Measurements of fluctuating flame temperature

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MEASUREMENTS

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FLUCTUATING

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TEMPERATURE

MASANAGA K U N U G I and H I R O S H I J I N N O INTRODUCTION Performance evaluation and control of industrial furnaces, as well as the fundamental research of the related combustion phenomena, depend considerably upon a knowledge of the steady temperature of the flame in the furnaces. However, the flames used in such a field are commonly turbulent diffusion flames. The main characteristic of turbulent phenomena is their irregular random nature, and an understanding of temperature fluctuations is one of the more important problems. The authors made an attempt to measure the fluctuating gas temperature in turbulent diffusion flames with bare thermocouples. Shadow and schlieren pictures of the flames were also taken for a better understanding of turbulence. Shepard and Warshawsky 1 have developed the electrical techniques for time lag compensations of thermocouples used in the temperature measurement of jet engine gas whose velocity with respect to the thermocouple corresponds to a Mach

number ranging from 0.1 to 0.9. In our experiments the velocity of flame gases is low with a Mach number of less than 0.1. E X P E R I M E N T A L P R O C E D U R E AND RESULTS The flames were formed by city gas issuing vertically from circular tubes into stagnant air with various velocities. Two sizes of burner port were used, of 4.8 mm and 6.4 mm inside diameter. The Reynolds number in our experiments has a maximum value of 8,000 in the burner tube. For dynamic measurement of fluctuating gas temperatures it is desirable to use a thermocouple of sufficiently small size to achieve adequately rapid response. Such small elements, however, have inadequate service life, so that it is necessary to use somewhat larger elements with appreciable lag. For example, the thermocouples used were made by welding platinum and platinum/13 per cent rhodium wires, each 0.06 mm in diameter

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FLUCTUATING FLAME TEMPERATURE MEASUREMENTS

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I N S T R U M E N T A T I O N IN COMBUSTION RESEARCH schlieren photographs of the flame is shown in

and about 1.5 m m long and attached to 0.5 m m support wire of the same composition. The bare thermocouple was inserted approximately perpendicular to the gas stream into the flame. With this technique, the flame was not visibly disturbed. T h e measurements for the aerodynamic conditions in the flame, such as Reynolds number range of 1,500 to 8,000 and M a c h number of less than 0.1, were carried out at various points across the flame and at various heights above the burner. THERMO

Figure 3. From the pictures it is suggested that in the lower part of the flame the boundary between the unburnt gas and combustion products and the flame envelope are characterized by steep density gradients; and the burning zone lies between these two regions. In the upper part of the flame the appearance is that of irregular turbulence. For a measurement of the time constant, an alternating electric current was passed through the

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The compensator for the bar-wire thermocouple used was set and adjusted to match the thermocouple time constant. T h e value of the therrriocouple time constant, however, is a function of the velocity and the density of the gas stream. In practice, therefore, a certain amount of mis-match may exist between the thermocouple time constant and the compensator. T h e authors corrected experimentally the mis-match under several sets of aerodynamic conditions in the flame as described later. The typical characteristics of the compensator are shown in Figure 1 with the frequency response curve predicted from the heat transfer relations for the thermocouple wire of 0.06 m m in diameter in the flame. From the combined response curve, shown by the dotted line in Figure 1, it is seen that the combined response is flat from 10 to about 1,000 cycles per second. In order to determine also the spectrum of fluctuating temperature, the output from the compensating amplifier was fed into a variablefrequency band pass amplifier~ The complete system, shown schematically in Figure 2, consists of the bare thermocouple, preamplifier, negative feedback amplifier with compensating network, band pass amplifier, and the cathode ray oscilloscope or recorder. T o visualize the structure of the flame used in our experiment an example of shadow and

thermocouple wire in the flame to heat the junction to about 100~ above the gas temperatu~'e. The temperature rise A T and the heating current I were measured. The time constant ~- is given by T = 1.8 x 10-4(AT/P). T h e circuit for this method is presented schematically in Figure 4. The typical time average values of the time constant obtained at various points across a flame and at two heights above the burner are shown in Figure 5 as a plot of time constants in seconds against radial distance, r mm, from the flame axis. From the figures it is seen that the value of the time constant in the u p p e r part of the flame is much larger than that in its lower part. This is probably due to the fact that the flame broadens with increasing height and this causes a decrease of the axial velocity of the flame. The representative fluctuation profiles of gas temperature occurring in turbulent diffusion flames are illustrated in Figure 6, in which temperature-frequency relations obtained across the flame are presented. Figures 6a and 6b indicate the data measured at two different heights, 6.4 m m and 19.2 mm, of the flame of city gas issuing from a burner port of 6.4 m m in diameter. T h e Reynolds number is 5,700 in the burner tube. Figure 6c refers to the case of the burner port of 4.8 m m diameter. The vertical axes indicate'the amplitude in ~ and the abscissa the frequency in cycles per second. Tm and r in the figures represent the

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FLUCTUATING

FLAME

TEMPERATURE

MEASUREMENTS

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INSTRUMENTATION IN COMBUSTION RESEARCH time-mean temperature in ~ and the radial into the boundary, and at another time the comdistance in mm from the flame axis. In Figure 6c bustion products are replaced by fresh air, so that the vertical axis on the left refers to the data the large variation of temperature at that portion obtained inside the flame, namely, r = 0, 5, 10, 20 is to be expected. ram, and that on the right to the curve shown by the dotted line (r = 15 mm). CONCLUSION The curves in Figure 6a show that the flame in its The fluctuating gas temperature in turbulent and lower part is characterized by the three regions laminar diffusion flames was determined by a bare having different frequencies of the temperature thermocouple of 0.06 mm diameter combined with variations, namely, (1) in the inner jet of unburnt compensating networks. From the results obtained gas (the curves ofr = 0, r = 2 mm) the amplitude it may be concluded that the turbulent diffusion of the temperature is less than 20~ and the typical flame is characterized by three typical regions, frequency of the variation seems to be about namely, the inner jet of unburnt gas, the burning 1,000 c/s, (2) around the burning zone (the curves zone, and the flame envelope, with the different of r = 6, r = 8 mm) the fundamental frequency peaks at frequencies of approximately 20, 100, 500, of the variation is 500 c/s, (3) at the boundary and 1,000 c/s respectively in temperature-fre(r = 10, r = 12 mm) between the ambient air and quency relations. the envelope of hot gases the peak in the curves The difference of the frequency and amplitude appears at a frequency of approximately 20 c/s. of temperature variation in the unburnt gas region For the upper portion of the flame Figure 6b and between turbulent and laminar flames can be Figure 6c indicate that the typical temperature explained by the fact that the unburnt gas stream fluctuation is represented by two peaks in the is affected by the Reynolds number in the burner curves situated at frequencies of about 20 and tube. 100 c/s, the former occurs at the flame envelope In such a flame, fuel and oxygen have to diffuse and the latter inside the flame. against the stream of combustion products in order In Figure 7a and 7b the results obtained for to mix and to react. The air necessary for coma laminar flame are shown. bustion is drawn in from the surroundings. In the lower part of the flame the curves in Accordingly, the characteristics of the temperature Figure 7a are seen to be fairly similar to that given fluctuation near the flame envelope may be in Figure 6a, except near the flame axis. The upper influenced by the coarseness of turbulent mixing; part of the flame, however, is characterized by the however, in the burning zone it is believed that the peak of temperature variation at about 50 c/s and molecular diffusivity and rate of chemical reaction its amplitude of 500-600~ as is evident in are more effective. Figure 7b. The occurrence of such amplitude is The authors are indebted to Professor L Sawai for probably related to the flickering of the flame advice with regard to the experimental technique and to caused by the low axial velocity of the flame. Mr. M. Takada for his assistance in carrying out the The amplitude of the temperature fluctuation in experiments. the flame envelope also seems to be too large. This may be mainly due to the following fact. As is REFERENCES well known, the rate of burning along the diffusion 1 SHEPARD,C. E. and WARSnAWSKY,I. Tech. Note flame depends on the entrainment of air from the Nat. Adv. Comm. Aero. Wash. No. 2703 NACA surroundings. By analysing the gas composition TN2703 (1952); Instrum. Soc. Amer. Proc. 7 near the flame envelope, it has been found that the (1952) 149 average sample contains substantial amounts of 2 HOTTEL, H. C. and HAWTHORNE, W. R. unburned fuel gas and free oxygen2. At some time Third Symposium (International) on Combustion, p. 266. 1949. Baltimore; Williams and Wilkins combustion products and fuel gases are transported

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