Boundary Velocity Gradient Research Articles

Experimental study on real-time detection of artificial blood vessel micro-leakage via triboelectrification effect

Artificial blood vessels are prone to breakage in the form of apertures or fractures during periodic blood pulsation and installation. It has caused great threats and hidden danger to the treatment and rehabilitation of patients. Therefore, it is of great research value and practical significance to develop a rapid real-time detection method with high detection accuracy and good response characteristics for artificial blood vessels. Based on the triboelectrification(TE) effect and fluid boundary layer theory, this paper proposes and systematically studies a liquid–solid contact single-electrode mode sensing device for micro-leakage detection of vascular prosthesis (MLDVP). By real-time monitoring and analyzing the voltage and frequency characteristics of the output electrical signal, the damage form prediction, damage degree estimation, and leakage area location can be carried out quickly and effectively. The results show that the boundary layer structure and velocity gradient of the inner wall of the tube change when the artificial blood vessel is damaged, which changes the directional movement law of carriers at the liquid–solid interface, resulting in slow growth of the output open-circuit voltage. Further research shows that under the same damage type, with the gradual increase of damage degree, the output current of the external circuit detected by the system decreases gradually, and the corresponding output voltage change ratio also decreases. Therefore, the real-time monitoring and analysis of the open-circuit voltage and short-circuit current output by MLDVP can effectively realize the rapid detection and estimation of the damage location, damage type, and damage degree of the artificial blood vessel.

Stability Analysis of Mass Transfer Effects on the Unsteady Stagnation-Point Flow Past a Permeable Sheet

This study involves the problem of mass transfer effects on unsteady stagnation-point flow through a permeable stretching/shrinking sheet. Similarity transformations in the form of nonlinear partial differential equations to ordinary differential equations were used to reduce the basic governing equations. The resulting equations were then solved numerically using the bvp4c function and shooting method. The programming codes for bvp4c function were built using Matlab software, whereas the programming codes for shooting method were built using Maple software. The effects of the Schmidt number, stretching/shrinking, suction/injection, unsteadiness and the reaction rate parameter on the flow and mass transfer characteristics were numerically analysed. The boundary layer thickness and velocity gradient are also influenced by these parameters. Dual solutions were figured out for the problems involving stretching/shrinking sheet at the stagnation-point. To determine the stability of the solutions obtained, stability analysis was performed by solving the linear eigenvalue problems using the bvp4c function. The analysis showed that for the first solution, there was an initial decay of disturbance, while the second and third solutions were opposite, where an initial growth of disturbance were detected for both. This result indicated that the first solution was more stable and physically realizable compared to others.

The impact of hydrogen addition to natural gas on flame stability

Injecting hydrogen into existing natural gas networks is a promising step to mitigate global warming. However there is little evidence showing that how much degree of hydrogen admixture can be accepted. Experiments on populations of gas appliances are not available at present, thus a unified estimation method is required. First, a single-port burner was experimentally evaluated using methane-hydrogen mixtures (hydrogen percentage: 0–50%vol, unburned mixture temperature:373–573K, equivalence ratio:0.8–2). Mathematical formulae of the critical boundary velocity gradient, which quantitatively depict the relationships between flashback, lifting and hydrogen percentage, temperature were provided. A multi-ports burner was subsequently assessed, and the results of the two burners showed good agreement. An analysis method for changes in the flame stability region and gas source replacement decision was proposed based on curves of the flashback and lifting limits. This work provides an important basis for introducing hydrogen into gas networks and the design and adjustment of gas appliances.

Lateral Velocity Gradients in the African Lower Mantle Inferred From Slowness Space Observations of Multipathing

AbstractLarge low‐velocity provinces (LLVPs) are hypothesized to be purely thermal features or possess some chemical heterogeneity but which exactly remains ambiguous. Regional seismology studies typically use travel time residuals and multipathing identification in the waveforms to infer properties of LLVPs. These studies have not fully analyzed all available information such as measuring the direction and inclination of the arrivals. These measurements would provide more constraints of LLVP properties such as the boundary velocity gradient and help determine their nature. Here, we use array seismology to measure backazimuth (direction) and horizontal slowness (inclination) of arriving waves to identify structures causing multipathing and wavefield perturbation. Following this, we use full‐wavefield forward modeling to estimate the gradients required to produce the observed multipathing. We use SKS and SKKS data from 83 events sampling the African LLVP, which has been extensively studied providing a good comparison to our observations. We find evidence for structures at heights of up to 600 km above the core‐mantle boundary causing multipathing and wavefield perturbation. Forward modeling shows gradients of up to 0.7% δVs per 100 km (0.0005 km s−1 km−1) can produce multipathing with similar backazimuth and horizontal slowness to our observations. This is an order of magnitude lower than the previous strongest estimates of −3% δVs per 50 km (0.0044 km s−1 km−1). As this is lower than that predicted for both thermal and thermochemical structures, lateral velocity gradients capable of producing multipathing are not necessarily evidence for a thermochemical nature.

On the correlation of inverted flame blow-off limits with the boundary velocity gradient at the flame holder surface

Conductive heat losses from the base of a lean methane–air inverted flame stabilized behind the trailing edge of a thin rod have been experimentally evaluated. The results favor the view that the heat losses to the flame holder play a crucial role in the inverted flame stabilization and blow-off. Simple estimations have been performed, which indicate that the well-established correlation between the mixture composition and the boundary velocity gradient at the flame holder, usually considered as a proof of the flame stretch theory of blow-off, can be explained without involving the flame stretch concept. The suggested explanation of this correlation is based on the assumption that the heat loss to the flame holder is the main factor that determines the inverted flame blow-off behavior and on the similarity between the mechanisms of energy and momentum diffusion in gases (Pr≈ 1).

Effects of combustor size and filling condition on stability limits of premixed H2‐air flames in planar microcombustors

An experimental study on stability limits of premixed hydrogen‐air flames in planar microcombustors (H = 1 mm and 1.5 mm) partially filled with porous medium is carried out, focusing on the effects of combustor sizes and filling conditions. Critical conditions for blow‐off, flashback, and breaking through the porous medium are experimentally measured. The blow‐off limits are nearly independent of combustor sizes and filling conditions, while the flashback limits are strongly influenced by the combustor size and the filling conditions. Critical values for breaking through are identified with two different methods, and it is shown that standing combustion waves are settled over a range of velocities, instead of a fixed value of filtration velocity, which is considered an important characteristic of microcombustion. Most results can be explained by the classic boundary velocity gradient theory by von Elbe and Lewis, and thus the validity of the theory to the present channel spacings is confirmed. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2571–2580, 2015

Dynamic behaviors of premixed hydrogen–air flames in a planar micro-combustor filled with porous medium

The micro-combustor is a key component of the micro-thermophotovoltaic (TPV) system, and the flame stability limits are very important to its design and operation. An experimental study on the dynamic behaviors of premixed hydrogen–air flames in an H=1mm planar micro-combustor is undertaken. The combustor is partially filled with porous medium to enhance heat transfer and flame stabilities. Critical conditions (Φ1 for blow-off, Φ2 for flashback, and Φ3 for breaking through the porous medium) are experimentally identified for different combustor configurations. It is shown that the blow-off limits are nearly independent of the combustor configuration, while the flashback limits are strongly affected by the combustor configuration as well as the parameters of the inserted porous medium, including the position (Lout), the width (W) and the porosity (ε) of the porous medium. Most of the results can be explained by the classic boundary velocity gradient theory by von Elbe and Lewis in a qualitative sense. Based on the experimental findings, widening the operation window of the planar micro-combustor partially filled with porous medium is possible when the parameters are chosen properly.

О КОРРЕЛЯЦИИ МЕЖДУ ПРЕДЕЛАМИ СТАБИЛИЗАЦИИ ОБРАЩЕННОГО ПЛАМЕНИ И ГРАДИЕНТОМ СКОРОСТИ ВБЛИЗИ ПОВЕРХНОСТИ СТАБИЛИЗАТОРА

О КОРРЕЛЯЦИИ МЕЖДУ ПРЕДЕЛАМИ СТАБИЛИЗАЦИИ ОБРАЩЕННОГО ПЛАМЕНИ И ГРАДИЕНТОМ СКОРОСТИ ВБЛИЗИ ПОВЕРХНОСТИ СТАБИЛИЗАТОРА

The effect of hydrogen containing fuel blends upon flashback in swirl burners

Lean premixed swirl combustion is widely used in gas turbines and many other combustion Processes due to the benefits of good flame stability and blow off limits coupled with low NOx emissions. Although flashback is not generally a problem with natural gas combustion, there are some reports of flashback damage with existing gas turbines, whilst hydrogen enriched fuel blends, especially those derived from gasification of coal and/or biomass/industrial processes such as steel making, cause concerns in this area. Thus, this paper describes a practical experimental approach to study and reduce the effect of flashback in a compact design of generic swirl burner representative of many systems. A range of different fuel blends are investigated for flashback and blow off limits; these fuel mixes include methane, methane/hydrogen blends, pure hydrogen and coke oven gas. Swirl number effects are investigated by varying the number of inlets or the configuration of the inlets. The well known Lewis and von Elbe critical boundary velocity gradient expression is used to characterise flashback and enable comparison to be made with other available data.Two flashback phenomena are encountered here. The first one at lower swirl numbers involves flashback through the outer wall boundary layer where the crucial parameter is the critical boundary velocity gradient, Gf. Values of Gf are of similar magnitude to those reported by Lewis and von Elbe for laminar flow conditions, and it is recognised that under the turbulent flow conditions pertaining here actual gradients in the thin swirl flow boundary layer are much higher than occur under laminar flow conditions. At higher swirl numbers the central recirculation zone (CRZ) becomes enlarged and extends backwards over the fuel injector to the burner baseplate and causes flashback to occur earlier at higher velocities. This extension of the CRZ is complex, being governed by swirl number, equivalence ratio and Reynolds Number. Under these conditions flashback occurs when the cylindrical flame front surrounding the CRZ rapidly accelerates outwards to the tangential inlets and beyond, especially with hydrogen containing fuel mixes. Conversely at lower swirl numbers with a modified exhaust geometry, hence restricted CRZ, flashback occurs through the outer thin boundary layer at much lower flow rates when the hydrogen content of the fuel mix does not exceed 30%. The work demonstrates that it is possible to run premixed swirl burners with a wide range of hydrogen fuel blends so as to substantially minimise flashback behaviour, thus permitting wider used of the technology to reduce NOx emissions.

An experimental study of the stabilization mechanism of burner flames (2nd report, Examination of the stabilization mechanism)

In order to elucidate the blowoff mechanism of burner flames, mass burning velocities, tangential components of mass velocities to the ignition front and stretching near the flame base were examined. Experimental results show that the mass burning velocity of a stabilizing point in the flame base at the blowoff limit is larger than that at the lean limit of inflammability. Therefore, it is not a valid interpretation that blowoff occurs as the fuel concentration of the stabilizing point approaches that of the lean limit of inflammability, because of dilution with the surrounding air. Since stretching of the flame at the stabilizing point has a tendency to decrease when approaching the blowoff, the flame-stretch concept can not be applied to it. A model for the stabilization mechanism of premixed flames, in which the boundary velocity gradient theory is expressed algebraically, is proposed to explain the blowoff the burner flame, as well as the inverted flame reported previously by the authors.

Explanation of the blowoff of inverted flames by the area-increase concept

An experimental study of a laminar, two-dimensional inverted flame established in the wake of a thin plate inserted into the interior of the premixed gas stream was performed, and the effects of the radius of curvature of the very base of the flame on the blowoff limit were examined in detail. Experimental results show that as the boundary velocity gradient at the trailing edge of the plate is increased, the distance from the trailing edge to the flame base increases, whereas the radius of curvature of the flame base decreases. The critical radii of curvature at the blowoff limits tend to approach a constant value independent of the plate thickness when it is small. A single curve for the critical area-increase factors-namely, for the fractional area increases from the ignition plane to the front edge of the preheat zone for the flame base at the blowoff limits-can be drawn through the data points except for the thicker plate, and the blowoff is more directly correlated to the factor based on the area-increase concept than to the socalled Karlovitz number that has been used so far.

Blow-off Limit of Bunsen Flame Accompanied with Forced Secondary Air Flow

Many studies on the stability of Bunsen flame have so far been done by many investigators. However there are little studies on the flame stability accompanied with forced secondary air flow around the burner tube. Nomaguchi and coworkers related the blow-off limits of such flames with the critical boundary velocity gradient theory at the flame base and explained the mechanism of blow-off limits. The blow-off mechanism is, however, considered to be different according to the range of secondary air flow velocity. There-fore, analyzing the data of our experiments with the Maezawa's momentum theory, the following results were obtained:(1) In the case of small secondary air flow, Gi/Go=1-kGp0.5 (1)(2) In the case of large secondary air flow, Gi/Go=1-k'Gp0.1 (2)In Eqs.(1), (2), G is the momentum of gaseous fluid and subscripts i, 0 and p show with secondary air flow, without secondary air flow and secondary air flow, respectively.

Blowoff of inverted flames

Blowoff data have been obtained for methane-air flames stabilized on the edge of a thin plate forming a common long side of twin rectangular slits. Good correlations of the data, using the concepts of critical boundary velocity gradient and Karlovitz flame stretch factor, are possible, but different correlations are obtained for each stabilization plate thickness. The values obtained with very thin plates probably approximate the true values most closely, and with these plates blowoff tends to occur at a constant value of flame stretch factor independent of mixture composition. The results thus provide additional support for the flame stretch theory of blowoff. It is suggested that twin-slit inverted flame burners with extremely thin stabilization plates should permit the accurate determination of critical values of Karlovitz flame stretch factor.

Flame stabilization on rectangular burners

Disagreement exists between the relationships previously published for correlating the stability limits of rectangular burners in which the flow is fully developed. It is now found that flashback limits in laminar flow can be correlated satisfactorily by the boundary velocity gradient close to the corners of the burner port. The blow-off limits in both laminar and turbulent flow are best correlated by the average velocity gradient around the perimeter of the port. Blow-off limits in laminar flow can also be reasonably correlated by the boundary velocity gradient close to the corners. The velocity gradients are all calculated assuming that the flow is undistorted by the presence of the flame. The correlations are applied to a considerable body of new experimental data and also to the results of the earlier workers.

Flame stretch—A connecting principle for blow-off data

A new physical basis for the phenomenon of blow-off of aerated burner flames is proposed. This follows from the observation that combinations of velocity and velocity gradient at the base of this type of burner flame are similar to those which have been shown to result in extinction of a combustion wave. In such cases extinction results from excessive stretch of the combustion wave. Previously unpublished data on the burning velocity and stability of ethylene air flames over a range of temperatures and pressures together with similar data on other gases already available in the literature, are used to demonstrate that the phenomenon of blow-off may be accounted for on the basis of excessive flame stretch in the stabilizing region and that a single equation gb = (0.23 ρ cpSu2k)[1 − (1 − X6.4)α] correlates almost all of the data. This equation is valid for the following range of experimental parameters: atmosphere: air and inert: mixture composition: 0·7 to 1·36 of stoichiometric fuel: ambient pressure: 200 mm of mercury to atmospheric: mixture temperature: atmospheric to 693 K: flow state: laminar, transitional and turbulent Deviations from the equation are discussed; these arise when there are marked preferential diffusion effects. In such cases the relevant value of the burning velocity is not known and therefore the stretch factor cannot be calculated. It is shown that the well known correlation between mixture composition and the boundary velocity gradient at the blow-off condition, which is normally associated with the physical picture presented in the boundary velocity gradient theory of blow-off, is a special case of the flame stretch theory of blow-off. Suggestions are made for applying the similarity criterion known as the Karlowitz stretch factor, to various burner stability problems which are of particular importance to the gas industry at the present time.

Solutions to Some Steady- and Transient-Flow Problems Relevant to Flow Birefringence and Dichroism of Suspensions

Hydrodynamic equations are solved giving the flow velocity for the types of transient and steady flow that exists in various apparatus used in the study of flow birefringence and dichroism of suspensions. The time involved in starting and stopping the apparatus is taken into account. Formal solutions are given for general cases, and these are worked out in detail for the special cases of constant rate of change of boundary velocity or pressure gradient.

Studies of industrial high intensity gas combustors

This paper describes theoretical and experimental studies of high intensity gas combustors, providing information from which practical combustors have been designed and applied in industrial furnaces and heating equipment. The combustors consist of combinations of an injector, in which low pressure air injects fuel gas, followed by a refractory-lined combustion chamber in which the air/gas mixture is burned, and the burned gases, at or near flame temperature, are accelerated through a refractory exit nozzle. The studies included investigations of the fluid dynamics of injection, the burner pressure loss characteristics, flame stability, completeness of combustion and combustion intensity. In designing a combustor of this type, the objective is to make the best possible use of the air supply pressure to obtain the highest exit velocity of the burned gases, or alternatively, the highest air/gas mixture velocity at the mixture nozzle (if a burner of wide stability range is required, i.e., high turndown). This necessitates minimizing the pressure losses in the whole burner, from fuel gas inlet to combustion exit. The pressure losses in the system may be characterized by defining a dimensionless pressure efficiency, i.e., the ratio of the dynamic pressure of combustion gases in the exit nozzle to the dynamic pressure of air in the injector air nozzle. Examples of the calculation of pressure efficiency have been given for both incompressible and compressible conditions. Experimental determinations of flashback rates have been carried out over a wide range of burner sizes and the results in the laminar flow range have correlated with the concept of a critical boundary velocity gradient. In the turbulent region no such correlation has been found. Compositions of combustion products at the burner exit have been determined experimentally and compared with the calculated equilibrium values to find the combustion chamber length required for complete combustion.

Kinetic parameters for the hydrogen and ethylene flames from flashback measurements

Values for the flame activation energy and an overall reaction order have been obtained for premixedhydrogen-oxygen and ethylene-oxygen flames with nitrogen and with argon as diluents; the values come from measurements of the change with pressure and flame temperature of the critical boundary velocity gradient for flashback. Measurements have been made for rich and stoichiometric hydrogen flames and for lean and stoichiometric ethylene flames. For hydrogen flames with nitrogen diluent an overall order of 2·3 is found; for ethylene flames with nitrogen diluent, the order obtained is 1·8. With argon diluent, values for the order are about 10 per cent lower for each flame. With nitrogen diluent the flame activation energy apparently increases with flame temperature; thin increase is not found with argon diluent. For hydrocarbon flames the assumption that the initial equivalence ratio equals the effective equivalence ratio in the flame leads to an order of reaction with respect to oxygen that is considerably larger than unity. For hydrogen flames, that assumption is consistent with an oxygen order of unity.

Effect of Initial Temperature on Flash Back of Laminar and Turbulent Burner Flames

The effect of elevated initial temperature on the critical boundary velocity gradient for the flashback of laminar and turbulent flames has been measured for flames of hydrogen and ethylene with air and propane and ethylene with mixtures of 50 percent oxygen and 50 percent nitrogen by volume. For each fuel-oxidant system measurements have been made at a single equivalence ratio close to that giving the maximum gradient. For each system the dependence of the critical boundary velocity gradient on initial temperature is the same for laminar and turbulent conditions. This is consistent with the concept that a turbulent burner flame near flashback is stabilized in a near-laminar portion of the boundary layer. The extent of variation of the critical gradient with initial temperature differs markedly among the several systems studied.

The flashback of laminar and turbulent burner flames at reduced pressure

The effect of reduced pressure on the critical boundary velocity gradient for flashback has been studied for laminar and turbulent burner flames of hydrogen and propane with a variety of oxygen-containing mixtures. The proportionality between the boundary velocity gradient and the burning velocity divided by the quenching distance is examined. Detailed examination is difficult because of uncertainties in pressure exponents, particularly for burning velocity. However, flame reaction orders based on flashback are in general agreement with those based on quenching and burning velocity. Comparison of flashback gradients in the turbulent and laminar regions suggests that a turbulent burner flame near flashback is stabilized in the laminar sublayer. This interpretation requires, however, that a flame in a field of pipe turbulence shall penetrate about three times closer to a cold wall than a laminar flame.