The experimental results concerning heat transfer characteristics and flow pattern visualization in narrow annular gaps will be presented. On this account, experiments were carried out using two different test sections. An outer heated concentric annulus allowed the calculation of the heat transfer coefficients through the measurement of the outer wall temperatures using infrared thermography. On the other hand, an inner heated annulus with an outer glass tube, having the same gap size, 500 μm, was employed for flow observations using a high speed camera. By these means, both measurement methods can be combined in order to correlate the trends of the heat transfer coefficients as function of vapor quality with the observed boiling regimes.

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