Research of Boiling Heat Transfer and Flow Pattern of Refrigerant in Horizontal Small-Diameter Tubes

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Boiling heat transfer to HFC134a and flow pattern were investigated in horizontal small-diameter tubes, 0.51 mm, 1.12 mm and 3.1mm I.D. Boiling heat transfer coefficients were obtained for mass velocity of 150 〜 400 kg/m^2s, and heat flux of 5〜30 kW/m^2. The flow patterns of plug, slug, wavy and annular flows were observed at inlet qualities ranging from zero to 0.7 for the range of mass velocity of 100〜400 kg/m^2s. For the low inlet quality and high heat flux, the oscillated refrigerant flow occurred and oscillations of wall temperature and pressure were observed in the heated tube. The boiling heat transfer coefficients were affected by the flow patterns, and were considerably reduced for the oscillated and slug flow. It was shown that the contribution of the forced convective evaporation to boiling heat transfer decreases, as the tube diameter decreases. A heat transfer deterioration model and a dryout prediction model were proposed, and predictive value and experimental value are correlated well. The pressure drop in 3.1 mm tube agreed well with the prediction by the correlation of Lockhart-Martinelli, but in the case of the tube diameter 0.51 mm, it agreed well by the homogenous model.