Radiant heat distribution

Radiative Heat Transfer Heat-transfer equipment using the radiative mechanism for divided solids is constructed as a table which is stationary, as with trays, or moving, as with a belt, and/or agitated, as with a vibrated pan, to distribute and expose the burden in a plane parallel to (but not in contacl with) the plane of the radiant-heat sources. Presence of air is not necessary (see Sec. 12 for vacuum-shelf dryers and Sec. 22 for resubhmation). In fact, if air in the intervening space has a high humidity or CO9 content, it acts as an energy absorber, thereby depressing the performance. 

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 ... 

The inner chamber of the oven has curved walls for smooth circulation of air the radiant heat from the sample injection port units and the detector oven is completely isolated. These factors combine to provide demonstrably uniform temperature distribution. (The temperature variance in a column coiled in a diameter of 20cm is less than 0.75°K at a column temperature of 250°C). 

Knowledge of the distribution and density of people is necessary to assess the impact of radiant heat and smoke from fires. This allows an estimate to be made of the risk to which the population in and around the facility may be exposed. Extensive population data is necessary where an estimate of societal risk is required. Where only an estimate of individual risk is desired, extensive population data may not be required. However, it is still necessary to determine the location of the people whose individual risk is being estimated. 

Combustion of aluminum particle as fuel, and oxygen, air, or steam as oxidant provides an attractive propulsion strategy. In addition to hydrocarbon fuel combustion, research is focussed on determining the particle size and distribution and other relevant parameters for effectively combusting aluminum/oxygen and aluminum/steam in a laboratory-scale atmospheric dump combustor by John Foote at Engineering Research and Consulting, Inc. (Chapter 8). A Monte-Carlo numerical scheme was utilized to estimate the radiant heat loss rates from the combustion products, based on the measured radiation intensities and combustion temperatures. These results provide some of the basic information needed for realistic aluminum combustor development for underwater propulsion. 

Results of an experimental program in which aluminum particles were burned with steam and mixtures of oxygen and argon in small-scale atmospheric dump combustor are presented. Measurements of combustion temperature, radiation intensity in the wavelength interval from 400 to 800 nm, and combustion products particle size distribution and composition were made. A combustion temperature of about 2900 K was measured for combustion of aluminum particles with a mixture of 20%(wt.) O2 and 80%(wt.) Ar, while a combustion temperature of about 2500 K was measured for combustion of aluminum particles with steam. Combustion efficiency for aluminum particles with a mean size of 17 yum burned in steam with O/F) / 0/F)st 1-10 and with residence time after ignition estimated at 22 ms was about 95%. A Monte Carlo numerical method was used to estimate the radiant heat loss rates from the combustion products, based on the measured radiation intensities and combustion temperatures. A peak heat loss rate of 9.5 W/cm was calculated for the 02/Ar oxidizer case, while a peak heat loss rate of 4.8 W/cm was calculated for the H2O oxidizer case. 

The goal of the present study is to provide the information needed for design of a practical underwater propulsion system utilizing powdered aluminum burned with steam. Experiments are being conducted in atmospheric pressure dump combustors using argon/oxygen mixtures and steam as oxidizers. Spectrometer measurements have been made to estimate combustion temperatures and radiant heat transfer rates, and samples of combustion products have been collected to determine the composition and particle size distribution of the products. 

Based on the measured distribution of radiation intensity along the combustor length, the average radiant heat loss rate for the entire combustor volume for the 02/Ar case is estimated at about 4.0 W/cm, corresponding to a total radiant heat loss of about 20 kW or 160 cal/g of combustion products. Heat loss of 160 cal/g, together with an estimated combustion products temperature of 3050 K, implies that the reactedness of the mixture is on the order of 60% to 70%. The lower estimate of reactedness corresponds to a case in which the unburned aluminum never ignited. The higher estimate of reactedness corresponds to a case where the unburned aluminum is at the same temperature as the rest of the mixture, which implies that it is in vapor phase. The actual reactedness should fall between these two extremes. 

The Lateral Ignition and Flame spread Test (LIFT) apparatus was developed primarily for lateral flame spread measurements. The apparatus, test procedures, and methods for data analysis are described in ASTM E 1321. A sample of 155 x 800 mm is exposed to the radiant heat of a gas-tired panel. The panel measures 280 x 483 mm. The heat flux is not uniform over the specimen, but varies along the long axis as a function of distance from the hot end as shown in Figure 14.6. The flux distribution is an invariant of distance when normalized to the heat flux at the 50 mm position. When methane or natural gas is burnt, the upper limit of the radiant heat flux is 60-65 kW/m2. The lower limit is approximately 10kW/m2 since the porous ceramic tile surface of the panel is only partly covered with flame at lower heat fluxes. 

A piece of beef steak is cooked either in a microwave oven or a radiant heating oven. Sketch temperature distributions at specific times during the heating and the cooling processes in each oven. 

In some cases it is not possible to consider the modes separately. For example, if a gas, such as water vapor or carbon dioxide, which absorbs and generates thermal radiation, flows over a surface at a higher temperature, heat is transferred from the surface to the gas by both convection and radiation. In this case, the radiant heat exchange influences the temperature distribution in the fluid. Therefore, because the convective heat transfer rate depends on this temperature distribution in the fluid, the radiant and convective modes interact with each other and cannot be considered separately. However, even in cases such as this, the calculation procedures developed for convection by itself form the basis of the calculation of the convective part of the overall heat transfer rate. 

The nonuniform heat distribution in the radiant section of a nine-burner refinery process fluid heater... 

Premix radiant wall burners are mounted horizontally through the wall of a furnace. The burner tip, which penetrates only a few inches inside the hot face of the furnace wall, fires radially along the wall. These burners are almost exclusively used in ethylene cracking furnaces, either alone or in conjunction with floor-mounted, wall-fired burners to provide a uniform heat distribution to the process tubes. In some cases several hundred radiant wall burners are installed in a furnace to fine-tune the radiant heat flux to the process tubes. Figure 18.11 is a rendering of a radiant wall burner. 

Using the approximate radiant tube surface determined previously, choose firebox dimensions to accommodate the required total tube length. The exact proportions depend on judgment and past experience. Long furnaces minimize the number of return bends required, thus decreasing total cost. Shorter and wider fireboxes, on the other hand, usually give more uniform heat distribution and lessen the probability of flame impingement on the tube surface. 

The heat transfer to the product and to the trays is by radiation, which in the easiest way safeguarding a correct and an evenly distributed heat transfer to the material during the process. The radiant heat is produced by horizontal heater plates grouped in temperature zones. Each tray remains for a fixed period of time in each temperature zone in such a way that the drying time is minimized. 

The Radiant Flooring Panel Test apparatus consists of an air-gas-fiieled radiant heat panel inclined at 30° to and directed at a horizontally mounted floor covering system specimen. The radiant panel generates a heat flux distribution along the 1-m length of the test specimen from a nominal maximum of 10 kW/m (1 W/cm ) to a minimum of 1 kW/m (0.1 W/cm ). The test is initiated by open-flame ignition from a pilot burner. The heat flux at the location of maximum flame propagation is reported as the critical radiant flux. 

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