When using optical diagnostics to measure sprays and other two-phase flows it is common to assume that each detected photon has scattered from only one droplet or particle. However in dense particle fields the number which have scattered more than once may be significant. These multiply scattered photons carry information about more than one region in the spray and as a result the measurement or image becomes difficult to interpret and the accuracy of the measurement is reduced. We have written a flexible Monte Carlo photon transport simulation code capable of handling any 3D geometry and particularly suited to two-phase flows. The code has been used to simulate photon transport in typical dense spray imaging experiments, to quantify the error introduced by multiple scattering. The results show that up to 50% of the photons reaching the camera are multiply scattered. Under the conditions modelled, camera aperture is ineffective in reducing the contribution of multiple scattering to the image. The particles modelled here are non-absorbing, and their scattering is forward dominated. As a result multiple scattering causes little blur or position-dependent attenuation in the images. For particles nearer the Rayleigh limit, forward scattering is not so dominant and blur may be more serious. For thicker systems and systems with absorbing particles the effects are also expected to be more serious.
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