Search Results (16)

Modern aircraft increasingly utilizes highly integrated electronic equipment, driving continuously increasing heat dissipation demands. Vapor compression refrigeration systems demonstrate stronger alignment with future aircraft thermal management trends, leveraging their superior volumetric cooling capacity, high energy efficiency, and independence from engine bleed air. This paper reviews global research progress on aircraft vapor compression refrigeration systems, covering performance optimization, dynamic characteristics, control strategies, fault detection, and international development histories and typical applications. Analysis identifies emerging challenges under large-temperature-range cooling requirements, with comparative assessment establishing zeotropic mixture auto-cascade vapor compression refrigeration systems as the optimal forward-looking solution. Finally, recognizing current research gaps, we propose future research directions for onboard auto-cascade vapor compression refrigeration systems: optimizing refrigerant mixtures for flight conditions, achieving efficient gas-liquid separation during variable overloads and attitude conditions, and developing model predictive control with intelligent optimization to ensure reliability. Full article

The flow boiling heat transfer characteristics of zeotropic mixture R290/R601a in a horizontal mini-channel with an inner diameter of 2 mm were experimentally studied. The experiments were conducted at saturation pressure from 1 to 1.5 MPa, mass flux from 100 to 500 kg/(m²·s), heat flux from 20 to 30 kW/m², and vapor quality from 0 to 1. The effects of mass fraction, mass flux, saturation pressure, heat flux, and vapor quality on the flow boiling heat transfer coefficient in a mini-channel were analyzed. The experimental results show that the boiling heat transfer coefficient initially decreases and then increases with a decrease in the R290 mass fraction. The boiling heat transfer coefficient increases with the increase in mass flux and heat flux and decreases with the increase in saturation pressure. In addition, due to the dry-out phenomenon, the boiling heat transfer coefficient first increases and then decreases with the increase in vapor quality. The experimental data were compared and evaluated with existing correlations. Finally, a new prediction correlation for the boiling heat transfer coefficient is proposed, and the mean absolute relative deviation is 13.7%. This work provides key data for the development of green refrigeration technology, which is helpful in promoting the application of low-GWP natural refrigerants in new refrigeration systems. It also offers experimental guidance for the energy efficiency optimization of the ORC system and the structural design improvement of the compact heat exchanger. Full article

The organic Rankine cycle (ORC) is a valuable method for harnessing low-temperature waste heat to generate electricity. In this study, two dual-pressure auto-cascade ORC systems driven by low-grade geothermal water are proposed in series and parallel configurations to ensure high thermal efficiency and power output. The energy and exergy analysis models for two systems are developed for comparative and parametric analysis, which uses a zeotropic refrigerant mixture of R134a and R245fa. The findings indicate that, with a heat source temperature of 393.15 K, the thermal efficiency and exergy efficiency of the series auto-cascade ORC reach 10.12% and 42.07%, respectively, which are 27% and 21.9% higher than those of the parallel auto-cascade ORC. However, the parallel cycle exhibits a higher net power output, indicating a better heat source utilization. The exergy analysis shows that evaporator 1 and the condenser possess the highest exergy destruction in both cycles. Finally, the parameter analysis reveals that the system performance is affected significantly by the heat source and heat sink temperature, the pinch temperature difference, and the refrigerant mixture concentration. These findings could provide valuable insights for improving the overall performance of ORCs driven by low-grade energy when using zeotropic refrigerant mixtures. Full article

The last decade (2013–2023) was the most prolific period of organic Rankine cycle (ORC) research in history in terms of both publications and citations. This article provides a detailed review of the broad and voluminous collection of recent internal combustion engine (ICE) waste heat recovery (WHR) studies, serving as a necessary follow-on to the author’s 2013 review. Research efforts have targeted diverse applications (e.g., vehicular, stationary, and building-based), and it spans the full gamut of engine sizes and fuels. Furthermore, cycle configurations extend far beyond basic ORC and regenerative ORC, particularly with supercritical, trilateral, and multi-loop ORCs. Significant attention has been garnered by fourth-generation refrigerants like HFOs (hydrofluoroolefins), HFEs (hydrofluoroethers), natural refrigerants, and zeotropic mixtures, as research has migrated away from the popular HFC-245fa (hydrofluorocarbon). Performance-wise, the period was marked by a growing recognition of the diminished performance of physical systems under dynamic source conditions, especially compared to steady-state simulations. Through advancements in system control, especially using improved model predictive controllers, dynamics-based losses have been significantly reduced. Regarding practically minded investigations, research efforts have ameliorated working fluid flammability risks, limited thermal degradation, and pursued cost savings. State-of-the-art system designs and operational targets have emerged through increasingly sophisticated optimization efforts, with some studies leveraging “big data” and artificial intelligence. Major programs like SuperTruck II have further established the ongoing challenges of simultaneously meeting cost, size, and performance goals; however, off-the-shelf organic Rankine cycle systems are available today for engine waste heat recovery, signaling initial market penetration. Continuing forward, next-generation engines can be designed specifically as topping cycles for an organic Rankine (bottoming) cycle, with both power sources integrated into advanced hybrid drivetrains. Full article

The widespread use of automobiles and the increased duration spent within automobiles equipped with air conditioning systems have prompted various countries to enforce regulations that advocate for eco-friendly cooling substances (refrigerants) characterized by a slight global warming potential (GWP) and the absence of an ozone depletion potential (ODP). The imperative for automobiles to possess air conditioning systems that are both high-performing and eco-conscious has emerged as a means to mitigate their ecological impact, reduce fuel usage, and minimize carbon emissions. Zeotropic refrigerants, with a lower GWP than traditional alternatives, contribute to sustainability in car air conditioning by reducing the environmental impact and enhancing the energy efficiency, aligning with global regulations and fostering innovation in the automotive industry. This shift signifies a commitment to mitigating climate change and adopting environmentally conscious practices. The objective of the present study is to introduce blends of zeotropic refrigerants based on CO₂ (R-744), namely R455A (a blend of R-744, R-32, and R-1234yf), R469A (a blend of R-744, R-32, and R-125), and R472A (a blend of R-744, R-32, and R-134a), to enhance the thermodynamic performance of pure CO₂ refrigerant. Through the utilization of the Aspen HYSYS V11 software, an investigation is carried out involving thermodynamic energy and exergy analyses, as well as system optimization for an automotive air conditioning (AAC) system utilizing these novel zeotropic refrigerant blends, in comparison with the use of R-744 as the refrigerant. The study delves into the impact of parameters such as average evaporator temperature, condenser/cooler pressure, refrigerant flow rate, and condenser/cooler outlet temperature on AACs’ output parameters and subsequently presents the findings. The outcomes reveal that, under equivalent operational circumstances, the adoption of R455A, R469A, and R472A offers improvements in coefficient of performance (COP) by 35.4%, 18.75%, and 2%, respectively, when compared to R744. This shift is advantageous as it mitigates leakage-related issues stemming from the elevated operational pressure of R744 and eliminates the need for cumbersome equipment. R455A and R469A obtain the greatest COP and exergy efficiency (η_ex) values, measuring 4.44 and 4.55, respectively, at the identical operating conditions with optimal condenser/cooler pressures of the examined blends. Furthermore, eco-friendly refrigerants R455A and R472A are recommended for integration into AAC systems in vehicles, as they help combat global warming and protect natural surroundings and leakage issues. Full article

The influence of flow step ratio (FSR) on the startup characteristics of instant heat pump water heaters (IHPWHs) with natural mixture M (R744/R290 (12/88)) under nominal conditions was studied experimentally to verify the feasibility of a new quick startup method. The results show that the FSR had a marked effect on the startup time of system performance parameters. Under the optimal FSR of 0.6, the shortest system startup time and available hot water supply time were 700 s and 250 s, respectively, which were markedly shorter than those in the conventional startup. Therefore, rapid startup of the system and rapid production of usable domestic hot water can be realized by controlling the flow step. The influence of flow step on the variation trend of system performance parameters was obviously different, and there was no slow warming section for the heat sink outlet temperature (HSOT) under three FSRs. The HSOT, heating capacity, and high pressure side pressures had the maximum values in the quick startup, and the maximum values were obviously affected by the FSR. The FSR had no marked effect on the minimum suction pressure. The refrigerant pressures and refrigerant temperatures fluctuated markedly in both rapid and conventional starts. Full article

The popularity of vehicles and the increased time spent in cars with air conditioning systems has led to regulations in many countries that require the use of environmentally friendly refrigerants with minimal global warming and zero ozone depletion potential (GWP and ODP). Cars need high-performance, eco-friendly air conditioning systems to reduce their impact on the environment, lower fuel consumption, and decrease carbon emissions. The aim of the current work was to propose CO₂-based blend zeotropic refrigerants, R-455A (R-744/32/1234yf) and R-463A (R-744/32/125/1234yf/134a), to improve the thermodynamic performance of pure CO₂ refrigerants. The thermodynamic energy and exergy analysis and system optimization of an AAC system for the new zeotropic refrigerant blends compared to carbon dioxide (R-744), using Aspen HYSYS software, were investigated. The influence of cooler/condenser pressure, average evaporator temperature, cooler/condenser outlet temperature, and refrigerant flow rate on the cycles’ COP and exergy efficiency were conducted and are presented. The results showed that, at the same operating condition parameters, the cycle COP improved by 57.6 and 76.5% when using R455A and R463A instated of R744, respectively, with the advantage of reducing leakage problems due to the higher operating pressure of R744 (5–7 times higher than those of R455A and R463A), as well as requiring heavy equipment, but at optimal operating condition parameters, R744 and R-463A had a maximum COP of 14.58 and 14.19, respectively. The maximum COPs of R744, R455A, and R463A based on the optimal pressure of the cooler/condenser were 3.1, 4.25, and 5.4, respectively. Additionally, regarding the need for environmentally friendly air conditioning systems with acceptable performance in cars due to their impact on the environment and their contribution to global warming, the blend R455A is recommended for use as a refrigerant in AAC systems. Full article

The cooling capacity needed by ultra-low temperature apparatus cannot be reached economically with a single vapor compression refrigeration cycle due to the constraint of the high compressor pressure ratio. The auto-cascade refrigeration cycle is a good alternative. In this work, a novel concept that applies the principle of the auto-cascade refrigeration cycle to store cold energy is conducted. The environment-friendly refrigerants of R600a/R290/R170 zeotropic mixtures are used to study the performance of the modified auto-cascade refrigeration cycle (MACRC) as an alternative for cold-energy applications. The simulation results show that a cooling capacity of 500 W can be provided below −60 °C. The mixture with a mass fraction of 0.25/0.35/0.40 yields a COP of 0.695 and an exergy efficiency of 0.262 at −66 °C. The performance of the MACRC system was investigated at an ambient temperature of 20 to 40 °C for indoor small-scale applications. It is concluded that the performance would be improved by decreasing the ambient temperature. The results of the work should be helpful for the design and optimization of auto-cascade systems. Full article

In this study, modeling and thermodynamic analysis of the combined double flash geothermal cycle generation was conducted using zeotropic fluid as the working fluid in the Organic Rankine Cycle (ORC). The analysis was performed based on the first and second laws of thermodynamics. Hexane, cyclohexane, isohexane, R245fa, and R236ea exhibit good performance at higher temperatures. In this study, three fluids—hexane, cyclohexane, and isohexane—were used. First, the model results for the pure fluids were compared with those of previous studies. Then, the important parameters of the cycle, including the efficiency of the first law of thermodynamics, the efficiency of the second law of thermodynamics, net productive power, and the amount of exergy destruction caused by changing the mass fraction of the refrigerant for the zeotropic fluids (investigated for the whole cycle and ORC), were obtained and compared. Full article

The presented results show that the presence of refrigerant significantly deteriorates the lubricating properties of compressor oil under starved lubrication conditions (with a small amount of oil). The change can be 40–120% compared to the properties of the oil alone. Additionally, in the group of oils that are substitutes (operational alternatives) compatible with a given refrigerant, the effect of the refrigerant on the lubricating properties varies. The differences can be as much as 25%. In order to evaluate and properly select compressor oils for the refrigerant, the lubricating properties should be tested in a mixture with the refrigerant under conditions similar to actual operation. Such an evaluation of lubricating properties is made possible by the author’s method of testing the wear of the block-on-ring friction node. The obtained rankings of lubricating properties for oils (due to the wear volume) can provide good guidelines for the suitable selection of a lubricant for refrigeration compressors (especially for new, environmentally friendly refrigerants, such as R452A). The research was carried out for mixtures of zeotropic refrigerants (R404A, R452A) with polyester oils (POE) and natural refrigerant (R600a) with mineral oils (MO). In each group of refrigerants, different mechanisms of oil–refrigerant mixture formation occur. Each refrigerant was tested with three different compressor oils recommended for each other for alternative uses in refrigeration systems. Full article

Regulations and restrictions against high global warming potential (GWP) refrigerants have been introduced to encourage the adoption of environmentally friendly refrigerants and mitigate the environmental impact of the HVAC industry. R448A, a zeotropic blend with a GWP of 1390, has recently been proposed as a drop-in replacement for R404A and R410A in commercial systems. In this study, the heat transfer coefficient and pressure drop characteristics of R448A within a multiport mini-channel tube were experimentally investigated. The experimental ranges of the mass and heat fluxes were 100 to 500 kg/(m²s) and 3–15 kW/m², respectively. Additionally, the range of quality from 0 to 1 was considered at two fixed saturated temperatures of 3 and 6 °C. The heat transfer coefficient increased with mass flux. Under low mass flux condition, the heat flux increased the heat transfer coefficient, but there was no noticeable effect of the saturated temperature on the heat transfer coefficient. At high mass flux, heat flux had no major effect on heat transfer, while a decrease in the saturated temperature was found to increase the heat transfer coefficient. Moreover, the pressure drop increased with an increase in the mass flux and vapor quality, whereas the heat flux did not affect the pressure drop. The heat transfer coefficient and pressure drop performance of R448A was compared with that of R410A inside the same tube. Finally, correlations for heat transfer coefficient and pressure drop were proposed for the prediction of heat transfer coefficient and pressure drop in practical applications. Full article

An experimental rig of a zeotropic mixture separation condensation-based dual-temperature refrigeration cycle is built and the mixture R290/R600a is used as the refrigerant. Compared with a conventional cycle, the proposed refrigeration system demonstrates its application advantages under an on–off operation mode. Furthermore, the on–off periodic operation behaviors of this refrigeration system are experimentally investigated. The influence of a different refrigerant charge, a refrigerant mass fraction, a throttling valve opening, and ambient temperature are explored to evaluate the cyclic operation characteristics. The results reveal that the compressor average power for the duration of the compressor startup increases and the compressor duty cycle first declines then increases with the rise of the refrigerant charge. The average compressor during an on-period decreases from 104.5 W to 79.2 W and meanwhile the compressor duty cycles fluctuates between 72.1% and 96.9% as the R600a-charged concentration increases from 30% to 70%. The average power of the compressor during the on-period and the duty cycle are also sensitive to the freezer valve opening variation. Thus, the minimum energy consumption of 1.60 kWh·24 h⁻¹ is achieved at the refrigerant charge of 300 g, a R600a-charged mass fraction of 50%, and a freezer throttling valve opening of 10%. A higher ambient temperature deteriorates heat transfer during condensation and increases the cabinets’ heat load, the compressor duty cycle, and eventually affects the daily power consumption. Generally, the present study offers an in-depth insight of cyclic operation characteristics of a separation condensation-based hydrocarbon mixture dual-temperature refrigerator under two parallel evaporators’ concurrent cooling process. Full article

This paper is an introduction to the cycle proposed by the authors related to research directions concerning the problems of condensation of zeotropic refrigerant mixtures. For over a hundred years, research has been conducted on the search for new working fluids in the cycles for cooling devices and heat pumps. Initially, the natural refrigerants used were replaced with homogeneous synthetic refrigerants, followed by mixtures of two or more refrigerants. Among the mixtures, there are azeotropic and zeotropic mixtures. In the case of an azeotrope mixture, a liquid solution of two or more chemical compounds is in thermodynamic equilibrium with the saturated vapor resulting from this mixture. The chemical composition of the liquid and vapor is identical. A zeotropic mixture is a liquid-vapor system in which the composition of a liquid mixture (solution) of two or more chemical compounds is always different from that of the saturated vapor generated from this liquid. This is due to the different boiling and condensation temperatures of the individual components of the mixture at the same pressure. There is a so-called temperature glide. The phase transformations of individual components do not run simultaneously, which means that the boiling or condensation phase transition temperature changes during the process being carried out. This raises a number of computational, design, and operational problems for power equipment. Today, however, zeotropic mixtures find an alternative to refrigerants with a high GWP potential. Despite the disadvantage of temperature glide, they also have advantages. These include ecological, energy, and economic indicators. As a result, they are increasingly used in the energy economy. This prompts researchers to conduct further research in the field of a detailed description of the phenomenon of boiling and condensation phase transformations of zeotropic mixtures under temperature glide, searching for new computational relationships, new design solutions, and applications. It is still an insufficiently recognized research problem. Bearing the above in mind, the authors made an attempt to review the state of knowledge in this area. Particular attention was paid to the progress in modeling the condensation phenomenon of zeotropic mixtures for application in compact heat exchangers. Miniaturization of cooling devices creates great application possibilities in this area. Full article

This paper presents experimental research and mathematical modeling data concerning the impact of unit dynamic instabilities on the phase-transition condensation processes of the zeotropic mixtures R404A and R448A and azeotropic R507A refrigerants in pipe minichannels. The R507 refrigerant is currently used as a temporary substitute for R404A, whereas R448A is a sustainable prospective substitute for R404A. The study presents experimental testing data for the condensation processes of these refrigerants in pipe minichannels and a proposal for the use of dimensional analysis, including the Π-Buckingham theorem, to determine the regression relationship explaining the propagation of unit dynamic instabilities. Based on the experimental studies performed, regression computational models were developed and showed satisfactory agreement in the range of 20% to 25%. They give the possibility to identify, in a utilitarian, way the speed of propagation of temperature and pressure instabilities during the liquefaction of refrigerants. The study was carried out on pipe minichannels with an internal diameter of di = 3.3, 2.3, 1.92, 1.44 and 1.40 mm. Full article

An experimental evaluation of an autocascade refrigeration (ACR) system was carried out. A zeotropic mixture of isobutane and CO₂ was employed as a working fluid in an autocascade refrigeration (ACR) system. An experimental system was designed and built to study the influence of the recuperative heat exchanger (RHX) and openings of the throttle valves on the system performance. The use of RHX facilitated the condensation process and improved the cycle characteristics. The working mass concentration of CO₂ was higher, as it was closer to the nominal concentration and the discharge pressure was lower by 19% to even 39% when the RHX was employed in the system. An increase of up to 20% in the coefficient of performance (COP) was observed. Furthermore, the effects of the openings of the throttle valves on the system characteristics were studied. The change in the openings of the expansion valves affected the mass flows and the working mixture composition. The working CO₂ mass fraction increased with higher openings of the evaporator throttle. The subcooling degree of liquid CO₂-rich refrigerant increased with higher openings of the expansion valve under the phase separator. The results of the present work should be helpful for design and optimization of autocascade systems working with natural and synthetic refrigerants. Full article