Thermal investigations of an experimental active magnetic regenerative refrigerator operating near room temperature

In this paper, numerical and experimental investigations on a magnetic refrigeration device based upon the active magnetic regeneration (AMR) cycle operating near room temperature are presented. A numerical 1D model based on the transient energy equations is proposed for modelling the heat exchange between the magnetocaloric material and the carrier fluid in the regenerator bed. The validity of 1D AMR-numerical model is investigated through the recently developed magnetic cooling demonstrator by Clean Cooling Systems SA (CCS) at the University of Applied Sciences of western Switzerland (HES−SO). The obtained results including the temperature span, the coefficient of performance and the cooling power are presented and discussed. In general, good agreements have been noted between the experimental and numerical results.     

A nonintrusive flow measurement technique to validate the simulated laminar fluid flow in a packed container with vented walls

A nonintrusive flow measurement technique (particle image velocimetry) was used to determine the airflow field in a package with a container-to-product diameter ratio of less than 10. The complexity and uneven distribution of the measured flow field supported the requirement of a geometrical and mathematical model capable of describing the geometry and physics of flow within the package. Using novel computational fluid dynamics (CFD) codes, an accurate model of the packed structure was developed and the 3D Navier–Stokes equations were solved. A good agreement was obtained between experimental and predicted velocities. The detailed insight on the airflow pattern provided by the CFD analysis makes this approach an ideal tool to analyze the effect of different vent designs in the airflow field distribution in complex packaging systems.     

The chemistry of stratospheric ozone: its response to natural and anthropogenic influences

The current rapid changes in the composition of the atmosphere reflect both the metabolism of the biosphere and the broad range of influencing human activities. However, the only known sources of CFCs are industrial. In predicting future changes in ozone depletion and global warming it is necessary to understand the role of the biosphere in regulating emissions as well as future industrial emissions. This paper discusses theoretical predictions and trends in global ozone concentrations from 1969–1986. Antarctic and Arctic ozone trends and the global implications of the Antarctic ozone hole are described in detail. It is concluded that even if the control measures of the Montreal Protocol are implemented by all nations, the atmospheric abundance of chlorine will at least double during the next few decades. Future global ozone depletion could be larger than originally predicted because the PSC-induced chemical reactions that cause ozone depletion were not included in the stratospheric ozone assessment models on which the Montreal Protocol was based. To return the Antarctic ozone layer to its natural state will require very strong measures, including a complere phase out of all fully halogenated CFCs, halons, carbon tetrachloride and methyl chloroform, as well as careful use of HCFC substitutes.     

The use of CFD to improve the performance of a chilled multi-deck retail display cabinet

Maintaining food temperatures below critical values is the key to maximising the high quality display life of chilled foods. Studies were carried out to see if computational fluid dynamic (CFD) modelling could be used to rapidly identify the changes that would be required to an existing multi-deck display cabinet so that it would meet a higher test specification. Implementing the changes on a Pastorfrigor MV 200TP display cabinet reduced the average power consumption from 1.37 to 1.29 kW as well as significantly reducing the number of test packs which spent any time above 4 °C, from 12 to 1.     

Charles Marville and the Landscapes of the Carrières d’Amérique

Charles Marville’s photographs of Paris preserve the look of streets slated for demolition under Georges-Eugène Haussmann. This article examines his photographs of the Carrières d’Amérique, or America Quarries – gypsum quarries in the outlying Parisian neighbourhood of Belleville. At a time when the rezoning of districts that were formerly extra muros was still controversial, Belleville was seen as a crime-ridden area. Marville’s photographs become documents that refute contemporary narratives of criminality. Borrowing Walter Benjamin’s view that the city ‘opens up’ to the flâneur ‘as a landscape’, the article analyses Marville’s landscapes of the Carrières d’Amérique as images that juxtapose the city, the work site, and the no-man’s land or terrains vagues at Paris’s outer limits.     

Comparison of CFD analysis to empirical data in a commercial vortex tube

This paper presents a comparison between the performance predicted by a computational fluid dynamic (CFD) model and experimental measurements taken using a commercially available vortex tube. Specifically, the measured exit temperatures into and out of the vortex tube are compared with the CFD model. The data and the model are both verified using global mass and energy balances. The CFD model is a two-dimensional (2D) steady axisymmetric model (with swirl) that utilizes both the standard and renormalization group (RNG) k-epsilon turbulence models. While CFD has been used previously to understand the fluid behavior internal to the vortex tube, it has not been applied as a predictive model of the vortex tube in order to develop a design tool that can be used with confidence over a range of operating conditions and geometries. The objective of this paper is the demonstration of the successful use of CFD in this regard, thereby providing a powerful tool that can be used to optimize vortex tube design as well as assess its utility in the context of new applications.     

Grid deformation strategies for CFD analysis of screw compressors

Customized grid generation of twin screw machines for CFD analysis is widely used by the refrigeration and air-conditioning industry today, but is currently not suitable for topologies such as those of single screw, variable pitch or tri screw rotors. This paper investigates a technique called key-frame re-meshing that supplies pre-generated unstructured grids to the CFD solver at different time steps. To evaluate its accuracy, the results of an isentropic compression-expansion process in a reciprocating piston cylinder arrangement have been compared. Three strategies of grid deformation; diffusion equation mesh smoothing, user defined nodal displacement and key-frame remeshing have been assessed. There are many limitations to key-frame re-meshing. It requires time consuming pre-processing, has limited applicability to complex meshes and leads to inaccuracies in conservation of calculated variables. It was concluded that customized tools for generation of CFD grids are required for complex screw machines.     

Numerical assessment of steam ejector efficiencies using CFD

Ejector efficiencies for the primary nozzle, suction, mixing and diffuser were determined for the first time, according to their definitions, using an axi-symmetric CFD model. Water was considered as working fluid and the operating conditions were selected in a range that would be suitable for an air-conditioner powered by solar thermal energy. Ejector performance was estimated for different nozzle throat to constant section area ratios. The results indicated the existence of an optimal ratio, depending on operating conditions. Ejector efficiencies were calculated for different operating conditions. It was found that while nozzle efficiency can be considered as constant, the efficiencies related to the suction, mixing and diffuser sections of the ejector depend on operating conditions.     

Experimental and numerical study of the compact heat exchanger with different microchannel shapes

Experimental and numerical analysis of heat transfer and fluid flow in the compact heat exchanger has been done in this paper. In an open circuit wind tunnel, developed on purpose for this investigation, the measurement of working media temperatures and mass flow rates for heat exchanger with microchannel coil has been accomplished. In accordance with the heat exchangers used for experiments, numerical 3D simulation of adequate geometry shapes has been done. With utilization of air/water side numerical simulation, more detailed results have been achieved in relation to the simulation that assumes constant temperature or constant heat flux on the pipe wall. Good agreement between experimentally measured and numerically calculated results has been accomplished. The influence of different microchannel shapes on heat transfer effectiveness and pressure drop has been studied numerically. Comparison of results has been made accompanied by the discussion and final conclusions.     

Optimal design of a light commercial freezer through the analysis of the combined effects of capillary tube diameter and refrigerant charge on the performance

In this work, a discussion on a methodology to optimize the performance of a commercial freezer by using a simulation tool is presented. In order to provide a practical tool for deciding the best combination of refrigerant charge and capillary tube diameter, the results of the numerical studies are shown in the form of two-dimensional maps. The usefulness of this type of representation lies in the possibility of selecting the best operating point of the system, taking into account not only the efficiency or the power consumption but also the technical constrictions imposed by parameters such as the suction line temperature, the condenser subcooling, the evaporator superheat, and the run-time ratio. The discussion leads to the conclusion that the useful performance map is drastically reduced when all the operation requirements must be satisfied. Once the system design had been optimized, an additional numerical study, aimed at identifying the influence of the external conditions on the system behavior, was performed. The results show that the performance reduction can be effectively minimized modifying the refrigerant charge.