Terms Of Energy Transfer Research Articles

Numerical simulation of natural convection in a horizontal enclosure: Part I. On the effect of adiabatic obstacle in middle

In this study, an effect of a three-dimensional obstacle of natural convection in a horizontal enclosure was discussed. Geometry which was taken into account was horizontal enclosure with unit aspect ratio and length of π along spanwise direction. The enclosure was heated from the bottom wall, and then was cooled down from above. An obstacle was located in the middle of the enclosure to examine its effect. A three-dimensional solution was obtained using Chebyshev spectral multi-domain methodology for different Rayleigh number at which the thermal behavior was evolved from a steady state to a chaotic pattern. As the geometry was elongated in conjunction with periodic boundary conditions to allow lateral freedom for the convection cells, longitudinal geometry along spanwise direction was discretized through a Fourier series expansion with a uniform mesh configuration. An adiabatic obstacle played a different role in determining the thermal behavior: No-slip condition of the surface of the obstacle disturbed the overall plume behavior in terms of the momentum transfer, whereas the adiabatic boundary condition did not influence significantly in terms of energy transfer. At a low Rayleigh number, thermal behavior in three-dimensional enclosure showed steady invariant solution along spanwise direction which is identical to two-dimensional result. With increasing buoyant force, spanwise invariance of longitudinal roll cell was collapsed and three-dimensional mode was obtained following flow regime transition. After undergoing periodically oscillatory phase, a chaotic flow transition occurred. At a high Rayleigh number, three-dimensional thermal plume oscillates freely in elongated geometry and consequently yields higher heat transfer rate. In addition, the thermal flow field was captured by visualizing the three-dimensional vortical structure. The chaotic three-dimensional flow behavior was quantitatively examined by obtaining the turbulent statistics.

A Kármán–Howarth–Monin equation for variable-density turbulence

We present a generalisation of the Kármán–Howarth–Monin (K–H–M) equation to include variable-density (VD) effects. The derived equation (i) reduces to the original K–H–M equation when density is a constant and (ii) leads to a VD analogue of the $4/5$-law with the same value of constant ($=4/5$) appearing as the prefactor of the dissipation rate. The equation is employed to understand negative turbulent kinetic energy production in a $\text{SF}_{6}$ turbulent round jet with an initial density ratio of 4.2. From a Reynolds-averaged Navier–Stokes (RANS) perspective, negative production means that the mean flow is strengthened at the expense of the energy of turbulent fluctuations. We show that the associated energy transfer is accomplished by the deformation of smaller turbulent eddies into large ones in the development region of the jet and is captured by the linear scale-by-scale energy transfer term in the VD K–H–M equation. The nonlinear transfer term of the VD K–H–M equation depicts a conventional forward cascade for all eddies having a size less than the Eulerian integral length scale, regardless of their orientation. The net effect is a retarded energy cascade in the non-Boussinesq jet that has not been accounted for by existing turbulence theories. Implications of this observation for turbulence modelling are discussed.

Deciphering hierarchical features in the energy landscape of adenylate kinase folding/unfolding

Hierarchical features of the energy landscape of the folding/unfolding behavior of adenylate kinase, including its dependence on denaturant concentration, are elucidated in terms of single-molecule fluorescence resonance energy transfer (smFRET) measurements in which the proteins are encapsulated in a lipid vesicle. The core in constructing the energy landscape from single-molecule time-series across different denaturant concentrations is the application of rate-distortion theory (RDT), which naturally considers the effects of measurement noise and sampling error, in combination with change-point detection and the quantification of the FRET efficiency-dependent photobleaching behavior. Energy landscapes are constructed as a function of observation time scale, revealing multiple partially folded conformations at small time scales that are situated in a superbasin. As the time scale increases, these denatured states merge into a single basin, demonstrating the coarse-graining of the energy landscape as observation time increases. Because the photobleaching time scale is dependent on the conformational state of the protein, possible nonequilibrium features are discussed, and a statistical test for violation of the detailed balance condition is developed based on the state sequences arising from the RDT framework.

Luminescence Decay of Colloidal Quantum Dots and Stretched Exponential (Kohlrausch) Relaxation Function

AbstractThe room temperature non-exponential luminescence decay of colloidal quantum dots is investigated theoretically in an attempt to identify the underlying physical mechanisms responsible for the shape of the decay. It is shown that a stretched exponential functional form of the luminescence decay can be understood in terms of long-range resonance energy transfer from quantum dots to acceptors (molecules, quantum dots, or anharmonic molecular vibrations) or by contact quenching by quenchers (surface traps, molecules or quantum dots) distributed statistically (Poisson distribution) on the surface of the dots. These non-radiative transition processes are assigned to different ranges of the stretching parameter β.

Saturation scalings of toroidal ion temperature gradient turbulence

The emerging understanding of instability-driven plasma-turbulence saturation in terms of energy transfer to stable modes in the same scale range as the instability is employed to derive a saturation theory for the toroidal branch of ion temperature gradient turbulence that provides the scaling of turbulence and zonal flow levels for all physical parameters. The theory is based on the eigenmode decomposition of a nonlinear fluid model, which is subjected to a statistical closure and simplified via an ordering expansion consistent with zonal-flow catalyzed energy transfer from the unstable mode to the stable mode at large scale. Solution of the closed energy balance equations yields a turbulence level that is proportional to the ratio of the zonal flow damping rate and the inverse of the triplet correlation time of the zonal-flow catalyzed wavenumber triplet interaction. The zonal flow energy is proportional to the ratio of the growth rate and the inverse triplet correlation time. The saturation scalings are applied to the ion heat flux, showing that it has a factor proportional to the quasilinear heat flux and a correction factor that includes the inverse of the triplet correlation time and a reduction due to the stable mode.

RECOMBINATION CHAMBERS-DO THE OLD IDEAS REMAIN USEFUL?

Recombination methods were invented almost contemporaneously with the introduction of the ICRU/ICRP concept of a radiation quality factor expressed in terms of linear energy transfer. Since then, recombination chambers and related methodologies have been extensively developed, along with new concepts of dosimetry and radiation protection. This article briefly presents the main features of recombination chambers and their possible applications in practical systems for measuring dosimetric quantities.

Acceleration of lean aqueous hydroformylation in an innovative jet loop reactor concept

The lean biphasic aqueous hydroformylation without chemical additives of the long chain olefin 1-octene using the simple ligand triphenyl trisulfonate sodium salt was investigated in an innovative jet loop reactor on miniplant scale. This elaborate reactor type offers a unique mixing characteristic and is optimised in terms of mechanical energy transfer into the reaction medium through a jet nozzle and organic phase volume fraction. Both variables are important factors in the emulsification of two immiscible liquids. It is subsequently compared to a standard stirred reactor which was also individually optimised. Both concepts showed individual advantages in terms of productivity and catalytic activity. While the jet loop reactor reached high turnover frequencies of 7640h−1 the stirred tank reactor showed high space time yields of up to 12.6·10−5kmolm−3s−1. At very low amounts of dispersed aqueous catalyst phase even very high catalytic activity (TOF=10,960h−1) was observed in the jet loop reactor. High reactivity could also be preserved in a long time run leading to a total turnover number of 76,500. The jet loop reactor enabled high mass transfer rates for the substrate gases of up to 0.28s−1 for H2 and 0.21s−1 for CO resulting in a mass transfer multiple times faster than the obtained reaction rates. To increase regio-selectivity the bidentate ligand sulfoxanthphos was applied which increased the regio-selectivity up to l:b=53 but reduced catalytic activity.

A molecular dynamics study on the thermal energy transfer and momentum transfer at the solid-liquid interfaces between gold and sheared liquid alkanes

Solid-liquid (S-L) interfaces of linear alkane liquids contacting two parallel solid walls of gold with three types of face-centered cubic (FCC) of (100), (110) and (111) crystal planes were examined using nonequilibrium molecular dynamics simulations (NEMD). The liquid alkanes were sheared by the two parallel solid walls sliding at a constant speed and in opposite directions, which generates viscous heating in the liquid. The effect of the molecular length of the linear alkane liquids, methane, butane, octane, and tetracosane, was investigated in terms of the thermal energy transfer and momentum transfer at the S-L interfaces. The gap distance between the surface layer of the solid atoms and the adsorption layer of the liquid molecules was measured as a key feature of the interface. The slip length, which was defined as the extrapolated velocity of liquid into the solid walls where the tangential velocity vanishes, was measured and the mechanism that determines its magnitude was examined. The gap distance and the slip length were correlated with the molecular length of the liquid alkanes. The thermal boundary resistance at the S-L interfaces was measured and it was also correlated with the gap distance and molecular length of liquid alkanes. It was found that the differences in the surface structure of the solid walls between the three types of crystal planes affect the slip length at the S-L interfaces. The present results suggest that the factors affecting the thermal energy transfer and momentum transfer at the S-L interfaces were the gap distance, which differs significantly depending on the molecular length of liquid, and the surface structure of the solid walls.

Modelling and Experimental Analysis of Noise Transmission through Wall of a Railway Vehicle

The paper presents results of a grant research project dealing with acoustic properties of the walls and floor of a railway passenger coach investigated by means of the experimental analysis of noise produced during train runs and corresponding simulations. In the research, conditions and theoretical approaches for possible reconstructions of walls and floor of the railway passenger coach using new fibrous-microstructure acoustic materials were defined, with the aim of increasing the sound reduction index. The properties of the proposed materials were compared in terms of the acoustic energy transfer through the porous microstructure calculated by means of simulations. The final acoustic effects of different solutions for the incorporation of the materials in the floor structure and in the complete body of the rail passenger coach were evaluated. In this paper the effects on interior noise of the introduction of a new material for floor and wall are presented.

Lumped parameter model and numerical model of vapor bubble–driven valve-less micro-pump

A lumped parameter model for a vapor bubble–driven valve-less micro-pump is established, in which the mass transition process is considered, and a numerical model is built to validate it. The lumped parameter model consists of a bubble dynamic model based on Rayleigh–Plesset equation, a phase-change model based on thermodynamics, and a pump chamber pressure model derived from the mass conservation law accounting for the resistance of the nozzle and diffuser. The numerical model is established in ANSYS Fluent. Source terms of mass and energy transfers caused by phase change are taken into the control equation using user-defined function. The result shows that these two models have a good agreement with each other, in terms of bubble dynamics and flow performance. The proposed analytical model allows a convenient tool to study the vapor bubble–driven valve-less micro-pump and has a potential to guide the design of the micro-pumps.

Kinetic Energy Transfer between First Proper Orthogonal Decomposition Modes in a Mixing Tank

AbstractTwo‐dimensional particle image velocimetry (PIV) was applied to study hydrodynamics in a mixing tank at medium Reynolds numbers. Post‐processing of PIV data was based on proper orthogonal decomposition (POD) which enabled to reconstruct organized motion induced by impeller blades without performing angle‐averaged sampling. Energy transfers between POD modes were analyzed. In particular, a local analysis of the modal interactions was developed in terms of energy transfer between the first mode, i.e., mean flow, and successive modes, i.e., organized motion. Eventually, derivation of transfer of energy between the second mode and fourth or fifth ones reveals a double pulsation.

On the origin of stretched exponential (Kohlrausch) relaxation kinetics in the room temperature luminescence decay of colloidal quantum dots.

The non-exponential room temperature luminescence decay of colloidal quantum dots is often well described by a stretched exponential function. However, the physical meaning of the parameters of the function is not clear in the majority of cases reported in the literature. In this work, the room temperature stretched exponential luminescence decay of colloidal quantum dots is investigated theoretically in an attempt to identify the underlying physical mechanisms associated with the parameters of the function. Three classes of non-radiative transition processes between the excited and ground states of colloidal quantum dots are discussed: long-range resonance energy transfer, multiphonon relaxation, and contact quenching without diffusion. It is shown that multiphonon relaxation cannot explain a stretched exponential functional form of the luminescence decay while such dynamics of relaxation can be understood in terms of long-range resonance energy transfer to acceptors (molecules, quantum dots, or anharmonic molecular vibrations) in the environment of the quantum dots acting as energy-donors or by contact quenching by acceptors (surface traps or molecules) distributed statistically on the surface of the quantum dots. These non-radiative transition processes are assigned to different ranges of the stretching parameter β.

Spectral analysis of energy transfer in turbulent flows laden with heated particles

In this study we consider particle-laden turbulent flows with significant heat transfer between the two phases due to sustained heating of the particle phase. The sustained heat source can be due to particle heating via an external radiation source as in the particle-based solar receivers or an exothermic reaction in the particles. Our objective is to investigate the effects of fluid heating by a dispersed phase on the turbulence evolution. An important feature in such settings is the preferential clustering phenomenon which is responsible for non-uniform distribution of particles in the fluid medium. Particularly, when the ratio of particle inertial relaxation time to the turbulence time scale, namely the Stokes number, is of order unity, particle clustering is maximized, leading to thin regions of heat source similar to the flames in turbulent combustion. However, unlike turbulent combustion, a particle-laden system involves a wide range of clustering scales that is mainly controlled by particle Stokes number. To study these effects, we considered a decaying homogeneous isotropic turbulence laden with heated particles over a wide range of Stokes numbers. Using a low-Mach-number formulation for the fluid energy equation and a Lagrangian framework for particle tracking, we performed numerical simulations of this coupled system. We then applied a high-fidelity framework to perform spectral analysis of kinetic energy in a variable-density fluid. Our results indicate that particle heating can considerably influence the turbulence cascade. We show that the pressure-dilatation term introduces turbulent kinetic energy at a range of scales consistent with the scales observed in particle clusters. This energy is then transferred to high wavenumbers via the energy transfer term. For low and moderate levels of particle heating intensity, quantified by a parameter $\unicode[STIX]{x1D6FC}$ defined as the ratio of eddy time to mean temperature increase time, turbulence modification occurs primarily in the dilatational modes of the velocity field. However, as the heating intensity is increased, the energy transfer term converts energy from dilatational modes to divergence-free modes. Interestingly, as the heating intensity is increased, the net modification of turbulence by heating is observed dominantly in divergence-free modes; the portion of turbulence modification in dilatational modes diminishes with higher heating. Moreover, we show that modification of divergence-free modes is more pronounced at intermediate Stokes numbers corresponding to the maximum particle clustering. We also present the influence of heating intensity on the energy transfer term itself. This term crosses over from negative to positive values beyond a threshold wavenumber, showing the cascade of energy from large scales to small scales. The threshold is shown to shift to higher wavenumbers with increasing heating, indicating a growth in the total energy transfer from large scales to small scales. The fundamental energy transfer analysis presented in this paper provides insightful guidelines for subgrid-scale modelling and large-eddy simulation of heated particle-laden turbulence.

Using TCAD Modeling to Compare Heavy-Ion and Laser-Induced Single Event Transients in SiGe HBTs

Silicon-germanium heterojunction bipolar transistor (SiGe HBT) models are used in technology computer aided design (TCAD) to investigate single event transients induced by heavy-ion broadbeam and pulsed-laser two-photon absorption sources. A comparison between transient waveforms is provided, the proper extraction of heavy-ion broadbeam transients is discussed (along with circuit implications), and basic laser strike profiles are implemented in TCAD to provide insight into future design practices for simulation software to be used to describe laser-induced upsets in terms of an effective linear energy transfer (LET).

Comparison of excitation energy transfer in cyanobacterial photosystem I in solution and immobilized on conducting glass

Excitation energy transfer in monomeric and trimeric forms of photosystem I (PSI) from the cyanobacterium Synechocystis sp. PCC 6803 in solution or immobilized on FTO conducting glass was compared using time-resolved fluorescence. Deposition of PSI on glass preserves bi-exponential excitation decay of ~4–7 and ~21–25 ps lifetimes characteristic of PSI in solution. The faster phase was assigned in part to photochemical quenching (charge separation) of excited bulk chlorophylls and in part to energy transfer from bulk to low-energy (red) chlorophylls. The slower phase was assigned to photochemical quenching of the excitation equilibrated over bulk and red chlorophylls. The main differences between dissolved and immobilized PSI (iPSI) are: (1) the average excitation decay in iPSI is about 11 ps, which is faster by a few ps than for PSI in solution due to significantly faster excitation quenching of bulk chlorophylls by charge separation (~10 ps instead of ~15 ps) accompanied by slightly weaker coupling of bulk and red chlorophylls; (2) the number of red chlorophylls in monomeric PSI increases twice—from 3 in solution to 6 after immobilization—as a result of interaction with neighboring monomers and conducting glass; despite the increased number of red chlorophylls, the excitation decay accelerates in iPSI; (3) the number of red chlorophylls in trimeric PSI is 4 (per monomer) and remains unchanged after immobilization; (4) in all the samples under study, the free energy gap between mean red (emission at ~710 nm) and mean bulk (emission at ~686 nm) emitting states of chlorophylls was estimated at a similar level of 17–27 meV. All these observations indicate that despite slight modifications, dried PSI complexes adsorbed on the FTO surface remain fully functional in terms of excitation energy transfer and primary charge separation that is particularly important in the view of photovoltaic applications of this photosystem.

Energy Balance Analysis to Assess Efficiency Improvements with Low Heat Rejection Concepts Applied to Low Temperature Combustion

ABSTRACTSimultaneously decreasing diesel engine emissions while increasing engine efficiency can be a challenge. The results of recent computational and experimental studies, however, have indicated that emissions can be decreased while simultaneously increasing efficiency through the application of low heat rejection (LHR) techniques to low temperature combustion (LTC). One such recent experimental study showed simultaneous decreases in nitrogen oxides and smoke from a light-duty, multi-cylinder diesel engine and serves as motivation to the present study. The major conclusion from that study is that combining LHR with LTC substantially increases LTC brake fuel conversion efficiency through increases in both combustion efficiency (i.e., decreased hydrocarbon and carbon monoxide emissions) and brake thermal efficiency. LTC is realized through high levels of exhaust gas recirculation and retarded injection timings while different degrees of LHR are achieved by means of higher coolant temperatures, which should serve to decrease the temperature gradients across the cylinder walls. It is clear that the increase in brake thermal efficiency is the significant contributor to the increase in brake fuel conversion efficiency; it is not clear, however, what necessarily drove the increase in brake thermal efficiency. Seeking clarity to this issue is the motivation of the current study. This study explores in more detail the increase in brake thermal efficiency in an attempt to understand what influence LHR has on LTC efficiency improvements. This study is satisfied through analysis of the quantification of various efficiencies (i.e., net indicated thermal, net indicated fuel conversion, and brake fuel conversion efficiencies) and the many energy transfer terms (i.e., energy balance analysis). The energy balance analysis specifically reveals insight in how energy transferred through the coolant compares to those changes observed with power and exhaust energy. Specifically, the energy balance confirms important insights offered by the efficiency analysis as related to the operation of LTC at high engine coolant temperature compared to low coolant temperature. It is concluded that the chief reason for efficiency improvement with LTC at high engine coolant temperature (i.e., the LHR condition), compared to low engine coolant temperature, is improved combustion phasing; combustion efficiency is also improved. The results suggest that LTC efficiency improvements can be realized with higher operating engine coolant temperatures under light-load engine conditions.

Adlayer structure dependent ultrafast desorption dynamics in carbon monoxide adsorbed on Pd (111)

We report our ultrafast photoinduced desorption investigation of the coverage dependence of substrate-adsorbate energy transfer in carbon monoxide adlayers on the (111) surface of palladium. As the CO coverage is increased, the adsorption site population shifts from all threefold hollows (up to 0.33 ML), to bridge and near bridge (>0.5 to 0.6 ML) and finally to mixed threefold hollow plus top site (at saturation at 0.75 ML). We show that between 0.24 and 0.75 ML this progression of binding site motifs is accompanied by two remarkable features in the ultrafast photoinduced desorption of the adsorbates: (i) the desorption probability increases roughly two orders magnitude, and (ii) the adsorbate-substrate energy transfer rate observed in two-pulse correlation experiments varies nonmonotonically, having a minimum at intermediate coverages. Simulations using a phenomenological model to describe the adsorbate-substrate energy transfer in terms of frictional coupling indicate that these features are consistent with an adsorption-site dependent electron-mediated energy coupling strength, ηel, that decreases with binding site in the order: three-fold hollow > bridge and near bridge > top site. This weakening of ηel largely counterbalances the decrease in the desorption activation energy that accompanies this progression of adsorption site motifs, moderating what would otherwise be a rise of several orders of magnitude in the desorption probability. Within this framework, the observed energy transfer rate enhancement at saturation coverage is due to interadsorbate energy transfer from the copopulation of molecules bound in three-fold hollows to their top-site neighbors.

Nonsymmetrized noise in a quantum dot: Interpretation in terms of energy transfer and coherent superposition of scattering paths

We calculate the nonsymmetrized current noise in a quantum dot connected to two reservoirs by using the non-equilibrium Green function technique. We show that both the current auto-correlator (inside a single reservoir) and the current cross-correlator (between the two reservoirs) are expressed in terms of transmission amplitude and coefficient through the barriers. We identify the different energy-transfer processes involved in each contribution to the auto-correlator, and we highlight the fact that when there are several physical processes, the contribution results from a coherent superposition of scattering paths. Varying the gate and bias voltages, we discuss the profile of the differential Fano factor in light of recent experiments, and we identify the conditions for having a distinct value for the auto-correlator in the left and right reservoirs.

Optical characterization of ZnSe/CdSe nanocrystals with π-conjugated organic ligands

Organic–inorganic hybrid systems having the excitation resonance state are predicted to exhibit unique optical properties owing to a new type of excitons, i.e., hybrid excitons. The formation of the hybrid excitons requires the resonance between the excited states in organic and inorganic parts and the strong interactions with each other. In this study, we synthesized an organic–inorganic hybrid system (ZnSe/CdSe–2-aminoanthracene (AAn)) in order to realize the excitation resonance state. For the ZnSe/CdSe–AAn system in which PL wavelengths of the organic and inorganic parts are almost the same, the PL peak shifts by ∼20 nm toward longer wavelength and the decay time constant decrease with the addition of AAn. These changes in the hybrid cannot be explained simply in terms of self-absorption or excitation energy transfer. These results strongly suggest that a new excited states or new decay route was formed by the resonance of the excited states.

Closing the energy cycle in an ocean model

An effort is discussed to construct a realistic ocean model in Boussinesq approximation which features a closed energy cycle up to numerical precision errors. In such a model, the energy related to the mean variables interacts with all parameterised forms of energy without any spurious energy sources or sinks. First, the concept of the energetics of the model in terms of resolved and unresolved energy variables is outlined using potential and dynamical enthalpy instead of internal and potential energy and without use of the concept of available potential energy. The role of energy transfer terms due to the non-linear, compressible equation of state is clarified. Second, a discretisation of the primitive equations is described in which energy transfers of viscous dissipation and mixing parameterisations are exactly calculated. Third, the model performance is documented using idealised and realistic global model configurations.