Visualization and model development of Marangoni convection in NH3–H2O systemVisualization et développement d'un modèle de convection de Marangoni dans un système à NH3–H2O

The objectives of this paper are to obtain experimental data of surface tension and interfacial tension, and to develop a new model of Marangoni convection for the best selection of heat transfer additive in ammonia–water absorption systems. The basic mechanism of Marangoni convection in absorption systems was reviewed from the viewpoints of the surface tension and the interfacial tension gradients. Marangoni convection was successfully visualized using a shadow graphic method. The solubility limits of the additives in ammonia–water solution ranged from 500 to 3000 ppm depending on the heat transfer additives. These values are much higher than those in LiBr–H₂O solution in which the solubility ranged from 70 to 400 ppm. The temperature gradient of the surface tension should not be a criterion for Marangoni convection inducement in NH₃–H₂O system. The concentration and temperature gradients of the interfacial tension should not be a criterion for Marangoni convection inducement in NH₃–H₂O system. The magnitude of the interfacial tension did not affect the occurrence of Marangoni convection either. It was found that addition of the heat transfer additive beyond the solubility limit assisted Marangoni convection occurrence, but should not be a criterion for Marangoni convection inducement. It was proposed that the radical-out model should be a criterion for Marangoni convection inducement within the solubility limit in NH₃–H₂O system.     

A phenomenological theory of polycomponent interactions in non-ideal mixtures. Application to NH3/H2O and other working pairsInteractions entre les composants de mélanges non-idéaux : théorie et application au fluide actif NH3 / H2O et à d'autres fluides actifs

The paper proposes an original linear phenomenological theory (Ph T) of evolution physical mono-, bi- and particular polycomponent gas–liquid interactions with non-ideal mixture. The expressions of the phenomenological factors (entropy source, force, coefficient and coupled heat and mass transfer currents) are deduced. The theory is particularized to the NH₃/H₂O and other gas–liquid systems used in the thermal absorption technology. The work's conclusions are listed next. The paper raises the problem of ammonia bubble absorption which is difficult to answer with current theory of interface mass transfer and absorption as a surface phenomenon. The heat and mass transfer at the gas–liquid interface is governed by the thermodynamic force, which applies also to solid–liquid, solid–gas, or liquid–liquid, gas–gas type interactions and continuous or discontinuous media. The paper mentions a postulate referring to the force behavior approaching an ideal point, previously formulated by the author. According to its consequence, the mass and heat currents suffer an ideal point approaching (i.p.a.) effect, not mentioned so far in the specialized literature, consisting in a continuous increase of their absolute value by several percent (for a pure component), to several hundred times (for a binary system) when the interacting system approaches an ideal state, as compared to the values of states which are far from the same ideal point. In this way, “far from equilibrium” becomes synonymous to “low interaction”. The classic assessment of the interface mass transfer by analogy with heat transfer lacks basic physics. The (Ph T) satisfactorily explains the problem of ammonia bubble absorption. Absorption is a mass phenomenon, not a surface one. An intensive way of improving absorption is emphasized, which seeks to promote the i.p.a. effect appearance rather than the extensive way currently used, based on increasing gas–liquid interaction area. To this extent, the bubble absorber is hereby proposed for efficient absorption. The i.p.a. effect existence offers an additional chance for a satisfactory explanation of the Marangoni effect.     

The role of surfactant adsorption rate in heat and mass transfer enhancement in absorption heat pumps

The importance of heat and mass transfer additives in absorption chillers and heat pumps has been recognized for over three decades. However, a universally accepted model for the mechanisms responsible for enhanced absorption rates has yet to be proposed. The Marangoni effect—an instability arising from gradients in surface tension at the liquid-vapor interface—is generally accepted as the cause of the convective flows that enhance transfer rates. Certain surfactant additives can significantly improve absorption rates and thus reduce the overall transfer area required by a given machine. Any means available that can increase the efficiency and acceptability of absorption machines is to be welcomed, as this technology provides an alternative to vapor compression systems which is both environmentally friendly and more versatile with regards to energy sources. This study investigates the rate at which a surfactant additive adsorbs at a liquid-vapor interface. The residence time of the falling liquid solution in an absorber is quite short. An effective additive must not only reduce the surface tension of the solution; it must do so quickly enough to cause the Marangoni instability within the short absorption process time. The entrance region of an absorber features a freshly exposed interface at which no surfactant has adsorbed. A numerical model is used to analyze surfactant relaxation rates in a static film of additive-laced solution. Kinetic parameters for the combination of the working pair LiBr-H₂O and the additive 2-ethyl-1-hexanol are derived from data in the literature for static and dynamic surface tension measurements. Bulk, interfacial and boundary parameters influencing relaxation rates are discussed for surfactant adsorption occurring in the absence of absorption, as well as for concurrent adsorption and stable vapor absorption. Initial solution conditions and absorption driving force are shown to impact the potential for instability in the effect they have on the rate of interfacial additive adsorption.     

Effects of gas phase and additive properties on Marangoni instability for absorption process in a horizontal fluid layer

The objectives of this study are to investigate theoretically the effect of additive on the onset of Marangoni convection and to find the meaningful relationships between the important parameters. The propagation theory in which the penetration depth is chosen as the length scale and the scaling analysis are adapted in the present study. It is found that the combined absorbate Marangoni number MB is linearly related to the modified Biot number Bi^* and there is a critical Biot number Bi to cause the most unstable state of liquid layer. It is concluded that it is not always advantageous to increase the mass transfer coefficient in the gas phase in order to enhance the absorption rate. It is interesting that the additive Marangoni number Ma_A and the relative surface diffusivity of additive R_A which represent the additive properties act as the stabilizer and the destabilizer respectively for the onset of Marangoni convection.     

Two-phase flow heat transfer of CO2 vaporization in smooth horizontal minichannels

Experiments were performed on the convective boiling heat transfer in horizontal minichannels with CO₂. The test section is made of stainless steel tubes with inner diameters of 1.5 and 3.0 mm and with lengths of 2000 and 3000 mm, respectively, and it is uniformly heated by applying an electric current directly to the tubes. Local heat transfer coefficients were obtained for a heat flux range of 20–40 kW m⁻², a mass flux range of 200–600 kg m⁻² s⁻¹, saturation temperatures of 10, 0, −5, and −10 °C and quality ranges of up to 1.0. Nucleate boiling heat transfer contribution was predominant, especially at low quality region. The reduction of heat transfer coefficient occurred at a lower vapor quality with a rise of heat flux, mass flux and saturation temperature, and with a smaller inner tube diameter. The experimental heat transfer coefficient of CO₂ is about three times higher than that of R-134a. Laminar flow appears in the minichannel flows. A new boiling heat transfer coefficient correlation that is based on the superposition model for CO₂ was developed with 8.41% mean deviation.     

The effect of heat transfer additive and surface roughness of micro-scale hatched tubes on absorption performance

The objectives of this paper are to investigate the effect of heat transfer additive and surface roughness of micro-scale hatched tubes on the absorption performance and to provide a guideline for the absorber design. Two different micro-scale hatched tubes and a bare tube are tested to quantify the effect of the surface roughness on the absorption performance. The roughness of the micro-scale hatched tubes ranges 0.39–6.97 μm. The working fluid is H₂O/LiBr solution with inlet concentration of 55, 58 and 61 wt.% of LiBr. Normal Octanol is used as the heat transfer additive with the concentration of 400 ppm. The absorber heat exchanger consists of 24 horizontal tubes in a column, liquid distributor at the liquid inlet and the liquid reservoir at the bottom of the absorber. The effect of heat transfer additive on the heat transfer rate is found to be more significant in the bare tube than that in the micro-scale hatched tubes. It is found that the absorption performance for the micro-hatched tube with heat transfer additive becomes up to 4.5 times higher than that for the bare tube without heat transfer additive. It is concluded that the heat transfer enhancement by the heat transfer additive is more significant than that by the micro-scale surface treatment.     

Experimental correlation of combined heat and mass transfer for NH3–H2O falling film absorptionCorrélation expérimentale entre le transfert de chaleur et de masse pour de l'absorption au NH3–H2O à film tombant

In this article, experimental analysis was performed for ammonia–water falling film absorption process in a plate heat exchanger with enhanced surfaces such as offset strip fin. This article examined the effects of liquid and vapor flow characteristics, inlet subcooling of the liquid flow and inlet concentration difference on heat and mass transfer performance. The inlet liquid concentration was selected as 5%, 10% and 15% of ammonia by mass while the inlet vapor concentration was varied from 64.7% to 79.7%. It was found that before absorption started, there was a rectification process at the top of the test section by the inlet subcooling effect. Water desorption phenomenon was found near the bottom of the test section. It was found that the lower inlet liquid temperature and the higher inlet vapor temperature, the higher Nusselt and Sherwood numbers are obtained. Nusselt and Sherwood number correlations were developed as functions of falling film Reynolds Re₁, vapor Reynolds number Re_v, inlet subcooling and inlet concentration difference with ±15% and ±20% error bands, respectively.     

Surfactants with NH3H2O

To avoid rivulet flow in compact absorbers, which consist of compact heat exchanger plates, the surface tension of the ammonia-water solution has to be reduced by surfactants. The influence of these additives on the absorption of ammonia into water is investigated for two anionic tensides, two non-ionic tensides and the alcohol 1-octanol in a stagnant pool measuring cell. All four tensides had no influence on the absorption in the concentration range investigated (0.25–0.91 wt%), although they were able to reduce the surface tension of water down to 30mN m⁻¹. In order to increase the mass transfer, 1-octanol was used in three concentrations, which probably lead to a convection in the liquid layer, called Marangoni convection. The experiments have shown that the presence of surfactant islands is not necessary to induce Marangoni turbulence. The best result was achieved with 50 ppm 1-octanol dissolved in water.     

表面张力对流对 BaB2O4晶体生长界面边界层的作用

利用高温光学实时观察方法，实时地观察了BaB2O4（BBO）高温熔体的表面张力对流效应以及BBO单晶的旋转生长过程，计算了固液界面附近的浓度、温度以及动量边界层厚度δc，δT和δv，并研究了热毛细对流对边界层厚度的影响．结果发现，浓度边界层厚度远远小于温度以及动量边界层厚度，说明晶体生长过程中，质量扩散在界面输运过程中起着主导性作用，同时发现，边界层厚度随体系无量纲Marangoni数的增大而线性地减小．     

Flow boiling of ammonia in a plain and a low finned horizontal tubeEbullition en éncoulement d'ammoniac dans un tube lisse ou un tube ailetté

Experimental data of the local heat transfer coeffcient of flow boiling ammonia in dependence of vapor fraction, mass flux and local heat flux is presented. Two horizontal test sections of 450 mm length and an inner diameter of 10 mm have been used, one being a plain tube, one being a spirally low finned tube. A constant wall temperature boundary has been aimed for the test section by heating with a fluid condensing on the tube outside. Local heat transfer coeffcients and pressure drops have been measured in the range −40 < T_sat < 4°C, 0 < x< 0.9, 50 < < 150 kg/m² s and 2 < ΔT_w < 15 K with resulting heat fluxes of 17 < < 75 kW/m². The vapor quality is denoted as x, is the mass flux and ΔT_w the wall superheat. The measured data is carefully evaluated using a finite element model of the tube with regard to the circumferential heat flow distribution. The smooth tube results are compared with recently published data and the correlation from Zürcher (Zürcher, O., Thome, J.R., Favrat, D. Evaporation of ammonia in a smooth horizontal tube: heat transfer measurements and predictions. Journal of Heat Transfer, 1999;121:89–101), and with the correlations of Steiner (Steiner D. Strömungssieden gesättigter Flüssigkeiten. VDI-Wärmeatlas, vol. 8. VDI-Verlag, 1997) and Kattan (Kattan N, Thome JR, Favrat D. Flow boiling in horizontal tubes: part 3 — development of a new heat transfer model based on flow pattern. Transactions of the ASME, 1998;120). The results of the low finned tube are not matched by any known correlation.     

Development of an advanced A-TIG (AA-TIG) welding method by control of Marangoni convection

A new type of tungsten inert gas (TIG) welding has been developed, in which an ultra-deep penetration is obtained. In order to control the Marangoni convection induced by the surface tension gradient on the molten pool, He gas containing a small amount of oxidizing gas was used. The effect of the concentration of O₂ and CO₂ in the shielding gas on the weld shape was studied for the bead-on-plate TIG welding of SUS304 stainless under He–O₂ and He–CO₂ mixed shielding gases. Because oxygen is a surface active element for stainless steel, the addition of oxygen to the molten pool can control the Marangoni convection from the outward to inward direction on the liquid pool surface. When the oxygen content in the liquid pool is over a critical value, around 70 ppm, the weld shape suddenly changes from a wide shallow shape to a deep narrow shape due to the change in the direction of the Marangoni convection. Also, for He-based shielding gas, a high welding current will strengthen both the inward Marangoni convection on the pool surface and the inward electromagnetic convection in the liquid pool. Accordingly, at a welding speed of 0.75 mm/s, the welding current of 160 A and the electrode gap of 1 mm under the He–0.4%O₂ shielding, the depth/width ratio reaches 1.8, which is much larger for Ar–O₂ shielding gas (0.7). The effects of the welding parameters, such as welding speed and welding current were also systematically investigated. In addition, a double shielding gas method has been developed to prevent any consumption of the tungsten electrode.     

Shaped single crystal growth and scintillating application of Yb:(Gd,Lu)3(Ga,Al)5O12 solid solutions

Shaped single crystals of (Yb_0.05Lu_xGd_0.95−x)Ga₅O₁₂ (0.0x0.9) and Yb_0.15Gd_0.15Lu_2.7(Al_xGa_1−x)O₁₂ (0.0x1.0) were grown by the modified micro-pulling-down method. Continuous solid solutions with garnet structure and a linear compositional dependency of crystal lattice parameter in the system Yb:(Gd,Lu)₃(Ga,Al)₅O₁₂ are formed. Measured optical absorption spectra of the samples show 4f–4f transitions related to Gd³⁺ ion at 275 and 310 nm, and also an onset of charge transfer transitions from oxygen ligands to Gd³⁺ or Yb³⁺ cations below 240 nm. A complete absence of Yb³⁺ charge transfer luminescence under X-ray excitation in any of the investigated samples was explained by the overlapping of charge transfer absorption of Yb³⁺ by that of Gd³⁺ ions. For specific composition of Lu_1.5Gd_1.5Ga₅O₁₂ an intense defect––host lattice-related emission, which achieve of about 40% integrated intensity compared with Bi₄Ge₃O₁₂, was found.     

Review of advanced absorption cycles: performance improvement and temperature lift enhancement

The objective of this study is to propose and evaluate advanced absorption cycles for the coefficient of performance (COP) improvement and temperature lift enhancement applications. The characteristics of each cycle are assessed from the viewpoints of the ideal cycle COP and its applications. The advanced cycles for the COP improvement are categorized according to their heat recovery method: condensation heat recovery, absorption heat recovery, and condensation/absorption heat recovery. In H₂O–LiBr systems, the number of effects and the number of stages can be improved by adding a third or a fourth component to the solution pairs. The performance of NH₃–H₂O systems can be improved by internal heat recovery due to their thermal characteristics such as temperature gliding. NH₃–H₂O cycles can be combined with adsorption cycles and power generation cycles for waste heat utilization, performance improvement, panel heating and low temperature applications. The H₂O–LiBr cycle is better from the high COP viewpoints for the evaporation temperature over 0°C while the NH₃–H₂O cycle is better from the viewpoint of low temperature applications. This study suggests that the cycle performance would be significantly improved by combining the advanced H₂O–LiBr and NH₃–H₂O cycles.     

Experimental researches on characteristics of vapor–liquid equilibrium of NH3–H2O–LiBr system

An improved system of NH₃–H₂O–LiBr was proposed for overcoming the drawback of NH₃–H₂O absorption refrigeration system. The LiBr was added to NH₃–H₂O system anticipating a decrease in the content of water in the NH₃–H₂O–LiBr system. An equilibrium cell was used to measure thermal property of the ternary NH₃–H₂O–LiBr mixtures. The pressure–temperature data for their vapor–liquid equilibrium (VLE) data were measured at ten temperature points between 15–85 °C, and pressures up to 2 MPa. The LiBr concentration of the solution was chosen in the range of 5–60% of mass ratio of LiBr in pure water. The VLE for the NH₃–H₂O–LiBr ternary solution was measured statically. The experimental results show that the equilibrium pressures reduced by 30–50%, and the amount of component of water in the gas phase reduced greatly to 2.5% at T=70 °C. The experimental results predicted much better characteristics of the new ternary system than the NH₃–H₂O system for the applications.     

Optical Conductivity of La1.875Ba0.04Sr0.085CuO4

The in-plane optical conductivity and dc resistivity of La_1.875Ba_0.125–YSr _Y CuO₄ with y = 0.085 was measured from 30 to 20,000 cm^–1 and from 295 to 8 K. A strong extra-Drude absorption at finite frequency is detected both in the normal and superconducting state, and remains unchanged when the free carriers condensate. The present results confirm and extend recent infrared data on La_2–xSr_xCuO₄.     

Experimental correlation of pool boiling heat transfer for HFC134a on enhanced tubes: Turbo-E

The objectives of this paper are to study the heat transfer characteristics for enhanced surface tubes in the pool boiling and to provide a guideline for the design conditions for the evaporator using HFC134a. The shape of tube surfaces, the wall superheat, and the saturation temperature are considered as the key parameters. Copper tubes (d_o = 19.05 mm) are treated with different helix angles and the saturation temperatures are controlled from 3 to 16 °C. It is found that the pool boiling heat transfer coefficient decreases with increasing the wall superheat. It is also found that boiling heat transfer coefficients for Turbo-II and Turbo-III are 1.5–3.0 times and 1.2–2.0 times higher than that for Turbo-I without the helix angle, respectively. The higher heat transfer performance from Turbo-II and Turbo-III can be explained by the “bubble detention” phenomenon on the surface without the helix angle for the Turbo-I. The experimental correlations for the pool boiling heat transfer on the present enhanced tubes without (Type I) and with the helix angle (Type II and Type III) are developed with the error bands of ±30%, respectively.     

The effect of chemical surfactants on the absorption performance during NH3/H2O bubble absorption process

The objectives of this paper are to visualize the bubble behavior by shadow graphic method, to examine the effect of surfactants on the bubble type absorption, and to find the optimal conditions to design highly effective compact absorber for NH₃/H₂O absorption system. The initial concentrations of NH₃/H₂O solution and the kinds and the concentrations of surfactants are considered as key parameters. By measuring the absorption rate for each condition, two effects of the addition of surfactants, the Marangoni and the barrier effect, are compared with each other. The results show that the addition of surfactant enhances the absorption performance up to 4.81 times. The experimental correlations of the effective absorption ratio for each surfactant, 2-ethyl-1-hexanol, n-octanol, and 2-octanol, are suggested within ±15, ±10, and ±10%, respectively.     

Experimental studies on the characteristics of an absorber using LiBr/H2O solution as working fluidEtudes expérimentales sur les caractéristiques d'un absorbeur utilisant comme fluide actif le LiBr/H2O

The absorber is an important component in absorption machines and its characteristics have significant effect on the overall efficiency of absorption machines. This article reports on the results of experimental studies on the characteristics for a falling film absorber which is made up of 24 row horizontal smooth tubes. It shows that while the mass transfer coefficient is increased with the increase of spray density, the heat transfer coefficient is increased only in small spray density range. There is an optimum spray density between 0.005 and 0.055 kg s⁻¹ m⁻¹ spray density at which the heat transfer coefficient is maximum. The heat transfer coefficient (Nusselt number), which is traditionally expressed using Reynolds number and Prandtl number, was modified taking the effect of inlet solution concentration into account. The results can be used to optimize the future design of absorption machines having a falling film absorber and using LiBr/H₂O as working fluid.

Abstract

L'absorbeur est un composant important des systèmes à absorption et ses caractéristiques exercent des effets significatifs sur l'efficacité des machines à absorption. Cet article présent des résultats obtenus dans des études expérimentales sur les caracteristiques d'un absorbeur à film tombant composé d'une rangée de 24 tubes lisses horizontaux. Les auteurs montrent que le coefficient de transfert de masse augmente avec la densité de pulvérisation, le coefficient de transfert de chaleur augmente uniquement dans la gamme des densités de pulvérisation faibles. Il existe une densité de pulvérisation optimale (0,005–0,055 kg s⁻¹ m⁻¹) pour laquelle le coefficient de transfert de chaleur est maximal. Le coefficient de transfert de chaleur (nombre de Nusselt), qui est généralement exprimé en utilisant le nombre de Reynolds et le nombre de Prandtl, a été modifié en tenant compte l'effet de la concentration de la solution à l'entrée. A l'avenir, les résultats peuvent être utilisés pour optimiser la conception des systèmes à absorption à absorbeur à film tombant utilisant le LiBr/H₂O comme fluide actif.     

表面活性剂对溴化锂吸收式制冷机吸收过程强化机理的研究概况

吸收式制冷技术具有环保、节电和利用余热等不可替代的优点，在我国应用广范。溴化锂机组的吸收器是系统中换热面积最大、成本最高的换热部件，采用添加剂强化吸收器传热传质是一种不可缺少的手段。但是添加剂的强化机理却一直没有研究清楚，各国对添加剂的强化机理的研究很重视，已经有了不少研究成果，本文对国外添加剂对溴化锂制冷机吸收器的强化机理的研究进行简要介绍和分析。     

Heat and mass transfer enhancement of binary nanofluids for H2O/LiBr falling film absorption process

The objectives of this study are to measure the vapor absorption rate and heat transfer rate for falling film flow of binary nanofluids, and to compare the enhancement of heat transfer and mass transfer under the same conditions of nanofluids. The key parameters are the base fluid concentration of LiBr, the concentration of nanoparticles in weight %, and nanoparticle constituents. The binary nanofluids are H₂O/LiBr solution with nanoparticles of Fe and Carbon nanotubes (CNT) with the concentrations of 0.0, 0.01 and 0.1 wt %. The vapor absorption rate increases with increasing the solution mass flow rate and the concentration of Fe and CNT nanoparticles. It is found that the mass transfer enhancement is much more significant than the heat transfer enhancement in the binary nanofluids with Fe and CNT. It is also found that the mass transfer enhancement from the CNT nanoparticles becomes higher than that from the Fe nanoparticles. Therefore, the CNT is a better candidate than Fe nanoparticles for absorption performance enhancement in H₂O/LiBr absorption system.