Development of a slug flow absorber working with ammonia-water mixture: part I—flow characterization and experimental investigation

This study deals with an experimental investigation for a counter-current slug flow absorber, working with ammonia–water mixture, for significantly low solution flow rate conditions that are required for operating as the GAX (generator absorber heat exchanger) cycle. It is confirmed that the slug flow absorber operates well at the low solution flow rate conditions. From visualization results of the flow pattern, frost flow just after the gas inlet, followed by slug flow with well-shaped Taylor bubble, is observed, while dry patch on the tube wall are not observed. The liquid film at the slug flow region has smooth gas–liquid interface structure without apparent wavy motion. The local heat transfer rate is measured by varying main parameters, namely, ammonia gas flow rate, solution flow rate, ammonia concentration of inlet solution and coolant inlet conditions. The heat transfer rate while absorption is taking place is higher than that after absorption has ended. The absorption length is greatly influenced by varying main parameters, due to flow conditions and thermal conditions.     

Mass transfer characteristics of a structured packing for ammonia rectification in ammonia–water absorption refrigeration systems

In ammonia–water absorption refrigeration systems a purification process of the vapours produced in the generator is required. One type of equipment to carry out the purification process is a packed column. However, detailed experimental studies at the normal operating conditions found in ammonia–water absorption refrigeration systems have not been found. An experimental facility has been designed and built to study the ammonia–water rectification in packed columns. Experimental tests have been performed at the normal operating conditions found in the high-pressure stage of a small power ammonia–water absorption refrigeration system. In this paper, the experimental set-up is described and experimental results of the height equivalent to a theoretical plate (HETP) and the volumetric mass transfer coefficient of a rectifying section with the Sulzer BX packing are presented. The HETP values and the experimental mass transfer coefficients are compared with different data and correlations proposed in the literature; it has been found that the differences are appreciable.     

Dynamic simulation of a single-effect ammonia–water absorption chiller

A lumped-parameter dynamic simulation of a single-effect ammonia–water absorption chiller is performed. Modeling is based on the continuity of species constituting the ammonia–water mixture and the conservation of energy for each component of the absorption chiller. Ordinary differential equations governing the response of each component and the algebraic equations describing the constitutive relation are solved in parallel by numerical integration. The model has been applied to a commercially available 10.5 kW absorption chiller to study the transients of temperature, pressure, concentration, and void fraction of each component during the start-up operation. The time constant of the absorption chiller is also investigated. The parameters considered are the bulk concentration of the ammonia–water solution, the mass of the solution filled, and the volumes of key components of the absorption chiller. In addition, the reduction of the time constant by a stepwise turn-up and turn-down of the flue gas flow rate during the primary stage of start-up period is demonstrated.     

Development of a slug flow absorber working with ammonia–water mixture: part II—data reduction model for local heat and mass transfer characterization

This study deals with a data reduction model for clarifying experimental results of a counter-current slug flow absorber, working with ammonia–water mixture, for significantly low solution flow rate conditions. The data reduction model to obtain the local heat and mass transfer coefficient on the liquid side is proposed by using the drift flux model to analyze the flow characteristics. The control volume method and heat and mass transfer analogy are employed to solve the combined heat and mass transfer problem. As a result, it is found that the local heat and mass transfer coefficient on the liquid side of the absorber is greatly influenced by the flow pattern. The heat and mass transfer coefficient at the frost flow region is higher than that at the slug flow region due to flow disturbance and random fluctuation. The solution flow rate and gas flow rate have influence on the local heat and mass transfer coefficient at the frost flow region. However, it is insignificant at the slug flow region.     

Analytical investigation of two different absorption modes: falling film and bubble types

The objectives of this paper are to analyze a combined heat and mass transfer for an ammonia–water absorption process, and to carry out the parametric analysis to evaluate the effects of important variables such as heat and mass transfer areas on the absorption rate for two different absorption modes — falling film and bubble modes. A plate heat exchanger with an offset strip fin (OSF) in the coolant side was used to design the falling film and the bubble absorber. It was found that the local absorption rate of the bubble mode was always higher than that of the falling film model leading to about 48.7% smaller size of the heat exchanger than the falling film mode. For the falling film absorption mode, mass transfer resistance was dominant in the liquid flow while both heat and mass transfer resistances were considerable in the vapor flow. For the bubble absorption mode, mass transfer resistance was dominant in the liquid flow while heat transfer resistance was dominant in the vapor region. Heat transfer coefficients had a more significant effect on the heat exchanger size (absorption rate) in the falling film mode than in the bubble mode, while mass transfer coefficients had a more significant effect in the bubble mode than in the falling film mode.     

Experimental characterization of the rectification process in ammonia–water absorption systems with a large-specific-area corrugated sheet structured packing

In this paper, the mass transfer performance of a large-specific-area corrugated sheet structured packing for ammonia–water absorption refrigeration systems (AARS) is reported. An experimental facility was used to test the performance of the packing. Experimental results of the temperature, ammonia concentration and mass flow rate of the rectified vapour are presented and discussed for different operating conditions including reflux ratio values from 0.2 to 1. The volumetric vapour phase mass transfer coefficient is calculated from the measured data and compared with different correlations found in the literature. A new correlation is proposed which was fitted from the experimental data. Finally, a comparison is made between the actual packing height used in the experimental setup and the height required to obtain the same ammonia rectification in AARS with different packings previously tested by the authors.     

Characteristics of the membrane utilized in a compact absorber for lithium bromide–water absorption chillers

This study aims at investigating experimentally and analytically the characteristics and properties of a membrane utilized to design compact absorbers for lithium bromide–water absorption chillers. The main focus of this study are the factors that influence the water vapor transfer flux into a lithium bromide–water solution in confined narrow channels under vacuum conditions, as well as the properties limits for utilization in compact absorber design. The results indicate that the desired membrane characteristics for this application are as follows: high permeability to water vapor, hydrophobic to the aqueous solution with high liquid entry pressure (LEP) to avoid wettability of the membrane pores and no capillary condensation of water vapor to avoid blocking of the pores. For practical use, this membrane should have a thin hydrophobic microporous active layer with a thickness up to 60 μm, mean pore sizes around 0.45 μm and a porosity of up to 80%. The active layer should be attached to a porous support layer to meet the mechanical strength requirements needed for practical use in the absorber of lithium bromide water absorption chillers application.     

Simulation of ternary ammonia–water–salt absorption refrigeration cyclesSimulation des cycles frigorifiques à absorption à ammoniac / eau / sel

This paper proposes a new working fluid for refrigeration cycles utilizing low temperature heat sources. The proposed working fluid consists of the ammonia–water working fluid mixture and a salt. The salt is used to aid the removal of ammonia from the liquid solution. This effect is a manifestation of the well known “salting-out” effect. While the addition of salt improves the generator performance, it also has a detrimental effect on the absorber. The overall effects on the performance of three absorption cycles using the NH₃–H₂O–NaOH working fluid have been investigated using computer simulations. The results indicated that salting out can lower the generator operating temperature while simultaneously improving the cycle performance. Furthermore, limiting the salt to the generator suggests potential for further improvement in cycle performance.     

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.     

Twin screw oil-free wet compressor for compression–absorption cycle

The performance of a twin screw compressor operating under wet (two-phase) compression conditions in an ammonia–water compression absorption heat pump cycle is investigated both theoretically and experimentally. The paper reports on the influence of the location of liquid intake or, depending what applies, injection angle and mass flow rate of the injected liquid on compressor performance. Labyrinth seals separate the oil-free process side from oil lubricated bearing housing. Labyrinth seals leakage is modelled and its impact on performance is theoretically and experimentally investigated. The need for liquid injection from the discharge side to obtain acceptable performance is discussed based on experimental results.     

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.     

Experimental analysis of ammonia–water rectification in absorption systems with the 10 mm metal Pall ring packing

In this paper, the mass transfer performance of the 10 mm metal Pall ring packing for ammonia rectification in ammonia–water absorption refrigeration systems (AARS) is investigated. The experimental setup is described and the experimental procedure and data reduction method are explained. Experimental results of the top vapour temperature, concentration and mass flow rate are presented for different operating conditions, including reflux ratio values from 0.4 to 1 (total reflux conditions). Vapour phase mass transfer coefficients are calculated from the measured data and the results are compared with different mass transfer correlations found in the open literature. In this paper, a correlation is proposed for the packing analysed which was fitted from the experimental data. Finally, a comparison is made between the actual packing height used in the experimental setup and the height required to obtain the same ammonia rectification in AARS with the first generation packings: ½″ ceramic Berl saddles, 15 mm glass Raschig rings and ½″ ceramic Novalox saddles. It was found that a packing height reduction between a factor of 2.5 and 3 is attained with 10 mm metal Pall rings.     

Thermodynamic modeling of an ammonia–water absorption chiller

This article develops a general thermodynamic framework for the modeling of an irreversible absorption chiller at the design point, with application to a single-stage ammonia–water absorption chiller. Component models of the chiller have been assembled so as to quantify the internal entropy production and thermal conductance (UA) in a thermodynamically rigorous formalism, which is in agreement with the simultaneous heat-and-mass transfer processes occurring within the exchangers. Local thermodynamic balance (viz. energy, entropy, and mass balance) and consistency within the components is respected, in addition to the overall thermodynamic balance as determined by the inlet and outlet states of the components. For the absorbers, Colburn-and-Drew mass transfer equations are incorporated to describe the absorption process. Furthermore, the impact of various irreversibilities on the performance of chiller is also evaluated through the use of a general macroscopic equation.     

Research on the condensation of the ammonia–water mixture on a horizontal smooth tube

This paper reports on the experimental research and the theoretical analysis conducted to study the condensation of the ammonia–water mixture on a horizontal smooth tube. Experiments were carried out with ammonia concentrations and wall subcoolings ranging from 62% to 95% and from 45 °C to 90 °C, respectively. Experimental results of the overall condensation heat transfer coefficients (HTCs) are reported and discussed. A theoretical model based on the analytical method proposed by Colburn and Drew was developed. The model was able to predict the trends of the experimental HTCs for the ranges of concentrations and wall subcoolings considered in the experiments. The heat flow and the overall condensation HTCs were slightly overestimated with mean errors of 9.3% and 11.2%, respectively. The theoretical results revealed that the ammonia mass transfer in the vapour phase has a significant effect on the heat and mass transfer coefficients and, consequently, on the overall condensation HTCs. Finally, a calculation procedure was established to estimate the vapour mass and heat transfer coefficients from experimental data. The results are shown as dimensionless correlations.     

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.     

Energy and exergy analysis of single effect and series flow double effect water–lithium bromide absorption refrigeration systems

In this paper, the energy and exergy analysis of single effect and series flow double effect water–lithium bromide absorption systems is presented. A computational model has been developed for the parametric investigation of these systems. Newly developed computationally efficient property equations of water–lithium bromide solution have been used in the computer code. The analysis involves the determination of effects of generator, absorber and evaporator temperatures on the energetic and exergetic performance of these systems. The effects of pressure drop between evaporator and absorber, and effectiveness of heat exchangers are also investigated. The performance parameters computed are coefficient of performance, exergy destruction, efficiency defects and exergetic efficiency. The results indicate that coefficient of performance of the single effect system lies in range of 0.6–0.75 and the corresponding value of coefficient of performance for the series flow double effect system lies in the range of 1–1.28. The effect of parameters such as temperature difference between heat source and generator and evaporator and cold room have also been investigated. Irreversibility is highest in the absorber in both systems when compared to other system components.     

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.     

A minichannel aluminium tube heat exchanger – Part I: Evaluation of single-phase heat transfer coefficients by the Wilson plot method

A prototype liquid-to-refrigerant heat exchanger was developed with the aim of minimizing the refrigerant charge in small systems. To allow correct calculation of the refrigerant side heat transfer, the heat exchanger was first tested for liquid-to-liquid (water-to-water) operation in order to determine the single-phase heat transfer performance. These single-phase tests are reported in this paper. The heat exchanger was made from extruded multiport aluminium tubes and was designed similar to a shell-and-tube heat exchanger. The heat transfer areas of the shell-side and tube-side were approximately 0.82 m² and 0.78 m², respectively. There were six rectangular-shaped parallel channels in a tube. The hydraulic diameter of the tube-side was 1.42 mm and of the shell-side 3.62 mm. Tests were conducted with varying water flow rates, temperature levels and heat fluxes on both the tube and shell sides at Reynolds numbers of approximately 170–6000 on the tube-side and 1000–5000 on the shell-side, respectively. The Wilson plot method was employed to investigate the heat transfer on both the shell and tube sides. In the Reynolds number range of 2300–6000, it was found that the Nusselt numbers agreed with those predicted by the Gnielinski correlation within ±5% accuracy. In the Reynolds number range of 170–1200 the Nusselt numbers gradually increased from 2.1 to 3.7. None of the previously reported correlations for laminar flow predicted the Nusselt numbers well in this range. The shell-side Nusselt numbers were found to be considerably higher than those predicted by correlations from the literature.     

Spray absorbers in absorption systems using lithium nitrate–ammonia solution

In the present work, the spray absorption method is studied for the absorption of ammonia refrigerant vapour by lithium nitrate–ammonia solutions. Mass transfer coefficients attainable using the spray absorption method are estimated. In this study the low-pressure absorber of a double-stage absorption refrigeration system is considered. Results show that the mass transferred is maximum (about 60% of the total) during the deceleration period of the drops. This period represents about 13.4% of the time required to reach the equilibrium state at the end of the absorption chamber. The results show that a time-average mass transfer coefficient equal to k_m=18.6×10⁻⁵ m s⁻¹ may be attained.     

A minichannel aluminium tube heat exchanger – Part II: Evaporator performance with propane

This paper presents heat transfer data for a multiport minichannel heat exchanger vertically mounted as an evaporator in a test-rig simulating a small water-to-water heat pump. The multiport minichannel heat exchanger was designed similar to a shell-and-tube type heat exchanger, with a six-channel tube of 1.42 mm hydraulic diameter, a tube-side heat transfer area of 0.777 m² and a shell-side heat transfer area of 0.815 m². Refrigerant propane with a desired vapour quality flowed upward through the tubes and exited with a desired superheat of 1–4 K. A temperature-controlled glycol solution that flowed downward on the shell-side supplied the heat for the evaporation of the propane. The heat transfer rate between the glycol solution and propane was controlled by varying the evaporation temperature and propane mass flow rate while the glycol flow rate was fixed (18.50 l min⁻¹). Tests were conducted for a range of evaporation temperatures from −15 to +10 °C, heat flux from 2000 to 9000 W m⁻² and mass flux from 13 to 66 kg m⁻² s⁻¹. The heat transfer coefficients were compared with 14 correlations found in the literature. The experimental heat transfer coefficients were higher than those predicted by many of the correlations. A correlation which was previously developed for a very large and long tube (21 mm diameter and 10 m long) was in good agreement with the experimental data (97% of the data within ±30%). Several other correlations were able to predict the data within a reasonable deviation (within ±30%) after some adjustments to the correlations.