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• Oil retention models in various types of pipelines are established. • Hierarchical iterative algorithm based on graph theory is developed. • Oil retention in various pipelines is measured for model validation. • Mass flow rate or evaporation temperature increase may reduce oil retention. A multi-split air conditioning system may suffer damage caused by oil starvation in its compressor when the lubricating oil largely retains in long pipelines, and thus a reasonable oil charge based on accurate prediction of the oil retention in pipelines is important. The purpose of this study is to present an oil retention model for pipelines and to experimentally validate this model. The structure of pipelines connecting the outdoor unit and the indoor units with arbitrary numbers installed on separated floors is described as a combination of straight pipe types, joint types, and bend types. In modeling the oil retention, all the components of straight pipes, joints, and bends are numbered to conveniently distinguish each component which is further divided into several control volumes with equal lengths along the flow direction, and the equations of oil retention, pressure drop and enthalpy difference for the control volumes are established to describe the refrigerant-oil flow behavior. A hierarchical iterative algorithm based on the graph theory is developed in order to solve the models quickly. An experimental rig is built to measure the oil retention in four types of straight pipes, and the validation results show that the predicted oil retention in straight pipes agrees with at least 74.0 % of the experimental data within a deviation of ±20 %, and the mean deviation is lower than 15.4 %. Based on the model, the effects of oil retention reduction by increasing the total mass flow rate or the evaporation temperature of refrigerant are quantitatively analyzed. [ABSTRACT FROM AUTHOR]

• Develop the 3-D numerical model of sub-Kelvin porous media heat exchanger. • Obtain the optimal structure parameters of porous media for the best performance. • Propose the partial-width liquid gap that shows better performance than the conventional full-width one. A dilution refrigerator is an essential apparatus for achieving sub-Kelvin temperatures. Porous-media heat exchangers used inside dilution refrigerators at temperatures below 100 mK are crucial components that determine their optimal performance. In this study, a numerical model of a porous-media heat exchanger is developed. The local thermal non-equilibrium heat transfer model captures the Kapitza heat transfer between solid and fluid components, whereas the coupled Brinkman-Forchheimer and Navier-Stokes equations describe fluid flows in porous media. Results indicate that a liquid gap can enhance the thermal conduction between the flow channel and porous media, thus improving the heat-exchange efficiency of the porous media. For a constant-volume and -thickness porous media, the most suitable length-to-width ratio is 5. The concentrated phase exhibits a lower outlet temperature, whereas the dilute phase exhibits an appropriate pressure drop. For a constant-volume and -length porous media, the optimal width-to-thickness ratio is 10, which provides the best thermal and hydrodynamic performances. For a full-width liquid gap, increasing its thickness enhances both the thermal and hydrodynamic performances but also increases the usage of 3He. Under a constant volume of liquid gap, a partial-width liquid gap can improve the heat-exchange efficiency, thus achieving better thermal and hydrodynamic performances than the conventional full-width one. [ABSTRACT FROM AUTHOR]

• A novel dual piston linear compressor prototype is presented. • The experimental results verify the established numerical model. • Piston operation is a direct control variable, which means the volumetric efficiency can be regulated. • The output characteristics, especially linear motor efficiency, depend heavily on the operating frequency and stroke. A novel dual-piston linear compressor consisting of two piston-cylinder assemblies and a linear electric motor was constructed. To explore the performance of the dual-piston linear compressor, a numerical model that contains a thermodynamic sub-model, dynamic sub-model, three-phase linear motor sub-module, and control system sub-model was built. Several piston trajectories and different operating conditions were assessed to investigate the performance of the dual piston linear compressor. Results from both the simulation and experiments indicate that the system with the triangle curve trajectory exhibits the best performance. The output characteristics (mass flow rate, motor efficiency, mechanical efficiency, and overall efficiency) depend heavily on the operating frequency and the stroke of the piston. For the dual-piston linear compressor system that was designed, the overall efficiency can reach up to 57.59% when the system works at a frequency of 12 Hz and a stroke of 26 mm. [ABSTRACT FROM AUTHOR]

• The model for predicting the water bridge deformation between fins was developed. • The model for calculating the particle mass adhered on water bridge was developed. • The experimental apparatus was established to validate the numerical model. • The comparisons between experiment data and predicted results were performed. A typical room air conditioner uses fin-and-tube heat exchanger as evaporator, and its performance may be dramatically deteriorated by the dusty-bridge between fins, which is resulted from dehumidification and dusty particle adhesion. Design of a fin-and-tube heat exchanger to avoid dusty-bridge need to predict the forming process of dusty-bridge between fins. In this paper, a numerical model is developed to simulate the dusty-water bridge formation between fins, which can well predict the shape of dusty-water bridge and mass of the dusty particles adhered by water bridges. Prediction of the dusty-water bridge shape is realized by calculating the mean curvature of the water bridge interface at first and then predicting the effect of air flow and gravity on the water bridge shape. The prediction method of the mass of the dusty particles adhered by water bridges between fins is to simulate the motion trajectories of all particles in the air flow at first and then to choose those particles intersecting with the water bridge surface as adhered. The validation results show that, the water bridge shape predicted by the model agrees well with the experimental image, and the predicted adhered particle mass agree with 92% of the experimental data within the deviation of ±20% and the mean deviation is 14.8%. [ABSTRACT FROM AUTHOR]

• A novel dual piston linear compressor is presented. • The influence of typical displacement profiles is studied by a numerical model. • Triangle profile has the highest isentropic and electrical efficiency. • The influence of back electromagnetic force cannot be ignored. A novel dual piston linear compressor has been proposed in this paper, which is suitable for various applications such as refrigeration and gas compression owing to capacity modulation by stroke control. A numerical model for the proposed dual piston linear compressor was built to investigate the influence of different piston displacement profiles on the performance of the linear compressor. Sub-models on the piston dynamics, cylinder gas thermodynamics, motor force and frictional force were presented using MATLAB/Simulink. The piston was controlled to oscillate with the 5 typical piston displacement profiles. From the simulation results, it is found that the displacement profile of a triangle curve shows the highest compression efficiency and highest electrical efficiency. Moreover, the piston velocity and the current in the coil of the linear motor are considered to be necessary for feedback signals in the real-time control system to adjust the motor force and to reduce the effect of the back electromagnetic force. [ABSTRACT FROM AUTHOR]

Highlights • Influence of the inlet vapour superheating in the condenser performance. • Opposite effect between the convective coefficient and the temperature difference. • Comparison between experimental data and a numerical model. • Influence of the condensing superheating zone included in the numerical model. Abstract In this investigation the influence of the degree of superheating at the inlet of a R134a condenser on its performance is studied. The degree of superheating is controlled by liquid injection from the condenser outlet by a membrane pump. The physical phenomena of influence are a reduction in the mean temperature difference and an increment in the overall heat transfer coefficient. An experimental rig has been built and a number of operating conditions tested as a function of the compressor capacity, evaporator power and the degree of inlet superheating. A numerical model has been developed for the experimental setup, the result of which has been compared with the experimental data. Additionally, the model has been used to understand the evolution of the heat transfer inside the condenser. It has been concluded that the overall condenser efficiency may slightly increase at high evaporator loads and high compressor capacities. [ABSTRACT FROM AUTHOR]

Highlights • Two-phase numerical model of absorption of vapour into a falling film with a co-current gas-phase. • Applied to LiBr aqueous solution and water vapour. • Fully-coupled robust marching solution produces velocity, temperature and mass fraction in the liquid film plus velocity and temperature in the vapour. • Carefully compared with previous results. • Detailed results presented that can be suitable for model validation by others. • New results for the effect of gas flow in a vertical channel. Abstract A two-phase numerical simulation is performed to investigate the absorption of water vapour into an aqueous lithium bromide solution inside a parallel plate vertical channel. The liquid film and vapour phase interactions and concurrent heat and mass interchange is modelled using a two-dimensional steady laminar flow model and a fully-coupled marching numerical scheme. The capability of this new model is examined by comparisons to the previous numerical results for a liquid film flowing over a cold wall. The numerical predictions are examined for the effect of the following operating parameters: the liquid film Reynolds number, the inlet mass fraction, and the wall temperature. New results are provided on the impact of the gas phase Reynolds number variation from 1000 to 3000 on the liquid film development along the channel. [ABSTRACT FROM AUTHOR]

Highlights • An analytical model of particle-deposited fin efficiency was developed. • Fin efficiency expressions of annular fin and rectangular fin were obtained. • Heat transfer rate of fouled heat exchangers was predicted based on the model. • Experimental validation of the proposed model was performed. Abstract Fins of heat exchangers are gradually deposited by dust particles in most application scenarios, and so a model of particle-deposited fin efficiency is needed for heat transfer prediction. In this study, the expression of fin efficiency for a particle-deposited annular fin was developed by simplifying the fin as two two-dimensional coating layers on a one-dimensional clean fin firstly, and then this expression was extended to rectangular fins with circular holes by the equivalent radius transformation. The analytical solutions for a typical rectangular fin show that the attenuation rate of fin efficiency may increase up to 23.9% if the fin surface is covered by a fouling layer with an average thickness of 4.0 mm. This model can be used to predict the heat transfer rate of fouled heat exchanger with multi-fins, and the validation results show that the accuracy of the proposed model is acceptable in engineering applications. [ABSTRACT FROM AUTHOR]

This paper treats a control technique designed to maximise the working of a rotary permanent magnet magnetic refrigerator (RPMMR). The method, named ANNTEO, is based on the use of the artificial neural networks (ANNs), which have demonstrated to predict well the energy performances of an actual RPMMR obtaining a good agreement with the experimental tests. The ANN gives the possibility to carry out a working map, and then, applying an optimisation process, it is possible to catch the optimal working point regarding the number of revolution of the magnets per minute and the volumetric flow rate of the regenerating fluid. In particular, the optimisation can be processed with the aim to maximise the COP (energy saving) or the cooling capacity (time-saving). As a proof of the concept, this paper reports an example of an application of the ANNTEO. Also, new perspectives on the use of the ANNs in the magnetic refrigeration field are proposed. [ABSTRACT FROM AUTHOR]

Two-phase expansion devices are a practicable solution for improving the energy efficiency of heat pumps. The study of the fluid behavior in such machines has not yet been investigated in detail. In the present study, a radial piston expander was used in a heat pump operating with R134a in place of the throttling valve. Special attention was paid to investigate both numerically and experimentally the behavior of the fluid inside a cylinder within a full cycle of the engine, at different inlet thermodynamic conditions and rotational speeds. The measured indicated cycles were compared with those predicted by a quasi-steady model and a rather large difference between them was noticed. Therefore, a non-isentropic model to predict the in-cylinder pressure trend during the expansion phase at closed valves, developed on the basis of a model proposed for water at atmospheric conditions, was applied. The results achieved with the improved non isentropic model showed a significantly higher accuracy in predicting the experimental results along the expansion phase at closed valves, thus filling the gap registered with the full isentropic models, which lack in this part of the cycle. [ABSTRACT FROM AUTHOR]

Dust particle deposition on fin surface has a significant influence on the performance of fin-and-tube heat exchangers, and the purpose of this study is to develop a numerical model for predicting the particle deposition rate on fin surface. In the model, the particle trajectories were calculated by the particle motion equation; the particle deposition on the fin surface was described based on the critical impact angle and the critical sticking velocity of incident particles; the particle deposition on the formed fouling layer was described based on the critical impact angle, the critical sticking velocity and the critical removal velocity of incident particles. The particle distributions on fin surface predicted by the model agree well with the images captured in the visualization experiment. The predicted particle deposition weight per unit area can describe 88% of the experimental data within a deviation of ±20% and the mean deviation is 12.8%. [ABSTRACT FROM AUTHOR]

A flexible one-dimensional model was created in Python to determine the periodic steady state cooling power of an active magnetocaloric regenerator. Several features of the model provide advantages when performing large parametric studies. Geometry, material properties, and operation parameters are read in as input. Fluid flow and magnetization profiles are parameterized, and thermodynamic consistency is forced in magnetocaloric material properties. A dynamic time step is used, and three associated model constants are set to work over a wide range of input parameter combinations. Derivative gain is adjusted to minimize convergence time resulting in an average convergence time reduction of approximately 50%. [ABSTRACT FROM AUTHOR]

A numerical simulation by using a commercial software named COMSOL MULTIPHYSICS @ V4.4 was carried out to predict the heat transfer process in fresh-cut leaves of Brassica chinensis during vacuum cooling. The temperature distribution and the average surface temperatures of the leafy part and the petiole were predicted and compared with the experimental data. Both the experimental and simulated results followed the same time–temperature curves. The evaluated results show that the R 2 values were 0.9837 and 0.9876 for the average surface temperatures of the leafy part and the petiole, respectively. Further, the maximum differences between the experimental and simulated average surface temperatures at any time did not exceed 0.91°C and 0.83°C for the leafy part and the petiole, respectively. This indicates that the simulation outcomes are in great agreement with the experimental results. [ABSTRACT FROM AUTHOR]

In this paper, a distributed parameter (DP) numerical model with the new proposed flow boiling heat transfer correlation was established for parallel flow minichannel (PFMC) evaporator. DP model validation was made by comparing the measured values obtained on experimental studies, which were conducted under refrigerant mass flow rate range of 34.6–245.6 kg h −1 and evaporation pressure of 200–500 kPa. The effects of four different flow boiling heat transfer correlations on DP model performance were investigated. Results showed that the new correlation predicted 99% of experimental data in ± 30% error bands. Moreover, the DP model with the new correlation yielded the mean absolute error (MAE) of 1.5%, 9.1%, 18.8%, 14.2% and 19.8% in prediction of cooling capacity, outlet air temperature, refrigerant superheat, air side and refrigerant side pressure drop, respectively. The presented DP model can be implemented to evaluate the performance of PFMC evaporator, and therefore can save efforts on component and system design and optimization. [ABSTRACT FROM AUTHOR]

The aim of this article is to present a distributed numerical model that simulates the thermal and fluid-dynamic phenomena inside non-adiabatic capillary tubes. The resolution approach is based on a two-phase flow model where the fluid domain is discretized in a one-dimensional way, and the governing equations (continuity, momentum, and energy) are solved by means of a step-by-step algorithm. The model explained herein consists of an improved and extended version of previous works (Escanes et al., 1995; García-Valladares et al., 2002a,b; Ablanque et al., 2010) including two additional features. On the one hand, it allows the simulation of the two typical geometric arrangements found in capillary-tube/suction-line heat exchangers (i.e. concentric and lateral). On the other hand, it has an enhanced capability to address the convergence difficulties found in distributed models at the near-saturation zone. This document presents the major numerical adaptations done to the model, a comprehensive validation of the two geometric configurations, the model performance when tackling the aforementioned numerical difficulties and finally, some numerical studies. [ABSTRACT FROM AUTHOR]

This paper presents a model of shell and tube evaporator with micro-fin tubes using R1234yf and R134a. The model developed for this evaporator uses the ε-NTU method to predict the evaporating pressure, the refrigerant outlet enthalpy and the outlet temperature of the secondary fluid. The model accuracy is evaluated using different two-phase flow boiling correlations for micro-fin tubes and comparing predicted and experimental data. The experimental tests were carried out for a wide range of operating conditions using R134a and R1234yf as working fluids. The predicted parameter with maximum deviations, between the predicted and experimental data, is the evaporating pressure. The correlation of Akhavan– Behabadi et al. was used to predict flow boiling heat transfer, with an error on cooling capacity prediction below 5%. Simulations, carried out with this validated model, show that the overall heat transfer coefficient of R1234yf has a maximum decrease of 10% compared with R134a. [ABSTRACT FROM AUTHOR]

The flow and heat transfer characteristics of binary refrigerant mixtures in a heated horizontal tube were investigated numerically. The pressure drop, temperature profile, and heat transfer coefficient for non-azeotropic and near-azeotropic mixtures of different bulk compositions were obtained. It is found that the non-linear physical properties of the mixtures strongly affect the pressure drop characteristics. Both the fluid properties and mass transfer resistance are responsible for the heat transfer characteristics. The mass transfer resistance has a more significant influence on the nucleate boiling than the convective evaporation for non-azeotropic mixtures, while the resistance can be neglected for near-azeotropic mixtures. [ABSTRACT FROM AUTHOR]

Abstract: A high-performance magnetic refrigeration device is considered as a potential technology for in-vehicle air conditioners in electric vehicles. The high power consumption of a conventional air conditioner in an electric vehicle has considerable impacts on cruising distance. For this purpose the demands on cooling power density, temperature difference between hot and cold side, transient properties and COP, will be high. In this paper the potential to reach these demands are explored for two technologies, firstly a conventional AMR device and secondly a novel magnetocaloric device based on control of the axial thermal conductivity. [Copyright &y& Elsevier]

Abstract: Objective. – The objective was to implement a predictive numerical model of periprosthetic healing supported by histomorphometric studies in canine implants. Material and methods. – The hypothesis was to couple cellular migration (osteoblasts) with growth factor diffusion in porous media to predict the periprosthetic tissue healing. The multiphasic macroscopic model involved solid and fluid fractions, cellular and growth factor concentrations. The axisymetric model of the implant permitted to predict the osteoblast migration (diffusion, convection, haptotaxis, chemiotaxis), the osteoid deposit, the interaction with growth factor diffusion and clinical conditions (drill hole). The numerical results have been compared with the in vivo histomorphometric data. Results. – The numerical model permitted to predict a generic healing pattern involving two main oscillations of the osseous solidity distribution. The analysis of solidity peak in terms of magnitude and location led to a classification into three main healings: homogenous with high mineralization, heterogeneous, homogeneous with low mineralization. Conclusion. – The numerical and histomorphometric correlations have been very satisfying in consideration of the complexity of mechanobiological phenomena. The model permitted to predict heterogeneous healing generally preceding the implant loosening. Now, the impact evaluation of clinical relevant parameters such as growth factor concentration and operative settings (drill hole) can be study to improve the long-term survival of the arthroplasty. [Copyright &y& Elsevier]