Effect Of Recirculation Research Articles

Hemodynamic and fluid flow analysis of a cerebral aneurysm: a CFD simulation

In this study, we investigate the hemodynamics parameters and their impact on the aneurysm rupture. The simulations are performed on an ideal (benchmark) and realistic model for the intracranial aneurysm that appears at the anterior communicating artery. The realistic geometry was reconstructed from patient-specific cerebral arteries. The computational fluid dynamics simulations are utilized to investigate the hemodynamic parameters such as flow recirculation, wall shear stress, and wall pressure. The boundary conditions are measured from the patient using ultrasonography. The solution of the governing equations is obtained by using the ANSYS-FLUENT 19.2 package. The CFD results indicate that the flow recirculation appears in the aneurysms zone. The effect of the flow recirculation on the bulge hemodynamics wall parameters is discussed to identify the rupture zone.

The Effect of Flue Gas Recirculation on CO, PM and NOx Emissions in Pellet Stove Combustion

Pellet stoves are popular appliances because they are an affordable technology and because the fuel is easy to store and to use. The increasing concern for environmental issues, however, requires a continuous effort to reduce pollutant levels in the atmosphere. This experimental work focuses on flue gas recirculation (FGR) as a possible way to improve combustion and decrease the emissions of carbon monoxide CO, particulate matter PM, and nitrogen oxides NOx in order to fulfill European and Italian emission requirements, for NOx in particular. A pellet stove has been tested in several experimental sessions with and without FGR. Pollutant emissions have been measured and analyzed in terms of statistical summaries and instantaneous trends. With FGR, the average CO and PM emissions were found to be 80% and 45% lower than the corresponding emissions without FGR. Results for PM are significant since FGR reduces emissions well below the most restrictive limits enforced in Italy. The analysis of instantaneous emissions in relation to excess air indicated that FGR can considerably reduce emissions, especially at the extremities of the oxygen O2 content range. Optimal ranges of excess air, in terms of O2 in flue gas, were identified for both the tested configurations, in which CO and PM emissions are minimized. The optimal range is 8–9% without FGR, and it decreases to 5–7% with FGR. Finally, a reduction in NOx emissions by about 11% has been observed in the configuration with FGR. Although this reduction seems modest as compared to CO and PM, it is important in that it lowers the emission level to the most severe limit in Italian regulations and indicates an improved FGR system as the solution for further reduction.

Co-digestion of food waste and hydrothermal liquid digestate: Promotion effect of self-generated hydrochars

Hydrothermal treatment (HTT) can efficiently valorize the digestate after anaerobic digestion. However, the disposal of the HTT liquid is challenging. This paper proposes a method to recover energy through the anaerobic co-digestion of food waste and HTT liquid fraction. The effect of HTT liquid recirculation on anaerobic co-digestion performance was investigated. This study focused on the self-generated hydrochars that remained in the HTT supernatant after centrifugation. The effect of the self-generated hydrochars on the methane (CH4) yield and microbial communities were discussed. After adding HTT liquids treated at 140 and 180 °C, the maximum CH4 production increased to 309.36 and 331.61 mL per g COD, respectively. The HTT liquid exhibited a pH buffering effect and kept a favorable pH for the anaerobic co-digestion. In addition, the self-generated hydrochars with higher carbon content and large oxygen-containing functional groups remained in HTT liquid. They increased the electron transferring rate of the anaerobic co-digestion. The increased relative abundance of Methanosarcina, Syntrophomonadaceae, and Synergistota was observed with adding HTT liquid. The results of the principal component analysis indicate that the electron transferring rate constant had positive correlationships with the relative abundance of Methanosarcina, Syntrophomonadaceae, and Synergistota. This study can provide a good reference for the disposal of the HTT liquid and a novel insight regarding the mechanism for the anaerobic co-digestion.

Effects of flue gas recirculation on the thermal performance of the gas remediation well

Gas thermal remediation (GTR) is a promising novel soil remediation technology. Many problems such as high energy consumption and unreasonable temperature distribution are still commonly found in the application of the gas remediation well. External flue gas recirculation (e-FGR) was applied to improve the thermal performance of the well. A 3D CFD model of the gas remediation well coupled with e-FGR was developed to analyze the heat transmission and combustion process. Four representative e-FGR ratios, 15 %, 30 %, 45 %, and 60 %, were selected to analyze the relationship between the e-FGR ratio and the well thermal performance. The results show that limited impact is obtained at a low e-FGR ratio and excellent results can only be achieved when the e-FGR ratio reaches 60 %. The maximum flame temperature was reduced by about 28 % at an e-FGR ratio of 60 % when compared with conventional combustion conditions. Localized high-temperature areas on the well surface are eliminated. The total heat transfer of the well increased from 4.69 kW to 6.16 kW when the e-FGR ratio increased from 0 to 60 %, which means a 23.9 % reduction in the remediation cycle and fuel consumption can be achieved at an e-FGR ratio of 60 %.

Effects of exhaust gas recirculation (EGR) rates on emission characteristics of ethanol and methanol diesel blended fuels

The introduction of ever more stringent diesel emission standards is associated with a reduction in greenhouse gases. These limitations push many researchers working in this field to look for alternative solutions with the aim of lower greenhouse gas emissions. One of the alternatives is the use of alternative fuels and additives in order to reduce the consumption of fossil fuels. The use of alternative fuels such as ethanol, methanol and others in combination with traditionally used technologies such as exhaust gas recirculation (EGR) is one of the solutions that would lead to the limitation of diesel engines emissions. The aim of the present study is to investigate the performance of a diesel engine with ethanol and methanol additives combined with different degrees of recirculation on engine emissions. For this purpose, a model of a diesel engine with EGR with the possibility of working with ethanol and methanol additives has been developed.

Effect of Flue Gas Recirculation and Oxygen Addition on Combustion Parameters in the Brayton Cycle

Effect of Flue Gas Recirculation and Oxygen Addition on Combustion Parameters in the Brayton Cycle

Ducted fuel injection with Low-Net-Carbon fuels as a solution for meeting future emissions regulations

Several studies have proven how ducted fuel injection (DFI) reduces soot emissions for compression-ignition engines. Nevertheless, no comprehensive study has investigated how DFI performs over a load range in combination with low-net-carbon fuels. In this study, optical-engine experiments were performed with four different fuels—conventional diesel and three low-net-carbon fuels—at low and moderate load, to measure emissions levels and performance. The 1.7-liter single-cylinder optical engine was equipped with a high-speed camera to capture natural luminosity images of the combustion event. Conventional diesel and DFI combustion were investigated at four different dilution levels (to simulate exhaust-gas recirculation effects), from 14 to 21 mol% oxygen in the intake. At a given dilution level, with commercial diesel fuel, DFI reduced soot by 82 % at medium load, and 75 % at low load without increasing NOx. The results further show how DFI with dilution reduces soot and NOx without compromising engine performance or other emission types, especially when combined with low-net-carbon fuels. DFI with the oxygenated low-net-carbon blend HEA67 simultaneously reduced soot and NOx by as much as 93 % and 82 %, respectively, relative to conventional diesel combustion with commercial diesel fuel. These soot and NOx reductions occurred while lifecycle CO2 was reduced by at least 70 % when using low-net-carbon fuels instead of conventional diesel. All emissions changes were compared with future emissions regulations for different vehicle sectors to investigate how DFI can be used to facilitate achievement of the regulations. The results show how the DFI cases fall below several future emissions regulation levels, rendering less need for aftertreatment systems and giving a possible lower cost of ownership.

Experimental and Computational Investigation upon Combustion Characteristics of Liquid Fuel in a Novel Combustor with Hybrid Swirl and Recirculation Bowl.

In the present study, a novel hybrid swirl combustor is designed to reduce the size and complexity of a conventional gas turbine combustor. In this combustor, a dual-swirl pattern is adopted by providing the central vane swirler (45° vane angle) and circumferential tangential injection scheme to achieve higher recirculation of heat and combustion products inside the combustor. Numerical and experimental studies are carried out to understand the flow patterns and combustion characteristics in this high mass-heat interacting environment. Initially, computational studies were carried out to find the optimum geometry for greater recirculation and interaction among the reacting species inside the combustor. Liquid fuel (kerosene) is sprayed into the combustor for two thermal inputs of 25 and 50 kW. Three cases were studied to analyze the effect of bowl recirculation and tangential air inputs in addition to the swirlers. The hybrid swirl, formed by the counter-flow pattern, helps in achieving low and uniform temperature throughout and assists in flame anchoring. The tangential air flow provides a push to the combustion products from the downstream to the central recirculation zone of the combustor. The combined effect of central and tangential swirlers helps in attaining a more distributed combustion. The CO and NO emissions reduced with the use of hybrid swirl.

Effects of central catheter recirculation during extracorporeal photopheresis

Effects of central catheter recirculation during extracorporeal photopheresis

Effect of Exhaust Gas Recirculation on a CRDI Engine Fuelled with Biodiesel, Ethanol and Butanol

The global community has focused on the sustainability of petroleum-based fuel supply due to increased usage in various sectors, depletion of petroleum resources and volatile crude oil market prices. We are looking for an alternative fuel. As a result, we have chosen biodiesel. In this paper, a diesel blend containing biodiesel, ethanol and butanol was created to test the characteristics of a common rail direct injection (CRDI) diesel engine equipped with an exhaust gas recirculation (EGR) system. The blends of diesel-biodiesel and diesel-biodiesel-ethanol-butanol were designated as B40, B40 E10 BUT10. The experiment is carried out at a rate of 20% EGR for biodiesel blends of B40, B40 E10 BUT10. The experimental results show that mechanical efficiency is higher in biodiesel blends than in diesel. The use of an EGR system with these blends reduces NOx by lowering the oxygen concentration in the combustion chamber.

Estimating Residence Time Distributions in Industrial Closed-Circuit Ball Mills

This paper compares two deconvolution methodologies used to estimate residence time distributions (RTD) in industrial closed-circuit ball mills. Parametric and non-parametric deconvolution techniques were evaluated. Both techniques allowed for direct RTD estimates from inlet and outlet tracer measurements in the mills, with no need for mass balances nor assumptions to correct the effect of the tracer recirculation in the grinding circuits. Measurements of inlet and outlet concentrations were conducted by radioactive solid tracers and on-stream detectors. The parametric deconvolution was applied assuming the N-perfectly-mixed-reactors-in-series model, whereas the non-parametric deconvolution consisted of a constrained least squares estimation subject to non-negativity. The shapes of the estimated RTDs were consistent between these methodologies, showing mound-shaped distributions in all cases. From the parametric approach, mixing regimes described by 2–4 perfect mixers in series were observed, which indicated significant differences regarding perfect mixing. The mean (τmean) and median (τ50) residence times were more consistent with the RTD shapes when applying the parametric deconvolution. The non-parametric approach was more sensitive to noise, a disadvantage leading to mean residence times significantly higher than the median, and less consistent with the RTD locations. From the comparisons, the estimation strategies proved to be applicable in industrial closed-circuit ball mills. The parametric deconvolution led to better overall performances for τ50 = 1.7–8.3 min, given a suitable model structure for the RTDs.

Effect of frequency of the wavy wall on turbulent wall jet heat transfer characteristics

The thermal and hydraulic characteristics of a sinusoidal wavy wall are studied numerically to increase the heat transfer rate of the turbulent wall jet. To do so, the sinusoidal profile (y=A∗sin(ωNx)) is used for the wavy wall, where ωN=2πN/L is the frequency with suffix N denoting the number of cycles for the given length L and A is the amplitude. The amplitude “A” is varied from 0 to 0.8a and ωN is changed from ω4 to ω12, to find out the optimum amplitude and frequency for which the heat transfer rate is maximum. The low Reynolds number two equations RNG k−ϵ model is used for the detailed study and to understand the impact of a re-circulation zone on the heat transfer rate. The inlet velocity is uniform and equals to 10.95 m/s for all the cases to achieve the inlet Reynolds number 15000 for a 20 mm nozzle height. At the inlet, jet is heated to a temperature of 330 K and the wavy wall on which the jet flows is provided an isothermal condition with 300 K temperature. With these conditions, the results for streamwise maximum velocity decay, skin friction coefficient, re-circulation area and local Nusselt number have been compared for different amplitudes and frequencies of the wavy wall. From the results, it has been observed that with the increasing amplitude and frequency the Umax increases, the area of re-circulation zone increases and the thermal potential core decreases. For the wavy wall amplitude A = 0.8 and frequency ω12, the peak of Umax is 170% more than the plane wall jet, the area covered by re-circulation zone is 58.2% of the total area and the thermal potential core reduces to X=3.7 which is about 50% of the plane wall jet. The local Nusselt number also increases with the increasing amplitude and frequency in the near field, but in the far field it starts decreasing as the re-circulation effect dominates there. For the wavy wall, the maximum increase in heat transfer rate is 23.23% with respect to the plane wall case.

Co-production of biooil and biochar from microwave co-pyrolysis of food-waste and plastic using recycled biochar as microwave susceptor

Microwave (MW) co-pyrolysis of food-waste (FW) and plastic with recycled biochar as MW susceptor was investigated for co-production of biooil and biochar. Effect of biochar recirculation (3–10 wt%) was analysed on end product yield and properties and compared with conventional MW susceptor (Granular Activated Carbon, GAC). Highest biooil yield (52 wt%) along with 30 wt% of biochar yield were obtained with 7 wt% biochar recirculation. pH and density of biooil were 4.12 and 1.09 g/mL, respectively. Increase in biochar recirculation caused substantial reduction in phenols, acids, alcohols, furans and oxygenated compounds in biooil due to rapid feedstock volatilization, end-chain β-scission and/or disengaging of the long carbon chain of FW and LDPE and transference of oxygenated functional groups into the gaseous phase. Similarly, with increased biochar recirculation, biochar showed decrease in volatile matter (by 47 %) with a simultaneous increase in fixed carbon (by 10 %), carbon (by ∼ 6 %), higher heating value (by ∼ 27 %) and morphological properties. 70 % of biooil energy yield and 45 % of biochar energy yield (obtained with 7 wt% biochar recirculation) accounted for the highest net energy efficiency of 115 %, which was comparable to that of GAC (117 %).

Effectiveness of Exhaust Gas Recirculation on Low-Load Combustion Efficiency of a Reactivity Controlled Compression Ignition Engine

Effectiveness of Exhaust Gas Recirculation on Low-Load Combustion Efficiency of a Reactivity Controlled Compression Ignition Engine

Effect of Exhaust Gas Recirculation and Spark Timing on Combustion and Emission Performance of an Oxygen-Enriched Gasoline Engine.

Oxygen-enriched combustion (OEC) technology in SI engines can greatly improve the degree of constant volume combustion, increase the torque output, and reduce HC and CO emissions but lead to a sharp increase in NO x emissions. Simultaneously, the high temperature from OEC would lead to high nucleation particle emissions. Under the OEC mode, except the oxygen content, spark timing and engine load are important influencing factors on emissions. Exhaust gas recirculation (EGR) technology has been proven to reduce NO x emissions effectively. This research investigates the effects of EGR on combustion and emission performance under an oxygen-enriched ratio (OER) of 25% with five EGR ratios (0-20%) for the initial throttle opening of 14% (at an EGR ratio of 0%) with an engine speed of 1500 rpm. The study shows that when the OER is 25%, the output torque increases with the increase of the EGR ratio. At the proper spark timing, the EGR ratio over 15% can obtain lower NO x emissions and particle emissions than the baseline (OER of 21%). Although HC emissions increase with the EGR ratio, they are still lower than the baseline. Overall, the OER of 25% coupled with the EGR ratios of 15-20% is the predominant combustion mode to improve power and emission performance in SI engines.

Numerical optimization of guaco leaves extraction based on pre‐heat treatment

AbstractThe mass yield of the extraction is influenced directly by the pre‐treatment conditions of the raw material. In the case of leaves, drying is required to ensure their biological stability and is a pre‐processing step that demands large amounts of thermal energy. In the present study, the oil extraction process was numerically optimized, considering the influence of the pre‐heat treatment of the guaco leaves. An operating range between 30°C and 70°C, 1.0 and 5.0 m/s, and recirculation between 0 and 1 was evaluated. The minimum ratio of energy consumption of drying by the amount of oil obtained after extraction (4.05 × 105 kWh/kg) was obtained under the optimal conditions of 30.02°C, 1.0 m/s, and .129 of air recycling flow rate. With our results, it is possible to guarantee safety in the storage of guaco leaves, in addition to optimizing the extraction process, maximizing the obtaining of the oil mass, and reducing energy consumption in the drying step.Practical applicationsTea is one of the most consumed beverages in the world and is obtained by extraction. Between tea leaves, guaco (Mikania glomerata Sprengel) is mainly applied to obtain therapeutic properties as an expectorant and bronchodilator. While guaco leaves are perishable, they must be dried to ensure their biological stability. As others heat and mass transfer processes, the efficient design is mandatory to reduce energy consumption. One of the more efficient drying processes is the operation under exhaust air recirculation. Thus, there is a need to evaluate optimal drying operation conditions to understand the effects of exhaust air recirculation on the energy consumption of drying and the amount of extract obtained in the extraction. Our numerical results indicated that it is possible to guarantee safety in storing guaco leaves and optimize its extraction process.

Characteristics of hollow bluff body-stabilized natural gas-air premixed flames with heat recirculation

The combined effects of flow recirculation and preheating on the premixed flame stability limits were employed via two flow configurations. In the first configuration, the natural gas-air mixture was set to flow around a bluff body whose orientation, number of vertices and vertex angle were varied. In the second configuration, an innovative design was developed where the fuel–air mixture was set to flow inside the bluff body as two opposing jets whose resultant flow is redirected to pass around the bluff body. The peak turbulent kinetic energy (T. K. E.) decreased from 7.24 to 4.94 m2/s2 with the star shape of 4 vertices as its orientation changed from 45 to 0° and such decrease was limited to 5.27 m2/s2 upon having a smaller vertex angle. A smaller drop from 7.24 to 6.0 m2/s2 was experienced when the 4 vertices were replaced by 5 vertices at 0° angle of orientation, while the drop continued to 4.3 m2/s2 upon having 6 vertices due to the reduction in the space available for eddies. A smaller peak T. K. E. of 2.33 m2/s2 was found with the star shape of 3 vertices because the sites for turbulence development were limited. With the star cross-section of 4 vertices at 45° angle of orientation, a maximum blow-off velocity as high as 51.5 m/s and a minimum lean limit as low as 0.40 were obtained. A further extension in the flame stability limits was provided via the second flow configuration due to the favorable effects of impingement and prolonged path of preheating. As the distance between the bluff body and the combustor separating wall decreased to 1.7 times the jet diameter, the blow-off velocity increased to 79.1 m/s; while the lean limit decreased to 0.31 with the star shape of 4 vertices at 45° angle of orientation.

Flame-vortex interaction during turbulent side-wall quenching and its implications for flamelet manifolds

In this study, the thermochemical state during turbulent flame-wall interaction of a stoichiometric methane-air flame is investigated using a fully resolved simulation with detailed chemistry. The turbulent side-wall quenching flame shows both head-on quenching and side-wall quenching-like behavior that significantly affects the CO formation in the near-wall region. The detailed insights from the simulation are used to evaluate a recently proposed flame (tip) vortex interaction mechanism identified from experiments on turbulent side-wall quenching. It describes the entrainment of burnt gases into the fresh gas mixture near the flame’s quenching point. The flame behavior and thermochemical states observed in the simulation are similar to the phenomena observed in the experiments. A novel chemistry manifold is presented that accounts for both the effects of flame dilution due to exhaust gas recirculation in the flame vortex interaction area and enthalpy losses to the wall. The manifold is validated in an a-priori analysis using the simulation results as a reference. The incorporation of exhaust gas recirculation effects in the manifold leads to a significantly increased prediction accuracy in the near-wall regions of flame-vortex interactions.

Structure and dynamics of the turbulent flow through a central baffle

Central baffle flume (CBF) can be utilized as a control structure to measure flow discharge in irrigation channels under free and submerged flow conditions. Stage-discharge relationship has been extensively studied for various geometrical parameters and flow conditions, whereas internal structure of the flow around a baffle has not been investigated in the literature. To address this need, the present work investigates the turbulent flow around a central baffle through high-resolution numerical simulations using an open source computational model. Velocity measurements were conducted in a laboratory flume to setup and validate the numerical model. Comparison of the numerical results with the experimental measurements proves that the present numerical model can predict water depth and velocity field. Longitudinal distance from the apex to the intersection point of water and critical depths can be estimated as Lxc = 2Le, where Le is the longitudinal length of the guide walls. A horseshoe vortex system identified in front of the baffle produces a significant bump on the free-surface and rib vortices generated from the baffle extend up to the sidewalls of the channel. The vertical separation layer observed downstream of the baffle results in a reverse flow and a vortex pair is formed by the impingement of the resulting reverse flow on the back of the baffle. Reverse flow, plunging flow structure, splash and rebounding wave events observed at the downstream produce substantial hydrodynamic effects on the baffle. Geometry of the central baffle was modified to suppress recirculation effects based on the insights into the complete flow structure around the baffle. Eventually, vortex structures were suppressed and the length of the recirculation zone was reduced by 76%.

Ultra-lean dynamics of holder-stabilized hydrogen-enriched flames in a preheated mesoscale combustor near the laminar critical limit

This work experimentally investigates the ultra-lean dynamics of a 40% H2–60% CH4 flame near the laminar critical limit in a preheated mesoscale combustor with a flame holder. These experiments are conducted to verify a conjecture we proposed in a previous publication and reveal the ultra-lean flame dynamics under the synergistic effects of heat and flow recirculation. Notably, not only is our conjecture confirmed, but also some novel flame behaviors are found. As the equivalence ratio ϕ is decreased from 0.500 to 0.320, the conventional stable flame, stable residual flame, periodic residual flame with repetitive local extinction and re-ignition (periodic RFRER), and periodic oscillating residual flame are observed in sequence. For the stable residual flame (0.370 ≥ ϕ ≥ 0.355), the left and right flame roots reside directly behind the flame holder, and the flame tip stays near the combustor exit. For the periodic RFRER (0.350 ≥ ϕ ≥ 0.340), observed experimentally for the first time, the flame roots reside at almost the same location, but the flame tip oscillates up and down over time with pinch-off events. For the periodic oscillating residual flame (0.335 ≥ ϕ ≥ 0.320), found for the first time, the stable flame roots also reside at almost the same location, but the residual flame tip oscillates up and down over time without a pinch-off event. When ϕ decreases to 0.315, the oscillating residual flame extinguishes, and its blow-off dynamics are revealed in detail.