8.1 Concluding Remarks
First, we have carried out experiments to unravel the saturated flow boiling heat transfer characteristics and associated bubble characteristics of R-134a in a horizontal narrow annular duct. The effects of the refrigerant mass flux, imposed heat flux, and refrigerant saturated temperature have been examined in detail. A summary of the major findings from the present results is given in the following.
(1). The temperature undershoot at ONB are insignificant for the saturated flow boiling.
(2). The saturated boiling heat transfer coefficient of R-134a is found to increase with a decrease in the gap size. Besides, raising the imposed heat flux can also cause a significant increase in the saturated flow boiling heat transfer coefficient. However, the effects of the refrigerant mass flux and saturated temperature on the boiling heat transfer coefficient are also small.
(3). The results from the flow visualization of the bubble behavior in the saturated flow boiling show that the mean diameter of the bubbles departing from the heating surface decreases slightly at increasing refrigerant mass flux. Besides, at a high imposed heat flux many bubbles generated from the cavities in the heating surface tend to merge together to form a big bubbles. The mean bubble departure frequency is higher in the smaller duct.
(4). Empirical correlations for the flow boiling heat transfer coefficients for R-134a in the narrow annular duct has been provided. Correlating equations have also been proposed for some bubble characteristics in the saturated boiling of R-134a in the narrow annular duct.
Then, in the small circular tubes we have experimentally measured the heat transfer coefficient and pressure drop for the R-407C evaporation in the small circular tubes (Di=2mm). The influences of the refrigerant mass flux, heat flux, saturated temperature and vapor quality on the evaporation heat transfer and frictional pressure drop have been
investigate in detail. The major results obtained can be briefly summarized as follows.
(1). Both the evaporation heat transfer coefficient and frictional pressure drop increase with the mass flux and vapor quality except at low mass fluxes. A rise in the imposed heat flux results in a significant increase in the evaporation heat transfer coefficient except at low heat fluxes. Nevertheless the influence of the imposed heat flux on the frictional pressure drop is rather slight. The increase in the saturated temperature of R-407C results in a smaller increase in the evaporation heat transfer coefficient when compared that due to a rise in the mass flux. But the trend in the frictional pressure drop is the opposite. The decrease in the R-407C saturated temperature results in a noticeable increase in the frictional pressure drop.
(2). Empirical correlations for the heat transfer coefficient and friction factor for the R-407C evaporation in the small tubes were provided.
We have also carried out experiments to unravel the saturated flow boiling heat transfer characteristics and associated bubble characteristics of R-407C and R-410A in a horizontal narrow annular duct. The effects of the refrigerant mass flux, imposed heat flux, refrigerant saturated temperature and refrigerants have been examined in detail. A summary of the major findings from the present results is given in the following.
(1). The temperature undershoot at ONB are insignificant for the saturated flow boiling for all refrigerants used in the horizontal narrow annular duct.
(2). The saturated flow boiling heat transfer coefficients of R-410A are better than R-407C and R-134a. Besides, raising the imposed heat flux can cause a significant increase in the boiling heat transfer coefficient. However, the effects of the refrigerant mass flux and saturated temperature on the boiling heat transfer coefficient are small.
(3). The results from the flow visualization show that the mean diameter of the bubbles departing from the heating surface decreases slightly with increasing refrigerant mass flux. Besides, at a high imposed heat flux many bubbles generated from the cavities in the heating surface tend to merge together to form big bubbles. The bubble departure frequency increases with the increasing refrigerant mass flux and saturated temperature. The active nucleation site density is much lower at a higher refrigerant mass flux and lower saturation temperature. The higher active nucleation site density on the heating surface for the R-410A flow is due to its smaller surface tension and is
the main reason that the flow boiling heat transfer performance of R-410A is better than R-407C and R-134a.
Finally during the present study three articles have been published in the International Journal of Heat and Mass Transfer up to this point. They are included here for more detailed reference of the boiling of R-134a, R-407C and R-410A in small channels.
1. Y.M. Lie and T.F. Lin, Saturated flow boiling heat transfer and associated bubble characteristics of R-134a in a narrow annular duct, International Journal of Heat and Mass Transfer 48(25-26)(2005)5602-5615.
2. Y.M. Lie and T.F. Lin, Subcooled flow boiling heat transfer and associated bubble characteristics of R-134a in a narrow annular duct, International Journal of Heat and Mass Transfer 49(13-14)(2006)2077-2089.
3. Y.M. Lie, F.Q. Su, R.L. Lai and T.F. Lin, Experimental study of evaporation heat transfer characteristics of refrigerants R-134a and R-407C in horizontal small tubes , International Journal of Heat and Mass Transfer 49(1-2)(2006)207-218.
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