The effect of nano-particles on the bubble absorption performance in a binary nanofluid

The objectives of this paper are to examine the effect of nano particles on the bubble type absorption by experiment 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 nano particles are considered as key parameters. The results show that the addition of nano particles enhances the absorption performance up to 3.21 times. Moreover, the absorption rate increases with increasing concentration of nano particles and the nano particles are more effective for lower absorption potential solution. The potential enhancement mechanism for binary nanofluid is suggested. The experimental correlations of the effective absorption ratio for each nano particles, Cu, CuO, and Al₂O₃, are suggested within ±10% error-band.     

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 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.     

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.     

Experimental investigation on the performance of NH3/CO2 cascade refrigeration system with twin-screw compressor

This paper describes the experiment carried out to analyze the performance of a refrigeration system in cascade with ammonia and carbon dioxide as working fluids. The effect of operation parameters, such as the evaporating temperature of the low temperature cycle, the condensing temperature of low temperature cycle, temperature difference in cascade heat exchanger and superheat degree, on the system performance was investigated. Performance of the cascade system with NH₃/CO₂ was compared with that of two-stage NH₃ system and single-stage NH₃ system with or without economizer. It was found that the COP of the cascade system is the best among all the systems, when the evaporating temperature is below −40 °C. Also, the cascade system performance is greatly affected by evaporating temperature, condensing temperature of low temperature cycle, temperature difference in cascade heat exchanger and is only slightly sensitive to superheat degree. All the experimental results indicate that the NH₃/CO₂ cascade system is very competitive in low temperature applications.     

An experimental investigation on performance of bubble pump with lunate channel for absorption refrigeration system

An experimental research on the performance of the bubble pump for absorption refrigeration units was made. The bubble pump provides the drive for the absorption cycle and is a decisive component of the absorption refrigeration unit. The bubble pump's property determines the efficiency of the absorption refrigeration system. A continuous experimental system with different size of bubbles pumps were designed, constructed and successfully worked. The experiments were performed by changing some of the parameters affecting the bubble pump performance. The experimental results shows that the performance of the bubble pump depends mainly on the driving temperature, the solution head and the combining tube diameters. With the suitable size of section area of the pump tubes the net elevating height of solution is 2.5 times as high as the solution submergence. The lunate channel has several outstanding characteristics, such as low starting temperature (minimum 68 °C), wide operating temperature range and lower requirement for vacuum condition (under 10 kPa). Then the elevating capability of the bubble pump with lunate channel is much better than others currently. It would provide well foundation for practical applications.     

Mass transfer correlation of NH3–H2O bubble absorption

The objectives of this paper are to study the effect of key parameters on absorption performance and to develop an experimental correlation of mass transfer coefficient for ammonia–water bubble absorption. The orifice diameter, liquid concentration and vapor velocity are considered as the key parameters. This study successfully visualized the bubble behavior and measured the volumetric diameter of bubbles during the bubble absorption process. The bubble absorption is grouped into two processes, bubble growth (process I) and bubble disappearance (process II), respectively. The following conclusions were drawn from the present study. A new experimental correlation for the volumetric bubble diameter was proposed with ±15% error band, which could be applied to calculate the mass transfer coefficient. The mass transfer coefficient increased with a decrease of the liquid concentration. In process II, the mass transfer coefficient increased with an increase of the Galileo number. Finally, experimental correlations of mass transfer coefficient were developed for processes I and II with ±18% error bands.     

Evaluation of the column components size on the vapour enrichment and system performance in small power NH3–H2O absorption refrigeration machines

This paper presents an analysis of the influence of the distillation column components size on the vapour enrichment and system performance in small power NH₃–H₂O absorption machines with partial condensation. It is known that ammonia enrichment is required in this type of systems; otherwise water accumulates in the evaporator and strongly deteriorates the system performance and efficiency. The distillation column analysed consists of a stripping adiabatic section below the column feed point and an adiabatic rectifying packed section over it. The partial condensation of the vapour is produced at the top of the column by means of a heat integrated rectifier with the strong solution as coolant and a water cooled rectifier. Differential mathematical models based on mass and energy balances and heat and mass transfer equations have been developed for each one of the column sections and rectifiers, which allow defining their real dimensions. Results are shown for a given practical application. Specific geometric dimensions of the column components are considered. Different distillation column configurations are analysed by selecting and discarding the use of the possible components of the column and by changing their dimensions. The analysis and comparison of the different column arrangements has been based on the system COP and on the column dimensions.     

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.     

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 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.     

Absorption performance enhancement by nano-particles and chemical surfactants in binary nanofluids

The objectives of this paper are to visualize the bubble behavior during the NH₃/H₂O absorption process with chemical surfactant and nano-particles and to study the effect of nano-particles and surfactants on the absorption characteristics. Binary nanofluid which means binary mixture with nano-sized particles is tested to apply nanofluid to the absorption system. Cu, CuO and Al₂O₃ nano-particles are added into NH₃/H₂O solution to make the binary nanofluids, and 2-ethyl-1-hexanol, n-octanol and 2-octanol are used as the surfactants. The concentration of ammonia in the basefluid, that of nano-particles in the nanofluid, and that of surfactants in the nanofluid are considered as the key parameters. The results show that the addition of surfactants and nano-particles improves the absorption performance up to 5.32 times. It can be concluded that the addition of both surfactants and nano-particles enhances significantly the absorption performance during the ammonia bubble absorption process.     

Experimental investigation of a vapor absorption refrigeration system

An experimental investigation of the performance of a commercially available vapor absorption refrigeration (VAR) system is described. The natural gas-fired VAR system uses aqua-ammonia solution with ammonia as the refrigerant and water as the absorbent and has a rated cooling capacity of 10 kW. The unit was extensively modified to allow fluid pressures and temperatures to be measured at strategic points in the system. The mass flow rates of refrigerant, weak solution, and strong solution were also measured. The system as supplied incorporates air-cooled condenser and absorber units. Water-cooled absorber and condenser units were fitted to extend the VAR unit's range of operating conditions by varying the cooling water inlet temperature and/or flow rates to these units. The response of the refrigeration system to variations in chilled water inlet temperature, chilled water level in the evaporator drum, chilled water flow rate, and variable heat input are presented.     

Numerical design of ammonia bubble absorber applying binary nanofluids and surfactants

The objectives of this paper are to analyze the combined heat and mass transfer characteristics for the ammonia bubble absorption process and to study the effects of binary nanofluids and surfactants on the absorber size. The ammonia bubble absorbers applying binary nanofluids and surfactants are designed and parametric analyses are performed. In order to express the effects of binary nanofluids and/or surfactants on the absorption performance, the effective absorption ratios for each case are applied in the numerical model. The values of the effective absorption ratio are decided from the previous experimental correlations. The kinds and the concentrations of nano-particles and surfactants are considered as the key parameters. The considered surfactants are 2-ethyl-1-hexanol (2E1H), n-octanol, and 2-octanol and nano-particles are copper (Cu), copper oxide (CuO), and alumina (Al₂O₃). The results show that the application of binary nanofluids and surfactants can reduce the size of absorber significantly. In order to reach 16.5% ammonia solution under the considered conditions, for example, the addition of surfactants (2E1H, 700 ppm) can reduce the size of absorber up to 63.0%, while the application of binary nanofluids (Cu, 1000 ppm) can reduce it up to 54.4%. In addition, it is found that the effect of mass transfer resistance is more dominant than that of heat transfer resistance. That is, the enhancement of mass transfer performance is more effective than that of heat transfer performance.     

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.     

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.     

Thermodynamic analysis of optimal condensing temperature of cascade-condenser in CO2/NH3 cascade refrigeration systems

This study thermodynamically analyzed a cascade refrigeration system that uses carbon dioxide and ammonia as refrigerants, to determine the optimal condensing temperature of the cascade-condenser given various design parameters, to maximize the COP and minimize the exergy destruction of the system. The design parameters include: the evaporating temperature, the condensing temperature and the temperature difference in the cascade-condenser. The results agreed closely with the reported experimental data. The optimal condensing temperature of the cascade-condenser increases with T_C, T_E and ΔT. The maximum COP increases with T_E, but decreases as T_C or ΔT increases. Two useful correlations that yield the optimal condensing temperature of the cascade-condenser and the corresponding maximum COP are presented.     

Circulation concentration of CO2/propane mixtures and the effect of their charge on the cooling performance in an air-conditioning system

CO₂ and propane mixtures are considered as alternative refrigerants due to their negligible direct global warming potentials and favorable thermodynamic properties. To properly evaluate the system performance using zeotropic mixtures, the circulation concentration was measured and the cause for its shift from the charged concentration was discussed. The circulation concentration of CO₂/propane mixtures has increased CO₂ fraction than its charged concentration. In addition, the effect of refrigerant charge on the cooling performance was tested for the transcritical cycle of CO₂ and the subcritical cycle of CO₂/propane mixtures of 75/25 and 60/40 by the charged mass percentage. It is shown that CO₂ refrigeration system could operate without a significant impact on its COP over a relatively wider range from the optimum charge.     

Experimental comparison of the sorption and refrigerating performances of a CaCl2 impregnated composite adsorbent and those of the host silica gel

In this paper, the adsorption and refrigerating performances of a composite adsorbent (S40) and its host microporous silica gel matrix (S0) are investigated comparatively in which water is used as refrigerant. The composite adsorbent is developed by impregnating the silica gel (S0) with calcium chloride. A lab-scale single-bed adsorption chiller system, functioning without any valve on its refrigerant circuit, is designed and used as test rig. The mass ratio (MR), defined as the ratio of the specific cooling power (SCP) of S40 to that of S0, is found to be higher than 2, while the COP has been improved by 25%, in average. The S40 has been tested to have, not only the capacity of adsorbing water vapour more than twice as much as the S0 does, but also, kinetically, to adsorb and desorb faster. The cycled amounts of refrigerant (CAR), calculated from measured isobaric adsorption levels, further show that the S40 can be regenerated at lower temperatures, with respect to S0.     

Composite adsorbent of CaCl2 and expanded graphite for adsorption ice maker on fishing boats

Adsorption performances and thermal conductivity were tested for three types of adsorbent: Pure CaCl₂ powder, simple composite adsorbent and consolidated composite adsorbent. The simple composite adsorbents show better adsorption performance because the additive of expanded graphite in CaCl₂ powder has restrained the agglomeration phenomenon in adsorption process and improved the adsorption performance of CaCl₂. The consolidated composite adsorbent are suitable to be used as adsorbent for ice maker on fishing boats because they have higher thermal conductivity, larger volumetric cooling capacity, higher SCP values and better anti-sway performance than simple composite adsorbents. Thermal conductivity of the consolidated composite adsorbent is 6.5–9.8 W m⁻¹ K⁻¹ depending on the molding pressure, ranging from 5 to 15 MPa, which is about 32 times higher than the thermal conductivity of CaCl₂ powder. The volumetric cooling capacity of consolidated composite adsorbent is about 52% higher than the best result obtained for CaCl₂ at the evaporating temperature of −10 °C. The SCP of the consolidated adsorbent increases of about 353% than CaCl₂ powder from simulation results at T_ad=30 °C and T_ev=−10 °C. The consolidated composite adsorbents have good anti-sway performance and they are not easy to be scattered out when the fishing boats sway on the sea.