Development of novel global rainbow technique for characterizing spray generated by ultrasonic nozzle

Abstract

The energy crisis leads to the increasing demand of other alternative fuel instead of petrol. Among others, many attentions are dedicated to biodiesel which can be synthesized from many different plants. For Thailand, the interest is focused on palm oil and Jatropha. As a key issue to optimize the energy extraction and to reduce the pollutant formation from liquid fuel combustion, it is essential to control heat up and evaporation of fuel droplets. However, it has not been yet systematically studied. In order to obtain good engineering design of stable spray combustors, it is essential to develop a complete understanding of the fundamental phenomena of droplets that influence and control the overall combustion process. Also it would be advantageous to measure the fuel droplet temperature. Therefore, the objective of this work is to develop a novel global rainbow technique to determine the droplet temperature and size distrubution in a free spray before and during combustion. The spray taken into account is generated by ultrasonic nozzle which could provide a relatively small mono-disperse droplets with low velocities. Rainbow and global rainbow techniques are developed based on measurement of absolute angular position of the rainbow angle depending on the refractive index of droplets, which is in turn affected by droplets temperature and composition. A new processing concept based on a development of Nussenzveing's theory is introduced to gain information of the scattered light intensity. this approach could provide accurate results comparable with Lorenz-Mie's theory and be also as fast as Airy' theory. With this new tool, improvements of rainbow and global rainbow techniques are obtained and reported in this work. For the measurement of individual droplet, using the classical rainbow technique, it has been proved that measurement accuracy of 0.01 [micrometre] can be obtained for droplet diameter and of 0.0001 for the refractive index (corresponding to 1ํC). Meanwhile, the global rainbow technique has been developed to measure the temperature of droplet sprays. The developed global rainbow setup could be employed to record the scattered light in a solid angle of about 30ํ, permitting to measure the variation of the refractive index on a large range, corressponding to about an evolution of 300ํC for kerosene. Finally, the developed global rainbow technique is applied to measure the temperature of biodiesel spray created by an ultrasonic nozzle under ambient and combustion conditions. To our knowledge, measurements of droplets temperature in a pure biodiesel spray flame are successfully achieved for the first time.