การศึกษาโครงสร้างเปลวไฟและการถ่ายเทความร้อนของเจ็ทเปลวไฟหมุนควงแบบผสมก่อน

Abstract

The impinging flame jet is the method for a heating surface with high heat transfer rate and can increase the temperature on the surface rapidly. In general, the impinging flame was mainly used for the flame jet from the pipe nozzle or orifice nozzle which the impinging flame provides a high heat transfer rate only the jet impingement area. This results in non-uniform heat distribution on the surface. In this research, the flame structure and heat transfer characteristics of swirling flame jet had been studied, which the swirling flame jet can improve the uniform heat distribution and can enhance heat transfer rate on the surface. In the experiment, the nozzle with a cylindrical chamber of 20mm in diameter was applied for swirl flow generation. The chamber was connected with double inlets tangentially. The inlet has a square cross-section with D=5 mm in height. The inlets were injected with a mixture of LPG and air into the chamber for generating swirling flame jet. In this study, the experimental parameters consisted of equivalence ratio between LPG and air at ф=0.8, 1.0, and 1.2 with Reynolds number Re=2,000, 4,000, and 6,000. The effect of chamber height for swirling flame structure was studied at H=2.2D, 3.4D, 4.6D, 5.8D and 7.0D. The structure of swirling free flame jet was studied by the digital camera while the heat distribution in swirling flame jet using Schlieren imaging technique. In addition, the flame temperature distribution in the swirling flame structure was measured using a thermocouple type-B. For the heat transfer on the surface with swirl impinging flame jet, the nozzle to the impingement plate distance was studied at L=4D, 6D, 8D, and 10D. The measurement can be divided into two parts; Including the overall average heat transfer measurement and local heat transfer measurement using the heat flux sensor, it can be known heating performance from the swirling flame. Also, the heat transfer enhancement on the surface using swirling flame jet were compared with the flame jet from a pipe nozzle at a Reynolds number Re=2,000. In the result, the swirling flame structure can be divided into three zones, including the neck zone near the chamber exit, the reaction zone, and the post-combustion flame zone. The flame structure for all zones changed with the increase of Reynolds number and the increase of equivalence ratio affected to the color in the flame and the flame length. The height of the swirl chamber at H=2.2D, 3.4D, 4.6D, 5.8D, and 7.0D affected to the swirling flame structure. The flame spreading was changed and divided into two flames which the swirl angle of the flame to the jet axial was varied at swirl angle 24.5°, 18.5°, 15.5°, 13.5°, and 10.5°, respectively. Also, when Increasing the height of the swirl chamber, the swirling velocity becomes and increasing the jet spreading rate near the chamber exit. The flame was mixed and entrained with the ambient air. It was found that the case of chamber height at H=4.6D provided the heat distribution. This corresponded to the swirling flame structure recorded by the digital camera and agreed with the results of flame temperature measurement. When the swirling flame impinges on the surface, the flame covered broadly on the impingement surface and resulted in uniform heat transfer on the surface. The case of chamber height H=4.6D at impingement distance L=4D provided the highest average heat transfer on the surface. It was found that the equivalence ratio at Q=1.2 provided the highest heat transfer for all the cases and heating performance was increased up to 67%. This is due to the air entrainment providing the complete combustion near the impingement surface. In this study, the heat transfer for swirling impinging jet and impinging jet from pipe nozzle was also compared. It was found that for the case of impingement distance less than L=6D, the swirling flame jet provided higher heat transfer rate than the impinging jet from pipe nozzle for average heat transfer and local heat flux at center of the nozzle. However, when increasing the impingement distance larger than L=6D, due to the flame structure from pipe nozzle, the heat transfer give highest heat transfer rate at the center point of impingement which makes the heat transfer is lower than swirling impinging flame jet about 30% for all equivalence ratio and for the case of impingement distance larger than L=8D. Since the flame structure of swirling impinging jet spread broadly and impinged on the surface with a large flame covering the area. The average heat transfer for swirling impinging jet was higher than the case of the flame jet from pipe nozzle about 40% for all equivalence ratios and impingement distances.