Summary and Conclusion

From the study of the CO2 flow boiling heat transfer coefficients in the 6.1 and 3.5 copper tubes, the

following conclusions can be inferred:

• The CO2 heat transfer coefficients for the 6.1 mm tube are the strong function of heat flux, whereas

they are not strongly dependent on quality and mass flux except at the low mass flux condition of 100 kg/m2 _{s. Consequently, they show the nucleate dominant heat transfer mechanism.}

• The CO2 heat transfer coefficients for 3.5 mm tube at the mass flux of 200 kg/m2 s indicate the

dominance of nucleate boiling, however those at the mass flux of 400 kg/m2 _{s present that the}

convective boiling contribution to heat flux is significant especially at low heat flux condition. • The comparison of the CO2 heat transfer coefficients for 6.1 and 3.5 mm tubes shows that the change

of tube diameter does not influence much to heat transfer coefficient at the mass flux of 200 kg/m2 _{s. It}

also demonstrates that the smaller tube does not always bring out a higher heat transfer coefficients at an identical condition.

• The reduction of CO2 heat transfer coefficients with increasing mass flux in low quality regions is

experimentally shown.

• The CO2 heat transfer coefficients for the 6.1 mm tube at the mass flux of 100 kg/m2 s decrease with

the increase of quality because the mass flux is insufficient to wet the entire inner surface area of the tube. When designing the CO2 heat exchangers for low temperature applications, the mass flux should

be high enough to sufficiently wet the inside area of the tube.

• For the evaporation temperature of –15 and –30 °C, the Gungor and Winterton (1986) and Wattelet et al. (1994) correlations can predict the CO2 heat transfer coefficients in the 6.1 mm tube relatively well

for most of the flow conditions except the mass flux of 400 kg/m2_{s and heat flux of 5 kW/m}2 _{at the}

evaporation temperature of –30 °C.

• For the evaporation temperature of –15 °C, the Gungor and Winterton (1986) correlation is

recommendable to determine the CO2 heat transfer coefficients in the 3.5 mm tube for the mass flux of

200 kg/m2_{s and the Wattelet et al. (1994) correlation can predict very well for the mass flux of 400}

kg/m2_s.

• For the evaporation temperature of –30 °C, the Wattelet et al. (1994) correlation shows acceptable bias errors for predicting the CO2 heat transfer coefficients in the 3.5 mm tube at the mass flux of 200

kg/m2_{s and the Liu and Winterton (1991) and Shah (1982) correlations predict relatively well for the}

mass flux of 400 kg/m2_s.

• The surface roughness of the 6.1 and 3.5 mm tube are presented by SEM and AFM images and surface profiles, and it is shown that the rougher surface of the 6.1 mm tube can affect the flow boiling heat transfer.

From the investigation of the CO2, R410A, and R22 flow boiling heat transfer coefficients in the 6.1 mm

copper tube, the following conclusions can be proposed:

• The investigation of flow boiling heat transfer coefficients and pressure drop is performed in the horizontal smooth tube of 6.1 mm inner diameter for CO2, R410A, and R22.

• Flow boiling heat transfer for CO2 is much higher than those for R410A and R22 especially at low

• The lower molecular weight and the higher reduced pressure of CO2 than those of R410A and R22

result in higher flow boiling heat transfer coefficients by enhancing the nucleate boiling heat transfer contribution.

• For the evaporation temperatures of –15 and –30 °C, the Gungor and Winterton (1986) can predict R410A heat transfer coefficients relatively well for all mass fluxes except the heat flux of 5 kW/m2_.

• For the evaporation temperature of –15 °C and the heat flux of 5 kW/m2_{, the Liu and Winterton (1991)}

correlation is recommendable to predict the R410A heat transfer coefficients for the mass flux of 100 and 200 kg/m2_{s, and the modified Gungor and Winterton (1987) shows good predictions for the mass}

flux of 400 kg/m2_s.

• For the evaporation temperature of –30 °C and the heat flux of 5kW/m2_{, the modified Gungor and}

Winterton (1987) is most recommendable to predict R410A heat transfer coefficients for the mass flux of 100 kg/m2_{s, and no correlation can correctly predict for the mass fluxes of 200 and 400 kg/m}2_s.

From the research of the CO2 flow boiling heat transfer coefficients in the 0.89 mm aluminum tube at the

heat flux of 5 kW/m2_{, the following conclusions can be suggested:}

• The CO2 heat transfer coefficients in the 0.89 mm tubes increase with the increase of vapor quality for

all mass fluxes, which means that the convective boiling is an active heat transfer mechanism. Also, they are not significantly increased, compared with the heat transfer coefficients in 3.5 and 6.1 mm tubes.

• The CO2 heat transfer correlations for macro-scale tubes tend to overpredict the heat transfer

coefficients for all measurement conditions. • For the mass flux of 200 kg/m2_{s, the CO}

2 heat transfer coefficients can be calculated relatively well by

the Liu and Winterton (1991) and the Shah (1982) correlations with the bias error from 0 to 40%. • Due to the limited data base to develop the heat transfer correlations for mini-scale tubes, their

accuracy is not better than the accuracy of the correlations for macro-scale tubes.

• It can be recommendable to modify the Tran et al (1996) correlation with vapor qualities for developing a heat transfer correlation for the measured values in this study.

Chapter 6. Pressure Drop

Two-phase flow pressure drop is a critical design parameter for heat exchanger designers because the performance of a system is strongly influenced by pressure drop in heat exchangers. As a result, two-phase flow pressure drop for numerous kinds of fluids has been measured under various conditions and empirical or semi- empirical models to calculate pressure drop have been proposed by many researchers. Fundamental theory including some general correlations about pressure drop for two-phase flow is presented in section 2.2.

In this chapter, two-phase pressure drop of CO2 is investigated with experimental data and numerical

results based on some correlations to predict two-phase flow pressure drop. CO2 pressure drop is measured in 6.1

and 3.5 mm copper tubes at low saturation temperatures and the measured pressure drop is compared with predicted values. In order to examine the pressure drop characteristics of CO2, the pressure drop of R410A and R22 is

measured in a 6.1 mm copper tube and the pressure drop is compared with CO2 pressure drop. Finally, the CO2

pressure drop in a small channel, the diameter of 0.89 mm, is investigated by experimental and predictive methods. In this study, the two-phase flow pressure drop is measured in horizontal tubes under diabetic conditions. As a result, the presented pressure drop data show the frictional pressure drop without any acceleration or gravitational effect on the pressure drop.

6.2 CO2 Pressure Drop in 6.1 and 3.5 mm Tubes