Stability of an annular viscous liquid jet in compressible gases with different properties inside and outside of the jet

A spatial linear instability analysis is conducted on an annular viscous liquid jet injected into compressible gases and a three-dimensional model of the jet is developed. The model takes into account differences between the velocities, densities of the gases inside and outside of the liquid jet. Theoretical analysis reveals that there exist 9 dimensionless parameters controlling the instability of the liquid jet. Numerical computations reveal some basic characteristics in the breakup and atomization process of the liquid jet as well as influences of these relevant parameters. Major observations and findings of this study are as follows. The Mach number plays a destabilizing role and the inner Mach number has a greater effect on the jet instability than the outer Mach number. The Reynolds number always tends to promote the instabilities of the liquid jet, but its influence is very limited. The Weber number and the gas-to-liquid density ratio also have unstable effects and can improve the atomization of liquid jets. Furthermore, the effects of the Weber number and gas-to-liquid density ratio on the maximum growth rates of axisymmetric and non-axisymmetric disturbances and corresponding dominant wave numbers are manifested in a linear way, while that of the Mach number is non-linear. The effect of Reynolds on the maximum growth rates is non-linear, but the dominant wavenumber is almost not affected by the Reynolds number.     

Response of microscopic bubbles to sudden decrease of ambient pressure in viscous compressible liquid

Intpoduction'Sudden Changes Of liquid flow velocity in pipingsystems are accompanied by high amplitode Pressareoscillations. In some phases of osdllations the Pressarecan reach negative Values (tensile sacsses). la thesecases the microscopic bubbles dispersed in the liquidgrow tO large cavihes. ~ some delay, when theliquid pressure inereases, they collapse again.The dynndcs Of a single gas or gas-vapour bubblein v~ ambient Pressal has been a subject ofnumerous stUdies over the years. Main …     

Aerodynamic and mass transfer characteristics of an annular bistable impinging jet with a fluidic flip-flop control

An annular nozzle has been designed on the basis of fluidic principles. The nozzle forms actively controlled air jet. Numerical and experimental investigations were performed in several subsequent steps, namely numerical simulation (using a commercial CFD code FLUENT), geometry adaptation, models manufacturing, flow visualization, hot-wire measurement, and mass transfer (naphthalene sublimation) experiment. A “jet switching” possibility has been discussed, an undesirable hysteresis effect has been suppressed. Present collaborative numerical and experimental investigations have resulted in a better understanding of mechanisms involved in controlled impinging jets, as well as in a further improvement of the particular nozzle geometry.     

Dynamic behaviour of non-isothermal compressible natural gases mixed with hydrogen in pipelines

This paper focuses on non-isothermal transient flow in mixed hydrogen–natural gas pipelines. The effect of hydrogen injection into natural gas pipelines has been investigated in particular the pressure and temperature conditions, Joule–Thomson effect, linepack and energy consumption of the compressor station. The gas flow is described by a set of partial differential equations resulting from the conservation of mass, momentum and energy. Real gas effects are determined by the predictive Soave–Redlich–Kwong group contribution method. The Yamal-Europe gas pipeline on Polish territory has been selected as case study.     

Recursive formulation on thermal analysis of an annular fin with variable thermal properties

This study introduces a discrete model which is pertinent for calculating thermal performance of singular annular fin with variable thermal properties. The singular annular fin can be divided into several circular sections, and each section can take its variable thermal properties, such as heat transfer coefficient and thermal conductivity, into account. The result from each section can be combined and calculated together by a recursive formula. Then, the solutions of temperature distribution and heat transfer rate on singular annular fin can be quickly obtained. For this model, the recursive formulae for both conditions with and without heat transfer on fin tip are demonstrated. Finally, some examples including composite materials for an annular fin have been successfully simulated through the present approach.     

Forced convection heat transfer inside an anisotropic porous channel with oblique principal axes: Effect of viscous dissipation

An analytical study on laminar and fully developed forced convection heat transfer in a parallel-plate horizontal channel filled with an anisotropic permeability porous medium is performed. The principal axis of the anisotropic porous medium is oriented from 0 to 90 degrees. A constant heat flux is applied on the outer wall of the channel. Both clear (Newtonian) fluid and Darcy viscous dissipations are considered in the energy equation. Directional permeability ratio parameter A¹ is defined to combine both the effect of the dimensionless permeability ratio parameter K¹=(K₁/K₂) and orientation angle φ into one parameter. The effects of the parameter A¹, the Darcy number Da and the modified Brinkman number Br¹ on the heat transfer and fluid flow characteristics in the channels are investigated and presented in graphs. The obtained results show that the parameters A¹, Da and Br¹ have strong effects on the dimensionless normalized velocity and temperature profiles as well as on the Nusselt number. It is found that for a particular value of A¹, called as critical value A_cr¹, the external heat applied to the surface of the channel is balanced by the internal heat generation due to viscous dissipation and the bulk mean temperature approaches the wall temperature. Hence, the Nusselt number approaches infinity for the critical values A_cr¹.     

电场作用下气泡内外的速度场分析

在蠕动流假设的基础上计算了电场作用下气泡内外的速度场分布,并与无电场作用但有来流时气泡内外的速度场进行了比较。结果表明,无电场作用时,来流速度只能使气泡内部形成两个球涡,而作用电场后即使无来流,气泡内部的流体运动仍然加剧,可以形成四个球涡。这表明外电场作用也有利于加剧气泡内部流体的运动,从而有助于核态沸腾换热过程中气泡底部微液层液体的蒸发。     

Numerical simulation on the behavior of a liquid jet into an air flow

A numerical simulation has been performed to clarify the effects of turbulence in a liquid on the deformation of the liquid jet surface into an air flow. The turbulences in the liquid jet were simulated by the Rankin vortices, and the liquid jet surface was tracked numerically by the volume of fluid method. By numerical simulations, the onset of the protrusions on the liquid jet surface is caused by the vortices in the liquid, and the surrounding air flow plays an important role in the amplification of the protrusions. The amplification rate of the trough displacement is proportional to the air‐to‐liquid velocity ratio. At large imposed vortex intensities, the trough displacement increases with the vortex intensity. On the other hand, at small imposed vortex intensities, the amplification of the trough displacement is also affected by factors other than vortex intensity. © 2001 Scripta Technica, Heat Trans Asian Res, 30(6): 473–484, 2001     

Experimental study of vented hydrogen deflagration with ignition inside and outside the vented volume

Experiments were carried out inside a 25 m³ vented combustion test facility (CVE) with a fixed vent area sealed by a plastic sheet vent. Inside the CVE, a 0.64 m³ open vent box, called RED-CVE was placed. The vent of the RED-CVE was left open and three different vent area were tested. Two different mixing fans, one for each compartment, were used to establish homogeneous H₂ concentrations. This study examined H₂ concentrations in the range between 8.5% vol. to 12.5% vol. and three different ignition locations, (1) far vent ignition, (2) inside the RED-CVE box ignition and (3) near vent ignition (the vent refers to the CVE vent). Peak overpressures generated inside the test facility and the smaller compartment were measured. The results indicate that the near vent ignition generates negligible peak overpressures inside the test facility as compared to those originated by far vent ignition and ignition inside the RED-CVE box. The experiments with far vent ignition showed a pressure increase with increasing hydrogen concentration which reached a peak value at 11% vol. concentration and then decreased showing a non-monotonic behaviour. The overpressure measured inside the RED-CVE was higher when the ignition was outside the box whereas the flame entered the box through the small vent.     

Convective boiling and particulate fouling of stabilized CuO-ethylene glycol nanofluids inside the annular heat exchanger

Thermal performance of convective flow boiling heat transfer and particulate fouling of CuO/EG nanofluids has been experimentally studied inside the annular heat exchanger. CuO nanoparticles were well-dispersed and stabilized using a new combinational method (adding surfactant, stirring, pH control and sonication) in ethylene glycol (EG) as the base fluid in different weight fractions of nanoparticles (0.1–0.4%). Despite stabilizing the nanofluids, a considerable boiling heat transfer reduction due to the fouling resistance has been reported. Subsequently, scale formation and particulate fouling of nanofluids in term of fouling resistance has experimentally been investigated. Influences of operating parameters on the fouling resistance and heat transfer coefficient are investigated and briefly discussed.     

Combined effect of variable fluid properties on nonisothermal flow of an inelastic fluid with viscous heating

This study investigates the effect of variable thermal conductivity and variable viscosity on the flow characteristics and flow rates of an inelastic fluid, including viscous heating terms in the analysis. The exponential dependence of viscosity on temperature is modeled through Arrhenius law and shear rate dependence is modeled through Carreau rheological equation while it is assumed that the thermal conductivity varies linearly with temperature. Flow governing motion and energy balance equations are coupled and the solution is found iteratively facilitating a pseudospectral method based on the Chebyshev polynomial expansions. Effect of variable thermal conductivity and viscosity as well as the effect of power law index, material time constant, Brinkman and Peclet numbers on the velocity and temperature profiles are presented. Influence of the above parameters on the flow rate is also calculated.     

Numerical simulation of the quenching process in liquid jet impingement

Direct numerical simulation is performed for quenching of a hot plate in liquid jet impingement. The flow and thermal characteristics associated with the quenching process, which includes film boiling in the fluid region as well as transient conduction in the solid region, are investigated by solving the conservation equations of mass, momentum and energy in the liquid, gas and solid phases. The liquid–vapor and liquid–air interfaces are tracked by the sharp-interface level-set method modified to treat the effect of phase change. The computations demonstrate that the boiling curve of wall heat flux versus temperature does not depend on the transient or steady-state heating conditions. The effects of initial solid temperature and solid properties on the quenching characteristics are quantified.     

瑞士dimaco CTB--170立式内外圆仿形机床改造

1引言CTB-170立式活塞环内外圆仿形车铣床。是瑞士dimaco公司于上世纪80、90年代研发的。专用于活塞环生产加工的设备,问世后经过几代改良。该设备集活塞环内外圆同时仿形加工、开口于一体化的机加工机床。克服了过去车内圆、车外圆、开口三道工序必须由三台机床分开加工,而产生同心度差、物流中间过程时间长、效率低、产品质量不稳定等缺点。由于性能优异,被许多国内活塞环厂家引进,     

Experimental study of the fuel jet combustion in high temperature and low oxygen content exhaust gases

The performance of high temperature air combustion (HiTAC) depends on the heat regenerator efficiency and on the way fuel is mixed with furnace gases. In this work, combustion of a single fuel jet of gasol (>95% of propane) was investigated experimentally. Experiments were carried out in steady-state conditions using a single jet flame furnace. The jet of fuel was co-axially injected into high temperature exhaust gases generated by means of a gas burner also fired with gasol. Thus, instead of highly preheated and oxygen depleted air, which was normally used by other researches for such studies, this work has used high temperature and low oxygen content exhaust gases as the oxidiser. A water-cooled fuel nozzle was used to control fuel inlet temperature. Influence of the oxygen content in the oxidiser, at temperatures of 860–890 °C, on the flame visibility and the reactants composition was investigated. The combustion of gasol in hot flue gases appeared to be very stable and complete even at very low oxygen concentration. The oxygen concentration in the oxidiser was found to have a substantial effect on flame size, luminosity, colour, visibility and lift-off distance. Reduced oxygen concentration increases the flame size and lift-off distance, and decreases luminosity and visibility. The HiTAC flame first became bluish and then non-visible at sufficiently low concentration of oxygen in the oxidiser. In this work, results are presented for the constant ratio between fuel jet velocity and velocity of co-flowing flue gases. This ratio was equal to 26.     

Stability of conducting viscous film flowing down an inclined plane with linear temperature variation in the presence of a uniform normal electric field

Weakly nonlinear stability analysis of a thin liquid film falling down a heated inclined plane with linear temperature variation in the presence of a uniform normal electric field has been investigated within the finite amplitude regime. A generalized kinematic equation for the development of free surface is derived by using long wave expansion method. A normal mode approach and the method of multiple scales are used to investigate the linear and weakly nonlinear stability analysis of film flow, respectively. It is found that both Marangoni and electric Weber numbers have destabilizing effect on the film flow. The study reveals that both supercritical stability and subcritical instability are possible for this type of film flow. It is interesting to note that both the Marangoni and electric Weber numbers have qualitatively same influence on the stability characteristics but the effect of Marangoni number is much stronger compare to the electric Weber number. Scrutinizing the effect of Marangoni and electric Weber numbers on the amplitude and speed of waves it is found that, in the supercritical region amplitude and speed of the nonlinear waves increases with the increase in Marangoni and electric Weber numbers, while in the subcritical region the threshold amplitude decreases with the increase in Marangoni and electric Weber numbers. Finally, we obtain that spatially uniform solution is side-band stable in the supercritical region for our considered parameter range.     

Numerical study of compressible convective heat transfer with variations in all fluid properties

The effects of property variations in single-phase laminar forced micro-convection with constant wall heat flux boundary condition are investigated in this work. The fully-developed flow through micro-sized circular (axisymmetric) geometry is numerically studied using two-dimensional continuum-based conservation equations. The non-dimensional governing equations show significance of momentum transport in radial direction due to μ(T) variation and energy transport by fluid conduction due to k(T) variation. For the case of heated air, variation in C_p(T) and k(T) causes increase in Nu. This is owing to: (i) reduction in T_w, (T_w ? T_m), and (?T/?r)_w and (ii) change in ?T_m/?z results in axial conduction along the flow. The effects of ρ(p,T) and μ(T) variation on convective-flow are indirect and lead to: (i) induce radial velocity which alters u(r) profile significantly and (ii) change in (?u/?r)_w along the flow. It is proposed that the deviation in convection with C_p(T), k(T) variation is significant through temperature field than ρ(p,T), μ(T) variation on velocity field. It is noted that Nu due to variation in properties differ from invariant properties (Nu = 48/11) for low subsonic flow.     

An experimental and theoretical study of solidification in a free-convection flow inside a vertical annular enclosure

The natural convection and solidification in an annular enclosure has been studied experimentally and theoretically. Here the inner cylinder of the annular enclosure was cooled below the solidification temperature of the water, while the outer cylinder was kept on a uniform temperature well above 0 °C. The problem of the unsteady growth of the ice-layer on the inner cold cylindrical surface is studied theoretically and an approximate solution has been obtained for the quasi-steady development of the ice-layer thickness. In addition, it has been found that the influence of the contact layer between the frozen layer and the cold surface is of significant importance for the solidification process. Results are presented and compared between the experimental and the analytical investigation.     

Flow structure,wall pressure and heat transfer characteristics of impinging annular jet with/without steady swirling

The detailed impinging annular jet characteristics with/without induced swirling motion are experimentally studied. Obtained flow and wall pressure data confirm the classical observation for conventional annular impinging jet, with the flow exhibiting simple round impinging jet-like features at sufficiently large separation distances. In contrast, no such transition occurs in the presence of swirling. At short and intermediate separation distances, the swirling annular jet causes more non-uniform distributions of wall pressure and heat transfer on the impingement plate compared to the conventional annular jet. However, the reverse holds at large separation distances.     

Numerical modelling of isothermal release and distribution of helium and hydrogen gases inside the AIHMS cylindrical enclosure

Hydrogen is a highly flammable gas and accidental release in confined space can pose serious combustion hazards. Numerical studies are required to assess the formation of flammable hydrogen cloud within confined spaces. In the present study, numerical investigations on the release of helium and hydrogen gases as high-velocity jets and their subsequent distribution inside an unventilated cylindrical enclosure (AIHMS facility) has been carried out as a first step towards numerical studies on hydrogen distribution in confined spaces for safety assessments. Experimental data for jet release of helium at volume Richardson number 0.1 and subsequent distribution has been used as benchmark data. Sensitivity studies on the influence of grid sizes, time-steps and turbulence models are performed. The performance of the validated numerical model is evaluated using statistical performance parameters. Similarity relations are used to determine input parameters for hydrogen jet for corresponding experimental data with helium jets. Finally, the mixing and flammability aspects of hydrogen distribution inside the enclosure are studied using four numerical indices that quantify mixing and deflagration potential of a distribution. It is concluded that the helium experiments can be used for validation of numerical models for hydrogen safety studies and any one of the similarity relationships, viz., equal buoyancy, equal volumetric flow, or equal concentration can be used for assessing the behaviour of hydrogen release and distribution within confined spaces.     

Effect of stochastic thermal input on elastic and thermal properties of an infinitely long annular cylinder

Random elastic and thermal properties for an infinitely long solid conducting circular cylinder are investigated under the effect of random thermal input. The problem is considered in the context of a generalized thermoelasticity theory with one relaxation time. The lateral surface of the solid is traction free and subjected to known stochastic temperature, driven by an additive Gaussian white noise. Laplace transform technique is used to obtain the solution in the transformed domain. Statistically, we derive and analyze the mean and variance for temperature, displacement and stress. Numerical inversion of the transformed solution is carried out, represented graphically and discussed.