Convection Velocity Research Articles

Experimental and Statistical Determination of Convective Heat and Mass Transfer Coefficients for Eucalyptus nitens Sawn Wood Drying

This study aimed to develop a model using experimentally obtained convective heat and mass transfer coefficients to predict the effect of temperature, humidity, and drying rate on wood drying. Tangential wood samples of Eucalyptus nitens (H. Deane & Maiden) were used in the investigation. The experimental design consisted of two temperature levels (40 °C and 55 °C), two relative humidity levels (55% and 75%), and two air velocity settings (2 m·s−1 and 3 m·s−1). The experiments were conducted under a constant evaporation rate, spanning the maximum and critical moisture content in the wood. A statistical model using multivariate regression was created to predict the convective heat and mass transfer coefficients. The results indicated that the experimental data and empirical correlations exhibited an error margin of 37.77% and 37.86%, respectively. A significant positive correlation was found between the convective heat transfer coefficient and air velocity, temperature, and relative humidity, while the convective mass transfer coefficient showed a significant positive correlation only with air velocity and temperature. The model predicted the convective heat and mass transfer coefficients with high accuracy and statistical significance. Using the proposed method, we successfully obtained both convective coefficients, which enable accurate description of heat and mass flow during the convective drying of Eucalyptus nitens wood.

Shell mergers in the late stages of massive star evolution: new insight from 3D hydrodynamic simulations

ABSTRACT One-dimensional (1D) stellar evolution models are widely used across various astrophysical fields, however they are still dominated by important uncertainties that deeply affect their predictive power. Among those, the merging of independent convective regions is a poorly understood phenomenon predicted by some 1D models but whose occurrence and impact in real stars remain very uncertain. Being an intrinsically multi-D phenomenon, it is challenging to predict the exact behaviour of shell mergers with 1D models. In this work, we conduct a detailed investigation of a multiple shell merging event in a 20 M$_\odot$ star using 3D hydrodynamic simulations. Making use of the active tracers for composition and the nuclear network included in the 3D model, we study the merging not only from a dynamical standpoint but also considering its nucleosynthesis and energy generation. Our simulations confirm the occurrence of the merging also in 3D, but reveal significant differences from the 1D case. Specifically, we identify entrainment and the erosion of stable regions as the main mechanisms that drive the merging, we predict much faster convective velocities compared to the mixing-length theory velocities, and observe multiple burning phases within the same merged shell, with important effects for the chemical composition of the star, which presents a strongly asymmetric (dipolar) distribution. We expect that these differences will have important effects on the final structure of massive stars and thus their final collapse dynamics and possible supernova explosion, subsequently affecting the resulting nucleosynthesis and remnant.

Offline tagging of radon-induced backgrounds in XENON1T and applicability to other liquid xenon time projection chambers

This paper details the first application of a software tagging algorithm to reduce radon-induced backgrounds in liquid noble element time projection chambers, such as XENON1T and XENONnT. The convection velocity field in XENON1T was mapped out using Rn222 and Po218 events, and the rms convection speed was measured to be 0.30±0.01 cm/s. Given this velocity field, Pb214 background events can be tagged when they are followed by Bi214 and Po214 decays, or preceded by Po218 decays. This was achieved by evolving a point cloud in the direction of a measured convection velocity field, and searching for Bi214 and Po214 decays or Po218 decays within a volume defined by the point cloud. In XENON1T, this tagging system achieved a Pb214 background reduction of 6.2−0.9+0.4% with an exposure loss of 1.8±0.2%, despite the timescales of convection being smaller than the relevant decay times. We show that the performance can be improved in XENONnT, and that the performance of such a software-tagging approach can be expected to be further improved in a diffusion-limited scenario. Finally, a similar method might be useful to tag the cosmogenic Xe137 background, which is relevant to the search for neutrinoless double-beta decay. Published by the American Physical Society 2024

Assessing The Potential Of PIV Data To Resolve Hidden Frequency Scales

The present work investigates the performance of an advection-based flow reconstruction model to increase the temporal resolution of PIV measurements recorded in a turbulent, planar jet. Using a semi-Lagrangian technique in combination with Rapid Distortion Theory, a modified trajectory tracking procedure is implemented. The method introduces a specification for a two-dimensional convective velocity based on the least squares minimization of the linearized advection equation, in contrast to the previously introduced one-dimensional mean velocity profile outlined in Vocke et al. (2023). The new implementation is based on local flow measurements, making it well-suited towards streamwise heterogeneity and spatially developing flows. With this, the flow at some unknown time can be estimated from the know flow measurements at the forward and backward time. Spectral analysis illustrates the model's proficiency in recovering spatiotemporal information far exceeding the Nyquist frequency, with spectral reconstruction errors of less than 5% for the most extreme case. It is demonstrated that the spectral content can be estimated at least two orders of magnitude beyond the sampling frequency of the original recording. Improved performance compared to alternative methods is demonstrated with only minor impact on computational time. This indicates the approach may be used as a tool for experimental researchers a) having no access to a high-speed PIV or b) with high-speed PIV to increase the spectral resolution even further.

Fluid Dynamics Effects of a Porous Blockage on Laminar Flow in the Interior Subchannels of a 61-Pin Wire-Wrapped Rod Bundle

Flow blockages in Liquid Metal Fast Reactor (LMFR) fuel assemblies that can cause fuel pin cladding failures require further investigation for the development and optimization of these Generation IV reactor designs. The objective of this study was to experimentally evaluate the effects of a porous blockage accident scenario on laminar flow behavior in the interior subchannels of a prototypical 61-pin wire-wrapped rod bundle. The laminar flow condition is considered because of its importance to natural circulation and loss-of-coolant-accident operating scenarios as well as the limited availability of experimental data. The matched-index-of-refraction method was utilized to obtain two-dimension two-component time-resolved particle image velocimetry (TR-PIV) measurements at three planes centered on blocked and neighboring subchannel regions. Time-averaged first-order and second-order statistics, computed by Reynolds decomposition, were visualized including the mean and fluctuating streamwise and spanwise velocity components. Line profiles described the evolution of flow from upstream regions, to separated flow, and recombination downstream, while a modified Galilean decomposition distinguished differences between flow in the blocked measurement plane and flow in its neighboring counterpart region. Spatial-temporal cross-correlations were performed for the streamwise velocity fluctuations to characterize the convection velocity and decay of traveling vortices in the wake of the porous blockage. The isothermal TR-PIV measurements from this study provide high-fidelity experimental data sets for the validation of computational models and numerical studies to characterize complex fluid phenomena in wire-wrapped rod bundles during the potential accident scenario of a porous, interior flow blockage.

Impact of exposure time on optical-phase measurements in turbulence

In this paper, we explore the impact of exposure time on optical-phase measurements collected on light that has propagated through atmospheric-optical turbulence. We model the exposure time by phase averaging over a convective distance, and we quantify the associated impact of imposing an exposure time using the piston- and tilt-removed phase variance. We accomplish this analysis through the development of an analytic solution and wave-optics simulations. In turn, we show that the analytic solution and simulation results are in good agreement when U c τ/D≲0.25, where U c is the convective velocity, τ is the exposure time, and D is the aperture diameter. When U c τ/D≳0.25, the analytic solution underestimates the piston- and tilt-removed phase variance relative to the simulation results, and we discuss these differences. This work, at large, informs wavefront sensing and adaptive-optics efforts, where either the wind speed is high, the system is on a high-speed platform, the beacon is on a high-speed platform, or the beacon signal is very faint thereby requiring long-exposure data collections.

Investigating the convective flow of ternary hybrid nanofluids and single nanofluids around a stretched cylinder: Parameter analysis and performance enhancement

Within this research, we examined the convective flow of a blend of ternary hybrid nanofluids and single nanofluids with a stagnation point caused by a stretched cylinder. The purpose of this study is to investigate the effect of using a ternary hybrid nanofluid instead of a single nanofluid. Water is used as the base fluid in this investigation. The single nanofluid is made of copper and the ternary hybrid nanofluid is made of Molybdenum disulfide, copper, and silver. The impact of the curvature parameter, nanoparticle volume fraction, mixed convection parameter, velocity ratio, shape factor, conjugate number and Eckert number parameters on temperature and velocity profiles has been investigated. Differential equations with partial derivatives were transformed to equations of ordinary differential type. The ODEs were then solved using the RK5th method. The ternary hybrid nanofluid has greater advantages than the single nanofluid and enhances the velocity and temperature compared to the single nanofluid, according to this study. Furthermore, the results showed that when curvature parameter, volume fraction and shape factor improved, so did the velocity and temperature profile. The results showed that moving from a single nanofluid to a ternary hybrid nanofluid at shape factor equal to 16.2 (Lamina) boosts the velocity by 20 %. Also, in Eckert number equal to 0.45, and substituting the ternary hybrid nanofluid boosts the temperature tenfold.

Frequency-domain quadrupole correction for the permeable-surface Ffowcs Williams and Hawkings integration

The permeable-surface Ffowcs Williams and Hawkings (FW–H) integration for computing the far-field sound has the advantage of encapsulating the sources and nonlinear propagation inside the integral surface. However, it suffers from spurious sound when the volume integral for quadrupole term outside the permeable surface is conventionally ignored. The spurious sound is often suppressed by using two distinct approaches, which modifies the FW–H integration and acoustic variables/sources, respectively. This work clarifies the connection between the two approaches by analyzing the integral of the quadrupole sources. We show that the modification of the acoustic sources can be reformulated as a modification of the FW–H integration, which means that the two distinct approaches are interconvertible. A new quadrupole correction model for the FW–H integration is proposed by delicately modifying the acoustic sources. The modified acoustic sources consist of the filtered Lighthill stress tensor, where a convection operator is used to filter out the acoustically inefficient components. The proposed quadrupole correction model is consistent with the previous work on the modification of the FW–H integration under special conditions with the uniform convection velocity. The proposed model is validated by computing the sound pressure generated by laminar and turbulent flows over bluff bodies. It is found that the sensitivity of the acoustic pressure to the FW–H surface's position is suppressed and the accuracy of the predicted sound is improved. The results suggest that the modification of acoustic variables/sources can be a powerful method to construct new quadrupole correction models for the permeable FW–H integration.

Variability of convection velocities and structure inclination angles in wall-bounded turbulence

Variability of convection velocities and structure inclination angles in wall-bounded turbulence

Fast and Slow Crystallization-driven Convection in White Dwarfs

We investigate crystallization-driven convection in carbon–oxygen white dwarfs. We present a version of the mixing length theory that self-consistently includes the effects of thermal diffusion and composition gradients, and provides solutions for the convective parameters based on the local heat and composition fluxes. Our formulation smoothly transitions between the regimes of fast adiabatic convection at large Peclet number and slow thermohaline convection at low Peclet number. It also allows for both thermally driven and compositionally driven convection, including correctly accounting for the direction of heat transport for compositionally driven convection in a thermally stable background. We use the MESA stellar evolution code to calculate the composition and heat fluxes during crystallization in different models of cooling white dwarfs, and determine the regime of convection and the convective velocity. We find that convection occurs in the regime of slow thermohaline convection during most of the cooling history of the star. However, at the onset of crystallization, the composition flux is large enough to drive fast overturning convection for a short time (∼10 Myr). We estimate the convective velocities in both of these phases and discuss the implications for explaining observed white dwarf magnetic fields with crystallization-driven dynamos.

Experimental and numerical study of structural parameters on the heat transfer performance of submerged cooling system

Immersion direct contact cooling effectively transfers heat from electronic components to a cooling solution without additional thermal resistance, due to its high energy density, low power consumption, system simplicity, and integrated thermal management for electronic devices. Current scholarly research on immersed structures predominantly focuses on surface temperature effects, with a scant investigation into internal flow and temperature field distributions concerning boundary layers. This study addresses structural optimization in immersion cooling systems, employing experimental and simulation methods to elucidate the thermal mechanisms by which structural optimization impacts the performance of chip immersion cooling systems. The research examines the impact of panel spacing and liquid level height on the thickness of velocity and thermal boundary layers, as well as the combined effects of natural and forced convection under three distinct cooling modes. Analysis reveals that heat pipe immersion cooling can reduce the heat source junction temperature to 40.7 °C under a 60 W power dissipation, a 47.2 % decrease from the bare heat source temperature. Reducing panel spacing disrupts the thermal boundary, enhancing forced convection and lowering the junction temperature by up to 10.1 % (6.5 °C). As liquid level height increases, the junction temperature initially decreases and then rises, with an optimal height of 175 mm for the best overall heat transfer effect. At low flow velocities (0.04 m/s), the vertical buoyancy-induced flow velocity at the heat source surface (24.2 × 10-6 m/s) exceeds the horizontal forced convection velocity (0.78 × 10-6 m/s), indicating that buoyancy changes due to density differences are significant and must be considered.

Fingering convection in a spherical shell

We use $123$ three-dimensional direct numerical simulations to study fingering convection in non-rotating spherical shells. We investigate the scaling behaviour of the flow length scale, the non-dimensional heat and compositional fluxes $Nu$ and $Sh$ and the mean convective velocity over the fingering convection instability domain defined by $1 \leq R_\rho < Le$ , $R_\rho$ being the ratio of density perturbations of thermal and compositional origins and $Le$ the Lewis number. We show that the chemical boundary layers are marginally unstable and adhere to the laminar Prandtl–Blasius model, hence explaining the asymmetry between the inner and outer spherical shell boundary layers. We develop scaling laws for two asymptotic regimes close to the two edges of the instability domain, namely $R_\rho \lesssim Le$ and $R_\rho \gtrsim 1$ . For the former, we develop novel power laws of a small parameter $\epsilon$ measuring the distance to onset, which differ from theoretical laws published to date in Cartesian geometry. For the latter, we find that the Sherwood number $Sh$ gradually approaches a scaling $Sh\sim Ra_\xi ^{1/3}$ when $Ra_\xi \gg 1$ ; and that the Péclet number accordingly follows $Pe \sim Ra_\xi ^{2/3} |Ra_T|^{-1/4}$ , $Ra_T$ and $Ra_{\xi}$ being the thermal and chemical Rayleigh numbers. When the Reynolds number exceeds a few tens, we report on a secondary instability which takes the form of large-scale toroidal jets which span the entire spherical domain. Jets distort the fingers, resulting in Reynolds stress correlations, which in turn feed the jet growth until saturation. This nonlinear phenomenon can yield relaxation oscillation cycles.

On the natural transition of the natural convection boundary layer under a constant heat flux condition

This study investigates the thermo-fluidic characteristics of a natural convection boundary layer over a vertical plate heated under a constant heat flux condition. We used particle image velocimetry to observe the surface convection velocity at the proximity of the plate, which is submerged in a water chamber with Rayleigh number, Ra=7×109 and Prandtl number, Pr = 8.1. The velocity distributions indicated the presence of distinct stripe-like flow structures in the early transition (upstream) region and that of distinct negative–positive Λ-shaped flow structures in the downstream region. In addition, the temporal variation of spanwise and streamwise velocity components was presented. Furthermore, proper orthogonal decomposition (POD) analysis was used to reduce the complexity of the flow structure and retain the most salient characteristics of the flow field. Our results confirmed the existence of a very weak inceptive oscillation in the behaviour of POD time coefficients in the upstream region.

A Quantitative Analysis of the Uncertainties on Reconnection Electric Field Estimates Using Ionospheric Measurements

AbstractCalculating the magnetic flux transfer across the open‐closed boundary (OCB) per unit time and distance—the reconnection electric field—is an important means of remotely monitoring magnetospheric dynamics. Ground‐based measurements of plasma convection velocities together with velocities of the OCB are commonly used to infer reconnection rates. However, this approach is limited by spatial coverage and often lacks robust uncertainty quantification. In this paper, we assimilate Super Dual Auroral Radar Network convection measurements, ground magnetometer data, and estimates of the conductance derived from the Imager for Magnetopause‐to‐Aurora Global Exploration satellite imagers, using the Local mapping of polar ionospheric electrodynamics (Lompe) framework over a region in North America. We present a new method to assess various contributions to uncertainties in the derived reconnection electric fields, including a novel approach to estimate uncertainties in conductance from global auroral imaging. Our method is demonstrated on a substorm event with an associated pseudobreakup during a period of favorable observational coverage. In this case study, the uncertainties in the reconnection electric field are ∼5–10 mV/m at the peak of substorm expansion, roughly 15% of the peak reconnection electric field. We find that the main contributor to the reconnection electric field estimates after substorm onset is the OCB motion, whereas during the pseudobreakup the main contributor is ionospheric plasma convection.

Heat effects on supersonic jet screech: a linear stability analysis based on parabolized stability equations

In this paper, we use the parabolized stability equations (PSEs) to investigate heat effects on supersonic jet screech. The PSE is derived from the linear Euler equations, and the computations are performed on an empirical mean flow profile. Employing PSE, we can examine several important characteristics of the shear-layer instability waves, including the convection velocity, the growth rate, and the location where the instability waves attain the maximal total amplification. Equipped with these knowledge, we further explore their impacts on the jet screech. Specifically, using a newly proposed iteration scheme, we first identify a more suitable convective Mach number to predict the screech frequency in both heated and cold jets. Second, we find that the transition of the screech mode from the axisymmetric to the helical mode may be explained by the shift in the most amplified instability modes. Using this criterion, we can predict a transition Mach number for the mode staging. Third, by assuming that the effective source location is the position where the instability wave attains its maximal amplitude and using the newly obtained convective Mach number, we can predict the screech mode staging in cold and heated jets using Gao's model. The predictions agree well with the experimental data. Finally, we investigate the heat effects on screech amplitude. It is found that compared to cold jets, the difference between the frequency of the most amplified instability wave and the frequency of the jet screech is much bigger in heated jets, which may lead to a lower screech amplitude.

Significance of [formula omitted]- water nanofluid, buoyant strength and ohmic heating in the enhancement of microchannel efficiency

The present study investigates the influence of buoyant strength on magnetohydrodynamic flow of Titanium dioxide (TiO2) water based nanoliquid in a vertical microchannel with convective boundary and velocity slip conditions. Additionally, exponentially dependent heat source, Joule heating and viscous dissipation are considered to explore the flow and heat transfer fields. The buoyant forces are used to enhance fluid mixing and heat transmission by carefully planning the microchannel shape and fluid characteristics. Therefore, the numerical study investigates the effect of entropy generation on channel efficiency and discusses interesting results on microchannel efficiency. To solve the nonlinear dimensionless differential equations, the Runge-Kutta-Fehlberg fourth-fifth (RKF-45) order numerical approach in conjunction with shooting methodology is employed. The obtained parameters are examined and analyzed graphically with regards to their effects on flow, thermal, local entropy generation and Bejan number profiles. Results reveal that, the velocity profile shows a rising character with increasing Grashof and Brinkman numbers. On the other hand, when the volume fraction of nanoparticles drops, the temperature profile shows a declining trend. This finding suggests that nanoparticle concentration may be carefully controlled and manipulated to improve microchannel efficiency. Lowering the system's entropy generation improves the channel efficiency.

Traveling waves for monostable reaction-diffusion-convection equations with discontinuous density-dependent coefficients

This paper concerns wave propagation in a class of scalar reaction-diffusion-convection equations with p-Laplacian-type diffusion and monostable reaction. We introduce a new concept of a non-smooth traveling wave profile, which allows us to treat discontinuous diffusion with possible degenerations and singularities at 0 and 1, as well as only piecewise continuous convective velocity. Our approach is based on comparison arguments for an equivalent non-Lipschitz first-order ODE. We formulate sufficient conditions for the existence and non-existence of these generalized solutions and discuss how the convective velocity affects the minimal wave speed compared to the problem without convection. We also provide brief asymptotic analysis of the profiles, for which we need to assume power-type behavior of the diffusion and reaction terms.

Numerical Simulation of Rapid Heat Storage in Plate-Fin Phase Change Heat Exchanger

Rapid phase change heat storage has significant application value in high-power density thermal management systems. This paper presents a comparative numerical simulation study of the rapid heat storage and release dynamics of two types of finned plate heat exchangers with different fin shapes. The study reveals that, under the same fin thickness and volume fraction, the contact surface area of triangular fins is 1.6 times that of rectangular fins, but the heat storage rate of triangular fin heat exchangers is 2 times that of rectangular fin heat exchangers. Due to the small characteristic scale and high material viscosity of the heat exchanger, the natural convection velocity of the liquid phase during the heat storage process is low, and its influence on enhancing the heat storage rate is weak.

Variation of vortical structures across shock-wave/turbulent boundary-layer interaction region in a compression ramp flow

This paper describes direct numerical simulations of a shock-wave/turbulent boundary-layer interaction (STBLI) process in a compression-ramp flow with a ramp angle of 24° and a free-stream Mach number of Ma∞=2.9. Spectral analysis, two-point cross correlation, convection velocity statistics, and individual vortex identification are used to elucidate the streamwise variation of multiscale turbulent structures in the STBLI process. Typical Lagrangian coherent structures in the turbulent boundary layer before the STBLI region are characterized as hairpin-like vortical structures, with heads that rise together with the separated mean flow in the STBLI region. In the downstream region, the reattached turbulent boundary layer has a two-layer structure. The outer layer is characterized as an intensification of large-scale velocity structures, which is attributed to the shock-wave-induced compression effect on vortical structures. A viscous-dominated layer develops independently in the vicinity of the wall, leading to a gradual restoration of the wall-shear effect that accumulates the inner-layer dynamics of small-to-moderate-scale turbulent motions.

Debris-induced consequences on turbulence and vorticity in solar photovoltaic module-generated array wakes

Particle-laden flows in solar photovoltaic (PV) systems are inevitable, where wind-swept debris in open environments are carried by high winds and turbulence, coating panel surfaces or damaging structures. Particle deposition, or soiling, is a well-known issue for large-scale plants which rely on uninhibited solar rays for optimal production. But understanding the mechanisms leading to soiling requires a physical and fluid dynamics-centered focus, since turbulence dominates PV panel wakes and is also known to alter particle concentration and trajectories. This study presents an experimental campaign toward consequences of particle-laden flow between two model PV panels using time-resolved particle image velocimetry. The model array was subjected to varied particle volume fractions, including a tracer particle case and a water droplet case. Characterization of mean velocity, turbulence statistics, and mean kinetic energy within the single phase and, separately, particle phase flows showed modified features due to particle inertia. Images captured at a frequency of 1 kHz in the near wake of the upstream panel allow for a first experimental look at vorticity and convective velocity of vortex structures for single-phase and particle-phase flows which are crucial to debris transport and soiling in PV environments.