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.     

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.     

Thermoeconomic evaluation and optimization of a double-effect H2O/LiBr vapour-absorption refrigeration system

In this paper, the thermoeconomic concept is applied to the optimization of a double-effect H₂O/LiBr VAR system, aimed at minimizing its overall product cost. A simplified cost minimization methodology based on the thermoeconomic concept is applied to calculate the economic costs of all the internal flows and products of the system by formulating thermoeconomic cost balances. Once these costs are determined, the system is thermoeconomically evaluated to identify the effects of the design variables on cost of the flows and products. This enables to suggest changes of the design variables that would make the overall system cost-effective. Finally, an approximate optimum design configuration is obtained by means of an iterative procedure. The result shows significant improvement in the system performance. The sensitivity analysis shows that the changes in optimal values of the decision variables are negligible with changes in the fuel cost.     

The effect of micro-scale surface treatment on heat and mass transfer performance for a falling film H2O/LiBr absorber

The objectives of this paper are to develop a new method of wettability measurement, to study the effect of micro-scale surface treatment on the wettability across horizontal tubes and to apply it for numerical analysis of heat and mass transfer in a H₂O/LiBr falling film absorber. Three types of tubes with roughness are tested in a test rig. Inlet solution temperature (30–50 °C), concentration (55–62 wt.% of LiBr) and mass flow rate (0.74–2.71 kg/min) are considered as key parameters. Reynolds number ranged from 30 to 120 by controlling the inlet mass flow rate. The wettability on the roughened tubes was higher than that for the smooth tubes. The wettability decreased linearly along the vertical location but was proportional to the solution temperature and mass flow rate. The experimental correlations of the wettability for the smooth and the roughened tubes were developed with error bands of ±20 and ±10%, respectively. These are used for the heat and mass transfer analysis of absorbers with micro-scale hatched tubes.     

Experimental verification of H2O/LiBr absorber bundle performance with smooth horizontal tubes

A model for absorption of water vapor into LiBr flowing over horizontal smooth tubes is developed to predict the absorption performance of a tube bundle. The performance of a horizontal smooth tube absorber is calculated and compared with experimental data. The calculation results of absorber performance are found to vary considerably depending on mass diffusivity. The literature value of mass diffusivity, of the order of 10⁻⁹ m² s⁻¹, causes the model to deviate from the experimental results especially for solution side heat transfer coefficient. A parametric study varying mass diffusivity shows that a value of 1.0 × 10⁻¹⁰ m² s⁻¹ gives a better agreement with the experimental results. The effects of absorber capacity, heat transfer coefficient of droplets ejected from the system (slinging) and number of tubes in the bundle on the absorption performance are also discussed.     

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.     

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.     

Improved thermodynamic property fields of LiBr–H2O solution

This article presents a thermodynamically consistent set of specific enthalpy, entropy, and heat capacity fields for LiBr–H₂O solution. The temperatures span from 0 to 190°C, while the concentrations span from 0 to 75 wt%. The work is based on the empirical inputs of Dühring's gradient and intercept, specific heat capacity data at a reference concentration of 50 wt% and density data. These properties have been evaluated using most of the experimental data available in the literature. The present approach circumvents the issue of negative dew point at low temperatures and high concentrations. The information provided in this article could be useful for designers of absorption chillers.     

Vapor absorption by LiBr aqueous solution in vertical smooth tubes

Heat and mass transfer in a falling film vertical in-tube absorber was studied experimentally with LiBr aqueous solution. The presented results include the effect of solution flow rate, solution subcooling and cooling water temperature on the absorption in a smooth copper tube 16.05 mm I.D. and 400 mm long. The experimental data in the previous report for a 1200-mm-long tube was also re-examined and compared. It was demonstrated by the observation of the flow in the tube that the break down of the liquid film into rivulets leads to deterioration of heat and mass transfer at lower film Reynolds number or in longer tubes. An attempt to evaluate physically acceptable heat and mass transfer coefficients that are defined with estimated temperature and concentration at the vapor–liquid interface was also presented.     

The thermodynamic performance of a new solution cycle in double absorption heat transformer using water/lithium bromide as the working fluids

In this paper, a new solution cycle in the double absorption heat transformer is presented and the thermodynamic performance of this new cycle is simulated based on the thermodynamic properties of aqueous solution of lithium bromide. The results show that this new cycle is superior to the cycle being studied by some researchers. This new solution cycle has a wider range of operation in which the system maintains the high value of COP and has larger temperature lifts and operation stability. The relationship between the absorber and the absorbing evaporator is more independent and this makes the operation and control of the system more easier.     

Experimental evaluation of the performances of a H2O–LiBr absorption refrigerator under different service conditions

The paper describes an experimental plant aimed at simulating and verifying the performances of a single-stage H₂O–LiBr absorption machine. The machine is water cooled and it is supplied by hot water produced by an electrical boiler; it is possible to simulate different service conditions by varying the temperatures and the flow rate of water in the external circuits. Measurement facilities allow to record in real time all the main operating parameters of internal and external circuits (temperatures, pressures and flow rates). The paper illustrates the characteristics of the machine and of the plant and the results of various experimental campaigns. In particular, the acquisitions on the plant have tested different service conditions by varying the flow rate and the temperature of the supplying hot water; the energy and energy performances of the plant are presented and compared with data from literature and from a simulation code developed for the plant.The results show that the absorption machine can work, with acceptable efficiency, with input temperatures of about 65–70 °C; this result is interesting for a future supply of the machine with solar energy.     

Thermoeconomic optimization of a single effect water/LiBr vapour absorption refrigeration system

In this paper, the thermoeconomic theory is applied to the economic optimization of a single effect water/LiBr vapour absorption refrigeration system for air-conditioning application, aimed at minimizing its overall operation and amortization cost. The mathematical and numerical techniques based optimization of thermal system is not always possible due to plant complexities. Therefore, a simplified cost minimization methodology is applied to evaluate the economic costs of all the internal flows and products of the system by formulating exergoeconomic cost equations. Once these costs are determined, the system is thermoeconomically evaluated to identify the effects of design variables on costs and enables to suggest values of design variables that would make the overall system cost effective. Finally, an approximate optimum design configuration is obtained by means of sequential local optimization of the system, carried out unit by unit. The result compares this optimum with the base case and shows percentage variations in the system's operation and amortization cost.     

Thermodynamic properties for the quaternary system H2O + CH3OH + LiBr + ZnCl2

The thermodynamic properties (solubility, vapour pressure, density, viscosity, heat capacity and heat of mixing) of the H₂O + CH₃OH + LiBr + ZnCl₂ (9:1 H₂O:CH₃OH and 1:1 LiBr:ZnCl₂ by mass) system using H₂O + CH₃OH as the working media and LiBr + ZnCl₂ as the absorbents were measured. The solubility data were obtained in the temperature range from 270.35 to 389.55 K. The measurements of vapour pressure, density, viscosity and heat capacity were carried out at various temperatures and absorbent concentrations. The differential heat of dilution and differential heat of solution at 298.15 K were measured for solutionw with absorbent concentrations from 0 to 75.2 wt%. The integral heat of mixing data at 298.15 K were obtained from both sets of experimental data. The integral heats of mixing for this quaternary system showed exothermic behaviour. The vapour pressure data were correlated with an Antoine-type equation. An empirical formula for the heat capacity was obtained from experimental data. The experimental data for the basic thermodynamic properties of this quaternary system were compared with those of the basic H₂O + LiBr system.     

Model for absorption of water vapor into aqueous LiBr flowing over a horizontal smooth tube

A model for absorption of water vapor into aqueous LiBr flowing over a horizontal smooth tube is developed. The flow is divided into three regimes: (1) falling film in contact with the tube, (2) drop formation at the bottom of the tube, and (3) drop fall between the tubes. Governing equations are formulated for each flow regime, and the variations of solution temperature, LiBr mass fraction, mass absorption rate and heat transfer rate are discussed including the effect of inlet subcooling. It is shown that the temperature variation across the film exhibits a nonlinear profile near the top of the tube and this effect leads to the necessity of a two-dimensional formulation in the falling film regime for accurate prediction. As has been observed previously, the mass fraction boundary layer at the vapor/liquid interface is found to be very thin and this explains the low absorption flux. The model predicts that significant absorption takes place in the drop formation regime with a considerable variation of temperature and mass fraction.     

Modelling of the heat exchangers of a small capacity, hot water driven, air-cooled H2O–LiBr absorption cooling machine

General models for the design of the heat exchangers (absorber, generator, condenser and evaporator) of a prototype of an air-cooled absorption chiller of 2 kW for air-conditioning using the pair H₂O–LiBr have been developed. An absorption machine of such characteristics has been constructed to be used as a test facility for validating the results obtained from the mathematical models developed. The discrepancies considering the heat exchanged between numerical results and experimental data are under 15% in most cases for all these components except the condenser, where the discrepancies are higher. The conclusions reported will lead to: (i) future improvements of the mathematical simulation models and (ii) improvements in the experimental infrastructure.     

Visualization of bubble behavior and bubble diameter correlation for NH3–H2O bubble absorption

The objectives of this paper are to visualize the bubble behavior for an ammonia–water absorption process, and to study the effect of key parameters on ammonia–water bubble absorption performance. The orifice diameter, orifice number, liquid concentration and vapor velocity are considered as the key parameters. The departing bubbles tend to be spherical for surface tension dominant flow, and the bubbles tend to be hemispherical for inertial force dominant flow. A transition vapor Reynolds number is observed at a balance condition of internal absorption potential (by the concentration difference) and external absorption potential (by the vapor inlet mass flow rate). As the liquid concentration increases, the transition Reynolds number and the initial bubble diameter increase. The initial bubble diameter increases with an increase of the orifice diameter while it is not significantly affected by the number of orifices. Residence time of bubbles increases with an increase in the initial bubble diameter and the liquid concentration. This study presents a correlation of initial bubble diameter with ±20% error band. The correlation can be used to calculate the interfacial area in the design of ammonia-water bubble absorber.     

Heat transfer enhancement by Marangoni convection in the NH3–H2O absorption process

The objectives of this paper are to quantify the effect of Marangini convection on the absorption performance for the ammonia–water absorption process, and to visualize Marangoni convection that is induced by adding a heat transfer additive, n-octanol. A real-time single-wavelength holographic interferometer is used for the visualization using a He–Ne gas laser. The interface temperature is always the highest due to the absorption heat release near the interface. It was found that the thermal boundary layer (TBL) increased faster than the diffusion boundary layer (DBL), and the DBL thickness increased by adding the heat transfer additive. At 5 s after absorption started, the DBL thickness for 5 mass% NH₃ without and with the heat transfer additive was 3.0 and 4.5 mm, respectively. Marangoni convection was observed near the interface only in the cases with heat transfer additive. The Marangoni convection was very strong just after the absorption started and it weakened as time elapsed. It was concluded that the absorption performance could be improved by increasing the absorption driving potential (x_vb−x_vi) and by increasing the heat transfer additive concentration. The absorption heat transfer was enhanced as high as 3.0–4.6 times by adding the heat transfer additive that generated Marangoni convection.     

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.     

Numerical simulation of an advanced energy storage system using H2O–LiBr as working fluid, Part 2: System simulation and analysis

This paper is the second part of our study on the advanced energy storage system using H₂O–LiBr as working fluid. In the first part, the system working principle has been introduced, and the system dynamic models in the operation process have also been developed. Based on the previous research, this paper focuses on the numerical simulation to investigate the system dynamic characteristics and performances when it works to provide combined air-conditioning and hot water supplying for a hotel located near by Yangzi River in China. The system operation conditions were set as follows: the outdoor temperature was between 29 °C and 38 °C, the maximum air-conditioning load was 1450 kW, the total air-conditioning capacity was 19,890 kWh and the 50 °C hot water capacity for showering was 20 tons which needed heat about 721 kWh on a given day. Under these conditions, the system operation characteristics were simulated under the full- and partial-storage strategies. The simulation results predicted the dynamic characteristics and performances of the system, including the temperature and concentration of the working fluid, the mass and energy in the storage tanks, the compressor intake mass or volume flow rate, discharge pressure, compression ratio, power and consumption work, the heat loads of heat exchanger devices in the system and so on. The results also showed that the integrated coefficient of performances (COP_int) of the system were 3.09 and 3.26, respectively, under the two storage strategies while the isentropic efficiency of water vapor compressor was 0.6. The simulation results are very helpful for understanding and evaluating the system as well as for system design, operation and control, and device design or selection in detail.