OCR Text |
Show a Camot efficiency, but a thermal efficiency for the heat exchanger. The percent of 02 in the exhaust product was recorded and used to correct the raw NO ppm readings to 3% 02 using the following equation. NO NO * 0.03) / 02(meaSUred)) (corrected) = (measured) (1- -- (1- 0.21 0.21 (3) Regulations require this scaling in order to accurately compare emission readings with different dilution levels. RESULTS AND DISCUSSION The minimum swirl requirements (i.e. blowoff limits) for five open burners with f varied from 2 to 22 cm are shown in Figure 3. Also shown are the maximum and minimum swirl (flashback and blowoff) for the f =7 cm burner at three power levels. The blowoff limits for the five burners clearly show an increase in swirl requirement with increasing f. The likely cause of this trend is that with increasing f, swirl air added at the outside edges of the burner is allowed to diffuse into the core of the premixture and loses some of its tangential momentum. This leads to a decrease in the centrifugal force and thus the divergence of the premixture as it leaves the exit tube. Without adequate divergence, the local premixture velocity is higher than the turbulent burning velocity and the flame is blown away. Irregardless of the length, there is a steady increase in the swirl necessary to stabilize the flame as ~ decrease from stochiometry (~ = 1.0). As flame speeds diminish with decreasing ~, a greater divergence of the premixture is needed to lower the reactant velocity and balance the lower flame speeds. Thus, more swirl is needed at lower equivalence ratios. Therefore, in order to maximize the operating range (i.e. have the lowest blowoff limits) of the WSB, the shortest possible tube length should be used. However, this does not imply that the f = 2 cm burner is the optimum design. Repeated testing showed that its flame tended to be unsteady and could not operate for long periods of time. The f = 7 cm burner is much more stable and thus was chosen for the test station. 8 |