Gas Boundary Layer Research Articles

Longitudinal oscillation of a liquid sheet by parallel air flows

A liquid fuel sheet injected into the combustor of gas turbine engines is deformed and atomized by the complex interactions between the liquid sheet and air flows. Aiming at improving the control technology of a fuel spray, the oscillation phenomenon and the primary break-up process of a planar liquid sheet with air flows have been studied for many years. Based on the previous studies, we propose a new correlation on the longitudinal wavelength λLon given by λLonDLip=cMRLip and that on the oscillation frequency fLon of a liquid sheet given by fLon=c′VGρL/ρGDLDLip, where MRLip is the lip momentum ratio defined in this study. In addition to previous visualization experiments of a planar liquid sheet and parallel air flows with various densities of gas and liquid, gas and liquid velocities, liquid sheet thicknesses and lip thicknesses, we carry out an additional experiment with various gas velocities and liquid viscosities to cover all the effects of fluid properties, injector geometries including gas and liquid boundary layers on the deformation and the atomization characteristics of the oscillating liquid sheet. Image analysis is conducted to obtain fLon. As a result, we confirm that liquid viscosity does not affect fLon and λLon of the liquid sheet in a wide range of liquid Reynolds number. Finally, we verify the validity of the correlations of λLon whose constant c is 14.3 and fLon whose constant c’ is 0.095.

Effect of Viscous Gas on Stability of the Electrified Liquid Sheet

An electrified liquid sheet was investigated by the linear analysis method. The sheet was injected into a dielectric viscous gas bounded by two horizontal parallel flat plates with a transverse electric field. To take into account the gas boundary layer thickness, the velocity profile of the liquid sheet must be considered and derived. By analyzing the liquid and gas domain, the relation between the growth rate and the wavenumbers for electrified liquid sheets was derived, and solved using the spectral method whose accuracy is higher than those of the finite element method and the finite difference method. Two modes, namely the sinuous mode and varicose mode were also investigated. The results revealed that the maximum growth rate of the sinuous disturbance wave is greater than that of the varicose one for the electrified liquid sheet. Moreover, the effects of the electrified Euler number and other parameters on the instability of the electrified liquid sheets have been tested. It is found that the growth rate is faster for an electrified liquid sheet than a non-electrified one. The electrical Euler number, the liquid Reynolds number, the Weber number and the momentum flux ratio can promote the breakup of a liquid sheet. However, the increase in the ratio l of the distance between the liquid sheet and flat plate to the liquid sheet thickness has an opposite influence with the other parameters.

REYNOLDS' LIMIT FORMULA FOR DORODNITZYN'S ATMOSPHERIC BOUNDARY LAYER MODEL IN CONVECTIVE CONDITIONS

Atmospheric convection is an essential aspect of atmospheric movement, and it is a source of errors in Climate Models. Being able to generate approximate limit formulas and compare the estimations they produce, could give a way to reduce them. In this article, it is shown that it is enough to assume that the velocity's $L^2$-norm is bounded, has locally integrable, $L^1_{loc}$, weak partial derivatives up to order two, and a negligible variation of its first velocity's coordinate in direction parallel to the surface, to obtain a Reynolds' limit formula for a Dorodnitzyn's compressible gaseous Boundary Layer in atmospheric conditions.

Passivation Threshold for the Oxidation of Liquid Silicon and Thermodynamic Non-equilibrium in the Gas Phase

The present study focuses on a specific step of the metallurgical path of purification to provide solar-grade silicon: the removal of boron through the injection of H2O(g)-H2(g)-Ar(g) (cold gas process). A progressive increase of the oxidant H2O(g) concentration at injection increases the speed of the process until a silica layer appears at the surface of the liquid silicon to be purified. It then stops the purification. During the process, silica aerosols may form in the gas boundary layer. This modifies the flows of oxidants and the gas concentrations at the liquid silicon surface. Using a monodimensional model, this article shows that a hypothesis of thermodynamic equilibrium of silica aerosols with the gas phase in the boundary layer has to be dropped in order to explain the appearance of a silica passivating layer. The passivation threshold is defined as the limit concentration of the oxidant at the injection below which there is no silica on the liquid silicon surface and beyond which silica particles appear on the liquid silicon surface. Three experiments of estimation of the passivation threshold with the injection of water vapor are used to confirm an empirical criterion for the prediction of the appearance of the silica layer. Two other sets of experiments with the injection of Ar-O2 are also being studied where the kinetics of the formation of silica aerosols seems to be slower than when water vapor is used. An optimization of the speed of boron removal under the assumption of a maximal concentration of water vapor before the appearance of a passivating silica layer would require an increase of the liquid silicon surface temperature from the fusion temperature of silicon.

Theoretical Investigation For The Effect of Fuel Quality on Gas Turbine Power Plants

Gas turbine engine power generation is declined dramatically because of the reduction in thermodynamic parameters as a work of turbine, compressor ratio, compressor work, and air mass flow rate and fuel consumption. There are two main objectives of this work, the first is related with the effect of fuel kinds and their quality on the operation of fuel flow divider and its performance specifically gear pump displacement and fuel flow rate to the combustion chambers of gas power plant. AL-DORA gas turbine power plant 35MW was chosen to predict these effects on its performance MATLAB Software program is used to perform thermodynamic calculations.Fuel distribution stage before the process of combustion and as a result of the kind and its quality, chemical reaction will occur between the fuel and the parts of the gear system of each pump of the flow divider, which causes the erosion of the internal pump wall and the teeth of the gear system, thus hampering the pump operation in terms of fuel discharge.The discharge of fuel form the eight external gates of flow divider is decreased and varied when going to the combustion chambers, so that, flow divider does not give reliable mass flow rate due to absence of accurate pressure in each of eight exit pipes.The second objective deals with the stage of fuel combustion process inside the combustion chamber. A comparative study based upon performance parameters, such as specific fuel consumption for gas and gasoil and power generation. Fuel poor quality causes incomplete combustion and increased its consumption, so that combustion products are interacted with the surface of the turbine blades, causing the erosion and create surface roughness of the blade and disruption of gas flow. As a result of this situation, turbulence flow of these gases will increase causing the separation of gas boundary layers over the suction surface of the blade. Therefore the amount of extracted gas will decrease causing retreat work done by turbine, as a result decline of power and gas turbine power plant efficiency causing the drop in the level of electric generation. The fuel quality is found to be a strong function of specific fuel consumption and its effects on the power generation and the efficiency of the gas turbine power plants and hence, the cycle performance shifts towards favorable conditions.

Surface-Induced Near-Field Scaling in the Knudsen Layer of a Rarefied Gas.

We report on experiments performed within the Knudsen boundary layer of a low-pressure gas. The noninvasive probe we use is a suspended nanoelectromechanical string, which interacts with ^{4}He gas at cryogenic temperatures. When the pressure P is decreased, a reduction of the damping force below molecular friction ∝P had been first reported in Phys. Rev. Lett. 113, 136101 (2014)PRLTAO0031-900710.1103/PhysRevLett.113.136101 and never reproduced since. We demonstrate that this effect is independent of geometry, but dependent on temperature. Within the framework of kinetic theory, this reduction is interpreted as a rarefaction phenomenon, carried through the boundary layer by a deviation from the usual Maxwell-Boltzmann equilibrium distribution induced by surface scattering. Adsorbed atoms are shown to play a key role in the process, which explains why room temperature data fail to reproduce it.

Break-up length of liquid jets produced by short nozzles

Numerous experimental studies and theoretical investigations related to the break-up length of liquid jets can be found in the literature. Experimental results, given in this paper and obtained with nozzles having a very small length-to-diameter ratio, showed poor agreement with existing theoretical and experimental data. It was found that the discrepancies can be explained by differences in the ambient Reynolds number ranges covered. Therefore, the properties of the ambient gas boundary layer around the liquid jet had to be taken into account to describe theoretically the experimental stability curves. A modification of the model of Sterling and Sleicher (1975) is proposed, which involves treating their constant correction factor accounting for ambient viscous effects of 0.175 as a variable of the jet axial coordinate with 0.189zg*−0.5 where zg*=zνg/(Ua2) and z, νg, U and a are the axial distance from the jet origin, the kinematic ambient viscosity, the jet velocity and the jet radius, respectively.The decreasing branches of the experimental stability curves were found to be related to the second wind-induced jet break-up regime, since the curves coincide and show no dependence on the nozzle diameter. The critical points of the stability curves were found to lie within a small range of ambient Weber numbers, also indicating the onset of second wind-induced break-up.One of the investigated nozzles showed distinct deviations from the existing model and the proposed modification. In this case, further investigations are required for clarification.

Turbulence aspects of mass transfer in the thin interfacial region of the concentration boundary layer in gas–liquid systems

Turbulence aspects of mass transfer in the thin interfacial region of the concentration boundary layer in gas–liquid systems

Evolution of Electrolytic Hydrogen Bubbles on Microstructured Electrodes

Microstructured electrodes have been shown to be effective as light harvesting structures for solar-driven water electrolysis.[1] However, a better understanding of the fundamental physicochemical aspects of the gas evolution particularly at the micro- and mesoscale, and bubble formation during the electrolysis is needed in order to improve the efficiencies of hydrogen production.[2] We have micromachined superhydrophobic pits on top of single pillars to control the evolution of gas bubbles. Single pillars with radius Rp , ranging from 1.5 μm up to 15 μm in radius, are considerably smaller than the electrode surface (Re >> Rp , with Re the radius of the circular electrode surface). This is done to minimize bubble-bubble interaction and to optically study the interaction of bubbles on such structures. Pits entrap air upon submersion of the electrode in an electrolyte, and thus serve as predefined bubble nucleation site. During electrolysis, the bubbles grow from the pit, up to their detachment. Upon detachment, a gas pocket is left in the pit, ensuring a continuous cycle of bubble growth and detachment. Figure 1(b) shows the electrolytic growth of successive bubbles on a microstructured electrode at various current densities. It is observed that the size of detaching bubbles does not depend on the current density, but on a balance between interfacial tension and buoyancy forces. However, the time taken for bubbles to detach varies with the current density. In general, for successive electrolytic bubbles growing on electrodes, the bubble growth rate increases with time. The time evolution of the bubble radius R(t) can be described by R = βtx , where β is a growth coefficient determined by the supersaturation level of dissolved gas in the liquid phase and x a scaling exponent.[3] It is known that during electrolysis with a large electrode surface (Re >> Rd , with Rd the bubble radius at detachment), diffusive bubble growth is obtained. Diffusive growth, where x = 1/2, will be assumed for our studies. The growth coefficient is then defined as: β = d(R(t)/t1/2)/dt. A boundary layer of dissolved gas is formed near the electrode surface during electrolysis. We can accurately describe the phenomena associated to the boundary layer influencing the growth of bubbles by studying the growth coefficient. Successive generated bubbles experience a locally depleted gas boundary layer. When bubbles detach, they transport hydrogen away from the boundary layer. In Figure 1(c) the β value decreases at short times (tm < 684 s, with t m the mean time of the individual bubble growth period), because the evolved hydrogen had little time to diffuse in a region of the order of the bubble size. This depletion effect is more important in early stages because, as the reaction time increases, the boundary layer develops further, resulting in faster bubble growth and larger β values. Eventually, a roughly constant value is reached with prolonged electrolysis time (not shown in the figure). Next to the bubble evolution by electrolysis as described above, we also work on CO2 bubble evolution in supersaturated liquids on the same type of microstructured electrodes. Our ultimate aim is to understand the underlying fundamentals of bubble growth on any substrate. The obtained insight will be valuable for the design of novel surfaces in electrolysers, fuel cells, and solar-to-fuel devices. [1] R. Elbersen, W. Vijselaar, R. M. Tiggelaar, H. Gardeniers, and J. Huskens. Adv. Energy Mater. Vol. 6, Issue 3, pp. 1-6, 2016. [2] M. A. Modestino, D. F. Rivas, S. M. H. Hashemi, J. G. E. Gardeniers, and D. Psaltis, Energy Environ. Sci. Vol. 9, pp. 3381-3391, 2016. [3] N. P. Brandon and G. H. Kelsall, J. Appl. Electrochem. Vol. 15, No. 4, pp. 475-484, 1985. Acknowledgements This work was supported by the Netherlands Center for Multiscale Catalytic Energy Conversion (MCEC), an NWO Gravitation programme funded by the Ministry of Education, Culture and Science of the government of the Netherlands. Figure 1

Effects of extended surface and surface gold plating on condensation characteristics of steam with large amount of CO2

To promote steam condensation with high concentration of non-condensable gas, vapor mixture with 45–95% CO2 is adopted to experimentally investigate the effects of extended surface and surface gold plating on the heat-transfer enhancement. The results show that a gold-plated stainless vertical surface does not exhibit superior performance under the influence of less condensate and slower update frequency of the drops. A V-shaped fluted surface can enhance heat transfer under CO2 concentrations of 85% and 95%, but no further enhancement is shown after the gold-plating treatment on the surface. Although droplets formed at both the inclined surfaces and bottom of the groove, the disturbance of the droplet movement on the gas boundary layer is much weaker due to the lower heat flux. Both experimental results indicate that a proper increase in the fin spacing and height can improve the heat-transfer process on both untreated and gold-plated fluted surfaces.

Experimental study of turbulent natural-convective condensation on a vertical wall with smooth and wavy film interface

Pressurized water reactor nuclear plants, currently under construction, have been designed with passive containment cooling systems. Turbulent, natural-convective condensation, with high non-condensable mass fraction, on the walls of the containment vessel is a primary heat transfer mechanism in these new plant designs. A number of studies have been completed over the past two decades to justify use of the heat and mass transfer analogy for this scenario. These studies are founded upon natural-convective heat transfer correlations and, in many cases, apply a diffusion layer model to couple heat and mass transfer. Reasonable success in predicting experimental trends has been achieved when correction factors are applied. The corrections are attributed to mass transfer suction, film waviness or mist formation even though little experimental evidence exists to justify these claims. The purpose of this work is to investigate the influence of film waviness and mass transfer suction on the turbulent, natural-convective condensing flow with high non-condensable mass fraction. Testing was conducted using 0.457m×2.13m and a 0.914m×2.13m condensing surfaces suspended in a large pressure vessel. The test facility implements relatively high accuracy calorimetric and condensate mass flow measurements to validate the measured heat and mass transfer rates. Test results show a significant enhancement in heat transfer is caused by disruption of the gas boundary layer due to film waviness. This result has implications for the scalability of existing correlations. A new correlation is proposed and results compared to several datasets.

Tangential gas blowing and boundary layer stability of a compressible gas

The effect of distributed blowing of a gas mass through a porous surface on the stability characteristics of a supersonic boundary layer is studied at a moderate supersonicMach numberM= 2. Tangential blowing when only the U-component of the mean velocity is not equal to zero on the wall is considered. The effect of the porous surface parameters on vortex perturbations is investigated and a comparison with the variant of the so-called “cutoff” regime is carried out. Different-density gas blowing is simulated by means of variation of the temperature factor (wall heating or cooling), namely, blowing of a heavy gas is simulated via blowing of a cold gas and vice versa.

Two-Phase Dusty Fluid Flow Along a Rotating Axisymmetric Round-Nosed Body

This article is concerned with the class of solutions of gas boundary layer containing uniform, spherical solid particles over the surface of rotating axisymmetric round-nosed body. By using the method of transformed coordinates, the boundary layer equations for two-phase flow are mapped into a regular and stationary computational domain and then solved numerically by using implicit finite difference method. In this study, a rotating hemisphere is used as a particular example to elucidate the heat transfer mechanism near the surface of round-nosed bodies. We will investigate whether the presence of dust particles in carrier fluid disturbs the flow characteristics associated with rotating hemisphere or not. A comprehensive parametric analysis is presented to show the influence of the particle loading, the buoyancy ratio parameter, and the surface of rotating hemisphere on the numerical findings. In the absence of dust particles, the results are graphically compared with existing data in the open literature, and an excellent agreement has been found. It is noted that the concentration of dust particles’ parameter, Dρ, strongly influences the heat transport rate near the leading edge.

Study of in situ ethanol recovery via vapor permeation from fermentation

Vapor permeation (VP) is applied to be integrated with ethanol fermentation as an in situ product recovery technique which could avoid the ethanol inhibition in this study. The polydimethylsiloxane o(PDMS) membrane is used in separating ethanol from model ethanol/water mixture and fermentation integrated processes. And the performance of VP-fermentation process is investigated and compared to that of Pervaporation (PV)-Fermentation and batch fermentation processes. Experimental results show that the ethanol removal is successfully achieved, while it still suffers low flux. Then, in order to reduce the mass transfer resistance in gas boundary layer and thus improving the ethanol removal rate, gas stripping is coupled with VP process to form a new configuration of VP process, i.e. external mode. Benefited from its high ethanol removal rate, the ethanol productivity of External VP-Fermentation process reaches 3.33g/(L·h), much higher than 2.13g/(L·h) in batch fermentation and 2.90g/(L·h) in PV-Fermentation under its optimal condition, i.e. vapor cyclic flow of 1.3L/min and module temperature of 35℃. In VP-fermentation process, membrane deterioration is also efficiently alleviated, which commonly occurs in PV-fermentation process. It makes the membrane process more feasible for industrial fermentation process.

Experimental investigation of the particle distribution of gas-solid turbulent flow in the boundary layer

By the three-dimensional and no-contact optical technique digital holography, the particle distributions of a turbulent gas-solid boundary layer were studied in a fully developed horizontal channel, and the velocity field of the gas phase was obtained by particle image velocimetry (PIV) with main upstream speeds of Uh=2.1ms−1, 5.2ms−1, and 8.74ms−1. Carborundum powders with diameters of 38μm and 60μm were used as the particle phase and titanium dioxide nanoparticles were used as ghost particles. It was found that the peak of the particle distribution occurs in a logarithmic region. With an increase of particle size, the peak shifts toward the wall. As the velocity increases, the peak shifts toward the channel center. The gravity and sweeps have influence on the particle motion toward the wall; particle–wall collisions, ejections and the Magnus lift force have influence on the particle motion toward the channel center. Through analysis of the velocity field and shear strain distribution in the gas boundary layer, the existence of sweeps and ejections caused by quasi-flow vortices has been confirmed. Meanwhile, it was found that the peak of shear strain occurred near the wall and that wall-normal fluctuation and shear strain under high flow rate were significantly higher than at low flow rate; these characteristics were consistent with the distribution of particles in the boundary layer.

On computational experiments in some inverse problems of heat and mass transfer

The results of mathematical modeling of effective heat and mass transfer on hypersonic aircraft permeable surfaces are considered. The physic-chemical processes (the dissociation and the ionization) in laminar boundary layer of compressible gas are appreciated. Some algorithms of control restoration are suggested for the interpolation and approximation statements of heat and mass transfer inverse problems. The differences between the methods applied for the problem solutions search for these statements are discussed. Both the algorithms are realized as programs. Many computational experiments were accomplished with the use of these programs. The parameters of boundary layer obtained by means of the A.A.Dorodnicyn's generalized integral relations method from solving the direct problems have been used to obtain the inverse problems solutions. Two types of blowing laws restoration for the inverse problem in interpolation statement are presented as the examples. The influence of the temperature factor on the blowing restoration is investigated. The different character of sensitivity of controllable parameters (the local heat flow and local tangent friction) respectively to step (discrete) changing of control (the blowing) and the switching point position is studied.

On the possibility of control restoration in some inverse problems of heat and mass transfer

The hypersonic aircraft permeable surfaces effective heat protection problems are considered. The physic-chemical processes (the dissociation and the ionization) in laminar boundary layer of compressible gas are appreciated in mathematical model. The statements of direct problems of heat and mass transfer are given: according to preset given controls it is necessary to compute the boundary layer mathematical model parameters and determinate the local and total heat flows and friction forces and the power of blowing system. The A.A.Dorodnicyn's generalized integral relations method has been used as calculation basis. The optimal control - the blowing into boundary layer (for continuous functions) was constructed as the solution of direct problem in extreme statement with the use of this approach. The statement of inverse problems are given: the control laws ensuring the preset given local heat flow and local tangent friction are restored. The differences between the interpolation and the approximation statements are discussed. The possibility of unique control restoration is established and proved (in the stagnation point). The computational experiments results are presented.

Effect of mass transfer induced velocity slip on heat transfer of viscous gas flows over stretching/shrinking sheets

In this paper, effect of mass-transfer induced velocity slip on heat transfer of laminar boundary layer gas flows above a linearly stretching/shrinking sheet at macroscale is theoretically studied. Our exact similarity solutions demonstrate that mass-transfer induced velocity slip is able to appreciably change the gas flow velocity, which subsequently leads to non-negligible changes in temperature distribution and wall heat flux through thermal convection process.

Spatial instability of viscous double-layer liquid sheets

This paper investigates the spatial instability of a double-layer viscous liquid sheet moving in a stationary gas medium. A linear stability analysis is conducted and two situations are considered, an inviscid-gas situation and a viscous-gas situation. In the inviscid-gas situation, the basic state of the entire gas phase is stationary and the analytical dispersion relation is derived. Similar to single-layer sheets, the instability of double-layer sheets presents two unstable modes, the sinuous and the varicose modes. However, the result of the base-case double-layer sheet indicates that the cutoff wavenumber of the dispersion curve is larger than that of a single-layer sheet. A decomposition of the growth rate is performed and the result shows that for small wavenumbers, the surface tension of all three interfaces and the aerodynamic forces of both the lower and upper gases contribute significantly to the unstable growth rate. In contrast, for large wavenumbers the major contribution to the unstable growth rate is only the surface tension of the upper interface and the aerodynamic force of the upper gas. In the viscous-gas situation, although the majority of the gas phase is stationary, gas boundary layers exist at the vicinity of the moving liquid sheet, and the stability problem is solved by a spectral collocation method. Compared with the inviscid-gas solution, the growth rate at large wavenumber is significantly suppressed. The decomposition of growth rate indicates that all the aerodynamic and surface tension terms behave consistently throughout the entire unstable wavenumber range. The effects of various parameters are discussed. In addition, the effect of gas viscosity and the gas velocity profile is investigated separately, and the results indicate that both factors affect the maximum growth rate and the dominant wavenumber, although the effect of the gas velocity profile is stronger than that of the gas viscosity.

A CFD study of wave influence on film steam condensation in the presence of non-condensable gas

Steam condensation plays an important role in removing heat from the containment of a nuclear plant during postulated accidents. However, due to the presence of non-condensable gases such as air and hydrogen in the containment, the condensation rate can decrease dramatically. Under certain conditions, the condensate film on the cold containment walls can affect the overall heat transfer rate. The wavy interface of the condensate film is a factor and is usually believed to enhance the condensation rate, since the waves can both increase the interfacial area and disturb the non-condensable gas boundary layer. However, it is not clear how to properly account for this factor and what is its quantitative influence in experiments. In this work, a CFD approach is applied to study the wave effects on film condensation in the presence of non-condensable gas. Wavy surfaces with and without moving velocities are used to replace the wavy interface of the falling film. A condensation model is incorporated in the ANSYS FLUENT simulation and a realizable k–ε turbulence model is applied. Various wavy surfaces with different wave heights and wavelengths are selected to conduct numerical experiments with a wide range of gas velocities. The results show that the wave structure can enhance condensation rate up to ten percent mainly due to the alteration of local flow structures in the gas phase. The increments of the condensation rate due to the wavy interface can vary with different gas velocities. The investigation shows that a multiplication factor accounts for the wave effects on film condensation should be added in the containment heat transfer analysis where the film Reynolds number is large.