The size of the channels in a compact heat exchanger can significantly affect the performance of the exchanger, suggesting that the channel confinement effects on the two-phase flow in the loop are important. In classifying the channel size Kandlikar [3,4]
proposed ： (1)Dh >3mm for the conventional channels, (2) 200μm< Dh <3mm for the mini-channels, (3) 10μm< Dh <200μm for the micro-channels, (4) 1μm< Dh <10μm for the transitional micro-channels, (5) 0.1μm< Dh <1μm for the transitional nano-channels, and (6) Dh ≦0.1μm for the molecular nano-channels. Cornwell and Kew [5] investigated refrigerant R-141b boiling in a horizontal tube with its inner diameter ranging from 1.39 to 3.69 mm. In this study, they introduced a new dimensionless group named as the confinement
number,
( ( ) )
1/ 2≡ , which represented the importance of the restriction of
the flow by the small size of the channel. They showed that the confined boiling occurred whenNconf >0.5. Accordingly, the effects of the tube confinement can be significant for two-phase flow in micro and mini channels.
1.2.1 Single-Phase Heat Transfer in Small Channels
In 1995, Peng and Wang [6] studied single-phase heat transfer for liquid methanol in a small rectangular channels with Dh=0.31, 0.51 and 0.646 mm. Their experimental data showed that liquid velocity, liquid properties, and geometry of the channels all exhibited significant effects on the heat transfer performance. Specifically, increasing the liquid velocity
and channel number caused enhancement in heat transfer. In a continuing investigation for water [7] they also found that the geometric configuration of channels had a significant effect on the single-phase convective heat transfer and flow characteristics. Besides, the turbulent heat transfer was noted to depend on a new dimensionless variable, Z defined as
)
H and W are the height and width of the channel, respectively.
Recently in an experiment for R-134a liquid flow in mini-channels for a vertical liquid up-flow, Agostini et al. [8] compared their heat transfer data with some existing correlations.
They concluded that the correlation of Gnielinsky correlation [9] was good for 2300< Re <106 and 0.6< Pr <105, and the Dittus-Boelter correlation [10] was good for Re >105 and 0.7< Pr
<16700.
1.2.2 Evaporation Heat Transfer in Conventional Channels
In the following the relevant literature on the evaporation heat transfer of Refrigerants in conventional and other enhanced tubes is reviewed.
Lazarek and Black [11] studied evaporative heat transfer, pressure drop and critical heat flux in a vertical tube with R-113 (Dh=31 mm). They showed that the R-113 evaporative heat transfer coefficient increased with the heat flux for the vapor quality up to about 60%.
Experimental data was taken for the two-phase heat transfer characteristics of R-22/R-407C in a smooth tube (Dh=6.5 mm) by Wang and Chiang [12] for the vapor quality varied from 0.1 to 0.9 with the refrigerant saturated temperature Tsat= 2℃. The R-22 evaporation heat transfer coefficient was found to increase with increasing mass flux and vapor quality. However, the R-407C evaporation heat transfer coefficient increased with increasing mass flux but
under forced flow conditions in a horizontal tube (Dh=7.2 mm) was examined by Hartnett and Minkowycz [13]. Their results indicated that the evaporation heat transfer coefficients of R-1314a increased with increasing vapor quality for the vapor quality varied from 0.1 to 0.8 with the refrigerant saturated temperature Tsat= 4℃ and 25℃. At low refrigerant vapor quality for x < 0.35 and low saturation temperature, the refrigerant mass flux was found to have almost no effect on the evaporation heat transfer coefficient. For the vapor quality ranging from 0.35 to 0.6, the evaporation heat transfer coefficient increased slightly at increasing mass flux. When the vapor quality was much higher than 0.6, the evaporation heat transfer coefficient increased evidently with the refrigerant mass flux. Choi et al. [14]
investigated the evaporation heat transfer of R-32, R-134a, R-32/134a, and R-32/125/134a inside a horizontal smooth tube (Dh=7.75 mm) for the vapor quality ranging from 0.05 to 0.95 with the refrigerant saturated temperature varied from -12℃ to 17℃. They found that the evaporation heat transfer coefficient increased also with increasing mass flux and vapor quality. Besides, at low vapor quality the evaporation heat transfer coefficient increased strongly with heat flux. But at high vapor quality, the heat flux, however, does not exhibit noticeable effect on the evaporation heat transfer coefficient. Yu et al. [15] studied the heat transfer and flow pattern in flow boiling of R-134a in horizontal smooth and microfin tubes (Dh=10.7 mm). Their results also indicated that the evaporation heat transfer coefficient increased with increasing mass flux and vapor quality for the vapor quality up to 0.7 with Tsat= 6℃. Characteristics of condensing and evaporating heat transfer using hydrocarbon refrigerants were investigated by Lee et al. [16] for R-290, R-600a, R-1270 and R-22 (Dh=12.7 mm) covering the vapor quality from 0.1 to 0.99 with Tsat= 14℃. They reported that the evaporation heat transfer coefficient increased with the vapor quality. Besides, the evaporation heat transfer coefficient is highest for R-1270 and lowest for R-22. Moreover, R-290 has higher evaporation heat transfer coefficient than R-600a. Wongwises and
Polsongkram [17] investigated the evaporation heat transfer and pressure drop of HFC-134a in a helically coiled concentric tube-in-tube heat exchanger (Dh=7.2 mm). Their results show that the evaporation heat transfer coefficient increases with increasing vapor quality, mass flux, heat flux and saturation temperature for the vapor quality ranging from 0.1 to 0.9 with Tsat= 10℃ to 20℃. Park and Hrnjak [18] studied the CO2 and R-410A flow boiling heat transfer, pressure drop, and flow pattern at low temperatures in a horizontal smooth tube (Dh=6.1 mm). They found that the evaporation heat transfer coefficient of R-410A increased with the mass flux and vapor quality. Besides, the evaporation heat transfer coefficient of CO2
is higher than R-410A for the vapor quality varied from 0.1 to 0.8 with Tsat= -15℃ and -30℃.
Condensation heat transfer of R-134a inside a microfin tube with different tube inclinations (Dh=8.92 mm) examined by Akhavan-Behabadi et al. [19] indicated that the condensation heat transfer coefficient increased also with increasing mass flux and vapor quality for the vapor quality ranging from 0.2 to 0.8 with Tsat= 26 ~ 32℃.
1.2.3 Evaporation Heat Transfer in Small Channels
A comprehensive review of saturated flow boiling in small passages of compact heat-exchangers conducted by Watel [20] suggests that the heat transfer mechanism in the evaporation of refrigerants in small channels is dominated by conduction and convection through the liquid film and the interfacial vaporization. The evaporation heat transfer coefficient depends mainly on the refrigerant mass flux and vapor quality.
Evaporation heat transfer and pressure drop of refrigerant R-134a in a bank of 28 small pipes (Dh=2.0 mm) were experimentally investigated by Yan and Lin [21,22] for the vapor quality ranging from 0.1 to 0.95 with the refrigerant saturated temperature Tsat= 5℃ to 31℃.
low vapor quality. But the evaporation heat transfer coefficient decreased with increasing heat flux at high quality. Besides, an increase in the refrigerant saturated temperature results in an increase in the evaporation heat transfer coefficient. Yun et al. [23] studied the convective boiling heat transfer characteristics of CO2 in small channels (Dh=1.08 and 1.54 mm) covering the vapor quality from 0.15 to 0.9 for Tsat= 0℃ to 10℃. Their results indicated that before the liquid film dryout the evaporation heat transfer coefficient increased with heat flux.
Besides, the evaporation heat transfer coefficient increased with increasing refrigerant saturated temperature and decreasing channel hydraulic diameter. Moreover, the CO2
evaporation heat transfer coefficient was higher than R-134a by 53%. An experimental study was carried out by Lie et al. [24] to examine the evaporation heat transfer characteristics of refrigerants R-134a and R-407C in horizontal bank of small tubes (Dh=0.83 and 2.0 mm) for the vapor quality ranging from 0.2 to 0.8 at the refrigerant saturated temperature Tsat= 5℃ to 15℃. They showed that the evaporation heat transfer coefficient increased at increasing heat flux, saturated temperature and mass flux. Besides, the evaporation heat transfer coefficient increases almost linearly with the vapor quality. Moreover, the evaporation heat transfer coefficient of R-407C was higher than R-134a. Evaporative heat transfer and pressure drop of R-410A in small channels (Dh=1.33 and 1.44 mm) were investigated by Yun et al. [25] for the vapor quality varied from 0.05 to 0.9 at the saturated temperature Tsat= 0℃ to 10℃. Their results indicated that before the dryout, the evaporation heat transfer coefficient was independent of the refrigerant saturated temperature, heat flux and mass flux. But after the dryout the evaporation heat transfer coefficient increased at increasing refrigerant saturated temperature, heat flux and mass flux. Choi et al. [26] studied the flow boiling heat transfer of R-22, R-134a, and CO2 in horizontal minichannels (Dh=1.5 and 3.0mm) covering the vapor quality from 0.05 to 0.95 at the refrigerant saturated temperature Tsat= 10℃. They reported
that the heat transfer coefficient increased with the heat flux. Besides, they found that the dryout quality became lower for a higher heat flux.