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1.
Thermoacoustic convection, that is, convection generated in a compressible fluid due to rapid heating of one of the enclosing walls, was studied numerically. Acoustic waves were found to be generated by sinusoidal heating of the wall as well as by a step function. In a finite layer of fluid the pressure wave is reflected back and forth until dissipated by viscosity. The fluctuating velocity generated by the pressure wave greatly enhances the rate of heat transfer over that for pure transient conduction. The same qualitative behavior but widely varying quantitative behavior was computed for different conditions. The simple theory of adiabatic waves, which neglects the diffusion of momentum and energy, accurately predicts the inter-relationship between the various dependent variables but neither the strength of the wave nor the rate of decay. Hence, these two approaches are complementary. 相似文献
2.
Stuart W. Churchill 《Chemical engineering science》2004,59(6):1377-1392
The rapid heating of a wall bounding a semi-infinite region of gas generates a slightly supersonic compressive wave and thereby increases the rate of heat transfer to and through the gas. Generalized correlating expressions are presented for the wave velocity and for the amplitude in pressure, temperature, and gas velocity as a function of distance and time for any specified exponential rate and extent of increase of the temperature of the wall. These expressions are based on idealized analytical solutions modified by virtue of finite-difference solutions and experimental data. The combination of all three of these elements proved to be essential in order to understand and correlate the behavior. Although a compressive wave is generated for any rate and extent of heating of the wall, significant effects are observed only for very rapid heating and very large temperature differences. A critical time constant for exponential heating is identified such that the strength of the wave is equivalent to that for an instantaneous step in wall temperature. 相似文献
3.
A computational fluid dynamics (CFD) study was conducted to model effects of coal properties on the potential for spontaneous heating in longwall gob (mined-out) areas. A two longwall panel district using a bleeder ventilation system was simulated. The permeability and porosity profiles for the longwall gob were generated from a geotechnical model and were used as inputs for the three-dimensional CFD modeling. The spontaneous heating is modeled as the low-temperature oxidation of coal in the gob using kinetic data obtained from previous laboratory-scale spontaneous combustion studies. Heat generated from coal oxidation is dissipated by convection and conduction, while oxygen and oxidation products are transported by convection and diffusion. Unsteady state simulations were conducted for three different US coals and simulation results were compared with some available test results. The effects of coal surface area and heat of reaction on the spontaneous heating process were also examined. 相似文献
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5.
Approximate as well as reasonably exact numerical solutions of the equation of conservation have previously predicted that sudden heating of a solid surface adjacent to a region of gas will generate a slightly supersonic wave with small positive amplitudes in pressure, temperature and density, and thereby a small mass velocity in the direction of wave propagation. If the gas is confined by a second parallel surface, the wave is predicted to be reflected repeatedly from both surfaces and to decay slowly due to viscous and thermal dispersion. This process, which has been termed thermoacoustic convection, is presumed to result in transient heating of the confined gas and heat transfer through it at rates greatly exceeding that of pure thermal conduction. The current work constitutes the first quantitative experimental confirmation of this behaviour. Numerical solutions obtained for support and guidance of the experimental work define for the first time the conditions required for the generation of a wave of significant strength. 相似文献
6.
SPAN style="FONT-FAMILY: 宋体 FONT-SIZE: .pt mso-ascii-font-family: Calibri mso-bidi-font-family: 宋体 mso-ansi-language: EN-US mso-fareast-language: ZH-CN mso-bidi-language: AR-SA mso-bidi-font-size: .pt mso-ascii-theme-font: minor-latin mso-fareast-theme-font: minor-fareast">雷雨 孙宝芝 李彦军 宋福元 《化工学报》2011,62(Z1):140-145
增压锅炉装置因其容积热负荷高、体积小、质量轻等优点广泛应用于大中型船舶主动力装置。利用分子动力学原理及流体分子微观理论与性质,分析了增压对烟气导热与热对流的影响机理,并利用Illes教授与前苏联的两种计算方法,分别计算了不同增压比下增压锅炉对流受热面烟气流速、密度以及传热系数等参数并进行了对比。经分析可知烟气压力的提高使分子碰撞频率增加、平均自由程减小、烟气密度增大,从而对流传热得到强化。同时计算结果表明烟气压力较低时两方法计算结果相差不大,但在压力接近0.3 MPa时,Illes教授推荐的热力计算方法所得传热系数可达到前苏联方法的1.3倍。 相似文献
7.
以去离子水为工质,对矩形通道内低压自然循环压降型脉动及其复合型脉动进行可视化实验研究,利用可视化手段拍摄脉动过程气相分布状态图像。通过流量脉动曲线与图像进行对照,分析流量脉动的物理过程及产生机理。实验过程中发现4类动态不稳定性:第Ⅰ类密度波(DWOⅠ)、压降型脉动(PDO)、复合型脉动(SPO)及第Ⅱ类密度波(DWOⅡ)。重点分析了压降型脉动与复合型脉动产生机理、影响因素及流动不稳定性边界。实验研究发现,受矩形通道挤压效应及稳压器内部可压缩空间影响,工质在稳压器和循环回路之间往复波动,形成压降型脉动,增大加热功率压降型脉动会叠加第Ⅱ类密度波形成复合型脉动。两类脉动的起始点会随入口过冷度增加呈现偏离趋势,并且压力增加,偏离趋势会随之增大。通过相变数、过冷度数等无量纲参数绘制了压降型脉动不稳定性边界。 相似文献
8.
Z. -Q. Fan H. -H. Ma Z. -W. Shen M. -J. Lin 《Combustion, Explosion, and Shock Waves》2014,50(3):354-361
The interaction of underwater explosion waves with aluminum foam plates with different cell types is investigated. It is shown that the transmitted wave that passed through a closed-cell sample can be decomposed into two parts, one with a low-frequency content, which corresponds to the gas pressure in the pores, and one with a high-frequency content, which is argued to be a coupling of waves transmitted by the matrix and wave interactions between the fluid and the matrix of the medium. 相似文献
9.
Richard W. Hanks 《Chemical Engineering Communications》1990,89(1):187-194
Stokes' second problem, the propagation and damping of waves into a semi-infinite fluid generated by harmonic oscillations of a flat pate on the surface, is solved for the simple Bingham Theological constitutive model. The solution reveals the existence of “windows” in the distance-time-stress space in which shearing is possible whereas outside these restricted regions no shearing can occur. Within these restricted regions the wave forms developed are exponentially damped, traveling waves which propagate from the excitation plane into the fluid and disappear along definitely prescribed boundaries determined by the yield condition. The most significant consequence of the existence of these “windows” of shear is that even very small yield stresses will radically modify the induced velocity wave patterns from that which would be expected based upon the classical Newtonian fluid solution of Stokes' second problem. At least in this physical setting, it is not necessary for shear to occur globally for motion to occur anywhere, as has been postulated in some recent studies of complex motions. Thus, the motion is consistent with a simple Bingham model which does possess a yield stress. 相似文献
10.
The formation and decomposition of methane+methylcyclohexane (MCH) hydrate in a static batch reactor, which was also designed as a high‐pressure microwave reactor, were investigated. The addition of 300 ppm sodium dodecyl sulfate (SDS) provides continuous formation of CH4+MCH hydrate under static conditions. Increasing the initial pressure within the narrow range of 2.7 to 4.6 MPa at 274 K enhances the formation rate by even several times. The gas storage capacity can be largely improved with partial coexisting of sI CH4 hydrate. Unlike a stirred formation, an increase of nonaqueous MCH inhibits the static formation of sH hydrate. The following regasification by 2.45 GHz microwave heating indicates that the dissociation is rate‐controlled by the parallel connection of efficient internal heating and conventional external heating. The multiphase convection characterized by osmotic dehydration and driven by intensified regasification is considered as the dominant mechanism affecting the quiescent dissociation. 相似文献
11.
Guillermo Terrones FranciscoJ. Souto MichaelW. Burkett RobertF. Shea EdwardS. Idar 《Propellants, Explosives, Pyrotechnics》2006,31(5):333-342
Plastic bonded explosives when exposed to prolonged heating environments undergo a variety of changes that affect their bulk chemical, thermophysical, and mechanical properties. During slow heating conditions, referred to as cook‐off, the thermal behavior of the polymeric binder plays an important role in the transformations of these composite energetic materials. The recently introduced Darcian flow hypothesis for PBX‐9501 implies that, during preignition, temperature gradients will lead to pressure gradients which in turn will drive convection of decomposition gases throughout the explosive, thus affecting ignition time and location. Here, we focus on the cook‐off behavior of PBX‐9501 and investigate its effects on bulk permeability to gases produced as a result of thermal decomposition. The concept of Darcian convection through porous media is defined and illustrated in detail by the derivation of the governing equations for a permeameter. Based on a systematic analysis involving: 1) our current understanding about binder behavior as a function of temperature, 2) the physics of the gas permeameter apparatus, 3) the concept of liquid drainage by gas, and 4) the experimental record of four permeameter experiments with cooked PBX‐9501, we conclude that samples heated up to 186 °C were not permeable in the Darcy‐flow sense. 相似文献
12.
The hydraulic-jump phenomenon of a thin fluid layer flowing down an inclined plane under an electrostatic field is explored
by using a global bifurcation theory. First, the existence of hydraulic-hump wave has been found from heteroclinic trajectories
of an associated ordinary differential equation. Then, the jump behavior has been characterized by introducing an intensity
function on the variations of Reynolds number and surfave-wave speed. Finally, we have investigated the nonlinear stability
of traveling shock waves triggered from a hydraulic jump by integrating the initial-value problem directly. At a given wave
speed there exists a certain value of Reynolds number beyond which a time-dependent buckling of the free surface appears.
Like the other wave motions such as periodic and pulse-like solitary waves, the hydraulic-jump waves are also found to become
more unstable as the electrostatic field is getting stronger. 相似文献
13.
This paper is to review the works on strains and suesses in materials during drying.The strains and suesses are caused when temperature and moisture gradients are generated in mterials whose volume changes with heating and moisture removal. In such materials. failure and irregular deformion may be generated which affect considerably the qudity of the products after drying. In the first part. modeling procedure is introduced for the analysis of the strain-stress behavior in elaslic. viscous and visccelastic materials combined with heat and moisture transfer. An overview of the works on swains and stresses and drying characteristics are presented for malerials such as porous media. clay. sol-gels. agricultural products and foods in the second part. There are some materials that show both elasticity of the solid phase and viscosity of the fluid phase ( water or solvena∥ or viscoelasticiry. The suesses are often correlated with a suction pressure of fluids in pores and the flow rate is based on Dacy's equation for the elastic and viscous tnedia and a kind of viscoelastic media. The general canstitulive equalions. for suains and svesses are often analyzed with the stain behavior given by a function of moisturr for some media ai well. The emohasis is on the inuoduclion of comprehensive criteria for undersunding the problems of strain and stress development in materials subjected to drying. 相似文献
14.
Thin liquid layers of polymer solutions, oligomers and monomers, and oligomer–monomer bilayers were used as model systems to explore the mechanism of formation of surface textures in solid coatings and films. It was shown that under particular conditions vertical temperature gradients imposed on these fluid layers induce in them various types of convection. We demonstrated that hexagonal patterns with a high degree of order and symmetry are generated in fluid films undergoing surface tension-driven convection. Various non-equilibrium textures have been trapped in the solid state either by vitrification induced by solvent evaporation from the fluid layer, or by carefully performed polymerization of monomers and oligomers. Finally, we demonstrated that convection patterns can be replicated in convection-passive fluids by bringing them in contact with a fluid layer undergoing convection. 相似文献
15.
A. Ashari 《Chemical engineering science》2009,64(9):2067-623
In this work, a two-scale two-phase modeling methodology is presented for studying fluid release from saturated/unsaturated thin fibrous media when brought in contact with a moving solid surface. Our macroscale model is based on the Richards’ equation for two-phase fluid transport in porous media. The required constitutive relationships, capillary pressure and relative permeability as functions of medium's saturation, are obtained through microscale modeling. At microscales, a 3-D model based on fiber diameter, fiber orientation, and medium's solid volume fraction (SVF), is generated to resemble the internal structure of the fibrous sheets and be used in full-morphology analysis as well as microscale permeability simulation. A mass convection boundary condition is considered here to model the fluid transport at the boundary in contact with the target surface. It was shown that the mass convection coefficient, kf, plays a significant role in determining the release rate and is expected to be in the range of 10-6<kf<10-9, depending on the properties of the fluid, fibrous sheet, the target surface as well as the speed of the relative motion, and remains to be determined experimentally. 相似文献
16.
ABSTRACT This paper is to review the works on strains and suesses in materials during drying.The strains and suesses are caused when temperature and moisture gradients are generated in mterials whose volume changes with heating and moisture removal. In such materials. failure and irregular deformion may be generated which affect considerably the qudity of the products after drying. In the first part. modeling procedure is introduced for the analysis of the strain-stress behavior in elaslic. viscous and visccelastic materials combined with heat and moisture transfer. An overview of the works on swains and stresses and drying characteristics are presented for malerials such as porous media. clay. sol-gels. agricultural products and foods in the second part. There are some materials that show both elasticity of the solid phase and viscosity of the fluid phase ( water or solvena∥ or viscoelasticiry. The suesses are often correlated with a suction pressure of fluids in pores and the flow rate is based on Dacy's equation for the elastic and viscous tnedia and a kind of viscoelastic media. The general canstitulive equalions. for suains and svesses are often analyzed with the stain behavior given by a function of moisturr for some media ai well. The emohasis is on the inuoduclion of comprehensive criteria for undersunding the problems of strain and stress development in materials subjected to drying. 相似文献
17.
18.
A. A. Buzukov 《Combustion, Explosion, and Shock Waves》2000,36(3):395-404
The efficiency of using an air-water drop curtain for protection from the force and noise action of an air shock wave generated
by an open explosion is studied experimentally. It is shown that the curtain generated by outburst of sprayed water after
an advanced underwater explosion of a demolition cord is a reliable means of pressure decrease at the shock-wave front. The
dependence of the “effective coefficient of charge-mass reduction” on the position of the curtain relative to the point of
explosion, its length, time of evolution, and other conditions was studied. Zones with local pressure increase or decrease
in the shock wave were found, which is explained by imposition of secondary compression and expansion waves on the shock wave.
Possible physical mechanisms that ensure the protective effect are considered.
Translated fromFizika Goreniya i Vzryva, Vol. 36, No. 3, pp. 120–130, May–June, 2000. 相似文献
19.
气液两相流流量变化瞬态特性 总被引:6,自引:4,他引:2
在气液混输管线中常发生流量变化引发的瞬态过程,在大型多相流实验环道上进行了流量变化的瞬态实验.气量突增产生压力过增量,它的产生与流型有关:分层流的过增量是由于气体的惯性;在气团流和段塞流下则是由于气体压缩性和液体惯性的联合作用.气量突降时,膨胀波沿程传播.气量变化的瞬态过程可能出现对应准稳态变化没有出现的流型.与气量突变相比,液量突变引起的压力和流型变化较平缓,瞬态效应小.流量变化后产生沿线传播的压力波和空隙波,二者与流型相关,决定瞬态过程的发展.压力变化的传播速度在气量变化时等于声速,在液量变化时等于空隙波波速. 相似文献
20.
Natural convection of a fluid contained in an infinitely long horizontal cylinder at large Prandtl number and unit-order Grashof number is analyzed. The motion is generated by an imposed cosine wall temperature distribution which includes an arbitrary phase angle. The phase angle is a measure of the location of the wall temperature extrema. From an asymptotic ordering of the energy and vorticity transport equations for large Prandtl number it is shown that the core region, which contains fluid surrounded completely by a boundary-layer flow along the cylinder wall, may assume either of two configurations. For heating angles near the heating-from-the-side case (wall temperature extrema at the ends of the horizontal diameter) linearized forms of the boundary-layer equations are developed which yield solutions that match the core configuration not considered previously. The form of the results agrees generally with experimental evidence for heating-from-the-side. 相似文献