In this study, we have experimentally measured the heat transfer coefficients and visualized the two-phase flow patterns for the R-134a evaporation in the horizontal narrow annular ducts with δ=1.0, 2.0 and 5.0 mm. The effects of the refrigerant saturated temperature, mass flux, imposed heat flux, vapor quality and gap size of R-134a on the evaporation heat transfer coefficients at the middle axial location of the duct have been examined in detail. A summary of the major findings from the present results is given in the following.
(1) The R-134a evaporation heat transfer coefficient in the small ducts(δ=1.0、2.0 mm) increases almost linearly with the vapor quality and the increase is more significant at a higher refrigerant mass flux. Besides, increases in the refrigerant mass flux and saturated temperature and imposed heat flux can substantially improve the evaporation heat transfer.
The effects of G and Tsat on hr are less pronounced for δ=1.0 mm.
(2) The effects of the refrigerant vapor quality, saturated temperature, imposed heat flux and mass flux in the wider annular duct with δ=5.0 mm on the R-134a evaporation heat transfer coefficient exhibit similar trends to that in the narrow duct for δ=1.0 and 2.0 mm.
But partial dryout of the liquid film on the heating surface at high xm and low mass flux occurs, leading to a reduction in hr at increasing xm.
(3) Reducing the gap size of the duct causes a significant increase in the R-134a evaporation heat transfer coefficient.
the heating surface is found to be important at low vapor quality. Besides, at low vapor quality merging of small bubbles to form big bubbles and merging of big bubbles into bubble slugs take place, which is more pronounced at the smaller duct gap for δ=1.0 mm.
Moreover, bubbles dispersed in a large liquid slug appear in the duct. At the intermediate vapor quality some bubble nucleation on the heating surface also exists and the flow in the duct is dominated by the vapor flow over thin liquid film around the inner pipe, an annular two-phase flow. Irregular waves appear at the vapor-liquid interface. At the very high vapor quality bubble nucleation can still be seen at high imposed heat flux although the liquid film covering the heating surface is relatively thin. At this high quality the duct is also dominated by the annular two-phase flow. The annular two-phase flow prevails in a larger portion of the duct at higher imposed flux, lower refrigerant mass flux, and higher refrigerant saturated temperature.
(5) In the duct with the wider gap of 5.0 mm the effects of the gravity on the evaporating flow are stronger, resulting a stratified two-phase flow with the vapor and liquid flows respectively dominated in the upper and lower parts of the duct. Besides, at low refrigerant mass flux and high vapor quality dryout of the liquid film on the heating surface occurs at some downstream locations. These dryout locations move upstream at higher vapor quality, higher imposed heat flux and lower refrigerant saturated temperature.
(6) Empirical equation to correlate the present data for the heat transfer coefficient for the R-134a evaporation in the annular ducts are provided.
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