Heat transfer, in combustion

Richter, W. "Parametric Screening Studies for the Calculation of Heat Transfer in Combustion Chambers" Topical Report, prepared for Pittsburgh Energy Technology Center, Department of Energy, Under Contract No. DE-AC22-80PC30297, 1982. 

R. Viskanta and M. P. Mengtif, Radiation Heat Transfer in Combustion Systems, Progress in Energy and Combustion Science, 13, pp. 97-160,1987. 

D., and Evans, D. D. "Structure and Radiation Properties of Large Two-Phase Flames." In Heat Transfer in Combustion Systems, edited by N. Ashgriz, J. G. Quintiere, H. G. Semerjian, and S. E. Slezak, 77-86. New York American Society of Mechanical Engineers, HTD-Vol. 122, 1989. 

Butler, B. W., and Webb, B. W. "Measurements of Local Temperature and Wall Radiant Heat Flux in an Industrial Coal-Fired Boiler." In Heat Transfer in Combustion Systems—1990, edited by B. Farouk, W. L. Grosshandler,... 

Viscanta R, Mengiic MP (1987) Radiation heat transfer in combustion systems. Prog Energy Combust Sci 13 97-160... 

Multi-stage preheating, pre-calciners, kiln combustion system improvements, enhancement of internal heat transfer in kiln, kiln shell loss reduction, optimize heat transfer in clinker cooler, use of waste fuels Blended cements, cogeneration... 

C. K. Law, Heat and mass transfer in combustion Fundamental concepts and analytical techniques. Prog. Energy Combust. Sci. 10 255-318,1984. 

A furnace bums a liquid coal tar fuel derived from coke-ovens. Calculate the heat transferred in the furnace if the combustion gases leave at 1500 K. The burners operate with 20 per cent excess air. 

After the flue gas leaves the combustion chamber, most furnace designs extract further heat from the flue gas in horizontal banks of tubes in a convection section, before the flue gas is vented to the atmosphere. The temperature of the flue gases at the exit of the radiant section is usually in the range 700 to 900°C. The first few rows of tubes at the exit of the radiant section are plain tubes, known as shock tubes or shield tubes. These tubes need to be robust enough to be able to withstand high temperatures and receive significant radiant heat from the radiant section. Heat transfer to the shock tubes is both by radiation and by convection. After the shock tubes, the hot flue gases flow across banks of tubes that usually have extended surfaces to increase the rate of heat transfer to the flue gas. The heat transferred in the radiant section will usually be between 50 and 70% of the total heat transferred. 

The inclusion of radiative heat transfer effects can be accommodated by the stagnant layer model. However, this can only be done if a priori we can prescribe or calculate these effects. The complications of radiative heat transfer in flames is illustrated in Figure 9.12. This illustration is only schematic and does not represent the spectral and continuum effects fully. A more complete overview on radiative heat transfer in flame can be found in Tien, Lee and Stretton [12]. In Figure 9.12, the heat fluxes are presented as incident (to a sensor at T,, ) and absorbed (at TV) at the surface. Any attempt to discriminate further for the radiant heating would prove tedious and pedantic. It should be clear from heat transfer principles that we have effects of surface and gas phase radiative emittance, reflectance, absorptance and transmittance. These are complicated by the spectral character of the radiation, the soot and combustion product temperature and concentration distributions, and the decomposition of the surface. Reasonable approximations that serve to simplify are ... 

Recess stabilization appears to have two major disadvantages. The first is due to the large increase in heat transfer in the step area, and the second to flame spread angles smaller than those obtained with bluff bodies. Smaller flame spread angles demand longer combustion chambers. 

The last point is worth considering in more detail. Most hydrocarbon diffusion flames are luminous, and this luminosity is due to carbon particulates that radiate strongly at the high combustion gas temperatures. As discussed in Chapter 6, most flames appear yellow when there is particulate formation. The solid-phase particulate cloud has a very high emissivity compared to a pure gaseous system thus, soot-laden flames appreciably increase the radiant heat transfer. In fact, some systems can approach black-body conditions. Thus, when the rate of heat transfer from the combustion gases to some surface, such as a melt, is important—as is the case in certain industrial furnaces—it is beneficial to operate the system in a particular diffusion flame mode to ensure formation of carbon particles. Such particles can later be burned off with additional air to meet emission standards. But some flames are not as luminous as others. Under certain conditions the very small particles that form are oxidized in the flame front and do not create a particulate cloud. 

Smoke is composed of combustion gases, soot (solid carbon particles), and unburnt fuel. For outdoor fires, the impact of smoke is usually a secondary consideration after the heat transfer. In many circumstances, the immediate thermal threat from the fire plume (jet, pool, or flash fire) overwhelms the smoke threat, particularly for personnel in close proximity to the event. There may be circumstances where personnel are in a downwind smoke plume where there is no immediate thermal threat. As a rule-of-thumb, all people within a smoke plume may be immediately or nearly immediately affected and at risk from a life safety standpoint (be it from lack of visibility or by toxic products). 

The heat transfer in the combustion wave structure of an energetic material is illustrated in Fig. 3.10. The heat flux feedback from zone III to zone II by conductive heat transfer, = kg (dTIdx), is given by Eq. (3.46), and the heat flux feedback from zone II to zone I by conduction heat transfer, dT/dx), is given by... 

Modern steam plants are quite elaborate structures that can recover 80% or more of the heat of combustion of the fuel. The simplified sketch of Example 1.2 identifies several zones of heat transfer in the equipment. Residual heat in the flue gas is recovered as preheat of the water in an economizer and in an air preheater. The combustion chamber is lined with tubes along the floor and walls to keep the refractory cool and usually to recover more than half the heat of combustion. The tabulations of this example are of the distribution of heat transfer surfaces and the amount of heat transfer in each zone. 

Since we are located far from the boundary we may neglect the influence of heat transfer in the process. We equate the combustion temperature Tg with the so-called theoretical combustion temperature Tt, calculated from thermodynamic data under the assumption of adiabatic combustion. For more detail, see 1.4 and 1.5. 

Completely lacking in the literature are indications of another form of losses—braking and heat transfer in the course of the chemical reaction (the quantities / and g in our formulas (32)-(34) in which incomplete combustion is denoted by g ). Meanwhile, as has already been mentioned, it is precisely these losses, which depend on the smallest chemical reaction rate and its total duration, that are the most significant. 

VanderSteen J.D.J., Pharoah J.G. (2004) The effect of radiation heat transfer in solid oxide fuel cells modeling. Combustion Institute/Canadian Section, Spring Technical Meeting, Queen s University, May 9-12, 2004. 

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