Development Of Convective Flow Research Articles

Heat transfer in inhomogeneous dispersed systems based on graphene oxide hydrogels

Based on the optical holography method, studies of the occurrence and development of convective flows in hydrogels of various concentrations with the addition of graphene oxide in relation to 3D-bioprinting technology have been performed. For quantitative measurement of temperature fields, the optical holography method was used in combination with the gradient thermometry method, based on the dependence of the refractive index on the properties of hydrogel systems modified with graphene oxide with different concentrations and temperatures. Under conditions of changes in the thermophysical properties of hydrogels, as well as the magnitude of the supplied heat flux, the features of heating the wall area are studied in order to determine the coefficients of thermal conductivity and heat capacity, as well as the nature of the formation of convective flows near the wall heated from below.

Research of heat transfer processes in hydrogels by holographic interferometry and gradient thermometry

The study of natural convection in structured optically transparent materials using pure and combined agarose-gelatin gels was carried out by optical holography. The article presents data on visualization of the occurrence and development of convective flows in such gels with non-stationary conductive heating from below. The similarities and differences of the conditions of heat transfer and the occurrence of convection in structured materials and droplet liquids are analyzed. For the first time experimentally obtained data on the effect of two interpenetrating and interacting structured media on the transition from conductive to convective heat transfer. Keywords: natural convection in gels, optical holography, hydrogels, three-dimensional bioprinting.

Исследование процессов теплопереноса в гидрогелях методами голографической интерферометрии и градиентной теплометрии

The study of natural convection in structured optically transparent materials using pure and combined agarose-gelatin gels was carried out by optical holography. The article presents data on visualization of the occurrence and development of convective flows in such gels with non-stationary conductive heating from below. The similarities and differences of the conditions of heat transfer and the occurrence of convection in structured materials and droplet liquids are analyzed. For the first time experimentally obtained data on the effect of two interpenetrating and interacting structured media on the transition from conductive to convective heat transfer

Influence of thermal gravitational-capillary convection on temperature fields in a thin wall

The development of convective flow in a layer of ethyl alcohol when heating one of the vertical walls of a rectangular cavity was investigated experimentally. Thermal films were obtained, whose processing allowed plotting in time the distribution of temperature and temperature gradients on the free surface of the liquid layer and the opposite thin vertical wall of the cavity after the flow of heated liquid on it.

The Influence of Real Gas Radiation on the Stability and Development of Benard Convection in a Two-Dimensional Layer

Abstract The influence of real gas radiation on the thermal and hydrodynamic stability of a two-dimensional layer of radiatively participating H2O and/or CO2 heated from below is investigated. The nongray radiation effects of the two species are treated rigorously using a global spectral approach, the Spectral Line Weighted-sum-of-gray-gases model. The phenomena are explored by solving the full coupled laminar equations of motion, energy, and radiative transfer from the low-Rayleigh number, pure conduction-radiation regime through the onset of buoyancy-induced flow to the developed Bénard convection regime. The evolution of the thermal, hydrodynamic, and radiative heating fields is studied, and the critical Rayleigh number is characterized as a function of participating gas species mole fraction, average layer gas temperature, layer thickness, wall emissivity, and total pressure. It is found that participating radiation in the medium has the effect of stabilizing the layer, delaying transition from a stable conduction regime to buoyancy-induced flow. The development of convective flow and temperature, along with the radiative heating are presented. It is found that the critical Rayleigh number in the radiatively participating gas layer can be more than an order of magnitude higher than the classical convection-only scenario. The onset of instability is found to depend on the species mole fractions, average gas temperature in the layer, wall emissivity, layer thickness, and total pressure. Generally, all other variables being the same, H2O has a greater stabilizing influence on the layer than CO2.

Experimental and numerical study of the natural convection in dispersed systems in a heated rectangular cell

In this work, we study the appearance and development of convective flows in a dispersed medium by the experimental methods and mathematical modeling. An experimental study of the dynamics of thermoconvective flows is carried out in a specially designed cell, which is a rectangular vertical cavity with the possibility of heating the walls. A series of experiments were carried out at different temperature gradients. Various modes of convective flow have been obtained. To simulate the dynamics of convective flows in a rectangular cavity, a mathematical model based on the equations of thermal convection in the diffusion approximation has been created. Convective flow patterns for a dispersed medium have been obtained. Numerical calculations coincide with the experimental results. Numerical simulation of the problem allows us to estimate the distribution of velocities, temperature, and concentration of the dispersed phase inside the cell during the formation of convective flows and the established convection regime.

Неустойчивость двойной диффузии при сонаправленной диффузии растворенных компонент

Diffusion can have a destabilizing effect on the development of convective flows in multicomponent liquid mixtures with different diffusion rates of components. The movement that arises in this case, which is generally called double (or differential) diffusive instability, is an important mechanism for intensifying mass and heat transfer in many natural and technological systems. As components that affect the overall density distribution in a liquid medium, heat and a dissolved substance or two dissolved substances can act. In the latter, isothermal case, the standard formulation of the problem includes the study of the stability of a two-layer system under counter diffusion of components dissolved in adjacent layers. The possibility of developing instability during co-directional diffusion of components having a different sign of the concentration dependence of density and initially dissolved in one of the layers was experimentally investigated for the first time. Such a statement of the problem is closest to a number of oceanological applications, where the development of instability is due to the uneven distribution of temperature and salinity of sea water. In present work, the development of double diffusive instability was found and it was shown that the structure of the resulting convective flow is determined by the ratios of the density increments due to the dissolution of the components and the diffusion coefficients. Based on the research results, a set of dimensionless parameters is proposed and a map of the regimes of the observed convective flows is constructed, which allows us to systematize the experimental data.

Multilayer system of films heated from above

The paper deals with the development of convective flows in a complex system of thin liquid layers occurring under the effect of thermal load. To study this process a mathematical model which is based on the decomposition of complex problem into unitary elements (modules) with some set of rules allowing their linkage with each other was constructed. Each module represents a monotypic local model. The fluid flow and heat transfer in each of these modules are described by the Navier-Stokes and thermal conductivity equations. The boundary conditions required to solve these systems of equations are represented explicitly and written in the form of conservation laws. The numerical analysis of effect of thermal load on the characteristics of the liquid layers movement depending on the Marangoni number is carried out. It is shown that physical and thermophysical properties of liquid layers play the decisive role. It is exactly they control the convective flows in layers and determine the position and shape of interfaces. The results of model problem solutions are presented.

Onset of convective flows in a near-wall granular layer during nonstationary liquid boiling

The onset and development of convective flows during nonstationary conductive heating of a cell with a layer of cylindrical pebbles placed in a liquid medium have been studied using a proposed and implemented optical method. The temperature fields have been measured using a holographic interferometry technique with immersion optical tomography elements, which is based upon the dependence of the index of refraction on the temperature and properties of a continuous medium. The mechanism of the pebble-bed influence on the time and character of microconvection formed near the cell wall heated from below has been studied for variable thermal properties of the liquid medium and pebbles in the granular layer and supplied heat flux. Data on the visualization of temperature fields and the development of convective flows in a pebble-bed reactor cell of complicated shape during nonstationary heating have been obtained for the first time.

Development of convection in near-wall granular bed during nonstationary heating

Based on the developed and realized optical method of the polarization of temperature fields, the results of experiments on the visualization of temperature fields and the development of convective flows during the nonstationary conductive heating of the cell wall involving a layer of pebble beds placed in liquid are presented. Upon variations in the heat physical properties of liquid and particles (pebbles) of the granular bed, as well as the value of the supplied heat flow, the mechanism of the pebble-bed effect on the time and character of the formation of free convection close to the wall heated from below has been studied. The calculation model of the process has been suggested.

Methods of visualization of hydrodynamics and non-stationary heat exchange in granular media

The paper presents the results of experiments on the visualization of the temperature fields and development of convective flows in the process of non-stationary conductive heating of the cell wall which includes a layer of filled balls placed into the liquid. The authors developed the calculation method for modeling of the process of warming up of cells and the development of convection under various boundary conditions.

Thermogravitational convection in a square cell involving a phase transition

The processes of freezing and thawing of a liquid in the presence of thermogravitational convection in a square cell with adiabatic vertical and isothermal horizontal walls are considered. The development of convective flow in time depends on the initial conditions, and therefore various scenarios of transition to a stationary solution are possible. The presence of phase transitions in the liquid makes it possible to freeze the transient structure of convective vortices. As a result, depending on the initial conditions, several stationary solutions that differ in both the structure of the convective motion and the magnitude of the heat transfer rate of the medium can exist.

Nonsteady end effects in Hele-Shaw cells.

Two new results are presented on the development of convective flow in a finite Hele-Shaw cell. First, it is shown that within a certain range of Rayleigh numbers there exist three regions in the fluid layer: the center region having a steady flow and the two end regions exhibiting time-dependent flow. Second, in the two end regions each flow oscillation is always found to be monoperiodic and the two are not correlated. Transition to a more complicated oscillation is not observed, and the flow never becomes chaotic. The time-dependent flow regime is bounded at low and high Rayleigh number by transitions to steady-state flow.