A numerical and experimental analysis of the process of water vapour absorption by a static lithium bromide solution

This paper describes a numerical and experimental analysis of the process of water vapour absorption by a static lithium bromide solution. In the experiment, the temperature evolution of the absorbent solution is measured at different heights. The numerical model solves the set of governing equations for the simultaneous heat and mass transfer inside the absorbent by means of the finite-volume method. An iterative method is used to take into account the strong coupling of heat and mass transfer at the interface and variations of thermophysical properties. A moving grid technique is employed to represent the increase of the solution volume. Model results are compared with our measurements and data reported in the literature. The influence of using constant properties is analysed by comparison with the variable properties and experimental results. It is found that this assumption provides acceptable results in the investigated pool absorption cases despite a strong underestimation of the increase of the solution volume in the course of the absorption process.     

Numerical simulation on vapor absorption by wavy lithium bromide aqueous solution films

Numerical simulation has been made on heat and mass transfer of vapor absorption by wavy lithium bromide aqueous solution films. The velocity fields and interface positions are obtained by VOF model. Solitary waves are generated by periodically disturbed inflow boundary. Based on these, the temperature and concentration fields are obtained with a stationary interface shape. The effect of solitary waves on the heat and mass transfer across the film is investigated. It is shown that due to the mixing of circulation and stretch of large film thickness, the gradient of concentration and absorption rate decrease for solitary wave region. The region of capillary waves shows a significant amount of absorption enhancement. The percentage of absorption for the different regions is quantified.     

动力减振器的理论解

以连续弹性体的振动理论为依据,推导了带有集中质量的简支梁的理论解. 在此基础上又推导了以简支梁为模型的动力减振器的理论解. 计算结果表明：如果不考虑简支梁跨中的集中质量,其误差成倍增加. 利用推导的计算公式,所得结论与实测结果吻合得很好.     

A simple physical model for steam absorption into a falling film of aqueous lithium bromide solution on a horizontal tube

For one horizontal tube in an absorber the Nusselt solution for film thickness and velocity distribution was applied, assuming steady state in heat transfer and a semi-infinite body’s concentration profile with unsteady state mass transfer. The model was applied to the absorption of steam into aqueous lithium bromide in absorption chillers. The results are compared to published experimental values and show fair agreement.     

Molecular dynamics study of the liquid-vapor interface of lithium bromide aqueous solutions

The molecular dynamics simulations of the liquid–vapor interface of LiBr aqueous solutions were carried out to investigate the structural and thermophysical properties. As concerns the structural properties, the results of molecular dynamics simulation show that the ions exist in the liquid apart from the surface and this tendency becomes strong as the solute concentration is lowered. This phenomenon is due to the desorption of ion. The calculated values such as density or surface tension agree with experimental ones. As concerns thermophysical properties, the number of water molecules in the bulk gas decreases with an increase of the solute concentration. This result represents the depression of vapor pressure. In addition, in order to investigate the dynamic process of water vapor absorption into LiBr aqueous solution, the molecular dynamics simulation under non-equilibrium condition was carried out. The results show that when the solute concentration is low and the temperature is also low, almost all incident water molecules become trapped at the solution surface and then easily diffuse into the bulk liquid, and when the solute concentration is high and temperature is also high, most incident water molecules become trapped at the solution surface, and the sequent processes are very complicated. Received on 28 September 1998     

Stabilization of a 1/3-order subharmonic resonance using nonlinear dynamic vibration absorber

The paper proposes a stabilization method for a 1/3-order subharmonic resonance with a new type of nonlinear vibration absorber using nonlinear coupling between a main system and the absorber. The main system with nonlinear restoring force and harmonic excitation, i.e., subjected to a sinusoidally changed magnetic force, is introduced as a model which produces a 1/3-order subharmonic resonance. A damped pendulum, whose natural frequency is tuned to be in the neighborhood of twice that of the main system, is connected through a link to the main system as a nonlinear vibration absorber. Theoretical results using the method of multiple scales show that only a stable nontrivial steady state is changed into an unstable one due to the effect of absorber. In addition, we numerically confirm the validity of the proposed absorber using Runge–Kutta method.     

Heat transfer to evaporating falling films of water and sugar/water solution

A series of heat transfer experiments is performed with pure water and sugar/water solution films flowing down on outside surface of a smooth or fluted vertical stainless steel tube near atmospheric pressure. Data on length-averaged heat transfer coefficient are presented in a range of Prandtl number 2<Pr<20 with wall heat fluxes up to 30 kW/m². The measured heat transfer coefficient for smooth tube does not agree very well with the well-known correlated of Chun & Seban. A new corrected correlation is presented. The fluted tube shows an increased heat transfer performance as compared to the corresponding smooth tube. The increase in the overall heat transfer coefficient turned out to be slightly higher than the increase in the corresponding heat transfer area. Film thickness and spectrum of wave frequencies were measured for smooth tube with a special probe assembly.

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Wärmeübergang bei Verdampfung von Wasser- und Zuckerwasser-Rieselfilmen

Zusammenfassung Es werden Ergebnisse von Verdampfungs-experimenten mitgeteilt, die an Rieselfilmen von Wasser und Zuckerwasserlösungen gewonnen wurden, welche an der Außenseite von glatten oder geriefelten Edelstahlrohren unter Atmosphärendruck abliefen. Sie beziehen sich auf den gemittelten Wärmeübergangskoeffizienten bei Wärmestromdichten von bis zu 30 kW/m² im Prandtl-Zahlenbereich 2<Pr<20. Letztere stimmen bei Glattrohren nicht zufriedenstellend mit den aus der wohlbekannten Korrelation von Chun und Seban folgenden Werten überein, weshalb eine verbesserte Beziehung vorgeschlagen wird. Im Vergleich mit Glattrohren liefern Rohre mit Riefen ein besseres Wärmeübertragungsverhalten, auch wenn die durch Riefelung bewirkte Vergrößerung der Oberfläche berücksichtigt wird. Filmdichte und Spektrum der Wellenfrequenzen wurden an Glattrohren mit einem speziellen Meßgerät ermittelt.

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Vibration analysis of undamped,suspended multi-beam absorber systems

The vibration analysis of an Euler-Bernoulli beam with an attached rotary unit is first carried out assuming no unbalance. For comparison purposes, two different beam end boundary conditions are considered: a simply-supported and a clamped-clamped condition. The problem is then extended to the vibration behavior of the initial beam when subjected to a harmonic load due to an unbalance in the rotary unit. To absorb the ensuing vibrations, a secondary passive beam system is suspended from the primary beam which consists of two continuous leaf springs and three discrete masses. The absorption frequency is obtained by exploring the deflection norm of the primary beam versus dimensionless frequencies of the system. To ensure the appropriateness of the procedure for similar multi-beam absorber systems, an experimental set-up is established and analytical results are verified.     

含斜裂纹剪切型橡胶减振垫的断裂分析

通过非线性有限元方法分别对含角斜裂纹和含边缘斜裂纹的剪切型橡胶减振垫分别进行了数值计算,橡胶材料采用Mooney本构模型。对受剪切载荷作用的减振垫,分别给出了减振垫的切向刚度和撕裂能随裂纹倾角、裂纹深度、载荷大小的变化规律。研究表明,随着裂纹长度的增加,撕裂逐渐增加,两者基本显非线性关系。     

Suppression of super-harmonic resonance response using a linear vibration absorber

Super-harmonic resonances may appear in the forced response of a weakly nonlinear oscillator having cubic nonlinearity, when the forcing frequency is approximately equal to one-third of the linearized natural frequency. Under super-harmonic resonance conditions, the frequency-response curve of the amplitude of the free-oscillation terms may exhibit saddle-node bifurcations, jump and hysteresis phenomena. A linear vibration absorber is used to suppress the super-harmonic resonance response of a cubically nonlinear oscillator with external excitation. The absorber can be considered as a small mass-spring-damper oscillator and thus does not adversely affect the dynamic performance of the nonlinear primary oscillator. It is shown that such a vibration absorber is effective in suppressing the super-harmonic resonance response and eliminating saddle-node bifurcations and jump phenomena of the nonlinear oscillator. Numerical examples are given to illustrate the effectiveness of the absorber in attenuating the super-harmonic resonance response.     

Exergy analysis of vapor compression refrigeration cycle with two-stage and intercooler

In this study, exergy analyses of vapor compression refrigeration cycle with two-stage and intercooler using refrigerants R507, R407c, R404a were carried out. The necessary thermodynamic values for analyses were calculated by Solkane program. The coefficient of performance, exergetic efficiency and total irreversibility rate of the system in the different operating conditions for these refrigerants were investigated. The coefficient of performance, exergetic efficiency and total irreversibility rate for alternative refrigerants were compared.     

Suppression of chaos in nonlinear oscillators using a linear vibration absorber

Meccanica - This paper proposes a nonfeedback control to suppress the chaotic response of nonlinear oscillators. A linear vibration absorber is used as nonfeedback method, whose key idea is to find...     

Optimal design of vibration absorber using minimax criterion with simplified constraints

In this paper,a minimax design of damped dynamic vibration absorber for a damped primary system is investigated to minimize the vibration magnitude peaks.Moreover,to reduce the sensitivity of the primary system response to variations of the forcing frequency for a twodegree-of-freedom system,the primary system should have two equal resonance magnitude peaks.To meet this requirement,a set of simplified constraint equations including distribution characteristics of the resonant frequencies of the primary system is established for the minimax objective function.The modified constraint equations have less unknown variables than those by other authors,which not only simplifies the computation but also improves the accuracy of the optimal values.The advantage of the proposed method is illustrated through numerical simulations.     

Catastrophe analysis of cylindrical lithium ion battery

The thermal runaway reactions in a lithium ion battery cause its temperature and pressure to increase sharply and even as a result explode in the worst conditions. This kind of explosion is thought of as a catastrophe phenomenon. The energy conservation equation for the discharging process of lithium ion battery was produced, to disclose the catastrophic mechanism of thermal runaway explosion. By the dimensionless method, the swallowtail catastrophe potential function of the lithium ion battery was obtained. The control variables of the potential function were discussed further and the thermal runaway zones and non-thermal runaway zones were obtained. The results indicate that the thermal runaway of lithium ion battery is a swallowtail catastrophe in essence, and thus the control methods of lithium ion battery thermal runaway can be designed from the view point of catastrophes in the future.     

Modeling and experimental analysis of an efficient absorber for air dehumidification

The performance of an air dehumidifier composed of a tube bank sprinkled with a LiCl/H₂O hygroscopic solution and cooled inside by water is studied under steady state conditions. An one-dimensional model simulating also the case of solution recirculation is presented together with the experimental procedure for the evaluation of the mass transfer coefficient for the apparatus. A series of tests on an experimental absorber validates the model. The behaviour of the apparatus together with a simplified parametric study are then presented for a full scale absorber with air and solution flowing both in countercurrent and in cocurrent. The external parameters controlling the apparatus performance are also defined.

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Mathematische Simulation und experimentelle Analysen eines leistungsfähigen Absorptionsgerätes für die Luftentfeuchtung

Zusammenfassung Es wird das Verhalten eines Luftentfeuchters bei Bedingungen stationären Zustandes studiert, bestehend aus einem versetzt angeordneten Rohrbündel mit einer hygroskopischen Lösung aus LiCl/H₂O und im Innern mit Wasser gekühlt. Es wird sowohl ein eindimensionales Modell vorgestellt, welches in der Lage ist, auch den Fall von Rezirkulation der Lösung zu simulieren, als auch ein experimentelles Verfahren für die Beurteilung des Koeffizienten des Stoffüberganges des Gerätes.Eine Serie von Prüfungen eines experimentellen Absorptionsgerätes bestätigen das Modell. Für den Fall eines Absorptionsgerätes in natürlichem Maßstab wird die Studie über das Verhalten des Gerätes zusammen mit einer vereinfachten Parameterisierung vorgestellt, sowohl für Luft und Lösung in Gegenals auch in Gleichstrom. Die äußeren Parameter, welche die Leistung des Gerätes kontrollieren, werden festgelegt.

     

A simplified solution for piled-raft foundation analysis by using the two-phase approach

In common practice, the pile–soil–raft interaction still remains a challenging problem in the analysis of piled-raft foundations. In the present study, a simplified analytical approach is introduced to analyze a vertically-loaded piled-raft foundation by using a developed homogenization technique called the two-phase approach. In spite of classical and simplified methods in the literature, the proposed method considers the pile–soil interaction. The other major advantage is the ability to predict the axial pile load along the pile length. The problem is solved in the domain of elasticity and simple closed-form solutions are presented for the prediction of the settlement and the pile load sharing of a piled raft as well as the pile's axial force distribution along its length. The applicability of the proposed method is validated by considering case studies and field measurements. A comparison of the results indicates that the method can be utilized safely in a proper, quick, and effective manner with the least computational effort in comparison with sophisticated numerical approaches. The raft settlement can be accurately predicted while the pile load sharing might be over/under estimated. A parametric study is also carried out to investigate the response of piled-raft foundations including the influence of the parameters of the soil and the geometric characteristics of the piles.     

The role of solution structure in apparent thickening behavior of dilute peo/water systems

An experimental investigation was undertaken to determine the role of solution structure on the apparent thickening exhibited by “dilute” polyethylene oxide/water solutions in extensional flow. Measurements of apparent relative viscosity were obtained as a function of wall shear rate for solutions flowing from a reservoir through a 0.1 mm internal diameter tube. As the wall shear rate was increased, slightly shear thinning behavior was observed up until a critical wall shear rate was exceeded at which point a large increase in relative viscosity was seen. Other researchers have observed these apparent thickening effects and have interpreted them in terms of individual polymer molecules undergoing coil-stretch transitions. However, in the systems used in this study, the critical wall shear rate and the degree to which relative viscosity increased wer both seen to be strong functions of solution aging time and concentration. These results are inconsistent with the simple picture of individual polymer coils undergoing a coil-stretch transition and instead are consistent with the picture of aggregated systems or micronetworks being stretched from their equilibrium configurations.     

Exergy efficiency of a counterflow air/air heat exchanger with vapour condensation

The exergy efficiency of a counterflow hot moist air/ambient air heat exchanger with vapour condensation as a function of the temperature and humidity of the hot moist air, the ratio of ambient air flow rate to hot moist air flow rate and the minimum temperature differences between the flows is determined. The ranges 20 to 120°C and 0.01 to 1.0 kg·kg^–1 for the temperature and humidity ratio of hot moist air respectively are considered. Constant ambient air conditions of 20°C and 0.01 kg·kg^–1 were assumed.

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Exergie-Wirkungsgrad eines Luft/Luft-Gegenstrom-Wärmetauschers mit Dampfkondensation

Zusammenfassung Der Exergie-Wirkungsgrad eines Gegenstrom-Wärmetauschers wird bestimmt für die Kombination: heiße Feuchtluft zu Umgebungsluft mit Dampfkondensation, und zwar als Funktion von Temperatur und Feuchtegrad der heißen Feuchtluft, des Verhältnisses von Umgebungsluft zu heißen Feuchtluftstrom und der minimalen Temperaturdifferenz zwischen beiden Strömen. Die Temperatur des heißen Feuchtluftstroms variiert im Bereich von 20 bis 120°C, sein Feuchtegrad im Bereich 0,01 bis 1,0. Bezüglich der Umgebungsluft wurden konstante Bedingungen angenommen, und zwar 20°C und ein Feuchtegrad von 0,01.