Search

A three-dimensional numerical analysis of the flow in a Gas-Solid Vortex Unit (GSVU) is carried out. The numerical model is first validated by comparing the bed pressure drop and solids velocity with experimental data. Next, the influence of gas flow rate, solids density, and particle diameter on the pressure drop, solids velocity, bed void fraction and slip velocity between the two phases is studied. A stable, solids bed is achieved for the entire range of gas flow rates tested (0.1–0.6 Nm3/s). No particle entrainment is observed when varying the solid density (1800–450 kg/m3) or the particle diameter (2–0.5 mm). A shift to bubbling fluidization regime is observed for small particle diameters (0.5 mm). The observed changes in the GSVU flow patterns are discussed by analyzing the changes in the cumulative centrifugal to drag force ratio over the bed.

The aim of this paper is to investigate in depth whether adding nanoparticles or nanotubes to a fluid enhances its heat transfer capabilities. For this reason, the thermal conductivities and viscosities of a selection of nanofluids were thoroughly examined. The systems studied were (a) ethylene glycol with added CuO, TiO2, or Al2O3 nanoparticles and (b) water with TiO2 or Al2O3 nanoparticles or multiwall carbon nanotubes (MWCNTs). All of the measurements were conducted at 298.15 K. In a very recent paper, it was shown that instruments employing the transient hot-wire technique can produce excellent measurements when a finite element method (FEM) is employed to describe the instrument for the geometry of the hot wire. Furthermore, it was shown that an approximate analytic solution can be employed with equal success, over the time range from 0.1 to 1 s, provided that four specific criteria are satisfied. Subsequently a transient hot-wire instrument was, designed, constructed, and employed for the measurement of the thermal conductivities of nanofluids with an uncertainty of about 2%. A second, validated technique, namely, a hot-disk instrument, was also employed to conduct measurements on some of the systems to provide mutual support for the tesults of the thermal conductivity measurements. To investigate the effect of any enhancement Of the thermal conductivity of the fluids on their application in practical heat transfer, the viscosities of typical concentrations of:several of the nanofluids were also measured. A parallel-plate rotational rheometer, able to measure the viscosities of Newtonian and non-Newtonian liquids with an uncertainty of better than 5%, was employed for, these measurements because most of the nano fluids considered showed behavior comparable to a Bingham plastic. All of these measurements have:allowed an investigation of the change in the heat transfer capability of the base fluid when nanoparticles or MWCNTs are added to it for a typical heat exchanger. It is shown that in general the combined changes in physical properties that accompany suspension of nanoparticles in fluids mean that the heat transfer benefits are all rather modest, even when they are achieved.

To clarify the three-dimensional structure of near-wall jets observed in disc-like gas vortex units, experimental and numerical studies are performed. The experimental results are obtained using Particle Image Velocimetry (PIV), Laser Doppler Anemometry (LDA), pressure probes and surface oil flow visualization techniques. The first three techniques have been used to investigate the bulk flow hydrodynamics of the vortex unit. Surface oil flow visualization is adopted to visualize streamlines near the end-walls of the vortex unit. The surface streamlines help determine the azimuthal and radial velocity components of the radial near-wall jets. Simulations of the vortex unit using FLUENT® v.14a are simultaneously performed, computationally resolving the near-wall jet regions in the axial direction. The simulation results together with the surface oil flow visualization establish the three-dimensional structure of the near-wall jets in gas vortex units for the first time in literature. It is also conjectured that the near-wall jets develop due to combined effects of bulk flow acceleration and swirl. The centrifugal force diminishes in the vicinity of the end-walls. The radially inward pressure gradient in these regions, no longer balanced by the centrifugal force, pushes gas radially inward thus developing the near-wall jets. This article is protected by copyright. All rights reserved.

Coincident (simultaneous) three‐component particle velocity measurements performed using two laser Doppler anemometry probes at the outlet section of a 9 m high cylindrical riser are for the first time presented for dilute flow conditions. Near the blinded extension of the T‐outlet a solids vortex is formed. Particle downflow along the riser wall opposite the outlet tube is observed, which is restricted to higher riser heights at higher gas flow rates. Increased velocity fluctuations are observed in the solids vortex and downflow region as well as at heights corresponding to the outlet tube. Contrary to the rest of the riser, in the downflow region time and ensemble velocity averages are not equal. Given the local bending of the streamlines, axial momentum transforms to radial and azimuthal momentum giving rise to the corresponding shear stresses. Turbulence intensity values indicate the edges of the downflow region. © 2016 The Authors AIChE Journal published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers AIChE J, 62: 3575–3584, 2016

A unique normalized radial pressure profile characterizes the bed of a gas‐solid vortex reactor over a range of particle densities and sizes, solid capacities, and gas flow rates: 950–1240 kg/m3, 1–2 mm, 2 kg to maximum solids capacity, and 0.4–0.8 Nm3/s (corresponding to gas injection velocities of 55–110 m/s), respectively. The combined momentum conservation equations of both gas and solid phases predict this pressure profile when accounting for the corresponding measured particle velocities. The pressure profiles for a given type of particles and a given solids loading but for different gas injection velocities merge into a single curve when normalizing the pressures with the pressure value downstream of the bed. The normalized—with respect to the overall pressure drop—pressure profiles for different gas injection velocities in particle‐free flow merge in a unique profile. © 2015 The Authors AIChE Journal published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers AIChE J, 61: 4114–4125, 2015

Coincident three-component particle velocity measurements are performed in an almost 9 m high cylindrical riser (internal diameter 0.10 m) of a pilot-scale cold-flow Circulating Fluidized Bed set-up, using two Laser Doppler Anemometry (LDA) probes. Experiments are performed with superficial gas velocities of 3.5 m/s and 5.3 m/s near the solids inlet line and with an average solids volume fraction of 0.0002. The particle flow is observed to be highly disturbed due to the asymmetrical position of both the gas and the Y-shaped solids inlet line. A jet-like particle flow and a by-passing streaming gas jet around the particles are formed. Just below the solids inlet line a downward oriented particle velocity is measured. Particles are immediately lifted by the ascending gas and spread over the entire cross-sectional tube area. Near the solids inlet line the axial and radial particle velocity fluctuations are of the same order of magnitude as the corresponding mean particle velocity components, showing that the flow is anisotropic. The radial particle velocity fluctuations decrease fast as the radial inflow of particles is transformed into axial particles flow. The turbulence intensity in the inlet section of the dilute riser flow is found to be extremely high, indicating that the flow is far from being fully developed. The total particle shear stress profiles near the solids inlet line indicate a high momentum transfer from radial to axial particle transport. The particle fluctuation energy values calculated based on experimental data are nearly double than the values corresponding in fully developed flow studied previously.

In a Gas–Solid Vortex Reactor (GSVR), also referred to as a Rotating Fluidized Bed in Static Geometry, a fluidized bed is generated in a centrifugal field by introducing the gas via tangential inlet slots to the reactor chamber. Better heat and mass transfer are observed, making this a promising reactor type for Process Intensification. Developing GSVRs on industrial scale requires, amongst other, a good insight and understanding of the hydrodynamics of the granular flow. In the present work experiments are performed over a wide range of operating conditions in a cold flow pilot-scale set-up. The set-up has a diameter of 0.54 m, a length of 0.1 m and 36 tangential inlet slots of 2 mm. Different materials with solids density between 950–1800 kg/m3 and particle diameters of 1–2 mm, at varying gas injection velocities from 55 to 110 m/s are tested between minimum and maximum solids capacities. All these operating conditions are used to follow the change of granular flow by performing PIV. The rotating fluidized bed can change from a smoothly rotating, densely fluidized bed to a highly bubbling rotating fluidized bed depending on the operating conditions. Bubbling diminishes with increasing solids density and particle diameter. Experimental measurements of azimuthal particle velocity fields in a GSVR are for the first time reported. Azimuthal solids velocity is found to decrease with higher solids density and larger particle diameter. The critical minimum fluidization velocity, that is the minimum velocity at which the complete bed is fluidized, is calculated and the centrifugal bed behavior is mapped in terms of a dimensionless radial gas velocity and a dimensionless particle diameter, as conventionally done for gravitational beds.

Coincident three-component solids velocity measurements are performed in a 8.7-m-high cylindrical riser (internal diameter 0.10 m) of a pilot-scale cold-flow Circulating Fluidized Bed set-up, using two Laser Doppler Anemometry (LDA) probes. Experiments are performed with superficial gas velocities of 3.5 and 5.3 m/s at riser heights from 0.9 to 5.5 m and with an average solids volume fraction of 0.0002. At the lower riser heights, the solids flow is observed to be highly disturbed due to the asymmetrical position of both the air and solids inlet. A fully developed parabolic axial solids velocity field is observed about 2 m above the solids inlet line. The measured radial and azimuthal particle velocity components are significantly smaller than the axial one. The particle velocity fluctuations in the axial flow direction are larger than in the radial and azimuthal directions, showing that the solids flow is anisotropic. However, both the radial and azimuthal velocity fluctuations are higher than the corresponding mean velocities, as observed when studying the turbulence intensity in the three directions. All fluctuating velocities are decreasing significantly with increasing riser height, i.e. as the flow becomes more developed. No average down-flow of solids is recorded, not even close to the wall, due to the highly diluted flow. The total particle shear stress profiles designate a mean transport of fluctuating momentum along the riser radius due to velocity fluctuations. The particle fluctuation energy calculated based on experimental data indicates that the solids flow is dominated by wall-particle rather than intra-particle collisions.

This experimental study of backward-facing step flow is focused on the transient flow regime ( R e h from 30 to 1800). The electrodiffusion technique is applied to measure the wall shear rate in a water channel with the most common expansion geometry ( E R = 2 ). The direction-sensitive wall probes are able to recognize the individual regions of flow separation and reattachment behind the step. The bottom and roof wall shear rate profiles are obtained under steady and pulsatile flow conditions at the inlet. The near-wall extent of primary (mean reattachment length) and secondary (corner and roof eddy location) flow-recirculation zones is determined from these profiles. The high level of wall shear rate is observed inside the reverse flow regions even at moderate Reynolds numbers. The spectral analysis of natural flow fluctuations reveals that the low frequencies characterized by S t h ∼ 0.15 dominate the reattachment region. The inlet flow pulsations affect strongly the overall flow structure behind the step. Up to 80% reduction of the reattachment length is achieved by applying flow pulsations at the most effective frequency. Two flow instability modes are found to be important to control this frequency: the “step mode” dominates the laminar flow regime, whereas the “shear layer mode” prevails at the transient flow conditions. The frequency of “step mode” coincides with the scaling recently suggested for the global instability of backward-facing step flows.

In the present work, the effect of the use of a nanofluid in a miniature plate heat exchanger (PHE) with modulated surface has been studied both experimentally and numerically. First, the thermophysical properties (i.e., thermal conductivity, heat capacity, viscosity, density and surface tension) of a typical nanofluid (CuO in water, 4% v/v) were systematically measured. The effect of surface modulation on heat transfer augmentation and friction losses was then investigated by simulating the existing miniature PHE as well as a notional similar PHE with flat plate using a CFD code. Finally, the effect of the nanofluid on the PHE performance was studied and compared to that of a conventional cooling fluid (i.e., water). The results suggest that, for a given heat duty, the nanofluid volumetric flow rate required is lower than that of water causing lower pressure drop. As a result, smaller equipment and less pumping power are required. In conclusion, the use of the nanofluids seems to be a promising solution towards designing efficient heat exchanging systems, especially when the total volume of the equipment is the main issue. The only drawbacks so far are the high price and the possible instability of the nanoparticle suspensions.

The efficacy of nanofluids as coolants is investigated in the present study. For the nanofluids tested, systematic measurements confirmed that the thermophysical properties of the base fluid are considerably affected by the nanoparticle addition. A typical nanofluid, namely a 4% CuO suspension in water, is selected next and its performance in a commercial herringbone-type PHE is experimentally studied. The new experimental data confirmed that besides the physical properties, the type of flow inside the heat exchanging equipment also affects the efficacy of a nanofluid as coolant. The fluid viscosity seems also to be a crucial factor for the heat exchanger performance. It is concluded that in industrial heat exchangers, where large volumes of nanofluids are necessary and turbulent flow is usually developed, the substitution of conventional fluids by nanofluids seems inauspicious.

Free flowing liquid layer characteristics, counter-current gas–liquid two-phase flow and incipient flooding were studied in small diameter inclined tubes (7 and 9 mm). Experiments were carried out at various inclination angles from the horizontal (30°, 45°, 60° and 75°), while several liquids covering a wide range of physical properties were employed. Fast video recordings and conductivity probes were used for liquid layer thickness measurements, from which mean layer thickness and its statistical quantities were calculated. The wall shear stress at the tube bottom was also measured using an electrodiffusion technique. The new experimental data confirm previous interpretation that in almost all cases the dominant flooding mechanism is wave growth and upward dragging by the gas phase. Consequently, incipient flooding is strongly affected by the liquid layer characteristics, which in turn are influenced by the liquid properties. New correlations based on dimensionless groups for the prediction of flooding in inclined small diameter tubes are proposed.

Changing production patterns towards waste reduction in a globalizing world can be considered a starting point towards sustainable development. The aim of the chemical plant designer is to reduce pollutant emissions, not by cleaning the effluents but by diminishing the production of the undesirable compounds. The case study examined is focused on reducing the CHF 3 emission of an existing difluorochloromethane (HCFC-22) plant by allocating the source of the problem and trying to decrease byproduct emissions by reducing their production. The effect of the operating conditions on the formation rate of both the product and the byproduct of the plant is studied and it is proved that the optimum result is accomplished simply by reducing the residence time in the fluorination reactor, that is, without the need for extra investment and/or energy consumption, a solution highly desirable from an economic point of view. The results of the study were applied to an existing plant leading to 50 % reduction of the CHF 3 emissions.