Radiative Heat Transfer Mechanisms Research Articles

Modelling temperature distribution in cylindrical crystal growth furnaces

This paper presents a simple mathematical formulation for determination of temperature profiles of crystal growth furnaces using radiative heat transfer mechanism. The influence of various furnace elements on the profiles are presented. Using this analysis, one can design furnace with desired temperature profile. The theoretically calculated temperature profiles are in good agreement with the experimentally measured profiles of furnaces fabricated in our laboratory which are currently being used to grow a variety of crystals.

Molecular dynamics simulation of the precision machining process including radiative and convective heat transfer mechanisms

During the machining process, extreme temperatures can occur in the various heat generation zones of the material. These temperatures can sometimes reach or exceed the melting point. To accurately represent the influence of this heating, heat transfer mechanisms must be incorporated into simulation models. The simulation model described in this paper provides a new feature that includes heat transfer to the environment by the application of a finite-temperature molecular dynamics (md) simulation technique. The modelled system is not adiabatic. The simulated thermal environment described herein produces realistic simulations of the material surfaces. The exposed surfaces are cooler than the bulk as expected. The process simulations obtain realistic levels of thermal activation, which affect the material properties and the machining process parameters, e.g. friction forces, etc. Heat transfer with the environment is found to be of the same order of magnitude as the thermostat boundary layer for the simulations performed. As expected, at the temperatures that occur during machining, radiative heat transfer dominates over convective transport. The thermal model also clearly shows the heat associated with the various machining regions, due to plastic deformation and friction.

Ignition studies of fuel droplet streams

Experiments have been conducted to identify the importance of transport mechanisms on fuel droplets ignition. An electrically heated flat plate was positioned in front of a vertical stream of decane droplets. The geometry and dynamic of the arrangement was photographically recorded to obtain time and space resolved information. The following results were observed: The ignition event depends on the temperature of the hot surface, the geometry of the arrangement, the physical properties of the fuel, the residence time of the droplets in the thermal boundary layer and the fuel vapor concentration of the carrier gas at room temperature. Comparisons of ignition delay times which translate into ignition delay distances show discrepancy with predicted values. The lack of a radiation heat transfer mechanism in the model and the enhanced convective heat transfer due to the thermal boundary layer of the vertical flat plate are assumed to be the reason for the mismatch between theory and experimental data.

Size and number density change of droplet populations above a quench front during reflood

The ‘precursory cooling’ in the immediate region above the quench front cannot be adequately explained using a three component (wall-droplet-vapor) phenomenological heat transfer analysis based on a single averaged droplet size. This phenomenon can be attributed to the fact that two distinct droplet generation mechanisms are present (fragmentation of bubble film and hydrodynamic) leading to a bi-spectral droplet population. A model is proposed to describe the bi-spectral droplet generation mechanism. The evaporation rate of these droplet populations is analyzed by taking into account the relevant convective and radiative heat transfer mechanisms. The rapid evaporation and diminution of the small droplet spectrum can explain the exponential rate of energy transfer from the cladding surface in the vicinity above the quench front, and by analogy the enhanced heat transfer rate downstream of flow restrictions.

NUMERICAL STUDY OF THE THERMAL PROCESSES IN A FURNACE

A detailed numerical study of the combined modes of heat transfer in a furnace is carried out. The furnace considered is a heat treatment furnace and the conjugate modes involving radiation )particularly gas radiation), conduction, and convection are considered in order to determine the transient temperature variation in the furnace and in the material undergoing heat treatment. The thermal processes are coupled through the boundary conditions and the property variations. Strong non-linearity is introduced due to the radiative heat transfer mechanisms. The nonlinear coupled system of partial differential equations is solved numerically by time marching, which allows study of the time-dependent temperature field in the system. The initial temperature of the system and the inflow temperature of the hot gases are the main inputs. Interest lies in determining the temperature decay in the gases as they flow from the inlet to the outlet and in the transient increase in temperature level in the furnace. By use o...

Numerical Study of the Thermal Processes in a Furnace

A detailed numerical study of the combined modes of heat transfer in a furnace is carried out. The furnace considered is a heat treatment furnace and the conjugate modes involving radiation )particularly gas radiation), conduction, and convection are considered in order to determine the transient temperature variation in the furnace and in the material undergoing heat treatment. The thermal processes are coupled through the boundary conditions and the property variations. Strong non-linearity is introduced due to the radiative heat transfer mechanisms. The nonlinear coupled system of partial differential equations is solved numerically by time marching, which allows study of the time-dependent temperature field in the system. The initial temperature of the system and the inflow temperature of the hot gases are the main inputs. Interest lies in determining the temperature decay in the gases as they flow from the inlet to the outlet and in the transient increase in temperature level in the furnace. By use o...

On the relative importance of the electronic and radiative contributions to the thermal conductivity of uranium dioxide

The relative contributions of radiative and electronic heat transfer mechanisms to the thermal conductivity of uranium dioxide are considered. Uncertainties in the extrapolation of data to high temperatures are discussed, with particular reference to the thermal conductivity of the liquid phase.

Guide and Data for Greenhouse Air Conditioning

Thermal radiation cooling during the night is one of the most significant phenomena for the greenhouse and the long wave net radiation exchange between the cold sky and the greenhouse surface has been analysed under the assumption that there is no significant temperature difference betwen the greenhouse surface and the surrounding ground surface.In order tlo calculate the air-conditioning load, especially the heating load of the greenhouse, it is indispensable to analyse the radiative heat transfer mechanism of the whole system which includes the green house and to calculate the amount of the heat flow of each section involved. The diagrammatic representation each radiation exhange path is given in Fig. 3.In the present study, the theoretical analysis is expanded to the thermal radiation cooling of the heated greenhouse as well as experimental verification. The view factor to the sky of the finite glass plane whose inclination angle is α is expressed as (1+cosα)/2 under the assumption that the long wave radiation from the sky is uniform and this factor is introduced as one of the important factor of the heat loss from the unheated greenhouse (Takakura 1967 a). Using this relationship, the ratio of the net radiation between the greenhouse surface and the sky to that between the sky and the horizontal surface whose area is the same as the greenhouse floor is expressed as (1+β)/2, where β is the ratio of the greenhouse floor area to the greenhouse surface area (Tachibana 1971).The net radiation between the greenhouse surface and the sky is a function of the water vapor pressure in the air and the angular height of the net radiation intensity. Therfore, the numerical integration of Kondrat'yev's equation has been conducted and one of the results, which is the curve for 6mmHg is given in Fig. 2. The deviation from this curve due to the variation of the water vapor pressure is given as vertical deviation bands at three inclination angles in the same figure instead of giving the family of curves. This curve is expressed by the function (1+cosα-0.23 sin2α)/2 under 2% error.Using this function, the ratio of the net radiation loss from the greenhouse surface to the net radiation of the horizontal surface is expressed as (0.77+β+0.23β2)+γ(1-β)/2, where γ is the ratio of the net radiation between the greenhouse wall and the surrounding ground surface to the net radiation between the greenhouse wall and the cold sky. Suppose that there is no temperature difference between the greenhouse wall and the surrounding ground surface, γ is equal to zero. On the other hand, if there is a significant temperature difference between them, e. g., 10 deg C, γ is approximately 0.5 and the value is increased 60% more than the value at γ is zero in the smaller range of β. The results are given in Fig. 4. Therefore, it is concluded that the thermal radiation cooling of the heated greenhouse should be calculated by Eqs. 13 and 14.The calculation results are verified experimentally.

Numerical Analysis of Overall Radiation Interchange Configuration Factor

The radiation heat transfer mechanism is one of the significant problems in engineering. This problem is centered on the determination of the overall radiation interchange configuration factor. In the past, Hottel, Yagi-Kunii, Sparrow and Gebhart introduced the methods of calculating the overall configuration factor in the enclosure. But these methods have been too complicated for practical application, and are not suitable for making an accurate analysis and calculation. In this paper, a new method for determining the overall configuration factor within an enclosure filled with absorbing medium is described. Even if the enclosure is composed of a complicated shape and a number of surfaces, the conversion method expressed as a matrix form leads to a systematic and efficient solution of the overall configuration factor.