Two-Phase Flow Analyses During Throttling Processes

This study presents an experimental investigation of the throttling process of saturated fluorocarbon refrigerants (such as R116, R218, and R610) inside capillary tubes. For the detailed two-phase flow analyses, refrigerant R218 was selected. A divided capillary tube was prepared with a set of precise pressure and temperature sensors providing detailed information about the refrigerant flow behavior inside the tube. The metastable flow regions of the superheated liquid and of the two-phase vapor-liquid mixture were clearly detected. A correlation for the ‘underpressure’ of vaporization applicable for capillary flow of fluorocarbon refrigerants was determined. New experimental data were compared with a modified numerical model simulating all four capillary flow regions. A negative effect of non-condensing gases present within the cooling circuit on the overall capillary tube performance was experimentally noted.     

Solubility of R22, R23, R32, R134a,R152a,R125 and R744 refrigerants in water by using equations of state

This paper tries to demonstrate the ability of VPT and CPA equations of state and modified mixing rules for predicting of solubility CHC1F₂ (R22), CHF₃ (R23), CH₂F₂ (R32), C₂H₂F₄ (R134a), C₂H₄F₂ (R152a), C₂HF₅ (R125) and CO₂ (R744) refrigerants in water at different temperatures and pressures. For this purpose, the fugacity of each component in gas and liquid phases is calculated by using VPT and CPA equations of state. Also in this work, the interaction parameters for mixing rules in each mixture are optimized by using two-phase equilibrium data (VL_W). Results of the two-phase flash calculation show good agreement between obtained solubility and the experimental data. The predicted solubility of the selected refrigerants in water agrees with the experimental data with accuracy of about 1.5% and 3.5% by VPT equation of state – modified mixing rule and CPA equation of state – Van der Waals classic mixing rule respectively.     

A Versatile Evaporative Cooling System Designed for Use in an Elementary Particle Detector

An evaporative cooling system developed for operation and qualification testing of silicon pixel and microstrip detectors for the inner tracking detector of the CERN ATLAS spectrometer is described. Silicon detector substrates must be continuously operated between 0 and − 7°C in the high radiation environment near the circulating beams at the CERN Large Hadron Collider (LHC). This requirement imposes unusual constraints on the cooling system and has led to the choice of perfluoro-n-propane (C₃F₈) refrigerant, which combines good chemical stability under ionizing radiation with high dielectric strength and nonflammability. Since the silicon detectors must also be of extremely light construction to minimize undesirable physics background, coolant tubes are of thin (200 μm) aluminum wall, while evaporative operation allows a very low circulating coolant mass-flow (1–3 g · s⁻¹/100W to evacuate). The assembled detector arrays will undergo qualification tests at room temperature before installation in the ATLAS spectrometer. The cooling system is “dual-fuel,” and can also be operated with perfluoro-n-butane (C₄F₁₀) refrigerant, offering a reduced evaporation pressure (1.9 bar) compared to that of C₃F₈ (6.5 bar at 15°C). Paper presented at the Seventeenth European Conference on Thermophysical Properties, September 5–8, 2005, Bratislava, Slovak Republic.     

Generalised critical free-surface flows

Nonlinear waves in a forced channel flow are considered. The forcing is due to a bottom obstruction. The study is restricted to steady flows. A weakly nonlinear analysis shows that for a given obstruction, there are two important values of the Froude number, which is the ratio of the upstream uniform velocity to the critical speed of shallow water waves, F _C>1 and F _L<1 such that: (i) when F<F _L, there is a unique downstream cnoidal wave matched with the upstream (subcritical) uniform flow; (ii) when F=F _L, the period of the cnoidal wave extends to infinity and the solution becomes a hydraulic fall (conjugate flow solution) – the flow is subcritical upstream and supercritical downstream; (iii) when F>F _C, there are two symmetric solitary waves sustained over the site of forcing, and at F=F _C the two solitary waves merge into one; (iv) when F>F _C, there is also a one-parameter family of solutions matching the upstream (supercritical) uniform flow with a cnoidal wave downstream; (v) for a particular value of F>F _C, the downstream wave can be eliminated and the solution becomes a reversed hydraulic fall (it is the same as solution (ii), except that the flow is reversed!). Flows of type (iv), including the hydraulic fall (v) as a special case, are computed here using the full Euler equations. The problem is solved numerically by a boundary-integral-equation method due to Forbes and Schwartz. It is confirmed that there is a three-parameter family of solutions with a train of waves downstream. The three parameters can be chosen as the Froude number, the obstruction size and the wavelength of the downstream waves. This three-parameter family differs from the classical two-parameter family of subcritical flows (i) but includes as a particular case the hydraulic falls (ii) or equivalently (v) computed by Forbes.     

Intensive parameter analysis of adiabatic capillary tube using approximate analytic solution

Parameter analysis is helpful to have an insight into the flow characteristics of capillary tubes. Based on the approximate analytic solutions developed by the author, influences of geometrical parameters (inner diameter and length) and inlet operating parameters (pressure, subcooling or quality) on the mass flow rate through an adiabatic capillary tube have been intensively studied in this work. Some simple theoretical relations have been developed. The relations show that the mass flow rate is the power function of the geometries. For the subcooled inlet, good generic linearity between (2−C₂) power of the mass flow rate and the inlet operating parameters was deduced. With further approximation, some well-known linear trends could be theoretically interpreted. For the low quality inlet, good linearity between (C₂−2) power of the mass flow rate and the inlet quality was proposed. Approximately, the reciprocal of the mass flow rate is linear with the inlet quality. Experimental data supplemented by numerical data for R22, R410A and R407C are employed to verify the relations.     

Viscosity of saturated liquid fluorocarbon refrigerants from 273 to 353 K

Viscosity measurements were carried out on saturated liquid fluorocarbon refrigerants using an improved capillary viscometer for 11 kinds of fluorocarbon refrigerants; CCl₃F (R11), CCl₂F₂ (R12), CHClF₂ (R22), CBrF₃ (R13B1), CH₃CHF₂ (R152a), CCl₂FCClF₂ (R113), CHCl₂CF₃ (R123), CHClFCClF₂ (R123a), CH₃CF₃ (R143a), CClF₂CCl₂F₂ (R114), and CH₂FCF₃ (R134a), in the temperature range from 273 to 353 K. An equation is given to represent the viscosity as a function of temperature.     

Visualization of the solid–liquid equilibria for non-flammable mixed refrigerants

Non-flammable mixed refrigerant (NF-MR) Joule Thomson (J–T) refrigerators have desirable characteristics and wide cooling temperature range compared to those of pure J–T refrigerators. However, the operating challenge due to freezing is a critical issue to realize this type of refrigerator. In this paper, the solid–liquid phase equilibria (i.e. freezing point) of the NF-MR which is composed of Argon (Ar), R14 (CF₄), and R218 (C₃F₈), has been experimentally investigated by a visualized apparatus. The accuracy of the apparatus is experimentally verified with pure refrigerants and selected binary mixed refrigerants. Freezing points of the ternary NF-MRs have been measured with the molar compositions from 0.1 to 0.8 for each component. Each test result is simultaneously acquired by a camcorder for visual inspection and temperature measurement during a warming process. Experimental results reveal that the specific MR, with R14 molar composition higher than 0.4, can achieve remarkably low freezing temperature even below 77 K. These unusual freezing point depression characteristics of the MR can be a useful information for designing a cryogenic MR J–T refrigerator to reach temperatures less than 77 K.     

Conversion between non-isothermal and isothermal transformation kinetics of γ to α for C–Mn and Nb microalloyed steels

Abstract

In the present paper, austenite γ to ferrite α transformation kinetics of steels at different cooling rates has been measured by thermal dilatation. The relationship between lnln[1/(1 ? X)] and ln(|C _R|) (C _R: cooling rate) was plotted using the data at early stage of cooling, which well fitted a linear relationship for the calculation of exponential values of n in the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation for isothermal kinetics using the method developed by Rios. The values of k in the JMAK equation obtained with the Rios method, however, have led to big discrepancies when the isothermal equations were used to predict the transformation kinetics during cooling. By assuming a Gaussian function of temperature, k was calculated using an optimisation method based on the rule of additivity. The isothermal transformation model was used to predict the transformation kinetics during cooling, showing good agreement with the measured data. It has been proved that even though the rule of additivity has to be relaxed to take into account the effects of cooling rates, precise conversion between non-isothermal and isothermal kinetics can still be realised.     

The Influence of Adsorption on PVT Measurements in the Gaseous Phase

PVT measurements of 1,1,1,2-tetrafluoroethane (C₂H₂F₄, HFC-134a) and its blend with octofluoropropane (C₃F₈, FC-218) have been performed in the gas phase near the dew curve. The experimental data were obtained by variable-volume and vibrating tube methods. Discrepancies in the behavior of isotherms from their classical behavior were experimentally observed. It was found that the phase transition does not go to completion at a single point of the thermodynamic surface but extends over a limited range of conditions. Obtained results are in accordance with a concept of adsorption of the vapor sample on the surface of the experimental cell. An increase in adsorption under the conditions close to condensation is caused by capillary condensation of the sample at the walls of the cell that initiates an early phase transition. The ranges of diffuse phase transitions were determined for 1,1,1,2-tetrafluoroethane as well as for its mixture with octofluoropropane at different thermodynamic parameters. The influence of selective adsorption on the change in the conditions of phase transition of the 1,1,1,2-tetrafluoroethane/octofluoropropane mixture was also experimentally studied.     

Surface modification of materials by dielectric barrier discharge deposition of fluorocarbon films

Dielectric barrier discharges have been used to deposit fluorocarbon (FC) films on various materials, such as paper, glass, and silicon substrates. The primary monomers used for plasma polymerization were difluoromethane (CH₂F₂), octafluoropropane (C₃F₈), and octafluorocyclobutane (C₄F₈). FC films were characterized using Fourier transform infrared spectroscopy, atomic force microscopy, static contact angle measurements, and scanning electron microscopy. Surface and structural properties of deposited films are strongly dependent on the plasma compositions and plasma parameters. FC films deposited on paper are to enhance its barrier properties and to achieve hydrophobic surfaces. Contact angle studies reveal that a minimum FC film thickness of about 200 nm on paper is required to completely cover surface and near-surface fibers, thereby providing the paper with long term hydrophobic character. In the C₃F₈ and C₄F₈ systems, the contact angles of the deposited films do not change appreciably with plasma parameters and are strongly dependent on the substrate roughness. Hydrogenated FC films deposited with CH₂F₂ plasmas show the relatively low contact angles due to the existence of CH_X (x = 1-3) groups.     

Assessment on the use of common correlations to predict the mass-flow rate of carbon dioxide through capillary tubes in transcritical cycles

Capillary tubes are extensively used in small refrigeration and air-conditioning systems with synthetic refrigerants and hydrocarbons. For CO₂ transcritical applications, it has been shown that the capillary tube demonstrates an intrinsic capability of adjusting the upper pressure close to the optimal value in response to changes of gas-cooler heat sink temperature. The CO₂ flow rate through four capillary tubes of various lengths, diameters and materials was measured in a test rig. Each capillary tube was tested with inlet pressure varying from 7.5 MPa to 11 MPa and inlet temperature from 20 °C to 40 °C. Outlet pressure varied from 1.5 MPa to 3 MPa. The experimental results were validated against different numerical and approximate analytical solutions of the capillary tube equations. These models give good predictions only if the friction factor of the capillary tube is calculated accounting for its dependence on the tube roughness.     

Effect of pressure and surface roughness on pool boiling of refrigerantsInfluence de la pression et de la rugosité superficielle sur l'ébullition libre des frigorigènes

Heat transfer from five horizontal copper plates with different surface finish to boiling refrigerants R11 (CFCI₃) and R115 (C₂F₅Cl) was measured at saturation pressures between 0.4 and 91% of the critical pressure. Although the relative pressure dependence of the heat transfer coefficient α is similar for the plates with different surface treatment, a detailed comparison of the experimental results shows that the influence of the different surface conditions achieved by mechanical and chemical treatment on the absolute value of α cannot be correlated by one of the roughness parameters used in the literature, within the whole range of pressures investigated.     

Ideal-Gas Heat Capacities and Virial Coefficients of HFC Refrigerants

Thermodynamic properties of HFC (hydrofluorocarbon) compounds have been extensively studied with worldwide interest as alternative refrigerants. Both quality and quantity in the experimental data far exceed those for the CFC and HCFC refrigerants. These data now provide a great opportunity to examine the validity of theoretical models, and vice versa. Among them, the ideal-gas heat capacity C _p ⁰ and virial coefficients derived from the experimental data are of particular interest, since they are directly related to the intramolecular and intermolecular potentials through the statistical mechanical procedure. There have been some discrepancies reported in the observed and theoretical C _p ⁰ for HFC compounds. We have performed new calculations of C _p ⁰ for several HFCs. The present results are consistent with the selected experimental values. The second (B) and third (C) virial coefficients have been reported for these HFC refrigerants from speed of sound data and Burnett PVT data. Often, a square well-type intermolecular potential is employed to correlate the data. However, the model potential cannot account consistently for both B and C coefficients with the same potential parameters. We have analyzed the data with the Stockmayer potential and obtained self-consistent results for various HFC (R-23, R-32, R-125, R-134a, R-143a, and R-152a) compounds with physically reasonable potential parameters.     

Effect of pressure and surface roughness on pool boiling of refrigerants

Heat transfer from five horizontal copper plates with different surface finish to boiling refrigerants R11 (CFCI₃) and R115 (C₂F₅Cl) was measured at saturation pressures between 0.4 and 91% of the critical pressure. Although the relative pressure dependence of the heat transfer coefficient α is similar for the plates with different surface treatment, a detailed comparison of the experimental results shows that the influence of the different surface conditions achieved by mechanical and chemical treatment on the absolute value of α cannot be correlated by one of the roughness parameters used in the literature, within the whole range of pressures investigated.     

Measured and predicted thermodynamic properties of selected halon alternative/nitrogen mixtures

Experimental measurements of selected thermodynamic properties of alternative agent/nitrogen mixtures were performed. The selected alternatives were HFC-227ea (C₃HF₇), CF₃I, FC-218 (C₃F₈), and HFC-125 (C₂HF₅); CF₃Br was also included as a reference. A thermodynamic model based on an extended corresponding states principle was used. The model predictions were generally found to be within 10% or less of the experimental measurements. The model can be used as a computational tool for designers of fire extinguishers to obtain pressure-temperature relationships for selected halon alternative/nitrogen mixtures.     

Design method of the layered active magnetic regenerator (AMR) for hydrogen liquefaction by numerical simulation

The design procedure of an active magnetic regenerator (AMR) operating between liquid nitrogen temperature and liquid hydrogen temperature is discussed with the selected magnetic refrigerants. Selected magnetic refrigerants (GdNi₂, Dy_0.85Er_0.15Al₂, Dy_0.5Er_0.5Al₂, and Gd_0.1Dy_0.9Ni₂) that have different transition temperatures are layered in an AMR to widen the temperature span. The optimum volume fraction of the layered refrigerants for the maximum COP with minimum volume is designed in a two-stage active magnetic regenerative refrigerator (AMRR) using one dimensional numerical simulation. The entropy generation in each stage of the AMR is calculated by the numerical simulation to optimize the proposed design. The main sources of the entropy generation in the AMR are pressure drop, convection and conduction heat transfers in the AMR. However, the entropy generation by the convective heat transfer is mostly dominant in the optimized cases. In this paper, the design parameters and the operating conditions such as the distribution of the selected refrigerants in the layered AMR, the intermediate temperature between two stages and the mass flow rate of heat transfer fluid are specifically determined to maximize the performance of the AMR. The proposed design method will facilitate the construction of AMR systems with various magnetic refrigerants and conditions such as AMR size, operating temperature range, and magnetic field variation.     

Core electron level structure in C<Subscript>60</Subscript>F<Subscript>18</Subscript> and C<Subscript>60</Subscript>F<Subscript>36</Subscript> fluorinated fullerenes

The structure of C1s and F1s core electron levels in C₆₀F₁₈ and C₆₀F₃₆ fluorinated fullerenes has been studied by X-ray photoelectron spectroscopy using synchrotron radiation. It is established that C1s levels of carbon atoms not bound to fluorine in these compounds are shifted down by 1.0 and 1.6 eV relative to the C1s level in the usual C₆₀ fullerene, so that the binding energies of the core electron levels in C₆₀F₁₈ and C₆₀F₃₆ amount to E _b (C1s, C-C) = 285.7 and 286.3 eV, respectively. These values are characteristic and can be used for the identification of both homogeneous fluorinated fullerenes and combined materials comprising a mixture of various fluorinated fullerenes with each other and with different carbon-containing based materials.     

Surface reaction processes in C4F8 and C5F8 plasmas for selective etching of SiO2 over photo-resist

Surface reactions induced by fluorocarbon plasmas were studied on Si substrates with SiO₂ and photo-resist overlayers using an inductively coupled plasma source. As source gases, C₄F₈ and C₅F₈ were employed to investigate their differences in the etching performance and the selectivity between SiO₂ and photo-resist. Deposition of fluorocarbon polymer was noticed in both gases by Fourier-transform infrared ellipsometric measurements when substrate bias was not applied. With the bias application, etching started on both substrate from certain threshold values of the bias voltage and the rate increased with increase of the voltage. However, in C₅F₈ plasma the increasing tendency on photo-resist was much less than on SiO₂, while in C₄F₈ plasma the difference is small. This difference is attributed to a larger deposition ability of C₅F₈ plasma with higher content of fluorine atoms in the polymer than that of C₄F₈ plasma as confirmed by X-ray photoelectron spectroscopy.     

Resonant nonlinearity enhancement in rubidium vapor with additional optical pumping

We investigate the enhancement of characteristic nonlinear phase shift in warm rubidium vapor for a signal beam using an additional pump laser at different frequency. A numerical model based on density matrix formalism is used. Experimental results of self-rotation and diffraction in a three-wave mixing process as well as beam amplification are shown. For ⁸⁷Rb transition D2 line with F_g?=?1, the nonlinearity to absorption ratio can be enhanced approximately two times with a pump beam tuned at F_g?=?2 transition in a co-propagating scheme, which produces strong nonlinear interaction for F_g?=?1 line at signal beam powers of ~10?mW.     

基于R600/R290混合制冷剂的立式冷冻箱的降温能力试验研究

为优化以混合制冷剂为工质的立式冷冻箱制冷系统降温能力,对使用R600/R290混合制冷剂的立式冷冻箱在32℃环境温度下进行降温能力试验,研究混合制冷剂充注量、制冷剂各组分配比、毛细管流量及风机转速对其性能的影响.结果 表明:混合制冷剂充注量过大会削弱制冷系统降温能力,并使压缩机功率增加;增大毛细管流量、风机转速及混合制...