Energy Balance Properties Research Articles

Burning and flame extinction patterns of PMMA under external heating and oxygen deficiency

The paper presents an experimental study investigating the impact of under-oxygenated conditions on the burning characteristics and flame extinction of horizontal transparent poly(methyl-methacrylate) (PMMA) slabs subjected to external heating. Small-sized solids are exposed to various constant levels of radiant heat flux in a nitrogen-diluted environment at ambient pressure. The study aims to understand the multivariable dependence of PMMA combustion on oxygen and heating levels. The analysis goes beyond the study of global and local variables to provide an understanding of the mechanisms involved, revealing a notable effect on burning characteristics and flame extinction mechanisms by blow-off. The results provide a detailed assessment of the surface energy balance and flame bulk properties. The results demonstrate good repeatability up to a certain oxygen limit, beyond which the flame combustion regime exhibits stochastic behaviour. The study of heat transfer mechanisms has revealed that flame radiation and external heating play a predominant role in the heating of horizontal solid fuels, In contrast, within the flame, convection is the main contributor to solid heating, rather than flame radiation. Solid characteristics show linear trends proportional to oxygen depletion, while gas phase characteristics show monotonic trends strongly influenced by the changing combustion regime. The surface temperature of the solid is highly sensitive to the test conditions that affect the energy balance. Dimensionless parameters are developed and data are correlated with existing literature. The analysis demonstrates that the linear or monotonic trends are maintained regardless of external heating. Finally, models are proposed and validated to predict both the extinction limit and the burning rate independently of irradiance and oxygen levels.

Experimental analysis of passivity‐based control theory for permanent magnet synchronous motor drive fed by grid power

AbstractControlling the Permanent Magnet Synchronous Motor (PMSM) can be challenging due to the nonlinearity of its dynamics, which makes it difficult to design control strategies that are both robust and effective. To address this challenge, this paper presents a novel control strategy rooted in the concept of passivity that combines field‐oriented control (FOC). This strategy compels the PMSM to accurately follow velocity and electrical torque trajectories. The approach, known as passivity‐based control (PBC), entails reshaping the inherent system energy while introducing the necessary damping to attain the desired objectives. A crucial aspect involves identifying workless force terms within the process model. Despite their presence, these terms do not impact the energy balance and stability properties. As a result, eliminating these terms is unnecessary. This simplicity in control architecture not only preserves system stability but also bolsters overall robustness. The system's overall stability and the current tracking error's exponential convergence have both been demonstrated analytically. In order to maintain stability, the controller accounts for the nonlinearities of the plant and approximates the unstructured dynamics of the PMSM. The proposed control is designed using thedqmodel of the PMSM, which avoids the model's structure destruction due to singularities, since thedqmodel does not depend explicitly on the rotor angular position. Experimental results shown further, illustrate speed and position control with a desired pair calculated by a filter or a proportional‐integral (PI) controller for speed control and a proportional‐integral‐derivative (PID) controller for position control. Also the correlation between practical and theoretical results is given as well as the robustness test in relation to the uncertainties of the PMSM's inertia moment. The results demonstrates the effectiveness of the proposed strategy in controlling the PMSM under different operating conditions, highlighting its potential for industrial applications.

Thermodynamics Analysis of a Novel Compressed Air Energy Storage System Combined with Solid Oxide Fuel Cell–Micro Gas Turbine and Using Low-Grade Waste Heat as Heat Source

As the next generation of advanced adiabatic compressed air energy storage systems is being developed, designing a novel integrated system is essential for its successful adaptation in the various grid load demands. This study proposes a novel design framework for a hybrid energy system comprising a CAES system, gas turbine, and high-temperature solid oxide fuel cells, aiming for power generation and energy storage solutions. The overall model of the hybrid power generation system was constructed in Aspen PlusTM, and the mass balance, energy balance, and thermodynamic properties of the thermal system were simulated and analyzed. The results demonstrate that the hybrid system utilizes the functional complementarity of CAES and solid oxide fuel cells (SOFCs), resulting in the cascade utilization of energy, a flexible operation mode, and increased efficiency. The overall round-trip efficiency of the system is 63%, and the overall exergy efficiency is 67%, with a design net power output of 12.5 MW. Additionally, thermodynamic analysis shows that it is advisable to operate the system under lower ambient temperatures of 25 °C, higher compressor and turbine isentropic efficiencies of 0.9, a higher fuel utilization of 0.62, and optimal SOFC/MGT split air flow rates of 1.1 kg/s. The results of this article provide guidance for designing innovative hybrid systems and system optimization.

An Interannual Drought Feedback Loop Affects the Surface Energy Balance and Cloud Properties

AbstractLong‐term groundwater droughts are known to persist over timescales from multiple years up to decades. The mechanisms leading to drought persistence are, however, only partly understood. Applying a unique terrestrial system modeling platform in a probabilistic simulation framework over Europe, we discovered an important positive feedback mechanism from groundwater into the atmosphere that may increase drought persistence at interannual time scales over large continental regions. In the feedback loop, groundwater drought systematically increases net solar radiation via a cloud feedback, which, in turn, increases the drying of the land. In commonly applied climate and Earth system models, this feedback cannot be simulated due to a lack of groundwater memory effects in the representation of terrestrial hydrology. Thus, drought persistence and compound events may be underestimated in current climate projections.

Surface energy balance of snow cover in the Japanese Alps is similar to that in continental Alpine climates

The dynamics of a seasonal snow cover in the temperate cryosphere are critical for discussing climate change and understanding Earth systems. The most basic information is the previously unknown surface energy balance of snow and ice that can describe the snow dynamics in Japanese Alps. We show the surface energy balance properties of seasonal snow cover in the Northern Japanese Alps: one is the net radiation controlling the surface energy balance variation, and the negative latent heat flux (sublimation). We found that the surface energy balance property in this region is similar to that in the continental climate region due to the specific climate of Japan (winter monsoon) and topographic conditions (steep elevation gradient) of the Japanese Alps. This is a novel finding because Japanese seasonal snow cover is thought to accumulate and ablate under a maritime climate. It has been reported that the sensitivity of snow ablation to global warming depends on current atmospheric conditions. The results offer vital context for discussing environmental changes in the temperate cryosphere and environment of the Japanese Alpine region.

Molecular Specificity and Proton Transfer Mechanisms in Aerosol Prenucleation Clusters Relevant to New Particle Formation.

Atmospheric aerosol particles influence the Earth's radiative energy balance and cloud properties, thus impacting the air quality, human health, and Earth's climate change. Because of the important scientific and overarching practical implications of aerosols, the past two decades have seen extensive research efforts, with emphasis on the chemical compositions and underlying mechanisms of aerosol formation. It has been recognized that new particle formation (NPF) contributes up to 50% of atmospheric aerosols. Nowadays, the general consensus is that NPF proceeds via two distinct stages: the nucleation from gaseous precursors to form critical nuclei of sub-1-2 nm size, and the subsequent growth into large particles. However, a fundamental understanding of both the NPF process and molecular-level characterization of the critical size aerosol clusters is still largely missing, hampering the efforts in developing reliable and predictive aerosol nucleation and climate models.Both field measurements and laboratory experiments have gathered convincing evidence about the importance of volatile organic compounds (VOCs) in enhancing the nucleation and growth of aerosol particles. Numerous and abundant small clusters composed of sulfuric acid or bisulfate ion and organic molecules have been shown to exist in ∼2 nm sized aerosol particles. In particular, kinetic studies indicated the formation of clusters with one H2SO4 and one or two organics being the rate-limiting step.This Account discusses our effort in developing an integrated approach, which involves the laboratory cluster synthesis via electrospray ionization, size and composition analysis via mass spectrometry, photoelectron spectroscopic characterization, and quantum mechanics based theoretical modeling, to investigate the structures, energetics, and thermodynamics of the aerosol prenucleation clusters relevant to NPF. We have been focusing on the clusters formed between H2SO4 or HSO4- and the organics from oxidation of both biogenic and anthropogenic emissions. We illustrated the significant thermodynamic advantage by involving organic acids in the formation and growth of aerosol clusters. We revealed that the functional groups in the organics play critical roles in promoting NPF process. The enhanced roles were quantified explicitly for specific functional groups, establishing a Molecular Scale that ranks highly hierarchic intermolecular interactions critical to aerosol formation. The different cluster formation pathways, probably mimicking the various polluted industrial environments, that involve cis-pinonic and cis-pinic acids were unveiled as well. Furthermore, one intriguing fundamental phenomenon on the unusual protonation pattern, which violates the gas-phase acidity (proton affinity) prediction, was discovered to be common in sulfuric acid-organic clusters. The mechanism underlying the phenomenon has been rationalized by employing the temperature-dependent experiments of sulfuric acid-formate/halide model clusters, which could explain the high stability of the sulfuric acid containing aerosol clusters. Our work provides critical molecular-level information to shed light on the initial steps of nucleation of common atmospheric precursors and benchmarks critical data for large-scale theoretical modeling to further address problems of environmental interest.

Vibrations of a beam between stops: Collision events and energy balance properties

<p style='text-indent:20px;'>We consider the model problem of an elastic beam vibrating between two stops. More precisely the beam is clamped at its left end while its right end may undergo contact and collision events with two stops. We model the interaction between the beam and the stops either with Signorini complementarity conditions when the stops are perfectly rigid or with a normal compliance contact law allowing some penetration within the stops and given by a linear relationship between the shear stress and the penetration at some positive power <inline-formula><tex-math id="M1">\begin{document}$ \beta $\end{document}</tex-math></inline-formula> when contact occurs. <p style='text-indent:20px;'>Motivated by computational issues we study the evolution of the energy functional defined as the sum of the kinetic energy and the potential energy of elastic deformation of the beam. When contact is modelled with a normal compliance law we prove an energy conservation property. Then we interpret the relationship between the shear stress and the penetration in case of contact as a penalization of the non-penetration condition. We show that the solutions of the penalized problems converge to a <i>strong solution</i> of the problem with Signorini conditions as defined in [<xref ref-type="bibr" rid="b26">26</xref>] and we prove that the limit satisfies an energy conservation property through instantaneaous collision events.

Radiation-constrained algorithms for Wireless Energy Transfer in Ad hoc Networks

We study the problem of efficiently charging a set of rechargeable nodes using a set of wireless chargers, under safety constraints on the electromagnetic radiation incurred. In particular, we define a new charging model that greatly differs from existing models in that it takes into account real technology restrictions of the chargers and nodes of the network, mainly regarding energy limitations. Our model also introduces non-linear constraints (in the time domain), that radically change the nature of the computational problems we consider. In this charging model, we present and study the Low Radiation Efficient Charging Problem (LREC), in which we wish to optimize the amount of “useful” energy transferred from chargers to nodes (under constraints on the maximum level of imposed radiation). We present several fundamental properties of this problem and provide indications of its hardness. Finally, we propose an iterative local improvement heuristic for LREC, which runs in polynomial time and we evaluate its performance via simulation. Our algorithm decouples the computation of the objective function from the computation of the maximum radiation and also does not depend on the exact formula used for the computation of the electromagnetic radiation in each point of the network, achieving good trade-offs between charging efficiency and radiation control; it also exhibits good energy balance properties. We provide extensive simulation results supporting our claims and theoretical results.

Effect of Hot Vapor Filter Temperature on Mass Yield, Energy Balance, and Properties of Products of the Fast Pyrolysis of Pine Sawdust

The fast pyrolysis in a fluidized-bed reactor using pine sawdust has been studied at 500 °C with ceramic hot vapor filter (HVF). The effects of HVF temperature (350, 400, 450, and 500 °C) on mass balance, energy balance, and physical properties of pyrolysis products were investigated. The results were reported that the maximum yields of bio-oil and organic liquid were 58.7 and 41.6 wt % at HVF temperatures of 350 and 400 °C, respectively. When HVF temperature increased from 350 °C to 500 °C, the maximum biochar yield was 25.3 wt % at 350 °C, while the minimum noncondensable gas (NCG) yield was 19.2 wt % at 400 °C. The energy consumption of pine sawdust pyrolysis was calculated as 2.38 MJ/kg. The optimized HVF temperature for energy balance was 450 °C. Under these conditions, the potentially recovered energy of bio-oil and total pyrolysis products reached maximum values of 7.49 MJ/kg and 17.91 MJ/kg, respectively. Maximum energy recovery ratios of total pyrolysis products and bio-oil were 0.92 and 0.39, re...

Spectrum of a jet-emitting disc: application to the microquasar XTE J1118+480

Under the framework of the magnetized accretion ejection structures, we analyze the energy balance properties, and study the spectral energy distributions (SEDs) of the Jet Emitting Disc (JED) model for black hole X-ray transients. Various radiative processes are considered, i.e. synchrotron, bremsstrahlung, and their Comptonizations, and external Comptonization of radiation from outer thin disc. With these cooling terms taken into account, we solve the thermal equilibrium equation self-consistently and find three solutions, of which the cold and the hot solutions are stable. Subsequently we investigate the theoretical SEDs for these two stable solutions.We find the hot JED model can naturally explain the spectra of the Galactic microquasars in their hard states. As an example, we apply this model to the case of XTE J1118+480.

Lysimetric evaluation of SEBAL using high resolution airborne imagery from BEAREX08

In this study, Surface Energy Balance Algorithm for Land (SEBAL) was evaluated for its ability to derive aerodynamic components and surface energy fluxes from very high resolution airborne remote sensing data acquired during the Bushland Evapotranspiration and Agricultural Remote Sensing Experiment 2008 (BEAREX08) in Texas, USA. Issues related to hot and cold pixel selection and the underlying assumptions of difference between air and surface temperature (dT) being linearly related to the surface temperature were also addressed. Estimated instantaneous evapotranspiration (ET) and other components of the surface energy balance were compared with measured data from four large precision weighing lysimeter fields, two each managed under irrigation and dryland conditions. Instantaneous ET was estimated with overall mean bias error and root mean square error (RMSE) of 0.13 and 0.15mmh−1 (23.8 and 28.2%) respectively, where relatively large RMSE was contributed by dryland field. Sensitivity analysis of the hot and cold pixel selection indicated that up to 20% of the variability in ET estimates could be attributed to differences in the surface energy balance and roughness properties of the anchor pixels. Adoption of an excess resistance to heat transfer parameter model into SEBAL significantly improved the instantaneous ET estimates.

Simplified modeling of active magnetic regenerators

A simplified model for predicting the general trends of active magnetic regenerator devices is presented. To reduce computational demands it is assumed the regenerator has sufficiently high convective interaction such that a one-phase regenerator approximation is sufficient. A corrective term is subsequently added to the thermal conductivity to account for the convective heat leak. Losses internal and external to the regenerator are also defined for a generalized AMR device. The steady state temperature span across the regenerator is then evaluated using an energy balance and real material properties with the cooling capacity, work inputs and COP post-calculated using the result. A comparison of the simplified model with experimental results was performed using single-layer and multilayer regenerators of Gd, Gd0.74Tb0.26 and Gd0.85Er0.15. The model showed good conformity in magnitude and sensitivity in all of the scenarios which varied by heat load, frequency and utilization.

Numerical calculation of thermal effect on cavitation in cryogenic fluids

A key design issue related to the turbopump of the rocket engine is that cavitation occurs in cryogenic fluids when the fluid pressure is lower than the vapor pressure at a local thermodynamic state. Cavitation in cryogenic fluids generates substantial thermal effects and strong variations in fluid properties, which in turn alter the cavity characteristics. To date, fewer investigate the thermal effect on cavitation in cryogenic fluids clearly by the numerical methods due to the difficulty of the heat transfer in the phase change process. In order to study the thermal effect on cavitation in cryogenic fluid, computations are conducted around a 2D quarter caliber hydrofoil in liquid nitrogen and hydrogen respectively by implementing modified Merkle cavitation model, which accounts for the energy balance and variable thermodynamic properties of the fluid. The numerical results show that with the thermal effect, the vapour content in constant location decreases, the cavity becomes more porous and the interface becomes less distinct which shows increased spreading while getting shorter in length. In the cavity region, the temperature around the cavity depresses due to absorb the evaporation latent heat and the saturation pressure drops. When the vapour volume fraction is higher, the temperature depression and pressure depression becomes larger. It is also observed that a slight temperature rise is found above the reference fluid temperature at the cavity rear end attributed to the release of latent heat during the condensation process. When the fluid is operating close to its critical temperature, thermal effects on cavitation are more obviously in both the liquid nitrogen and hydrogen. The thermal effect on cavitation in liquid hydrogen is more distinctly compared with that in liquid nitrogen due to the density ratio, vapour pressure and other variable properties of the fluid. The investigation provides aid for the design of the cryogenic pump of the liquid rocket.

Impact of wildfire on the thermal behavior of northern peatlands: Observations and model simulations

Wildfire represents the single largest disturbance to the ecohydrological function of northern peatlands. Alterations to peatland thermal behavior as a result of wildfire will modify the carbon balance of these important long‐term global carbon stores and regulate post‐fire ecosystem recovery. We simulate the 3‐D thermal behavior of a peatland that has been disturbed by wildfire to identify how changes in peat temperatures emerge from changes to the surface energy balance and peat thermal properties. Peat temperatures are simulated within two adjacent peatlands, one area having burned 4 years previously, the second which has been wildfire‐free for 75 years. We demonstrate that there is only a small alteration to the thermal response inSphagnum fuscum hummocks that are not severely burnt within the wildfire. In contrast, wildfire produces important changes to the energy balance of Sphagnumhollows. A large reduction in the latent heat flux post‐fire increases surface temperatures by up to 30°C during daytime summer conditions. However, temperatures through the peat profile are insensitive to these increases in surface temperature. The low surface moisture content of near‐surface peat insulates the profile from these higher temperatures and, at depths below 0.015 m, only small differences are identifiable between burned and unburned hollow temperatures. Nevertheless, we argue that these alterations to near‐surface temperatures and evaporation rates likely substantially influence the thermal and hydrological conditions post‐wildfire, impacting the peatland recovery.

Retrieving cloud geometrical extents from MIPAS/ENVISAT measurements with a 2-D tomographic approach

Clouds represent a critical factor in regulating the Earth's atmosphere and its energy balance. Satellite instruments can measure the energy balance and global atmospheric properties only through an accurate knowledge of the vertical profile of cloudiness, which is as yet one of the key shortages in atmospheric science. The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on-board the ENVISAT satellite, designed to infer the amount of atmospheric trace-gases, demonstrated also sensitivity to the radiation emitted from clouds. In order to model the effect of the geometrical extent of a cloud on MIPAS measurements, we developed a retrieval model capable to simulate cloud effects on broad spectral intervals accounting for the two-dimensional (2-D) variability of the atmosphere in the satellite orbit plane. The 2-D analysis revealed a sensitivity of MIPAS spectra to both the vertical and horizontal extents and the position of clouds along the instrument line of sight. One-dimensional models were found to underestimate Cloud Top Height (CTH) by approximating clouds as an infinite horizontal layer with a finite vertical extents. With the 2-D approach, we showed it is possible, for optically thin Polar Stratospheric Clouds (PSCs), to retrieve both CTH and horizontal dimension by analyzing simultaneously all the limb observations that come across the cloud with their field of view. For a selected case study we found a very good agreement for both PSC CTH and horizontal extents retrieved from MIPAS measurements and those retrieved from coincident CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarisation) measurements.

Parameterized energy–latency trade-offs for data propagation in sensor networks

We study the problem of greedy, single path data propagation in wireless sensor networks, aiming mainly to minimize the energy dissipation. In particular, we first mathematically analyze and experimentally evaluate the energy efficiency and latency of three characteristic protocols, each one selecting the next hop node with respect to a different criterion (minimum projection, minimum angle and minimum distance to the destination). Our analytic and simulation findings suggest that any single criterion does not simultaneously satisfy both energy efficiency and low latency. Towards parameterized energy–latency trade-offs we provide as well hybrid combinations of the two criteria (direction and proximity to the sink). Our hybrid protocols achieve significant performance gains and allow fine-tuning of desired performance. Also, they have nice energy balance properties, and can prolong the network lifetime.

Numerical modeling of two‐phase flow in the NaCl‐H2O system: 2. Examples

In an earlier paper, we presented a new multiphase numerical code for NaCl‐H2O fluid called FISHES. Here we present some example simulations using FISHES that show its applicability to 1‐D and 2‐D geometries that are commonly used in describing two‐phase hydrothermal flows. We simulate a 1‐D heat pipe with an open top upper boundary in order to study mass and energy balance properties within a region of phase separating fluid. The results show the formation of a dense brine layer at the base of the heat pipe as a two‐phase zone expands toward the top. We then simulate 2‐D circulation in a single‐pass geometry with a highly permeable discharge zone, a geometry that has often been used in the study of seafloor hydrothermal systems. We show that when active phase separation occurs at the base of the system, low‐salinity fluid emerges from the top as a dense brine layer forms at the base and that increased vent fluid temperatures result from decreasing the width of the discharge pipe. Following a period of active phase separation, we show that lowering the basal temperature results in flushing the brine from the system on a time scale similar to the period of active phase separation. Lowering the permeability does not significantly affect vent fluid salinities during active subsurface phase separation but inhibits the flushing of brine upon lowering of basal temperatures; also, low permeabilities increase the time scale for the evolution of the system. Finally, we conduct simulations of two‐phase flow in rectangular 1 km2 homogeneous fluid‐saturated open top boxes heated from below with permeabilities of 10−12 and 10−13 m2. During these simulations a strong plume forms above the region of strong lateral temperature change, and brine collects at the base of the system. Initially, low‐salinity fluids vent from the top of the box; eventually, higher‐salinity fluids emerge. Vent fluid salinities and temperatures associated with plumes vary with time, resulting in larger temporal variation in fluid properties than those obtained from single‐pass simulations.

Numerical study of cavitating flows with thermodynamic effect

Thermodynamic effects play an important role in the cavitation dynamics of cryogenic fluids. Such flows are characterized by strong variations in fluid properties with the temperature. A compressible, multiphase, one-fluid formulation that accounts for the energy balance and variable thermodynamics properties of the fluid is described. A preconditioning method for low Mach number areas is presented. Cavitation phenomenon is modelled by two different liquid–vapour mixture equations of state (EOS). Mathematical and thermodynamic properties are studied. Numerical results are given for a Venturi geometry with freon R-114 fluid and comparisons are made with experimental data.

Midpoint numerical technique for stochastic Landau-Lifshitz-Gilbert dynamics

The implicit midpoint time-integration technique is applied to the stochastic Landau-Lifshitz-Gilbert (LLG) equation. The numerical scheme converges to the Stratonovich solution in the limit of vanishing time step. It preserves the magnetization magnitude and the main energy balance properties of the LLG equation independently of the time step. The numerical technique is then applied to the study of superparamagnetic state in a small spheroidal particle, and the numerical results are compared with the theory.

Dynamics of shear-transformation zones in amorphous plasticity: Nonlinear theory at low temperatures

We use considerations of energy balance and dissipation to derive a self-consistent version of the shear-transformation-zone (STZ) theory of plastic deformation in amorphous solids. The theory is generalized to include arbitrary spatial orientations of STZs. Continuum equations for elasto-plastic material and their energy balance properties are discussed.