The effects of the refrigerant vapor quality, mass flux, saturated temperature and imposed heat flux on the evaporation heat transfer coefficient of refrigerant R-407C in the horizontal small circular tubes are shown in Figs. 5.2-5.10. The measured heat transfer coefficients are inspected by checking their variations with the vapor quality at the test section inlet
x . Since the tubes are short, the total vapor quality change
in Δ in the tubes for various cases in the present test is relatively small, ranging fromx
0.01 to 0.03. At first, Fig. 5.2 shows the variations of the evaporation heat transfer coefficient with the inlet vapor quality of the refrigerant at q = 5 kW/m2 for different mass fluxes and saturated temperatures of R-407C. The results show that at this low imposed heat flux the evaporation heat transfer coefficient only increases very slightly with the inlet vapor quality for given G, Tsat and q. But the increases of hr with the refrigerant mass flux cannot be ignored. For example in Fig. 5.2(a), at Tsat = 5℃ the quality-average evaporation heat transfer coefficient at G = 400 kg/m2s is about 2700W/m2℃ and is higher than 2050 W/m2℃ for G= 200 kg/m2s by 32%. It means that the effect of the mass flux on the R-407C evaporation heat transfer is significant. Next, the effects of the refrigerant saturated temperature on the R-407C evaporation heat transfer coefficient are shown in Fig. 5.3 again for the low heat flux of 5 kW/m2. Note that the increase of hr with Tsat is rather significant especially when Tsat is raised from 5℃ to 10℃. This is due to the lower latent heat of vaporization for the higher saturated temperature and liquid film vaporization is stronger, which in turn results in a higher vapor velocity and enhances the interfacial heat transfer between the liquid film and vapor flow. Thus, it results in a higher evaporation heat transfer coefficient.
The variations of the evaporation heat transfer coefficient with the inlet vapor quality of the refrigerant at the higher imposed heat flux with q = 10 kW/m2 are illustrated in Figs. 5.4 and 5.5. The data in Fig. 5.4 for the higher q clearly indicate that at higher mass fluxes of 300 and 400 kg/m2s the evaporation heat transfer coefficient increases substantially with the inlet vapor quality of the refrigerant in small tubes. For instance in Fig. 5.4(a), the heat transfer coefficient for G = 400 kg/m2s at
x = 0.78 is about 5800 W/m
in 2℃ and is higher than 3900 W/m2℃ forx
in= 0.21 by 48 %. This significant increase of hr with
x obviously results from the
in fact that at the high vapor quality the liquid film on the inside surface of the small tubes is thinner. This, in turn, reduces the resistance of heat transfer from the heating surface to the refrigerant. We also note that at the higher q the increase of hr with the refrigerant mass flux is also large. For example in Fig. 5.4(a) for , the heat transfer coefficient at G = 400 kg/m= 0.78
x
in2s is about 5800 W/m2℃ and is much higher than 2500 W/m2℃ for G = 200 kg/m2s by 132 %. This results from the simple fact that for the higher refrigerant mass flux the vapor velocity is higher and the convective evaporation is stronger. Then, the effects of the refrigerant saturated temperature on the evaporation heat transfer coefficient are inspected in Fig. 5.5. The results again
suggest that the evaporation heat transfer is much better at the higher Tsat.
Figures 5.6 and 5.7 further reveal the variations of the evaporation heat transfer coefficient with the inlet vapor quality of the refrigerant in the small tubes at an even higher imposed heat flux of 15 kW/m2 for different mass fluxes and refrigerant saturated temperatures. The data clearly indicate that the evaporation heat transfer coefficient also increases significantly with the refrigerant mass flux, saturated temperature and vapor quality. These trends are similar to those at the lower heat fluxes. Obviously, at different imposed heat fluxes the quantitative changes in hr with G and Tsat are somewhat different. For example, at q=15 kW/m2 and Tsat = 10℃ the quality-average evaporation heat transfer coefficients at G = 300 & 400 kg/m2s are respectively about 47 % and 86 % larger than that at G = 200 kg/m2s (Fig. 5.6(b)).
And at q=15 kW/m2 and G = 400 kg/m2s the quality-average evaporation heat transfer coefficient at Tsat = 10 & 15 ℃ are respectively about 24 % and 49 % larger than that at Tsat = 5℃ (Fig. 5.7(c)).
Next, results are presented in Figs. 5.8 to 5.10 to further illustrate the effects of the imposed heat flux on the R-407C evaporation heat transfer coefficient for the small circular tubes. These results clearly manifest that the increase of the evaporation heat transfer coefficient with the vapor quality is almost linear for all G, Tsat and q.
Moreover, the increase is rather significant at the high G, Tsat and q. Note that at the lower heat flux there is lower nucleation density on the heating surface, lower bubble generation frequency, and slower bubble growth. Besides, at the lower mass flux we have lower vapor and liquid velocity. Thus, at low q and G the variations of the evaporation heat transfer coefficient with the vapor quality are less significant.
Finally, the R-407C evaporation heat transfer coefficients measured here are compared with those from Yan and Lin [12] for R-134a in the same small tubes and
with Wang et al. [7] for R-407C in a larger tube of 6.5 mm in diameter. Due to the limited amount of the available data for R-134a in the small tubes which were conducted at similar conditions covered in the present study, the comparison is only possible for a few cases. Figure 5.11 compare the present R-407C evaporation heat transfer coefficients with those from Yan and Lin [12] for R-134a at selected cases for various imposed heat fluxes and refrigerant saturated temperatures at the same refrigerant mass flux. The comparison indicates that at the low q of 5 kW/m2 refrigerant R-407C has a slightly higher hr when the inlet quality is not high with
(Fig. 5.11(a)).But at the higher q of 10 and 15 kW/m
in 0.5
x
< 2 we have higher hr forR-407C only at a higher vapor quality for (Fig. 5.11(b) and (c)). It is of interest to mention that in the previous study for R-134a [12] the decline of h
in 0.65
x
>r with
x at a high vapor quality for the high imposed wall heat flux is attributed to the
inpossibility that at a higher vapor quality for R-134a the tube wall may become partially dry when the wall heat flux is high enough. Note that at the q = 5 kW/m2 and at the high vapor quality the R-134a evaporation heat transfer coefficient is higher than that of the R-407C (Fig. 5.11(a)). These reverse trends in different vapor quality ranges for the test heat fluxes are attributed mainly to the different thermal conductivities of the two refrigerants for the liquid and vapor phases. Specifically, the thermal conductivity of the liquid R-407C is higher than that of the R-134a by 17.7 %.
However, the thermal conductivity of the vapor R-407C is lower than that of the R-134a by 8 %. The much higher rise of the R-134a with the vapor quality is mainly due to the larger liquid-to-vapor density ratio when compared to that of the R-407C for the refrigerant pressures tested here. The liquid-to-vapor density ratios are respectively 31.6 and 30.6 for R-134a and R-407C for the cases examined in Fig. 5.11.
The comparison shown in Fig 5.12 reveals that the R-407C evaporation heat transfer coefficient in the small tubes (D = 2 mm) are much higher than that in a larger tube
(Di = 6.5 mm). Specifically, the quality-average heat transfer coefficient given in Fig.
5.12(a) is about 2700 W/m2℃ for Tsat = 5℃ in the small tubes and exceeds 1000 W/m2℃ for Tsat = 2℃ in the large tube by 180 %. Similarly, for the lower mass flux the quality-average heat transfer coefficient given in Fig. 5.12(b) is about 2400 W/m2℃ for Tsat = 5℃ in the small tubes and is also higher than 1200 W/m2℃ for Tsat
= 2℃ in the large tube by 100 %.