OCR Text |
Show In the work [8], heat transfer on the surface of a hemispherical dimple has been studied in details. The experiments have shown that local values of heat transfer coefficient a on the dimple surface are practically lower everywhere than those of a0 in front of the dimple. Maximal values of ctmax^Llcto correspond to the downstream part of the dimple while ct»0.8cto corresponds to the vortex epicentre (Figure 2). Minimal value of a * 0.4cto is observed on the dimple pole. The averaged 0.8 0.6 0.4 0.2 Figure 2. DISTRIBUTION OF HEAT TRANSFER COEFFICIENT ON THE HEMISPHERICAL DIMPLE S U R F A C E (LONGITUDINAL SECTION): 1 - E X P E R I M E N T A L D A T A [7], 2 - P R E S E N T S T U DY over the dimple surface value a comprises 0.75ao. The total heat flux from the dimple surface is approximately 1.5 times higher than that from the surface of a circle of the same diameter, as their surface are related as 2:1. However, it should be noted that, with the dimple on the flat surface, heat transfer also increases also around it. Therefore the total heat flux increases more than 1.5 times. Thus the available experimental data on flow pattern in spherical dimples enables to reason that the spherical dimples might be utilized to improve the gas combustion process. Facts that the secondary reverse flows exist and small values of flow velocity in the dimple occur are beneficial for flame stabilization. Such properties, like increased heat transfer rate in the spherical dimples together with the decrease of the resistance of the surfaces with the spherical dimples, may be used to devise highly efficient compact heat exchangers. A n effective technique of heat transfer through the wall from flame to coolant can be put into practice to establish the gas combustion on the heat-transfer surface. All these assumptions based on available experimental data have provided a basis for setting up a problem under the investigations. Before experimental study, it was not a priory known if the flame might be stabilized in a dimple, how large-scale vortex structures could influence on the combustion and flame stabilization, how insensitively the flame would be stabilized in the dimple when the external conditions changed, etc. Moreover, it was difficult to predict whether the large scale vortex flow pattern in the dimple, which takes place without combustion, would remain. Nevertheless, the authors hoped that the spherical dimples distinguished by the peculiarities in flow pattern and heat transfer phenomenon would aid in solving the problems of combustion improvement. |