Radiation heat transfer introduction

INTRODUCTION TO RADIATION HEAT TRANSFER 4.10A Introduction and Basic Equation for Radiation... 

Conduction is treated from both the analytical and the numerical viewpoint, so that the reader is afforded the insight which is gained from analytical solutions as well as the important tools of numerical analysis which must often be used in practice. A similar procedure is followed in the presentation of convection heat transfer. An integral analysis of both free- and forced-convection boundary layers is used to present a physical picture of the convection process. From this physical description inferences may be drawn which naturally lead to the presentation of empirical and practical relations for calculating convection heat-transfer coefficients. Because it provides an easier instruction vehicle than other methods, the radiation-network method is used extensively in the introduction of analysis of radiation systems, while a more generalized formulation is given later. 

This quantity is dependent on the emissivity e, and both temperatures T and Ts. The introduction of arad allows the influence of radiation to be compared to convection. As e < 1 is always true, it is immediately obvious that an upper limit for the radiative part of heat transfer exists. 

Thermal radiation is the subject of the fifth chapter. It differs from many other presentations in so far as the physical quantities needed for the quantitative description of the directional and wavelength dependency of radiation are extensively presented first. Only after a strict formulation of Kirchhoff s law, the ideal radiator, the black body, is introduced. After this follows a discussion of the material laws of real radiators. Solar radiation and heat transfer by radiation are considered as the main applications. An introduction to gas radiation, important technically for combustion chambers and furnaces, is the final part of this chapter. 

In this section a brief introduction to the fundamental concepts of thermal radiation modeling is given. The main purpose of this survey is to elucidate the basic assumptions involved deriving the conventional engineering model of thermal radiation fluxes. To this end the thermal radiation flux is determined in terms of a heat transfer coefficient. 

Howell, J. R. A Catalog of Radiation Configuration Faclar.v (McGraw-Hill, New York, 19S2) iNf RopERA, F. P. and DE Witt, D. P. Introduction to Heat Transfer, 4th edn (Wiley, New York. 1996,i. INCROPERA, F. P. and de Witt, D. P. Fundamentals of Heat and Mass Transfer (Wiley, New York, 1 85). Jakob, M. Heat Transfer, Vol. 1 (Wiley, New York, 1949). 

There are two main types of FTIR detection for GCs, in the gas-phase using an in-stream optical system and through vapor deposition with detection being away from the GG flow stream. In the first, a light pipe that can transmit IR radiation is positioned on either side of a detection cell. Transparent windows pass the IR radiation into the flow ceU. The whole assembly is maintained at temperatures of 250 °C to 350 °C to prevent deposition of sample molecules. Most interfaces for this type of GC-FTIR also have heated transfer lines to and from the flow cell to ensure that no deposition occurs before introduction into the spectrometer. 

Ernst Schmidt (1892—1975), the German scientist, is known for his pioneering works in the fields of thermodynamics and heat and mass transfer. Some of his noteworthy contributions to heat and mass transfer were developing the analogy between heat and mass transfer, first measurement of velocity and temperature fields in natural convection boundary layer and heat transfer coefficient in droplet condensation, introduction of aluminum foil radiation shielding, and solution of... 

Introduction. The use of fins or extended surfaces on the outside of a heat exchanger pipe wall to give relatively high heat-transfer coefficients in the exchanger is quite common. An automobile radiator is such a device, where hot water passes inside through a bank of tubes and loses heat to the air. On the outside of the tubes, extended surfaces receive heat from the tube walls and transmit it to the air by forced convection. 

Early bolometers used, as thermometers, thermopiles, based on the thermoelectric effect (see Section 9.4) or Golay cells in which the heat absorbed in a thin metal film is transferred to a small volume of gas the resulting pressure increase moves a mirror in an optical amplifier. A historical review of the development of radiation detectors until 1994 can be found in ref. [59,60], The modern history of infrared bolometers starts with the introduction of the carbon resistor, as both bolometer sensor and absorber, by Boyle and Rogers [12], The device had a number of advantages over the Golay cell such as low cost, simplicity and relatively low heat capacity at low temperatures. 

Elements such as As, Se and Te can be determined by AFS with hydride sample introduction into a flame or heated cell followed by atomization of the hydride. Mercury has been determined by cold-vapour AFS. A non-dispersive system for the determination of Hg in liquid and gas samples using AFS has been developed commercially (Fig. 6.4). Mercury ions in an aqueous solution are reduced to mercury using tin(II) chloride solution. The mercury vapour is continuously swept out of the solution by a carrier gas and fed to the fluorescence detector, where the fluorescence radiation is measured at 253.7 nm after excitation of the mercury vapour with a high-intensity mercury lamp (detection limit 0.9 ng I l). Gaseous mercury in gas samples (e.g. air) can be measured directly or after preconcentration on an absorber consisting of, for example, gold-coated sand. By heating the absorber, mercury is desorbed and transferred to the fluorescence detector. 

An important capability of Curie point pyrolysers should be that the sample does not suffer any modifications before the pyrolysis step itself. As previously indicated, the housing of the pyrolyser must be heated (commonly with electrical resistances) to avoid condensation or other modifications of the pyrolysate. However, because a waiting time is inherent between the moment of sample introduction in the pyrolyser and the start of the pyrolysis itself, the sample may be heated by radiation from the sample housing. Several Curie point pyrolysers [8b] have the capability to drop the ferromagnetic foil containing the sample from a cool zone into the induction area, which is pre-heated to avoid condensation. The pyrolysis takes place immediately after the sample is transferred into this induction area such that no uncontrolled preliminary sample decomposition takes place. 

A factor that must be considered with furnace pyrolysers as well as with the other types of pyrolysers is the achieving of short TRT values. A slow sample introduction in the hot zone of the furnace will end in a long TRT. A poor contact between the sample and the hot source may also lead to long TRT, most of the heat being transferred by radiation and convection and not by conduction. However, fairly short TRTs in furnace pyrolysers were reported in literature [16,17]. 

The products leaving the furnace radiation zone must be cooled as quickly as possible This operation is designed in particular to prevent the effluent composition from changing by the formation of heavy polymerization products and the increase in the gasoline content. It is important for the transfer line between the furnace and the quench boiler to be as short as possible to avoid additional residence of the effluents at elevated temperature. The heat of the furnace is fust recovered by indirect cooling in the quench boilers, and directly by the introduction in-line of a recycle using a heavy hydrocarbon cut called quench oil Fig. 2.11). 

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