Meso-scale Tube Research Articles

Experimental study on combustion characteristics of n-C4H10/air mixtures in a meso-scale tube partially filled with wire mesh

Liquid fuels are good candidates for combustion-based miniature power sources due to their large specific energy. In this study, combustion characteristics of n-C4H10/air mixtures in a tube (ID = 6 mm) partially filled with wire mesh was experimentally explored over an equivalence ratio of ϕ = 0.5–1.5. In most cases, partially submerged flames and surface flames were observed on the downstream side of the porous media. An external diffusion flame was established under high inlet velocities for fuel-rich mixtures of 1.2 ≤ ϕ ≤ 1.5. The flame with a center convex started to appear at a certain inlet velocity due to the relatively higher local velocity, which was a result of the rolling up method of the wire mesh. Moreover, a lower extinction limit of 0.1 m/s was achieved over 0.9 ≤ ϕ ≤ 1.2, whereas the blow-off limit rose almost linearly up to 0.8 m/s with the equivalence ratio even on the fuel-rich side. The underlying mechanisms for this tendency were comprehensively analyzed from the preheating effect, heat loss effect, volumetric expansion effect as well as the support effect from the external diffusion flame. Furthermore, for an identical inlet velocity, the wall temperature first rose until ϕ = 1.1 and then decreased when ϕ > 1.1. Under a constant equivalence ratio, both the maximum wall temperature and the exhaust gas temperature increased with an increasing inlet velocity. However, for an identical inlet velocity, the exhaust gas temperature at low and moderate inlet velocities remained almost invariable versus the equivalence ratio. This was speculated to be the relatively small variation of heat loss ratio versus equivalence ratio. In summary, these findings enriched the knowledge of n-C4H10/air combustion in small tubes partially filled with porous media.

Fe3+-assistantly carbon-impregnated porous SiO2 mesoscale tubes as Li-ion battery anode materials with highly enhanced performances

Porous SiO 2 mesoscale tubes (SiO 2 -pMT) impregnated with carbon in the pores within their tube walls were successfully prepared for the first time by making use of the Fe 3+ assisted anchoring of dopamine (DA) on the surface of SiO 2 as carbon source, and thereby endowed with highly enhanced anode performances for Li-ion batteries. Typically, a reversible specific capacity of 512.8 mAh g −1 was achieved after 500 cycles at 1 A g −1 when the SiO 2 -pMT impregnated with carbon by Fe 3+ assistance was used as anode material, which is about 10 times that of SiO 2 -pMT with no carbon impregnation and 25% higher than that of the SiO 2 -pMT with carbon impregnated at double the concentration of dopamine but in the absence of Fe 3+ cations. It has been found that the carbon impregnation of the SiO 2 -pMT may be achieved more easily and homogeneously with the assistance of Fe 3+ cations and, as a result, 3-dimensional porous carbon-networks with higher connectivity can be built up, which greatly facilitate the transport of electrons to more potential reaction sites where Li-ions are available and provide fast channel for Li-ions diffusion, leading to enhanced electrochemical kinetics, as unveiled by the differential capacity curves and electrochemical impedance spectra acquired at different intervals during the initial cycle. Furthermore, as shown by XPS data, such benefitial assistance of Fe 3+ cations is believed to arise from their coupling effects between the phenolic hydroxyl groups of dopamine molecules and the surface of SiO 2 through the chemical bondings of C-O-Fe and Fe-O-Si. Fe 3+ -assisted carbon impregnation of porous SiO 2 mesoscale tubes (SiO 2 -pMT) with dopamine as carbon source (via path 1) leading to remarkable enhancement in carbon content, structural homogeneity, and anode performance in Li-ion battery in contrast to that achieved with no Fe 3+ -assistance (via path 2). • Fe 3+ was firstly introduced into dopamine for the fabrication of SiO 2 -pMT@iC. • The carbon impregnation was achieved more homogeneously with the assistance of Fe 3+ . • The coupling effect of Fe 3+ exists through the chemical bonds of C-O-Fe and Fe-O-Si. • High connected porous carbon network enable SiO 2 -pMT@iC with enhanced performance.

Improvement of the Combustion Completeness of Hydrogen Jet Flames within a Mesoscale Tube under Zero Gravity

Microjet hydrogen flames can be directly used as micro heat sources or can be applied in micro propulsion systems. In our previous study, under zero gravity and without an active air supply, the combustion completeness of hydrogen jet flames within a mesoscale tube with an inner diameter of 5 mm was very low. In this study, we were dedicated to improving the combustion efficiency by using a convergent nozzle (tilt angle was around 68°) instead of the previous straight one, and the exit diameter was 0.8 or 0.4 mm. The numerical results demonstrate that the maximum combustion efficiency in the case of d= 0.8 mm was only around 15%; however, the peak value for the case of d = 0.4 mm was around 36%. This happened because with d = 0.4 mm, the fuel jet velocity was around four times that of the d = 0.8 mm case. Hence, the negative pressure in the combustor of d = 0.4 mm decreased to a much lower level compared to that of d = 0.8 mm, which led to an enhancement of the air entrainment ratio. However, the highest combustion efficiency of d = 0.4 mm was still below 36%; therefore, a slightly larger tube or an even smaller nozzle exit diameter will be necessary for further improvements to the combustion efficiency.

Acoustic parametric instability, its suppression and a beating instability in a mesoscale combustion tube

The present work reports thermo-acoustic instability in a mesoscale tube of diameter 8 mm (> quenching diameter) and length 702 mm. Mixtures with Lewis number, Le~ 0.8 (rich C2H4/O2/CO2), 1.05 (lean C2H4/O2/CO2) and 1.34 (lean C2H4/O2/N2) are used. Several new flame responses are observed. For lower burning velocity mixtures, flame extinction is observed due to heat loss when primary instability transforms to secondary instability for all Le mixtures. However, parametric cellular structures which are characteristic of parametric instability are observed only for Le of 0.8. It is proposed based on calculations and experiments that parametric structures will be observed only when diameter of tube is two times larger than the characteristic wavelength of parametric instability. If the tube diameter doesn't allow formation of parametric structure whirling and counter-rotating flames are observed instead of parametric structures. Suppression of acoustic parametric instability is observed for higher burning velocity mixtures with Le>1 and its mechanism is discussed. For a range of SL, a beating instability is observed for CO2 diluted mixtures of Le>1, where pressure oscillation and flame motion show beating oscillations of frequency around 15 Hz. This beating instability is believed to be caused by non-linear interaction of acoustic instability with pulsating instability of flame front which is caused due to combined radiative and convective heat loss. Due to CO2 dilution, the radiative heat losses could play a significant role in inducing pulsating instability.

Numerical investigations on behaviour bifurcation of premixed H2-air flames in mesoscale tubes

Unsteady numerical simulations with detailed kinetic chemistry have been reported for premixed H2-air flames in straight mesoscale tubes of various diameters to understand the effect of wall heat transfer conditions on flame transition and behaviour bifurcation leading to a change in flame shape and flame propagation velocity. A parabolic velocity profile is applied with no-slip boundary conditions at the walls. For certain wall heat transfer conditions, it was observed that the flame transforms from a concave- to convex-shaped flame during propagation through the channel, accompanied by a change in flame propagation speed, heat of reaction and heat release rates. The transition in flame shape is attributed to thermal wall coupling effect through wall heat transfer to surroundings, which affects the flow redirection in the vicinity of the flame due to heat-loss, resulting in flame behaviour transition. Due to these flame behaviour transitions, the heat release rates and flame propagation velocities change significantly as discussed in this paper.

The effects of hydrogen addition, inlet temperature and wall thermal conductivity on the flame-wall thermal coupling of premixed propane/air mixtures in meso-scale tubes

The effects of hydrogen addition, inlet temperature, wall thermal conductivity and wall thickness on the flame-wall coupling of the propane/air flames in a meso-scale tube are numerically investigated using a two dimensional model along with the detailed chemical mechanism. Higher wall thermal conductivity can result in preheating the fresh mixture uniformly in strongly flame-wall coupled system, which is vital to enhance the burning rate of fuel mixture. With the increase of wall thermal conductivity or hydrogen addition, the leading edge of the flame shifts from the wall to the axis, meanwhile the flame is more convex towards the unburned side near the leading edge. As the hydrogen addition and inlet temperature increase, the flame propagation speed increases significantly, while the maximum temperature and maximum total enthalpy decrease due to the reduced heat recirculation power. The flame propagation speed has a negative correlation with heat loss. The chemical reactions in preheat zone are enhanced at low wall thermal conductivity due to the higher inner wall temperature. Thinner combustor wall leads to higher flame speed and higher heat loss simultaneously. Results have implications on the choice of solid wall material and heat recirculation design in a stable meso-scale combustor for different fuels.

A factorial approach to understanding the effect of inner geometry of baffled meso-scale tubes on solids suspension and axial dispersion in continuous, oscillatory liquid–solid plug flows

Oscillatory flow reactors (OFRs) are a new generation of tubular mixing and reaction equipment uniquely capable of combining continuous near plug flow with homogeneous particle suspension, yet the design of OFRs for liquid–solid and multi-phase flow processes relies on rules established during the past two decades from single, liquid-phase studies. A Design of Experiment (DoE) approach was herein implemented for establishing the relationship between four key geometrical parameters of the inner tube baffles and both the suspension of particles and the axial dispersion for liquid–solid continuous flows in 10mminternal diameter (d) meso-scale tubes with periodic baffles. The parameters evaluated were the orifice open diameter, do=0.35d–0.50d; the open cross section, α=0.12d–0.25d, constriction spacing, l=1.5d–3.0d, and baffle shape (sharp vs smooth edged). A total of ten tubes were tested, five consisting of smooth periodic constrictions (SPC) and the other five of sharp edged periodic constrictions (SEPC) according to a complete 2×2 factorial design with 1 central point. Each tube was experimentally evaluated via optical imaging of suspended monodispersed polyvinyl chloride (PVC) particles. Both SPC and SEPC meso-tubes were capable of delivering a near plug flow behaviour and the values of axial dispersion coefficient (Dc) estimated for the solids were in the range of 1.0–2.2×10−4m2s−1. In contrast, the minimum (critical) fluid oscillation conditions required for full suspension of particles varied significantly, in general with the SPC tubes requiring up to 50% lower amplitude for full particles suspension. Overall, α revealed the dominant parameter in controlling solids backmixing, and the inner tube geometry requiring the lowest energy input for homogenous particle suspension and minimum Dc (i.e. sharpest residence time distribution) presented a l/d=3, do=0.35d, α=12% and SPC design. This study is believed to support the future design of optimised meso-scale OFR systems for continuous screening and manufacturing of value-added liquid–solid and multi-phase systems, such as catalytic and crystallisation processes.

설비공학 분야의 최근 연구 동향: 2014년 학회지 논문에 대한 종합적 고찰

This article reviews the papers published in the Korean Journal of Air-Conditioning and Refrigeration Engineering during 2014. It is intended to understand the status of current research in the areas of heating, cooling, ventilation, sanitation, and indoor environments of buildings and plant facilities. Conclusions are as follows. (1) The research works on the thermal and fluid engineering have been reviewed as groups of heat and mass transfer, cooling and heating, and air-conditioning, the flow inside building rooms, and smoke control on fire. Research issues dealing with duct and pipe were reduced, but flows inside building rooms, and smoke controls were newly added in thermal and fluid engineering research area. (2) Research works on heat transfer area have been reviewed in the categories of heat transfer characteristics, pool boiling and condensing heat transfer and industrial heat exchangers. Researches on heat transfer characteristics included the results for thermal contact resistance measurement of metal interface, a fan coil with an oval-type heat exchanger, fouling characteristics of plate heat exchangers, effect of rib pitch in a two wall divergent channel, semi-empirical analysis in vertical mesoscale tubes, an integrated drying machine, microscale surface wrinkles, brazed plate heat exchangers, numerical analysis in printed circuit heat exchanger. In the area of pool boiling and condensing, non-uniform air flow, PCM applied thermal storage wall system, a new wavy cylindrical shape capsule, and HFC32/HFC152a mixtures on enhanced tubes, were actively studied. In the area of industrial heat exchangers, researches on solar water storage tank, effective design on the inserting part of refrigerator door gasket, impact of different boundary conditions in generating g-function, various construction of SCW type ground heat exchanger and a heat pump for closed cooling water heat recovery were performed. (3) In the field of refrigeration, various studies were carried out in the categories of refrigeration cycle, alternative refrigeration and modelling and controls including energy recoveries from industrial boilers and vehicles, improvement of dehumidification systems, novel defrost systems, fault diagnosis and optimum controls for heat pump systems. It is particularly notable that a substantial number of studies were dedicated for the development of air-conditioning and power recovery systems for electric vehicles in this year. (4) In building mechanical system research fields, seventeen studies were reported for achieving effective design of the mechanical systems, and also for maximizing the energy efficiency of buildings. The topics of the studies included energy performance, HVAC system, ventilation, and renewable energies, piping in the buildings. Proposed designs, performance performance tests using numerical methods and experiments provide useful information and key data which can improve the energy efficiency of the buildings.

A numerical study of propane-air combustion in meso-scale tube combustors with concentric rings

Dwindling energy resources and strong demand for better power sources as compared to conventional batteries have sparked research interest in micro power generation. The invention of state-of-the-art electronic devices requires more energy capacity, shorter charging period and light in weight, characteristics of which batteries lack. Therefore, in recent years micro power generation systems have been seen as a potential alternative to batteries owing to the obvious advantages that it has. It is essential to fully understand the underlying factors that affect the combustion stability in meso and micro-scale combustors. One of the popular methods to examine these factors is by performing numerical simulations. This paper demonstrates an axisymmetric two-dimensional steady state numerical simulation of propane-air combustion in meso-scale cylindrical tube combustors with concentric rings. The inner diameter of the tube is set to 3.5 mm and the wall thickness is specified to 0.7 mm. The concentric rings are placed between the unburned and burned gas region. The main function of these rings is to act as a flame holder where a stable flame can be easily established. The wall thermal conductivity in the unburned and burned gas region is varied from 1 W/m/K to 1000 W/m/K and the results in terms of gas, inner wall, outer wall surface temperature distribution, the blowout limits and combustion efficiency are analyzed and presented. In addition, the effect of the inlet velocity and the equivalence ratio is also investigated. The results show that the inlet velocity and equivalence ratio have significant impacts on the flame temperature, which in turn change the wall temperature distribution. Although the wall thermal conductivity has minimal effect on the flame temperature, both inner and outer wall surface temperature are greatly affected. Consequently, this variation of wall temperature contributes to the significant changes on the blowout limits. It is also shown that the combustion efficiency is influenced by the wall thermal conductivity of the combustors.

Combustion of gaseous and liquid fuels in meso-scale tubes with wire mesh

Combustion of gaseous and liquid fuel in meso-scale tubes with wire mesh was studied experimentally. For C3H8/air mixtures, the flame can be stabilized near the mesh inside the tube without external heating even for smaller tube than the classical quenching diameter by heat recirculation from the burned gas to the unburned gas through heat conduction in the wall and mesh. The mesh enhances heat transfer from the heated wall to the unburned gas. For tubes larger than the classical quenching diameter, the flame can be stabilized inside the tube even outside the flashback limit by the heat recirculation. Within a specific range of flow velocity, the flame propagates at relatively high speed and extinction occurs near the mesh, but the flame can be stabilized near the mesh if the condition is gradually shifted from another condition with a stabilized flame. Therefore, the mesh can act as a combustion inhibitor or an enhancer. The two types of burning behavior are discussed based on two steady-state flames; normal flame and weak flame predicted by other researchers. Next, combustion of electro-sprayed ethanol/n-heptane mixtures inside a meso-scale tube with d=3.5mm with a single mesh and double meshes was studied. The flame can be stabilized near the mesh without external heating. If droplets passing through the mesh directly interact with the flame, heat loss and asymmetric flame shape occurs. The flame stabilization region with double meshes is larger than that with a single mesh and comparable to that for gaseous fuel combustor except for the wall wetting region.

Heat recirculation effects on flame propagation and flame structure in a mesoscale tube

Heat recirculation effects on flame propagation and flame structure are theoretically and experimentally examined in a mesoscale tube as the simplest model of heat-recirculating burners. Solutions for steady propagation are obtained using a one-dimensional two-temperature approximation. The results show that the low heat diffusivities of common solid materials permit significant heat recirculation through the wall only for a slowly-propagating condition, otherwise the flame behaves almost like a freely-propagating nonadiabatic flame. This limited heat recirculation sharply pinches and stretches two well-known branches of the freely-propagating nonadiabatic flame, resulting in the appearance of two slow-propagation branches. On the upper slow-propagation branch flames can reach superadiabatic temperatures and on the lower one, which is stretched from the classical unstable lower branch, flames can be stable. As the tube inner diameter decreases, another burning regime appears where flames are barely sustained by the heat recirculation. Further reduction of the tube inner diameter makes no flame exist. It is also revealed that a flame in a mesoscale tube has two length scales, i.e. the conventional flame thickness and a convective preheat zone thickness, and that the latter should be much larger than the former for significant heat recirculation. It is theoretically predicted that a heat-recirculating, even superadiabatic, flame with positive propagation velocity against the gas flow can exist in a mesoscale tube. It is also found that a flame transition from one branch to another in a given tube is well described by only one dimensionless parameter. Finally, these theoretical results show good qualitative agreements with experiments, especially for the transition behaviours.

J0801-2-5 メソスケールの細管内におけるボルテックスバースティング機構による火炎伝播([J0801-2]エンジン,動力システムにおける燃焼技術(2))

J0801-2-5 メソスケールの細管内におけるボルテックスバースティング機構による火炎伝播([J0801-2]エンジン,動力システムにおける燃焼技術(2))

SnO 2 meso-scale tubes: One-step, room temperature electrodeposition synthesis and kinetic investigation for lithium storage

SnO 2 meso-scale tubes were synthesized by anodic electrochemical deposition under ambient conditions. Controlled self-bubbling O 2 acted as both the template and the oxidizing agent for obtaining SnO 2 tube structures at the interface of the gas (O 2) and the liquid (electrolyte). Electrochemical testing showed that the meso-scale tubes have higher discharge capacity and better rate capability than the “microbowls” produced by varying the deposition conditions. From the Arrhenius plot, the apparent activation energies were calculated to be 58.4 and 90.1 kJ mol −1 for the meso-scale tubes and the microbowls, respectively, indicating that the meso-scale structure allows shorter diffusion routes for the lithium ions or for easier interaction with lithium.