Avi Levy - Academia.edu (original) (raw)
Papers by Avi Levy
Agriculture
Since leaf temperature (LT) is not a trivial measurement, deep-neural networks (DNN) and machine ... more Since leaf temperature (LT) is not a trivial measurement, deep-neural networks (DNN) and machine learning (ML) models were evaluated in this study as tools for estimating foliage temperature. Two DNN methods were used. The first DNN used convolutional layers, while the second DNN was based on fully-connected layers and was trained by cross-validation techniques. The machine learning used the K-nearest neighbors (KNN) method for LT estimation. All models used the meteorological and microclimatic parameters (hereafter referred to as features) of the examined greenhouses to determine the average foliage temperature. The models were trained on 75% of the collected data and tested on the remaining 25%. RMS and absolute error were used to evaluate the performance of the different models compared to the LT values measured by a thermal camera. In addition, after finding the correlation of each feature to the leaf temperature, the models were trained based on the high-correlated features onl...
Physics of Fluids
Convection over a wavy heated bottom wall in the air flow has been studied in experiments with th... more Convection over a wavy heated bottom wall in the air flow has been studied in experiments with the Rayleigh number of ∼108. It is shown that the mean temperature gradient in the flow core inside a large-scale circulation is directed upward, which corresponds to the stably stratified flow. In the experiments with a wavy heated bottom wall, we detect large-scale standing internal gravity waves (IGWs) excited in the regions with the stably stratified flow. The wavelength and the period of these waves are much larger than the turbulent spatial and time scales, respectively. In particular, the frequencies of the observed large-scale waves vary from 0.006 Hz to 0.07 Hz, while the turbulent time in the integral scale is about 0.5 s. The measured spectra of these waves contain several localized maxima that imply an existence of waveguide resonators for large-scale standing IGWs. For comparisons, experiments with convection over a smooth plane bottom wall at the same mean temperature differe...
Physics of Fluids
We perform an experimental study of transport of nanoparticles in convective turbulence with the ... more We perform an experimental study of transport of nanoparticles in convective turbulence with the Rayleigh number ∼108 in the air flow. We measure a temperature field in many locations by a temperature probe equipped with 11 E-thermocouples. Nanoparticles of the size ∼70 nm in diameter are produced by an advanced electrospray aerosol generator. To determine the number density of nanoparticles, we use a condensation particle counter. The joint action of turbulent effects and the large-scale circulations in convective turbulence, which are important in the core flow, and molecular effects, which are essential near the boundaries of the chamber, results in an effective accumulation of nanoparticles at the cold wall of the chamber. The turbulent effects are characterized by turbulent diffusion and turbulent thermal diffusion of nanoparticles, while the molecular effects are described by the Brownian diffusion and thermophoresis, as well as the adhesion of nanoparticles at the cold wall o...
An alternative utilization of a compressor or a mechanical pump in the traditional refrigeration ... more An alternative utilization of a compressor or a mechanical pump in the traditional refrigeration cycles is the diffusion absorption refrigeration (DAR) cycle operating with a bubble pump to circulate the binary working fluid. The bottle neck of the DAR cycle is the bubble pump and its performance is determined by the amount of separated refrigerant leaving the bubble pump and the circulation ratio (i.e., the ratio between the mass flow rates of separated refrigerant and rich solution). This work investigated the possibility of increasing bubble pump performance by using a set of parallel bubble pump lift tubes. A modular experimental continuous system was designed to characterize the performance of three parallel bubble pump lift tubes with an environmentally friendly binary solution of R134a-DMAC. The dependence of the number of bubble pump lift tubes and various operating conditions (i.e., refrigerant mass concentration and heat input) on the amount of the desorbed refrigerant was...
We consider a magnetohydrodynamic flow in a cylindrical vessel caused by frequency-modulated magn... more We consider a magnetohydrodynamic flow in a cylindrical vessel caused by frequency-modulated magnetic field traveling along vertical axis in order to estimate the field impact on the flow parameters. To increase the impact efficiency and intensify transfer processes, we propose to generate non-stationary structures of auto-oscillatory type in the melt against the background of a convective flow, whose parameters are connected with the parameters of the modulated electromagnetic field. The forced convective flow generated in liquid metal contributes to its homogenization and can improve the structure obtained at the melt crystallization.
Fluids, 2020
For several decades, magnetic nano- and microparticles have been used in various applications, as... more For several decades, magnetic nano- and microparticles have been used in various applications, as they can be attracted and controlled using external magnetic fields. Recently, carbonyl iron microparticles were used in a feasibility study of a new cardiac pacing application. The particles were inserted into a heart, attracted to its sidewall using a pulsating magnetic field, and applied pulsating pressure on its sidewall. The magnitude of the sidewall pressure is a critical parameter for the success and safety of the application, and it was evaluated analytically using a simplified model. In the present study, the behaviour of carbonyl iron microparticles in a water chamber was studied experimentally. Several masses of these particles were attracted to the sidewall of the chamber using an external pulsating magnetic field; the behaviours of the masses of particles, the particle–particle interaction, and the influence of fluid dynamics on them were examined during different periods o...
Proceedings of the 2nd World Congress on Mechanical, Chemical, and Material Engineering, 2016
The compressor or a mechanical pump of traditional refrigeration cycles is replaced by the bubble... more The compressor or a mechanical pump of traditional refrigeration cycles is replaced by the bubble pump that circulates the binary working fluid in the diffusion absorption refrigeration (DAR) cycle. The bottleneck of the DAR cycle is the bubble pump, and its performance is determined by the amount of separated refrigerant that leaves the bubble pump. This work investigated the influence of the structure of the generator on the amount of desorbed refrigerant in a bubble pump system, with three parallel lifting tubes. Two modular experimental systems were designed to characterize the performance of three parallel bubble pump lift tubes with an environmentally-friendly binary solution of R134a-DMAC. The difference between the systems was in the design of the generator. One generator had three inlets and three outlets, and another one had a common inlet and three outlets. All other operating conditions remained the same. The results showed that an experimental system containing three inlets to the generator, produced more refrigerant than the system containing a common inlet.
International Journal of Aerospace Innovations, 2010
A numerical study of the flow in overexpanded planar nozzles shows the existence of Mach reflecti... more A numerical study of the flow in overexpanded planar nozzles shows the existence of Mach reflection hysteresis inside an ideal nozzle while in tapered (constant angle) nozzles it does not appear. When including the geometry of the nozzle in the simulation it becomes evident that flow separation will occur before the transition from regular to Mach reflection for all relevant Mach numbers. The simulation reveals complex changes in the flow structure as the pressure ratio between the ambient and the stagnation is increased and decreased. Detailed examination of the pressure in the region of flow separation reveals an interaction between the shock cell structure and the location of the separation point. The pressure along the nozzle wall downstream of the separation point was found to be less than the ambient pressure with the affect being more pronounced in the case of the ideal nozzle. The formation of closed circulation bubbles may generate reverse flow separation. 221 Volume 2 • Number 4 • 2010 Figure 1. Similarity in flow structure between wedge flow (left) and nozzle flow (right). ISincident shock, RS-reflected shock, MS-Mach stem, JB-jet boundary, EFexpansion fan, SSslipstream [7]
Transport in Porous Media, 2005
A development is provided showing that for any phase, by not neglecting the macroscopic terms of ... more A development is provided showing that for any phase, by not neglecting the macroscopic terms of the deviation from the intensive momentum and of the dispersive momentum, we obtain a macroscopic secondary momentum balance equation coupled with a macroscopic dominant momentum balance equation that is valid at a larger spatial scale. The macroscopic secondary momentum balance equation is in the form of a wave equation that propagates the deviation from the intensive momentum while concurrently, in the case of a Newtonian fluid and under certain assumptions, the macroscopic dominant momentum balance equation may be approximated by Darcy's equation to address drag dominant flow. We then develop extensions to the dominant macroscopic Navier-Stokes (NS) equation for saturated porous matrices, to account for the pressure gradient at the microscopic solid-fluid interfaces. At the microscopic interfaces we introduce the exchange of inertia between the phases, accounting for the relative fluid square velocities and the rate of these velocities, interpreted as Forchheimer terms. Conditions are provided to approximate the extended dominant NS equation by Forchheimer quadratic momentum law or by Darcy's linear momentum law. We also show that the dominant NS equation can conform into a nonlinear wave equation. The one-dimensional numerical solution of this nonlinear wave equation demonstrates good qualitative agreement with experiments for the case of a highly deformable elasto-plastic matrix.
Applied Thermal Engineering, 2009
Predicted heat transfer coefficients (HTC) are widely used for calculations of various heat trans... more Predicted heat transfer coefficients (HTC) are widely used for calculations of various heat transfer processes. Various empirical and semi-empirical models of non-dimensional groups are used for calculating the heat transfer coefficients. The common accuracy of the predicted heat transfer coefficient is usually about ±25%; however, the accuracy is failing by the inaccurate predictions of the fluid properties. In the present study the heat transfer coefficient of subcooled organic mixture chlorodifluoromethane (R22)dimethylacetamide (DMAC) were predicted. While the solution's pressure-temperature-concentration, densitytemperature-concentration and viscosity-temperatureconcentration relations were previously measured in our laboratory and the heat capacity were calculated from the enthalpy of the solution, the only unknown property that was required for predicting the HTC was the mixture thermal conductivity. Various correlations and mixing rules for the mixture thermal conductivity were adopted for predicting the heat transfer coefficient. In order to validate the predicted heat transfer coefficients, an experimental system was designed, built and successfully operated. The heat transfer coefficient was measured experimentally and compared with the predicted one. Based on this comparison the best fitted thermal conductivity for the working fluid (R22-DMAC) is recommended. As a result, the deviations of the predicted heat transfer coefficients, obtained by the well-known Dittus-Boelter equation, from the experimental values were less than 15%.
Advanced Powder Technology, 2003
By modifying Grif th's theory, a theoretical fatigue model was built to explain how the fatigue s... more By modifying Grif th's theory, a theoretical fatigue model was built to explain how the fatigue strength of a single particle changes under repeated compressions. The predictions of the model were validated by experimental results for two kinds of crystal particles: NaCl and MgO. We also performed experiments on granular particles whose properties were unknown. Here too the fatigue trends were similar to that of the model. The results show that as the compression stresses acting on the particles and the number of compressions increase, the fatigue compression strength decreases. In addition, fatigue trends we observed for the various particles demonstrate dependence on the material's properties.
Physics of Fluids
It has been previously shown [Elperin et al., “Formation of large-scale semi-organized structures... more It has been previously shown [Elperin et al., “Formation of large-scale semi-organized structures in turbulent convection,” Phys. Rev. E 66, 066305 (2002)] that a non-rotating turbulent convection with nonuniform large-scale flows contributes to the turbulent heat flux. As a result, the turbulent heat flux depends explicitly not only on the gradients of the large-scale temperature, but also on the gradients of the large-scale velocity. This is because the nonuniform large-scale flows produce anisotropic velocity fluctuations, which modify the turbulent heat flux. This effect causes an excitation of a convective-wind instability and formation of large-scale semi-organized coherent structures (large-scale convective cells). In the present study, we perform mean-field numerical simulations of shear-free convection, which take into account the modification of the turbulent heat flux by nonuniform large-scale flows. We use periodic boundary conditions in horizontal direction as well as s...
Proceedings of the International Thermal Science Seminar Bled. Volume 1
Chemical Engineering Science
Abstract This paper presents a detailed investigation into the modeling of the total bend pressur... more Abstract This paper presents a detailed investigation into the modeling of the total bend pressure drop (TBPD) for all three orientations: horizontal-horizontal (H-H), horizontal-vertical (H-V), and vertical-horizontal (V-H) in dilute phase flow. Different types of materials were fed with a screw feeder and conveyed through two different sets of galvanized steel pipeline. The TBPD (all orientations) is the summation of the pressure drop due to the momentum change (reacceleration), the air only flow along the bend, and a function of the pressure drop due to the particles, which contributes to the bend only. A qualitative comparison was performed for blind-T and a radius bend (R/D = 10) using a particular material property. This comparison helped in obtaining a comprehensive understanding of TBPD.
Biosystems Engineering
This study examines the feasibility of using semi-transparent, flexible organic photovoltaic (OPV... more This study examines the feasibility of using semi-transparent, flexible organic photovoltaic (OPV) modules as greenhouse shading material. By using such modules, it may be possible to utilise existing greenhouse-based agricultural areas for electricity production. Using OPV modules to shade greenhouses and reduce excess solar energy may result in reduced heat load on the crop on the one hand, and use of renewable energy on the other. We examined the radiometric and thermal properties of an OPV module. Module transmissivity was measured under outdoor conditions at four different angles of radiation incidence: 0, 21, 41 and 46. Simultaneously, the open-circuit voltage, and short-circuit current of the module were recorded for power and efficiency calculations. Supplementary laboratory measurements of transmissivity, reflectivity and absorptivity were performed with a spectroradiometer. To further characterise the OPV module, its overall heat-transfer coefficient (U value) was determined. The examined module had about 20% transmissivity, 15% reflectivity and 65% absorptance in the photosynthetically active radiation (PAR) range. The mean daily power conversion efficiency of the module was about 0.8% and the overall heat transfer coefficient U, was about 6.0 Wm À2 K À1. The temperature of a module placed on the polyethylene cover of a greenhouse high tunnel was about 50e55 C at midday. Thermal images of the module revealed non-uniform heat distribution, with temperature differences between regions reaching up to 7.5 C. OPV modules appear to be suitable for greenhouse shading and electricity generation but currently they are too expensive and their life duration is relatively short.
Powder Technology
Abstract The behavior of particles in dilute phase pneumatic conveying is a rather complex phenom... more Abstract The behavior of particles in dilute phase pneumatic conveying is a rather complex phenomenon. For example, particle velocities and their trajectories owing to particle-particle and particle-wall collisions are unpredictable. In this study, we focus on particle velocity reduction in horizontal-vertical (H-V) and vertical-horizontal (V-H) bends. A new terminology, ‘bend point’, is introduced to compare different bends on common measures. A broad range of materials (Archimedes numbers = 102 to 106) was investigated using six different bends (blind-T, R/D = 1.5, 4.5, 6.6, 10, and 20) and wide range of conveying velocities (10 m/s
Israel Journal of Psychiatry and Related Sciences, 1986
APA PsycNET Our Apologies! - The following features are not available with your current Browser c... more APA PsycNET Our Apologies! - The following features are not available with your current Browser configuration. - alerts user that their session is about to expire - display, print, save, export, and email selected records - get My ...
Powder Technology, 1998
Tile effect o1' a bend on tile distribution of particles ill a pipe cross section and segregation... more Tile effect o1' a bend on tile distribution of particles ill a pipe cross section and segregation in pneumatic conveying systems has been investigated numerically. The numerical model solved tile linite-volume equation,,, for the conservation of mass and momentum for two phases. The predictions of the numerical model were compared qualitatively with various published experimental data. It was evident that the crosssectional concentration of the particles a few metres after a bend is not uniform and that the panicles tend to concentrate around the pipe wall. Various cross-sectional concentrations of particles were fimnd for different pipe-to-bend radius ratios, particle sizes and direction of gravity (i.e.. horizontal-to-vertical flow. and horizontal-to-horizontal flow). Based on the different cross-sectional concentrations fl)r different particle sizes, it was concluded that the paths taken by the particles after the bend were strongly dependent upon their sizes. Since a real paniculate phase (i.e.. powder, granular material, etc.) is composed of particles vdth a range of sizes, this would lead to segregation of the panicles in the pipe section tollowing a bend. Hence. a better understanding of the segregation process in a pneumatic conveying system was obtained.
Agriculture
Since leaf temperature (LT) is not a trivial measurement, deep-neural networks (DNN) and machine ... more Since leaf temperature (LT) is not a trivial measurement, deep-neural networks (DNN) and machine learning (ML) models were evaluated in this study as tools for estimating foliage temperature. Two DNN methods were used. The first DNN used convolutional layers, while the second DNN was based on fully-connected layers and was trained by cross-validation techniques. The machine learning used the K-nearest neighbors (KNN) method for LT estimation. All models used the meteorological and microclimatic parameters (hereafter referred to as features) of the examined greenhouses to determine the average foliage temperature. The models were trained on 75% of the collected data and tested on the remaining 25%. RMS and absolute error were used to evaluate the performance of the different models compared to the LT values measured by a thermal camera. In addition, after finding the correlation of each feature to the leaf temperature, the models were trained based on the high-correlated features onl...
Physics of Fluids
Convection over a wavy heated bottom wall in the air flow has been studied in experiments with th... more Convection over a wavy heated bottom wall in the air flow has been studied in experiments with the Rayleigh number of ∼108. It is shown that the mean temperature gradient in the flow core inside a large-scale circulation is directed upward, which corresponds to the stably stratified flow. In the experiments with a wavy heated bottom wall, we detect large-scale standing internal gravity waves (IGWs) excited in the regions with the stably stratified flow. The wavelength and the period of these waves are much larger than the turbulent spatial and time scales, respectively. In particular, the frequencies of the observed large-scale waves vary from 0.006 Hz to 0.07 Hz, while the turbulent time in the integral scale is about 0.5 s. The measured spectra of these waves contain several localized maxima that imply an existence of waveguide resonators for large-scale standing IGWs. For comparisons, experiments with convection over a smooth plane bottom wall at the same mean temperature differe...
Physics of Fluids
We perform an experimental study of transport of nanoparticles in convective turbulence with the ... more We perform an experimental study of transport of nanoparticles in convective turbulence with the Rayleigh number ∼108 in the air flow. We measure a temperature field in many locations by a temperature probe equipped with 11 E-thermocouples. Nanoparticles of the size ∼70 nm in diameter are produced by an advanced electrospray aerosol generator. To determine the number density of nanoparticles, we use a condensation particle counter. The joint action of turbulent effects and the large-scale circulations in convective turbulence, which are important in the core flow, and molecular effects, which are essential near the boundaries of the chamber, results in an effective accumulation of nanoparticles at the cold wall of the chamber. The turbulent effects are characterized by turbulent diffusion and turbulent thermal diffusion of nanoparticles, while the molecular effects are described by the Brownian diffusion and thermophoresis, as well as the adhesion of nanoparticles at the cold wall o...
An alternative utilization of a compressor or a mechanical pump in the traditional refrigeration ... more An alternative utilization of a compressor or a mechanical pump in the traditional refrigeration cycles is the diffusion absorption refrigeration (DAR) cycle operating with a bubble pump to circulate the binary working fluid. The bottle neck of the DAR cycle is the bubble pump and its performance is determined by the amount of separated refrigerant leaving the bubble pump and the circulation ratio (i.e., the ratio between the mass flow rates of separated refrigerant and rich solution). This work investigated the possibility of increasing bubble pump performance by using a set of parallel bubble pump lift tubes. A modular experimental continuous system was designed to characterize the performance of three parallel bubble pump lift tubes with an environmentally friendly binary solution of R134a-DMAC. The dependence of the number of bubble pump lift tubes and various operating conditions (i.e., refrigerant mass concentration and heat input) on the amount of the desorbed refrigerant was...
We consider a magnetohydrodynamic flow in a cylindrical vessel caused by frequency-modulated magn... more We consider a magnetohydrodynamic flow in a cylindrical vessel caused by frequency-modulated magnetic field traveling along vertical axis in order to estimate the field impact on the flow parameters. To increase the impact efficiency and intensify transfer processes, we propose to generate non-stationary structures of auto-oscillatory type in the melt against the background of a convective flow, whose parameters are connected with the parameters of the modulated electromagnetic field. The forced convective flow generated in liquid metal contributes to its homogenization and can improve the structure obtained at the melt crystallization.
Fluids, 2020
For several decades, magnetic nano- and microparticles have been used in various applications, as... more For several decades, magnetic nano- and microparticles have been used in various applications, as they can be attracted and controlled using external magnetic fields. Recently, carbonyl iron microparticles were used in a feasibility study of a new cardiac pacing application. The particles were inserted into a heart, attracted to its sidewall using a pulsating magnetic field, and applied pulsating pressure on its sidewall. The magnitude of the sidewall pressure is a critical parameter for the success and safety of the application, and it was evaluated analytically using a simplified model. In the present study, the behaviour of carbonyl iron microparticles in a water chamber was studied experimentally. Several masses of these particles were attracted to the sidewall of the chamber using an external pulsating magnetic field; the behaviours of the masses of particles, the particle–particle interaction, and the influence of fluid dynamics on them were examined during different periods o...
Proceedings of the 2nd World Congress on Mechanical, Chemical, and Material Engineering, 2016
The compressor or a mechanical pump of traditional refrigeration cycles is replaced by the bubble... more The compressor or a mechanical pump of traditional refrigeration cycles is replaced by the bubble pump that circulates the binary working fluid in the diffusion absorption refrigeration (DAR) cycle. The bottleneck of the DAR cycle is the bubble pump, and its performance is determined by the amount of separated refrigerant that leaves the bubble pump. This work investigated the influence of the structure of the generator on the amount of desorbed refrigerant in a bubble pump system, with three parallel lifting tubes. Two modular experimental systems were designed to characterize the performance of three parallel bubble pump lift tubes with an environmentally-friendly binary solution of R134a-DMAC. The difference between the systems was in the design of the generator. One generator had three inlets and three outlets, and another one had a common inlet and three outlets. All other operating conditions remained the same. The results showed that an experimental system containing three inlets to the generator, produced more refrigerant than the system containing a common inlet.
International Journal of Aerospace Innovations, 2010
A numerical study of the flow in overexpanded planar nozzles shows the existence of Mach reflecti... more A numerical study of the flow in overexpanded planar nozzles shows the existence of Mach reflection hysteresis inside an ideal nozzle while in tapered (constant angle) nozzles it does not appear. When including the geometry of the nozzle in the simulation it becomes evident that flow separation will occur before the transition from regular to Mach reflection for all relevant Mach numbers. The simulation reveals complex changes in the flow structure as the pressure ratio between the ambient and the stagnation is increased and decreased. Detailed examination of the pressure in the region of flow separation reveals an interaction between the shock cell structure and the location of the separation point. The pressure along the nozzle wall downstream of the separation point was found to be less than the ambient pressure with the affect being more pronounced in the case of the ideal nozzle. The formation of closed circulation bubbles may generate reverse flow separation. 221 Volume 2 • Number 4 • 2010 Figure 1. Similarity in flow structure between wedge flow (left) and nozzle flow (right). ISincident shock, RS-reflected shock, MS-Mach stem, JB-jet boundary, EFexpansion fan, SSslipstream [7]
Transport in Porous Media, 2005
A development is provided showing that for any phase, by not neglecting the macroscopic terms of ... more A development is provided showing that for any phase, by not neglecting the macroscopic terms of the deviation from the intensive momentum and of the dispersive momentum, we obtain a macroscopic secondary momentum balance equation coupled with a macroscopic dominant momentum balance equation that is valid at a larger spatial scale. The macroscopic secondary momentum balance equation is in the form of a wave equation that propagates the deviation from the intensive momentum while concurrently, in the case of a Newtonian fluid and under certain assumptions, the macroscopic dominant momentum balance equation may be approximated by Darcy's equation to address drag dominant flow. We then develop extensions to the dominant macroscopic Navier-Stokes (NS) equation for saturated porous matrices, to account for the pressure gradient at the microscopic solid-fluid interfaces. At the microscopic interfaces we introduce the exchange of inertia between the phases, accounting for the relative fluid square velocities and the rate of these velocities, interpreted as Forchheimer terms. Conditions are provided to approximate the extended dominant NS equation by Forchheimer quadratic momentum law or by Darcy's linear momentum law. We also show that the dominant NS equation can conform into a nonlinear wave equation. The one-dimensional numerical solution of this nonlinear wave equation demonstrates good qualitative agreement with experiments for the case of a highly deformable elasto-plastic matrix.
Applied Thermal Engineering, 2009
Predicted heat transfer coefficients (HTC) are widely used for calculations of various heat trans... more Predicted heat transfer coefficients (HTC) are widely used for calculations of various heat transfer processes. Various empirical and semi-empirical models of non-dimensional groups are used for calculating the heat transfer coefficients. The common accuracy of the predicted heat transfer coefficient is usually about ±25%; however, the accuracy is failing by the inaccurate predictions of the fluid properties. In the present study the heat transfer coefficient of subcooled organic mixture chlorodifluoromethane (R22)dimethylacetamide (DMAC) were predicted. While the solution's pressure-temperature-concentration, densitytemperature-concentration and viscosity-temperatureconcentration relations were previously measured in our laboratory and the heat capacity were calculated from the enthalpy of the solution, the only unknown property that was required for predicting the HTC was the mixture thermal conductivity. Various correlations and mixing rules for the mixture thermal conductivity were adopted for predicting the heat transfer coefficient. In order to validate the predicted heat transfer coefficients, an experimental system was designed, built and successfully operated. The heat transfer coefficient was measured experimentally and compared with the predicted one. Based on this comparison the best fitted thermal conductivity for the working fluid (R22-DMAC) is recommended. As a result, the deviations of the predicted heat transfer coefficients, obtained by the well-known Dittus-Boelter equation, from the experimental values were less than 15%.
Advanced Powder Technology, 2003
By modifying Grif th's theory, a theoretical fatigue model was built to explain how the fatigue s... more By modifying Grif th's theory, a theoretical fatigue model was built to explain how the fatigue strength of a single particle changes under repeated compressions. The predictions of the model were validated by experimental results for two kinds of crystal particles: NaCl and MgO. We also performed experiments on granular particles whose properties were unknown. Here too the fatigue trends were similar to that of the model. The results show that as the compression stresses acting on the particles and the number of compressions increase, the fatigue compression strength decreases. In addition, fatigue trends we observed for the various particles demonstrate dependence on the material's properties.
Physics of Fluids
It has been previously shown [Elperin et al., “Formation of large-scale semi-organized structures... more It has been previously shown [Elperin et al., “Formation of large-scale semi-organized structures in turbulent convection,” Phys. Rev. E 66, 066305 (2002)] that a non-rotating turbulent convection with nonuniform large-scale flows contributes to the turbulent heat flux. As a result, the turbulent heat flux depends explicitly not only on the gradients of the large-scale temperature, but also on the gradients of the large-scale velocity. This is because the nonuniform large-scale flows produce anisotropic velocity fluctuations, which modify the turbulent heat flux. This effect causes an excitation of a convective-wind instability and formation of large-scale semi-organized coherent structures (large-scale convective cells). In the present study, we perform mean-field numerical simulations of shear-free convection, which take into account the modification of the turbulent heat flux by nonuniform large-scale flows. We use periodic boundary conditions in horizontal direction as well as s...
Proceedings of the International Thermal Science Seminar Bled. Volume 1
Chemical Engineering Science
Abstract This paper presents a detailed investigation into the modeling of the total bend pressur... more Abstract This paper presents a detailed investigation into the modeling of the total bend pressure drop (TBPD) for all three orientations: horizontal-horizontal (H-H), horizontal-vertical (H-V), and vertical-horizontal (V-H) in dilute phase flow. Different types of materials were fed with a screw feeder and conveyed through two different sets of galvanized steel pipeline. The TBPD (all orientations) is the summation of the pressure drop due to the momentum change (reacceleration), the air only flow along the bend, and a function of the pressure drop due to the particles, which contributes to the bend only. A qualitative comparison was performed for blind-T and a radius bend (R/D = 10) using a particular material property. This comparison helped in obtaining a comprehensive understanding of TBPD.
Biosystems Engineering
This study examines the feasibility of using semi-transparent, flexible organic photovoltaic (OPV... more This study examines the feasibility of using semi-transparent, flexible organic photovoltaic (OPV) modules as greenhouse shading material. By using such modules, it may be possible to utilise existing greenhouse-based agricultural areas for electricity production. Using OPV modules to shade greenhouses and reduce excess solar energy may result in reduced heat load on the crop on the one hand, and use of renewable energy on the other. We examined the radiometric and thermal properties of an OPV module. Module transmissivity was measured under outdoor conditions at four different angles of radiation incidence: 0, 21, 41 and 46. Simultaneously, the open-circuit voltage, and short-circuit current of the module were recorded for power and efficiency calculations. Supplementary laboratory measurements of transmissivity, reflectivity and absorptivity were performed with a spectroradiometer. To further characterise the OPV module, its overall heat-transfer coefficient (U value) was determined. The examined module had about 20% transmissivity, 15% reflectivity and 65% absorptance in the photosynthetically active radiation (PAR) range. The mean daily power conversion efficiency of the module was about 0.8% and the overall heat transfer coefficient U, was about 6.0 Wm À2 K À1. The temperature of a module placed on the polyethylene cover of a greenhouse high tunnel was about 50e55 C at midday. Thermal images of the module revealed non-uniform heat distribution, with temperature differences between regions reaching up to 7.5 C. OPV modules appear to be suitable for greenhouse shading and electricity generation but currently they are too expensive and their life duration is relatively short.
Powder Technology
Abstract The behavior of particles in dilute phase pneumatic conveying is a rather complex phenom... more Abstract The behavior of particles in dilute phase pneumatic conveying is a rather complex phenomenon. For example, particle velocities and their trajectories owing to particle-particle and particle-wall collisions are unpredictable. In this study, we focus on particle velocity reduction in horizontal-vertical (H-V) and vertical-horizontal (V-H) bends. A new terminology, ‘bend point’, is introduced to compare different bends on common measures. A broad range of materials (Archimedes numbers = 102 to 106) was investigated using six different bends (blind-T, R/D = 1.5, 4.5, 6.6, 10, and 20) and wide range of conveying velocities (10 m/s
Israel Journal of Psychiatry and Related Sciences, 1986
APA PsycNET Our Apologies! - The following features are not available with your current Browser c... more APA PsycNET Our Apologies! - The following features are not available with your current Browser configuration. - alerts user that their session is about to expire - display, print, save, export, and email selected records - get My ...
Powder Technology, 1998
Tile effect o1' a bend on tile distribution of particles ill a pipe cross section and segregation... more Tile effect o1' a bend on tile distribution of particles ill a pipe cross section and segregation in pneumatic conveying systems has been investigated numerically. The numerical model solved tile linite-volume equation,,, for the conservation of mass and momentum for two phases. The predictions of the numerical model were compared qualitatively with various published experimental data. It was evident that the crosssectional concentration of the particles a few metres after a bend is not uniform and that the panicles tend to concentrate around the pipe wall. Various cross-sectional concentrations of particles were fimnd for different pipe-to-bend radius ratios, particle sizes and direction of gravity (i.e.. horizontal-to-vertical flow. and horizontal-to-horizontal flow). Based on the different cross-sectional concentrations fl)r different particle sizes, it was concluded that the paths taken by the particles after the bend were strongly dependent upon their sizes. Since a real paniculate phase (i.e.. powder, granular material, etc.) is composed of particles vdth a range of sizes, this would lead to segregation of the panicles in the pipe section tollowing a bend. Hence. a better understanding of the segregation process in a pneumatic conveying system was obtained.