OCR Text |
Show - 6 - 20/20 and 225 m/s and 800°C. In general the penetration depth at 225 m/s is around 0.1 to 0.2 m further through the combustion air flow compared to the cases at 75 and 125 m/s gas i~jection. The variation of the gas injection angle from 12 to 20° showed an increase in penetration of 0.1 to 0.2 m at different ga s injection velocities. Effect of gas velocity on in-flame temperature Another important aspect is the influence of the gas injection velocity on the in flame temperature. In general, an increase i n gas injection velocity results in an increase in jet penetration as described in the previous section . As will be shown in this section an increase in gas injection velocity also results in a n increase in in-flame temperature which competes with the je t penetration effect. The flame length, and flame surface area are determined by t he nozzle diameter and therefore, at constant thermal input, b y the gas injection velocity. Using a the typical value of 22° f or a jet divergence angle [3, 5, 6), the flame surface area for 0.5 MWt natural gas flame can be estimated: Afl 443 / Vg ( 3 ) The flame temperature is determined by the heat generation in t he flame and radiative heat loss to the surroundings. The f ame surface area and therefore the radiative exchange factor (3) between the flame and the surroundings are a function of the gas injection velocity. Based on the above approach, the effect: of gas injection velocity on the in-flame temperature can be estimated. The calculation technique is given in detai l in Reference 3. The approach is based on differentiation of a mass and heat balance over the flame volume which leads to : d(AflIFlfl- >l) OT ( 4 ) [ ~e CPav + 4 Afl IFI fl >rl T3bslJ where OT is the change in flame tempera t ure compa r ed to the baseline flame due to gas injection veloc i t y changes, Tbs l is the baseline in-flame temperature (K), Tl i s t he load t emperature (K), IFlfl->l is the exchange factor fr om the f lame to t he load and d ( A f II F I f 1- > 1 ) is the gas vel 0 c i t y d e pe n d en t c han gel n e total flame exchange factor, Me i s the rate of mass entr a i nmen into the flame (kg/s) and Cpa v the h e a t capaci t y of the entraineu gases (J/kgOC). The exchange fa cto r , / F/ f l->l ' varies with he flame emissivity and was not accurate l y k nown. Estimatlons o ver a range of emissivities resulted i n t e mpe r a ure reductions 0 70-120°C for 75 m/s gas injection compared to the base l ine cas (125 m/s) and temperature inc reases of 1 40-2 10°C a bove t he baseline case for 225 m/s gas i n j ec ion veloc i y. |