When compared with the convectional refrigerants, literatures concerning the effect of lubricant on the heat transfer perfor-mance of R-744 (CO2) is comparatively few. A variety of lubricants can be used in CO2 refrigeration systems. In certain systems, synthetic hydrocarbons such as alkylbenzenes (ABs) and poly-alphaolefins (PAOs) can still be used even though they have poor solubility with CO2 [78]. The poor solubility of the synthetic hydrocarbons is compensated by their excellent low temperature flow properties, which can be improved further by blending with more miscible lubricants ([79]). Kawaguchi et al. [80] reported that polyalkylene glycol (PAG) was the primary lubricant for CO2
systems since it is partially miscible with CO2. Seeton and Fahl [81]found that PAG reveals the best lubricity for trans-critical applications, and PAG is not miscible with CO2at high concentra-tions. Li and Rajewski[82]found that polyol ester (POE) lubricant was completely miscible with CO2. Ma et al.[83]suggested that POE was better than other lubricants for trans-critical CO2system, while Renz[84]reported that POE was particularly suitable for semi-hermetic reciprocating and screw compressors for CO2
cascade systems. They have a high viscosity index, good lubrica-tion behavior, acceptable solubility properties and favorable miscibility. For the effect of lubricant on the boiling heat transfer performance, Zhao and Bansal[78]had presented a comprehen-sive review and some of relevant details are summarized as follows:
3.1. Effect of lubricant on convective boiling of R744
Effect of oil concentration: Dang et al.[85]investigated the flow boiling heat transfer of CO2/PAG mixture in horizontal smooth tubes with oil concentrations varying from 0.5% to 5.0%. For a test tube size of 6 mm, they found that the addition of a small amount of lubricant resulted in a sharp decrease in the heat transfer coefficient. For instance, the HTC is reduced from 8–9 to 3–5 kW/
m2K when oil concentration is increased from 0% to 0.5%.
However, a further increase of oil concentration from 0.5% to 5%
casts almost negligible effect on heat transfer coefficient.
Similar influences are observed for smaller diameter tubes like 2 or 4 mm but the corresponding critical oil concentration is 0.5%
for 2 mm inner diameter tube and 1% for 4 mm inner diameter tube. Furthermore, the addition of lubricant seems to have no influence on the dryout quality and the post-dryout heat transfer coefficient. A possible explanation of this result may be attributed to most oil lubricant entrained at the post dry out region subject to high vapor shear after the post dry our region. and only a fixed amount of lubricant is deposited on the surface irrespective the lubricant concentration. As a result, the HTC remains unchanged for various oil concentration.
Analogous degradation of HTC were also reported by other investigators. For example, Gao et al.[86,87] found that HTC is decreased by about 50% (compared with pure CO2), when PAG oil concentration was more than 0.11% in a horizontal tube with 3 mm inner diameter at a saturation temperature of 10 1C.
However, an interesting feature is observed by Gao et al. [87]
who found that the heat transfer coefficient reveals a strong dependence of heat flux for the near pure CO2, indicating a dominate nucleate boiling. The results are applicable either for a smooth tube (Fig. 11(a)) or a microfin tube (Fig. 11(b)). But the strong dependence of heat flux is gone with the addition of PAG lubricant, suggesting a change of boiling mechanism from nucle-ate boing to convective evaporation. Gao et al.[86]noticed that in their test the PAG oil is separated from CO2liquid and becomes oil droplets and an oil film forms on the tube wall. This is due to PAG oil being immiscible or partially miscible with CO2. The cavities applicable for nucleate boiling are thus filled by oil film and a suppress of nucleate boiling is encountered accordingly. There-fore, the flow boiling heat transfer changes from nucleate boiling dominated regime to convective evaporation dominated regime, and the HTC decreases significantly due to the existence of PAG oil. Tanaka et al.[88]also observed that oil concentration more than 0.7% caused a drastic deterioration of HTC about 50%.
Katsuta et al.[89]investigated the flow boiling heat transfer of CO2–PAG mixture, and found that the HTC at 5% oil concentration is about 30% lower than that at 1% oil concentration.
Effect of vapor quality: The local oil concentration increases with vapor quality due to the nonvolatile nature of the lubricant despite its partial pressure in the vapor phase is usually negli-gible. In the low vapor quality region, the lubricant may increase the wetted surface due to its high surface tension and viscosity or due to the foaming effect. This is quite similar to the foregoing discussion about the effect of lubricant on the conventional refrigerant. But in high vapor quality region, the mixture viscosity and local oil concentration effect are quite significant. Once an oil-rich sublayer is formed near the heating surface, it may not only suppress boiling but also may introduce additional thermal
resistance to the heat transfer process ([4]). Zhao et al. [90]
reported considerable decrease of the boiling heat transfer coeffi-cient of CO2–lubricant mixture in high vapor quality region, especially for low oil concentrations less than 3%. Test results from Gao and Honda[87]and Dang et al.[85]also confirmed that the influence of oil concentration on HTC was higher at high vapor
X 0.0
h kW/(m2K)
0 5 10 15 20 25 30
h top q=10kW/m2 h bot h top q=20kW/m2 h bot h top q=30kW/m2 h bot
X h kW/(m2K)
0 10 20 30 40
q=10kW/m2 q=20kW/m2 q=30kW/m2
h kW/(m2K)
0 5 10 15 20 25
) Smooth tube (q=10kW/m2) Smooth tube (q=20kW/m2) Micro-fin tube (q=10kW/m2) Micro-fin tube (q=20kW/m2)
ωwt%
1.0 0.8
0.6 0.4
0.2
0.0 0.2 0.4 0.6 0.8 1.0
0.0 0.2 0.4 0.6 0.8
Fig. 11. Effect of heat flux on the smooth/microfin tube with and without lubricant oil[87]. (a) Effect of heat flux for smooth tube, G=780 kg/m2s, (b) Effect of heat flux for microfin tube, G=380 kg/m2s and (c) Effect of heat flux for smooth tube, subject to oil concentration.
quality region. But the dryout quality and post-dryout heat transfer were not influenced by the addition of oil. At high oil concentration around 7%, Zhao et al.[90]found that HTC was nearly independent of vapor quality. They explained that as the oil concentration increases, a rich oil layer forms on the wall along the whole test tube, which prevents the contact of liquid-phase refrigerant with the wall. Similar results are also seen for Gao et al.[86].
Effect of heat and mass fluxes: Most studies on the flow boiling heat transfer of pure CO2refrigerant showed that nucleate boiling dominates at low/moderate vapor quality prior to dryout (e.g., Gao et al. [86]and Yun et al.[91]). However, addition of lubricant in CO2results in higher convective boiling contribution in the low vapor quality region. The comparison of the four synthetic lubricants for CO2refrigeration. Zhao et al.[90]found that the HTC of CO2–oil mixture increased with mass flux, and this trend was more apparent at higher oil concentrations. Small oil concentrations enhanced the HTC more significantly at large mass fluxes. This phenomenon is similar to the conventional refrigerant. Test results from Gao et al. [86]also supported the foregoing statement but their test results for CO2/lubricant mixtures using microfin tube shows even more effect of mass flux. Dang et al.[85]also reported the effect of heat flux with 1%
oil concentration at two heat flux conditions. At a low heat flux of 18 kW/m2, HTC increased significantly with mass flux in the pre-dryout region. However, no obvious difference was observed at high heat flux of 36 kW/m2.
Effect of saturation temperature: Zhao et al.[90]experimentally investigated the effect of saturation temperature on the flow boiling heat transfer coefficient of CO2/lubricant mixture from 0 to 15 1C. They found that a high concentration of oil ( 43%) caused a larger heat transfer degradation at high saturation temperatures. For example, at 0 1C and a vapor quality of 0.05, the difference of HTC between 1% mixture and 7% mixture was only 0.5 kW/m2K, but it is increased to about 4 kW/m2K at 10 1C and 7 kW/m2K at 15 1C. The experimental data of Hassan[92]also found that the effect of lubricant on HTC decreased with decreas-ing saturation temperature. It is well known from many mono-graphs (e.g., Collier and Thome[93]) that the nucleate boiling HTC increased with the increasing saturation temperature, indicating higher contribution of nucleate boiling. On the other hand, the ratio of surface tension amid refrigerant/lubricant mixtures and pure refrigerant is also increased with the saturation temperature as shown inFig. 1. In this sense, the increased surface tension jeopardized the nucleate boiling as explained in Eq. (1), thereby leading to the deterioration of HTC increases with increasing saturation temperature.
Effect of tube configuration: Koyama et al.[94]compared the oil effect on the flow boiling heat transfer of CO2 in smooth and micro-fin tubes. They found that the deterioration rate of the heat transfer coefficient in micro-fin tube is smaller than that in smooth tube. They argued that micro-fin tubes can activate annular flow or semi-annular flow and suppress the foaming effect, leading to a gradual decrease of HTC with oil concentration.
Effect of microchannel: Siegismund [95] found a detectable reduction of the heat transfer coefficient of CO2with respect to POE lubricant during evaporation at 51C in microchannels (13x ID 0.8 mm).Fig. 12 shows that the reduction is dependent on the amount of oil, and at
o
¼3% the heat transfer is reduced by around 20%, while the reduction is around 50% when the oil concentration is increased to 9%. Though most studies for CO2reported a decline of HTC with addition of lubricant. However, an unusual phenomenon has been observed by Zhao et al.[90]for CO2–lubricant mixture flow boiling in a micro-channel at Ts¼10 1C. They found that large oil concentrations degrade the heat transfer coefficient significantly, for example, HTC with 7%
oil concentration was 60% lower than that of pure CO2, and by
contrast, smaller oil concentrations (o3%) at low vapor qualities (xo0.45) a marginally rise of the heat transfer coefficient (about 5% to 10%) is seen (shown inFig. 13). The moderate augmentation of the heat transfer coefficient may be attributed by the presence of oil for (i) promoting an earlier onset of annular flow and (ii) enhancing the nucleate boiling.
3.2. Effect of lubricant on R744 at supercritical state
Dang et al.[96–100] had conducted a systematic study about the effect of lubricant (PAG) on the heat transfer performance of CO2 at supercritical state. Their test tube size were, 2, 4, and 6 mm, respectively with mass flux ranging from 200 to 1200 kg/
m2s and oil concentration from 1% to 5%. They also performed flow visualization pertaining to lubricant influence. A short summary of their findings are given as follow:
Effect of lubricant on flow pattern:Fig. 14illustrates the observed flow pattern of supercritical CO2 flowing with PAG oil under the above-mentioned experimental conditions. Here, ‘‘V’’ denotes supercritical CO2, ‘‘D’’ denotes oil droplet, and ‘‘F’’ denotes oil film.
The observed flow pattern are: (a) mist flow (M), where a small amount of oil droplets flow with CO2and no oil film is observed;
(b) annular-dispersed flow (AD), where both oil droplets and an oil film are observed; (c) annular flow (A), where no or few oil droplets are observed; (d) wavy flow (W), where the oil film only exists at Fig. 12. Reduction of heat transfer as a function of oil content (by mass) at reduced pressure of 0.54 (¼ 5 1C evaporation temperature), mass flux 60–120 kg/
m2s and heat flux of 2.5 kW/m2. Comparison (ratio) at equal vapor fractions[96].
Fig. 13. Effect of oil concentration on HTC at G ¼ 300 kg/m2s, Ts¼10 1C and q¼ 11 kW/m2[90].
the bottom of the cross-section; (e) wavy-dispersed flow (WD), where oil droplets are observed flowing along with an oil film at the bottom of the cross-section. The flow pattern was found changing with the temperature, pressure, and oil concentration, as well as with the tube diameter.
At a low temperature of 25 1C and an oil concentration of 1 wt%, it is clear that the flow pattern is mist flow, with oil droplets flowing along with the bulk CO2at a slip ratio of about 0.7. The average diameter of the oil droplets ranges from 50 to 100
m
m. Under this condition, no distinct oil film flowing along the inner wall is observed. Since the solubility of CO2in the oil decreases with an increase in temperature, the viscosity and surface tension of the oil droplets increase with the temperature.As a result, the separated oil appears to adhere to the inner wall and forms an oil-rich layer at a high temperature with 20–60 wt%
CO2 dissolved inside it; this layer is visible as stripes at a temperature of 30 1C. With a further increase in temperature, the oil-rich layer becomes much thicker, and the oil droplets in the bulk region is not shown. The layer moves at a very low speed, resulting in a decrease in the heat transfer coefficient. A compar-ison of the flow pattern at tube diameters of 2 and 6 mm tubes is made and it is obvious from the comparison that the oil film for 2 mm tube is comparatively thicker than that of 6 mm tube, and the oil droplets are much larger with smaller number density.
The high viscosity of the deposited lubricant film give rise to the decreased turbulent disturbance, and becomes more severe with the decrease in tube diameter when compared under the same mass flux, temperature and pressure condition. As a consequence, the thicker oil film for small sized tube corresponds to a much more significant effect of increased thermal resistance from lubricant. On the other hand, the oil droplets flowing in the bulk region do not contribute significantly to the heat transfer dete-rioration[99].
Effect of lubricant on HTC subject to tube diameter: The effect of tube diameter on the HTC can be seen fromFig. 15. By introducing lubricant into CO2, it is seen that smaller diameter tube (2 mm) suffers more deterioration than the larger diameter tube (6 mm).
And the decline is especially vivid near the pseudocritical tem-perature. For an oil concentration of 5% and a smaller diameter of 2 mm (Fig. 15(a)), it appears that the drop in the heat transfer coefficient at 50 1C is much larger than that at 30 1C. From the visual observation, it is found that the flow pattern at low temperature is mist flow with a considerable amount of oil flowing alongside the bulk area and a very thin oil film. At high temperature, a thick oil film is observed, which corresponds to a sharp decrease in the heat transfer coefficient. The distinct
difference of the flow pattern is also related to comparatively rise of surface tension of lubricant which inevitably improve the wettability of the refrigerant/lubricant mixtures and lead to an oil film. This visual observation implies that the heat resistance due to the formation of the oil film is the main reason for the heat transfer deterioration.Fig. 15(b) and (c) also shows a comparison of the heat transfer coefficient and flow pattern for the 6 mm ID tube at two mass fluxes of 200 and 800 kg/m2s. The heat transfer coefficient decreases significantly at 800 kg/m2s due to the heat
Tbulk
Fig. 15. – Effect of tube diameter on the heat transfer performance of CO2–oil[99].
(a) 2 mm ID tube, (b) 6 mm ID tube, G=200 kg/m2s and (c) 6 mm ID tube,
Fig. 14. Classification of flow pattern for supercritical CO2with lubricant. M: mist flow; AD: annular-dispersed flow; A: annular flow; W: wavy flow; WD: wavy-dispersed flow.[99].
transfer resistance of the oil film flowing along inner wall.
By contrast, the drop of HTC is less pronounced when G ¼200 kg/m2s even for an oil concentration of 5%. From their flow observation, it can be attributed to the presence of wavy flow at a low mass flux and the oil film is present only at the bottom of the tube as opposed to entire film flow around the perimeter at a higher mass flux. In this regard, a significant drop in heat transfer performance is seen at a higher mass flux.
Effect of lubricant on HTC for grooved tube: Dang et al.[100]
performed an experimental study to examine the PAG oil on a 2 mm grooved tube having a helix angle of 6.31. with G ranging from 400 to 1200 kg/m2s. The drop in the heat transfer coefficient was 30–50% and 50–70% at oil concentrations of 1% and 3%, respectively. The heat transfer coefficient of CO2in the grooved tube was higher than that in the smooth tube at all tested oil concentrations. There are several reasons for the comparatively small deterioration of HTC in microfin tube, including a large heat transfer area, breaking up of the oil film by the grooved config-uration, and the oil film is normally so thin to entirely flood the grooved fin.
Effect of lubricant on HTC for microchannel: Siegismund [95]
investigated the heat transfer in microchannels at various POE oil concentrations. As shown inFig. 16, the heat transfer is drastically reduced when oil is present during heat rejection. InFig. 16, one can see the heat transfer coefficient is only 20% of the pure CO2at a pressure of 100 bar. The results are in line with the findings of Dang et al.[99]where smaller tube shows more reduction on HTC due to comparatively thicker of lubricant film. And test results for mini-channels from Yun et al.[101]also revealed similar characteristics.