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Abstract: In this study, a natural refrigerant based cascaded system, with nitrous oxide as the low temperature fluid and carbon dioxide as the high temperature fluid, is analyzed for simultaneous cooling and heating applications. Effects of significant design and operating parameters on system performance are studied. Optimization of intermediate pressure for maximum COP for various design and operating parameters are presented as well. Results show that use of internal heat exchanger has marginal influence on system performance. Due to similar thermodynamic properties of nitrous oxide and carbon dioxide, the optimized intermediate temperature turns out to be independent of the performance of gas cooler and evaporator for a given operating condition. Due to the same reason, N2O as low temperature fluid and CO2 as high temperature fluid in a cascade arrangement exhibit similar behavioural trends in a system where the fluids are swapped. [Copyright &y& Elsevier]

Abstract: The aim of this paper is the thermodynamic evaluation and optimisation of different two-stage transcritical carbon dioxide cycles. Five different cycles are studied: basic single-stage cycle, single-throttling with two-stage compression cycle, split cycle, phase separation cycle and single-stage cycle coupled with a gas cooling circuit. Each basic cycle is analysed for the effect of internal heat transfer between different streams of refrigerants. In the case of two-stage compression, intermediate cooling between the compressor stages is present. An analysis on the Plank cycle for intermediate pressure higher than critical one is performed. Each cycle is optimised with regards to energy performance, calculating the optimal values of both the upper and the intermediate pressures. In the case of split cycle, the ratio of the mass flow rate in the main stream to the one in the auxiliary stream is also optimised. [Copyright &y& Elsevier]

Abstract: This paper advances a study of the transcritical expansion of carbon dioxide (R-744, CO2) through adiabatic capillary tubes. The influence of both operating conditions (inlet and exit pressures, inlet temperature) and tube geometry (capillary diameter and tube length) on the CO2 mass flow rate was experimentally evaluated using a purpose-built testing facility with a strict control of the measured variables. A dimensionless correlation to predict the refrigerant mass flow rate as a function of tube geometry and operating conditions was developed. In addition, a theoretical model was put forward based on the mass, energy and momentum conservation principles. The model results were compared with experimental data, when it was found that the model predicts 95% of the measured refrigerant mass flow rate within an error band of ±10%. The model was also employed to advance the knowledge about the transcritical carbon dioxide flow through adiabatic capillary tubes. [Copyright &y& Elsevier]

Abstract: Capillary tubes are extensively used in small refrigeration and air-conditioning systems with synthetic refrigerants and hydrocarbons. For CO2 transcritical applications, it has been shown that the capillary tube demonstrates an intrinsic capability of adjusting the upper pressure close to the optimal value in response to changes of gas-cooler heat sink temperature. The CO2 flow rate through four capillary tubes of various lengths, diameters and materials was measured in a test rig. Each capillary tube was tested with inlet pressure varying from 7.5MPa to 11MPa and inlet temperature from 20°C to 40°C. Outlet pressure varied from 1.5MPa to 3MPa. The experimental results were validated against different numerical and approximate analytical solutions of the capillary tube equations. These models give good predictions only if the friction factor of the capillary tube is calculated accounting for its dependence on the tube roughness. [Copyright &y& Elsevier]

Abstract: This paper outlines an algebraic model for simulating the transcritical expansion of carbon dioxide through adiabatic capillary tubes. The model was put forward based on the analytical solution of the momentum conservation equation assuming an isenthalpic expansion process. The theoretical model predictions were compared with 66 experimental data points covering different operating conditions and tube geometries. A good agreement between the experimental and calculated mass flow rates was achieved, with more than 94% of the data points lying within an error band of ±10%. [Copyright &y& Elsevier]

Many species of disease-vector mosquitoes display vertebrate host specificity. Despite considerable progress in recent years in understanding the proximate and ultimate factors related to non-random host selection at the interspecific level, the basis of this selection remains only partially understood. Anopheles gambiae sensu stricto, the main malaria vector in Africa, is considered a highly anthropophilic mosquito, and host odours have been shown to play a major role in the host-seeking process of this species. Studies on host preference of An. gambiae have been either conducted in controlled conditions using laboratory reared mosquitoes and worn stockings as host-related stimuli, or have been done in the field with methods that do not account for internal (e.g. age of sampled mosquitoes) and/or environmental effects. We explored differential behavioural responses to host odours between two populations of the same sibling species, An. gambiae in semi-field conditions in Burkina Faso. The behavioural responses (i.e. degree of activation and strength of anemotaxis) were investigated using a Y-olfactometer designed to accommodate whole hosts as a source of odour stimuli. Two strains of An. gambiae (3 to 4-day-old female) from laboratory Kisumu strain, and from field-collected individuals were confronted to combinations of stimuli comprising calf odour, human odour and outdoor air. In dual-choice tests, field mosquitoes chose human odour over calf odour, outdoor air over calf odour and responded equally to human and outdoor air, while laboratory mosquitoes responded equally to human and calf odour, human odour over outdoor air and calf odour over outdoor air. Overall, no effect of CO2 exhaled by humans and calves neither on the proportion of activated mosquitoes nor on the relative attractiveness to odour stimuli was found. We report for the first time an intraspecific variation in host-odour responses. This study clearly suggests that there may be genetic polymorphism underlying host preference and emphasizes that the highly anthropophilic label given to An. gambiae s.s. must be carefully interpreted and refer to populations rather than the whole sibling species. [ABSTRACT FROM AUTHOR]

Abstract: Microfin tubes are widely used in air cooled and water cooled heat exchangers for heat pump and refrigeration applications during condensation or evaporation of refrigerants. In order to design heat exchangers and to optimize heat transfer surfaces, accurate procedures for computing pressure drops and heat transfer coefficients are necessary. This paper presents a new simple model for the prediction of the heat transfer coefficient to be applied to condensation in horizontal microfin tubes of halogenated and natural refrigerants, pure fluids or nearly azeotropic mixtures. The updated model accounts for refrigerant physical properties, two-phase flow patterns in microfin tubes and geometrical characteristics of the tubes. It is validated against a data bank of 3115 experimental heat transfer coefficients measured in different independent laboratories all over the world including diverse inside tube geometries and different condensing refrigerants among which R22, R134a, R123, R410A and CO2. [Copyright &y& Elsevier]

Abstract: The cooling performance and reliability of a transcritical CO2 cycle can be significantly improved by using a multi-stage compressor with gas injection because the CO2 cycle has a large pressure difference across a compressor. The objective of this study is to investigate the performance and operating characteristics of a two-stage CO2 cycle with gas injection. In this study, the performances of a two-stage CO2 cycle with gas injection (called as “two-stage gas injection cycle”) were measured as the amount of refrigerant charge, first- and second-stage compressor frequencies, and first- and second-stage EEV openings were varied in the cooling mode operation. The cooling COP of the two-stage gas injection cycle was maximally enhanced by 16.5% over that of the two-stage non-injection cycle in the experiments. In addition, when the first- and second-stage EEV openings were increased, the compression ratio decreased and this in turn, improved the cooling COP of the two-stage gas injection cycle. However, when the first-stage EEV opening was increased, the mass flow rate through the evaporator decreased, and this in turn, decreased the cooling capacity of the two-stage gas injection cycle. Therefore, in the two-stage gas injection cycle, an optimum control of both EEV openings is required. [Copyright &y& Elsevier]

Abstract: Local heat transfer coefficients and pressure drops during condensation of pure Dimethyl Ether (DME) and non-azeotropic mixtures of CO2 and DME inside a horizontal smooth tube have been measured experimentally. The mass fractions of CO2 and DME in the mixtures have been varied as CO2/DME (39/61, 21/79 mass%) and the refrigerant mass fluxes have been varied from 200 to 500kgm−2 s−1. The increase of mass fraction of CO2 in the mixture decreases the heat transfer coefficient and the pressure drop. At the high refrigerant mass flux, the effect of mass transfer resistance on the heat transfer is decreased. Existing prediction method for condensation of binary refrigerant mixtures, which considered the vapor core mass transfer resistance, has been used to predict the present experimental data. Changing of the two-phase frictional multiplier in that prediction method has improved the prediction of the present experimental data. [Copyright &y& Elsevier]

Abstract: A comprehensive analysis of heat transfer and pressure drop experimental data and correlations for supercritical CO2 cooling in macro- and micro-channels is presented in this article. First, the physical and transport properties of CO2 at supercritical conditions are discussed and then their influence on heat transfer and pressure drops. Next, a review of experimental studies on heat transfer and pressure drops of supercritical CO2 cooling is provided and detailed comparisons and analysis relative to the available heat transfer and pressure drop correlations for supercritical CO2 cooling were done where possible. Furthermore, noting the lack of all pertinent experimental details required to use the data published in many of these studies, comments are given on how to reduce and present supercritical CO2 experimental data properly in the future. In addition, the effect of oil on heat transfer and pressure drops for supercritical CO2 is shown to significantly decrease the former and to increase the latter. Comparison of experimental data to a selection of heat transfer correlations shows that the Fang et al. [2001b. Modeling and analysis of gas coolers. ASHRAE Trans. 107 (1), 4–13] correlation gives the closest values to the experimental data but is still not satisfactory. According to the comparison and analysis, it is recommended that further efforts be made to develop good heat transfer methods for supercritical CO2 cooling based on accurate database in the future. To achieve this, more careful experiments should be done over a wide range of test parameters to meet the requirement in practical applications. In addition, four experimental studies show that the Blasius equation works well for CO2 cooling in the near supercritical region. More careful experimental data are still needed to further validate this conclusion because some experimental data are much different from others. [Copyright &y& Elsevier]

Abstract: In order to clarify the heat transfer mechanism of supercritical carbon dioxide flowing with a small amount of lubricating oil, visualization experiments were conducted using two sight glass tubes with inner diameters of 2 and 6mm. The visualization images were recorded using a high-speed CCD camera with shutter speed changing from 500 to 10,000pps (pictures per second). PAG-type oil, which is partially miscible with supercritical carbon dioxide, was used. The experiments were conducted with lubricating oil concentrations of 1 and 5wt%, pressures between 8 and 10MPa, and mass fluxes between 200 and 1200kgm−2 s−1. The visualization images revealed that the two-phase flow pattern inside the gas cooler was determined by many factors, including the tube diameter, oil concentration, temperature, pressure, and mass flux. For a small size tube of 2mm ID, the formation of both oil droplets in the bulk region and an oil film along the inner wall of the tube was confirmed. At low temperatures, a large number of oil droplets were observed flowing with CO2 with a slip ratio of approximately 0.7. With an increase in the bulk temperature, both the dimension and number of oil droplets entrained with CO2 decreased, and the flow of the oil film became clearly visible. For a large tube of 6mm ID, the flow pattern at a low mass flow rate was a separated wavy flow; with an increase in mass flow, the flow pattern changed to annular flow, which corresponds to a distinct decrease in the heat transfer coefficient due to the heat resistance of oil layer. The transition of the flow pattern is considered due to the shear stress between the oil layer and bulk CO2. In addition, the dissolution of CO2 into PAG oil as well as the change in the solubility and thermodynamic properties of CO2 with the temperature and pressure makes the prediction of the flow patterns a challenging task. The relationship between the flow pattern and heat transfer characteristics was also discussed. [Copyright &y& Elsevier]

Abstract: This article presents an analytical study on the performance of a cascade refrigeration–heat pump system based on a model incorporating both internal and external irreversibilities. It further explores the optimum allocation of heat exchanger inventories in cascade refrigeration cycles for the maximization of performance and minimization of system cost. In a novel attempt, a CO2–propane cascade system, lately showing good promise as a future solution, has been numerically simulated comprehensively incorporating precision property routines for the working fluids and this numerical model subsequently has been employed for validation of the proposed analytical model. It has been established that the total system heat exchanger inventory should be optimally divided in the ratio of a factor K between the gas cooler and the evaporator heat exchangers for maximal system output and minimal mechanical power requirement for a specified total heat exchanger inventory. It can also be construed that a similar allocation is recommended for minimization of total heat exchanger inventory with specified total system output. A strong assertion of all the analytical model results has been exhibited through a completely independent numerical system simulation. [Copyright &y& Elsevier]

Abstract: A novel Carnot-type cycle is proposed to reach high performances for sub-critical and trans-critical applications. Two natural fluids, ammonia and carbon dioxide each one corresponding to one of the application, were chosen. Compared to conventional cycles, the Carnot-type cycle leads to COP increase by 4–70% depending on the application. For the most suitable case (CO2), a simpler design resulting from a trade-off between complexity and performances still exhibits high performances (92.5% of COPCarnot vs. 95.4% for Carnot-type cycle). Compared to conventional one, this simpler design appears to be more sensitive to the expander efficiency but still achieves higher performances: 80% vs. 50% of COPCarnot. The impact of the compression cooling efficiency was explored too. With a temperature pinch equal to 20K, the performances are still better than those of the conventional one (+21% of COP) despite a decrease by 24% of the COP. [Copyright &y& Elsevier]

Abstract: The classical substances as hydrochlorofluorocarbons (HCFCs) used as working fluids in the vapour compression plants have to be replaced by new substances because of their ozone depletion potential and their greenhouse effect. Carbon dioxide (CO2) is non-toxic, non-flammable, has zero ozone depletion potential and negligible global warming potential as refrigerant. Referring to a transcritical CO2 cycle working as a classical “split-system” to cool air in residential applications, the aim of this paper is the evaluation of the energy performances using an internal heat exchanger. The experimental plant employs a semi-hermetic compressor, plate-finned tube type heat exchangers, a back pressure valve electronically controlled and an expansion valve. Besides it is possible to control the flash gas produced in the liquid receiver thanks to another semi-hermetic compressor linked to an inverter. An increase of the coefficient of performance has been found using the internal heat exchanger. The comparison of the coefficients of performance of two cycles, working with and without the internal heat exchanger, is discussed. [Copyright &y& Elsevier]

Abstract: The aim of this paper is to explain observed correlations between temperature and carbon dioxide concentrations in Antarctic ices. Chemical characteristics of the carbonate system, biological and calcium carbonate pumps between superficial and deep ocean, CCD and lysocline allow a satisfactory representation of the present ocean. Sediment studies show that the biological pump was more efficient in glacial times and that the lysocline was at about the same depth. It is then possible to reconstruct the carbonate system at this period. The question is then to identify the processes involved in the carbonate system change: increase of deep phosphate concentration, decrease of coral reefs, increased competition of siliceous organisms versus calcareous ones, input of iron-rich dust in Southern ocean. None of these propositions allows a satisfactory explanation. [Copyright &y& Elsevier]

Abstract: Because of the ozone layer depletion and global warming, new alternative refrigerants are being developed. In this study, evaporation heat transfer characteristic and pressure drop of carbon dioxide flowing upward in vertical smooth and micro-fin tubes were investigated by experiment with regard to evaporating temperature, mass flux and heat flux. The vertical smooth and micro-fin tubes with outer diameter (OD) of 5mm and length of 1.44m were selected as a test section to measure the evaporative heat transfer coefficient. The tests were conducted at mass fluxes from 212 to 530kg/(m2 s), saturation temperatures from −5 to 20°C and heat fluxes from 15 to 45kW/m2, where the test section was heated by a direct heating method. The differences of heat transfer characteristics between the smooth and the micro-fin tubes were analyzed with respect to enhancement factor (EF) and penalty factor (PF). Average evaporation heat transfer coefficients for the micro-fin tube were approximately 111–207% higher than those for the smooth tube at the same test conditions, and PF was increased from 106 to 123%. [Copyright &y& Elsevier]

Abstract: The performance of CO2 refrigeration systems strongly depends on the operating conditions. The specific characteristics of low critical temperature and high operating pressure limit its applications and imply the implementation of different control strategies. This study compares the performance of different CO2 system solutions for supermarket refrigeration with R404A system. Some possible modifications and improvements on the CO2 system have been investigated. The COP of the investigated CO2 system solution can be improved by about 3–7% along the ambient temperature range of 10–40°C. The annual energy consumption calculations in three different climates; cold, moderate and hot, show that the centralized trans-critical CO2 system is good solution for cold climates whereas the NH3–CO2 cascade system has the lowest energy consumption in hot climates. Both systems proved to be good alternatives to R404A DX system for supermarket refrigeration. [Copyright &y& Elsevier]

Abstract: Using CO2 trans-critical system solutions in supermarket refrigeration is gaining interest with several installations already running in different European countries. Using a computer simulation model, this study investigates the performance of two main system solutions: centralized with accumulation tank at the medium temperature level and parallel with two separate circuits for low and medium temperature levels. Both system solutions are presented and the simulation model is described in details. Calculations have been performed to design the systems and optimize their performances where basic layout and size of each solution have been defined. For ambient temperature range of 10–40°C, the reference centralized system solution shows higher COP of about 4–21% than the reference parallel solution. Using two-stage compression in the centralized system solution instead of single stage will result in total COP which is about 5–22% higher than that of the reference centralized system and 13–17% higher than that of the improved two-stage parallel system. The two-stage centralized system solution gives the highest COP for the selected ambient temperature range. [Copyright &y& Elsevier]

Abstract: This study presents experimental results obtained from a transcritical R744 system using a refrigerant ejector. The results were compared to that of a conventional system with an expansion valve. For the test conditions considered, the cooling capacity and COP simultaneously improved by up to 8% and 7%, respectively. Experiments were analyzed to quantitatively assess the effects on system performance as a result of changes in basic ejector dimensions such as motive nozzle and diffuser sizing. Small angles of 5° yielded best results for the static pressure recovery of the high-speed two-phase flow entering the diffuser. Experiments confirmed that like in a conventional transcritical R744 system with expansion valve, the high-side pressure control integrated into the ejector could be used to maximize the system performance. Numerical simulation results helped identifying this basic trend. Due to difficulties in the ejector throat pressure measurements, a more practical performance metric was introduced in order to quantify overall ejector efficiencies. According to this definition, the prototype ejector was able to recover up to 14.5% of the throttling losses. [Copyright &y& Elsevier]

Abstract: In this paper, a cryogenic refrigeration method is described, which utilizes CO2 solid–gas two phase flow and the dry ice. The CO2 solid–gas two phase flow is achieved by expanding liquid CO2 and thus refrigeration process less than CO2 triple point −56.6°C can be available. The experimental work is divided into two parts and two experimental set-ups were designed, constructed and tested. The interest of the first experiment test is the feasibility of expanding liquid CO2 into CO2 solid–gas flow in a horizontal circular tube by expansion valve. The second experiment focuses on the feasibility of the refrigeration of liquid CO2 expanding into solid–gas two phase flows used in a prototype CO2 heat pump system. The results show that solid–gas two phase flows can be achieved by expanding liquid CO2 by expansion valve in a closed CO2 heat pump system loop and low temperature refrigeration below −56.6°C is achieved in the experiments, which give greater possibility to create a cryogenic refrigeration process below −56.6°C for food industries, bio-medical engineering, etc. [Copyright &y& Elsevier]

Abstract: A capillary tube based CO2 heat pump is unique because of the transcritical nature of the system. The transcritical cycle has two independent parameters, pressure and temperature, unlike the subcritical cycle. In the present study, a steady state simulation model has been developed to evaluate the performance of a capillary tube based transcritical CO2 heat pump system for simultaneous heating and cooling at 73°C and 4°C, respectively against optimized expansion valve systems. Capillary tubes of various configurations having diameters of 1.4, 1.5 and 1.6mm along with internal surface roughness of 0.001–0.003mm have been tested to obtain the optimum design and operating conditions. Subcritical and supercritical thermodynamic and transport properties of CO2 are calculated employing a precision in-house property code. It is observed that the capillary tube system is quite flexible in response to changes in ambient temperature, almost behaving to offer an optimal pressure control. System performance is marginally better with a capillary tube at higher gas cooler exit temperature. Capillary tube length turns out to be the critical parameter that influences system optimum conditions. A novel nomogram has been developed that can be employed as a guideline to select the optimum capillary tube. [Copyright &y& Elsevier]

Abstract: Using economizer in R-744 heat pump cycle is an effective way to improve the heating capacity in cold climates. In this paper, a modification construction of reciprocating compressor with economizer port, a Voorhees compressor was introduced and the heat pump cycle with Voorhees economizer was compared with the traditional screw or scroll economizer cycles. Both the R-744 transcritical heat pumps with and without Voorhees economizer were tested at the same conditions with different air mass flow rates and different evaporating temperatures. The results show that the heating capacity of the heat pump with Voorhees economizer can be two times higher than the transcritical heat pump without economizer at low evaporating temperature conditions. At the same capacity operation conditions, the efficiency of the heat pump with Voorhees economizer is higher at high refrigerant mass flow rate conditions. The optimum discharge pressure of the heat pump with Voorhees economizer is found to be higher than the heat pump without economizer at the same ambient conditions. For mobile heat pump application, CO2 transcritical heat pump with Voorhees economizer demonstrates better performance comparing to the conventional transcritical CO2 heat pump without economizer when the evaporating temperature is lower than −20°C, or when the mobile is idling with low compressor RPM. [Copyright &y& Elsevier]

Abstract: A thermodynamic analysis of carbon dioxide–ammonia (R744–R717) cascade refrigeration system is presented in this paper to optimize the design and operating parameters of the system. The design and operating parameters considered in this study include (1) condensing, subcooling, evaporating and superheating temperatures in the ammonia (R717) high-temperature circuit, (2) temperature difference in the cascade heat exchanger, and (3) evaporating, superheating, condensing and subcooling in the carbon dioxide (R744) low-temperature circuit. A multilinear regression analysis was employed in terms of subcooling, superheating, evaporating, condensing, and cascade heat exchanger temperature difference in order to develop mathematical expressions for maximum COP, an optimum evaporating temperature of R717 and an optimum mass flow ratio of R717 to that of R744 in the cascade system. [Copyright &y& Elsevier]

Abstract: This study examined convective boiling heat transfer in horizontal minichannels using R-22, R-134a, and CO2. The local heat transfer coefficients were obtained for heat fluxes ranging from 10 to 40kWm−2, mass fluxes ranging from 200 to 600kgm−2 s−1, a saturation temperature of 10°C, and quality up to 1.0. The test section was made of stainless steel tubes with inner diameters of 1.5mm and 3.0mm, and a length of 2000mm. The section was heated uniformly by applying an electric current to the tubes directly. Nucleate boiling heat transfer was the main contribution, particularly at the low quality region. An increasing and decreasing heat transfer coefficient occurred at the lower vapor quality with increasing heat flux and mass flux. The mean heat transfer coefficient ratio of R-22:R-134a:CO2 was approximately 1.0:0.8:2.0. Laminar flow was observed in the minichannels. A new boiling heat transfer coefficient correlation based on the superposition model for refrigerants in minichannels was developed with a mean deviation of 11.21%. [Copyright &y& Elsevier]

Abstract: Carbon dioxide among natural refrigerants has gained a considerable attention as an alternative refrigerant due to its excellent thermophysical properties. In-tube evaporation heat transfer characteristics of carbon dioxide were experimentally investigated and analyzed as a function of evaporating temperature, mass flux, heat flux and tube geometry. Heat transfer coefficient data during evaporation process of carbon dioxide were measured for 5m long smooth and micro-fin tubes with outer diameters of 5 and 9.52mm. The tests were conducted at mass fluxes of from 212 to 656kgm−2 s−1, saturation temperatures of from 0 to 20°C and heat fluxes of from 6 to 20kWm−2. The difference of heat transfer characteristics between smooth and micro-fin tubes and the effect of mass flux, heat flux, and evaporation temperature on enhancement factor (EF) and penalty factor (PF) were presented. Average evaporation heat transfer coefficients for a micro-fin tube were approximately 150–200% for 9.52mm OD tube and 170–210% for 5mm OD tube higher than those for the smooth tube at the same test conditions. The effect of pressure drop expressed by measured penalty factor of 1.2–1.35 was smaller than that of heat transfer enhancement. [Copyright &y& Elsevier]

Abstract: This paper presents an overview of the flow boiling heat transfer characteristics and the special thermo-physical properties of CO2 at low temperatures (down to −30°C). Subsequently, the boiling heat transfer of CO2 at low temperatures is experimentally investigated in a horizontal tube with inner diameter of 4.57mm. Due to the large surface tension, the boiling heat transfer coefficient of CO2 is found to be much lower at low temperatures but it increases with vapour quality (until dryout), which is contrary to the trend at high temperatures around 0°C. None of the empirical correlations from open literature were able to predict the boiling heat transfer coefficient for CO2 in good agreement with the experimental data, suggesting the need for further research in this area. [Copyright &y& Elsevier]

Abstract: In the present paper, the suitability of hydrate slurries in secondary refrigeration was investigated by the means of a new hydrate solid-fraction model. Considering the high melting enthalpy of CO2-containing hydrates, slurries presenting high hydrate solid fractions can carry sufficient latent heat to be useful for a two-phase secondary-refrigerant application. The model presented in this paper allowed to calculate the solid fraction of CO2 and CO2–THF hydrate from thermodynamic conditions of pressure and temperature. Contrary to a previous work on single CO2 hydrates in a closed system, the present model can take into account hydrate mixture and is well adapted to additional CO2 injections (opened system). By relying on the hydrate-conversion model results, the study of hydrates in suspension in a carrying liquid was also studied in an experimental loop and was based on a formation process by CO2 injection in a cooled aqueous solution. [Copyright &y& Elsevier]

Abstract: 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 3.0mm and with lengths of 2000 and 3000mm, 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 20–40kWm−2, a mass flux range of 200–600kgm−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. [Copyright &y& Elsevier]

Abstract: Analysis of an endoreversible two-stage cascade cycle has been implemented and optimum intermediate temperature for maximum exergy and refrigeration effect have been obtained analytically. Further, the heat reservoir temperatures has been optimised independently. A comprehensive numerical model of a transcritical CO2–C3H8 cascade system was developed with intent to verify the theoretical results. It is seen that the simulation results agree well for optimal T L but deviate modestly from the theoretical optimum of T H. It has also been observed that system performance improves as T H increases and unlike theoretical predictions, no optimal T H is present within feasible working temperatures. [Copyright &y& Elsevier]

The objective of this study was to evaluate the effects of elevated carbon dioxide and sucrose concentrations on Arabidopsis thaliana L. Heynh root growth, morphology, and architecture. Two levels of CO2, 360 (ambient) and 900 (elevated) μmol·mol–1, and various sucrose concentrations were used. A. thaliana plants grown on a phytagar medium in small chambers with elevated CO2 had longer roots, more lateral root growth and a more dichotomous branching pattern than plants grown in ambient CO2. Roots in elevated CO2 had wider root diameters, and showed considerably more secondary growth such as larger diameter vascular cylinders and better-developed periderm. Addition of sucrose to the media closely resembled the effects of elevated CO2. Further, the increase in sucrose concentration had a more pronounced effect on root morphology under ambient, than elevated CO2. Thus, both elevated CO2 and increased sucrose concentrations promote root growth by increasing their number, length, and diameter, and by changing the branching pattern from herringbone to dichotomous. [ABSTRACT FROM AUTHOR]

Abstract: Flow boiling heat transfer coefficient, pressure drop, and flow pattern are investigated in the horizontal smooth tube of 6.1mm inner diameter for CO2, R410A, and R22. Flow boiling heat transfer coefficients are measured at the constant wall temperature conditions, while pressure drop measurement and flow visualization are carried out at adiabatic conditions. This research is performed at evaporation temperatures of −15 and −30°C, mass flux from 100 to 400kgm−2 s−1, and heat flux from 5 to 15kWm−2 for vapor qualities ranging from 0.1 to 0.8. The measured R410A heat transfer coefficients are compared to other published data. The comparison of heat transfer coefficients for CO2, R410A, and R22 is presented at various heat fluxes, mass fluxes, and evaporation temperatures. The difference of coefficients for each refrigerant is explained with the Gungor and Winterton [K.E. Gungor, R.H.S. Winterton, A general correlation for flow boiling in tubes and annuli, Int. J. Heat Mass Transfer 29 (1986) 351–358] correlation based on the thermophysical properties of refrigerants. The Wattelet et al. [J.P. Wattelet, J.C. Chato, B.R. Christoffersen, J.A. Gaibel, M. Ponchner, P.J. Kenny, R.L. Shimon, T.C. Villaneuva, N.L. Rhines, K.A. Sweeney, D.G. Allen, T.T. Heshberger, Heat Transfer Flow Regimes of Refrigerants in a Horizontal-tube Evaporator, ACRC TR-55, University of Illinois at Urbana-Champaign, 1994], and Gungor and Winterton [K.E. Gungor, R.H.S. Winterton, A general correlation for flow boiling in tubes and annuli, Int. J. Heat Mass Transfer 29 (1986) 351–358] correlations give the best agreement with the measured heat transfer coefficients for CO2 and R410A. Pressure drop for CO2, R410A, and R22 at various mass fluxes, evaporation temperatures and qualities is presented in this paper. The Müller-Steinhagen and Heck [H. Müller-Steinhagen, K. Heck, A simple friction pressure drop correlation for two-phase flow in pipes, Chem. Eng. Process. 20 (1986) 297–308], and Friedel [L. Friedel, Improved friction pressure correlations for horizontal and vertical two-phase pipe flow, in: The European Two-Phase Flow Group Meeting, Ispra, Italy, 1979 (paper E2)] correlation can predict most of the measured pressure drop within the range of ±30%. The relation between pressure drop and properties for each refrigerant is described by applying the Müller-Steinhagen and Heck correlation. The observed two-phase flow patterns for CO2 and R410A are presented and compared with flow pattern maps. Most of the flow patterns can be determined by the Weisman et al. [J. Weisman, D. Duncan, J. Gibson, T. Crawford, Effect of fluid properties and pipe diameter on two-phase flow patterns in horizontal lines, Int. J. Multiphase Flow 5 (1979) 437–462] flow pattern map. [Copyright &y& Elsevier]

Abstract: Theoretical analyses of subcritical/transcritical heat pumps using four natural refrigerants, carbon dioxide, ammonia, propane and isobutane have been carried out for high temperature heating applications at different heating outlet temperatures and heat sources using computer models. The compressor discharge pressures have been optimized for transcritical and subcritical (with near critical operation of condenser) cycles. Results show that for subcritical heat pumps, use of subcooling is efficient for heating applications with a gliding temperature. Results also show that although propane yields better coefficient of performance (COP) in low temperature heating applications, ammonia performs the best in high temperature heating applications. Finally, design aspects of major components of all the four heat pumps for high temperature heating have been discussed, particularly with reference to suitability of available lubricants to the newly evolved operating conditions. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: A new binary mixture of R744 and R290 as an alternative natural refrigerant to R13 was first presented in this paper. Its environmental performance is friendly. It has an ODP of zero and GWP smaller than 20. Experimental studies for this mixture and R13 were performed on a cascade refrigeration system only with modification to capillary in low-temperature circuit. COP and refrigeration capacity of this binary mixture were higher than those of R13, at the same time, condensing pressure, evaporating pressure, compression ratio, and discharge temperature were also higher than that of R13 when the high-temperature circuit of cascade refrigeration system was kept invariable. The new binary mixture of R744 and R290 is considered as a promising alternative refrigerant to R13 when the evaporator temperature is higher than 201K. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: The heat transfer coefficient and pressure drop during gas cooling process of CO2 (R744) in a horizontal tube were investigated experimentally. The experiments are conducted without oil in the refrigerant loop. The main components of the refrigerant loop are a receiver, a variable-speed pump, a mass flow meter, a pre-heater and a gas cooler (test section). The water loop consists of a variable speed pump, an isothermal tank, and a flow meter. The refrigerant, circulated by the variable-speed pump, condenses in the inner tube while water flows in the annulus. The gas cooler of tube diameter is 6000mm in length, and it is divided into 12 subsections. The pressure drop of CO2 in the gas cooler shows a relatively good agreement with those predicted by Blasius''s correlation. The local heat transfer coefficient of CO2 agrees well with the correlation by Bringer–Smith. However, at the region near Pseudo-critical temperature, the experiments indicate higher values than the Bringer–Smith correlation. Based on the experimental data presented in this paper, a new correlation to predict the heat transfer coefficient of supercritical CO2 during in-tube cooling has been developed. The majority of the experimental values are within 18% of the values predicted by the new correlation. [Copyright &y& Elsevier]

Abstract: In this paper, an alternate tool, i.e. the artificial neural network technique has been applied for estimation of vapour liquid equilibria (VLE) for the binary system, carbon dioxide–difluoromethane, which is an attractive alternative to chlorofluorocarbons and hydrochlorofluorocarbons, normally used as refrigerants. The model can satisfactorily estimate the vapour liquid equilibrium pressure and mole fraction carbon dioxide in vapour phase in the temperature range 222.04–343.23K and in the pressure range 0.105–7.46MPa. The average absolute error for the system in the estimation of vapour phase mole fraction is 0.0086 and 0.056MPa for the pressure. [Copyright &y& Elsevier]

Abstract: A bottle-sublimation cooler is an open-cycle cooler of novel design combining the advantages of Joule–Thomson and sublimation coolers. The store of a refrigerant is bottled without heat leakages at ambient temperature. Sublimation of the refrigerant solid phase is the cold-generating process in the cooler. The cooler design and a discovered effect of the solid phase spontaneous capillary-porous structuring provide autoadjustability of the system operational characteristics. The results of experimental investigations of the stored-carbon dioxide bottle-sublimation cooler are reported in this paper. The experimental set-up and procedure of the system principal parameters measurement are covered. Temperature, thermal and overall size-mass parameters of the cooler are measured and reported. Performance specification of the cooler, which had been used for refrigerating of the IR module of a star spectrophotometer during astronomical observations, is presented. [Copyright &y& Elsevier]

Abstract: Conventional working fluids (refrigerants) are being phased out worldwide to combat with the twin menace of ozone layer depletion and global warming and natural refrigerants are fast gaining favour lately. Single stage and multi stage refrigeration systems fail to widen the gap between heat source and heat sink temperatures required in many industrial applications requiring simultaneous heating and cooling and cascaded systems appear to be the best alternative. Modest research has been done in cascaded systems based on natural refrigerants thereby offering good potential for research. In this paper, a cascaded system for simultaneous heating and cooling (refrigeration and heat pump system) with a carbon dioxide based HT cycle and propane based LT cycle for simultaneous refrigeration and heating applications has been analyzed. To facilitate prediction of optimum performance parameters, performance trends with variation in the design parameters and operating variables have been presented in this article. Relevant expressions have been developed to serve as guidelines to the user for selecting appropriate design parameters like intermediate temperature so that the system yields optimum performance. Independently developed property codes have been employed for both carbon dioxide and propane for higher accuracy. [Copyright &y& Elsevier]

Abstract: An ejector expansion transcritical CO2 refrigeration cycle is proposed to improve the COP of the basic transcritical CO2 cycle by reducing the expansion process losses. A constant pressure mixing model for the ejector was established to perform the thermodynamic analysis of the ejector expansion transcritical CO2 cycle. The effect of the entrainment ratio and the pressure drop in the receiving section of the ejector on the relative performance of the ejector expansion transcritical CO2 cycle was investigated for typical air conditioning operation conditions. The effect of different operating conditions on the relative performance of the ejector expansion transcritical CO2 cycle was also investigated using assumed values for the entrainment ratio and pressure drop in the receiving section of the ejector. It was found that the COP of the ejector expansion transcritical CO2 cycle can be improved by more than 16% over the basic transcritical CO2 cycle for typical air conditioning operation conditions. [Copyright &y& Elsevier]

Abstract: A simulation model for a piston-cylinder type expansion device in which the linear motion of the piston is converted to the rotational motion of a crank via a connecting rod was developed for its use in a transcritical CO2 cycle. The model is based on lubrication theory with an assumption that the piston ring is a slippery bearing. This model has been used to verify the proper operation of the device in a prototype application and to identify the loss mechanisms of the device in the design stage. In addition, an analysis of the device has been conducted using this theoretical model for the device as installed in a prototype cycle as reported in a companion part I paper. [Copyright &y& Elsevier]

Abstract: Carbon dioxide is receiving strong consideration as an alternative refrigerant substituting hydroclorofluorocarbons (HCFCs) and chlorofluorocarbons (CFCs) due to its zero ozone depletion potential and negligible global warming potential. The system performance of CO2 systems, however, is typically poor compared to the current conventional air conditioning systems using HCFC or CFC. One of the most effective ways to achieve parity with CFC and HCFC systems is to replace the expansion valve with an expansion device that minimizes entropy creation and allows for energy recovery during the expansion process. A piston-cylinder type work output expansion device was designed, constructed and tested as part of the study reported here. The first-cut prototype device is based on a highly modified small four-cycle, two-piston engine that is commercially available. The work-producing expander replaced the expansion valve in an experimental transcritical CO2 cycle and increased the system performance by up to 10% as characterized by COP. The prototype device was not meant to be a final product, but provided valuable insight and experimental results to validate a detailed simulation model of the device. The model and corresponding theoretical analysis are presented in a companion part II paper. [Copyright &y& Elsevier]

Abstract: This paper describes an analysis on the performances of a cascade refrigeration cycle operated with blends of carbon dioxide (CO2, or R744) and hydrofluorocarbons (HFC) as the low-temperature working fluid. The aim of this work was to study the possibility of using carbon dioxide mixtures in those applications where temperatures below CO2 triple point (216.58K) are needed. The analysis was carried out by developing a software based on the Carnahan–Starling–De Santis (CSD) equation of state (EoS) using binary interaction parameters derived from our experimental data. The properties of the investigated blends (R744/R125, R744/R41, R744/R32, R744/R23) were used to simulate the behavior of a cascade cycle using ammonia (R717) as the high-temperature-circuit working fluid and operating at evaporating temperatures down to -70°C. The use of a suction–liquid heat exchanger on the low-temperature side of the circuit was also investigated. Results show that the R744 blends are an attractive option for the low-temperature circuit of cascade systems operating at temperatures approaching 200K. [Copyright &y& Elsevier]

Energetic and exergetic analyses as well as optimization studies of a transcritical carbon dioxide heat pump system are presented in this article. A computer code has been developed to obtain sub-critical and super-critical thermodynamic and transport properties of carbon dioxide. Estimates for irreversibilities of individual components of the system lead to possible measures for performance improvement. The COP trends indicate that such a system is more suitable for high heating end temperatures and modest cold end temperatures. Expressions for optimum cycle parameters have been developed and these correlations offer useful guidelines for optimal system design and for selecting appropriate operating conditions. [Copyright &y& Elsevier]

We summarize (1) knowledge about the global pre-industrial cycle of CO2 and CH4, and (2) recent reconstructions of the concentration of CO2 and CH4 in the atmosphere (CO2atm and CH4atm) based on ice cores drilled in Antarctica and Greenland. We discuss critically some difficulties in the interpretation of these reconstructions and suggest studies that should permit to better understand the factors controlling CO2atm and CH4atm variations. To cite this article: O. Marchal, J. Chappellaz, C. R. Geoscience 336 (2004). [Copyright &y& Elsevier]

In recent years, the technology envelope that embraces critical fluids can involve a wide range of conditions, different types of pure and modified fluids, as well as processing options involving extractions, fractionations or reactions. Technological development drivers continue to be environmentally and consumer-benign processing and/or products, however in recent years expansion of the use of sub- and supercritical fluids has been catalyzed by applications in such opportune fields as nutraceuticals, conversion of biomass (bio-refining), and the ability to modify natural products by reactions. The use of critical fluid technology is an important facet of any sustainable development program, particularly when utilized over a broad, interconnected application platform. In this overview presentation, concepts and applications of critical fluids from the author’s research as well as the literature will be cited to support the above trends. A totally ‘green’ processing platform appears to be viable using carbon dioxide in the appropriate form, ethanol and water as intermediate co-solvents/reactants, and water from above its boiling point to supercritical conditions. These fluids can be combined in overall coupled unit processes, such as combining transesterification with hydrogenation, or glycerolysis of lipid moieties with supercritical fluid fractionation. Such fluids also can exploited sequentially for bio-refining processes or the segregation of value-added products, but may require using coupled fluid or unit operations to obtain the targeted product composition or purity. Changing the reduced temperatures and/or pressures of critical fluids offers a plethora of opportunity, an excellent example being the relative critical fluid state of water. For example, sub-critical water slightly above its boiling point provides a unique medium that mimics polar organic solvents, and has been used even for the extraction of thermally labile solutes or reaction chemistry. To cite this article: J.W. King, C. R. Chimie 7 (2004). [Copyright &y& Elsevier]

An updated version of the Kattan–Thome–Favrat flow pattern based, flow boiling heat transfer model for horizontal tubes has been developed specifically for CO2. Because CO2 has a low critical temperature and hence high evaporating pressures compared to our previous database, it was found necessary to first correct the nucleate pool boiling correlation to better describe CO2 at high reduced pressures and secondly to include a boiling suppression factor on the nucleate boiling heat transfer coefficient to capture the trends in the flow boiling data. The new method predicts 73% of the CO2 database (404 data points) to within ±20% and 86% to within ±30% over the vapor quality range of 2–91%. The database covers five tube diameters from 0.79 to 10.06 mm, mass velocities from 85 to 1440 kg m−2 s−1, heat fluxes from 5 to 36 kW m−2, saturation temperatures from −25 °C to +25 °C and saturation pressures from 1.7 to 6.4 MPa (reduced pressures up to 0.87). [Copyright &y& Elsevier]

Carbon dioxide among natural refrigerants has gained considerable attention as an alternative refrigerant due to its excellent thermophysical properties. In this study, transcritical refrigeration cycle using carbon dioxide is of great interest, and the evaporation process is investigated by experiment and analysis. This paper presents the measured heat transfer coefficients and pressure drop during evaporation process of carbon dioxide in a horizontal smooth tube. The test section was made of a seamless stainless steel tube with the inner diameter of 7.53 mm, and length of 5 m. Heat is provided by a direct heating method to the test section. Experiments were conducted at saturation temperatures of −4 to 20 °C, heat fluxes of 12 to 20 kWm−2 and mass fluxes of 200 to 530 kgm−2 s−1. A comparison of different heat transfer correlations applicable to evaporation of carbon dioxide has been made. Based on the experiments for the evaporation heat transfer, useful correlation is developed. [Copyright &y& Elsevier]

A new automated set-up based on the original Sturm test was developed for determining the biodegradability of proteic mulching films. This method is based on the measurement of released (respiratory metabolism of microorganisms) carbon dioxide to determine film biodegradability. Maintenance of constant aerobic conditions in thermoregulated bioreactors and continuous measurement of released CO2 with an infrared differential gas analyser were the main advantages of this set-up. It was verified with a plasticised proteic film that soil extracts could be used as microbial sources instead of activated sludge that was usually used in laboratory biodegradation tests. To cite this article: S. Lefaux et al., C. R. Chimie 7 (2003). [Copyright &y& Elsevier]

This paper presents the current results of the development of a Modelica library for CO2-refrigeration systems based on the free Modelica library ThermoFluid. The development of the library is carried out in a research project of Airbus Deutschland and the TUHH and is focused on the aim to obtain a library for detailed numerical investigations of refrigeration systems with the rediscovered refrigerant carbon dioxide (CO2). A survey of the concept of an integrated on-board cooling system of airliners, the modelling language Modelica™ and the CO2-library is given and the modelling of CO2-heat exchangers is described. A comparison with steady state results of heat exchangers shows a fair agreement. The presented transient simulation results are compared with experimental data showing also a fair agreement. [Copyright &y& Elsevier]