This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.Microprocessor and microchip speeds are continuously increasing with their shrinking sizes. For this reason, the size of subject related heat sinks are continuously decreasing from mini size to micro size. Among many microscale heat transfer cooling applications, the most practical and extensively used micro heat sinks are plain microchannels. This study addresses the lack of information about microchannel boiling instability phenomena and includes a parametric investigation in microtubes. Experimental data were obtained from a microtube having a 250~μm inner diameter, which was tested at low mass fluxes (78.9-276.3 kg/m2s) to reveal potential boiling instabilities. De-ionized water was used as a coolant, while the microtube was heated by Joule heating. Configurations prone to boiling instabilities (low system pressures, low mass velocities) were imposed to observe boiling instabilities in microtubes. After the experiments without any inlet restriction, experiments were conducted with the configuration with inlet restriction where pressure drop over inlet restriction element was 4 times as much as pressure drop over the microtube. Temperature and pressure drop fluctuation signals were recorded and processed once boiling instabilities were observed
