Turbulent Region Research Articles

Heat Transfer Enhancement of Regenerative Cooling Channel with Pyramid Lattice Sandwich Structures

To meet the requirements of high heat transfer efficiency and light weight, the pyramid lattice structures are applied to the regenerative cooling design of scramjets. This study numerically investigates heat transfer and fluid flow characteristics of kerosene in a regenerative channel with pyramid lattice sandwich structures. At the same porosity, the maximum wall temperature of the pyramid lattice sandwich regenerative cooling channel decreases by 30%. With increased porosity, the cooling capacity of the pyramid regenerative cooling channel decreases gradually. The numerical results show that a complex secondary flow appears in the flow field in the pyramid lattice structures. In the downstream region of each strut, a spiral vortex is formed which accelerates the flow velocity approaching the wall with heat transfer enhancement. Because of the disturbing effect of the strut, a high turbulent kinetic energy region is formed in the upstream region of each strut and the heat transfer is enhanced. In general, the local heat transfer in the upstream region of the pyramid strut is better than that in the downstream region. For pyramid lattice structures, the cooling structures with high convective heat transfer and relatively low pressure drop can be obtained by optimization of the design parameters.

Inlet gap effect on aerodynamics and tonal noise generation of a voluteless centrifugal fan

In this paper, the gap effects on the aerodynamics and tonal noise generation of voluteless centrifugal fans are studied based on different gap geometries. The study is motivated by the state of the art of this type of fan, for which the tonal noise generation due to the gap turbulence has not been addressed concerning the gap geometry, while a recent study reported that there is tonal noise at the blade passing frequency (BPF) from the gap turbulence. We simulate the configurations using a hybrid method coupling the improved delayed detached eddy simulation (IDDES) with Formulation 1A of Farassat. Our simulation shows regions with high vorticity magnitudes in the channel between two blades near the trailing edges close to the shroud. The turbulence renders a uniform pressure rise. By changing the gap design, the turbulent regions can be reduced. The configurations show a similar trend of the root mean square (RMS) pressure on the blade leading edge (BLE), largest at the shroud, and decays when the distance to the gap increases. The gap designs affect the amplitude of the RMS pressure, which is connected to the BPF. Spectral analysis is performed for the surface pressure fluctuations and the sound pressure upstream of the fan. The surface pressure fluctuations show that, for all cases, the regions with high energy are identical to the locations where the gap turbulence evolves and accounts for the impingement on the BLE. The amplitude of the tonal noise at the BPF differs between the cases.

Sub-parsec resolution cosmological simulations of star-forming clumps at high redshift with feedback of individual stars

ABSTRACT We introduce a new set of zoom-in cosmological simulations with sub-pc resolution, intended to model extremely faint, highly magnified star-forming stellar clumps, detected at z = 6.14 thanks to gravitational lensing. The simulations include feedback from individual massive stars (in both the pre-supernova and supernova phases), generated via stochastic, direct sampling of the stellar initial mass function. We adopt a modified ‘delayed cooling’ feedback scheme, specifically created to prevent artificial radiative loss of the energy injected by individual stars in very dense gas (n ∼ 103–105 cm−3). The sites where star formation ignites are characterized by maximum densities of the order of 105 cm−3 and gravitational pressures Pgrav/k >107 K cm−3, corresponding to the values of the local, turbulent regions where the densest stellar aggregates form. The total stellar mass at z = 6.14 is 3.4$\times 10^7~\rm M_{\odot }$, in satisfactory agreement with the observed stellar mass of the observed systems. The most massive clumps have masses of $\sim 10^6~\rm M_{\odot }$ and half-mass sizes of ∼100 pc. These sizes are larger than the observed ones, including also other samples of lensed high-redshift clumps, and imply an average density one orders of magnitude lower than the observed one. In the size–mass plane, our clumps populate a sequence that is intermediate between the ones of observed high-redshift clumps and local dSph galaxies.

Investigation of fouling mitigation using stationary and rotating twisted tapes

• Fouling mitigation is achieved using stationary and rotating twisted tapes. • Stationary twisted tapes offer 50% reduction in fouling resistance over plain tube. • Rotating twisted tape offered a maximum of 90% reduction in fouling resistance. • Fouled tube pressure drop is about 2–24% higher than unfouled tube. The fouling phenomenon is one of the critical issues to be solved in industrial heat exchangers, which can adversely affect the size of the heat exchanger, thermofluids performance, and operational lifetime. In this paper, a novel fouling mitigation technique using stationary and rotating twisted tapes is introduced. The particulate fouling phenomenon was experimentally studied using the accelerated fouling test. The aluminum oxide particle was used as a foulant material, and the experiments were performed for different twisted tape pitches (60 mm and 120 mm), flow rates (1.5–5 l min −1 ), and particle sizes (1.2 µm and 6.9 µm). The results showed that within the turbulent flow region, the fouling resistance decreased with an increase in flow rate. Moreover, compared to the plain tubes, a maximum of 68% and 90% reduction in fouling resistance was attained using the rotating twisted tapes for the 1.2 µm and 6.9 µm particle sizes, respectively. Compared to the tube with stationary twisted tape, the rotating twisted tape provided an additional 16% reduction in the fouling resistance for the 1.2 µm particle size. However, for both plain and twisted tape tubes, the pressure drop of the fouled tube is about 2–24% higher than the unfouled tube.

Effects of injection pressure on the flame front growth in an optical direct injection spark ignition engine

• Optical diagnostics performed at wide open throttle and 1200 rpm conditions. • Elevated injection pressure does not monotonically increase engine output. • Higher spatial variation in turbulence measured for very high injection pressure. • Local overmixing and increased mixture inhomogeneity are likely causes. • Despite decreased power, soot is still lower at very high injection pressure. The fuel injection pressure of direct injection spark ignition (DISI) petrol engines has increased significantly to improve engine efficiency and reduce air-polluting emissions. The present study performs a detailed analysis of spray penetration, burn duration, flame front vectors and turbulence intensity to provide scientific explanations about the optimal injection pressure observed for the maximum power output/efficiency in a single-cylinder optically accessible DISI engine. Five injection pressures of 5, 10, 15, 20 and 25 MPa were tested based on an equal-split double injection strategy using a side-mounted six-hole direct injector. A high-speed camera with a 20 kHz frame rate was used to record spray development, propagating petrol flame and soot luminosity. The Mie-scattering technique was used to image the fuel dispersion under varied injection pressure and to calculate spray penetration. For time-resolved, two-dimensional flame front vector extraction, a newly developed flame image velocimetry (FIV) was applied to high-speed natural combustion luminosity movies. A spatial filtering method was also used for the flow decomposition to acquire flame front turbulence intensity. A total of 100 combustion cycles were FIV processed for each test pressure to tackle the inherent cyclic variations. The spray images showed that higher fuel injection pressure leads to faster spray tip penetration and enhanced spray evaporation for better air/fuel mixing. However, the in-cylinder pressure and power output do not continue to increase with an increase in injection pressure; they peak at 15 MPa and decrease at higher injection pressure. The spatially averaged flame front vector magnitude and turbulence intensity also peak at 15 MPa, indicating the optimal injection pressure was due to the maximum turbulent flame front growth. The distribution of turbulence intensity along the flame boundary shows increased variations at injection pressure higher than 15 MPa, because local over-mixing caused low turbulent flame growth regions. Although the highest turbulent flame front growth was not measured at 25 MPa, the soot luminosity was much lower than that of 15 MPa, which explains the development trend towards higher injection pressure.

Experimental Study on the Influence of a Two-Dimensional Cosine Hill on Wind Turbine Wake

The accurate prediction of wind energy distribution on the terrain has been of great significance for wind farm selection. Therefore, this paper evaluated the influence of a two-dimensional hill with a relatively large slope (i.e., 0.83) on wind turbine wake. Firstly, the wakes on flat terrain and wind characteristics around a single hill were investigated by using a hot-wire anemometer. Subsequently, wake distributions combined with the hill were measured when the turbine was located 2D in front and on the hilltop. The results of the hill showed that a flow separation was formed within 6D of the leeward side (where D is the rotor diameter). The turbulence intensity increased initially and then decreased as height increased, leading to a high turbulence region 2.28 times the hill height, according to the experiment of a wind turbine combined with the hill. In conclusion, wake recovery was promoted on the windward side and 4D behind the hilltop. As the longitudinal coordinate increased, the intensity of the turbulence changed to a single peak, and the peak value was more than twice as high as on flat terrain. Based on this, it may be possible to optimize the design of wind turbines for better performance.

Ignition of hexane-air mixtures by highly under-expanded hot jets

We report an experimental study of ignition of flammable mixtures by highly unexpanded, supersonic hot jets. The high-pressure, hot-gas reservoir supplying the jet is created by impacting a projectile on a plunger to rapidly compress and ignite a rich n-hexane/air mixture, resulting in a peak reservoir pressure of more than 20 MPa. A locking mechanism was used to prevent the plunger from rebounding and the jet was created by rupturing a diaphragm covering a nozzle with an exit diameter between 0.25 and 1 mm. The jet development and ignition processes in the main chamber filled with hexane-air mixture were visualized using high-speed schlieren and OH* chemiluminescence imaging. The ignition threshold was determined as a function of composition in the jet and main chamber, the nozzle diameter, and the initial pressure in the main chamber. Unlike the case of subsonic jets in which ignition occurs at the shear layer near the nozzle exit, ignition of combustion in the main chamber was found to take place downstream of the Mach disk terminating the supersonic expansion and within the turbulent mixing region created by the startup of the supersonic jet. The results are interpreted using a constant-pressure, well-stirred reactor model simulating the mixing between the hot jet and cold ambient gas. The critical conditions for ignition are determined by the competition between energy release due to chemical reactions initiated by the hot jet gas and cooling due to mixing with the cold chamber atmosphere. The critical value (maximum for which ignition occurs) of the mixing rate was computed using a detailed chemical reaction model and found to be a useful qualitative guide to our observations.

Quasi-dynamic subgrid-scale kinetic energy equation model for large-eddy simulation of compressible flows

A quasi-dynamic subgrid-scale (SGS) kinetic energy one-equation eddy-viscosity model is introduced in this paper for large-eddy simulation (LES) of compressible flows. With the additional SGS kinetic energy equation, the SGS kinetic energy can be predicted properly. Then, using the dual constraints of SGS kinetic energy and the SGS kinetic energy flux, the eddy-viscosity model can be determined exactly. Taking a similar scheme as the expansion of the SGS stress, other unclosed quantities in the equations to be solved could be well modelled separately. Therefore, with the advance of the equations, all the model coefficients can be determined dynamically. Differing from the classic dynamic procedure, the new methodology needs no test filtering, and thus it could also be called a quasi-dynamic procedure. Using direct numerical simulation of compressible turbulent channel flow, the$a$ $priori$test shows that the key modelled quantities of the suggested model display high correlations with the real values. In LES of compressible turbulent channel flows of the Mach number being$1.5$and 3.0, the proposed model can precisely predict some important quantities, including the mean velocity, Reynolds stress and turbulent flux, and it can also supply more abundant turbulent structures. For the compressible flat-plate boundary layer, the new model can correctly predict the transition process, mean velocity and turbulence intensities in the turbulent region. The results show that the proposed model has the advantage of scale adaptivity. Finally, the new model is applied to LES of turbulent mixing in spherical converging Richtmyer–Meshkov instability, and the accurate results show that the new model has a good ability for LES of complex fluids.

Scalings of turbulence dissipation in space and time for turbulent channel flow

We investigate scalings of turbulence dissipation and turbulence length/time scales in the fully developed turbulent channel flow region of wall distances $y$ where the ratio of turbulence production to turbulence dissipation oscillates close to 1. First, we study averages over both time and wall-parallel streamwise ( $x$ ) and spanwise ( $z$ ) planes at $y$ . Turbulent channel flow data with friction velocity $u_{\tau }$ , and global Reynolds number $Re_{\tau }$ ranging from $550$ to $5200$ , suggest that the integral length scales of streamwise fluctuating velocities along the streamwise direction, and of wall-normal fluctuating velocities along the transverse direction, tend towards scaling with $y$ , and that the respective turbulence dissipation coefficients tend towards being constant with increasing $Re_{\tau }$ . However, the data for integral lengths of transverse fluctuating velocities in the transverse direction suggest that these lengths obey an asymptotic scaling $\sqrt {\delta y}$ (where $\delta$ is the channel half-width) with increasing $Re_{\tau }$ . The corresponding turbulence dissipation's scaling seems to tend towards $\sqrt {Re_{\tau }}/Re_{\lambda }$ , which is reminiscent of the non-equilibrium turbulence dissipation scaling found in boundary-free turbulent flows, $Re_{\lambda }$ being a $y$ -local Taylor-length-based Reynolds number. The data do not exclude minor corrections from these asymptotic scalings, and in fact, suggest finite Reynolds number deviations to them. Second, we remove time averaging and study time-fluctuating averages over wall-parallel planes at $y$ . We find that the time fluctuations of the turbulence dissipation coefficients and the Taylor-length-based Reynolds number are very strongly anti-correlated at all wall distances $y$ considered, reflecting a dominance of turbulent kinetic energy fluctuations at the lower frequencies, but a dominance of both turbulent kinetic energy and turbulence dissipation at the higher frequencies. In the case of the turbulence dissipation coefficient corresponding to the integral length of the wall-normal velocity along the transverse direction, it is possible to determine the cross-over frequency $f_c^*$ between these two behaviours, and we find $f_c^{*}\sim u_{\tau }/y$ for $Re_{\tau } = 950$ , but $f_c^* \sim u_{\tau }/\delta$ for $Re_{\tau }=2000$ , where there is evidence of very-large-scale motions.

Observation of Magnetic Reconnection in a Region of Strong Turbulence

We examine a rare and interesting observation of magnetic reconnection embedded in a large-scale region of strong turbulence in which magnetic field annihilation is energizing ions and electrons. The magnetic reconnection event is in Earth’s magnetotail and is associated with enhanced energetic particle fluxes indicating local particle acceleration. Despite substantial electric and magnetic field fluctuations throughout the surrounding, large-scale region, the ongoing magnetic reconnection has many similar properties to laminar, 2D magnetic reconnection including Hall electric fields, Hall magnetic fields, a thin electron current sheet, and ion and electron jets. Notably, the electron jet emerging from the electron diffusion region (EDR) appears to transport sufficient off-diagonal momentum to infer that off-diagonal electron stress can support the reconnection electric field in the EDR even in a turbulent environment. Although the electron jet appears to be briefly (∼1 s) deflected or possibly interrupted by an electromagnetic disturbance, the reconnection appears to otherwise continue for a long period (∼30 minutes) as evidenced by a persistent ion jet. This particular finding implies that the fundamental electron-scale processes inside of the EDR in turbulent magnetic reconnection are not necessarily distinct from those in laminar magnetic reconnection. These observations provide direct confirmation that magnetic reconnection can not only be responsible for but also can continue in regions of large-scale turbulence. Because the electric and magnetic fields of strong turbulence are linked to particle acceleration, it follows that particle acceleration also can continue as a consequence of turbulent magnetic reconnection.

Washing the air: enhanced condensation of water on aerosols to enable their removal

Previous theory (Clement 1985 Proc. R. Soc. A 398 , 307–339.) is applied to calculate water vapour condensation on aerosol in a chamber with a hot water base, a cooled roof and insulated walls. The fractional aerosol condensation of the water vapour is given by the surface condensation numbers, Cn , at the water surface and the roof. The magnitude of the aerosol condensation rate, M , per unit area of the base is then obtained from theory using heat transfer correlations. The central well-mixed turbulent region in the chamber is generally equilibrated, and maximum and minimum values of M are obtained from the limits of complete vapour condensation, and no condensation, in the boundary layers, respectively. Practically all their difference arises in the cold boundary layer where the supersaturation reaches up to 3 for no condensation. Results for M are obtained for water temperatures of 70 and 80°C and roof temperatures of 5–30°C, and range from 0.1 to 1.0 g m −2 s −1 . Chamber properties are compared to those of the Pi chamber used to investigate cloud condensation. Estimations for aerosol growth in the chamber are made for varying chamber heights and aerosol number concentrations, N . Passage of air through such a chamber could effectively grow aerosols of even the highest N up to an easily removable size range in a matter of seconds, enabling the cleaning of polluted air, including that containing viruses.

Numerical simulation study of nozzle bench mark axial velocity before sudden expansion using computational fluid dynamics

Food and drug administration (FDA) benchmark study for biomedical flow transition. An idealized medical device is presented and CFD predictions of pressure and velocity are compared against experimental measurements of pressure and velocity. The fluid flow transitions considered laminar (Re=500), transitional (Re =2000), and turbulent (Re=6500) with various turbulence fluid flow simulation models of laminar, k-omega, k-omega SST and k-epsilon based on inlet throat Reth. = 500, 2000 and 6500. Axial velocity at centreline for Reth = 500, 2000 and 6500 at line X =0, showed maximum difference of 77.4% for velocity at centerline at 0.08m and 19% for wall pressure at -0.09m sudden expansion at laminar region of Re = 500. Good agreement with simulation happened at 65.6% and 17.2% transition Re =2000. At turbulent region Re = 6500, all models were in good agreement at 49.6% velocity centerline and 8.10% pressure drop, But in laminar legion, downstream of the simulation of Reth =6500, other models disappeared which demonstrated K-epsilon model is best at higher Reynolds turbulent region. Emphatically, from 0 to -120N/m2 counterbalanced at Reth = 500 wall pressure showed negligible axial pressure gradient at centerline with drop in normalization point of experimental data.

Investigation of flow field and pollutant particle distribution in the aquaculture tank for fish farming based on computational fluid dynamics

Hydrodynamics is important for fish survival in the aquaculture tank systems, as it can directly affect the behaviour of farmed fish and the self-cleaning property of culture tank systems. The present study designs a new tank configuration, rectangular single-side arc angle tanks, to improve the performance of aquaculture vessels. The tank hydrodynamics was investigated by solving the Reynolds-Averaged Navier–Stokes (RANS) equations and combined with the discrete phase model (DPM) model to study particle motion. The velocity at the monitoring point and the discharge rate of particles from the tank were separately measured using Acoustic Doppler Velocimetry and the MATLAB image recognition method to validate the numerical model. We numerically tested the effect of culture tank configurations and the horizontal distance from the inlet pipe to the sidewall on hydrodynamics parameters, including velocity magnitude and uniformity, turbulence region and vortex distribution. Subsequently, to further understand the self-cleaning property of the culture tank systems for settleable particles, 300 cylindrical particles with a uniform size of 2.5 mm are tracked in the Lagrangian frame. Meanwhile, velocity distribution and pressure gradient have been employed to quantitatively evaluate the effects of hydrodynamics on the collection and discharge regularity of particles. Through CFD-assisted design verification, the tank configuration and the position of the inlet pipe layout have a considerable impact on the removal of particles from the tank and the energy to sustain the flow in the tank. Some key findings are found that 1) the hydrodynamics was improved significantly after rounding two right-angle edges in the rectangular culture tank, in which the configuration tanks of η = 0.2 and η = 0.3 could establish the strengthened rotational flow with better uniformity; 2) the inlet pipes proper far from the sidewall can enhance the self-cleaning property of the tank and the inlet pipe layout position of δ = 0.02–0.04 is the preferred interval.

Depth effects on the cavity induced transition at hypersonic speed by DNS

Cavity-induced transition (CIT) in a hypersonic boundary layer was simulated by employing direct numerical simulation (DNS). The upwind Roe scheme was discretized by high-order minimum dispersion and controllable dissipation (MDCD) reconstruction based on high-quality multi-block structured grids. The grid scale and time step are determined after evaluating the grid scale and nondimensional time step and comparing with the available measurements. Four depths, 1.0, 1.5, 2.0, and 2.5, multiplied by the baseline depth (D), are evaluated to determine their effects on the CIT on the wall after the cavity. The baseline is a typically “closed” cavity and the width of turbulent region at one cavity length downstream cavity (x = 2L) is 67 mm, caused by the strong adverse pressure gradient and horseshoe vortex instability. Typically, the other three are “open” cavities with no transition for the 1.5D cavity, a turbulent region width of 26 mm for the 2.0D cavity, and 45 mm for the 2.5D cavity at x = 2L were obtained after the cavity. Moreover, the relationship between the flow structures and a typical high heat flux region is explored. Reattachment supplies heat to the wall, and separation removes heat from the wall.

Two-dimensional Modeling of the Tearing-mode-governed Magnetic Reconnection in the Large-scale Current Sheet above the Two-ribbon Flare

We attempt to model magnetic reconnection during the two-ribbon flare in a gravitationally stratified solar atmosphere with the Lundquist number of S = 106 using 2D simulations. We found that the tearing mode instability leads to inhomogeneous turbulence inside the reconnecting current sheet (CS) and invokes the fast phase of reconnection. Fast reconnection brings an extra dissipation of magnetic field which enhances the reconnection rate in an apparent way. The energy spectrum in the CS shows a power law pattern and the dynamics of plasmoids govern the associated spectral index. We noticed that the energy dissipation occurs at a scale l ko of 100–200 km, and the associated CS thickness ranges from 1500 to 2500 km, which follows the Taylor scale l T = l ko S 1/6. The termination shock (TS) appears in the turbulent region above flare loops, which is an important contributor to heating flare loops. Substantial magnetic energy is converted into both kinetic and thermal energies via TS, and the cumulative heating rate is greater than the rate of the kinetic energy transfer. In addition, the turbulence is somehow amplified by TS, in which the amplitude is related to the local geometry of the TS.

Experimental research on atomization process and dust reduction performance of swirl pressure nozzle.

In this study, experimental studies on atomization process and dust reduction performance of four swirl nozzles with different inlet/outlet diameter ratio (D) were performed. The results of the atomization process study of the nozzle show that with the increase of D, the droplet breakup range of the spray field is gradually increasing, but the droplet breakup intensity of the spray field is gradually decreasing. At D = 3.33 and 3.63, droplet breakup occurs mainly in the range of 0-4mm in the strong turbulent region. At D = 3.75, droplet breakup occurs mainly in the range of 0-2mm in the strong turbulent region. At D = 3.96, droplet breakup occurs mainly in the range of 0-1mm in the strong turbulent region. Droplet breakup in the spray field at D = 3.33 and D = 3.67 was better than that at D = 3.75 and D = 3.96. From the dust reduction experimental results, the dust reduction efficiency increases and then decreases with the increase of D. The dust reduction efficiency is highest among the four nozzles at D = 3.67. Based on the dust reduction curves of four different D of nozzles, it is predicted that the optimal dust reduction condition will be achieved at D of 3.60, which provides a reference for the design and optimization of nozzles.

Tomographic Imaging Using Multi-Simultaneous Measurements (TIMes) For Single-Shot Emission Reconstructions In Turbulent Spray Flames

Tomographic imaging using multi-simultaneous measurements of emissions is applied to three operating conditions of the SpraySyn burner in order to investigate the turbulent flame morphology and its potential impact on the flame spray pyrolysis. Instantaneous and time-averaged three-dimensional fields of CH* chemiluminescence (flame front marker) and atomic Na emission from NaCl dissolved in the injected liquid solution (linked to hot products of the spray flame) are reconstructed using 29 cameras divided between them. The association between CH* and Na signals is analyzed using isosurface visualizations, combined slices, profiles and scattered plots. The instantaneous results unveil the complex reacting turbulent flow and regions of interplay between the flame front and the hot spray products. The time-averaged results show that the investigated spray flames are slightly asymmetric near the burner exit and evolve towards symmetric bell-shaped distributions further downstream. In the context of flame spray pyrolysis, the flame asymmetries potentially lead to inhomogeneous characteristics of the final synthesized material.

Real-time Wind Predictions for Safe Drone Flights in Toronto

Tall buildings in urban environments can negatively affect drone navigation, by creating turbulence and local regions of high-velocity winds. This jeopardizes the safety of the public in addition to the security of any items being transported by the drone such as medical equipment, packages, or expensive sensors. This study focuses on studying the effect of buildings located in the downtown area of the City of Toronto on the wind field. The City of Toronto was chosen due to the recent structures' development of and the increased usage of drones by hospitals and other businesses. The city geometry was divided into 20 zones and Computational Fluid Dynamics was utilized to study the effects of the local building geometry on the wind field. Wind velocities were extracted from the CFD data and coupled with historical meteorological data so that they could be compared to local weather station measurements. The coupled velocities and wind bearings from CFD were found to be within 12% and 8% RMSE respectively of the recorded data at the wind station for 85% of the sampled data. Wind velocities were then extracted at regular elevation intervals within the flight environment and coupled with meteorological weather data to generate historical, real-time, forecast, and statistical visualization of local wind fields. This study proposes a technique for obtaining real-time, historical, forecast, and statistical wind speeds and wind bearings in the drone flight environment or along a specified path. Visualization of flow fields is also presented to improve drone flight safety and efficiency.

Heat transfer enhancement of solar collector by placing wire mesh stainless porous material on the solar absorber plate of indirect forced convection solar dryer

Indirect forced convection solar dryers are a common technology in solar drying. The flat plate solar collector is an essential device that absorbs heat from solar radiation in order to heat the air flowing through. To improve the thermal utilization of this device, heat transfer enhancement by using the technique of changing airflow patterns into swirling or rotating flow was proposed in this research. In the experiment, wire mesh stainless porous material (WP) SUS304 with various porosities (X) of 0.747, 0.888 and 0.928, transverse pitch with duct height ratio (P/h) of 1.14, 1.43, and 1.71, rib height with duct height ratio (R/h) of 1, attack angle (α) of 30° was placed on the solar absorber plate and the investigation conducted under solar simulator of EN 12975–2 standard with a constant solar intensity of 780±20 W/m2. Air was used as the working fluid in the turbulent flow region at Reynolds number (Re) in the range of 3,700–30,000. The experiment results showed that Nusselt number (Nu), Friction factor (f) and Thermal enhancement factor (TEF) significantly increased when WP was placed on the absorber plate because the swirling airflow through WP occurred and disturbed the thermal boundary layer on absorber plate. The increase in Re resulted in increased Nu and decreased f. Nu and f increased with a decrease in X and P/h. The highest Nu and f of 303.01 and 5.14, respectively, were found at X = 0.747 and P/h = 1.14. The maximum TEF was 1.84. Besides, the developed correlations were in good agreement with the experimental results of predicting both Nu and f with the mean differences of 6.23% and 9.98%, respectively.

Electron Energization by High‐Amplitude Turbulent Electric Fields: A Possible Source of the Outer Radiation Belt

AbstractThe turbulent energy cascade that is characteristic of bursty bulk flow (BBF) braking regions in the Earth's magnetotail has been shown to be the energy source of large‐amplitude electric fields (>50 mV/m) which can, in turn, result in local energetic electron acceleration. These pre‐energized electrons can move inward to stronger magnetic fields being further accelerated and can eventually supply an energetic tail to electron distributions in the outer radiation belt. Using wave and plasma measurements from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites during four tail seasons from 2015 to 2019, we study the process of BBF magnetic and kinetic energy being transferred to electrons by turbulent electric fields from a statistical perspective. We identify turbulent BBF regions by the presence of high‐amplitude electric fields. We show that the high‐amplitude electric fields are associated with an increase in electron temperature by three times compared to quiet times and with a ten‐fold increase in temperature fluctuations. They are also associated with strong variations of energetic electron fluxes, indicative of local acceleration. We further discuss the implications of these findings and the role of this pre‐energized electron population in supplying the outer radiation belt.