The effects of the design of the cap of a natural gas-fired cooktop burner on flame stability

Results of an investigation aiming to study the effects of the burner cap design factors on dame stability are presented in this paper. Flame stability is an essential part of the operation of all domestic burners, including natural gas-fired cooktops. At high thermal inputs flame lifts are encountered above certain levels of primary aeration, whereas flashback only takes place at low thermal inputs, due to natural gas' low flame speed, above certain levels of primary aeration. In this work, flame lift limits were measured at 3.3 kW thermal input and the highest primary aeration above which flame lifts started to become visible was the stability limit at this thermal input. Around 60 % primary aeration was desired to minimize pollutant emissions. Turndown tests were done at 40 % primary aeration. The lowest thermal input below which the flame hashed back in less than 30 s was the turndown limit. A turndown ratio of at least 5 was aimed for (i.e. 0.67 kW or less would be desirable). The cap design factors studied were: the angle under the cap, the angle under the overlap, the shape under the cap, the size of the cap overlap, the height of overlap above the burner, cap material and cap thickness. The only feature of the burner head included in the tests was the angle at the top of the burner head, the part which comes into contact with the cap. The 'Factorial Experimental Design' method with statistical analysis was used. This enabled detection of interactions between factors as well as the effects of each single factor. The results show that combinations of factors which gave a balance between the performance at 3.3 kW and a satisfactory turndown ratio of 5 would be preferred. Those which gave very high turndown ratios were usually operable only at very low primary aerations (well below 60 %) at 3.3 kW. Overall, the preferred combination of factors would be: 15 degrees burner head top angle and angle under the cap, indent under the cap, 2.2 mm overlap with 15 degrees angle at zero height above the burner, and thin aluminium caps. For the burner used in the experiments, such a combination should have no problem operating at up to 61 % primary aeration at 3.3 kW and having a turndown ratio of 5.3. (C) 1998 John Wiley & Sons, Ltd.