Gas Side Research Articles

Flame self-interactions in globally stoichiometric spherically expanding flames propagating into fuel droplet-mists

A three-dimensional Direct Numerical Simulation (DNS) database of statistically spherical flames propagating into the mono-sized droplet-laden mixtures for an overall (i.e. gaseous + liquid phases) equivalence ratio of unity has been considered to investigate the distribution of flame topologies associated with flame self-interaction events. The analysis has been carried out for different values of initial droplet diameters and turbulence intensities to demonstrate their influences on the statistics of flame self-interactions. Spherically expanding stoichiometric premixed gaseous flames have also been additionally considered for the same initial burned gas radius and turbulence intensities for the purpose of comparison with the cases propagating into droplet-laden mixtures. It has been found that the topologies associated with flame self-interaction predominantly occur close to burned gas side of the flame front for turbulent premixed flames and also for droplet cases with small turbulence intensities, although tunnel closure/formation topologies are prevalent in premixed flames in contrast to unburned mixture pocket type topologies in the droplet cases. The distribution of the flame topologies has been observed to be influenced by both turbulence intensity and droplet diameter. The flame self-interaction events have mostly been found in the preheat zone for highly turbulent spray flames. It has been found that the occurrence of tunnel formation and closure topologies dominates over the availability of the unburned and burned mixture pockets in all cases except for the droplet cases under low turbulence intensities. The frequency of flame self-interaction events increases with increasing turbulence intensity and droplet diameter. The differences in the nature and distributions of flame self-interaction events between turbulent premixed and droplet cases are likely to have important implications on the possible extension of flame surface based modelling methodologies for simulating turbulent combustion in droplet-laden mixtures.

Simultaneous measurement of spatially resolved particle emissions in a pilot plant scale baghouse filter applying distributed low-cost particulate matter sensors

Baghouse filters are widely applied in industrial gas cleaning, for example in waste incineration plants and the cement industry, to meet particle emission standards and for product recovery. The global particle emission of pulse-jet cleaned surface filters is typically monitored end of pipe (e.g. in the stack). Since the particulate matter emission of baghouse filters originates often from leaks and incorrectly installed or damaged filter bags, operators would greatly profit from online measurement technology that monitors the emission contribution of individual filter bags or at least a subset of all installed filter elements to the total emission. Low-cost particulate matter sensors can be deployed inside filter houses in larger quantities due to their compact design and low asset cost compared to conventional aerosol measurement technology. The ability of several low-cost sensors to detect the characteristic PM emission behavior of surface filters has been shown in previous investigations in a filter test rig. This study shows first results regarding the emission contribution of individual filter bags of a pilot plant scale baghouse filter employing distributed low-cost sensors of the model OPC-N3 from the manufacturer Alphasense. A Promo® 2000 aerosol spectrometer with a welas® 2100 sensor serves as reference regarding the particulate matter concentration detected by the low-cost sensors and as end of pipe measurement equipment to monitor the global emission. The selected filter medium was a membrane filter medium with sealed seams to provide low emission levels and defined conditions on the clean gas side. The employed low-cost sensors detect an emission peak right after cleaning of the corresponding filter bag only. The global emission measured in the clean gas duct consists of the overlay of the individual emission peaks detected locally at the corresponding filter bags. By exchanging one filter bag with a filter bag made from a non-membrane filter medium without sealed seams, an increase of the total continuous emission can be detected, both end of pipe in the clean gas duct and locally via the low-cost particulate matter sensor. This demonstrates the applicability of the measurement technology for the detection and identification of leaks and damaged filter bags that serve as emission hotspots in baghouse filters.

Subsea natural gas dehydration in a membrane contactor with turbulence promoter: An experimental and modeling study

Subsea natural gas reservoirs are usually saturated with water vapor that causes corrosion in pipelines and forms hydrates blocking pipes and installations. In this work, the viability of the subsea natural gas dehydration in a non-porous membrane contactor was evaluated experimentally and by modeling.A flat sheet composite membrane is employed as the membrane interface and a structured packing turbulence promoter was installed at the gas side. The water flux and the outlet dew point were measured at different operating conditions (e.g., pressures, liquid and gas flow rates). To estimate simultaneously the permeability, overall mass transfer coefficient, and outlet water content, a systematic approach was applied and a 1D-2D model was developed bypassing the liquid phase mass transfer resistance and solved numerically coupled with a developed correlation for axial dispersion coefficient. The results show an increase in transmembrane water flux to the highest values of 72 g/m2h and 53 g/m2h with increasing gas flow rate and pressure to 500 ml/min and 11 bara, respectively. The effect of using a turbulence promoter was evaluated, and a good agreement between the simulated and experimental results was obtained. The mass transfer resistance of a dense layer was found to be dominating compared to that of gas and liquid phase and the high pressure and high gas flow rate are favored in this process. This study shows that membrane contactor enables effective water separation under subsea conditions. Installation of a turbulence promoter in the gas phase significantly reduced the water content in the outlet gas stream.

Affine Policies for Flexibility Provision by Natural Gas Networks to Power Systems

Using flexibility from the coordination of power and natural gas systems helps with the integration of variable renewable energy in power systems. To include this flexibility into the operational decision-making problem, we propose a distributionally robust chance-constrained co-optimization of power and natural gas systems considering flexibility from short-term gas storage in pipelines, i.e., linepack. Recourse actions in both systems, based on linear decision rules, allow adjustments to the dispatch and operating set-points during real-time operation when the uncertainty in wind power production is revealed. We convexify the non-linear and non-convex power and gas flow equations using DC power flow approximation and second-order cone relaxation, respectively. Our coordination approach enables a study of the mitigation of short-term uncertainty propagated from the power system to the gas side. We analyze the results of the proposed approach on a case study and evaluate the solution quality via out-of-sample simulations performed ex-ante.

Highly reversible oxygen redox in layered compounds enabled by surface polyanions

Oxygen-anion redox in lithium-rich layered oxides can boost the capacity of lithium-ion battery cathodes. However, the over-oxidation of oxygen at highly charged states aggravates irreversible structure changes and deteriorates cycle performance. Here, we investigate the mechanism of surface degradation caused by oxygen oxidation and the kinetics of surface reconstruction. Considering Li2MnO3, we show through density functional theory calculations that a high energy orbital (lO2p’) at under-coordinated surface oxygen prefers over-oxidation over bulk oxygen, and that surface oxygen release is then kinetically favored during charging. We use a simple strategy of turning under-coordinated surface oxygen into polyanionic (SO4)2−, and show that these groups stabilize the surface of Li2MnO3 by depressing gas release and side reactions with the electrolyte. Experimental validation on Li1.2Ni0.2Mn0.6O2 shows that sulfur deposition enhances stability of the cathode with 99.0% capacity remaining (194 mA h g−1) after 100 cycles at 1 C. Our work reveals a promising surface treatment to address the instability of highly charged layered cathode materials.

Analytical study of the direct initiation of gaseous detonations for small heat release

An analysis of the direct initiation of gaseous detonations in a spherical geometry is presented. The full set of constitutive equations is analysed by an asymptotic analysis in the double limit of Mach number close to unity (small heat release) and large thermal sensitivity. The quasi-steady curvature-induced quenching phenomenon is first revisited in this limit. Considering a realistic decrease rate of the rarefaction wave, the unsteady problem is reduced to a single nonlinear hyperbolic equation. The time-dependent velocity of the lead shock is an eigenfunction of the problem when two boundary conditions are imposed to the flow at the lead shock and at the burnt gas side. Following (Linan et al. , C. R. Mec. , vol. 340, 2012, pp. 829–844), the boundary condition in the quasi-transonic flow of burnt gas is expressed in terms of the curvature. Focusing our attention on successful initiation, the time-dependent velocity of the lead shock of a detonation approaching the Chapman–Jouguet regime is the solution of a nonlinear integral equation investigated for stable and marginally unstable detonations. By comparison with the quasi-steady trajectories in the phase space ‘propagation velocity versus radius’, the solution exhibits the unsteady effect produced upon the detonation decay by the long time delay of the upstream-running mode for transferring the rarefaction-wave-induced deceleration across the inner detonation structure from the burnt gas to the lead shock. In addition, a new and intriguing phenomenon concerning pulsating detonations is described. Even if the results are not quantitatively accurate, they are qualitatively relevant for real detonations.

Effect of Micro- and Nanobubbles on the Crystallization of THF Hydrate Based on the Observation by Atomic Force Microscopy

The air micro- and nanobubbles on a silicon wafer surface, generated by ethanol–water exchange method in THF solution, are found with anomalous small contact angles on the gas side due to the pinni...

Attempt of estimating flow characteristics from wall heat fluxes measured using a three-point micro-electro-mechanical systems sensor

In this study, it was attempted to estimate the flow characteristics in the vicinity of an engine inner wall from the instantaneous local heat fluxes measured using a micro-electro-mechanical systems sensor. As the sensor has three resistance temperature detectors with a size of 315 µm fabricated on a circumference with a diameter of 900 µm in rotational symmetry, it can measure local heat flux on the equivalent scale of the turbulence of in-cylinder flow. The advective velocity and turbulent eddy scale were estimated from heat flux fluctuations using a cross-correlation analysis, and these were compared with results of particle image velocimetry performed under motored operation conditions. As a result, it was found that the micro-electro-mechanical systems sensor has the potential to detect the gas side information such as the wall parallel flow velocity. Although further verification of the physical meanings of the estimated characteristics is necessary, the micro-electro-mechanical systems sensor will become a powerful tool for engine diagnostics.

Coupled simulation and validation of a utility-scale pulverized coal-fired boiler radiant final-stage superheater

The paper presents a coupled modelling methodology that employs a three-dimensional finite volume method computational fluid dynamics model to solve the boiler flue gas side, and a 1D FVM network approach to model the steam side flow and heat transfer of a final-stage radiant superheater. The CFD model includes the necessary upstream components that affect the thermal performance of the superheater, including combustion and heat transfer in the furnace and platen superheater. The methodology is applied to a 620MWe drum-type boiler of which the superheater was instrumented with thermocouples measuring the outlet steam temperatures on the different elements. This was augmented with boiler process data that was extracted from the distributed control system (DCS) for steady-state operation at 99% and 65% boiler loads.The results show that the approach can capture the details of the temperature maldistribution between legs in the same heat exchanger, between elements in the same leg and between the inner and outer loops of the same element for different load cases. It also captures the steam flow maldistribution between the inner and outer loops. On the flue gas side the results show that for the part-load case, the high gas temperature and velocity zones are less uniformly distributed compared to the full load case and also less symmetric than in the full load case.The calculated total heat transfer rates for the evaporator, platen superheater and final superheater compare well with the experimental values for both the 99% and 65% maximum continuous rating load cases. This shows that the coupled simulation models can capture the overall thermal performance of the three heat exchangers modelled here with good accuracy.

Research on gas side performance of staggered fin-tube bundles with different serrated fin geometries

This investigation conducted experiments on the gas-side performance of serrated spiral fin-and-tube heat exchanger with staggered layouts. The number of tube rows is 10 under the Reynolds numbers of 6000 to 12,000. The influences of varied water vapor content and fin geometry were explored. The results showed that the increase of water vapor content leads to an increase in the Nusselt number of the serrated fin and twisted serrated fin, and the effect on serrated fin was more significant. Besides, the decrease in the Euler number of both fin tube was observed with the water vapor content increasing. The twisting in the top of segment fin corresponded to a significant increase in the Nusselt and Euler numbers. Finally, the comparison of overall performance between the two types of fin tube was performed, and the results showed that the twisted serrated fin has better performance in the current experimental range.

An indirect online method for measuring the boiling rate of BFG boiler economizer

In blast furnace gas (BFG) boiler, high flow and large velocity of the flue gas will cause more heat to be absorbed by tail heating surfaces such as economizers. This will increase the probability of water boiling and vaporizing in economizer, which may cause safety accidents. Therefore, to facilitate the safe operation of boilers, it is of vital importance to grasp the boiling rate of economizer in real time. However, due to the complex state of medium in the piping, it is difficult to measure the boiling rate of economizer directly. Aiming at this issue, this paper presents an indirect online method for measuring the boiling rate of BFG boiler economizer through boiler operation parameters, based on heat transfer balance calculation. In this method, the boiling rate of economizer can be obtained indirectly through the boiler operation parameters on flue gas side and steam–water side. The measuring result can be used as the basis for boiler operation and adjustment, thus make the economizer running in a safe state for a long time, which can prolong the operation life of economizer.

Calculation studies of the ambient temperature variation influence on the gas tube boiler efficiency

The ambient air temperature variation influence on the efficiency of the gas tube hot water boiler operating in the heat supply system is considered in the paper. The dependency graphs of the fuel consumption, temperature in the near-wall layer of the convective bundle on the gas side, and aerodynamic losses along the length of the fire tubes in the convection bundle on the ambient temperature were drawn. The basic calculation formulas used for the calculation, as well as the temperature chart of the heat supply system operation depending on the ambient temperature are presented.

ОПТИМИЗАЦИЯ СТЕПЕНИ РЕГЕНЕРАЦИИ ДЛЯ ЦИКЛОВ МИКРОГАЗОТУРБИННЫХ УСТАНОВОК

A consideration subject in article is the mathematical model of pressure recovery factor of microgas turbine plants (MGTP) regenerators which considers dependence of hydraulic resistance of the heat–exchanger on the its surface area. Optimization of a regenerative cycle of MGTP and a cycle with regeneration and the turbocompressor utilizer for the purpose of further increase in their profitability is performed. It is established that use of the offered model of pressure recovery factor on the air and gas side allows to find degree of regeneration heattechnical optimum. This model can be used at simplified and predesign of MGTP.

Mechanism Analysis of Particle-Triggered Flashover in Different Gas Dielectrics Under DC Superposition Lightning Impulse Voltage

When DC GIL in operation endures the lightning impulse voltage, the charge accumulation at the gas-solid interface area will seriously affect the insulation performance of the spacer. Considering that gas side conduction is one of the important factors affecting charge accumulation, for the purpose of clarifying of the insulation characteristics of gaseous medium in the flashover process of gas-solid interface, an experimental platform for simulating the working conditions of the spacer is built. The spacer flashover tests were carried out with and without aluminum particle in SF6, 4% C3F7CN /96% CO2 and 20% SF6/80% N2 gas mixture. The measurement and analysis of surface potential distribution behavior of the spacer was conducted. The experiment results show that the gas dielectric is not the factor which dominate the potential distribution process without aluminum particle, and there is little difference in potential distribution with various gaseous conditions. When the linear aluminum particle appears on the surface of the insulator, it will cause severe electric potential distortion and these potential distorted areas are located around the end of the metal particle near the central conductor, and along with flashover pathway. It has also demonstrated that the gaseous dielectric has influence on the surface charge accumulation behavior especially with metallic particle adhere to spacer surface. Under the C3F7CN/CO2 gas mixture, the surface flashover voltage decrease percentage is about 16.82% and may be lower. Besides, the insulation strength of the gaseous dielectric itself is also a key factor affecting flashover.

Performance evaluation of wire mesh heat exchangers

Liquid-to-air cross-flow heat exchangers have a low heat transfer coefficient on the gas side due to poor thermal characteristics of air. One of the promising solutions is to attach extended surfaces on the gas side, thus increase the surface area of heat to transfer by convection. In this study, the experiment was carried out to evaluate the thermal and hydraulic performance of four modules of cross flow heat exchangers. One is bare copper tubes and three wire mesh heat exchangers with different mesh spacing. Hot distilled water was pumped into copper tubes at a constant flow rate of 9 l/min and inlet temperature of 80 °C. The heat exchanger subjected to external cross airflow inside an air duct of 20 cm × 20 cm cross-sectional area with an average air velocity of 2.8 m/s up to 14.9 m/s. Results showed an enhancement of Nusselt number for all wire mesh modules that is an increase of the volumetric heat transfer coefficient of up to 113.9% among the three tested modules compared to the bare module. The significance of this study is to use reasonably-priced wire mesh and maintain enhancement in the rate of heat transfer.

Microstructure driven design of porous electrodes for molten carbonate fuel cell application: Recent progress

This paper presents progress in development of microstructure in MCFC electrodes. Within these studies the influence of microstructure parameters of materials, such as porosity and pore size distribution on the fuel cell power density is analyzed using pure nickel.The results indicate that the optimal range of porosity for MCFC electrodes can be related with specific surface area, which reveals maximum for volume fraction of pores being 55–60%. The porosity of the cathode should be 5–10% higher due to in situ oxidation taking place during the startup procedure.Pore size distribution, PSD, was found to be especially important in MCFC, where liquid electrolyte infiltrates electrodes by capillary action. Through the application of specific porogens, multimodal PSD can be obtained, which significantly enhances reference cell power density.Optimization of basic microstructure parameters informed the design of a new concept MCFC cathode incorporating bi-layered materials, where each layer is appointed a different role. The application of commercial nickel foam as the gas side layer of the cathode increased power density and reduced the brittleness of the element, which is of key importance from the technological point of view.

Dynamic characteristics of a solar‐aided coal‐fired power generation system under direct normal irradiance (DNI) feedforward control

AbstractA solar‐aided coal‐fired power generation (SAPG) system is a new type of power generation system in the field of solar thermal power generation. However, the instability of solar radiation will cause fluctuations in steam temperature, which is unfavorable for the operation of boilers and steam turbines. Based on a 600 MW subcritical coal‐fired power generation unit, TRNSYS software is applied to establish a transient simulation model of SAPG in this paper. A dynamic feedforward control system for steam temperature is set up. Some essential parameters, such as the fuel consumption rate, solar‐heated steam extraction rate, water spraying flow rate, and flue gas damper opening, are under the control of direct normal irradiance (DNI). Then, the dynamic characteristics of SAPG are analyzed using typical weather data. The simulation results show that under the fuel‐saving mode, when solar irradiation changes, the SAPG system achieves a coal reduction of 4.9% by using the steam temperature control system with DNI feedforward control, and the main steam temperature fluctuation is less than 5°C with the reheat steam temperature reduced within a reasonable range. The power generation fluctuation is less than 10 MW, and the solar‐to‐electricity efficiency reaches 22.5%. Some parameters on both the flue gas side and the water/steam side in the utility boiler are simulated to demonstrate the effectiveness of DNI control.

Impact of the Heat Transfer Coefficient Determination Method on the Simulation of the Operation of Fin-and-Tube Cross-Flow Heat Exchangers at a Non-Uniform Inflow of Mediums

The work deals with a thermodynamic analysis of finned liquid-gas cross-flow heat exchangers. Analyzing such devices, it is important to determine the heat transfer coefficients on the liquid and the gas side. The analysis basic problem is determination of the of the heat transfer coefficient from the finned surfaces to the flowing gas due to a non-uniform flow of gas through the exchanger. The form and the scope of the gas inflow maldistribution were determined experimentally for two different fins, cross-flow heat exchangers with elliptical tubes. Six Nusselt number correlations were selected and were used to calculate the heat transfer coefficients for measured data. The values of heat transfer coefficient were also computed based on numerical simulations using numerical models of repetitive fragments of the tested heat exchangers. The Wilson plot method was also used. The heat transfer coefficient values were put into an in-house computer code and the experiments were simulated numerically. The comparison of experimental and computational data makes it possible to evaluate different methods of the heat transfer coefficient determination. The simplest Berman correlation for the Nusselt number as well as the computational fluid dynamics based approach gave the best results.

Achievement in ultra-low-load combustion stability for an anthracite- and down-fired boiler after applying novel swirl burners: From laboratory experiments to industrial applications

Deep peak shaving requires extremely high requirements for low-load combustion stability of boilers. In this study, a novel swirl burner (NSB) with an eccentric secondary air arrangement was proposed, and the validity and the progressiveness of the NSB in achieving ultra-low-load combustion stability for down-fired boilers (DFBs) were confirmed from laboratory experiments to industrial applications. Firstly, the cold-modelling experiments of gas/particle (GP) two-phase flow characteristics involving two combustion systems (i.e., the DFB with traditional swirl burners (TSBs) and the DFB with NSBs) were performed at an ultra-low load of 90 MWe. Compared with the original boiler with TSBs, the maximum horizontal recirculation velocity and the area of recirculation zone below arches significantly increase for the improved DFB with NSBs. The particle number concentration near furnace center for the improved DFB with NSBs is much higher than that for the original DFB with TSBs. The above two aspects will effectively guarantee timely ignition of anthracite. The downward depth of GP flows and the space utilization ratio of the lower furnace increase, which will be beneficial to promoting volume heat load and heat flux density in primary combustion zone. In addition, full-scale industrial-sized measurements aiming at a 300-MWe DFB improved by NSBs were carried out at ultra-low loads of 100 and 90 MWe. For the original DFB after improved by NSBs, the minimum load for stable combustion without oil support is reduced from 150 to 90 MWe, and the ultra-low-load combustion stability is achieved. For the improved DFB with NSBs, pulverized coal ignition distances for operating burners are about 2 and 2.6 m, respectively, and the signal strength of flame detectors for all operating burners is above 95% at 100 and 90 MWe. The flame fullness and combustion stability are good at the initial combustion stage. At ultra-low loads, furnace negative pressure, superheat steam pressure and oxygen concentration at furnace outlet fluctuate slightly. The temperature at air preheater inlet meets the needs of normal operation of denitrification system, and there is no problem of low temperature corrosion on the surface at flue gas side of air preheater. The unburned carbon in fly ash is about 4%. The maximum concentrations of NOx emission at furnace outlet are 714 and 687 mg/m3 (O2 = 6%) at 100 and 90 MWe, respectively, and ultra-low emission of NOx after denitrification system is achieved.

An additively manufactured novel polymer composite heat exchanger for dry cooling applications

The work presented in this paper focuses on the design and thermal characterization of a novel polymer composite heat exchanger (HX) produced by an innovative additive manufacturing process. The heat exchanger represents a gas to liquid configuration in which the gas side removes heat from the liquid side in a cross-flow arrangement. The novel HX utilizes a cross media approach in which, unlike the conventional HXs, the hot and cold sides are directly connected to each other through high conductivity metal fiber fins on the gas side protruding through the walls of the liquid side, thus eliminating the wall resistance separating the hot and cold sides. The HX demonstrates superior thermal performance at reduced pressure drops while also benefiting from the lighter weight and the lower cost that the polymer structure introduces. A 350-W water-to-air heat exchanger was fabricated using a fused filament fabrication (FFF) technique with a novel/patent pending printer head which was developed to produce the metal fiber composite structure of the heat exchanger. The results of the heat exchanger characterization tests show that it yields up to 220% and 125% improvement in heat flow rate over mass (Q/m) and heat flow rate over volume (Q/V), respectively, when compared to comparable state-of-the-art plate fins HX configurations. This study in particular demonstrates the impact of additive manufacturing in realizing potentially transformative heat exchanger technologies that may otherwise be very difficult to achieve with conventional fabrication methods.