Autoignition and early flame behavior of a spherical cluster of 49 monodispersed droplets

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Abstract

Autoignition and early flame behavior of a spherical cluster of 49 monodispersed droplets in a high-temperature air were examined in microgravity. The monodispersed suspended-droplet cluster (MSDC) model with which both droplet spacing and initial droplet diameter were well-controlled was developed, and the solidified-fuel fiber-suspension technique was utilized for making the MSDC model. The tested 3D MSDC models had the HCP (hexagonal closest packing) structure. Individual flames, which enveloped each droplet, or group flame, which enveloped the whole droplet cluster, were formed immediately after ignition. The flame changed from the group flame to a cluster of the individual flames either with increasing the droplet spacing or decreasing the initial droplet diameter. Each of the individual flames merged into the group flame with the lapse of time. Burning sphere diameter decreased at the beginning, and then increased. The transition from the individual flames to the group flame occurred around the time period at which the burning sphere diameter reached its minimum. The time period at which the burning sphere diameter reached its maximum was delayed and the expansion rate of the burning sphere was enhanced with decreasing the droplet spacing or with increasing the initial droplet diameter.

Introduction

Combustion of fuel sprays and arrays or matrices of fuel droplets has been studied experimentally, theoretically and numerically. The number of droplets in the droplet experiments is limited around 10 [1], [2], while the number in the spray experiments is quite large. On the other hand, theoretical or numerical studies have dealt with a wide range of droplet number in droplet clouds or clusters [3], [4], [5], [6], [7], [8], [9], [10]. Size and distribution of droplets, which were well-controlled in the droplets experiments, are difficult to be varied widely as experimental parameters in the spray experiments [11]. There is still lack of information on combustion of droplet clusters with several tens of fuel droplets, and additional experiments are needed.

For the final goal to bridge between knowledge on droplet combustion and on spray combustion, we have started an experimental study on evaporation, autoignition and combustion of multiple droplet clusters. The monodispersed suspended-droplet cluster (MSDC) model with which arrangement, spacing and initial diameter of the droplet are well-controlled has been developed. In the first phase of the experimental study [12], effects of the arrangement and the spacing on combustion characteristics of the droplet clusters which autoignite and burn in a high-temperature air were examined. The initial diameter of each droplet was kept constant. Combustion or evaporation of fuel droplets with diameters around 1 mm is largely affected by a buoyant flow induced under the normal gravity condition, while good optical resolution and easy handling are obtained with those droplet diameters. For reducing the effects of the buoyant flow and simplifying the phenomena, the experiments were performed using microgravity environments in a drop shaft. The arrangement effect was examined by changing dimension or droplet number. The results showed that ignition delay and burning time increased with decreasing the droplet spacing regardless of the arrangement. The three-dimensional arrangement showed rather longer ignition delay and much shorter burning time than the two-dimensional arrangement. Larger droplet number resulted in longer ignition delay and longer burning time. The observation of the flame behavior of the autoignited droplet clusters offered an interesting feature, which would be relevant to Chiu’s group combustion concept [3], [5], [7]. Individual flames were formed around each droplet with the larger droplet spacing clusters, while group flame was formed with the smaller droplet spacing clusters. When the droplet spacing was in the intermediate range, a transition of flame mode occurred with the lapse of time, from the individual flames to the group flame.

In the present study, effects of the initial droplet diameter as well as the droplet spacing on autoignition and early flame behavior of a spherical cluster of droplets in a high-temperature air were examined. The three-dimensional (3D) MSDC models of HCP (hexagonal closest packing) structure with 49 droplets were selected as the test clusters. The microgravity environment during the parabolic flight of aircraft, which provides rather longer microgravity duration than dropshaft facilities, was utilized for the experiments.

Section snippets

Experimental procedure

Figure 1 shows a schematic diagram of the experimental setup. The setup consisted of an electric furnace in which the droplet cluster was allowed to burn, a sample transfer unit, a sequence controller which enabled automatic operation of the apparatus, and optics. The electric furnace, 127 mm in inner diameter and 155 mm in inner height, was installed with a shutter at its opening in the bottom wall for shielding thermal radiation from the electric furnace to the droplet cluster on the sample

Results and discussion

In the previous study, the flame enveloped the whole cluster when the droplet spacing was small. This group flame would correspond to the external group flame presented in the theoretical and numerical studies on multiple droplet combustion [3], [5], [7]. Droplet flames were formed around each droplet in the cluster when the droplet spacing was large. These flames were named the individual flames. When the droplet spacing was in the intermediate range, the transition of flame mode occurred with

Conclusions

Effects of the initial droplet diameter as well as the droplet spacing on autoignition and early flame behavior of the spherical cluster of droplets in a high-temperature air were examined experimentally using microgravity environment. The solidified-fuel fiber-suspension technique was utilized for preparing the droplet clusters. The three-dimensional MSDC models of HCP (hexagonal closest packing) structure with 49 droplets were selected as the test droplet clusters. Essential conclusions drawn

Acknowledgments

This study has been carried out as a part of Grant-in-Aid for Scientific Research (B), The Ministry of Education, Culture, Sports, Science and Technology, Japan. The authors thank for the supports.

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