Onset of buoyancy-driven convection in melting from below

When a horizontal homogeneous solid is melted from below, convection can be induced in a thermally unstable melt layer. In this study the onset of buoyancy-driven convection during time-dependent melting is investigated by using similarly transformed disturbance equations. The critical Rayleigh numbers based on the melt-layer thickness are found numerically for various conditions. For small superheats, the present predictions approach the well known results of classical Rayleigh-Bénard problems, that is, critical Rayleigh numbers are located between 1,296 and 1,708, regardless of the Prandtl number. However, for high superheats the critical Rayleigh number increases with an increase in phase change rate but with decrease in Prandtl number.     

SURFACE TENSION AND BUOYANCY DRIVEN INSTABILITIES IN A LAYER OF LIQUID TIN HEATED FROM BELOW

The linear theory of Pearson (1958) and Nield (1964) is modified here to study liquid tin and include the finite thermal resistances of the bounding layers of boron nitride, copper and air (∼10^-2 torr) in the experiments of Ginde et al. (1989). The magnitude of the ΔT_c across the layer of liquid tin required for the onset of convection depends on the ratios of the thermal conductivities and thicknesses of the supporting layers of boron nitride and copper to those of the tin.

According to our theory surface tension contributes more than buoyancy to the instability observed experimentally. The critical ΔT_c observed required for the onset of convection in the layer of tin, is up to 25% lower than that predicted, which shows the layer is less stable than the theory indicates. Thus the surface of the tin was uncontaminated, or a significantly larger observed critical ΔT_c would be expected.

The boundary condition on the thermal fluctuations at the base of the supporting layer of copper does not appear to be important in these experiments. However, the thermal resistance of the boron nitride would have to be assumed to be unrealistically large to obtain agreement within experimental error with the theory.     

水下滑翔机总体设计与运动分析

水下滑翔机是一种浮力驱动的新型水下无人运载器,但是一直以来较低的巡航速度是制约其广泛应用的一个关键因素。为提高水下滑翔机的巡航速度,首先应通过对水下滑翔机的壳体和滑翔翼进行优化设计,并且从做功的角度规划出最佳滑翔路径,设计了一种新型的高速、高效水下滑翔机。最后,通过运动弹道的仿真分析,验证了水下滑翔机设计方法的有效性和总体性能的优越性。     

Buoyancy convection in a cavity with mutually orthogonal heated plates

Buoyancy driven convection in a square cavity induced by two mutually orthogonal arbitrarily placed heated thin plates is studied numerically under isothermal and isoflux boundary conditions. The flow is assumed to be two-dimensional. The coupled governing equations were solved by the finite difference method using the Alternating Direction Implicit technique and Successive Over Relaxation method. The steady state results are depicted in terms of streamline and isotherm plots. It is found that the resulting convection pattern is stronger for the isothermal boundary condition. A better overall heat transfer can be achieved by placing one of the plates far away from the center of the cavity for isothermal boundary condition and near the center of the cavity for isoflux boundary condition.     

Numerical investigation of supercritical LNG convective heat transfer in a horizontal serpentine tube

The submerged combustion vaporizer (SCV) is indispensable general equipment for liquefied natural gas (LNG) receiving terminals. In this paper, numerical simulation was conducted to get insight into the flow and heat transfer characteristics of supercritical LNG on the tube-side of SCV. The SST model with enhanced wall treatment method was utilized to handle the coupled wall-to-LNG heat transfer. The thermal–physical properties of LNG under supercritical pressure were used for this study. After the validation of model and method, the effects of mass flux, outer wall temperature and inlet pressure on the heat transfer behaviors were discussed in detail. Then the non-uniformity heat transfer mechanism of supercritical LNG and effect of natural convection due to buoyancy change in the tube was discussed based on the numerical results. Moreover, different flow and heat transfer characteristics inside the bend tube sections were also analyzed. The obtained numerical results showed that the local surface heat transfer coefficient attained its peak value when the bulk LNG temperature approached the so-called pseudo-critical temperature. Higher mass flux could eliminate the heat transfer deteriorations due to the increase of turbulent diffusion. An increase of outer wall temperature had a significant influence on diminishing heat transfer ability of LNG. The maximum surface heat transfer coefficient strongly depended on inlet pressure. Bend tube sections could enhance the heat transfer due to secondary flow phenomenon. Furthermore, based on the current simulation results, a new dimensionless, semi-theoretical empirical correlation was developed for supercritical LNG convective heat transfer in a horizontal serpentine tube. The paper provided the mechanism of heat transfer for the design of high-efficiency SCV.     

垂直矩形通道内的混合对流实验与数值研究

对耦合了热辐射的垂直矩形通道内的混合对流情况进行了实验研究和数值模拟分析.研究表明:空气在通道内向上流动时,随着浮升力作用的增大,对流换热能力表现出先减小后增强的趋势;热辐射在换热过程中起着重要的作用,并随着对流换热能力的减弱而增强.数值模拟在浮升力影响较小时可以给出较好的结果,当浮升力影响比较大时,数值模拟计算的结果与实验有较大的偏差.     

A Numerical Modeling of Rayleigh–Marangoni Steady Convection in a Non-Uniform Differentially Heated 3D Cavity

A numerical study of a buoyancy driven convection flow in presence of thermocapillarity has been developed. The fluid is a silicone oil (Prandtl number equal to 105) contained in a three-dimensional box bounded by rigid and impermeable walls with top free surface exposed to a gaseous phase. At the lateral box walls a different non-uniform temperature distribution is assumed so to induce horizontal convection and to keep separated thermocapillary and buoyancy effects. The vorticity-velocity formulation of the time-dependent Navier–Stokes equations for a non-isothermal incompressible fluid is used. A procedure based on a linearized fully implicit finite difference second order scheme has been adopted. We obtained very complex steady configurations for several values of the temperature difference at the lateral walls, T=30, 40 and 50°C. Along the direction perpendicular to the lateral walls, for T increasing, we observe a physically meaningful growth of heat transfer. Confidence in these results is supported by a comparison with recent experimental and numerical observations.     

Potential model for single-sided naturally ventilated buildings in China

The paper investigates a single-sided naturally ventilated buildings potential model considering number of factors in China. This model can be used to estimate potential of natural ventilation via local climate data and building parameters. The main goal of the model is to predict natural ventilation hours and hourly ventilation flow rate. In fluid model, formula of single-sided natural ventilation by coupling wind pressure and temperature difference was used to calculate air flow rate. Accordingly, the paper analyzed four typical cities in different climate region in China and calculated pressure difference Pascal hours (PDPH). The results show that single-sided ventilation has fewer adaptive comfort hours than two-sided ventilation and much less ventilation volume. This model provided quantitative information for early stage architectural natural ventilation design and building energy efficiency evaluation.     

Numerical computations and optical diagnostics of unsteady partially premixed methane/air flames

The structures and dynamics of unsteady laminar partially premixed methane/air Bunsen flames are studied by means of numerical simulations, OH and CH PLIF imaging, and high speed chemiluminescence imaging employing a high framing speed intensified charge coupled device camera. The Bunsen burner has a diameter of 22 mm. Rich methane/air mixtures with an equivalence ratio of 1.5 are injected from the burner into atmosphere at different flow speeds ranging from 0.77 to 1.7 m/s, with Reynolds numbers based on the nozzle flow ranging from 1100 to 2500. The numerical simulations are based on a two-scalar flamelet manifold tabulation approach. Detailed chemistry is used to generate the flamelet manifold tabulation which relates the species concentrations, reaction rates, temperature and density to a distance function G and mixture fraction Z. Two distinct reaction zones are identified using CH and OH PLIF imaging and numerical simulations; one inner reaction zone corresponds to premixed flames on the rich side of the mixture and one outer reaction zone corresponds to mixing controlled diffusion flames on the lean side of the mixture. Under normal gravity conditions both the inner premixed flames and the outer diffusion flames are unsteady. The outer diffusion flames oscillate with a flickering frequency of about 15 Hz, which slightly increases with the burner exit velocity. The inner premixed flames are more random with much more small-scale wrinkling structures. Under zero gravity conditions the outer diffusion flames are stable whereas the inner premixed flames are unstable and highly wrinkled. It appears that the outer diffusion flames are governed by the Rayleigh-Taylor instability whereas the inner premixed flames are dictated by Landau-Darrieus instability. The two-scalar flamelet approach is shown to capture the basic structures and dynamics of the investigated unsteady partially premixed flames.     

Turbulence and mixing across gravitationally unstable interface by tank-overturn experiment

Turbulence and mixing across gravitationally unstable interface were studied in the laboratory by overturning a tank of two-layer fluids initially of stable stratification. As the dense fluid fell under gravity to mix with the lighter fluid from below, highly unsteady exchange of fluids across the unstable interface was produced by the buoyancy force. The exchange was captured in the experiment by a video camera using dye in the dense fluid as tracer. Absorption of light by the dye determined the excess mass at every pixel of the digitized images. The position of the excess-mass center and the speed of the center were computed from the excess-mass profiles as parameters to characterize the mixing across the unstable interface. With positive feedback by the buoyancy force, mixing across the interface rapidly intensified. It increased linearly from zero to a maximum in an acceleration regime and then asymptotically toward a terminal state, as the total buoyancy in the layer stayed constant. In the terminal state, the excess-mass center advanced at a terminal speed in proportion to the square root of the layer's total buoyancy.