5754 and AISI 201). It was determined that Panknin’s bulge radius and Kruglov’s thickness calculation approaches are the best combination to accurately obtain the flow curves at both cold and elevated temperature conditions.     

Pressure drop and flashing mechanisms in refrigerant expansion devices

This paper examines a novel pressure drop mechanism as well as flow choking conditions that determine mass flow rate in refrigerant expansion devices. For this study, an ideal situation is considered where an expansion device such as a short tube orifice or a thermostatic expansion valve is modeled as an ideal isentropic nozzle. In addition, a liquid with a certain initial degree of superheat is first expanded in the converging nozzle down to the exit section without any phase transition. At the exit section where the metastable liquid jet flashes to produce a complex axisymmetric two-phase flow, a shock wave may terminate the overall expansion process. The model presented here is based on experimental observations in short nozzles, where the metastable liquid in the central core undergoes a sudden phase transition in the interfacial region, giving rise to a high-speed two-phase flow. A simple 1-D analysis of the radial evaporation wave based on the theory of discontinuities from gas dynamics leads to the Chapman–Jouguet (C-J) solution. Flow choking issues are examined and numerical examples are presented for three common refrigerants: R134a, R-22, and R-600a. Results suggest that the evaporation wave may be the flow controlling mechanism in these devices.     

音速喷嘴一元流动模型详解及其激波计算

针对文丘里音速喷嘴,阐述了如临界背压比等概念,指出了ISO 9300中背压比定义存在容易造成歧义的缺陷。然后基于一元等熵流动理论,从数学上证明了：当文丘里喷嘴喉部压力与上游滞止压力之比达到临界压比时,喉部产生音速,通过喷嘴的质量流量达到最大值;推导了实际条件下喷嘴的流量公式,导出的流量公式相较于ISO 9300给出的相应公式,增加了喉部状态参数下的压缩性系数修正项■。最后从气体动力学基本方程出发,讨论了在较大背压比范围内,喷嘴扩散段中产生激波的机理,给出了激波产生的位置、激波前、后的压力和马赫数的一元流动计算模型,并运用数值模拟方法对计算结果进行了验证,同时还与Craig A的实验数据作了对比。对最小出口压比对比的结果显示,一元流动模型与实验数据的最大误差≤17%。     

A one-dimensional model of a jet-ejector in critical double choking operation with R134a as a refrigerant including real gas effects

The geometrical simplicity of a jet-ejector is in stark contrast to the complexity of the flow phenomena occurring in ejector operation. The available flow models range from empirical models to models based on computational fluid dynamics, differing considerably in complexity and thus explanatory power. The one-dimensional flow models of semi-empirical nature are based on experiments at comparatively low motive pressures and thus on ideal gas equations. In contrast, a one-dimensional, experimentally validated model of a jet-ejector in critical double choking operation which includes real gas effects and relies on a single physically interpretable loss coefficient is introduced in the present paper. Real gas effects impact on the state quantities and the critical mass flow of the expansion in the supersonic nozzle in particular which should be considered in the determination of the entrainment ratio or the absolute motive and suction mass flows of the ejector and in ejector design.     

BEM simulation of compressible fluid flow in an enclosure induced by thermoacoustic waves

The problem of unsteady compressible fluid flow in an enclosure induced by thermoacoustic waves is studied numerically. Full compressible set of Navier–Stokes equations are considered and numerically solved by boundary-domain integral equations approach coupled with wavelet compression and domain decomposition to achieve numerical efficiency. The thermal energy equation is written in its most general form including the Rayleigh and reversible expansion rate terms. Both, the classical Fourier heat flux model and wave heat conduction model are investigated.The velocity–vorticity formulation of the governing Navier–Stokes equations is employed, while the pressure field is evaluated from the corresponding pressure Poisson equation. Material properties are taken to be for the perfect gas, and assumed to be pressure and temperature dependent.     

Studies on nucleate boiling crisis of Helium-I in channels of superconducting magnet systems

The experimental results of measuring the critical heat flux of Helium-I under natural circulation and forced flow are described in this paper. The direction of the experiments under natural circulation depends upon the real geometry of superconducting magnet cooling channels. The influence of the nonheated zones and local hydraulic resistances on the critical heat flux is discussed and some experimental results are given. The critical heat flux of saturated Helium-I under forced flow in vertical tube is measured in a wide range of pressure, quality, and mass velocities. The expressions have been obtained which describe the dependency of the critical heat flux upon the different conditions.     

旋转CVI制备C/SiC复合材料

旋转 ＣＶＩ是在 ＣＶＩ原理基础上发展的一种制备 Ｃ／ＳｉＣ复合材料的新工艺,通过石墨衬底的旋转,使预制体的制备与基体的沉积同步进行,能有效消除一般ＣＶＩ工艺过程中存在的“瓶颈”效应．在自制的旋转 ＣＶＩ设备上实验,探索了旋转 ＣＶＩ工艺参数中 ＣＨ_３ＳｉＣｌ_３（ＭＴＳ）的流量与浓度、沉积温度和Ｃ布缠绕线速度对ＳｉＣ基体沉积速度,以及沉积温度对基体结构的影响．并在低压（５ｋＰａ）、高温 （１１００℃）、 ４００ ｍＬ·ｍｉｎ^－１ Ｈ_２、 ２００ ｍＬ·ｍｉｎ^－１Ａｒ、 ＭＴＳ４０℃与Ｃ布以１．１～３．５ｍｍ·ｍｉｎ^－１的线速度连续旋转的沉积条件下,实现了单丝纤维间微观孔隙、纤维束之间以及Ｃ布层间宏观孔隙的致密化同步完成．     

Nonlinear buckling and postbuckling behavior of cylindrical nanoshells subjected to combined axial and radial compressions incorporating surface stress effects

In the present study, the Gurtin-Murdoch elasticity theory, as a theory capable of capturing size effects, is implemented to predict the nonlinear buckling and postbuckling response of cylindrical nanoshells under combined axial and radial compressive loads in the presence of surface stress effects. For this purpose, a size-dependent shell mode containing geometric nonlinearity is proposed within the framework of the classical shell theory. Because it is necessary to satisfy balance conditions on the surfaces of nanoshell, it is assumed that the normal stress component of the bulk varies linearly through the shell thickness. On the basis of a variational formulation using the principle of virtual work, the non-classical governing differential equations are derived. Subsequently, a boundary layer theory is employed including the nonlinear prebuckling deformations and the large deflections in the postbuckling regime. Then a two-stepped perturbation methodology is utilized to obtain the size-dependent critical buckling loads and the postbuckling equilibrium paths of nanoshells corresponding to the axial dominated and radial dominated loading cases. It is revealed that in the radial dominated loading case, a positive value of surface elastic constants leads to increase the critical buckling load but decrease the critical end-shortening of nanoshell. However, in the axial dominated loading case, surface elastic constants with positive sign causes to increase the both critical buckling load and critical end-shortening of nanoshell.     

Theoretical prediction of piloted ignition of polymeric fuels in microgravity at low velocity flows

A numerical model developed for the prediction of the piloted ignition delay of solid polymeric materials exposed to an external radiant heat flux is used to predict the ignition delay and critical heat flux for ignition of solid fuels in microgravity at low velocity flows. The model considers the coupled thermochemical processes that take place in the condensed phase, including oxidative and thermal pyrolysis, phase change, radiation absorption, and heat and mass transfer in a multi-phase and multi-composition medium. Ignition is considered to occur when a critical pyrolysate mass flow rate is reached at the sample surface. Microgravity experimental surface temperature and ignition delay data previously obtained in a KC-135 aircraft are used to infer, in conjunction with the theoretical analysis, the critical mass flow rate for ignition. This value is then used to predict the ignition delay as a function of the external radiant heat flux, and the critical heat flux for ignition. Calculations are made for Polymethylmethacrylate (PMMA) and a Polypropylene/Fiberglass composite at airflows of 0.09 and 0.15 m/s under microgravity conditions and at 1.0, 1.75 and 2.5 m/s under normal gravity. The experiments and theoretical predictions show that the ignition delay and critical heat flux for ignition decrease as the forced airflow velocity decreases. It is predicted that at the tested lower velocities, the critical heat flux for ignition is close to half the value measured in normal gravity. The results have important implications since they indicate that materials could ignite easier under the conditions expected in spacecraft, and consequently stricter design specifications may be needed for fire safety.     

Numerical-based non-dimensional analysis of critical flow of R-12, R-22, and R-134a through horizontal capillary tubes

Development of correlations predicting critical mass flow rate and critical pressure distribution through capillary tubes is presented. In order to accomplish such a work, the critical mass flow rate and pressure distribution for nearly 500 operational conditions for R-12, R-22, and R-134a are evaluated. Operational conditions include inlet pressure varying from 800 to 1500 kPa, inlet subcold temperature between 0 and 10 °C, length varying from 1 to 2 m, and inner diameter between 0.5 and 1.5 mm. By performing non-dimensional analysis on numerical data, general correlations are presented to predict the critical mass flow rate through capillary tubes. In addition, by utilizing numerical data for down-stream pressure, non-dimensional analysis is performed to present correlations to predict critical down-stream pressure and pressure distribution through capillary tubes.     

竖直矩形通道内不凝性气体对凝结换热特性的影响

本文以去离子水为工质,实验研究了竖直矩形窄通道内少量残余不凝性气体对蒸汽凝结换热特性的影响。采用热阻分离法得到凝结侧换热表面传热系数,分析了不凝性气体的含量、冷却水质量流速、进口温度和热流密度对蒸汽凝结侧表面传热系数的影响。结果表明:当热流密度为1.668 kW/m~2,即蒸汽质量流速较小时,2%体积分数的不凝性气体使凝结侧表面传热系数下降了33%,但当热流密度为3.887 kW/m~2,蒸汽质量流速较大时,2%体积分数的不凝性气体仅使凝结侧表面传热系数降低了14%,此外,凝结换热表面传热系数随冷水质量流速和不凝性气体分数的增加而变小,随冷水进口温度和热流密度的增加而变大。     

The gas flux distribution in the XHV chamber of the vacuum primary standard developed by PTB

The ultimate pressure in the XHV calibration chamber of the PTB primary standard CE3, whose operation is based on the continuous expansion of gas, has been reduced by the use of a relatively large orifice area. Thus, the gas density and flux distributions cannot be taken as uniform in the calibration chamber. To eliminate this drawback we examined the gas flux distribution by Direct Simulation using the Monte Carlo method. This paper describes the gas flux distribution over the chamber walls, which was simulated, recorded and related to the idealized conditions. This allows determining the gauge position correction factor K_gi for each of the gauge ports. Using these factors, it is possible to eliminate the systematic deviation of the gas flow parameters from those under the idealized Maxwellian conditions and to improve the uncertainty budget.     

球头体逆向喷流减阻的数值模拟研究

为研究逆向喷流对超声速球头体减阻的影响,该文结合标准k-ε湍流模型,通过求解轴对称和三维Navier-Stokes方程,数值模拟了超声速球头体逆向冷喷流流场,着重分析了喷口总压、喷口尺寸及攻角对流场模态和减阻效果的影响。计算结果显示:喷流能使球头体受到的阻力明显减小;随着喷流总压的增大,在不同喷口尺寸和攻角下,流场均先后经历长射流和短射流穿透模态;存在最大减阻临界喷流总压值,该值与喷口尺寸比呈近似的线性关系,在所研究参数范围内最大减阻可达54.7%;随着攻角的增大,流场的不对称性加强,减阻效果下降。该文的研究对超声速钝体减阻技术在工程上的应用具有一定的参考价值。     

温度极化对膜蒸馏过程的影响研究

采用直接接触式膜蒸馏，以纯水为料液，考察了材料及结构不同的5种微孔疏水膜的渗透性能．分析结果认为，膜的渗透系数和温度极化系数共同影响着膜通量的大小．实验还考察了料液温度和流率对膜通量及热效率的影响，利用膜及膜两侧边界层内传热传质理论对此进行了分析讨论．因温度极化系数随操作温度变化，可通过控制适宜操作温度获得大膜通量和高热效率．增大料液流率可强化传热，使温度极化减弱，膜通量增加．     

Gas gap thermal switches using neon or hydrogen and sorption pump

The very low pressure obtained thanks to adsorption phenomenon at low temperature can be used to build cryogenic heat switches, which offer the possibility to make or break thermal contact between two parts of a cryogenic system. The ON (conducting) and OFF (insulating) states of the switch are obtained by varying the gas pressure between two copper blocks separated by a gap of 100 μm. This pressure is controlled by acting upon the temperature of a small sorption pump (activated charcoal) connected to the gap space. For a “high” sorption pump temperature, the gas previously adsorbed in the sorption pump is released to the gap between the two blocks, allowing a good thermal conduction through the gas (ON state). On the opposite, cooling the sorption pump allows a very good vacuum between the copper blocks, which efficiently break the thermal contact (OFF state). Experimental thermal characteristics (Conductance in the ON and OFF state, ON-OFF switching temperature) of such a “Gas Gap Heat Switch” are described using hydrogen or neon as exchange gas and are compared with theoretical calculations.