Wall Heat Loss Research Articles

Evaluation of soot emission and wall heat loss for the fuels with high aromatic contents using the coupled analysis of chemical luminescence images with heat flux

The stricter regulations on the sulfur content of marine fuel oil in the general sea area that came into effect in January 2020 have led to a prediction of the increase of light cycle oil with high aromatic contents. The higher aromatic content is expected to result in higher soot in the exhaust gas. During combustion, soot emits radiant heat because of blackbody radiation, some of which passes through the walls of the combustion chamber and is released outside. Therefore, the exhaust emission characteristics and wall heat loss are affected if soot formation during combustion differs depending on the aromatic content of the fuel oil. This study developed an analytical method that combines the temporal and spatial distributions of C2 and OH radicals (these species contribute to soot formation and oxidation) to evaluate the emission and wall heat loss characteristics of different aromatics in fuel oils via combustion and heat flux analysis. The method was applied to test fuels containing different aromatic contents. Two test fuels containing 60 vol% each of toluene (monocyclic) and 1-methylnaphthalene (bicyclic) were evaluated in a rapid compression machine with a 100 mm wall-to-wall distance combustion chamber. The results suggested that the differences in soot emissions were observed because of differences in aromatics, and no differences in the wall heat flux were observed. The distributions of C2 and OH radical luminescence intensities between the walls were examined, and the differences in soot production were attributed to differences in the C2 radical production upstream of the flame in the region of low air entrainment. The lack of difference in the heat flux between the test fuels can be attributed to the the differences in the C2 and OH luminescence intensities, which are related to the heat release rate and heat flux, being almost nonexistent near the walls.

FGM modeling considering preferential diffusion, flame stretch, and non-adiabatic effects for hydrogen-air premixed flame wall flashback

Preferential diffusion plays an important role especially in hydrogen flames. Flame stretch significantly affects the flame structure and induces preferential diffusion. A problematic phenomenon occurring in real combustion devices is flashback, which is influenced by non-adiabatic effects, such as wall heat loss. In this paper, an extended flamelet-generated manifold (FGM) method that explicitly considers the preferential diffusion, flame stretch, and non-adiabatic effects is proposed. In this method, the diffusion terms in the transport equations of scalars, viz. the progress variable, mixture fraction, and enthalpy, are formulated employing non-unity Lewis numbers that are variable in space and different for each chemical species. The applicability of the extended FGM method to hydrogen flames is investigated using two- and three-dimensional numerical simulations of hydrogen-air flame flashback in channel flows. The results of the extended FGM method are compared with those of detailed calculations and other FGM methods. The two-dimensional numerical simulations show that considering both preferential diffusion and flame stretch improves the prediction accuracy of the mixture fraction distribution and flashback speed. The three-dimensional numerical simulations show that the prediction accuracy of the flashback speed, backflow region, and distributions of physical quantities near the flame front is improved by employing the extended FGM method, compared with the FGM method that considers only the heat loss effect. In particular, the extended FGM method successfully reproduced the relationship between the reaction rate and curvature. These results demonstrate the effectiveness of the extended FGM method.Novelty and Significance Statement The novelty of this research is the development of a flamelet-generated mani-fold (FGM) method that explicitly considers preferential diffusion, flame stretch, and non-adiabatic effects. To the best of the authors’ knowledge, no studies have performed numerical simulations of pure hydrogen flames using such an FGM method. The developed FGM method was applied to numerical simula- tions of hydrogen-air premixed flame flashback at an equivalence ratio of 0.5 and reproduced the flashback speed of lean hydrogen-air premixed flame. The applicability of the FGM method to the numerical simulation of hydrogen-air flashback is reported first. This research is significant because the FGM method is one of the most widely used combustion models for premixed combustion, and the development of an accurate FGM method will contribute to the engineering field. The accurate prediction of the flame flashback attempted in this study is particularly important for the development of hydrogen-fueled combustion devices.

Comparative study on the local hybrid combustion characteristics of the CH4-NH3 laminar premixed flames with/without the wall effects

The CH4-NH3 (50%:50%) premixed flame with/without the wall effects are simulated and compared at the stoichiometric condition in this study. The effects of cold wall and local flame dynamics on the flame temperature, heat release rate (HRR), CH4/NH3 oxidations and local CO/NO formations are analyzed quantitatively. Results show that the flame temperature and HRR are decreased and increased respectively along the flame front towards the axis of free flame, but they are both declined significantly towards the wall owing to suppressed chemical kinetics by the heat loss. The negative flame stretch and differential diffusion at the flame tip suppress the CH4 oxidation and the fast path (NH3→NH2→NH→N2) of NH3 oxidation effectively, but affect the slow path (NH3→NH2→HNO→NO→N2) of NH3 oxidation moderately. This contributes to suppressed CH4/NH3 oxidations and improved NO production at the flame tip of free flame. Based on NRR variations and ROP analysis, compared with CH4 oxidation, wall heat loss exerts stronger suppressions on the fast path of NH3 oxidation, while it decelerates the slow path of NH3 oxidation less evidently. Since the fast path is the primary pathway of the NH3 oxidation, the NH3 oxidation suffers more effective suppression of the wall heat loss than the CH4 oxidation. Furthermore, considering the significance of the slow path to the NO formation, the relatively enhanced importance of the slow path to the NH3 oxidation in the near-wall region predominates the comparatively effective NO formation in the CH4-NH3 flame compared with the pure CH4 flame under the influence of wall heat loss. For the CO/NO formations, the local CO production/oxidation in the wall-impinging CH4-NH3 flame are decelerated simultaneously due to the strong heat loss, while the local NO production/destruction are inhibited/improved by the wall heat loss.

Spatiotemporal Surface Temperature Measurements Resolving Flame-Wall Interactions of Lean H2-Air and CH4-Air Flames Using Phosphor Thermometry

AbstractSpatiotemporal wall temperature (Twall) distributions resulting from flame-wall interactions of lean H2-air and CH4-air flames are measured using phosphor thermometry. Such measurements are important to understand transient heat transfer and wall heat flux associated with various flame features. This is particularly true for hydrogen, which can exhibit a range of unique flame features associated with combustion instabilities. Experiments are performed within a two-wall passage, in an optically accessible chamber. The phosphor ScVO4:Bi3+ is used to measure Twall in a 22 × 22 mm2 region with 180 µm/pixel resolution and repetition rate of 1 kHz. Chemiluminescence imaging is combined with phosphor thermometry to correlate the spatiotemporal dynamics of the flame with the heat signatures imposed on the wall. Measurements are performed for lean H2-air flames with equivalence ratio Φ = 0.56 and compared to CH4-air flames with Φ = 1. Twall signatures for H2-air Φ = 0.56 exhibit alternating high and low-temperature vertical streaks associated with finger-like flame structures, while CH4-air flames exhibit larger scale wrinkling with identifiable crest/cusp regions that exhibit higher/lower wall temperatures, respectively. The underlying differences in flame morphology and Twall distributions observed between the CH4-air and lean H2-air mixtures are attributed to the differences in their Lewis number (CH4-air Φ = 1: Le = 0.94; H2-air Φ = 0.56: Le = 0.39). Findings are presented at two different passage spacings to study the increased wall heat loss with larger surface-area-to-volume ratios. Additional experiments are conducted for H2-air mixtures with Φ = 0.45, where flame propagation was slower and was more suitable to resolve the wall heat signatures associated with thermodiffusive instabilities. These unstable flame features impose similar wall heat fluxes as flames with 2–3 times greater flame power. In this study, these flame instabilities occur within a small space/time domain, but demonstrate the capability to impose appreciable heat fluxes on surfaces.

Simulation study of dynamic building insulation with transpired solar collectors

This study conducts a comprehensive numerical analysis using Ansys Fluent to investigate the thermal behaviour of transpired solar collectors (TSCs) focusing on temperature distribution, airflow dynamics, and heat recapture capabilities. The research evaluates TSC performance across various environmental conditions. A significant observation from the study is the TSCs' ability to recapture heat losses from the enclosures during nighttime, particularly under higher airflow conditions, where TSCs effectively redirect heat back into building spaces, contributing to energy conservation. Subsequently, the study reveals an inverse relationship between solar radiation levels and TSC efficiency in heat loss recapture. As solar radiation increases from 200 W/m2 to 1000 W/m2, the recaptured wall heat loss decreases by a percentage, ranging from 125 % to 241 % compared to the maximum observed during nocturnal periods. Additionally, the temperature analysis of the TSC backplate demonstrates a complex thermal response under different operational conditions. Solar radiation, airflow rates, and environmental properties influence this interplay. The study also uncovers a consistent linear increase in airflow velocity from the inlet to the outlet within the TSC air cavity, suggesting efficient airflow management. This insight is crucial for optimizing TSC design for improved heat exchange efficiency. In conclusion, the findings of this research offer valuable insights into the operational dynamics of TSCs, highlighting the importance of environmental factors in their design and functionality.

On the stabilization mechanism of high-speed deflagrations in narrow channels with heat loss

Statistically steady supersonic deflagrations are numerically investigated in narrow channels with strong thermal expansion and heat loss. Four modes of flame propagation are observed, namely, extinction, low-speed deflagration, high-speed deflagration, and DDT. It is determined that larger thermal expansion facilitates initiation of high-speed deflagrations while the heat loss can suppress the transition to detonation. The high-speed deflagration mode is shown to be the result of the dynamic balance between thermal expansion and wall heat loss. The limits of high-speed deflagration in terms of the thermal expansion and heat loss coefficients are determined. The statistically steady oscillatory high-speed deflagrations propagate at average velocities close to half of the CJ detonation velocity. The dynamics of the flame front and shock waves are visualized using numerical schlieren. Periodic acceleration and deceleration of the leading shock are identified, and the mechanism of DDT suppression is elucidated.

Waste heat recovery from household biomass gasifier stoves (HBGSs) for thermoelectric generators: Effect of the primary air valve angle

AbstractHousehold biomass gasifier stoves (HBGSs) are used in remote areas using natural firewood that is burned in the HBGS for general purposes, such as cooking and warming in winter. The heat dissipation from the walls is assumed to be heat loss. This paper evaluates the electrical performance of thermoelectric generators (TEGs) using heat recovered from the hot walls of an HBGS, considering the effect of three primary air valve angles of 0°, 9°, and 18°. The heat recovered from the HBGS that is electrically transferred through a TEG is assessed. The results showed that the wall heat loss increases with the primary air valve angle of the HBGS. The upper zone near the HBGS gas combustion zone exhibits higher wall temperatures than the middle and lower zones, with a maximum wall temperature of 140°C resulting in an output voltage of 2.55 V. However, the middle and lower zones are also considered as their capacities are consistent with the TEG temperature range. The heat loss from the HBGS is considerably larger than that from other heat sources, and the specific heating area of the heat source is a variable that should be considered to increase the number of TEGs to achieve the desired output values for practical applications. Furthermore, this research serves as a guideline for applying TEGs to recover waste heat from various applications.

Effect of the Cross-Section of a Porous Burner on the Combustion Stability Limit of Premixed Oxy-Methane Flames.

This study presents the idea of using a porous media burner to improve the oxy-methane combustion reaction rate and broaden the stability limit. Numerical studies on the premixed combustion of CH4/O2/CO2 in a two-layer porous medium burner using a two-dimensional symmetrical volume-average model with the skeleton mechanism based on OpenFOAM. The combustion characteristics of burners with variable cross-sectional (VC) and straight cylindrical (SC) structures were compared, including stable range, temperature field, thermal cycle efficiency, and CO emissions. It is confirmed that the cross-sectional structure is effective for broadening the stable range, and the broadening rate is more than 4 times. As more heat is lost from the outlet due to the increased cross-section, the solid-phase temperature of VC is lower than that of SC. As a result, the flame temperature of VC will be about 200 K lower than that of SC under the condition of insulated walls. It also leads to a reduction of about 2% in thermal cycle efficiency compared with SC. Meanwhile, it is shown that the VC structure slightly increases CO emissions at low thermal power but is smaller than that of SC at high power. When the wall heat loss of the burner is considered, the VC structure is still effective for improving the stable range. In addition, the VC structure slightly affects flame tilt and temperature uniformity at the burner outlet when the thermal power is large. It is confirmed that the VC structure is effective for increasing the power adjustment range and reducing pollutant emissions at high power.

Interactions of turbulence and flame during turbulent boundary layer premixed flame flashback under isothermal and adiabatic wall conditions using direct numerical simulation

In this work, lean hydrogen/air premixed flame flashback in a turbulent boundary layer along an isothermal wall and an adiabatic wall was simulated using direct numerical simulation. The general characteristics of flame flashback were analyzed. It was found that the propagating speed along the adiabatic wall is higher than that along the isothermal wall. The displacement speed near the isothermal wall is notably lower than that near the adiabatic wall, primarily due to the smaller value of the reaction component of the displacement speed near the isothermal wall. Furthermore, the properties of the boundary layer turbulence during flame flashback were examined in terms of the flow topology and the anisotropy of Reynolds stress. It was observed that the focal topologies tend to prevail near the adiabatic wall, and the volume fraction of stable topologies increases near the isothermal wall due to the increased compressed region caused by the wall heat loss. Finally, the influence of turbulence on the flame structure was highlighted. It was suggested that the unstable topologies dominate the heat release rate away from the wall. The proportion of heat release rate in focal topologies increases near the adiabatic wall, and the proportion in stable topologies increases near the isothermal wall. The impact of the low temperature wall on the flame structure is more pronounced in topologies with compression compared to those with expansion.

On the design of a hydrogen micro-rectangular combustor for portable thermoelectric generators

Micro-combustion based thermoelectric generators have been proposed to power micro-electromechanical units. The present study aimed to optimize the thermal design of micro-combustor to best suit the requirement of thermoelectric generators. An equivalent heat transfer coefficient was used to consider the arranged thermoelectric generators and its effect was elucidated. Increasing the coefficient enhances combustor wall heat loss, reducing flame stability and uniformity index of temperature. It is thus impossible to constrain combustor outer wall temperature below the maximum allowable temperature of commercial thermoelectric generators (623K) simply through increasing equivalent heat transfer coefficient. To address this issue, three simple strategies of combustor channel modification, adding copper layer and using flue gas heat recirculation were developed. A notable improvement in uniformity index of temperature over 6.5% is achieved for the latter two strategies. Nevertheless, none of these three strategies can ensure outer wall temperature below 623K. Motivated by the different mechanisms to improve temperature uniformity and reduce wall temperature for the two simple strategies of adding copper layer and using flue gas heat recirculation, a hybrid strategy combining them was proposed. The hybrid strategy can maintain flame stability, improve uniformity index of temperature approach to 100% and control combustor outer wall temperature below 623K, making it a qualified strategy for designing high-performance micro-combustors.

Computational Fluid Dynamics Modeling of HIsarna Off-Gas System—Wall Modeling and Heat Loss Prediction

The HIsarna off-gas system wall is a cooling jacket made of cooling pipes arranged in the radial direction and in a circular pattern. Part of the off-gas system cooling pipes are isolated using a low-thermal-conductivity refractory material to protect the cooling pipe from melting and thermal stresses. During long runs and due to thermomechanical stresses, the refractory material is lost, and its thickness is reduced. It is possible to measure the thickness of the refractory layer only during shutdown, which is a disadvantage during long runs. The aim is to investigate the possibility of predicting the thickness of the refractory material by using other parameters that are possible to measure during the operation. A combination of FEM and CFD modeling is used to develop a methodology for detailed wall modeling and refractory material loss prediction. Finite element method (FEM) analysis is used to obtain the thermal properties of the wall using detailed geometries for variable refractory thickness. The obtained properties are then used to build CFD models to study the effect of refractory thickness on wall heat loss, temperature and composition profiles. The proposed procedure is validated against the plant measurement, and according to the findings, it is possible to relate the wall thickness to measured parameters such as heat loss through the walls, temperature and carbon conversion.

Correlation of wall heat loss with quenching distance for premixed H2/Air flames during unsteady Flame-Wall interaction

Flame-wall interaction (FWI) of a laminar, premixed hydrogen/air flame perturbed by a pulsating inflow with excitation frequency (f) is investigated using fully resolved simulations employing finite-rate chemistry and detailed molecular diffusion. The setup is a two-dimensional (2D) V-shaped flame interacting with an isothermal wall on one side and a freely propagating flame on the other side, exhibiting a side-wall quenching and a freely-propagating mode. The impact of unsteady flame stretch on flame dynamics is characterized by the Damköhler number (Da), which is defined by the ratio of the turnover time of the oscillating inflow to the flame transit time. Da is found to directly describe the degree of flame deformation during flame propagation. During FWI, flame pocket formation is suppressed at conditions with Da close to unity. The correlation between wall heat flux (WHF) and quenching Peclet number (Peq) shows a time delay that depends on Da. A negative linear correlation can be recovered between time-averaged wall heat flux (WHF¯) with the quenching Peclet number (Peq¯), and its slope depends on the fuel equivalence ratio. For the lean hydrogen flame, the flame stretch near the quenching point leads to a decrease in flame intensity during FWI and therefore different slope of the correlation of WHF vs. Peq.

Velocity nonuniformity and wall heat loss coupling effect on supersonic mixing layer flames

With urgent demand for reliable flame stabilization in air-breathing high-speed aircrafts' combustors under a wide range of operational scenarios, understanding flame characteristics under various inflow and wall thermal conditions is crucial for mixing layer-stabilized flames. In this paper, the effect of inflow velocity nonuniformity, wall heat loss and their coupling on flame structures, flame lift-off and stabilization mechanisms in the supersonic mixing layer are analyzed. An approach based on the Beta distribution model is innovatively proposed to generate the pseudo nonuniform inflow velocity profiles. Different flame modes, the regime diagram and key controlling parameters are quantitatively analyzed via the active subspace method. The results show that the coupling of velocity nonuniformity and wall heat loss can lead to three flame modes, i.e., the heat loss-dominant flame, the intermediate flame, and the nonuniformity-dominant flame, as the flame stabilizes from upstream to downstream regions with the normalized flame lift-off length Lnorm in the range of [0,0.50], (0.50,0.58], and (0.58,1.0], respectively. This finding is consistent with the autoignition flame stabilization mechanism. In addition, the fuel burnt-out ratio of the flame is found to be nonlinearly correlated with flame lift-off length and peaks approximately with Lnorm=0.8, owing to the subtle interactions of mixing and chemical reactions in the mixing layer. The viscous heating in the boundary layer may have a pronounced impact on the flame lift-off depending on the level of wall heat loss, and the predicted Lnorm variation could be approximately 26 percent of the combustor length with/without viscous effect under the isothermal wall condition.

Wall heat loss effect on the emission characteristics of ammonia swirling flames in a model gas turbine combustor

Ammonia (NH3) combustion is regarded as one of the most promising solutions to realize zero CO2 emission. However, its low reactivity, low heat release result in a strong thermal effect (wall heat loss effect) which significantly affects flame macro structure and NOx emission characteristics. In this study, an air film cooling on the swirling combustion chamber was designed to investigate the wall heat loss effect on NOx emission. The flame structure and NO production were measured with the OH-/NO-PLIF techniques. The NOx emission was analyzed by the Gasmet DX4000 Fourier Transform Infrared (FTIR) gas analyzer. Large eddy simulation was also conducted with detailed chemistry to extend the understanding of the experimental findings. For the current combustion chamber, a larger convection heat loss was obtained when increasing the cooling air flow rate. NO emission shows a decreasing trend with heat loss which is mainly caused by the chemical reactions at near all region. This can be verified by the relatively farther NO profile from the combustor wall. By analyzing the LES results at near all region, the combustion efficiency is decreased due the lower temperature. As a result, NO production from HNO and NH pathway is suppressed due to the local OH decreasing in near-wall region. The larger unburned NH3 in the exhausted gas may also consumes NO by the NO reduction reactions. In comparison, N2O, around 300 times global warming potential than that of CO2, significantly increases as the heat loss increases. This is mainly because that the production of N2O from NO is promoted and N2O decomposition is suppressed within the thermal boundary. This study suggests that N2O might become a more serious emission component in NH3 fueled gas turbines. Furthermore, the heat loss effect on the NOx production should be fully considered when designing the gas turbine combustion chamber with strong wall cooling.

Experimental Investigation of Heat Transfer in a Flat-wall Impinging Diesel Spray Flame under Diesel-like Conditions

The heat loss through the walls of Internal Combustion Engines (ICEs) needs to be minimized and improved since it has a detrimental impact on the overall thermal efficiency of the engine. This study focuses on investigating heat transfer in a flat-wall impinging diesel spray flame, simulating diesel conditions, to optimize engine parameters. The effects of factors such as various injection pressures, nozzle hole diameters, impingement distances, and oxygen concentrations are analysed in combined and individually. Experimental techniques, such as high-speed imaging, heat flux sensors, and thermocouples, are used to visualize spray flame, measure heat flux profiles and temperature distributions, respectively. Preliminary results and their implications on heat transfer and heat loss are discussed. Regarding the parametric studies investigating the effect on wall heat loss under conditions similar to those of a small diesel engine, it was observed that the transferred heat on the wall was significant in certain conditions. These results emphasize the effectiveness of manipulating the injection rate profile as a viable step to suppress the heat transfer through the wall.

Effect of solid surface curvature and wall heat loss on the downward flame spread along the edge of thin PMMA sheets

Flames propagating along the sample edges (edge flame) will transform two-dimensional flame spread into three-dimensional flame spread, causing a significant interference with results of material flammability test. A potential method to minimize above effect is utilizing an inert wall to inhibit the side surface of sample with a designed air gap distance. In this work, the downward flame spread along the sample edge with various air gap distances is systematically studied. Cast PMMA with thicknesses from 0.4 to 2.5 mm were used as samples. The general trend of flame spread rate (FSR) at edge with respect to air gap distance can be summarized into three regimes: when the air gap distance is less than 2 mm, the edge flame is quenched by wall heat loss, and the overall flame spread behavior is two-dimensional. Then, the edge flame appears after the air gap distance exceeds 2 mm, resulting in a sharp increase in FSR. This critical air gap distance for the onset of edge flame is less sensitive to the sample thickness. Finally, as the air gap distance continues to increase, the FSR gradually approaches the asymptotic value of the uninhibited sample. With the increase in air gap distance, thermally-thin assumption loses validity as a result of the reduction in residence time of solid phase (larger FSR). To interpret the general trend of FSR with respect to air gap distance, a modified flame spread model based on conventional thermal theory is developed. The effect of air gap distance on flame spread along sample edge is characterized by two correction factors, one of which describes solid surface curvature at sample edge, the other of which is the effective flame temperature considering the heat loss from the spreading flame to the metal wall. The new model well reproduces the experimental results, and further supports the significance of heat loss in the trend of flame spread rate. This work helps to understand the flame spread mechanism under effects of solid surface curvature and heat loss, while also providing scientific support for optimization of material flammability testing standards.

Effects of turbulence–heat loss interactions on detonation development in end gas and its resulting knock intensity

The main objective of the present work is to investigate the end-gas autoignition and detonation development in a confined space with the presence of wall heat loss by two-dimensional numerical simulations with a hydrogen/air mixture. The effects of turbulence–heat loss interactions, initial temperature, equivalence ratio, and wall temperature on end-gas combustion modes are analyzed in detail. The results show that with the presence of wall heat loss, end-gas autoignition takes place in the hot core regions away from the walls, and the autoignition fronts touching the wall can lead to a much larger wall heat flux than that induced by main flame–wall interactions. In the base cases, increasing the turbulence intensity promotes the end-gas autoignition mode transition from thermal explosion-detonation to thermal explosion-deflagration and finally to no-autoignition, whereas detonation takes place in all cases regardless of the turbulence intensity after the initial temperature or equivalence ratio is raised. However, in these cases with a low equivalence ratio, the detonation propagation is unstable, which can be easily decoupled spontaneously after it encounters the cold flow. It is further found that for the cases with unstable detonation propagation, the burned mass fraction (BMF) dominates the knock intensity, whereas for the cases with stable detonation propagation, the maximum pressure in a chamber will extremely depend on the local and instantaneous interactions between the pressure/shock waves, but the effect of BMF becomes minor.

Experimental and numerical study on the thermocline behavior of packed-bed storage tank with sensible fillers

Packed-bed thermocline tank with sensible fillers is a cost-effective option for thermal energy storage (TES). In real charging and discharging, the thermocline stability is disturbed and thermocline expansion occurs due to various factors, leading to the decreased global performances of storage tank. The purpose of this work is to experimentally and numerically study the impact of inlet configuration and insulation on the thermocline expansion in a lab-scale tank under various operating parameters. Firstly, a DC-3P model considering the wall effect and heat loss is developed and validated by experimental results, capable of precisely predicting the performance of storage tank under the tested conditions. Especially, the temperature difference between the fluid and solid filler center is detected both in experiments and in modeling, indicating the existence of temperature gradient inside large solid particles. Results also show that the injecting flow causes the radial temperature non-uniformity and thermocline expansion at the near-entrance region of the tank, but its effect on the global performance of the storage tank is rather limited. Good insulation of the tank could reduce the thermocline thickness by about 20%, and increase the energy efficiency and the capacity ratio by 5–7% and 3–5%, respectively.

Combustion characteristics of steam-diluted decomposed ammonia in multiple-nozzle direct injection burner

In line with the decarbonisation of power sector, carbon-free fuels are currently being investigated. In particular, green ammonia or e-ammonia is a candidate fuel which will be playing a key role in many energy-intensive industries. It calls for an in-depth understanding of eFuels combustion characteristics in the fuel flexible combustors. Therefore, the present work for the first time numerically investigates the combustion regimes of steam-diluted, decomposed eNH3 in a novel multi-nozzle direct injection (MDI) burner. Although the MDI burner is not equipped with a conventional swirler, strong flow-flame interaction is observed. The two-layer, angled channels create swirling flows featuring swirl numbers larger than 0.9 in general. The centre recirculation region can help stabilise highly steam-diluted decomposed ammonia with a maximum steam-to-air ratio of 74%. This highest H2% containing, wettest ammonia flame case is found to emit the lowest total emission (NH3+NO + NO2+N2O) of ∼400ppmvd@15%O2 at stoichiometric conditions. The wall heat loss is confirmed responsible for the formation of N2O in distributed flame, suggesting the need of reducing pollution through good chamber wall insulation. However, for flames sitting in the conventional regimes, the impact of wall heat loss is found insignificant. Further, extensive data and flame regime analyses show that NNH can always accurately mark the high heat release region of all types of flames, while OH is only an effective marker for thin flames.

Experimental study of thermal efficiency and NOx emission of turbocharged direct injection hydrogen engine based on a high injection pressure

The direct injection (DI) hydrogen-fueled internal combustion engine (H2-ICE), as a promising carbon-neutral power device, has attracted more and more attention. However, the effects of the hydrogen injection strategy on the performance of H2-ICE lack in-depth analysis, even under the high injection pressure. Therefore, the present work investigated the thermal efficiency and NOx emissions with a single-hole injector over a wide range of high injection pressure and injection timing in a 3-cylinder turbocharged DI H2-ICE. The results show the strong dependence of high thermal efficiency on turbocharging and lean-burn strategies. The late-injection strategy can achieve higher thermal efficiency than the early-injection strategy, which can be applied at a wide range of loads. The effect of injection strategy on the brake thermal efficiency (BTE) mainly depends on the wall heat loss. Hydrogen late injection can reduce the wall heat loss by about 2% through strong mixture stratification. Furthermore, hydrogen late injection is always accompanied by a sharp increase in NOx emissions, even by orders of magnitude. This trade-off relationship between thermal efficiency and NOx emissions can be well optimized by a moderate late-injection strategy with an end-of-injection (EOI) of about 40 °CA. At the given load and start of injection (SOI), increasing the injection pressure from 5 MPa to 15 MPa can reduce the hydrogen injection duration by about 3 times. Under the higher injection pressure, the BTE performs a strong sensitivity to SOI. The BTE is improved by about 0.5% by optimizing SOI at PInj = 5 MPa mainly due to the reduced compression loss caused by further delay to the SOI allowed by higher injection pressure, while this improvement increases to about 2.3% at PInj = 15 MPa. However, at a given SOI, the highest BTE at PInj = 5 MPa is nearly 1.5% higher than that at PInj = 15 MPa, which implies the potential of low-pressure injection coupled with large-flow injectors. Finally, the DI H2-ICE performs well in terms of efficiency with a maximum BTE of 44.08%.