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• A dynamic model of civil aircraft air-cycle refrigeration system was developed • A dynamic solution method was introduced for dynamic mass transfer solution. • A near-logarithm heat transfer temperature difference function is presented. • A pressure-flow decoupling method is presented to obtain ACM pressure parameters. Dynamic simulation model of air-cycle refrigeration system s 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. [ABSTRACT FROM AUTHOR]

The temperature control loop of the actuating air following the heat exchanger is considered. The control loop consists of a heat exchanger, adjustable air intake, circulation fan, temperature sensor, and temperature controller. A dynamic model of the control loop that takes into account the automatic control system is created. Transient processes that arise in the control loop as a consequence of variations of the altitude of the aircraft are considered. Results of the variation in the actuating air temperature at a temperature control point are obtained. Partial optimization of the coefficients of the controller is performed. Optimal coefficients of the controller are selected. [ABSTRACT FROM AUTHOR]

Abstract The present investigation is a continuation of a previous study conducted by the authors that presented a dynamic-simulation model of an Ericsson engine equipped with a gas-to-gas heat exchanger. This model is here used to investigate the effect of compressor and expander valve timing variation. The investigation begins with an un-optimized case and examines various improvements in terms of valve timings. This results to an optimized case that improves power and thermal efficiency. The optimized case serves as reference to study the effects of variations in the valve timings of the compressor and expander. The inlet valve closing timing of the expander (IVC e), which determines the intake duration of the expander, necessitates the readjustment of some of the other valve timings, since it significantly affects the pressure levels in all devices. The IVC e variation led to the potential of using a single heater with multiple compressor-expander pairs that are sequentially and periodically coupled to the heater. This increases the specific power output compared to using several compressor-heater-expander triplets, since it reduces the total volume of the engine. The number of compressor-expander pairs that can be coupled to a single heater is largely determined by the IVC e timing. Highlights • Valve timing effects are analyzed in terms of flow conditions and performance. • Expander inlet valve closing greatly affects pressure levels and performance. • Multiple compressor-expander pairs coupled to a single heater increase specific power. • The number of compressor-expander pairs depends on expander inlet valve closing. [ABSTRACT FROM AUTHOR]

The aircraft air-conditioning system, which consumes the engine bleed air to provide stable and comfort environment for passengers, has a high demand of system reliability and energy efficiency. For analyzing system performance and control effect under a wide range of operating scenarios, this paper presents a dynamic model of the aircraft air-conditioning system. A pressure-node based method is proposed to decouple the system architecture, and a modelling framework is established with reflecting the interdependencies between component-level modules. Dynamic sub-models including pressure nodes, heat exchangers and the air cycle machine are built to predict all major dynamics in the system. The model is verified by the measured data of an airborne testing. The predicted temperatures show good agreement with the measured data, of which the average deviations at the compressor outlet and system outlet are 4.60 °C and 3.49 °C, respectively. The effectivity of the proposed model is investigated under various conditions, including different control signal inputs, standard pull up/pull down conditions and an entire flight case. The simulation results indicate that the model can be successfully applied for a wide range of aircraft operating scenarios, and provide insight towards control design and fault detection of the aircraft air-conditioning system. • A pressure-node based modelling framework for aircraft ACS is proposed. • Fully dynamic component-level models are built to predict all transient features. • Model predictions show good agreements with the airborne testing data. • System response characteristics are analyzed under various operation conditions. • Cabin temperature control effect is examined by simulating an entire flight case. [ABSTRACT FROM AUTHOR]

The supercritical cycle has a better variable-temperature match between the working fluid and heat source, potentially providing higher efficiency. Accurate simulation of the transient performance of supercritical heat exchangers is significant in advanced thermal system control. This paper proposes a dynamic model of supercritical heat exchangers based on the moving boundary method. The model redefines the concept of "interface" in the traditional moving boundary model. The new interfaces are determined by the thermophysical properties of the working fluid, called "virtual interfaces". A control-volume separation method is proposed based on the virtual interface. Additionally, the specific enthalpy is selected as the switching criterion for the configuration of the control volumes, so the model can dynamically adjust the number of control volumes. The integrity check results show that the simulation accuracy is at least equivalent to the finite volume model with 20 control volumes; however, the time cost is approximately 1/5 of that of the finite volume model with the same precision. An experimental study on the convective heat transfer of supercritical n -decane in an electrically heated vertical tube was conducted. The maximum absolute error of the steady-state outlet temperature is ±1.5 K, and the relative error is ±3% under different pressures. • Mass and energy exchange occur in heat exchangers in the thermal system. • Heat transfer has crucial influence on the dynamic performance of thermal systems. • Under supercritical pressures, the working fluid does not have a phase interface. • The proposed model shows a significant advantage in simulation efficiency. • The time cost of the model is approximately 1/5 of that of the FV model. [ABSTRACT FROM AUTHOR]

Modelling and simulation is presented for a finned cross-flow heat exchanger with the aim to heat cold air to be fed to air conditioning batteries for marine purposes. The model employed in this paper is finalized to simulate the dynamic behavior of air and water temperatures fed to the air conditioning batteries operating in cold environments, in order to predict possible troubles owing to the change in input parameters, such as unwanted flow rate variations due to system malfunctions. In the investigated model, heat balance equations are presented and discretized by Laplace transform, which has the advantage to easily account for the different structures of heaters used for the purpose of validation. The model was implemented in the Matlab-Simulink environment for its high capacity of dealing with dynamic systems. The results of the model are satisfactory, as the dynamic behavior of the air stream temperature is correctly reproduced, as compared to experimental data, providing a suitable parameter for malfunctions prediction. [ABSTRACT FROM AUTHOR]

Abstract: The European Energy Efficiency Building Directive 2002/91/CE, as well as other acts and funding programs, strongly promotes the adoption of passive strategies for buildings, in order to achieve indoor thermal comfort conditions above all in summer, so reducing or avoiding the use of air conditioning systems. In this paper, the energy performances achievable using an earth-to-air heat exchanger for an air-conditioned building have been evaluated for both winter and summer. By means of dynamic building energy performance simulation codes, the energy requirements of the systems have been analysed for different Italian climates, as a function of the main boundary conditions (such as the typology of soil, tube material, tube length and depth, velocity of the air crossing the tube, ventilation airflow rates, control modes). The earth-to-air heat exchanger has shown the highest efficiency for cold climates both in winter and summer. The possible coupling of this technology with other passive strategies has been also examined. Then, a technical-economic analysis has been carried out: this technology is economically acceptable (simple payback of 5–9 years) only in the cases of easy and cheap moving earth works; moreover, metallic tubes are not suitable. Finally, considering in summer a not fully air-conditioned building, only provided with diurnal ventilation coupled to an earth-to-air heat exchanger plus night-time ventilation, the possible indoor thermal comfort conditions have been evaluated. [Copyright &y& Elsevier]

Abstract: To account for microbial reduction and food quality changes, first-order chemical reaction was added to a model for axial-dispersed plug flow (ADPF), which was recently developed for efficient (In the terminology of this paper, efficient is related to computation time for a given level of accuracy.) computation of time-dependent, large flow systems. It was shown that the extended ADPF model gives accurate results for typical parameter values in liquid-food applications. The analysis was based on Laplace transforms and the models were written in the object-oriented language Modelica, as objects in a library structure which is being developed to simulate complex liquid-food process lines and their control systems. The models were also implemented in such a way as to efficiently account for non-isothermal reactions in heat exchangers. Furthermore, to account for changes in flow rates, a model of dynamically calculating the “true” holding time in a channel was developed. Simulations showed that the model performs better than a straightforward alternative. [Copyright &y& Elsevier]