Numerical Study of Rotating Thermal Convection on a Hemisphere

Numerical simulations of rotating two-dimensional turbulent thermal convection on a hemisphere are presented in this paper. Previous experiments on a half soap bubble located on a heated plate have been used for studying thermal convection as well as the effects of rotation on a curved surface. Here, two different methods have been used to produce the rotation of the hemisphere: the classical rotation term added to the velocity equation, and a non-zero azimuthal velocity boundary condition. This latter method is more adapted to the soap bubble experiments. These two methods of forcing the rotation of the hemisphere induce different fluid dynamics. While the first method is classically used for describing rotating Rayleigh–Bénard convection experiments, the second method seems to be more adapted for describing rotating flows where a shear layer may be dominant. This is particularly the case where the fluid is not contained in a closed container and the rotation is imposed on only one side of it. Four different diagnostics have been used to compare the two methods: the Nusselt number, the effective computation of the convective heat flux, the velocity and temperature fluctuations root mean square (RMS) generation of vertically aligned vortex tubes (to evaluate the boundary layers) and the energy/enstrophy/temperature spectra/fluxes. We observe that the dynamics of the convective heat flux is strongly inhibited by high rotations for the two different forcing methods. Also, and contrary to classical three-dimensional rotating Rayleigh–Bénard convection experiments, almost no significant improvement of the convective heat flux has been observed when adding a rotation term in the velocity equation. However, moderate rotations induced by non-zero velocity boundary conditions induce a significant enhancement of the convective heat flux. This enhancement is closely related to the presence of a shear layer and to the thermal boundary layer just above the equator.

Calculating the load on the room cooling ceiling panel at convective heat gains

A well-known statement of the theory of thermal stability asserts that when a harmonically time-changing convective heat flux enters a room, it can only be assimilated by a radiant cooling system if the assimilating flow exceeds the perturbing convective flow in magnitude. However, in engineering practice, there are no purely radiant systems. Therefore, the article has considered a ceiling cooling panel as a room cooling system, the heat flow from which is of a radiant-convective nature. The convective heat access to the room is constant during the working hours from 9 a.m. to 6 p.m. The task of determining the load on the cooling system has been performed by calculations.. Herewith, the rooms of different internal thermal stability have been considered. According to the calculation results, it has been found that, since the amount of the heat gains remains constant for a long period of time, the process of the room cooling comes almost to a stationary state and does not depend on the room thermal stability with assimilated heat gains, the value of which in different options varied from 100 W to 1000 W. It has been found, that when the temperature difference between the panel surface and the surrounding surfaces increases, the proportion of the convective assimilating flow becomes bigger.

Особенности испарения капли при лучистом и конвективном нагреве

New method for determining the evaporation rate of single levitating drop at radiant heat flux has been proposed. The results of experimental study of the evaporation rate of distilled water drop upon heating by radiant and convective heat flux in the range of q = (0.25–1.5) W/cm^2 are presented. Comparative analysis of the features of the drop evaporation during radiant and convective heating has been carried out.

Regression rate response in spin-stabilized solid fuel ramjets

ABSTRACT The regression rate of the solid fuel in the spinning solid fuel ramjet is investigated here using numerical simulations. The finite volume solver of the reacting turbulent flow is developed to study the flow field in the back-step combustion chamber where the burning rate of the solid fuel is computed using the conjugate heat transfer. The dependence of the burning rate on the circumferential velocity of the ramjet is studied, and it is shown that the spin augments the burning rate due to the enhancement of the convective heat flux along the fuel grain. So, the spin can be used to improve the performance of the solid fuel ramjets. In addition, the effect of rapid change in spin velocity on the regression rate of the fuel is investigated, which shows the transient-burning behavior. The results show that although the spin may increase the burning rate by ∼10% in steady-state operation of the ramjet, the spin acceleration may cause the overshoot in burning rate with the peak value >30% in the unsteady operation.

Thermomagnetic Convection of Ferrofluid in an Enclosure Channel with an Internal Magnetic Field

Ferrofluid is a colloidal liquid in which magnetic nanoparticles such as Fe3O4 are dispersed in a nonconductive solution, and the average diameter of the nanoparticles is 10 nm. When a magnetic field is applied, the ferrofluid generates magnetization, which changes the physical properties of the fluid itself. In this study, characteristics of the thermomagnetic convection of ferrofluid (Fe3O4) by the permanent magnet in the enclosure channel were studied. To effectively mix the ferrofluid (Fe3O4) and disturb the boundary layer, the heat dissipation of the heat source depending on the strength of the magnetic field and the shape of the enclosure channel was numerically studied. In particular, four different enclosure channels were considered: Square, separated square, circle, and separated circle. The hot temperature was set at the center of the enclosure channel. The ferrofluid was affected by the permanent magnet in the center of the channel. The magnetic field strength in the region close to the permanent magnet was enhanced. The magnetophoretic (MAP) force increased with increasing magnetic field strength. The MAP force generated a vortex in the enclosure channel, disturbing the thermal boundary. The vortex occurs differently, depending on the shape of the enclosure channel and affects the thermomagnetic convection. The temperature and velocity fields for thermomagnetic convection were described and the convective heat flux was calculated and compared. Results show that when the magnetic field strength was 4000 kA/m and the shape of the enclosure channel was a circle, the maximum convective heat flux of 4.86 × 105 W/m2 was obtained.