| OCR Text |
Show Btulft2h), which are closer to those achieved in induction furnaces than to those achievable in conventional gas-fired furnaces, were measured in some experiments with refractory temperatures of about 1380 (2024) to 1400 K (2060 F). This allows the use of relatively inexpensive refractory materials even at very high firing rates. Even at the maximum firing rates, the average combustion gas temperatures do not exceed 1900 K (2960 F) (see Figure 8), whereas in high velocity tunnel burners with nearly complete combustion within the tunnel, these temperatures may often exceed 2000 to 2200 K (3140° - 3500 F) when using preheated air. The excellent mixing and the relatively low combustion temperatures provide low levels of nitrogen oxidation rates. The NOx concentration in the final combustion products was only 25 to 40 vppm at 2 % 02. The experimental convective heat transfer coefficients as a function of the nozzle exit velocity are shown in Figure 9. The heat transfer coefficients increase with velocity to the power 0.54 (a _UO.54 ) and are comparable to values for isothermal multi-jet systems at the same Reynolds numbers and at the same ratio of total nozzle exit area to impingement surface area. SUMMARY OF RESULTS AND CONCLUSION The investigation of a high velocity multi-jet DFI combustion system, carried out in a directfired experimental furnace, shows that it is an efficient natural gas-fired rapid heating process, in which approximately 60% of total heat is transferred by convection. • If properly designed, the system allows utilization of very high velocity levels (up to Mach = 1) without creating flame instability. • A distance of about 100 mm (4.0 in.) between the furnace refractory lining (or nozzle tip) and the load is sufficient to reach 1400 -1800 K (2060 -2780 F) temperature in the impingement zone, thus ensuring high convective heat fluxes to the load. The furnace internal space is compact and comparable with induction furnaces. • Decay of the dynamic pressure along the flame axis is significantly less compared to isothermal jets. The relatively low temperature core of the flame jet occupies less than 1 % of the furnace internal space. The rest of the space is occupied by uniform high temperature circulating combustion products, containing minimal combustibles. • The high velocities and temperatures in the impingement zone provide efficient convective heat transfer to the load with total heat flux values up to 500 kW/m2 (160,000 Btulft2h), while refractory temperatures do not exceed 1400 K (2060 F). • The measured NOx levels of 25 to 40 vppm were significantly lower than those found in conventional furnaces equipped with tunnel burners (...., 150 vppm). • A simple 2-D numerical simulation method for turbulent flow, combustion, and heat transfer characteristics has been developed and found to have good agreement with the measured values 13 |
