Surface textures induced by convection in thin films of polymeric and polymerizable fluids

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.     

Stability of buoyancy-driven convection in directional solidification of a binary melt

In directional solidification, compositional convection can be driven by an unstable density gradient in the melt. In this paper convective instabilities in liquid and mushy layers during solidification of a horizontal binary melt are analyzed by using the propagation theory. The self-similar stability equations are used to examine the boundary-mode and mushy-layer-mode of convection. The effects of velocity conditions at the liquid-mush interface on the onset of convection are discussed. The critical Darcy-Rayleigh number for the convection in the mushy layer decreases with increasing the temperature of a cooled boundary. This paper is presented on the occasion of professor Chang Kyun Choi’s retirement from the school of chemical and biological engineering of Seoul National University.     

Lattice Boltzmann equation for axisymmetric thermal flows

A simple lattice Boltzmann equation (LBE) model for axisymmetric thermal flow is proposed in this paper. The flow field is solved by a quasi-two-dimensional nine-speed (D2Q9) LBE, while the temperature field is solved by another four-speed (D2Q4) LBE. The model is validated by a thermal flow in a pipe and some nontrivial thermal buoyancy-driven flows in vertical cylinders, including Rayleigh-Bénard convection, natural convection, and heat transfer of swirling flows. It is found that the numerical results agree excellently with analytical solution or other numerical results.     

Predictions of buoyancy-induced flow in various across-shape concave enclosures

The present study was conducted to numerically investigate the steady laminar buoyancy-driven and convection heat transfer characteristics within three different across-shape concave enclosures for the Prandtl number of 0.71 and 4, the Grashof number range 10⁴ ≤ Gr ≤ 2 × 10⁵_, and the gap range 0 ≤ H₁/H₂ ≤ 0.25. The steady Navier-Stokes equations, governing the flow under Boussinesq approximation, are solved with the dimensionless stream function-vorticity formulation in terms of curvilinear coordinates using the finite difference method. The results show that the effects of various shapes, the strength of the vortex is relatively bigger in the rectangular-rectangular concave enclosure than in the rectangular-circular concave enclosure at the same Grashof number. Heat transfer from the different across-shape concave enclosures is evaluated, and flow and heat transfer characteristics are discussed.     

Onset of buoyancy-driven convection in the horizontal fluid layer subjected to ramp heating from below

The onset of buoyancy-driven convection in an initially isothermal, quiescent fluid layer heated from below with a constant heating rate is analyzed by the propagation theory. Here the dimensionless critical time τ_c to mark the onset of convective motion is presented as a function of the Rayleigh number Ra_φ and the Prandtl number Pr. The present stability analysis predicts that for a given large Ra_φ, τ_c decreases with increasing Pr and it is independent of the conditions of the upper boundary. For deep-pool systems, the deviation of the temperature profile from conduction state occurs starting from a certain time τ_o≅4τ_c. The present predictions are compared with other models and existing experimental results in the whole time domain.     

Natural convection in an air-filled cavity: Experimental results at large Rayleigh numbers

A large-scale experimental setup is built and instrumented. It consists in a 4 m-high cavity with a horizontal cross-section equal to 0.86 × 1.00 m². Two opposite vertical walls are heated and cooled down; other walls (lateral walls, ceiling and floor) are made of insulating medium covered with a thin and low-emissivity film designed to minimize radiative effects into the cavity. The temperatures of active walls are imposed, constant and equally distributed around the ambient temperature in order to reduce heat losses. The temperature difference between the hot and cold walls is chosen to respect the Boussinesq approximation. Under these assumptions, Rayleigh number values up to 1.2 × 10¹¹ (ΔT = 20 °C) can be obtained. The centre-symmetry is verified on the thermal stratification. Influence of the temperature difference and of wall emissivities on the stratification parameter (dimensionless vertical temperature gradient) is discussed. Velocity measurements allow the velocity field to be obtained and provide information on flows encountered in the cavity. Temperature measurements are also carried out in the whole cavity. In the paper, a complete experimental characterization is provided: airflow inside the cavity is analyzed and the Nusselt number along the hot and the cold wall is presented.     

Buoyancy-driven single-sided natural ventilation in buildings with large openings

Full-scale experimental and computational fluid dynamics (CFD) methods were used to investigate buoyancy-driven single-sided natural ventilation with large openings. Detailed airflow characteristics inside and outside of the room and the ventilation rate were measured. The experimental data were used to validate two CFD models: Reynolds averaged Navier-Stokes equation (RANS) modeling and large eddy simulation (LES). LES provides better results than the RANS modeling. With LES, the mechanism of single-sided ventilation was examined by turbulence statistical analysis. It is found that most energy is contained in low-frequency regions, and mean flow fields play an important role.     

Evaluation of buoyancy-driven ventilation in respect of exergy utilization

Our work aimed to analyze and evaluate the buoyancy-driven ventilation based on the exergy analysis. We took the exergy load as a desired output for this consumer system and used the functional exergy efficiency to evaluate the ventilation performance. Through the numerical case studies for a high-rise building with a tall atrium, we found that the results from the energy and exergy analysis are quite different from each other, but the latter reveals the real essence of energy utilization in ventilation systems. The results showed that the exergy efficiency of the buoyancy-driven ventilation system is very poor, only 16.9% of the exergy input is effectively utilized and the exergy destruction counts for 83.1% of the total input. However, the exergy efficiency of the mechanical ventilation system is 100% because the input shaft work is entirely utilized to undertake the exergy load; no extra exergy losses are produced. We also analyzed the relationships between the temperature difference and the exergy efficiency. Furthermore, we found that the total radiation-to-exergy efficiency is 3.5 and 15% for ventilation systems equipped with solar collectors and solar cells respectively, it is concluded preliminarily that the latter is more efficient to utilize solar energy to create ventilation.     

The onset of convective instability in the thermal entrance region of plane Poiseuille flow heated uniformly from below

The onset condition of regular longitudinal vortex rolls in the thermal entrance region of plane Poiseuille flow heated from below is analyzed. Under propagation theory the stability equations are produced self-similarly, based on scale analysis. The onset position of secondary flow, which represents the starting point of mixed convection, is predicted as a function of the Prandtl number, Reynolds number and Rayleigh number. As expected, the critical position moves upstream as the Rayleigh number increases and an increase in Reynolds number makes the system more stable. The present predictions compare favorably with existing experimental data of water and air.     

Thermal instability of viscoelastic fluids in porous media

A theoretical analysis of thermal instability driven by buoyancy forces is conducted in an initially quiescent, horizontal porous layer saturated by viscoelastic fluids. Modified Darcy’s law is used to explain characteristics of fluid motion. The linear stability theory is employed to find the critical condition of the onset of convective motion. The results of the linear stability analysis show that the overstability is a preferred mode for a certain parameter range. Based on the results of linear stability analysis, a nonlinear stability analysis is conducted. The onset of convection has the form of a supercritical and stable bifurcation independent of the values of the elastic parameters. The Landau equations and the Nusselt number variations are derived for steady and oscillatory modes.