OCR Text |
Show geometry of the immersion tube. Although the program performs the calculation in unsteady flow, we were interested only in the asymptotic flow tending towards steady flow. STUDY OF THE INFLUENCE OF SEVERAL PARAMETERS The imposed dimensions for the equipment under study are indicated in figure 2. The adjustable geometric values (for this application) are : - the outside diameter of the internal recirculating tube (corresponding to the standard diameters available on the market) : d1• - the length of the space between the burner and the internal tube: 11' - the length of the flow return space: 12, - the input of the burner (sum of the input of the fuel to the burner and of the air preheating input) : Pee The maximum accepted temperature for the recirculating tube was set at 1323K (ie. 10s0°C) in order to take account of any possible maladjustment in industrial operation. HYPOTHESES AND PHYSICAL DATA - The temperature of the internal surface of the shell is one of the boundary conditions necessary to the calculation. It is a function of the thermal conducti vi ty of the ceramic material used to make the shell and of the power transmitted to the exterior medium (zinc bath at 7s0K). For a transmitted power of sOkW, it is approximately 1100K (823°C). The radiating properties of the walls have been measured ; we therefore set the total hemispherical emissivity of the 310 steel (after several hours of operation) at 0.85, and that of the ceramic material used for the shell at 0.90. The constant eddy viscosity model was chosen. It is assumed that no chemical reactions take place in the combustion products leaving the burner (when combustion is completed the air factor is equal to 1.1). If Qb is the mass flowrate of the combustion products leaving the burner and Qr is the recirculated mass flow rate (cf. figure 2), then in all that follows the title "recirculation ratio" will be given to the ratio r where : Qb + Qr (21) r = Qb This ratio is representative of the aspiration created by the jet leaving the burner. INFLUENCE OF THE LENGTH 11 - The test concerning the influence of 11 was performed for an internal tube with a thickness of 4mm. Figure 3 shows, f~r an input (P e) of 63kW, the variation, according to the value of 11' of the recirculation ratio, of the power transmitted to the outer shell (Pt ) and of the maximum temperature attained on a mesh of the recirculation tube (Tmax) • It is observed that the recirculation ratio rises rapidly with 11 before reaching an asymptote slightly above 2. On the other hand, the power transferred decreases gradually and stabi- 131 lises at around 44kW. The maximum temperature reached by the recirculating tube decreases when 11 increases. It falls below 1323K for 21 1, Pt(kW) Tmax(K) 2,5 50 1500 LPt 40 2 1400 30 1,5 20 1300 10 0 1200 0 11 211 31 1 411 51 1 '1 Fig. 3 - Influence of length 11 INFLUENCE OF d1 - The number of diameters of type AISI 310 stainless steel pipes available on the French market, which can be used in this ceramic shell, is quite limited. Only two diameters are suitable for this application. Table N°1 summarizes the results obtained for a tube of one type compared to those obtained for a tube with a diameter 1.12 times greater. It can be observed that the results calculated are practically identical. Tests carried out elsewhere have shown that the , ratio r is maximum when the ratio of the diameter of the internal tube to that of the adjacent shell is 0.6 ; this is true for the two cases under study. Table 1 - Influence of diameter d1 d1 Input Pe (kW ncv) Recirculation ratio r Power transmitted Pt (kW) Tube temperature Tmax (K) 1,00 63,0 2,08 44,2 1314 1,12 63,0 2,06 44,40 1307 INFLUENCE OF 12 - Tests using several values for 12 have been performed. The results of the calculations have shown that the influence on the fields of pressure and velocity of the combustion products is very slight. INFLUENCE OF P e - Figure 4 represents the influence of the input of the burner, Pe on Pt, rand Tmax for set values d1 , 11 and 12, There is a continuous increase in the recirculation ratio r with the input Pe supplied through an orifice with a constant cross-section (diameter do)' The role of the impulse of the jet in the induction of the recirculation flowrate is confirmed by the calculation. Tmax exceeds the boundary value of 1323K for a value of Pe greater than 63 kW. |