Nonluminous flames

Transparent (Invisible) Flames Some materials have nearly nonluminous flames, which may not be visible, especially in the daytime. For example, hydrogen has a nearly nonvisible flame in the daytime. A person may walk unaware into a hydrogen leak flame. Some other materials, including some alcohols such as methanol, also have nearly nonluminous flames and may be unusually hazardous because the flames cannot be seen in the daytime. 

The fundamentals of thermal radiation modeling are treated in Chapter 3. The value for emissive power can be computed from flame temperature and emissivity. Emissivity is primarily determined by the presence of nonluminous soot within the flame. The only value for flash-fire emissive power ever published in the open literature is that observed in the Maplin Sands experiments reported by Blackmore... 

Test 2. Heat niclosamide on a copper wire in a nonluminous flame. The flame becomes green [6]. 

Recently revised test methods have revealed that nonluminous or barely luminous combustion reactions are occurring where previously it was reported that no combustion activity was occurring based on flame observance. 

Pre-Flame Reaction - A slow nonluminous gas phase reaction of the sample or its decomposition products with an oxidant. 

In many process fires, heat transfer by radiation is the dominant form of heat transfer. The heat radiated from a flame is emitted by gases, in particular the products of combustion and by soot. Aflame in which the radiation is emitted solely from the gaseous products of combustion is termed nonluminous and a flame in which there is soot is termed luminous (i.e., yellow or visible). 

The use of the infrared pyrometric method extended significantly the range of measurement. Infrared pyrometry has the advantage of being equally adaptable to luminous low-transparency and to nonluminous flames. 

A. A 1 20 aqueous solution in 2.7 A hydrochloric acid imparts a transient yellow-red color to a nonluminous flame. 

A. When sufficiently heated, a sample chars and emits flammable vapors having an odor resembling that of burning sugar. At a higher temperature and with free access to air, the heat consumes the carbon of the black residue, and a white, fused mass of potassium carbonate remains that imparts a red-purple color to a nonluminous flame. 

Potassium compounds impart a violet color to a nonluminous flame if not masked by the presence of small quantities of sodium. In neutral, concentrated or moderately concentrated solutions of potassium salts, sodium bitartrate TS (10%) slowly produces a white, crystalline precipitate that is soluble in 6 N ammonium hydroxide and in solutions of alkali hydroxides or carbonates. The precipitation may be accelerated by stirring or mbbing the inside of the test tube with a glass rod or by the addition of a small amount of glacial acetic acid or alcohol. 

Dissolve 0.1 g of the sodium compound in 2 mL of water. Add 2 mL of 15% potassium carbonate, and heat to boiling. No precipitate is formed. Add 4 mL of potassium pyroantimo-nate TS, and heat to boiling. Allow to cool in ice water, and if necessary, rub the inside of the test tube with a glass rod. A dense precipitate is formed. Sodium compounds impart an intense yellow color to a nonluminous flame. 

Wind a 1.5- x 5-cm strip of 20-mesh copper gauze around the end of a copper wire. Heat the gauze in a nonluminous flame of a Bunsen burner until it glows without coloring the... 

The actual flame temperature is lower than the adiabatic equilibrium flame temperature because of heat loss from the flame. The actual flame temperature is determined by how well the flame radiates its heat and how well the combustion system, including the load and the refractory walls, absorbs that radiation. A highly luminous flame generally has a lower flame temperature than a highly nonluminous flame. The actual flame temperature will also be lower when the load and the walls are more radiatively absorptive. This occurs when the load and walls are at lower temperatures and have high radiant absorptivities. These effects are discussed in more detail in Chapter 4. As the gaseous combustion products exit the flame, they... 

It has been argued that the efficiency of transferring heat from the flame to the load may be increased using OEC.34 In a nonluminous flame, the flame emissivity is higher for an oxygen-enhanced flame, compared with an air/fuel flame. This is due to the higher concentrations of C02 and H20, which are the gases that radiate in a flame.35 There is no radiation from the N2 in the flame. These effects are discussed in Chapter 4. 

The use of OEC generally reduces the likelihood of emitting soot into the exhaust products because of the intensified combustion (see Figure 2.15) and higher flame temperatures, compared with air/fuel systems. Older style, high-intensity oxy/fuel burners usually had a very hot, bluish flame which was very nonluminous. The newer style oxy/fuel burners are much lower intensity and have a much higher luminosity. However, soot carryover into the exhaust has not been identified as a problem for OEC. 

K. The flame can be divided into three regions 1) the primary flame, 2) the dark zone, and 3) the secondary flame. The dark zone is a nonluminous region between the primary and the secondary flame, and is characterized with a temperature plateau. The concentrations of HCN, NO, and HiO in the dark zone appeared to be similar to those of pure nitramine propellants [3]. The rapid conversion of HCN and NO to N2 and CO in the secondary flame zone were successfully predicted. The dominant net reactions in this stage are as follows. 

Yellowish-white, slightly lustrous lumps body-centered cubic structure somewhat malleable very easily oxidizable must be kept under petroleum or other oxygen -free liquid to exclude air. d 3.6, mp approx 710 . bp approx 1600. E° (aq) BaJ + /Ba —2.91 V. Description of reactions which are characteristic of alkaline earth metals see Calcium. Solns of sol barium salts give a white ppt with HjS04 or sol sulfates they also color nonluminous flame green. 

P. B. Taylor and P. J. Foster, The Total Emissivities of Luminous and Nonluminous Flames, International Journal of Heat Mass Transfer, 17, p. 1591,1974. 

M. E. Kounalakis, Y. R. Sivathanu, and G. M. Faeth, Infrared Radiation Statistics of Nonluminous Turbulent Diffusion Flames, ASME Journal of Heat Transfer, 113, pp. 437-445,1991. 

Typical nonluminous flame. (From Baukal, C. E., ed.. Heat Transfer in Industrial Combustion, Boca Raton, FL CRC Press, 2000 courtesy of John Zink Co. LLC.)... 

One method that has been used to measure nonluminous radiation is to directly measure the radiation from free jet flames with a radiometer. An elevation and plan view of a radiometer aimed at a flame are shown in Figure 6.15 and Figure 6.16, respectively. In one study, a... 

Radiation heat transfer mechanisms in flame impingement (a) luminous flame, (b) nonluminous flame, and (c) surface radiation. 

The presence of soof particles in a flame can dominate the thermal radiation. A flame containing a significant amount of soof can radiate fhree to four times as much radiation as a nonluminous flame [1,9]. The radiation... 

Example of a nonluminous flame. (Courtesy of John Zink Co. EEC.)... 

CHEMICAL PROPERTIES highly polar liquid bums with nonluminous, bluish flame reacts vigorously with oxidizing agents FP (12°C, 54°F) LFLAJFL (6.0%, 36.5%) AT (470°C, 878°F) HC (723 kJ/mol). 

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