Numerical analysis of flow pattern of impinging liquid sprays in a cold model for cooling a hot flat plate

This paper treats the numerical analysis of two-phase mist jet flow, which is commonly adopted to cool the solidified shell in the secondary cooling zone of the continuous casting process. Flow structures of the two-phase subsonic jet impinging on a flat plate normal to flow, corresponding to the present cooling situation, are solved on the assumption that particles are perfectly elastically reflected from a surface. Again, the numerical experiments concerning mist flows composed of air and water-droplets are made in a cold model. The flow fields for both gas and particle phases strongly depend upon the particle size. When waterdroplets mixing in the mist are very small, the impinging particles travel very closely to the surface. With increasing particle size, particles are reflected from the surface in a far distance. Therefore, also, the case is analysed where a low velocity annular gas-only flow surrounding a round nozzle co-axially is present so that such idle particles may be pushed back to the surface again. This is considered to result in an improvement of the mist cooling efficiency.     

Visualization of high-speed gas jets and their airblast sprays of cross-injected liquid

A planar and instantaneous visualization study of high-speed gas jets and their airblast sprays was performed to qualitatively examine the different atomization performances of different gas nozzles. For the visualization of high-speed gas jets (with no liquid injected), Nd:YAG pulsed laser sheets imaged the clustered vapor molecules in the Rayleigh range (d?λ), condensed from the natural humidity during the isentropic gas expansion through a nozzle. This method visualized both underexpanded sonic gas jets from a converging nozzle (SN-Type) and overexpanded supersonic gas jets from a converging-diverging nozzle (CD-Type). When liquid is cross-injected, the same laser sheet images the spray droplets of relatively large sizes (d?λ). The present visualization results show that the SN-Type nozzle develops a wider spray than the CD-Type nozzle, quite probably because the SN-Type nozzle has a wider gas jet (in the absence of liquid) than the CD-Type. Also, the wider spray of the SN-Type nozzle lowers the probability of droplet coalescence and generates finer sprays compared to the CD-Type nozzle. These visualization results qualitatively agree with the previous quantitative finding of the different atomization characteristics of the two types of nozzles (Park et al. 1996).     

Mathematical treatment of the discharge of a laminar hot gas in a stagnant colder atmosphere

We study the boundary-layer approximation of the classical mathematical model that describes the discharge of a laminar hot gas in a stagnant colder atmosphere of the same gas. We prove the existence and uniqueness of solutions to a nondegenerate problem (without zones of stagnation of gas temperature or velocity). The asymptotic behavior of these solutions is also studied __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 49, No. 4, pp. 192–205, July–August, 2008.     

Aerodynamic behavior of liquid sprays

An analysis is presented for the effect of entrained gas flows on drop trajectories and spray distributions from liquid atomizing nozzles. In particular, the effect of the pressure (or density) of the environment into which the liquid is sprayed is examined. The contraction of atomized sprays at elevated pressure which has been observed by various workers is explained, and the analysis is substantially confirmed by their data and by new data presented here. Both the data and the theory show that the amount of spray contraction increases with increasing ambient pressure and nozzle pressure drop, and decreases with increasing nozzle diameter and drop size. The theory examines the entrained gas flow around and into a spray and its subsequent effect on the trajectories of the liquid droplets comprising the spray.     

Use of an extended gas glow discharge in a closed-cycle CO2 laser with convective cooling

An analysis is made of the operating characteristics of a powerful glow discharge used in a CO₂ laser system for the pumping of an extended uniform stream of working medium circulating through a closed circuit. The velocity of the medium is 30 m/sec and the pressure is about 20 mm Hg. Difficulties connected with instability and the possibility of pinching of the discharge (conversion into an arc) are overcome. The results of systematic experimental studies of the operating conditions of the discharge are presented. It is shown that the power level achieved (10 W/cm²) depends on the choice of resonator construction and is not limiting for the system selected.     

Kinetic cooling of a moving gas

A study is made of the effects that arise when a moving gas absorbs electromagnetic radiation whose frequency is in resonance with the frequency of the center of a spectral line of a vibrational-rotational transition of molecules of the mixture. It is shown that the variation of the gas-dynamic parameters depends on the relationships between the rates of the stimulated transitions, intramolecular V — V exchange, and V — T relaxation, and the maximal effects are attained in the neighborhood of the sonic point.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 127–138, May–June, 1982.     

Measurement of the liquid phase mass in gas-liquid sprays by X-ray attenuation

An optical method for measuring the mass of liquid phase in a spray has been developed into an X-ray method. An X-ray of about 6 keV is used to measure the mass distribution of liquid phase in the spray formed by a concentric injector with water and gaseous nitrogen at one atmosphere of pressure. The possibility of measuring the mass of gaseous oxygen and hydrogen is discussed.     

The liquid deposition fraction of sprays impinging vertical walls and flowing films

The impingement of coarse sprays with a mean diameter in the order of millimeters on vertical walls with and without an additionally supplied wall film was studied at conditions well below the Leidenfrost limit. The fraction of the sprayed liquid deposited on the wall was determined experimentally and theoretically for various impingement angles with the help of a flat fan spray directed against the wall. The deposition fraction shows a distinct minimum in the range of intermediate impingement angles. This fact cannot be described by single-droplet-based deposition-splash criteria when considering the droplet’s impact momentum alone. The investigation demonstrates that the measurement results can be explained by including the collision of splashed droplets with incoming ones. In principle, the entrainment of the primary spray’s fine fraction in the gas flow field may also be of relevance. For the coarse and relatively sparse sprays investigated, the importance of the collisions in determining the overall balance of deposited and splashed liquid was estimated by event statistics derived from Monte Carlo simulations. The main outcome of wall interaction for the coarse spray is splashing. The splashed droplets form a secondary spray. When the impingement angle is steep, the splashed liquid is redirected towards the wall as a result of the collision between the incoming primary spray and splashed droplets.     

Corona discharge in a moving gas

Some aspects of the theory of corona discharges in a moving medium are considered. Two situations are analyzed: a corona discharge at a negative electrode under the condition that in the region of electrogasdynamic flow exterior and interior regions of the discharge can be distinguished, the motion of the gas being taken into account only in the exterior region, and corona discharge at a negative electrode under the condition that the effects of the motion of the gas are important in both the exterior and the interior regions of the discharge. For the first situation, a mathmatical generalization is proposed of the traditional model of the interior region, and dimensional and similarity methods are used to obtain functional relationships for the current—voltage characteristics of the discharge in the moving medium. The second situation is investigated for the example of a corona discharge between cylindrical electrodes through which gas is blown or sucked. In this case, the solution to the problem is found without dividing the flow region into exterior and interior regions of the discharge, a system of kinetic equations describing the flow in the complete interelectrode gap being used.     

Investigation of high-speed combustible gas ignited by a hot gas jetproduced in the shock tube

The high-speed combustible gas ignited by a hot gas jet, which is induced by shock focusing, was experimentally investigated. By use of the separation mode of shock tube, the test section of a single shock tube is split into two parts, which provide the high-speed flow of combustible gas and pilot flame of hot gas jet, respectively. In the interface of two parts of test sections the flame of jet was formed and spread to the high-speed combustible gas. Two kinds of the ignitions, 3-D “line-flame ignition” and 2-D “plane-flame ignition”, were investigated. In the condition of 3-D “line-flame ignition” of combustion, thicker hot gas jet than pure air jet, was observed in schlieren photos. In the condition of 2-D “plane-flame ignition” of combustion, the delay time of ignition and the angle of flame front in schlieren photos were measured, from which the velocity of flame propagation in the high-speed combustible gas is estimated in the range of 30–90m/s and the delay time of ignition is estimated in the range of 0.12–0.29ms. PACS 47.40.Nm; 82.40.FpPart of this paper was presented at the 5th International Workshop on Shock/Vortex Interaction, Kaohsiung, October 27–31, 2003.     

The corona ignition voltage of an electrical discharge in a gas

The corona ignition voltage of an electrical discharge in air of atmospheric pressure depends on the presence of (moisture) particles, which may increase the corona losses. A relation between the corona ignition voltage and the particle size when tested shows unexpected results. With the corona ignition voltage in air as observed by Rose and Wood our calculations do not give particle sizes of 50 m (as used by Rose and Wood), but sizes of about 1 Å corresponding to the diameters of the molecules of the component gases in air. Our conclusion is that these molecules align in a conductive channel from the axial wires to the particle considered. In this way the charge transfer from the axial wire to the particles may be explained.     

Dynamics of a liquid film sheared by a co-flowing turbulent gas

Consider the dynamics of a thin laminar liquid film flowing over an inclined wall in the presence of a co-flowing turbulent gas. The solution to the full two-phase flow problem poses substantial technical difficulties. However, by making appropriate assumptions, the solution process can be simplified and can provide valuable insights. The assumptions allow us to solve the gas and liquid problems independently. Solving for the gas flow reduces to finding perturbations to pressure and tangential stresses at the interface, influencing the liquid problem through the boundary conditions. We analyze the effect of gas flow on the liquid problem by developing an integral-boundary-layer model, which is valid up to moderate liquid Reynolds numbers. We seek solitary-wave solutions of this model under the influence of gas flow via a pseudo-arclength continuation method. Our computations demonstrate that as a general trend, the wave speed increases with increasing the gas shear and the liquid flow rate. Further insight into the problem is provided via time-dependent computations of the integral-boundary-layer model.     

Perturbation of a downward liquid flow by a stationary gas slug

The effect of a stationary gas slug on the hydrodynamics of a downward liquid flow in a pipe is investigated. The electro-diffusion technique is used to measure the mean and fluctuating viscous stresses on the wall below the gas slug and in the liquid behind the slug base. It is shown that at a constant liquid velocity the friction beneath the slug is determined by the distance from its nose. The flow structure perturbation due to a single slug is recorded at distances greater than 25 pipe diameters from the slug base.     

Indirect temperature determination of a hot gas stream

The method for indirect temperature measurement of a hot flowing gas presented in this paper evolved out of the need to determine experimentally the temperature of hot gas leaving a hydrogen production solar thermal water dissociation reactor. The method is based on Fanno line choked flow theory. It enables gas-temperature determination from gas flow rate and pressure measurement. The reliability of the method was tested by applying it to determine the temperature of a relatively cold gas stream and by comparing the result with the gas temperature measured directly with a thermocouple.     

Breakup of gas bubbles in a liquid by shock waves

The nature of the propagation of shock waves in various media is related to the characteristics of the latter, including their compressibility, thermophysical properties, the presence of multiple phases, etc. The structure of a shock wave varies appreciably as a function of the properties of the medium. The most significant property of a liquid mixture with gas bubbles is the compressibility of the latter under the influence of an externally applied pressure, for example, in a shock wave propagating in the liquid—gas medium. The transfer of momentum and energy between phases and the pressure variation behind the wave depends on the behavior of the gas bubbles behind the shock front.     

A comparison between spray cooling and film flow cooling during the rewetting of a hot surface

The paper is dealing with a research carried out at the Institute of Thermal-Fluid Dynamics to investigate the rewetting of a hot surface. The rewetting of the hot surface by spray cooling has been analyzed in previous works. After the droplet impingement, the liquid film falls along the surface, and rewetting by falling film takes place. The experiment was characterized by a 1-dimensional liquid spray, i.e., drops having a uniform, constant diameter, impinging on the heated surface. The cooling rate of the hot surface has been detected as a function of wall temperature, drop diameter and velocity, and impact point of the spray. The working feature of the spray is based on the varicose rupture of the liquid jet: imposing a periodic (symmetrical) perturbation with appropriate amplitude and frequency on the jet surface, the flow is “constrained” to break soon after leaving the nozzle, eventually obtaining constant diameter drops, depending on the nozzle diameter and liquid velocity. In this paper, previous results with spray cooling are compared with experimental runs in which the spray injection is replaced with a falling film all along the test section. The rewetting velocity has been calculated from the response of the thermocouples placed on the heated wall and using a digital image system based on the video image registered during the runs.     

Initiation of cavitation by injection of hot steam into a cold liquid jet

The problem of the dynamics of a hot steam bubble in the nonuniform flowfield of a plane cold liquid jet is considered. The motion of the bubble along the symmetry axis is analyzed with allowance for nonequilibrium condensation and heat conduction by the steam and the liquid. The domain of jet and steam parameters corresponding to the dynamic cavitation bubble initiation mode, is evaluated.Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 5, pp. 91–100, September–October, 1995.     

Effect of successive sprays on liquid distribution in fluidized beds

In processes such as Fluid Coking^?,agglomerate formation should be minimized since it reduces the yield of valuable products,and degrades operability because of the fouling of internals.An experimental model,consisting of an aqueous solution of gum arabic with a dye,has been successfully developed to simulate the formation of agglomerates in the Fluid Coking^?process,where bitumen is sprayed into a fluidized bed of coke particles The particles wetted by a spray could be predicted by assuming that all the particles in the wake of bubbles formed from the tip of the spray jet have been wetted by the injected liquid.The transfer of liquid from particles wetted with the spray to dry bed particles was relatively ineffective,as the number of wet particles increased by only 50%.With successive liquid injections,the proportion of the liquid trapped in agglomerates increases in latter injections:large agglomerates from earlier injections accumulate above the grid and are carried by gas bubbles into the spray jet cavity,where they seed fresh agglomerates.     

The stability of viscous liquid jets in a swirling gas

Based on the linear analysis of stability, a dispersion equation is deduced which delineates the evolution of a general 3-dimensional disturbance on the free surface of an incompressible viscous liquid jet injected into a gas with swirl. Here, the dimensionless parameterJ _e is again introduced, in the meantime, another dimensionless parameterE called as circulation is also introduced to represent the relative swirling intensity. With respect to the spatial growing disturbance mode, the numerical results obtained from solving the dispersion equation reveal the following facts. First, at the same value ofE, in pace with the changing ofJ _e , the variation of disturbance and the critical disturbance mode still keep the same characters. Second, the present results are the same as that of S.P. Lin whenJ _e >1; but in the range ofJ _e <1, it's no more the case, the swirl decreases the axisymmetric disturbance, yet increases the asymmetric disturbance, furthermore the swirl may make the character of the most unstable disturbance mode changed (axisymmetric or asymmetric); the above action of the swirl becomes much stronger whenJ _e ≪1. The project supported by the National Natural Science Foundation of China