A novel miniature Swiss-roll combustor (left figure) was designed for non-premixed combustion. Experimental investigation demonstrated that the flame structure and outer wall temperature distribution ...(right figure) are almost symmetric under most conditions. Second, the combustor can run at very low equivalence ratio and fuel flow rate. As a result, the wall temperature level is suitable for micro thermoelectric devices using low-temperature materials.
Display omitted
•A novel miniature Swiss-roll combustor was developed for non-premixed combustion.•Stable flames can be sustained under a nominal equivalence ratio less than 0.3.•This combustor can run stably under very small flow rates.•The flame structure and outer wall temperature distribution are symmetric.•The new combustor can be used as a heat source for micro thermoelectric generators.
In the past decades, combustion in miniature Swiss-roll combustors has widely investigated for applications to micro power generation systems. However, all the existing Swiss-roll combustors are designed for premixed combustion mode, in which the flame is prone to flash back or become extinguished at low flow rates. In the present work, a miniature Swiss-roll combustor was designed specifically for non-premixed combustion, which has two inlets for counterflows of methane and air, and two outlets for exhaust gas. Preliminary experimental investigation was conducted on a model combustor with a transparent quartz cover, which demonstrated that this new configuration brings about several merits. First, the flame structure and outer wall temperature distribution under most conditions are almost symmetric. Second, this combustor can run at a very low equivalence ratio (<0.3) and a very small fuel flow rate (˜0.029 L/min). As a result, the wall temperature level is relatively low, which makes this combustor a potential heat source for miniature thermoelectric devices using low-temperature materials such as Bi2Te3. Furthermore, the exhaust gas can be controlled at a very low temperature level (˜390–450 K), which means this micro combustor has a good heat recirculation performance.
The effects of aspect ratio (α = 1, 2, 3 and 4) of mesoscale rectangular channels on CH4-air flame stability were numerically studied. The results show that at relatively high inlet velocity, the ...flame is convex (with respect to the upstream) when α ≤ 3; instead it shifts to a W-shaped appearance at α = 4. In addition, the flame blowout limit exhibits a non-monotonic variation and it peaks at α = 3. Our analysis reveals that when α is raised from 1 to 3, both of the heat loss ratio and hydrodynamic perturbation rate are reduced, which favors a larger blowout limit. However, for the channel with α = 4, the perturbation rate of hydrodynamic instability is significantly increased at the flame tip because of the increased wave number, which results from the W-shaped flame front. Consequently, the intensified hydrodynamic instability will lead to a reduction in the flame blowout limit of the channel with α = 4.
For rectangular mesoscale channel with a relatively large aspect ratio (e.g., α = 4), the flame exhibits an unstable W-shaped appearance at high inlet velocity. Theoretical analysis reveals that the perturbation rate of hydrodynamic instability is significantly increased at the central flame tip, which leads to a reduction in the flame blowout limit of wide channels. Display omitted
•The effects of aspect ratio of mesoscale channels on flame stability were studied.•The flame shows a W-shaped appearance in the channel with an aspect ratio of α = 4.•Flame blowout limit exhibits a non-monotonic variation and it has a peak at α = 3.•As α rises from 1 to 3, heat loss ratio and hydrodynamic perturbation decrease.•For the channel with α = 4, the hydrodynamic instability of flame is intensified.
A Y-shaped micro combustor consisting of three quartz tubes with an identical inner diameter of 2.0 mm was designed. Effects of total flow rate and fuel/air ratio on the propagation behaviors of ...non-premixed hydrogen-air flames were investigated experimentally. Six distinct modes were observed, including flames with repetitive extinction and ignition. Noise emission was detected during flame propagation towards upstream under relatively high mixture velocities. The mean flame propagation speed decreased monotonically with an increasing mixture velocity, however, it exhibited a non-monotonic variation with the nominal equivalence ratio and peaked at ϕ = 0.9. Stable edge flames were obtained at the intersection under suitable conditions, and its length increased as the mixture velocity was increased, but varied non-monotonically with the nominal equivalence ratio and had a maximum value at ϕ = 0.9. The regime diagram of the six flame propagation modes was drawn, which can provide an overview of the relationship between the inlet condition and flame propagation mode. Qualitative analysis showed that flame dynamics of non-premixed micro-combustion was closely associated with the total flow rate and fuel/air ratio by influencing the mixing effectiveness, residence time, heat release rate and heat loss rate.
•Diffusion H2-air combustion in a Y-shaped micro combustor was experimentally studied.•Six flame propagation modes were observed by varying flow rate and fuel/air ratio.•Flame with repetitive extinction and ignition was captured in diffusion combustion.•Stable edge flame can be established at the intersection under suitable conditions.•Flame regime diagram was drawn in the “velocity-equivalence ratio” coordinate system.
Combustion efficiency of lean H2/air flames (left figure) can be notably improved at high inlet velocity in a double-layered micro-combustor compared to single-layered ones. This is attributed to the ...enhancement of heat recirculation through inner wall and the reduction of heat loss from outer wall, which can alleviate flame tip opening phenomenon and fuel leakage (right figure).
Display omitted
•A micro-channel combustor with double-layered walls and cavities was proposed.•The double-layered combustor can achieve higher efficiency than single-layered ones.•Heat recirculation is enhanced and heat loss is reduced in double-layered combustor.•Flame tip opening and fuel leakage can be alleviated in double-layered combustor.
In this study, a micro combustor with double-layered walls was proposed for micro energy and power systems. SiC and quartz were selected for the inner and outer layers of combustor walls, respectively. Numerical investigation demonstrates that the double-layered micro combustor can achieve a higher combustion efficiency at large inlet velocity than single-layered SiC and quartz micro combustors. It is revealed that the heat recirculation effect of double-layered micro combustor is greatly enhanced compared to quartz combustor; meanwhile, the heat loss rate is decreased to some extent compared to SiC combustor. Moreover, the negative stretch effect at flame tip is reduced in the double-layered combustor. Consequently, combustion process can be intensified and flame tip opening can be alleviated, leading to a higher combustion efficiency in the double-layered micro combustor.
In the present study, dynamics of non-premixed hydrogen-air flames in two Y-shaped cylindrical micro combustors of different horizontal channel lengths (L = 100 and 200 mm) were experimentally ...compared. The inner diameters of the micro-combustors are 2 mm. Unburned mixture was ignited by heating the near-exit wall with a butane torch. The results show that six and three flame propagation modes in the 200-mm and 100-mm micro-combustors were observed, respectively. Moreover, it is found that the flame oscillation duration is much longer with a larger noise intensity in the 200-mm micro-combustor. As a result, the mean propagation speed under L = 100 mm is much larger. In addition, the edge flame is longer on the lean side under L = 100 mm and almost identical on the rich side for the two combustors. Furthermore, the luminosity of edge flame in the 100-mm micro-combustor is much brighter. Numerical analysis reveals that the deflection of propagating flame in the Y-shaped micro-combustor is determined by the stoichiometric line. In summary, the short combustor has a smaller heat loss ratio and a stronger flame-wall thermal coupling, which can enhance the combustion intensity and increase the flame propagation speed.
•Effect of channel length on diffusion H2-air flames in micro-combustors was stduied.•More flame propagation modes were observed in the longer micro-combustor.•Flame oscillation is more intense in the longer micro-combustor.•Mean flame propagation speed is much larger for the shorter micro-combustor.•Luminosity of edge flame in the shouter micro-combustor is much brighter.
Two-dimensional (2D) numerical models are frequently adopted to investigate combustion and thermal performances in rectangular micro-channels for micro-thermophotovoltaic and thermoelectric devices. ...However, large error may exist by applying a simple 2D model. In the present work, the outer wall temperature distributions predicted by 3D model and simple 2D model were compared. The results showed that the maximum relative error of the simple 2D model depends significantly on the aspect ratio (α) of the micro-channel. To be specific, the maximum relative error was >30% for α = 1 and > 10% for 2≤α ≤ 4. However, it was <5% for α ≥ 9. A new 2D model was proposed to modify the underestimated heat loss ratio. The new computational results demonstrated that the maximum relative error of α = 1 decreased to 8.07% and for micro-channels with α ≥ 2, all the maximum relative errors are <5%. In summary, the modified 2D numerical model can achieve a satisfactory prediction with low computation cost.
•Prediction error using simple 2D numerical model was assessed for micro-channels.•The maximum error of the simple 2D model was above 30% for the square channel.•The maximum error of the simple 2D model is <5% when the aspect ratio is ≥ 9.•The simple 2D numerical model underestimates the heat loss ratio by up to 22.8%.•A modified 2D model was proposed which reduced the maximum error to 8.07%.
In the present work, the effects of inlet velocity and channel height (H0 = 0.6 mm, 1.0 mm and 1.4 mm) on the mixing performance, flame stability limit and combustion efficiency of H2 and air in a 2D ...planar micro-combustor with a separating plate were studied numerically. The results demonstrate that improved mixing can be achieved with a decrease in inlet velocity and channel height. Moreover, the flame blow-off limit is the largest for a micro-combustor with H0 = 0.6 mm; the flame becomes inclined at a high velocity and the direction varies with the inlet velocity. Furthermore, a micro-combustor with a medium height (H0 = 1.0 mm) can achieve the largest blowout limit among the three cases. Finally, for identical inlet velocities, the combustion efficiency increases with decreasing combustor height. In summary, these findings can provide a guideline for the optimal design of such micro-combustors.
•The mixing of hydrogen and air becomes better with the decrease of channel height.•The heat loss ratio rises up almost linearly with the reduction of channel height.•The micro combustor with a narrower channel height has a larger blow-off limit.•The micro combustor with a medium channel height has a largest blowout limit.•The combustion efficiency decreases with the increase of the channel height.
The study aims to investigate the levels of serum NLRP3 along with its effector molecules (Caspase-1, IL-1β, and IL-18) in the mid-pregnancy in pregnant women with hyperglycemia, and explore the ...relationship between NLRP3, along with its effector molecules (Caspase-1, IL-1β, and IL-18) and insulin resistance, as well as pregnancy outcomes.
The levels of serum NLRP3 along with its effector molecules (Caspase-1, IL-1β, and IL-18) in three groups of pregnant women with gestational diabetes mellitus (GDM), pregestational diabetes mellitus (PGDM) and normal glucose tolerance (NGT) were measured in mid-pregnancy, and their relationship with insulin resistance and pregnancy outcomes was analyzed. The ROC curve was also used to evaluate the predictive value of serum NLRP3 inflammasome and its effector molecules for pregnancy outcomes.
There were no statistical differences in the general clinical data of the three groups, and the concentrations of serum NLRP3 along with its effector molecules were higher in the GDM and PGDM groups than in the NGT group, and NLRP3 along with its effector molecules were positively correlated with fasting blood glucose, fasting insulin, and insulin resistance index in both groups (
> 0,
< .05). The incidence of preterm delivery, hypertensive disorders of pregnancy, premature rupture of membranes, neonatal hypoglycemia and macrosomia was significantly higher in both groups than in the NGT group (
< .05). The value of the combined serum NLRP3 and its effector molecules in mid-pregnancy to predict adverse pregnancy outcomes was highest, and the AUCs for the combined prediction of late hypertensive disorders of pregnancy, premature rupture of membranes, preterm delivery, neonatal hypoglycemia and macrosomia were 0.84 (95% CI 0.79-0.88,
< .001), 0.81 (95% CI 0.75-0.85,
< .001), 0.76 (95% CI 0.70-0.81,
< .001), 0.76 (95% CI 0.70-0.81,
< .001) and 0.72 (95% CI 0.63-0.81,
< .001), respectively.
Increased serum NLRP3 along with its effector molecules in pregnant women with hyperglycemia are associated with the levels of insulin resistance and the subsequent development of adverse pregnancy outcomes.
Dynamic mechanical analysis was used to investigate the damping behavior of a U–Nb shape memory alloy in various states, including water quenched (WQ), aging (AG), and cold rolling (CR). Internal ...friction peaks in the U–Nb alloy were identified, as well as their sensitivity to microstructure. The effects of amplitudes on internal friction and storage modulus were more pronounced in the WQ and AG samples than in the CR samples. A relaxation peak at 200 K was discovered in the U–Nb alloy, and its intensity decreased with aging, as did the activation energy for the relaxation event. It may be that the interaction between H and twin boundaries became more active as a result of the local concentration of Nb with aging. There are three transformation peaks in the current U–Nb alloy in the WQ and AG states, corresponding to phase transformations of α’’→γ0 and γ0→γ, respectively. The damping behavior of the coarse-grained and fine-grained samples differs only slightly, and grain size (grain boundaries), is not believed to be directly responsible for the difference; instead, twin boundaries as damping sources may dominate the damping behavior of the U–Nb alloy.
