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Show E X P E R I M E N T A L R E S U L T S A N D DISCUSSION In experimental investigation of flow structure and heat transfer in a dimple without combustion, the problems associated with the influence of different external factors were of primary interest. That was important in order to compare results with combustion and without it, as well as to determine the extent to which the combustion influence on the flow structure. Experimental results without combustion have shown the following. For all values of the free stream velocity reverse flows are observed in the hemispherical dimple while large scale vortices are formed in its front part. The vortices are alternately ejected into the free stream. Frequency/of these ejections depends on both the flow velocity U0 and various external factors. For small values of free stream turbulence intensity, dimensionless Strouhal number Sh=fd/U0 approximates 0.08. As the free stream turbulence intensity Tu0 increases from 0.5% to 15%, the ejection frequency decreases approximately twice, whereas for 7w0=19.8%, the vortex stabilizes in either position on the dimple surface. With the increase of free stream turbulence intensity Tu0 from 0.5% to 15%, the total heat flux from the dimple surface increases approximately 1.5 times. Moreover, heat transfer rate on the dimple is approximately trebled while that of the vortex epicentre region and of the dimple downstream part is virtually not affected. The pressure drop exerts considerable influence on flow structure and heat transfer in the dimple. So, for the favourable pressure drop corresponding to the acceleration parameter K=5x IO"6, vortex ejection frequency is/=10-15Hz for d=50 m m and tV0=18.5 m/s. For adverse pressure gradient corresponding to shape function F=2x 10"3, the value of the frequency isf=\ Hz. As the adverse pressure drop increases, the heat transfer in dimple decreases compared to that of the case when pressure drop is not imposed. So, for the shape function |F | =2xl0"3, the total heat flux from the dimple surface is approximately 3 0 % less than that of the case F=0. In the accelerating flow the heat transfer rate in the dimple increases as compared to the case of zero pressure drop. So, for the acceleration parameter K=5x\0'6, the total heat flux in the dimple is approximately twice as high as that for K=0. In this case the heat transfer especially considerably increases in the area of vortex structure epicentres. Thus, the experimental data obtained allows us to assess the flow pattern and heat transfer rate in the dimple on exposure to different external factors. During the investigation of the combustion process in the hemispherical dimple, regimes of steady state gas combustion were primarily determined. For this purpose, free stream and gas supply velocities were varied in the experiments. The holes for gas supply into the dimple were brought into different positions with respect to the air free stream direction. To set in these positions, the dimple was rotated around its axis. The results of the experiments have shown the following. Natural gas feeding into the front, with respect to the free stream direction, area of the dimple always resulted in a flame-out: the 6 |
