Fountain Region Research Articles

Analyze the influence of nozzle shape and orientation on inline arrangement of multiple jets impingement on a concave surface

ABSTRACT The present study reports the thermal behavior of multiple jets on a hot concave surface with different nozzle geometries such as circular and elliptical and different orientation. Tests are performed with five different nozzles at different values of nondimensional nozzle to surface distance (z/d = 2–10) and Reynolds number (5000–28,000). Here, the parameter coefficient of variation (COV) is used to estimate the non-uniformity in the local Nu variation. For N-2, the value of CO V y , non − dim is found to be 50–60% higher compared to N-3 at z/d = 2; while at z/d ≥ 8, for N-3, the value of CO V y , non − dim is found to be 10–15% higher compared to N-2. The higher temperature uniformity ( CO V x , non − dim ) is noted in the fountain region compared to impingement region for the circular and elliptical nozzles. The thermal performance of elliptical jets is found to be sensitive toward the jet orientation at lower z/d values. The magnitude of temperature non-uniformity decreases with the increase in the z/d values. Among the tested nozzles, Nozzle N-2 consistently demonstrates superior performance across all nozzle-to-plate distances and with an increase in the z/d value improvement with nozzle N-2 reduces.

Experiment, CFD simulation and field synergy characteristics analysis of hot-air drying process in a spouted bed

Combining experimental and simulation methods, the hot-air drying process in the spouted bed was studied from the perspective of heat and mass transfer and multiphase physical field synergy. Based on the experimental system, the rate of drying curve was determined, and a drying prediction model was established and verified. The analysis of the drying characteristics shows that the heat and mass transfer in the spouted bed mainly occurred in the center of the spouting and fountain region, followed by the annulus region, and finally on both sides of the fountain region. The dimensionless parameter σ that simultaneously considered the synergy angle of velocity-velocity gradient and velocity-pressure gradient indicated that the gas drag was mainly concentrated near the gas inlet. The synergy coefficient SC of the velocity-temperature-concentration field was more affected by the variation coefficient of the velocity-concentration field and less affected by the variation coefficient of the velocity-temperature field.

Effect of Relative Jet Temperature in Supersonic Dual-Impinging Jets

Short takeoff and vertical landing aircraft configurations involve multiple jets that impinge onto the deck surface in tandem and lead to several adverse effects. This study reports on the experimental characterization of supersonic dual-impinging jets by systematically varying their relative jet temperature. A sonic converging and Mach 1.5 converging–diverging (CD) nozzles are employed. The expansion ratio of the converging nozzle is maintained at 0.96, 1.19, and 1.59, and the CD nozzle is operated at a fixed nozzle pressure ratio of 3. The temperature ratio of the jet from the CD nozzle is varied from 1.0, 1.3, and 1.7. For a fixed momentum of the jet pair, an increase in jet temperature intensified the nearfield noise and unsteadiness on the impingement surface. At short impingement heights, resonance in the heated jet was the primary source of unsteadiness. At a fixed impingement height, an increase in jet temperature led to a systematic increase in tonal frequency, while jet instability mode shapes were retained. Furthermore, the mean flowfield of sonic jet and fountain regions remain unaffected, and an increase in supersonic jet velocity is observed. This is also accompanied by an increase in unsteadiness in the fountain upwash.

Two-fluid simulation of spout-fluid bed with multiple chambers: Flow structure, hydrodynamic characteristics and heat transfer

Spout–fluid beds are widely applied in the process industry due to their advantages of efficient contact between particles and gas. In this paper, a TFM (two-fluid model) is set up to simulate the hydrodynamics and heat transfer of a 3D spout-fluid bed with single, double and triple chambers. Simulation results show that scale-up by aligning multiple chambers is complex in the regimes with low SF (spout-fluid ratio) due to chamber-chamber interaction. In the regimes with high SF , however, scale-up is simple because chamber-chamber interaction is hardly noticed. With increasing SF , gas and particle fluxes on inter-chamber interfaces prefer to occur in the annulus regions instead of in the fountain region. The kinetic heat flux of the solid phase has a similar preference. The molecular heat flux of the solid phase always prefers to pass through the inter-chamber interface in the annulus region. The preference however weakens with increasing SF .

Effect of Nozzle Spacing in Supersonic Twin Impinging Jets

The study of supersonic twin impinging jets is of significant interest to scientists and engineers for the fundamental understanding of flow physics and its applications. The present experimental investigation examines flow and acoustics characteristics of Mach 1.5 twin impinging jets as a function of internozzle spacing. The global flowfield was visualized using the shadowgraph technique, and mean pressures in the jet impingement and fountain region were measured using a pressure scanner. In addition, near-field acoustic measurements were performed in two different directions relative to the nozzle exit, and the velocity field was measured using planar particle image velocimetry. The results indicate that changing the internozzle spacing alters the fountain-flow strength, while the jet impingement characteristics remain relatively unchanged. The broadband and discrete high-amplitude tones are a function of nozzle spacing and show unique directivity. The velocity distributions agree well with the acoustic measurements and are a strong function of nozzle spacing.

CFD-DEM study of mitigation of alternating spout deflection in a spout fluidized bed: A geometry perspective

Alternating spout deflection is a common instability problem in spout-fluidized beds. Geometrical modification to the reactor is a feasible way to mitigate the deflection. In this work, three kinds of geometrical designs are studied to mitigate the alternating spout deflection by the CFD-DEM approach. Design I is to add baffles at the top of two annulus regions. The results demonstrate that this design can significantly reduce the amplitude of spout deflection by reducing the stamping of falling particle flow on the annulus regions. Design II is to install two zigzag-like baffles in the fountain region, which is expected to disperse the particle distribution in the fountain region. However, this solution leads to non-alternating spout deflection, which is undesirable in the application. Design III is to modify the slot angle of the reactor bottom. The results show that 30° slot angle increases the frequency of the alternating spout deflection but rarely affects its amplitude. Non-alternating spout deflection occurs with 45° slot angle. This work benefits the hydrodynamics stability in spout fluidized beds.

Effect of operating conditions on the hydrodynamics in fountain confined conical spouted beds

Spouted bed stability and operation is greatly affected by particle features. Accordingly, the hydrodynamic behaviour of conical spouted beds has been studied for fine particles differing in size and density in a wide range of inlet air flow rates. This knowledge is essential for a successful scaling up and industrial implementation of the spouted bed. Therefore, the effect air velocity and solid properties (density and size) have on local solid velocity has been ascertained in a fountain confined conical spouted bed using a borescope technique (Particle Tracking Velocimetry, PTV) applied to several bed configurations. The results show a close relationship between the inlet air velocity and the local solid velocity, with the gas-solid contact being especially vigorous in the configurations without draft tube and with the open-sided draft tube. The solid circulation flow rate is lowest when a nonporous draft tube is used due to the low solid vertical velocities in the annulus, even at high air flow rates. Nevertheless, vertical velocities in the annular zone increase when particle size and density are increased, although these velocities are lower in the spout and fountain regions due to the higher momentum exchange required for their acceleration.

Pyrolysis of plastic wastes in a fountain confined conical spouted bed reactor: Determination of stable operating conditions

The performance of both fluidized and spouted bed reactors in the pyrolysis of waste plastics is conditioned by particle agglomeration phenomena, which worsens the quality of the gas–solid contact and eventually lead to defluidization. The objective of this work is to determine the optimum conditions for stable operation (without defluidization) in a bench scale plant fitted with a fountain confined conical spouted bed reactor and equipped with a nonporous draft tube, which operates in continuous mode. The insertion of these devices enhances the gas–solid contact, especially in the fountain region, and leads to a highly stable hydrodynamic regime, with these features being of especial relevance for the in situ catalytic pyrolysis of waste plastics. This paper deals with the effect different variables have on the minimum temperature for stable operation by avoiding defluidization. The variables analyzed are as follows: plastic type (HDPE, LDPE, PP, PS, PET and PMMA), plastic feed rate, mass of inert material in the bed, spouting velocity and use of catalyst. The results show that polymers whose chains decompose at low temperatures or have high degrees of branching require low operating temperatures. Besides, as the ratio of bed mass to plastic feed rate (Wbed/Qplastic) and/or spouting velocity were increased, the temperature required to avoid defluidization was also reduced. The use of a catalyst also reduced the temperature required for stable operation, as the activation energy of cracking reactions is greatly reduced, and so reaction rate is increased.

Particle-scale evaluation of the pyrolysis process of biomass material in a reactive gas-solid spouted reactor

Biomass pyrolysis is numerically studied in a spouted bed reactor. Thermophysical processes with complex reactions are considered for gas and solid phases under Eulerian and Lagrangian frameworks. After thorough model validations with experimental measurements, particle-scale evaluation of sand and biomass species combined with the segregation phenomenon is presented. The results show that the axial segregation significantly affects the spatial distribution of particle-scale information in three distinct regions. Particle heat transfer coefficient (HTC) has the largest value in the spout region, and then fountain region. A continuous decrease of particle HTC along the bed height appears. Particles close to the spout-annulus interface have a comparatively smaller temperature. Different distribution patterns of the HTC for sand and biomass can be observed. The increase of pyrolysis temperature raises the biomass temperature and HTC, while the effect of gas velocity is negligible. The dispersion intensity of biomass species is obviously larger than that of sand particles. A smaller sand size results in a smaller temperature, but a larger HTC, and a reduced dispersion coefficient.

Particle-scale evaluation of the biomass steam-gasification process in a conical spouted bed gasifier

Based on the multiphase particle-in-cell approach, the steam gasification of biomass in a lab-scale spouted bed gasifier is simulated to explore the particle-scale transport behavior in three distinct regions of the bed. The numerical results are firstly validated with the experimental data, followed by assessing the particle-scale features of sand and biomass species in the gasifier. The results demonstrate that biomass particles in three regions of spouted bed behave with different constituent content, heat transfer coefficient and temperature. The intensive heat transfer occurs in the spout region and fountain region. Biomass particles in the spout have a small temperature, large mass, small carbon fraction, and large volatile fraction. Heterogeneous reactions of the biomass particles mainly occur in the lower part of the fountain region but can be negligible in the spout and annulus region. The water-gas reaction is two-order of magnitude faster than the methanation reaction and Boudouard reaction. The spatial distribution of particle-scale level information of both particle species is nonuniform due to the presence of three regions and biomass accumulation. The horizontal and vertical dispersion coefficient of both sand and biomass species are at the scale of 10 −4 m 2 /s and 10 −2 m 2 /s, respectively. Spatial distribution of biomass particles colored with the carbon and volatile content in the spouted bed • Particle-scale transport behavior of biomass in spouted bed gasifier revealed. • Most intensive heat transfer occurs in the spout region. • Heterogeneous reactions of biomass mainly occur in the lower part of the fountain. • Operating parameters influence the biomass particles released from the bed outlet.

Numerical investigation of gas thermal property in the gasification process of a spouted bed gasifier

Steam-biomass gasification in a three-dimensional lab-scale spouted bed gasifier is numerically simulated via the multiphase particle-in-cell approach. The spatial distribution of gas thermal properties (i.e., temperature, conductivity, specific heat capacity, density, and viscosity) together with the impact of operating parameters are comprehensively studied. The results show that distinct flow hydrodynamics in the three regions of the spouted bed gasifier lead to different spatial distributions of gas thermal properties. The gas thermal properties show significant transitions in the spout-annulus and fountain-annulus interfaces. Elevating the bed height decreases the gas temperature, density, and viscosity but increases the gas thermal conductivity and specific heat capacity. Among all operating parameters, the bed temperature exerts the most significant influence on the spatial distribution of gas thermal properties. Due to the existence of dead zone in the annulus region of the bed, bed height, biomass diameter, and steam to biomass ratio exert negligible influences on the gas thermal properties in the dense annulus region but noticeable impacts in the dilute fountain region. Based on the relationship between solid transportation and gas thermal property variations, the current work also provides a guideline for the optimization of spouted bed gasifiers, for example, the dead zone is recommended to be optimized by adjusting the inclination angle of the conical part to enhance the solid internal circulation and gasification performance.

Effects of liquid content and surface tension on fluidization characteristics in a liquid-containing gas–solid fluidized bed: A CFD–DEM study

The fluidization process within gas–solid fluidized bed reactor can be intensified by adding liquid, as it can enhance mass and heat transfer between gas phase and solid phase. With the help of computational fluid dynamics(CFD) and discrete element methods(DEM), fluidization characteristics are investigated at different liquid contents(V^) and surface tension(Bog). To be specific, the bed expansion ratio, particle concentration and particle velocity profiles are compared and analyzed. Numerical simulations show that Bog should be less than 5 at V^ = 0.013. At this recommended range, liquid addition makes interparticle force increase and has little effect on fluidization performance. From transient particle agglomeration behavior, the Bog increase is more likely to enhance interparticle force, resulting in obvious particle agglomerations during fluidization process. The variation of stagnation zone height can be reflected from the abrupt increase of particle velocity in annulus region. It can be found that V^ increase reduces particle velocity at critical height but Bog increase is more likely to cause larger stagnation region. By analyzing the distribution of spout, annulus and fountain regions at different V^ and Bog, the annulus region expands with V^ increase below the initial bed height while the spout region expands with Bog increase.

A Numerical and Experimental Study of Staggered Submerged Liquid Jet Arrays Using Variable Angle Discharge Manifolds

Increasing numbers of power electronics modules are becoming standard in both commercial and military equipment. In order to function reliably, these technologies require a dedicated and dynamic cooling system, such as liquid jet impingement. In a jet array, the spent fluid from upstream jets interacts with the downstream jets, degrading their performance. In this study, in order to counteract this effect, an expanding manifold, with a larger area for flow downstream, was used to allow the spent fluid from upstream jets to be diverted, reducing degradation of the heat transfer coefficients downstream. A series of numerical and experimental studies of liquid jet impingement utilizing water as the working fluid were performed to examine the heat transfer rate in staggered jet arrays. The simulations performed examined manifold angles between 0° and 10°, jet Reynolds numbers between 5600 and 14 000, and pitches of 2.5, 3, 4, 4.5, and 6 nozzle diameters. The experimental technique features a unique method of translation in two orthogonal axes to provide experimentally measured 2-D maps of heat transfer coefficient. The simulations, modeled in ANSYS Fluent, revealed details of the complicated interaction between the jets, their fountain regions, and the crossflow. The angled manifold systems had greater temperature uniformity and increased heat transfer coefficients compared to systems with constant area.

Effect of number of round jets on impingement heat transfer from a heated cylinder

The details of an experimental study carried out on a single jet and multiple jets emerging from round nozzles of diameter, d and impinging over a uniformly heated cylinder of diameter, D are presented. The number of jets are varied from one to six in the parametric study. In the multi-jet configurations, the jets are arranged circumferentially around the heated cylinder. The range of Reynolds number, Red considered is 5000 – 20,000. The curvature ratio, D/d is varied from 5.1 to 20.4 and the nozzle-to-cylinder spacing, H/d is varied from 2 to 12. The present results show that the Nusselt number at the stagnation point (Nustag) is not affected when the number of jets is increased. However, in multi-jet configurations with four and six jets, the interaction between the wall jets increases heat transfer in the fountain regions. The number of round jets required for the uniform cooling increases with increase in the curvature of the target cylinder, D/d. Based on the experimental study, correlations are given for the stagnation point Nusselt number (Nustag) and average Nusselt number (Nuavg).

Experimental investigation of fountain height in a shallow rectangular spouted bed using digital image analysis

The formation of a fountain is a unique phenomenon of spouted beds, which plays a significant role in the reactor design as well as the physical and chemical efficiency of the whole system. The present work investigates the fountain height (Hf) behavior in a rectangular spouted bed (30.2 mm × 101.6 mm) using four different kinds of particles classified in both Geldart groups D (Nylon and Alumina) and B (Glass beads (GB) and High density polyethylene (HDPE)), which is intended as a gap-bridging attempt, given the fact that most of the published fountain height studies were conducted in the cylindrical bed geometry. First, with the determination of Hf by systematic image processing methods, the effects of different variables including, superficial gas velocity (Ug), nozzle size (Deqn), and initial static bed height (H0) on Hf of the Geldart D and B particles are addressed. Secondly, because of the intrinsic periodical behavior of the absolute fountain height (Hfa, i.e. the vertical distance between the fountain surface to the bed bottom), the Fast Fourier Transform (FFT) is applied to characterize the variation of the Hfa for the four particle types. The period of particles’ entry into the spout and the “residence time” of particles in the fountain region are proposed as two possible factors that affect the fountain frequency. Finally, correlations to predict the Hf for both Geldart D and B particles are developed respectively, based on the discussion of different influence factors and frequency analysis. The correlation developed for Geldart B particles indicates that the effect of the nozzle size (Di) can be neglected when the ratio of particle size (dp) to nozzle size (Di) is less than 0.1. The predicted values show good agreement with the experimental data in the present study. In addition, the employment of the equivalent cross-sectional area diameters of the nozzle and rectangular column, namely Di and Dc, ensures the potential extension of the application of the developed correlations to cylindrical spouted beds.

Evaluating the new mechanistic scale-up methodology of gas-solid spouted beds using gamma ray computed tomography (CT)

In this work, we implemented the gamma-ray computed tomography (CT) technique to evaluate the new mechanistic scale-up methodology of gas-solid spouted beds by locally measuring the time-averaged cross-sectional distribution of solids and gas holdups, as well as their radial profiles along the bed height. The new scale-up methodology of geometrically similar beds is based on maintaining a similar radial profile of the gas holdup at a height within the bed, to achieve local and global similarity in dimensionless hydrodynamic parameters since the gas dynamic dictates the hydrodynamics of the gas-solid spouted bed. Two sizes of spouted beds (with diameters of 0.076 m and 0.152 m) were used with conditions that provided close magnitudes and trends of radial profiles of gas holdup; and with conditions that provide mismatches in magnitudes of the gas holdup radial profiles. The results clearly show the validity of the new method for scale-up, where the solids and gas holdup cross-sectional distributions and profiles are close to each other at all the heights and in the three regions (spout, annulus and fountain) of spouted beds, when the radial profiles of the gas holdup are close at one height between the two studied beds. In addition, the CT results clearly identified three regions mentioned above (spout, annulus, and fountain regions). In the spout region, the solids holdup increases along the spout height. When the radial profiles of gas holdups are different in two spouted beds, the differences are noticeable in local solids and gas holdups in all the heights, which further endorse the new mechanistic scale-up methodology.

Spouting behavior of binary particle mixtures of different densities: Fluid dynamics and particle segregation

Numerous industrial applications of spouted beds involve a mixture of granular particles, where differences in particle size, density, and shape can cause particle segregation, affecting the quality of flow and decreasing product homogeneity. The aim of this study was to investigate the particle segregation and fluid dynamics of a conical spouted bed operating with a binary mixture of particles of different densities. Using glass beads and polyethylene particles with a diameter of 4mm, we determined the effects of the mass fraction of denser particles and static bed height on the minimum spouting condition and mixture index. The pressure drop and airflow in the minimum spouting condition increased with an increase in bed height or jetsam concentration. Regarding particle segregation, the jetsam particles tended to concentrate near the bottom, at the spout–annulus interface because they have a shorter trajectory than the lighter particles in the fountain region. Jetsam-rich mixtures with high initial static bed heights exhibited more efficient particle mixing, excluding those at the bottom of the bed.

Investigation of hydrodynamics and heat transfer in pseudo 2D spouted beds with and without draft plates

Abstract In the present study, hydrodynamics and gas to particle heat transfer in pseudo two dimensional spouted beds (2DSB) with and without draft plates were investigated using the Eulerian-Eulerian approach. The main objective of the study was to provide an understanding of effects of the presence of draft plates on the hydrodynamics and heat transfer behavior of solid particles in the spouted beds. To validate the model, the predicted mean particle vertical velocity at the bed axis, the lateral profiles of vertical particle velocity at different bed heights for both systems, and the particle velocity vector fields in the beds were compared with the experimental measurements. A close agreement between the CFD results and the experimental data was found for both systems. The simulation results showed that the particle volume fraction in the spout and fountain regions of the spouted bed with draft plates is considerably lower than that in a conventional spouted bed (without draft plates). Simulation results also showed significant differences between the temperature distributions of gas and solid phases in spouted beds with and without draft plates.

Assessment of a conical spouted with an enhanced fountain bed for biomass gasification

This study pursues the development and characterization of a fountain enhanced spouting regime. This novel gas solid-contact method combines the advantages of fountain confined conical spouted beds with those of draft tube conical spouted beds. The aim of confining the fountain, and therefore attaining a clearly differentiated regime, is to progress towards a highly efficient conical spouted bed reactor for biomass gasification. Accordingly, and in order to delve into the knowledge of the regimes attained in this contact method, a study has been conducted by analyzing the influence operating parameters (temperature, gas flow rate, particle size and bed mass) and draft tube geometry (tube diameter and entrainment zone height) have on hydrodynamics. Fountain confinement allows greatly enlarging the fountain region, especially the height, which improves the contact between reacting gases and the catalyst. Moreover, the residence time distribution, and therefore the average residence time, may be optimized by confining and enlarging the fountain zone. These features promote tar cracking and so increase biomass conversion efficiency, which are highly relevant facts for use of conical spouted bed reactors in gasification.

Investigation of cross-sectional gas-solid distributions in spouted beds using advanced non-invasive gamma-ray computed tomography (CT)

The successful operation and safety of the Very-High-Temperature Nuclear Reactors (VHTR) extremely depend on the quality of the TRISO nuclear fuel coated particles. Hence, the fuel coating technology of TRISO particles, based on chemical vapor deposition (CVD) process, performed in gas-solid spouted beds, is of utmost importance. However, the deposition of the coating layers surrounding the kernel is a delicate process, and impacted by the hydrodynamics of spouted beds. Therefore, in this work, we have applied an advanced non-invasive gamma-ray computed tomography (CT) technique for the first time to investigate the hydrodynamics of spouted beds. In particular, we study the effect of particle density, particle size, bed size, and superficial gas velocity on the gas-solid cross-sectional distributions of spouted beds. The color distributions of the cross-sectional images clearly identified the three regions of spouted beds: the spout, the annulus, and the fountain regions. Interesting results and findings are presented, discussed and analyzed in the article. For example, the results demonstrated that the summation that operating spouted beds at stable spouting state would lead to achieving uniform coating layers of the particles in the TRISO fuel coating process is not adequate. Further, it has been found that increasing the superficial gas velocity much higher above the minimum spouting velocity increases the gas holdup in the annulus above the gas holdup value at the loss-packed bed state, contrary to common assumptions presented in the literature. This study represents an original experimental investigation required both for advancing the understanding of TRISO particles spouted bed and providing benchmark data to validate computational fluid dynamics (CFD).