Heat losses in a smouldering system: The key role of non-uniform air flux

Smouldering combustion is emerging as a valuable tool for energy conversion purposes. However, the effects of radial/lateral heat losses, while critical to its viability, are not well understood. It is known that heat losses weaken the smouldering reaction near the walls. It is less known that these losses generate non-uniform air flux across the system cross-section, potentially changing conversion rates and quenching limits. This study integrated: (i) highly instrumented smouldering experiments across numerous scales, (ii) a novel method of estimating non-uniform air flux in the experiments, (iii) analytical modelling to predict non-uniform cooling, and (iv) energy balance calculations to quantify the non-uniform heat of smouldering. Altogether, this work demonstrates that heat loss-induced non-uniform air flux is significant, affecting key smouldering propagation and cooling characteristics. The uniform air flux injected at the base became redistributed with a ~50% decrease at the centreline and a ~50% increase at the wall. This was shown to cause a concave (in the direction of air flow) smouldering front and a concave cooling front. The former was shown to cause radial heat transfer inwards, leading to super-adiabatic heating towards the centre of the reactor. The latter was shown to inhibit cooling along the centreline, which progressed ~40% slower than expected during propagation. Altogether, the multiple and integrated analyses used reveal the magnitude and significance of heat losses in smouldering systems. This insight is valuable to better harness smouldering for engineering applications

Heat losses in applied smouldering systems: Sensitivity analysis via analytical modelling

As applied smouldering systems gain popularity for a variety of energy conversion purposes, there is a strong interest in optimizing the reactor design to support robust smouldering. Heat losses play a critical role in the energy balance of smouldering systems, and therefore have strong implications toward understanding propagation limits and reactor design. Heat losses in an applied smouldering system were approximated by adapting the analytical model from Kuznetsov (1996), originally developed for unsteady local thermal non-equilibrium heat transfer in a porous cylinder, to simulate the cooling zone trailing the smouldering front. The analytical model was adapted to a smouldering system by solving on a domain that lengthens as the cooling zone expands at the rate of the smouldering velocity. The results are incorporated into a global energy balance on the smouldering system, thereby providing an inexpensive and rapid method to estimate the system energy efficiency. Confidence in the analytical model was provided by demonstrating its predictions compare well with existing experimental and numerical estimates of heat losses from similar smouldering systems. The model was then used to quantify the sensitivity of the heat losses to two key reactor design parameters: radius and insulation quality. The system energy efficiency was shown to be highly sensitive to improved insulation and increased radius up to ~0.1 m (i.e., laboratory-sized reactors). However, this sensitivity diminished with size. Beyond 0.4 m radius, the predicted system energy efficiency was high (~85-95%) and relatively insensitive to reactor radius and insulation quality. Therefore, commercial, batch treatment smouldering reactors do not need to be larger than 0.4 m in radius to protect against heat losses and maximize their energy efficiency. This threshold design radius is considerably less than used in current reactors and therefore can provide valuable cost savings

Investigation of Co2_2FeSi: The Heusler compound with Highest Curie Temperature and Magnetic Moment

This work reports on structural and magnetic investigations of the Heusler compound Co$_2$FeSi. X-Ray diffraction and M\"o\ss bauer spectrometry indicate an ordered $L2_1$ structure. Magnetic measurements by means of X-ray magnetic circular dichroism and magnetometry revealed that this compound is, currently, the material with the highest magnetic moment ($6 \mu_B$) and Curie-temperature (1100K) in the classes of Heusler compounds as well as half-metallic ferromagnets

Half-metallic ferromagnetism with high magnetic moment and high Curie temperature in Co2_2FeSi

Co$_2$FeSi crystallizes in the ordered L2$_1$ structure as proved by X-ray diffraction and M\"o\ss bauer spectroscopy. The magnetic moment of Co$_2$FeSi was measured to be about $6\mu_B$ at 5K. Magnetic circular dichroism spectra excited by soft X-rays (XMCD) were taken to determine the element specific magnetic moments of Co and Fe. The Curie temperature was measured with different methods to be ($1100\pm20$)K. Co$_2$FeSi was found to be the Heusler compound as well as the half-metallic ferromagnet with the highest magnetic moment and Curie temperature.Comment: conference contribution, MMM200

Elucidating the characteristic energy balance evolution in applied smouldering systems

Applied smouldering systems are emerging to solve a range of environmental challenges, such as remediation, sludge treatment, off-grid sanitation, and resource recovery. In many cases, these systems use smouldering to drive an efficient waste-to-energy process. While engineers and researchers are making strides in developing these systems, the characteristic energy balance trends have not yet been well-defined. This study addresses this topic and presents a detailed framework to uncover the characteristic energy balance evolution in applied smouldering systems. This work provides new experimental results; a new, validated analytical description of the cooling zone temperature profile at steady-state conditions; insight into the characteristic temperature changes over time; a re-analysis of published data; and a robust framework to contextualize the global energy balance results from applied smouldering systems. Altogether, this study is aimed to support researchers and engineers to better understand smouldering system performance to further the development of environmentally beneficial applications

Scaling up self-sustained smouldering of sewage sludge for waste-to-energy

Self-sustained smouldering combustion presents strong potential as a green waste-to-energy technique for a range of wastes, especially those with high moisture content like wastewater sewage sludge. While well-demonstrated in laboratory experiments, there is little known about scaling up this process to larger, commercial reactors. This paper addresses this knowledge gap by systematically conducting and analyzing experiments in a variety of reactors extending beyond the laboratory scale. This work reveals a robust treatment regime; however, it also identifies potential complications associated with perimeter heat losses at scale. Two key impacts, on the smouldering reactions and the air flow patterns, are shown to potentially degrade treatment if not properly understood and managed. Altogether, this study provides novel insight and guidance for scaling up smouldering science into practical, waste-to-energy systems

Benchmarking the performance of portfolio optimization with QAOA

We present a detailed study of portfolio optimization using different versions of the quantum approximate optimization algorithm (QAOA). For a given list of assets, the portfolio optimization problem is formulated as quadratic binary optimization constrained on the number of assets contained in the portfolio. QAOA has been suggested as a possible candidate for solving this problem (and similar combinatorial optimization problems) more efficiently than classical computers in the case of a sufficiently large number of assets. However, the practical implementation of this algorithm requires a careful consideration of several technical issues, not all of which are discussed in the present literature. The present article intends to fill this gap and thereby provide the reader with a useful guide for applying QAOA to the portfolio optimization problem (and similar problems). In particular, we will discuss several possible choices of the variational form and of different classical algorithms for finding the corresponding optimized parameters. Viewing at the application of QAOA on error-prone NISQ hardware, we also analyze the influence of statistical sampling errors (due to a finite number of shots) and gate and readout errors (due to imperfect quantum hardware). Finally, we define a criterion for distinguishing between "easy" and "hard" instances of the portfolio optimization problemComment: 28 pages, 8 figure

Geometric, electronic, and magnetic structure of Co2_2FeSi: Curie temperature and magnetic moment measurements and calculations

In this work a simple concept was used for a systematic search for new materials with high spin polarization. It is based on two semi-empirical models. Firstly, the Slater-Pauling rule was used for estimation of the magnetic moment. This model is well supported by electronic structure calculations. The second model was found particularly for Co$_2$ based Heusler compounds when comparing their magnetic properties. It turned out that these compounds exhibit seemingly a linear dependence of the Curie temperature as function of the magnetic moment. Stimulated by these models, Co$_2$FeSi was revisited. The compound was investigated in detail concerning its geometrical and magnetic structure by means of X-ray diffraction, X-ray absorption and M\"o\ss bauer spectroscopies as well as high and low temperature magnetometry. The measurements revealed that it is, currently, the material with the highest magnetic moment ($6\mu_B$) and Curie-temperature (1100K) in the classes of Heusler compounds as well as half-metallic ferromagnets. The experimental findings are supported by detailed electronic structure calculations

Design of magnetic materials: Co2_2Cr1−x_{1-x}Fex_{x}Al

Doped Heusler compounds Co$_2$Cr$_{1-x}$Fe$_{x}$Al with varying Cr to Fe ratio $x$ were investigated experimentally and theoretically. The electronic structure of the ordered, doped Heusler compound Co$_2$Cr$_{1-x}$Fe$_{x}$Al ($x=n/4, n=0,1,2,3,4)$ was calculated using different types of band structure calculations. The ordered compounds turned out to be ferromagnetic with small Al magnetic moment being aligned anti-parallel to the 3d transition metal moments. All compounds show a gap around the Fermi-energy in the minority bands. The pure compounds exhibit an indirect minority gap, whereas the ordered, doped compounds exhibit a direct gap. Magnetic circular dichroism (MCD) in X-ray absorption spectra was measured at the $L_{2,3}$ edges of Co, Fe, and Cr of the pure compounds and the $x=0.4$ alloy in order to determine element specific magnetic moments. Calculations and measurements show an increase of the magnetic moments with increasing iron content. The experimentally observed reduction of the magnetic moment of Cr can be explained by Co-Cr site-disorder. The presence of the gap in the minority bands of Co$_2$CrAl can be attributed to the occurrence of pure Co$_2$ and mixed CrAl (001)-planes in the $L2_1$ structure. It is retained in structures with different order of the CrAl planes but vanishes in the $X$-structure with alternating CoCr and CoAl planes.Comment: corrected author lis

A Single-Photon Server with Just One Atom

Neutral atoms are ideal objects for the deterministic processing of quantum information. Entanglement operations have been performed by photon exchange or controlled collisions. Atom-photon interfaces were realized with single atoms in free space or strongly coupled to an optical cavity. A long standing challenge with neutral atoms, however, is to overcome the limited observation time. Without exception, quantum effects appeared only after ensemble averaging. Here we report on a single-photon source with one-and-only-one atom quasi permanently coupled to a high-finesse cavity. Quasi permanent refers to our ability to keep the atom long enough to, first, quantify the photon-emission statistics and, second, guarantee the subsequent performance as a single-photon server delivering up to 300,000 photons for up to 30 seconds. This is achieved by a unique combination of single-photon generation and atom cooling. Our scheme brings truly deterministic protocols of quantum information science with light and matter within reach.Comment: 4 pages, 3 figure