Development of a Probe-Type Freezing Method for Droplet-Size Measurement

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A so-called probe-type freezing method was invented, and applied to sprays with smaller size droplets produced from a humidifier (ultrasonic vibrator type) and a nebulizer (tens of thousands micro-holes combined with ultrasonic vibrator, used for medical sucking). Followings were revealed.(1) With the probe-type freezing method, sphere shaped frozen particles could successfully be produced, and it is useful to be applied for droplet-size measurement.(2) With the probe-type freezing method, it is useful to measure droplet size measurement under the condition that the effect of evaporation and coalescence could be minimized, and thus several physical phenomena were clarified related to them.(3) The probe-type freezing method has an advantage in the measurement of droplets less than 10 micro-meter comparing with a conventional phase Doppler anemometry and a narrow-angle forward scattering technique, since the droplet diameter distribution in smaller range for the freezing method is more t … More han those obtained with a phase Doppler and a scattering technique.However, in the probe-type freezing method, when the frozen droplets were observed directly during their falling process with a digital microscope, the enlargement rate greater than 1000 caused seriously narrower focusing depth, and the average measurable number of droplets captured on one photograph was only five. Thus, there appeared a problem of less data rate. In order to solve this problem, a direct freezing sampling method was developed. This method introduced spray directly into a sampling dish in a thermally insulated vessel. Especially, since a modern phase Doppler anemometry system was available in the last year, followings were revealed.(1) For smaller droplets less than 10 micro-meter of the Sauter mean diameter (SMD) produced by a humidifier and a nebulizer, the newest phase Doppler anemometer gives more droplets less than 2 micro-meter than the direct freezing sampling method. This must be due to the improvement or the progress of the phase Doppler anemometer system. However, there was pretty well consistency of droplet size distribution for droplets greater than 2 micro-meter between this simple direct freezing sampling method and the newest phase Doppler anemometer. This verifies the practical validity of this simple freezing method.(2) For larger droplets more than 50 micro-meter of SMD for a hollow cone spray produced by a swirl nozzle, the droplet size distribution of the direct-freezing sampling method shifted to greater size than that of the newest phase Doppler anemometer. It would be due to the limited dynamic range for the newest phase Doppler anemometer, which must be a great advantage of this freezing method.(3) Thus, this direct-freezing sampling method has better performance of smaller droplet measurement than a conventional phase Doppler anemometer but has inferior than a newest one. It has better dynamic range than both phase Doppler anemometers. Less