Downcomer Section Research Articles

Effect of heater and condenser orientations on fluid flow behaviour in a two-phase natural circulation loop

A Numerical investigation on the influence of heater and condenser orientation and its location on steady state performance of a Two Phase Natural Circulation Loop (TPNCL) is presented. In the present study, four possible orientations are considered in TPNCL namely, Vertical Heater and Vertical Condenser (VHVC), Vertical Heater and Horizontal Condenser (VHHC), Horizontal Heater and Horizontal Condenser (HHHC) and Horizontal Heater and Vertical Condenser (HHVC). An in house MATLAB code is developed to solve the systems of equations. Homogeneous equilibrium model (HEM) is used in two phase regions. It is observed that steady state mass flow rate of the loop is significantly influenced by the developed pressure gradient in the riser and downcomer sections, as well as available vapour quality in the riser section. Moreover, it is also noticed that condenser position and orientation significantly effects the thermo-hydraulic characteristic of loop compared with heater orientation.

Local volumetric mass transfer coefficient estimation for Scenedesmus microalgae culture in a cylindrical airlift photobioreactor

AbstractA plug flow model without axial dispersion was utilized to evaluate the mass transfer coefficient in Scenedesmus microalgae culture grown in a split airlift photobioreactor. The local volumetric mass transfer coefficient and the local interfacial area also were calculated at various gas velocity (1.0, 2.0 and 2.8 cm s–1). In the riser and the downcomer section, the local volumetric mass transfer coefficients were dominated by interfacial area and gas holdup values, and were favored by lower optical densities (ODs) and higher values of superficial gas velocities of microalgae cultures. The OD effects on the liquid side and the local volumetric mass transfer coefficient were analyzed over the growth period divided into three zones: Zone I, for OD ≤0.08; Zone II, OD between 0.08 and 0.19; and Zone III, OD between 0.19 and 0.30. Although the local volumetric mass transfer coefficient was constant axially in the riser, it decreased axially on moving through the downcomer. The estimated local volumetric mass transfer coefficient was found always to be higher than that estimated from the available correlations. © 2020 Society of Chemical Industry (SCI)

Heat recovery from the Downcomer of a pressurized circulating fluidized bed during various transient conditions

An enormous amount of heat energy is available in the cyclone and downcomer of a circulating fluidized bed (CFB). The downcomer section can thus be a major source of heat recovery. There is scope for investigation on prospects of recovering heat along the downcomer. In the present work, an attempt is made to utilize the heat from the downcomer of a laboratory-scale Pressurized circulating fluidized bed (PCFB) set up by incorporating a spiral type of heat exchanger inside it. Transient behaviour of the heat recovery for various transient operating conditions such as superficial air velocity and power input which are operated under either step and ramp increase or decrease are reported. The PCFB unit in the present study consists of a riser of 2000 mm height and 54 mm inner diameter. The length of the downcomer is 1465 mm with an inner diameter of 54 mm. The results indicated that a maximum amount of around 350 W of heat could be recovered in the downcomer. It is also observed in some cases that this heat recovery fluctuates in phase with the transients in various operating conditions while it fluctuates with delays and phase shifts in others.

Computational fluid dynamics of rectangular external loop airlift reactor

AbstractThe novel design of a rectangular external loop airlift reactor is at present the most used large-scale reactor for microalgae culture. It has a unique future for a large surface to volume ratio for exposure of light radiation for photosynthesis reaction. The 3D simulations have been performed in rectangular EL-ALR. The Eulerian–Eulerian approach has been used with a dispersed gas phase for different turbulent models. The performance and applicability of different turbulent model’s i.e., K-epsilon standard, K-epsilon realizable, K-omega, and Reynolds stress model are used and compared with experimental results. All drag forces and non-drag forces (turbulent dispersion, virtual mass, and lift coefficient) are included in the model. The experimental values of overall gas hold-up and average liquid circulation velocity have been compared with simulation and literature results. It is seemed to give good agreements. For the different elevations in the downcomer section, liquid axial velocity, turbulent kinetic energy, and turbulent eddy dissipation experimental have been compared with different turbulent models. The K-epsilon Realizable model gives better prediction with experimental results.

Development of drift–flux correlation for predicting void fraction in downcomer regions

ABSTRACT Accurate prediction of interfacial drag in the downcomer annulus is crucial for the assessment of downcomer void fraction for the loss of coolant accident analysis. The downcomer annulus is the gap between reactor pressure vessel (RPV) exterior and the inner wall of pressure containment vessel (PCV). Based on the previous research, occurrence of the nonuniform two-phase flow in downcomer section is reported, which is partly due to the large wall temperature difference between RPV exterior and the inner wall of PCV. In RELAP5, interfacial drag term in downcomer section is calculated using Kataoka–Ishii and churn-turbulent drift–flux correlations. It has been pointed out that this traditional calculation approach for calculating downcomer void fraction needs modification. The purpose of the current study is to assess the behaviors of drift–flux parameters in downcomer section and to propose an improved distribution parameter model that is suitable for donwcomer boiling analysis.

Characterization of kinetic parameters and mass transfer resistance in an aerobic fixed-bed reactor by in-situ respirometry

An airlift reactor filled with Raschig rings in the downcomer section only was designed to allow for the characterization of biofilm biomass at several superficial liquid velocities. The reactor was inoculated with activated sludge and operated with synthetic wastewater containing acetate. After steady state was reached, the kinetic parameters of the process were determined at each superficial liquid velocity by an in-situ pulse respirometry method. Subsequently, the biofilm contained in the reactor was disaggregated, resuspended in a mineral medium, and reintroduced in the reactor, without Raschig rings, i.e., a standard airlift reactor. The kinetic parameters of the bioprocess were determined again, by the same respirometry method. That strategy allowed for the estimation of apparent and intrinsic kinetic parameters; i.e., with and without mass transfer limitation, respectively. Moreover, the method also allowed for the determination of the internal and external effectiveness factors and mass transfer resistances. The results shown that the hydrodynamic conditions had a strong effect on the kinetic parameters but none on the stoichiometric parameters. The external mass transfer accounted for up to 70% of the total mass transfer resistance. It is concluded that respirometry is a useful tool for the characterization of biofilms, in actual random packing reactors.

The Challenging Thermophysical Issues of Fast Reactors

The article presents and analyzes the results from topical thermophysical investigations aimed at substantiating the characteristics and assessing the safety of liquid metal cooled fast reactors: an advanced largecapacity fast sodium-cooled reactor and a fast lead-cooled reactor. We also outline the results from experimental investigations into hydrodynamics and heat transfer for stratified flow of coolant, into thermal hydraulics of a large-modular “sodium–water” steam generator in different modes of reactor operation, and into the fuel assembly degradation process in the course of a severe accident involving loss of sodium flowrate in a fast reactor. The basic mechanisms governing degradation of dummy fuel rod claddings are identified; the distribution of marker materials over the fuel assembly height in its ultimate state is estimated, and the phenomena of blocking the fuel assembly flow pass section and the marker materials being thrown to beyond the assembly boundaries are studied. The article demonstrates the advisability of using a combined sodium purification system built into the reactor vessel that contains cold traps as a mandatory element along with hot traps, which serve to perform accelerated removal of oxygen during the NPP operation at its nominal parameters. The results from thermal–hydraulic tests of the lead-cooled reactor core carried out on the 6B experimental setup and of the steam generator carried out on the SPRUT experimental setup installed at the SSC RF–IPPE are presented.1 It has been found from the experiments on the thermal–hydraulic model of the lead-cooled reactor’s steam generator that the steam temperatures at the outlets from both headers are identical with each other and so are the lead temperatures at the downcomer section outlet and in the main lead path. The experiments did not show pulsations of feedwater flowrate or pressure in the loops, which points to the stable nature of operation at partial load. The state of the technology of heavy metal coolants and the prospects for its future development are considered. It is shown that there is a possibility in principle to realize the required parameters of a high-temperature sodium-cooled fast reactor for producing large quantities of hydrogen, e.g., based on one of the thermochemical cycles or high-temperature electrolysis with a high electricity thermal utilization factor. The problems that must be solved during the further thermophysical investigations are analyzed.

Global hydrodynamic of hybrid external loop airlift reactor: Experiments and CFD modelling

In this study, the mixing and mass transfer of air-water system were characterized in hybrid external loop airlift reactor using the computational fluid dynamics (CFD). This hybrid configuration involves the advantages of both airlift and torus reactors. To predict the flow in hybrid geometry, an Eulerian-Eulerian approach has been adopted with per-phase interaction of the standard k–ε turbulence model. To simplify the solution, the bubbles were considered to have approximately the same size. The global flow characteristics were experimentally determined by a conductimetric method. Gassing-out method was applied to determine gas-liquid mass transfer coefficients. The findings highlighted a uniform gas flow distribution in the riser tube, despite the riser included two different sections. It was noticed that beyond the superficial gas velocity, 0.0253 m/s, the fine rotational bubbles were dragged away by the liquid recirculation in the downcomer section. The STD k–ε turbulence model with per-phase effect, estimated correctly the hydrodynamic of homogenous and transition regimes except for high superficial gas velocity, Ug = 0.0703 m/s. For superficial gas velocities ranging from 0.0253 to 0.0703 m/s, there was a good agreement between simulated and experimental values; the error was less than 6%. Optimal conception of this hybrid geometry and its industrial scale use need further research and testing.

Investigation of local gas holdup and bubble dynamics using four-point optical probe technique in a split-cylinder airlift reactor

A four-point optical fiber probe technique was employed in a split-cylinder airlift reactor with an air-water system. Effect of superficial gas velocities- 0.3, 1.0, 2.0, and 2.8 cm/s- was studied on the local gas holdup and bubble dynamic properties namely, bubble passage frequency, interfacial area, bubble chord length, and bubble rise velocity in both the riser and downcomer sections of the reactor. The bubble chord length and bubble velocity in the riser and downcomer followed log-normal distribution and normal distribution respectively. The gas holdup and interfacial area in the riser increased by 900 and 800% for a superficial gas velocity increase from 0.3 to 2.8 cm/s, while those at the top of the downcomer by 500, and 400% for a superficial gas velocity increase from 1.0 to 2.8 cm/s, respectively. At each superficial gas velocity, the bubble passage frequency, gas holdup, and interfacial area did not vary significantly along the axis of the riser. An axial variation in bubble passage frequency, gas holdup, and interfacial area was observed in the downcomer. A correlation for the variation of the gas holdup in the riser was developed based on superficial gas and liquid circulation velocities. The correlation for the gas holdup in the downcomer was developed to account for the axial variation observed.

Hydrodynamics and mixing in airlift contactors: Experimental work and CFD modelling

Fluid flow was examined in an airlift contactor equipped with a central baffle operated at superficial air velocities between 0.07 and 0.34m/s. Measurements were made of the Bubble Size Distribution (BSD), profiles of the average local hold-up and upwards liquid velocity, the overall hold-up and the mixing time. Overall hold-up was similar to that for a bubble column while profiles of the average local hold-up and average upwards liquid velocity were markedly asymmetric (unlike those measured when this pilot-plant was operated as a bubble column). Mixing times ranged from 7 to 27s, decreasing as the superficial velocity increased. Neither the tracer addition point nor the measurement location had a major impact on the observed mixing time, this contrasts with work in bubble columns where the tracer addition and measurement locations had a large impact on the mixing time while the influence of the superficial velocity was relatively small. Associated CFD modelling generally offered good agreement with our experimental measurements of the average local hold-up and upwards liquid velocity profiles. However, it was found that the model underestimated the extent to which air is entrained into the downcomer section and the mixing time at low superficial velocities. This study examined an airlift system operated at high superficial gas velocities for which there are very limited data available in the open literature; comparing experimental and computational results is instructive in defining the strengths and limitations of employing a single bubble size CFD model.

Scaling analyses for the open flashing-driven natural circulation system

Transient simulating and scaling analyses are carried out to reproduce the transient operating behaviors of the prototypical big-scale open natural circulation system, which is designed for the Passive Containment Cooling System (PCCS). Firstly, a transient numerical code to simulate the operating behaviors of the open natural circulation system is developed and validated. Than, an effective and simplified 1:1 height scaling analyses based on the steady-state flow are conducted to model the operating behaviors of the prototypical open natural circulation system. The scaling process consists of heat transfer scaling of the heat exchanger and the pressure resistance scaling of the riser and downcomer sections, the purpose of which is to keep the driven force and pressure resistance of the model same with those of the prototype by changing the dimensions of the pipes. When choosing the geometrical parameters of the heat transfer tubes, the Reynolds number of fluid inside the heat transfer tube has a great influence on the modeling results. Thus, the flow regime inside the tube in the model system should be the same as that in the prototype system when choosing the geometrical parameters of heat transfer tube. The dimensions of the riser and downcomer section are scaled separately to keep the pressure resistances of them in the model same with those in the prototype respectively. From the comparison results of the model and prototype it can be found that the scaling open natural circulation system can reproduce the transient operating behaviors of the prototype very well and the scaling method in this study can be used for the engineering applications.

A combined CFD/visualized investigation of two-phase heat and mass transfer inside a horizontal loop thermosiphon

A horizontal two-phase loop thermosiphon (HLTS) which incorporates a U-turn section to prevent bidirectional flow was investigated in this study via flow visualization and three-dimensional computational fluid dynamics (CFD). The experimental device consisted of a glass-water HLTS which was tested under a range of filling ratios and operating conditions to visualize the two-phase flow in the loop. At the same time, a three-dimensional, volume of fluid (VOF) CFD model was developed to further elucidate the underlying heat and mass transfer regimes for this HLTS. It was found that by increases the filling ratio, the flow regime changes from smooth unidirectional flow to geyser oscillation flow. At a filling ratio of <70%, nucleate boiling and drop-wise condensation occur in the presence of clear unidirectional flow. At >83% filling ratios, geyser oscillation occurs, in which ascending/descending flow of the working fluid was repeatedly observed. During ascending flow, a plug flow pattern was observed in the riser and a bubble flow pattern was observed in the downcomer section. During descending flow, an annular flow pattern was observed in the riser, while plug-to-annular flow was observed in the downcomer section. The simulation results revealed these same two-phase flow regimes and predicted and measured temperatures and thermal resistances deviated by less than 9%. Overall, this valid CFD model can easily be extended to evaluate the heat and mass transfer mechanisms and performance of any geometrically similar HLTS systems across the range of filling ratios and flow regimes studied in this paper.

CFD analysis of flow regimes in airlift reactor using Eulerian‐Lagrangian approach

AbstractThe hydrodynamic aspects of the concentric tube airlift reactor have been studied using two‐phase CFD simulations with a Eulerian‐Lagrangian approach. The three‐dimensional CFD simulations are carried out with the experimental geometry of airlift reactor (H/D = 12) reported in the published literature. The standard k‐ϵ turbulence model is used with additional consideration of production and dissipation of buoyancy‐induced turbulence. Two different bubble size distributions (BSDs) and their Sauter mean diameters have been considered to represent the gas distribution at sparger in the reactor. This study shows that the BSD1 (1–5–10 mm) containing a high fraction of small bubbles (≤ 5 mm) represents the hydrodynamics of flows appropriately as compared to the BSD2 (5–10–15 mm) containing a high fraction of large bubbles (≥ 10 mm) or the single size bubble diameters (5.25 mm and 10 mm). Further, three regimes of operation such as no gas bubbles in the downcomer (regime I), stationary gas bubbles in the downcomer (regime II), and gas bubble recirculation from the downcomer section to the riser section (regime III), are verified using CFD simulations with Lagrangian particle tracking. Satisfactory agreement (within 15 % deviation) with the experimental data was observed for parameters such as the gas holdups in the riser and downcomer and the liquid circulation velocity in the flow regimes I and II for BSD1.

Ozone assisted electrocoagulation in a rectangular internal-loop airlift reactor: Application to decolorization of acid dye

Decolorization of synthetic wastewater containing Acid Brown 214, a typical triazo acid dye, was carried out by ozone assisted electrocoagulation as an advanced oxidation process. Ozonation and electrocogulation were performed simultaneously in the riser and downcomer sections of a lab-scale rectangular airlift reactor, respectively, where ozonated gas had no influence on dielectric constant. A good mixing was achieved with the liquid circulation induced by gaseous ozone, without the need of mechanical mixing. Taguchi experimental design method with orthogonal arrays L27 and the “larger is better response” category were applied to determine optimum conditions for dye removal. The operating variables considered were the initial pH, initial dye concentration, salt concentration, current density, superficial gas velocity and decolorization period with three levels. Under optimum conditions, the synergistic effect of the combined process reaching complete decolorization in 30min with energy consumption 7.4kWh/kg dye removed. Results indicated that combination process in innovative configuration of single airlift reactor is cost effective as compared with that obtained individually in separate conventional reactors.

Mass Transfer And Hydrodynamics In A Split-Column Airlift Photobioreactor

Overall gas-liquid mass transfer coefficients of oxygen gas (KLa) at different axiallocations of the riser and downcomer of a split-column Airlift Photobioreactor wereinvestigated using FOXY Fiber Optic oxygen sensor supported by NeoFox viewer software.The effect of bubble dynamics on mass transfer was considered. A 4-point optical probe wasused to measure local interfacial area (a) and other bubble dynamics parameters for both theriser and the downcomer of a split-column airlift photobioreactor. Having both overall masstransfer (KLa) and interfacial area (a) measured at different locations, local oxygen masstransfer coefficient (KL) can be determined for each location along the riser and thedowncomer of the airlift bioreactor. Effects of different operating conditions (0.3 – 2.8 cm/ssuperficial gas velocity) on both mass transfer and bubble dynamics of the riser and thedowncomer have been investigated for air-water system. It was found that both local masstransfer and gas-liquid interfacial area are considerably varying from the bottom to the top ofthe riser and the downcomer of the bioreactor. It was found that local mass transfer coefficientvaries by 100% from the bottom to the top of the riser at low superficial gas velocity (1 cm/s)and by 30% at relatively high velocities (2-2.8 cm/s). The Trend of the variation of the KLa inthe downcomer section of the column is very different from that of the riser, as it was noticedthat insignificant mass transfer takes place at the lower part of the downcomer. The results ofmass transfer and bubble dynamics in the riser and the downcomer of the studied conditionswill be presented and discussed.

The influence of granular flow rate on the performance of a moving bed granular filter

The goal of this study is to evaluate the effect of granular flow rate on the performance of a moving bed granular filter designed for hot gas filtration of fine char particles (dust) produced during fast pyrolysis of biomass. The filter employs a counter-current configuration, in which down-flowing granular material spreads out at the bottom of the filtration vessel to form an interfacial area where the dusty gas enters the granular bed and much of the gas cleaning is hypothesized to occur. This study uses a real-time particle counter to measure the instantaneous filtration efficiency during cold flow tests of the filter. Differential pressure measurements at various locations within the granular bed are used to assess the level of char dust hold-up over time. These experiments reveal a critical granular residence time below which the filter must be operated to achieve filtration efficiencies exceeding 99%. Operating above the critical value causes the filter to “clog” and decrease in efficiency. The clogging is characterized by a critical dust volume fraction as determined through a fixed bed filtration test. The filter is found to accumulate most of the dust at the interfacial region. Also the interfacial region is more efficient than the downcomer section of the granular bed in removing dust. Decreasing residence time of granular material in the filter reduces the hold-up of char dust in the filter, which is expected to mitigate coking reactions of organic vapors when the filter is used to remove char from fast pyrolysis gas streams.

Mass production of the entomopathogenic nematode, Steinernema carpocapsae CABA01, through the submerged monoxenic culture in two internal-loop airlift bioreactors with some geometric differences

The submerged monoxenic culture of the entomopathogenic nematode, Steinernema carpocapsae CABA01, was carried out in two internal-loop airlift reactors (R1 and R2) with some geometric differences. Two runs each reactor were carried out at operating conditions V L = 4.22 L, Q = 1–1.5 vvm, T = 22 °C. The culture broth apparent viscosity changed from 2 to 12 mPa s determining global changes in both hydrodynamics (expressed as the dimensionless Reynolds number, Re) and oxygen transfer conditions (expressed as the k L a coefficient). Such changes were 170 < Re < 3080; 1.9 < k L a (10 −2 s −1) < 3.5 for R1 experiments, and 400 < Re < 10,060; 1.9 < k L a (10 −2 s −1) < 3.5, for R2 experiments. Infective juvenile stage (IJ) concentrations of 250,000/mL were achieved in R2 experiments whereas notably lower ones were obtained in R1 experiments (110,000 IJ/mL, approximately). The hydrodynamic conditions in both riser and downcomer sections of R2 experiments involved higher inertial forces than the corresponding ones to R1 runs, implying values of (Re R2/Re R1) riser from 2.3 to 3.3 and (Re R2/Re R1) downcomer from 1.8 to 2.5. These results suggest that higher inertial forces would promote the mating process among the vigorous adult nematodes of the first generation, probably due to encourage female–male encounters in response to a better mixing of the culture broth. Further, the nematode reproduction appeared to be greatly influenced by the reactor geometry.

Mathematical modeling of a horizontal tubular loop bioreactor for biomass production from natural gas

In this study, mathematical modeling of a horizontal tubular loop bioreactor (HTLB) was considered for biomass production from natural gas. Gas inlet segments, static mixers, gas–liquid separator, and liquid pump of the HTLB were mathematically modeled according to the ideal stirred reactors, and the horizontal parts, riser, and down-comer sections were modeled in line with the dispersed plug-flow reactors as well. The set of ordinary and partial differential equations were coupled to calculate the oxygen and methane concentrations in the liquid through the length of bioreactor and time. Moreover, the tuned kinetic and hydrodynamic parameters of SCP process in the HTLB were determined based on the mathematical model at various operational conditions. The model was validated by considering experimental dissolved oxygen, methane, and biomass concentrations in liquid at different ratios of air to methane and liquid flow rates. The results showed satisfactory agreement between the developed model and the experimental data.

Detection of agglomeration and gradual particle size changes in circulating fluidized beds

The monitoring of fluidized bed processes is valuable for the optimization of operational efficiency and avoiding unscheduled shut-downs in industrial practice. We here focus on two important applications for the industrially widely applied circulating fluidized beds (CFBs): the detection of small, gradual changes in particle size and the early detection of agglomeration. Based on high-frequency (200 Hz) pressure fluctuation measurements we have applied the attractor comparison method in different CFBs to investigate its suitability as a monitoring tool for the two proposed applications. Attractor comparison has shown to be generally sensitive to small, gradual particle size changes in a lab-scale CFB. The sensitivity normally depends on the measurement location (downcomer, return system, riser) and is often larger in the downcomer section. From the results, detection limits for the particle size have been identified. In an industrial installation the sensitivity and applicability of the method for measurements in the riser has been confirmed. With the help of model agglomerates it has been shown that attractor comparison can also detect the presence of small shares of agglomerates on lab-scale; here, filtering the pressure fluctuation data before the analysis has shown to improve the method. The suitability of the method for agglomeration detection has been confirmed with an agglomeration incident in a hot lab-scale facility; in this case the approaching defluidization has been detected with an early time of about 30 min based on measurements in horizontal return-leg and lower riser.

Model‐Aided Design of a Three‐Phase Gas‐Lift Reactor for Oxidation Accompanied by Catalyst Reversible Deactivation

AbstractA mathematical model of a three‐phase gas‐lift reactor (GLR) is developed to aid the design of a target reactor for simultaneous substrate catalytic oxidation in riser and a deactivated reactivation catalyst in the downcomer section of the multifunctional reactor. In the GLR model, the hydrodynamics of a real GLR and the kinetics of glucose oxidation by air over a palladium catalyst are incorporated. The GLR model searches for the optimal geometry of the target reactor. With regard to the GLR optimal geometry, the reactor productivity is maximal for given input operational conditions. An algorithm of the GLR model is presented together with simulation results of the target GLR and with insight into the parametric sensitivity of the model. Effects of the reaction components concentrations and the gas‐phase superficial velocity on the location of the target reactor optimal geometry and on the reactor productivity are discussed.