Contents
Introduction
Heat transfer radiation occurs via electromagnetic waves. It is 90-95% of the total heat transfer in furnaces of steam boilers, electric arc steel-making furnaces, 80-90% of plasma arc furnaces and chambers of the reheating furnaces. Electromagnetic waves propagated in a straight line at the speed of light and are subject to the laws of optical refraction, absorption, reflection. Radiation occurs when a quantum transition of atoms and molecules of the stationary states with greater vigour in stationary states with less energy. For continuous radiation body must receive power from outside. Because transitions of atoms and molecules from one state to another are diverse, radiation has different wavelengths. Thermal radiation is a type of electromagnetic oscillations with wavelength from 0.4 up to 40 microns. Heat transfer radiation in industrial furnaces plays a dominant role.
1. Theme urgency
The evolution of technology in the modern era is characterized by the widespread intensification processes in various facilities and devices that require improvement and creation of qualitatively new designs to ensure high physicotechnical characteristics. This applies to boiler installations, the most common of which are the metallurgical furnaces. Efficiency of most heat exchangers can be enhanced through intensification of heat radiation.
2. Goal and tasks
Purpose of the project is the analysis of mathematical model of absolute black body and support on that basis of radiation heat transfer calculation of effective methods that provide additional opportunities to improve the efficiency of the entire installation.
3. Main provisions
Radiant heat transfer occurs as a result of the processes of making internal energy in energy radiation, radiation energy transfer and absorption of the substance. Radiation heat transfer processes flow is determined by mutual arrangement in space, sharing warmth, properties, separating the bodies. Radiant heat transfer essential difference from other types of heat transfer (heat conduction, convection) is that it may proceed in the absence of the material environment, sharing the heat transfer surface, as is the spread of electromagnetic radiation. Any body temperature is absolute zero emits energy due to heating of the body. This radiation is called its own radiation body and is characterized by its own radiation Qown. Own radiation of the unit body surface is called the density of own radiation, or emissivity of a body. Ratio of emissivity of a body to the emissivity of black body at the same temperature is called the degree of blackness. Absolute black body is the body, pull down the greedy all electromagnetic radiation at all wavelengths, and indicating nothing physical is idealization, radiation by a factor equal to 1. Spectral characteristics of absolute black body corresponds to the limit, the maximum possible heat radiation at a given temperature. The model of completely black body can be described as a closed cavity walls which have the same temperature. If the cavity, do a small hole, much less than the surface area of the walls of the cavity, then exit hole of radiation energy can't substantially change the thermodynamic equilibrium in the cavity, and radiation of heat emission hole would be completely black body at the temperature of the walls. Model diagram shown in Figure 1. 1 (scheme a).
Picture 1
Consider a flat vertical surface where the holes will be done lots of spherical form in Figure 3. 1 (scheme b). To determine the intensity of such a surface, you first need to find the resulting heat flow at the exit of the hole world. The inner surface of the globe split to bars and imagine a polygonal pattern is shown in Figure 2:
Picture 2 - Location model of internal surfaces of the globel
The surface of a polygon has the same temperature and the same degree of blackness. Hole 1-2 see how black body. You must define the resulting thread that goes from holes 1-2. To calculate the heat transfer radiation between two surfaces, you must determine the portion of the total energy of radiation emanating from one surface and a second surface up directly. Define angle radiation factors, as part of the total energy of radiation emanating from the first surface and a second surface falling directly on. Value of the angular coefficient radiation will search using the tensed strings. This method of calculating angle factor for two-dimensional surfaces, indefinitely extended in one direction and characterized by identity all cross sections, normal to the direction of infinite length. Figure 3 shows two surfaces that meet the above geometric constraints.
Picture 3 - Method of the strained filaments
According to this method, the angular coefficient is equal to the sum of the lengths of intersecting threads, strained between the edges of the two surfaces, minus the sum of the lengths of two disjoint lines divided by twice the length (L). Heat exchange system, including gray surface is more complex than system consisting only of ferrous surfaces. When radiation is radiation of surface gray and it must be taken into account in the balance of power. All surface of isothermal, and pull down on every surface radiation evenly. Grey surface are diffuse and in accordance with the law, Kirchhoff, the ability of the surface emissivity is equal to its reflectivity. If such a surface put an imaginary plane (Figure 4), to ensure a balance of power in stationary conditions, the resulting energy that you need to tell the gray surface to maintain its temperature, must be equal to the difference between the energy emanating from the surface and energy falling on its radiation.
Picture 4 - Balance of energy on an imaginary plane near a grey surface
The resulting heat flow from the grey surface of the designated index i will be equal to the difference of heat flow effectively and the incident radiation to the surface:
We elaborate matrix A [1 .. 12.12.1], the value of the diagonal elements:
The values of other elements we determine on a formula:
For a black surface there are a values diagonally of element :
Values of other elements :
We make the column of sizes of B[1.12]:
For a black surface:
We find the values of effective streams of radiation of Eeff, deciding the system of equalizations of A[i, j]- Eeff [i]=B[i], by the method of Gausse.
We find a resulting stream for grey surfaces on a formula:
For a black surface:
If the temperature of the inner surface of the sphere is equal to 500K (if black level equal to 0.8)-the resulting heat flow is increased by 25%
Conclusion
Comparing the value of the resulting flow of Erez to pad 1-2 and stream emitted by the plate is a similar length, temperature, and degree of blackness as the internal surface of the ball, it can be concluded that the proposed configuration allows you to increase the overall result heat flow and intensified heat transfer radiation in General
This master's work is not completed yet. Final completion: December 2012. The full text of the work and materials on the topic can be obtained from the author or his head after this date.
References
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