The effect of the generation and handling in the acquired electrostatic charge in airborne particles

The measurement of the charge distribution in laboratory generated aerosols particles was carried out. Four cases of electrostatic charge acquisition by aerosol particles were evaluated. In two of these cases, the charges acquired by the particles were naturally derived from the aerosol generation procedure itself, without using any additional charging method. In the other two cases, a corona charger and an impact charger were utilized as supplementary methods for charge generation. Two types of aerosol generators were used in the dispersion of particles in the gas stream: the vibrating orifice generator TSI model 3450 and the rotating plate generator TSI model 3433. In the vibrating orifice generator, a solution of methylene blue was used and the generated particles were mono-dispersed. Different mono-aerosols were generated with particle diameters varying from 6.0 × 10^− 6 m to 1.4 × 10^− 5 m. In the rotating plate generator, a poly-dispersed phosphate rock concentrate with Stokes mean diameter of 1.30 × 10^− 6 m and size range between 1.5 × 10^− 7 m and 8.0 × 10^− 6 m was utilized as powder material in all tests. In the tests performed with the mono-dispersed particles, the median charges of the particles varied between − 3.0 × 10^− 16 C and − 5.0 × 10^− 18 °C and a weak dependence between particle size and charge was observed. The particles were predominantly negatively charged. In the tests with the poly-dispersed particles the median charges varied fairly linearly with the particle diameter and were negative. The order of magnitude of the results obtained is in accordance with data reported in the literature. The charge distribution, in this case, was wider, so that an appreciable amount of particles were positively charged. The relative spread of the distribution varied with the charging method. It was also noticed that the corona charger acted very effectively in charging the particles.     

Characterizing the effect of substrate surface roughness on particle-wall interaction with the airflow method

The effect of surface roughness on particle-wall interaction was studied by the airflow method. Five kinds of monodispersed spherical particles (D_p50 = 11-41 μm) and six test pieces with different surface roughness (Ra = 0.01-1.64 μm) were used in the experiments. The particles were dispersed on the test pieces to form a monolayer, and entrained in a rectangular air channel. The air velocity increased at a constant rate, and the entrained particles were detected with a laser dust monitor. Microscopic observations showed that particle entrainment occurred in discrete and intermittent events during experiment, thus a statistical parameter, i.e. the particle entrainment efficiency as a function of the air velocity, was defined for evaluating the particle-wall interaction force distribution. The experimental results showed that the air velocity for particle entrainment decreases with the increase of the surface roughness within submicron-scale and reaches a lower limit, while increases to some extent for micron-scale surface roughness. It was also found that the effect of the substrate surface roughness depends on the particle diameter.     

Investigation of the intra-particle sintering kinetics of a mainly agglomerated alumina powder by using surface area reduction

An alumina precursor was prepared by the aluminium sulphate (0.20 M) and excess urea reaction in boiling aqueous solution. The precursor was calcined at 900 °C for 2 h and then δ-Al₂O₃ powder having volumetric agglomeration degree of 80% was obtained. Cylindrical compacts having diameter of 14 mm were prepared under 32 MPa by axial pressing using oleic acid as binder. Each compact was fired isothermally at various temperatures between 950 and 1400 °C. The firing time was changed from zero to 2 h. The fired compacts were examined by scanning electron microscopy (SEM) and nitrogen adsorption techniques. The specific surface areas (S/m² g^− 1) of the samples were calculated using the Brunauer, Emmett, and Teller (BET) procedure. The rate constant (k) and mechanism-characteristic parameter (n) were obtained for different temperatures between 950 °C and 1150 °C from the application of the neck-growth sintering rate (NGRM) model on the surface area reduction data. An Arrhenius equation and the parameter n for the sintering were found in the forms of k = (7.648 × 10⁶ h^− 1) exp (− 186,234 J mol^− 1 / RT) and n = 4.0 × 10^− 7 T³-1.7 × 10^− 3 T² + 2.3 T − 1030.8 respectively. The parameter n changes in the interval 0.61 <  n < 1.34 with rising temperature having maximum at about 1025 °C. Based on the SEM images and NGRM data, the intra-particle sintering was discussed.     

Prediction of single particle settling velocities through liquid fluidized beds

Single particle settling velocities through water fluidized beds of mono-sized glass spheres (d_p = 0.645, 1.20, 1.94, 2.98 and 5 mm in diameter) were studied experimentally using a column, 40 mm in diameter. The settling spherical particles (D_p = 10 and 19.5 mm) had different densities (1237 to 8320 kg/m³), while the settling particles (D_p = 5 and 2.98 mm) were glass spheres. The pseudo-fluid model, which considers a liquid fluidized bed as a homogenous pseudo-fluid, predicts single particle settling velocities quite well if the ratio D_p/d_p is larger than about 10. With decreasing ratio D_p/d_p, the overall friction between the settling particle and the fluidized media increases. A method for predicting single particle settling velocities through a liquid fluidized bed is proposed and discussed. Following the approach of Van der Wielen et al. [L.A.M. Van der Wielen, M.H.H Van Dam, K.C.A.M. Van Luyben, On the relative motion of a particle in a swarm of different particles, Chem. Eng. Sci. 51 (2006) 995-1008], the overall friction is decomposed into a particle-fluid and a particle-particle component. The effective buoyancy force is calculated using the transition function proposed by Ruzicka [M.C. Ruzicka, On buoyancy in dispersion, Chem. Eng. Sci. 61 (2006) 2437-2446]. A simple model for predicting the collision force is proposed, as well as a correlation for the collision coefficient. The mean absolute deviation between the experimental and calculated slip velocities was 5.08%.     

Evaluation of flow properties of toner powder using conical rotor

In order to accurately evaluate the dynamic flow properties of toner powder, a new rotary shearing tester with a conical rotor was developed. This instrument was equipped with an automatic pressing system to compress toner powder. The tester could simultaneously measure torque and compression load during the intrusion and rotation of the conical rotor into the same packed toner powder. The optimum rotational speed and intrusion rate of the conical rotor for the characterization of the flow properties of toner powder were discussed based on test results; their values were calculated as 0.017 s^− 1 and 0.083 mm/s, respectively. The torque of toner powder changed in proportion to the cube of the depth of intrusion in the toner powder bed. The surfaces of toner powder samples prepared from polymer resin and carbon pigment particles were coated with fine particles (SiO₂, TiO₂) under a different condition. The flow characteristics of toner powder with a different particle surface were evaluated based on the relationship between the shearing torque and the void fraction of packed toner. In the present case, the Rumpf model was applied to estimate the shearing force H at the contact point between two particles of toner powder. The value of H for toner powder with a rough particle surface, which was covered with fine particles (SiO₂, TiO₂), was 41 nN, while that for toner powder with a smooth particle surface, which was not covered with fine particles, was 357 nN. Further the effects of the particle shape of the toner on the torque of toner powder after compression under the same conditions were investigated. The torque of toner powder decreased with an increase in circularity.     

Small capacity decay of lithium iron phosphate (LiFePO4) synthesized continuously in supercritical water: Comparison with solid-state method

Nanosize lithium iron phosphate (LiFePO₄) particles are synthesized using a continuous supercritical hydrothermal synthesis method at 25 MPa and 400 °C under various flow rates. The properties of LiFePO₄ synthesized in supercritical water including purity, crystallinity, atomic composition, particle size, surface area and thermal stability are compared with those of particles synthesized using a conventional solid-state method. Smaller size particles ranging 200-800 nm, higher BET surface area ranging 6.3-15.9 m² g⁻¹ and higher crystallinity are produced in supercritical water compared to those of the solid-state synthesized particles (3-15 μm; 2.4 m² g⁻¹). LiFePO₄ synthesized in supercritical water exhibit higher discharge capacity of 70-80 mAh g⁻¹ at 0.1 C after 30 cycles than that of the solid-state synthesized LiFePO₄ (60 mAh g⁻¹), which is attributed to the smaller size particles and the higher crystallinity. Smaller capacity decay at from 135 to 125 mAh g⁻¹ is observed during the 30 cycles in carbon-coated LiFePO₄ synthesized using supercritical water while rapid capacity decay from 158 to 140 mAh g⁻¹ is observed in the carbon-coated LiFePO₄ synthesized using the solid-state method.     

On the spinel formation in Co1 − xO/Co2TiO4 composites via reactive sintering, exsolution and oxidation

The formation mechanism and microstructural development of the spinel phases in the Co_1 − xO/Co₂TiO₄ composites upon reactive sintering the Co_1 − xO and TiO₂ powders (9:1 molar ratio) at 1450 °C and during subsequent cooling in air were studied by X-ray diffraction and analytical electron microscopy. The Co₂TiO₄ spinel occurred as inter- and intragranular particles in the matrix of Ti-doped Co_1 − xO grains with a rock salt-type structure during reactive sintering. The submicron sized Co₂TiO₄ particles were able to detach from grain boundaries in order to reach an energetically favorable parallel orientation with respect to the host Co_1 − xO grains via a Brownian-type rotation/coalescence process. Upon cooling in air, secondary Co₂TiO₄ nanoparticles were precipitated and the Ti-doped Co_1 − xO host was partially oxidized as Co_3 − δO₄ spinel by rapid diffusion along the {1 1 1} and {1 0 0}-decorated interphase interface and the free surface of the composites.     

Hydrogen oxidation kinetics on model Pd/C electrodes: Electrochemical impedance spectroscopy and rotating disk electrode study

This work reports on the kinetics of the hydrogen oxidation reaction (HOR) on model Pd nanoparticles supported on a low surface area carbon substrate. Two Pd/C samples, with the average particle size 2.6 and 4.0 nm were used. The structure of the catalysts was characterized with the ex situ (electron microscopy) and in situ (electrochemical) methods. We utilized the electrochemical impedance spectroscopy (EIS) and the rotating disk electrode (RDE) voltammetry to study the kinetics of the HOR on Pd/C. The relevance of these techniques for elucidating the kinetics and the mechanism of the HOR on Pd/C was explored. The experimental results suggest that the catalytic activity of Pd in the HOR is more than 2 orders of magnitude lower than that of Pt, and does not depend on the particle size in the range from 2.6 to 4.0 nm. Computational modeling of the experimental steady-state (RDE) and non-steady-state (EIS) data shows that the reaction kinetics can be adequately described within Heyrovsky-Volmer mechanism, with the rate constants υ_0H = (8.8 ± 1.5) × 10⁻¹⁰ mol cm⁻² s⁻¹ and υ_0V = (1.0 ± 0.3) × 10⁻⁸ mol cm⁻² s⁻¹. The model suggests that underpotentially deposited hydrogen H_UPD is unlikely to be the active intermediate H_ad of the HOR. It is concluded that the surface coverage of H_ad deviates from that of H_UPD with increasing overpotential, and the lateral interactions within H_ad adlayer are weak.     

Preparation and characterization of carbon-coated LiFePO4 cathode materials for lithium-ion batteries with resorcinol-formaldehyde polymer as carbon precursor

LiFePO₄/C composites were synthesized by two methods using home-made amorphous nano-FePO₄ as the iron precursor and soluble starch, sucrose, citric acid, and resorcinol-formaldehyde (RF) polymer as four carbon precursors, respectively. The crystalline structures, morphologies, compositions, electrochemical performances of the prepared powders were investigated with XRD, TEM, Raman, and cyclic voltammogram method. The results showed that employing soluble starch and sucrose as the carbon precursors resulted in a deficient carbon coating on the surface of LiFePO₄ particle, but employing citric acid and RF polymer as the carbon precursors realized a uniform carbon coating on the surface of LiFePO₄ particle, and the corresponding thicknesses of the uniform carbon films are 2.5 nm and 4.5 nm, respectively. When RF polymer was used as the carbon precursor, the material showed the highest initial discharge capacity (138.4 mAh g^− 1 at 0.2 C at room temperature) and the best rate performance among the four materials.     

Well-dispersed surfactant-stabilized Pt/C nanocatalysts for fuel cell application: Dispersion control and surfactant removal

A simple surfactant-stabilized method was investigated for the preparation of well-dispersed platinum nanoparticles supported on carbon black (Pt/C), using 3-(N, N-dimethyldodecylammonio) propanesulfonate (SB12) as the stabilizer. First, TEM analysis demonstrated that Pt dispersion can be improved by the increase of molar ratio of SB12 to Pt precursor. Moreover, pH environment plays a crucial role in Pt dispersion, and the optimal dispersion with an average Pt particle size of 2.2 nm was obtained under neutral or slightly alkaline environment. Pt dispersion mechanism was shown to involve the electrosteric stabilization of Pt nanoparticles by the zwitterionic surfactant SB12, which is highly pH-dependent. At pH ≥ 7, a stable electrosteric repulsion exists between the Pt particles covered by SB12, where the positively charged part is adsorbed on the particle surface and the negatively charged part (SO₃⁻) and the bulky alkyl chain (C₁₂H₂₅) are pointed away from particles. At pH < 7, protons H⁺ directly interact with Pt particles or SO₃⁻ groups of SB12, resulting in the destruction of the electrosteric stabilization and the following agglomeration of Pt nanoparticles. Furthermore, XPS and cyclic voltammetry showed that the surfactant on Pt particles can be efficiently removed by ethanol wash without any destruction on the dispersion and particle size of Pt, when compared to heat treatment and centrifugation. Electrochemical measurements showed that the ethanol-washed pH-controlled Pt/C catalyst has higher electrochemical surface area and catalytic performance than the commercial one.     

Density measurement of fine aerosol fractions from wood combustion sources using ELPI distributions and image processing techniques

Aerosols from combustion sources are of high concern since they present a risk for health and environment. Particle size distribution of aerosols and in particular number size distribution are easily and quickly obtained using an Electrical Low Pressure Impactor (ELPI). However, this technique is depending of aerosol density; ρ, which may lead to biased particle size distributions. Aerosol density from combustion sources is usually not well known and depends on several parameters. Aerosol density cannot be measured with usual methods since there is generally not enough matter collected on each stage of the ELPI. Our approach uses electronic microscopy to evaluate ρ at each impaction stage in order to increase the accuracy of the number size distributions resulting from the ELPI measurements.Particles were collected on glass substrates deposited on each impaction stages. Images were obtained using a scanning electron microscope and image processing tools were applied.This method was first tested with silica particles resulting from a combustion process which have a constant density found to be comprised between 2.2 and 2.4 g cm⁻³ for stages 2 (57 and 95 nm) and 3 (95 and 158 nm), respectively. Once validated, this method was used to determine the density of wood combustion aerosols. The results match well for fly ashes from wood combustion with densities varying from 1.1 to 3.0 g cm⁻³ for particles of mean equivalent diameter ranging from 69 to 157 nm, respectively.     

Diffusional mass transfer model for the adsorption of food dyes on chitosan films

The adsorption kinetics of erythrosine B and indigo carmine on chitosan films was studied by a diffusional mass transfer model. The experimental curves were obtained in batch system under different conditions of stirring rate (80–200 rpm) and initial dye concentration (20–100 mg L⁻¹). For the model development, external mass transfer and intraparticle diffusion steps were considered and the specific simplifications were based on the system characteristics. The proposed diffusional mass transfer model agreed very well with the experimental curves, indicating that the surface diffusion was the rate limiting step. The external mass transfer coefficient (k_f) was dependent of the operating conditions and ranged from 1.32 × 10⁻⁴ to 2.17 × 10⁻⁴ m s⁻¹. The values of surface diffusion coefficient (D_s) increased with the initial dye concentration and were in the range from 0.41 × 10⁻¹⁴ to 22.90 × 10⁻¹⁴ m² s⁻¹. The Biot number ranged from 17.0 to 478.5, confirming that the intraparticle diffusion due to surface diffusion was the rate limiting step in the adsorption of erythrosine B and indigo carmine on chitosan films.     

Influence of silica nanoparticles added to an organosilane film on carbon steel electrochemical and tribological behaviour

The electrochemical behaviour and tribological properties of carbon steel coated with bis-[trimethoxysilylpropyl]amine (BTSPA) filled with SiO₂ were evaluated. The silane film filled with SiO₂ was prepared by adding different SiO₂ concentrations. The electrochemical behaviour of the coated steel was mainly evaluated by means of open-circuit potential (E_OC), electrochemical impedance spectroscopy (EIS) and polarization curves, in 0.1 mol L⁻¹ NaCl solution. Structural and morphological characterizations were made by optical, electron and atomic force microscopy (AFM). E_OC and EIS data showed that sample filled with 300 ppm SiO₂ presented the highest E_OC and total impedance value. AFM measurements showed a homogeneous particle distribution of SiO₂ particles. Nanohardness measurements showed SiO₂ promoted an increase of the hardness mean value (1.70 ± 0.11 GPa to non-filled BTSPA and 2.21 ± 0.05 GPa for sample filled with 300 ppm SiO₂). Silane films when filled with SiO₂ particles improved the corrosion resistance of the steel substrate. The optimum SiO₂ particles concentration in silane solution is 300 ppm SiO₂. Incorporation of an extra amount of silica into BTSPA film led to degradation of the corrosion protection of the film to the substrate.     

Limestone particle attrition and size distribution in a small circulating fluidized bed

Limestone particle attrition was investigated in a small circulating fluidized bed reactor at temperatures from 25 to 850 °C, 1 atm pressure and superficial gas velocities from 4.8 to 6.2 m/s. The effects of operating time, superficial gas velocity and temperature were studied with fresh limestone. No calcination or sulfation occurred at temperatures ?580 °C, whereas calcination and sulfation affected attrition at 850 °C. Increasing the temperature (while maintaining the same superficial gas velocity) reduced attrition if there was negligible calcination. Attrition was high initially, but after ∼24 h, the rate of mass change became constant. The ratio of initial mean particle diameter to that at later times increased linearly with time and with (U_g − U_mf)², while decreasing exponentially with temperature, with an activation energy for fresh limestone of −4.3 kJ/mol. The attrition followed Rittinger’s surface theory [Beke B. Comminution. Budapest: Akademiai Kiado, 1964; Ray YC, Jiang TS, Wen CY. Particle attrition phenomena in a fluidized bed. Powder Technol 1987a; 49:193-206]. The change of surface area of limestone particles was proportional to the total excess kinetic energy consumed and to the total attrition time, whereas the change of surface area decreased exponentially with increasing temperature. At 850 °C, the attrition rate of calcined lime was highest, whereas the attrition rate was lowest for sulfated particles. When online impact attrition was introduced, the attrition rate was about an order of magnitude higher than without impacts.     

Structure & strength of silica-PDMS nanocomposites

Silica nanoparticles having specific surface area (SSA) 50-300 m² g⁻¹ were admixed into vinyl-terminated dimethylsiloxy monomer with a dual asymmetric centrifuge (planetary mixer) and cured to form PDMS-based nanocomposites containing up to 12 vol% SiO₂. Thin sections of cured nanocomposites obtained by a cryostate-microtome were analyzed by TEM while small and ultra small angle X-ray scattering (U/SAXS) was used to determine nanocomposite structure: filler primary particle, aggregate (chemically or sinter-bonded particles) and agglomerate (physically-bonded particles) size as a function of mixing duration and filler concentration. More aggregated silicas with higher SSA exhibited denser crosslinking than less aggregated ones regardless of crosslinker content as determined by swelling nanocomposites in toluene at equal filler content. The nanocomposite strength was determined by tensile tests (Young's modulus and elongation at break). Consistent with “bound rubber” theory, the Young's modulus of the nanocomposites increased non-linearly with increasing filler volume fraction.     

Formation enthalpy and magnetic properties of Bi-YIG powders

The formation enthalpy and magnetic properties of bismuth-doped yttrium iron garnet powders were investigated. The formation enthalpy and the crystallization temperature both decreased with increasing bismuth substitution for Bi_xY_3−xFe₅O₁₂ (0.25 ≦ x ≦ 1.25) powders prepared by the coprecipitation process. Bi substitution for Y can significantly reduce crystallization temperature for bismuth-doped yttrium iron garnet powders, and the magnetic properties (saturation magnetization, remanence, and coercive force) are independent of Bi substitution amounts. The average particle size has been determined by the specific surface area. As Bi substitution for Y increased, the average particle size also increased, while the specific surface area decreased.     

Influence of composite LiCl-KCl molten salt on microstructure and electrochemical performance of spinel Li4Ti5O12

A series of spinel Li₄Ti₅O₁₂ samples were synthesized via a composite molten-salt method (CMSM) using the mixtures of LiCl and KCl with different L values (L is defined as the molar ratio of LiCl:KCl) as the reaction media. It is found that the melting point of the composite molten salt can effectively influence the formation of particles, and leads to different electrochemical performances of the as-prepare Li₄Ti₅O₁₂. The investigations of X-ray diffraction (XRD), particle size distribution (PSD), Brunauer-Emmet-Teller (BET) surface area, and scanning electron microscopy (SEM) indicate that the as-prepared Li₄Ti₅O₁₂ with L = 1.5 is a pure phase, and has uniform homogeneous octahedral shape particles, rather narrow PSD, and high BET surface area. Electrochemical tests show that the optimized Li₄Ti₅O₁₂ with L = 1.5 has an initial discharge capacity of 169 mAh g⁻¹ and an initial charge-discharge efficiency of 94% at 0.2 C rate, and achieves good rate performances from 0.2 C to 5 C.     

The role of textural characteristics and oxygen-containing surface groups in the supercapacitor performances of activated carbons

It is suggested that the specific capacitance C₀ of activated carbons at low current densities (d ∼ 1 mA cm⁻²) consists, to a good first approximation, of two contributions. For the H₂SO₄ electrolyte they correspond to approximately 0.080 F m⁻² from the total accessible surface area and an additional pseudo-capacitance of 63 F mmol⁻¹ from the surface species generating CO in thermally programmed desorption (TPD). The new correlation proposed here is an alternative to Shi's earlier approach which considered contributions from the microporous and the external surface areas. Furthermore, it appears that the variation of the specific capacitance C at high current densities d (up to 100-150 mA cm⁻²) depends essentially on the CO₂-generating surface groups and on 1/L₀, the inverse of the average micropore width.     

The nucleation, crystallization and dispersion behavior of PET-monodisperse SiO2 composites

To more accurately investigate the nucleation, crystallization and dispersion behaviors of silica particles in polymers, the composites of PET with monodisperse SiO₂-PS core-shell structured particles were prepared with SiO₂ size from 380 nm to 35 nm.For these SNPET samples, DSC results showed that the nucleation rate of silica particles increased as their size decreased, in which 35 nm SiO₂ particles produced the most obvious nucleation effect. At 2.0 wt.% load of 35 nm silica, Avrami equation proved that the isothermal crystallization rate G of SNPET was ca. 30% higher than that of pure PET and the crystallization activation energy for SNPET was −218.7 kJ mol⁻¹ lower than −196.1 kJ mol⁻¹ for PET. While, the non-isothermal crystallization ΔE for SNPET was −199.8 kJ mol⁻¹ lower than −185.5 for PET.On non-isothermal crystallization, Jeziorny equation presented the primary and secondary crystallization stages in PET and SNPET, in which nano SiO₂ accelerated the crystallization rate. Their Ozawa number m was from 2.1 to 2.7, which was smaller than that of Avrami number n.The nucleation and dispersion behaviors of SiO₂ particles were directly observed. POM results demonstrated that SNPET samples crystallized more quickly from melt and their crystallization rate increased as silica load increases but accelerated at 2-3 wt.%. The spherulites grew well in PET but their size was smaller in SNPET due to the silica barrier on their growth. SEM and TEM observed the homogeneous silica dispersion morphology and the vivid ordered patterns formed in SNPET. The monodisperse particles are highly expected to give more accurate and valuable references than multi-scale ones in obtaining novel advanced PET composites.     

Heat transfer in a pulsed bubbling fluidized bed

Surface-to-bed heat transfer and pressure measurements were carried out in a 0.17 m ID pulsed bubbling fluidized bed with glass bead and silica sand particles having mean diameters ranging from 37 μm to 700 μm to investigate the effects of flow pulsation on heat transfer and bed hydrodynamics. A solenoid valve was used to supply pulsed air to the bed at 1 to 10 Hz. The bed surface was found to oscillate with the frequency of pulsation, the oscillation's amplitude decreasing with frequency. The standard deviation of the bed pressure drop in the pulsed bed was found to be larger than that in the conventional bed due to the acceleration force imposed by pulsation. For both Geldart B and A particles, high frequency pulsation (7, 10 Hz) enhances the heat transfer compared to continuous flow, the enhancement diminishing with superficial gas velocity and particle size. For Geldart B particles, the effect of pulsation on heat transfer ceases around U_o/U_mf = 3.5, whereas 24% improvement in heat transfer coefficient was obtained for 60 μm glass bead particles (Group A) at superficial gas velocities as high as U_o/U_mf = 27. Furthermore, in the fixed bed (U_o/U_mf < 1) for Geldart B particles, 1 Hz pulsation was found to be very effective resulting in two- to three-fold increase in heat transfer coefficient compared to continuous flow at the same superficial gas velocity. The flow pulsation loses its effect on heat transfer with increasing static bed height, i.e., when H_bed/D > 0.85.