Refractive index measurements using a dual-mode phase Doppler anemometer

Ping Bu Loke, Purdue University

Abstract

Refractive index measurements for a mono-disperse droplet chain and poly-disperse spray under ambient conditions were conducted using a dual-mode PDA. Results were within 1% of refractometer readings. Alignment of the mono-disperse droplet chains with the PDA measurement volume within 0.2mm was required to avoid the slit or Gaussian beam effects. Droplets below 30ìm have larger refractive index scatter in the measured phase differences because of the Mie scattering effect. The resolution of the refractive index was 0.01 due to scattering angle uncertainties. Further experiments were then conducted in a hot gas tunnel with a poly-disperse spray. Data showed that under evaporating conditions, the technique can resolve the trend that droplet refractive index increases as droplet size decreases and temperature increases. Turbulent flow conditions and evaporation created more droplets under 30 μm than in the ambient poly-disperse case. As a result, the already large scatter in the phase differences for these droplets became more pronounced. A non-linear spread of measured refractive index on the order of +0.3/-0.2 was observed for these droplets. Measured droplet refractive indices are helpful in determining droplet concentration because they are monotonically related to each other. However, non-isothermal situations decrease the accuracy of droplet concentration calculation based on refractive index, since refractive index varies as much as -0.02 for a droplet temperature increase of 70°C. The technique is hence expected to be more accurate for droplets that experience minimal impact of temperature on refractive index. To remove uncertainties of refractive indices due to droplet heat up, droplet temperature has to be measured.

Degree

M.S.M.E.

Advisors

Sojka, Purdue University.

Subject Area

Mechanical engineering

COinS