Air Cycle Refrigeration Research Articles

Performance simulation and diagnosis of faulty states in air-cycle refrigeration systems in civil aircrafts

In the present study, performance simulation models and diagnosis method of faulty states in air-cycle refrigeration systems in civil aircrafts are developed, and the effects of the training data set on the diagnosis accuracy were analyzed. A base model for normal state simulation of air-cycle refrigeration system (ACS) was established based on the existing method with several modifications to improve accuracy and performance. Dedicated models for faulty states were developed according to the causes of fault. Based on the established models, a data set was generated. Preliminary training of multiple two-layer neural network models is performed with the existing data set, and the trained model achieved a validation accuracy of 99.00 % which confirms the feasibility of using double-layer neural network for fault identification and classification based on simulation data. The effects of the training data set to the model are analyzed, and it was concluded that: a data set of about 50,000 entries can be used to train a satisfactory diagnostic model; the proportion of normal state data needs to be reduced appropriately to improve the sensitivity of the model to faulty states; it is possible to largely reduce the number of sensors with smaller effect to the diagnosis accuracy.

Design and analysis of integrated environmental control system for Plateau cold chain transport vehicles

Plateaus, one of the most important terrain types in China, contain high-quality agricultural and livestock resources. However, due to their natural conditions, they host major deficiencies in cold chain logistics. Existing studies have focused on improving the temperature regulation capabilities of cold chain vehicle refrigeration or air conditioning systems, but have neglected the unique low-pressure environment faced by the Plateau cold chain transport vehicles. Using zoning design, air and vapor cycle refrigeration technologies, this study proposed a bootstrap-type integrated environmental control system (BIECS) and simple-type integrated environmental control system (SIECS) to realize vehicle “micro-environmental control.” The analytical solution of the thermal characteristics of the system was obtained based on the scaling-endoreversible thermodynamic analysis model. Moreover, the influence of ambient humidity on the thermal performance of the system was evaluated based on the enthalpy method. According to the typical working conditions, the system performances using different refrigerants, R404A, R448A, and R452A, were analyzed at different trailer temperatures, cooling capacities, and altitudes. Our results showed that, under the same design requirements, the coefficient of performance (COP) of the BIECS was slightly higher than that of the SIECS. Higher the trailer temperature and cooling capacity, higher is the COP of the system. Comparing the three refrigerants under the same working conditions, compared with R404A, the COP of the BIECS and SIECS increased by 13.4% and 10.9%, respectively, using R484A, and increased by 6.1% and 5.0%, respectively, using R452A. The performance of the systems that used R448A was the best. The system could ensure a 2.4-km cab pressure altitude at an ambient altitude of 5 km, but the COP could decrease by 31.8–35.6%, compared to that on the plains. This study provides and comprehensively evaluates a new technical solution for the environmental control of Plateau cold chain transportation vehicles. It also provides comprehensive health protection measures for drivers and technical support for the promotion and development of Plateau cold chains.

Dynamic simulation model for three-wheel air-cycle refrigeration systems in civil aircrafts

Dynamic simulation model of air-cycle refrigeration systems in civil aircrafts is essential for revealing the operation mechanism during flight process. In the present study, a dynamic model of three-wheel air-cycle refrigeration system was developed. A near-logarithm heat transfer temperature difference function is utilized to improve the stability of the heat transfer solution. A dynamic solution method based on continuity equation and theorem of momentum was presented as the framework of the dynamic mass transfer solution. A pressure-flow decoupling method is presented to solve the dynamic pressure in the high-pressure zone, resolving the absence of key pressure parameters. The model has been validated based on experimental data, with a pack discharge temperature deviation of 0.05K. Dynamic simulations have been conducted with actual flight data to obtain transient performance under airborne condition. In the takeoff and landing phase, the drastic change in the environment results in low discharge temperature, endangering the condenser with icing risk, which can be avoided by enabling the TCV. In the cruise phase, the discharge temperature slightly exceeds the upper limit for a couple of minutes. The optimization by slightly extending the length of heat exchanger channels succeeded in eliminating the excessive discharge temperature with minimal side effects.

Thermodynamic Analysis of Air-Cycle Refrigeration Systems with Expansion Work Recovery for Compartment Air Conditioning

As a requirement for sustainable development, air-cycle refrigeration has received wide attention as a candidate for environmentally friendly air conditioning technology. In this study, the thermodynamic performance of air refrigeration cycles is investigated in compartment air conditioning. The effects of compressor efficiency, expander efficiency, ambient humidity, all-fresh-air supply and ambient pressure on the cycle performance are presented. The effects of compressor arrangement in the high-pressure cycle and the low-pressure cycle are compared. An open-loop high-pressure cycle has a larger COP than that of an open-loop low-pressure cycle but requires larger heat exchange. The performance of air refrigeration cycles with full fresh air is studied, and the influence of fresh air is discussed. Schemes for condensed water recirculation with wet compression are proposed, which can improve the COPs of open-loop low-pressure cycles by 44.7%, 48.8% and 48.4%. In the air conditioning of plateau trains, open-loop high-pressure cycles have slightly lower COPs, but they can supply air with elevated pressure and oxygen concentration.

Parameter-Matching Algorithm and Optimization of Integrated Thermal Management System of Aircraft

The integrated thermal management system of aircraft is essential to maintain a suitable environment for the cabin crew and devices. The system is composed of the air-cycle refrigeration subsystem, the vapor-compression refrigeration subsystem, the liquid-cooling subsystem and the fuel-cycle subsystem, which are coupled with each other through heat exchangers. Due to the complex structure and large number of components in the system, it is necessary to design a corresponding parameter-matching algorithm for its special structure and to select the appropriate optimization design method. In this paper, the structure of an integrated thermal management system is analyzed in depth. A hierarchical matching algorithm of system parameters was designed and realized. Meanwhile, a sensitivity analysis of the system was performed, where key parameters were selected. Besides, a variety of optimization algorithms was used to optimize the design calculations. The results show that the particle swarm optimization and genetic algorithm could effectively find the global optimal solution when taking the fuel penalty as the objective function. Furthermore, the particle swarm optimization method took less time.

Performance of an Integrated Thermal Management System for helicopter

The traditional environment control systems for helicopter are mainly the air cycle or evaporative refrigeration system and the engine bleed air heating system. But they have low exergy efficiency and high fuel mass penalty in order to work in all climatic conditions. In this paper, an Integrated Thermal Management System (ITMS) is proposed for helicopter to satisfy the heating and refrigerating demands. This ITMS is a Heat Pump Air Conditioning System (HPACS) based on waste heat recovery from the lubricating oil system. A performance evaluation method is set up to investigate its working performances using exergy, energy level and fuel mass penalty. In a hot day, the input exergy and exergy efficiency of ITMS are 3.2 kW and 34%, respectively, the energy level difference is 0.0274, and the total fuel mass penalty is 22.75 kg. In a cold day, the input exergy and exergy efficiency of ITMS are 3.8 kW and 53%, respectively, the energy level difference is 0.026, and the total fuel mass penalty is 24.76 kg. Therefore, the comparison results show the performance advantages of the proposed ITMS in different climate conditions.

Simulation and test study on split four-wheel air cycle refrigeration system

Simulation and test study on split four-wheel air cycle refrigeration system

Optimization on conventional and electric air-cycle refrigeration systems of aircraft: A short-cut method and analysis

The air-cycle refrigeration system is widely used in commercial and military aircraft, and its efficiency greatly affects aircraft performance. Nowadays, this system requires a more efficient design and optimization method. In this paper, a short-cut optimization method with high efficiency and effectiveness is introduced for both conventional and electric air-cycle refrigeration systems. Based on the system characteristics, a four-layer parameter matching algorithm is designed which avoids computational difficulty caused by simultaneous equations. Fuel penalty is chosen as the objective function of optimization; design variables are reduced based on sensitivity analysis to improve optimization efficiency. The results show that the 3-variable optimization of the conventional air-cycle refrigeration system can obtain almost the same results as the traditional 6-variable optimization in that these two optimizations can both significantly reduce the fuel penalty. However, the computer running time of the 3-variable optimization is much shorter than that of the 6-variable optimization. The optimal fuel penalty of the electric air-cycle refrigeration system is lower than that of the conventional one. This study can provide reference for optimizing the air-cycle refrigeration system of aircraft.

Evaluation of the Thermodynamic Properties of Air Cycle Refrigeration System

Evaluation of the Thermodynamic Properties of Air Cycle Refrigeration System - written by Onovoh Johnpaul Nkemjika , Ujam Amechi Joseph published on 2019/05/04 download full article with reference data and citations

An easy-to-implement thermal test system for large deployable antennas

Large deployable antennas (LDAs), having a wide range of applications in aerospace engineering, encounter extreme thermal conditions when subjected to the space environment. Ground thermal test facilities are used to validate LDA deployability and accuracy under extreme thermal conditions. General antenna thermal test facility is thermal vacuum test facility, which is complex and costly as the vacuum test chamber has to be pumped down and large enough to accommodate LDAs. In this paper, an easy-to-implement thermal test system is presented, which simulates atmospheric thermal environment for LDAs using an air cycle refrigeration system and electric heaters. Additionally, a series of measures are used to ensure uniform temperature and limit airflow turbulence. Test results show that the test system can provide dry, uniform temperature and small disturbance thermal environment for LDAs.

Thermodynamic Investigation of Integrated Air Cycle Refrigeration Systems

An analysis of air cycle refrigeration (ACR) systems based on the first and second laws of thermodynamics has been carried out. Cooling the gas cooler outlet enhances the system efficiency, hence the use of integrated refrigeration systems (IRSs). The IRS consists of an ACR, an absorption chiller and a prime mover. The waste heat from the prime mover operates the absorption chiller, which provides additional cooling that is used to cool down the air exiting the gas cooler. LiBr-H2O and NH3-H2O are used as fluid pairs in absorption section. The prime mover can either be gas turbine or solid oxide fuel cell (SOFC). Results indicate that an IRS with NH3-H2O and two recuperators has a better first and second law efficiency compared to other considered systems. It is also concluded that the energy utilization factor and second law efficiency are higher for the system with SOFC as a prime mover.

Comparative performance study of an integrated air-cycle refrigeration and power generation system

This study presents comparative energy and exergy analysis of an integrated air-cycle refrigeration and power generation system with four embodiments as developed in the literature. The energy and exergy efficiencies, energy and exergy inputs and outputs and major exergy destructions in the components are determined and compared for four embodiments. Parametric studies are also performed to observe the effects of different influential parameters namely: ambient temperature, mass flow rate and inlet air pressure of the air. The energy and exergy efficiencies of system 1 and system 2 are found to be 61.6% and 58.1%, and 64.3% and 60.7%, respectively. The energy and exergy efficiencies for system 3 are found to be the highest in the order of 74% and 68.5%, whereas for system 4 energy and exergy efficiencies are 64.8% and 61.2%, respectively. Moreover, system 3 produces the maximum net work done 4164kW, whereas system 1 produces minimum net work done 2585kW.

A Thermodynamic Analysis of Three and Four-Wheel Air Cycle Machines for Aeronautical Applications

Air cycle refrigeration is an attractive alternative that has been employed to provide aircraft cabin cooling. The primary function of the cabin air conditioning and pressurization system is to maintain an aircraft environment that will ensure safety and comfort of the passengers and crew during all fly operational conditions and provide adequate avionics cooling. The energy to drive these machines comes from the compressed air bleed from the compressor of the aircraft propulsion turbine. Usually, Air Cycle Machine (ACM) architecture includes mainly two compact air-to-air heat exchangers, a compressor and a expander with one or two expansion stages (called three-wheel or four-wheel configuration, respectively). In this context, this work focuses on a comparative thermodynamic study of three and four-wheels air-conditioning aircraft air cycle machines. This comparison will be performed taking account some design features that affect the ACM performance as: aircraft flight Mach number, cabin altitude, heat exchanger effectiveness and compressed bleed air conditions. Results showed that the second level of expansion from four-wheel air cycle machine increases its thermodynamic efficiency. As the amount of rejected heat in the secondary heat exchanger (SHX) is greater for the 4-wheel architecture, it is possible to have the same cooling effect than the 3-wheel machine but with a more compact SHX. Moreover, the implemented computational tool to solving the air cycle refrigeration mathematical model allows a better understanding of the ACM performance.

Empirical models for efficiency and mass flow rate of centrifugal compressors

Centrifugal compressors are widely used in refrigeration systems, heat, power and chemical plants, turbochargers, and vehicle engines, including flight vehicles. Empirical models of efficiency and mass flow rates of centrifugal compressors are required for the design, energy analysis, performance simulation, and control- and/or diagnose-oriented dynamic simulation of such compressors and systems. This work presents a comprehensive review of empirical models for efficiency and mass flow rates of centrifugal compressors. An experimental investigation of a centrifugal compressor for air-cycle refrigeration systems of aircraft is carried out. Then, the available models are evaluated with the experimental data of the refrigeration centrifugal compressor and two turbocharger centrifugal compressors. It is found that the most models for centrifugal compressors of vehicle engines and turbochargers are not satisfactory for refrigeration centrifugal compressors, and thus accurate models for refrigeration centrifugal compressors need to be developed.

航空機の空調システム設計（第1報：システム作動における高圧除湿系統の機器能力の影響）

The purpose of this study is to clarify the effect of a component for high-pressure water separation, which is included in the air conditioning system of the general commercial aircrafts, on the air-cycle operation (reverse Brayton cycle); the high-pressure water separation consists of water separator and two heat exchangers called reheater and condenser, respectively. This paper briefly describes the typical system configuration of the air-cycle refrigeration progressed with evolutions in turbofan engine. Since nowadays, 4-Wheel air-cycle has been a trend on the aircraft air conditioning system, this paper theoretically shows its advantage on the basis of the thermodynamic cycle study. In the following chapters, an analysis was conducted on the interaction between a reheater and a condenser when each heat transfer performance changes from the baseline operating point. Also, the comparative study was made on the system performances by the 3-wheel and 4-wheel air-cycle. In addition, this paper describes how the heat transfer performance is designed for the reheater and the condenser under the system requirements for the entire air conditioning system.

Thermodynamic analysis of regenerated air-cycle refrigeration in high and low pressure configuration

A thermodynamic analysis is performed on open, regenerated, inverse air-cycles, focussing on low temperature refrigeration, in order to provide general optimization criteria, support for a rational configuration choice and potential performance improvement for this specific application. The effect of high regenerator effectiveness (0.95) is explored, showing that it may double the air cycle COP in the case at hand. Low and high pressure configurations (i.e. featuring only one heat exchanger on the cold or hot cycle side) are compared. High pressure configuration, normally preferred in the field of low temperature refrigeration, has a lower efficiency, but the gap reduces as regenerator effectiveness increases. Avoidance of a cold heat exchanger eliminates frosting problems and power input for the circulation fan, so that the small COP decrease suffered by the high pressure cycle can easily be compensated for and eventually yield higher average plant efficiency. The feasibility of such a highly effective regenerator, already reported in the literature, is demonstrated using classic Kays and London (1964) data.

Thermodynamic performance of a hybrid air cycle refrigeration system using a desiccant rotor

Due to the concern on global warming, the demand for a system using natural refrigerant is increasing and many researches have been devoted to develop systems with natural refrigerants. Among natural refrigerant systems, an air cycle system has emerged as one of alternatives of Freon gas system due to environmentally friendly feature in spite of the inherent low efficiency. To overcome the technical barrier, this study proposed combination of multiple systems as a hybrid cycle to achieve higher efficiency of an air cycle system. The hybrid air cycle adopts a humidity control units such as an adsorber and a desorber to obtain the cooling effect from latent heat as well as sensible heat. To investigate the efficacy of the hybrid air cycle, the cooling performance of a hybrid air cycle is investigated analytically and experimentally. From the simulation result, it is found that COP of the hybrid air cycle is two times higher than that of the conventional air cycle. The experiments are conducted on the performance of the desiccant system according to the rotation speed in the system and displayed the feasibility of the key element in the hybrid air cycle system. From the results, it is shown that the system efficiency can be enhanced by utilization of the exhausted heat through the ambient heat exchanger with advantage of controlling the humidity by the desiccant rotor.

공기사이클 냉동시스템의 성능해석

The objective of this study is to analyze theoretically the performance of an open air-cycle refrigeration system in which environmental concerns increase. The pressure ratio of the external compressor and efficiencies of the components that compose of the system are selected as important parameters. As the pressure ratio of the external compressor increases, the pressure ratio of the ACM compressor is determined high, the refrigerating temperature and capacity increase, the COP decreases, and the total entropy production rate increases. The effect of heat exchanger effectiveness and turbine efficiency on the performance are greater than that of the ACM compressor efficiency. Also the performance of the air-cycle refrigeration system with two heat exchangers has been enhanced like high COP and low total entropy production rate, compared to the system with one heat exchanger.

Off-design operating characteristics of an open-cycle air refrigeration system

Air cycle refrigeration based on the reverse Brayton cycle is a potential solution to environmental problems. In this study, the operating characteristics of an open-air cycle refrigeration system were simulated and analyzed. First, optimal design conditions were found through a design parametric study. Then, the operation of the system was simulated using off-design models. Two partial load operating strategies were compared: fixed shaft speed operation and variable speed operation. Fixed shaft speed operation requires a decrease in the refrigeration temperature as the refrigeration capacity decreases. Variable speed operation can operate with any specified refrigeration temperature. Both operations accompany a reduction in the coefficient of performance compared to the design value, but variable speed operation exhibits much less reduction at the same refrigeration capacity. We presented system performance maps and demonstrated that all of the operating conditions, which differ in refrigeration capacity and refrigeration temperature, can be addressed by varying both the speed and target refrigeration temperature simultaneously.

Development of an empirical model of turbine efficiency using the Taylor expansion and regression analysis

The empirical model of turbine efficiency is necessary for the control- and/or diagnosis-oriented simulation and useful for the simulation and analysis of dynamic performances of the turbine equipment and systems, such as air cycle refrigeration systems, power plants, turbine engines, and turbochargers. Existing empirical models of turbine efficiency are insufficient because there is no suitable form available for air cycle refrigeration turbines. This work performs a critical review of empirical models (called mean value models in some literature) of turbine efficiency and develops an empirical model in the desired form for air cycle refrigeration, the dominant cooling approach in aircraft environmental control systems. The Taylor series and regression analysis are used to build the model, with the Taylor series being used to expand functions with the polytropic exponent and the regression analysis to finalize the model. The measured data of a turbocharger turbine and two air cycle refrigeration turbines are used for the regression analysis. The proposed model is compact and able to present the turbine efficiency map. Its predictions agree with the measured data very well, with the corrected coefficient of determination R c 2 ≥ 0.96 and the mean absolute percentage deviation = 1.19% for the three turbines.