RADIATION HEAT TRANSFER: BLACK SURFACES

RADIATION HEAT TRANSFER: BLACK SURFACES

So far, we have considered the nature of radiation, the radiation properties of materials, and the view factors, and we are now in a position to consider the rate of heat transfer between surfaces by radiation. The analysis of radiation exchange between surfaces, in general, is complicated because of reflection: a radiation beam leaving a surface may be reflected several times, with partial reflection occurring at each surface, before it is completely absorbed. The analysis is simplified greatly when the surfaces involved can be approximated as blackbodies because of the absence of reflection. In this section, we con- sider radiation exchange between black surfaces only; we will extend the analysis to reflecting surfaces in the next section.

Consider two black surfaces of arbitrary shape maintained at uniform temperatures T1 and T2, as shown in Fig. 22–18. Recognizing that radiation leaves a black surface at a rate of Eb = sT 4 per unit surface area and that the view factor F1 ® 2 represents the fraction of radiation leaving surface 1 that strikes surface 2, the net rate of radiation heat transfer from surface 1 to surface 2 can be expressed as

RADIATION HEAT TRANSFER -0047

which is the desired relation. A negative value for Q1 ® 2 indicates that net radiation heat transfer is from surface 2 to surface 1.

Now consider an enclosure consisting of N black surfaces maintained at specified temperatures. The net radiation heat transfer from any surface i of this enclosure is determined by adding up the net radiation heat transfers from surface i to each of the surfaces of the enclosure:

RADIATION HEAT TRANSFER -0048

Again a negative value for Q indicates that net radiation heat transfer is to surface i (i.e., surface i gains radiation energy instead of losing). Also, the net heat transfer from a surface to itself is zero, regardless of the shape of the surface.

RADIATION HEAT TRANSFER -0049RADIATION HEAT TRANSFER -0050RADIATION HEAT TRANSFER -0051

Related posts:

Air only relationships:Pipeline pressure drop and Flow parameters and properties.
Gas–solid separation devices:Separation devices and Gravity settling chambers.
Air movers:Pre-cooling systems
BASIC DIAGRAMS AND SYSTEMS:Automatic Venting at End of Cycle.
Erosive wear:Wear patterns and deflecting flows.
Operating problems:Existing plant and Positive pressure systems
Design procedures:The use of test data in system design
Material property influences:Material degradation effects
Conveying capability:High pressure conveying – Part II
HYDRAULIC FLUIDS:PURPOSE OF THE HYDRAULIC FLUID
Pressure and flow:Fluid types
Control components in a hydraulic system:Flow control valves.
Air compressors, air treatment and pressure regulation.
BASICCONCEPTS OF THE RMODYNAMICS:PROPERTIES OF A SYSTEM
ENERGY TRANSFER BY HEAT,WORK,AND MASS:NONMECHANICAL FORMS OF WORK

Leave a comment

Your email address will not be published. Required fields are marked *