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• The conjugate heat transfer in flow boiling simulation plays an important role and cannot be ignored. • Various bottom wall materials and thicknesses are analyzed during the bubble growth in microchannel. • There exists an optimum thickness for each solid material in the phase-change application. The conjugate heat transfer of bubble growth during flow boiling in microchannel has a significant effect on the flow field and heat transfer performance but few studies analyzed it before. In this study, the volume of fluid (VOF) method, Hardt's phase-change model, conjugate heat transfer between solid and fluid domains are adopted within an OpenFOAM solver to investigate the bubble growth and heat transfer performance in a microchannel with changed wall thickness from 5 μm to 160 μm and materials including silicon, aluminum, and copper. The results reveal that even if uniform heat flux is applied to the bottom wall, heat flux is not uniform at the solid-fluid interface due to the phase-change process in the channel. Conjugate heat transfer between the fluid and solid domain plays an important role in transferring the uniform heat flux from the bottom wall to the solid-fluid interface and homogenizing the solid-region temperature distribution, which cannot be ignored in the simulation of the phase-change phenomenon. When using different wall thicknesses, the bubble growth period differs by over two times. An optimum thickness exists for each material because the increasing wall thickness leads to a faster bubble growth rate but higher thermal resistance. With the same bottom wall thickness, the solid material with higher thermal diffusivity owns a faster bubble growth rate, thus a higher heat transfer coefficient. The optimum thickness decreases with increasing thermal diffusivity of the solid-domain material. [ABSTRACT FROM AUTHOR]

• Thermal performance of a plate heat exchanger under a magnetic field is studied. • Heat transfer performance is significantly improved by applying a magnetic field. • Pressure drop grows negatively for a vertical magnetic field. • Average Nusselt number under magnetic field arrangements deviates below 3.4%. • Pressure drop is reduced 10% with a side-by-side vertical magnet arrangement. This paper experimentally investigates the effect of various magnetic fields on the performance of a plate heat exchanger filled with ferrofluids. Spherical nanoparticles Fe 3 O 4 with an average diameter of 20 nm are dispersed into DI-water to synthesize the ferrofluid. Thermal performance and flow characteristics of the ferrofluid with 0.1% particle concentration are investigated based on various arrangements of magnets outside the plate heat exchanger. Effects of magnetic field strength and distribution are thoroughly studied concerning the performance of the heat exchanger with various ferrofluid flow rates. Results indicate that with a vertical arrangement of two magnets side by side outside the sidewalls, 21.8% increase in average Nusselt number and 10.0% reduction in average pressure drop are achieved compared to the cases without a magnetic field. Novel configurations of magnets are first discussed in a plate heat exchanger. Ferrofluid flow control is achieved under a measurable magnetic field strength and different flow rates. It is well known that enhancement of thermal performance in the plate heat exchanger is accompanied with a reduction of resistance loss. Deposition of magnetic particles and blockage in the channel of the plate heat exchanger will also be weakened based on results from this research. [ABSTRACT FROM AUTHOR]

Electrolyte-supported solid oxide cells are often used for steam electrolysis. Advantages are high mechanical stability and a low degradation rate. The aim of this proof of concept study was to use a femtosecond laser to process the electrolyte of an electrolyte-supported solid oxide cell and evaluate the effect of this laser treatment on the electrochemical performance. The femtosecond laser treatment induces a macroscopic and a superimposed microscopic structure. It can be proven that the electrolyte remains gas tight and the electrochemical performance increases independently of the laser parameters. The initial area-specific resistance degradation during a constant current measurement of 200 h was reduced from 7.9% for a non-treated reference cell to 3.2% for one of the laser-treated cells. Based on electrochemical impedance measurements, it was found that the high frequency resistance of the laser-treated cells was reduced by up to 20% with respect to the reference cell. The impedance spectra were evaluated by calculating the distribution of relaxation times, and in advance, a novel approach was used to approximate the gas concentration resistance, which was related to the test setup and not to the cell. It was found that the low frequency polarization resistance was increased for the laser-treated cells. In total, the area-specific resistance of the laser-treated cells was reduced by up to 14%. [ABSTRACT FROM AUTHOR]

Different findings about suitable correlations to describe nanofluid viscosity can be explained based on the research presented in this paper. The effective viscosity of nanofluids is crucial when nanofluids are considered as heat carrier fluids. Despite many publications, however, no consensus about suitable correlations could be found in past years. Particularly, the impact of the shear rate on the viscosity is being discussed controversially. It is shown in this paper that these different findings can be explained considering the theory for the rheology of suspensions. Any measurement of viscosity over shear rate only shows a section of the entire rheological behavior. Thus, experimental results of shear thinning, Newtonian behavior and shear thickening of nanofluids can all be a part of this overall range of possible shear rates. This hypothesis is validated based on viscosity data from the literature and viscosity measurements over a wide range of shear rates for different nanofluids showing all three types of behavior. [ABSTRACT FROM AUTHOR]

The calculation of the entropy production rate within an operational high temperature solid oxide fuel cell (SOFC) is necessary to design and improve heating and cooling strategies. However, due to a lack of information, most of the studies are limited to empirical relations, which are not in line with the more general approach given by non-equilibrium thermodynamics (NET). The SOFC 1D-model presented in this study is based on non-equilibrium thermodynamics and we parameterize it with experimental data and data from molecular dynamics (MD). The validation of the model shows that it can effectively describe the behavior of a SOFC at 1300   K . Moreover, we show that the highest entropy production is present in the electrolyte and the catalyst layers, and that the Peltier heat transfer is considerable for the calculation of the heat flux in the electrolyte and cannot be neglected. To our knowledge, this is the first validated model of a SOFC based on non-equilibrium thermodynamics and this study can be extended to analyze SOFCs with other solid oxide electrolytes, with perovskites electrolytes or even other electrochemical systems like solid oxide electrolysis cells (SOECs). [ABSTRACT FROM AUTHOR]

Reaction kinetic behaviour and cycling stability of the 2LiBH4–MgH2 reactive hydride composite (Li-RHC) are experimentally determined and analysed as a basis for the design and development of hydrogen storage tanks. In addition to the determination and discussion about the properties; different measurement methods are applied and compared. The activation energies for both hydrogenation and dehydrogenation are determined by the Kissinger method and via the fitting of solid-state reaction kinetic models to isothermal volumetric measurements. Furthermore, the hydrogen absorption–desorption cycling stability is assessed by titration measurements. Finally, the kinetic behaviour and the reversible hydrogen storage capacity of the Li-RHC are discussed. [ABSTRACT FROM AUTHOR]

Thermodynamic and heat transfer properties of the 2LiBH4-MgH2 composite (Li-RHC) system are experimentally determined and studied as a basis for the design and development of hydrogen storage tanks. Besides the determination and discussion of the properties, different measurement methods are applied and compared to each other. Regarding thermodynamics, reaction enthalpy and entropy are determined by pressure-concentration-isotherms and coupled manometric-calorimetric measurements. For thermal diffusivity calculation, the specific heat capacity is measured by high-pressure differential scanning calorimetry and the effective thermal conductivity is determined by the transient plane source technique and in situ thermocell. Based on the results obtained from the thermodynamics and the assessment of the heat transfer properties, the reaction mechanism of the Li-RHC and the issues related to the scale-up for larger hydrogen storage systems are discussed in detail. [ABSTRACT FROM AUTHOR]