Void fraction measurements are used to characterize two-phase flow within a microchannel. Limitations of typical void fraction measurement systems include disruption of the flow (intrusive optical probe) and non-continuous data acquisition. Using the principles of total internal reflection, a Non-Intrusive Void Incidence Sensor (NIVIS) has been developed to determine the void incidence (frequency of a vapor bubble passing a known position in the channel). Continuous data can be recorded with a typical computer. A light beam is introduced through a fiber optic to the outside of a transparent channel wall at a critical angle. This critical angle is designed such that when a vapor bubble is present at the specified location, total internal reflection will occur. An output fiber is fixed in a determined position to receive light in the case of total internal reflection. Without the presence of a vapor bubble, the light beam will be reflected past the output fiber. A substantial increase in output signal is noticed when total internal reflection occurs. Characteristics of the NIVIS, proven during non-intrusive testing, include: continuous data acquisition of bubble incidences, measurements within a 100 micron wide channel, and bubble boundary differentiation.

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