Boilers slagging when burning wood pellets

A promising trend for upgrading wastes from timber cutting, processing and treatment is their granulation. It allows to increase their specific heats of combustion by 2.5– 3.5 times and their portability characteristics by 3–4 times, to reduce transportation costs by 6– 10 times and to improve all the operations stages. The construction and commissioning of boiler facilities operating on refined biofuel made it possible to form a stable domestic market for wood pellets. However, 0.5 – 1.5 MW nominal capacity hot water boilers equipped with furnaces and profiled burners at the bottom, in cold seasons had fast accumulation of focal residues deposits in the burners and on the furnace chambers lining. The process was complicated by these deposits hardening due to their melting and sintering. These circumstances cause a decrease in the energy and environmental performance of heat-generating installations and their reliability, and also leads to the unplanned shutdowns to clean the boiler furnaces. To find out the reasons for these negative phenomena and to develop recommendations for their elimination, a set of research operations was carried out with wood pellets shipped by the manufacturer and supplied to the burners of the boilers under the analyses; with focal residues accumulated in the burners and on the lining of the furnace chambers; as well as an analysis of the heat generating facilities operation modes. The studies carried out made it possible to identify the main factors that caused these negative phenomena and to develop the recommendations for their elimination.

A TOOL FOR THE ASSESSMENT OF THERMAL EFFICIENCY OF IDEAL DIESEL CYCLES, PROVIDED TO FUTURE MARINE ENGINEERS

Diesel engines are a type of internal combustion engine widely spread in the maritime sector. In a world depending on fossil fuels, the challenge faced by scientists and professionals consists in the efficient use of this kind of fuels. The future of these engines strongly depends on the efficiency enhancement. Future mechanical engineers have to be trained in order to gain engineering judgement and enterprising attitude. In this respect, this paper deals with a theoretical study focused on the improvement of the efficiency of the ideal Diesel cycle. This type of study was recently introduced in Constanta Maritime University, within Thermodynamics seminar activities, delivered to future marine engineers, enrolled in their third semester. This study will reveal the fact that the efficiency of Diesel cycles is affected by the variation of compression ratio, cut off ratio and specific heats ratio. The results obtained will show that, for the ranges taken into discussion, thermal efficiency might be enhanced by the rise of compression ratio and specific heat ratio values and by lowering cut off ratio values. The study is a predecessor of expensive and time costing experiences and it is also a guidance in the hand of future professionals – able to train them for real solutions.

Rotating spacetimes generalizing Lifshitz black holes

We present a spinning black hole solution in d dimensions with a maximal number of rotation parameters in the context of the Einstein–Maxwell-Dilaton theory. An interesting feature of such a solution is that it accommodates Lifshitz black holes when the rotation parameters are set to zero. We verify the rotating nature of the black hole solution by performing the quasi-local analysis of conserved charges and defining the corresponding angular momenta. In addition, we perform the thermodynamical analysis of the black hole configuration, show that the first law of thermodynamics is completely consistent, and obtain a Smarr-like formula. We further study the thermodynamic stability of the constructed solution from a local viewpoint, by computing the associated specific heats, and from a global perspective, by using the so-called new thermodynamic geometry. We finally make some comments related to a pathology found in the causal structure of the obtained rotating black hole spacetime and compute some of its curvature invariants.

Characterization of Fatty Acids as Biobased Organic Materials for Latent Heat Storage

This work aims to characterize phase change materials (PCM) for thermal energy storage in buildings (thermal comfort). Fatty acids, biobased organic PCM, are attractive candidates for integration into active or passive storage systems for targeted application. Three pure fatty acids (capric, myristic and palmitic acids) and two eutectic mixtures (capric-myristic and capric-palmitic acids) are studied in this paper. Although the main storage properties of pure fatty acids have already been investigated and reported in the literature, the information available on the eutectic mixtures is very limited (only melting temperature and enthalpy). This paper presents a complete experimental characterization of these pure and mixed fatty acids, including measurements of their main thermophysical properties (melting temperature and enthalpy, specific heats and densities in solid and liquid states, thermal conductivity, thermal diffusivity as well as viscosity) and the properties of interest regarding the system integrating the PCM (energy density, volume expansion). The storage performances of the studied mixtures are also compared to those of most commonly used PCM (salt hydrates and paraffins).

The role of nano-sized intergranular phases on nickel alloy 725 brittle failure

Although precipitation-hardened nickel alloys, such as Alloy 725, are extensively used in the most aggressive oil and gas production conditions given their a priori superior environmentally assisted cracking resistance, recent failures associated with hydrogen embrittlement (HE) cast doubts about their performance in H-bearing environments. These high-profile incidents have led to extensive research to elucidate the failure mechanism and develop methods to reduce risks. However, the microstructural features that made only specific heats of Alloy 725 susceptible to HE have yet to be agreed upon by the scientific community. Herein, our results showed that the abundant precipitation of F phase, i.e., a recently discovered nano-sized and embrittling phase, at grain boundaries increased cracking susceptibility. A continuous F phase network offered a low-energy path for crack opening by matrix-precipitate interfacial decohesion. Additionally, we correlated Alloy 725 microstructure with HE resistance, which is essential for thermomechanical processing optimization and the design of alternative qualification tests to obtain embrittlement-resistant nickel alloys.

Logarithmic Generalization of the Lambert W Function and Its Applications to Adiabatic Thermostatistics of the Three-Parameter Entropy

A generalization of the Lambert W function called the logarithmic Lambert function is introduced and is found to be a solution to the thermostatistics of the three-parameter entropy of classical ideal gas in adiabatic ensembles. The derivative, integral, Taylor series, approximation formula, and branches of the function are obtained. The heat functions and specific heats are computed using the “unphysical” temperature and expressed in terms of the logarithmic Lambert function.

Removal of Hydrogen Sulfide (H2S) Using MOFs: A Review of the Latest Developments

The removal of hydrogen sulfide (H2S) from gas streams with varying overall pressure and H2S concentrations is a long-standing challenge faced by the oil and gas industries. The present work focuses on H2S capture using metal–organic frameworks (MOFs), in an effort to shed light on their potential as adsorbents in the field of gas storage and separation. MOFs hold great promise as they make possible the design of structures from organic and inorganic units, but also, they have provided an answer to a long-time challenging issue, i.e., how to design extended structures of materials. Moreover, the functionalization of the MOF’s surface can result in increased H2S uptake. For example, the insertion of 1% of a fluorinated linker in MIL-101(Cr)-4F(1%) allows for enhanced H2S capture. Although noticeable efforts have been made in studying the adsorption capacity of H2S using MOFs, there is a clear need for gaining a deeper understanding in terms of their thermal conductivities and specific heats in order to design more stable adsorption beds, experiencing high exothermicity. Simply put, the exothermic nature of adsorption means that sharp rises in temperature can negatively affect the bed stability in the absence of sufficient heat transfer. The work presented herein provides a detailed discussion by thoroughly combining the existing literature on new developments in MOFs for H2S removal, and tries to provide insight into new areas for further research.

Subtle metastability of the layered magnetic topological insulator MnBi2Te4 from weak interactions

Layered quantum materials can host interesting properties, including magnetic and topological, for which enormous computational predictions have been done. Their thermodynamic stability is much less visited computationally, which however determines the existence of materials and can be used to guide experimental synthesis. MnBi2Te4 is one of such layered quantum materials that was predicted to be an intrinsic antiferromagnetic topological insulator, and later experimentally realized but in a thermodynamically metastable state. Here, using a combined first-principles-based approach that considers lattice, charge, and spin degrees of freedom, we investigate the metastability of MnBi2Te4 by calculating the Helmholtz free energy for the reaction Bi2Te3 + MnTe → MnBi2Te4. We identify a temperature range (~500–873 K) in which the compound is stable with respect to the competing binary phases, consistent with experimental observation. We validate the predictions by comparing the calculated specific heats contributed from different degrees of freedom with experimental results. Our findings indicate that the degrees of freedom responsible for the van der Waals interaction, lattice vibration, magnetic coupling, and nontrivial band topology in MnBi2Te4 not only enable emergent phenomena but also play a crucial role in determining its thermodynamic stability. This conclusion lays the foundation for the future computational material synthesis of novel layered systems.

Numerical Study of Characteristics of Underexpanded Supersonic Jet

Correlations for the supersonic jet characteristics, the mean shock cell length and the supersonic core length, have been obtained in terms of the jet parameters. The jet parameters considered in this study are the exit diameter of the nozzle (de), the design Mach number (Me), the nozzle pressure ratio (NPR) and the ratio of specific heats of the medium (γ). The parameters were varied as follows: exit diameters, from 0.5 to 25 mm; Mach number from 1 to 3; the NPR from 2.14 to 35. Initially, working fluid used is cold air and then effect of variation of γ is taken into consideration. The computational model has been validated and then used for all the numerical simulations. A quadratic fit for both characteristics has been obtained which is applicable to any supersonic jet. The correlations developed are valid within the respective ranges of the parameters stated above.