2. Effect of lubricant on the heat transfer of conventional refrigerants
2.2. Effect of lubricant on convective boiling
Similar to the influence of lubricant on the pool boiling heat transfer performance, the existing data concerning the effect of lubricant on convective boiling is also inconclusive. The presence
#1, 1435 fins/m
#3, 1696 fins/m
#2, 1561 fins/m
#4, 1761 fins/m
Fig. 5. Test results for Ji et al.[21](a) Schematic of the enhanced tube structure by Ji et al.[21], enhanced tube geometry; (b) Variation of the relative boiling heat transfer coefficient of R-134a/lubricant. HTC vs. oil concentration.
q = 100.8 kW/m2, TS = −5°C, ω = 0.75%
q = 50.4 kW/m2, TS = −5°C, ω = 0.75%
q = 33.4 kW/m2, TS = −5°C, ω = 0.75%
Fig. 4. Boiling pattern for Ts¼ 5 1C, ando¼0.75%[14].
of lubricant has two counter effects which normally offset with each other, resulting in certain inconsistency. First, lubricant promote wettability, causing refrigerant mixture to spread around the periphery and an early transition to annular flow pattern. This is especially beneficial upon stratified/wavy flow pattern. By contrast, the presence of lubricant increases the viscosity which normally lessen the degree of mixing of refriger-ant mixtures, leading to a decline of heat transfer performance.
There are also other important factors affecting the convective boiling performance and are summarized in the following.
Oil concentration effect: In general, the presence of lubricant oil above 1% in mass reduces the heat transfer coefficient and this degradation increases with the oil concentration. Lubricant oil at high concentrations (above 5%) drastically decreases flow boiling heat transfer. However, as aforementioned previously, some studies (for example, [32,33] and [34]) observed that some lubricant oils increase flow boiling heat transfer when the oil concentrations are less than 3%. The enhancement apparently depends on the type of lubricant oil, heat flux, flow rate, flow patterns, the type of tube and other parameters, yet the exact enhancement mechanism was not clearly identified. Worsoe-Schmidt [35]was the first one to study the effect of lubricant on the flow pattern. His test data depicted in Fig. 8showed an appreciable rise of heat transfer coefficient with oil concentration, and an heat transfer enhancement of 50% for R-12 with
o
¼1.9%at a saturation temperature of 15 1C. When
o
¼8.0%, the heat transfer coefficient presented a strong enhancement in the first meter of the test section, but above this length the HTC dropped drastically while the heat transfer coefficient for pure refrigerant remained unchanged. The significant rise of HTC of Worsoe-Schmidt’s data arises from two aspects. First, their test was performed at low mass flux (30–100 kg/m2s), a relatively low heat flux, and at a low temperature conditions which resembling pool boiling condition to some extent. From the foregoing discussion of the effect of lubricant on pool boiling, one can see that there exists a positive effect on HTC at a low evaporation temperature provided the concentration is comparatively low.Fig. 7. Photos of the metal foam tested by Zhu et al.[26].
Fig. 6. Variation in the relative boiling heat transfer coefficient of lubricant-mixed refrigerant reported in literatures ([21]) (a) plain/integral tubes; (b) Highly struc-tured surfaces.
Evaporator Length [m]
h ( W / m2K )
0 500 1000 1500 2000 2500
A - 0 % oil B - 1.9 % oil C - 3.8 % oil D - 8.0 % oil
4 3
2 1
0
Fig. 8. Local HTC as a function of evaporator length at 15 1C ([35]).
Second, the mass flux was quite low where stratified/wavy flow prevails as the major flow pattern, and this was also confirmed from his flow pattern observation. As a consequence, the rise of surface tension from lubricant addition normally promotes wet-ting characteristics and resulwet-ting in an early transition to annular flow pattern and a considerable rise of heat transfer coefficient accordingly. Third, with the rise of oil concentration, he also observed detectable foaming which may easily touch and wet the periphery and assists the heat transfer performance. The enhanced level caused by lubricant oil of Worsoe-Schmidt’s data is moderately higher than others[32–34]. This is probably due to its hairpin configuration which comprise a series of return bends.
The presence of return bend further accentuates the occurrence of annular flow pattern[36], prolonging the influence of lubricant at a higher enhancement level.
The early transition to annular flow pattern for refrigerant/oil mixture had been observed by Wongwises et al. [37] who conducted two-phase flow visualization experiments with R-134a with lubricant oil mixtures inside a 7.8 mm horizontal tube. A schematic of the progress of flow pattern subject to the influence of lubricant oil is shown inFig. 9. They found that the small amount of the liquid layer is pushed up around the tube wall perimeter by the momentum of vapor when the surface wave rolls along the flow direction, and the presence of foaming, when comparing to that of pure refrigerant, results in a higher effective liquid level that may easily reach the upper wall. This liquid level and the high viscosity of the attached ‘‘oil-rich’’ layer Kim and Katsuta[38]may result in an ‘‘annular-like’’ flow pattern.
It should be mentioned that Worsoe-Schmidt[35]and Manwell and Bergles[39]also reported that the oil was found to increase the wetted portion of the tube wall.
The above-mentioned results may imply an early transition from wavy to annular flow pattern at a lower value of UGS. According to Taitel and Dukler[40], the transition from stratified to intermittent or annular flow will occur when
UGS4 1HL
where D is the pipe diameter and HLthe depth of liquid in the pipe. From Eq. (2), with an increase of effective liquid level, one can see the transition boundary from wavy to annular flow region may be shifted to a lower value of UGS.
Viscosity Effect: The decrease in the convective evaporation due to the presence of oil can be attributed to the increased mixture viscosity and the oil mass transfer resistance effect. On the other hand, the high viscosity accelerates the formation of annular flow ([4]). This factor is likely to benefit the flow boiling at low and intermediate qualities, while it impairs the flow boiling at high quality. Hambraeus et al. [41] studied three ester-based lubri-cants mixed with R-134a. They observed that the largest flow boiling degradation corresponded to the lubricant with the largest viscosity. McMullan et al. [42] reported the flow boiling heat transfer of R-12 mixed with three lubricants. During the tests, the authors found that at an oil concentration of 1%, the overall evaporator performance degraded with an increase of the oil viscosity. However, at an oil concentration of 3%, the trend was reversed. The authors claimed that the reversed trend is associated with the contribution of surface tension which pro-vides better wetting, albeit increased viscosity impairs the heat transfer performance in both concentrations. The optimum oil concentration for heat transfer was determined by the trade-off between the reduced convective heat transfer and the increased wetted surface. In addition, the miscibility of the refrigerant mixture also plays certain role in this phenomenon. As depicted inFig. 10which is taken from Kim and Katsuta[38], despite the
presence of lubricant promote an early transition to annular which normally enhances the corresponding heat transfer, the upper liquid layer that is an oil-rich layer containing very few refrigerant. The flow pattern is known as tear flow pattern and were observed from some studies at high oil concentration (e.g.,[38] and [35]). In this regard, the heat transfer performance is comparable or even lower than the oil-free case even the flow pattern is similar to annular flow.
Mass velocity effect: Some authors verified that high mass velocities promote a more uniform refrigerant/oil mixture, which can reduce the performance loss caused by the lubricant oil and lessen its non-equilibrium effects. Many researchers had reported that HTC increased with the mass velocity, implicating the same behavior as seen in pure refrigerants. It is also important to note that at low mass velocities (stratified/wavy flow), the foaming formation happens on the liquid–vapor interface. In the mean-time, at high mass velocities (annular flow), it is possible to observe froth flow like those observed by Wongwises et al.[37].
Vapor quality effect: Since the lubricant oil is a nearly non-volatile component, its partial pressure in the vapor phase is usually neglected. The lubricant oil remains in the liquid phase and its concentration increases with vapor quality as the refrig-erant evaporates. With a nominal lubricant oil concentration around 5.0%, at the end of the evaporation process the local concentration can reach values of the order of 90% in the remaining liquid. Therefore, the increase of the viscosity of refrigerant/oil mixture and the non-equilibrium effect become rather significant. This becomes especially pronounced when the mass flux is low and high oil concentration regime where stratified/wavy flow pattern prevails. With the presence of tear like pattern, the oil-rich layer in the upper portion of the horizontal tube may impair the heat transfer performance con-siderably. Thome[43]observed that in many experimental results the effect of lubricant oil concentration was not important at vapor qualities below about 85%. On the other hand, in direct expansion evaporators all the refrigerant is supposed to be evaporated and hence the evaporation process passes through the high vapor quality region where the adverse oil effect is very profound. In fact, it is not feasible to completely drive all of the refrigerant out of the solution into the vapor phase unless elevated temperatures are applied ( 4300 1C).
Effect of geometry of tube: Since microfin tubes promote the annular flow pattern even at low mass velocities (or pseudo annular, because the grooves tend to convey liquid to the upper regions of the tube, promoting its wetting and the flow pattern thus has a close resemblance with the annular flow pattern in smooth tubes, with a liquid layer of thickness possibly higher than that of the microfin covering the whole surface of the tube), the presence of lubricant oil in the refrigerant can lose its benefit to induce annular flow as observed in smooth tubes. Ha and Bergles[44]performed experiments for R-12/oil mixtures in an electrically heated microfin tube with
o
ranging from 0% to 5%.They found a much larger HTC degradation for microfin tube. On the other hand, Nidegger et al.[45]also reported an appreciable falloff of HTC for microfin tube at the mass flux of 100 kg/m2s with the presence of lubricant oil, but the trend is reversed where oil increased the HTC relative to pure R-134a when tested at the highest mass flux (G ¼300 kg/m2s). They argued that the degra-dation of HTC at low mass flux is associated with the holdup of the lubricant oil at the microfins. The comments seem feasible for it also can explain the reversed trend shown in higher mass flux.
And this degradation is even worse for highly porous coated surface as reported in the recent study by Dawidowicz and Cies´lin´ski[46].
Correlations to predict the convective boiling heat transfer coeffi-cient subject to influence of lubricant: Hu et al. [47]provided an
extensive comparison of their measured R-410A/oil data against existing correlations. These correlations are all empirically based and the correlations were developed based on a specific database,
extending the applicability outside their database is generally not recommended. One of the major reasons of the limited applicable range of the existing correlations is due to lack of rational Wavy flow (W)
Plug flow (P)
Flow direction
Slug flow (S)
Froth/wavy flow (F/W)
Froth/wavy/annulular flow (F/W/A)
Foth flow (F)
Wavy/annular flow (W/A)
Annular flow (A)
Fig. 9. Sketch of flow patterns subject to influence of lubricant[32].
parameters. For instance, as aforementioned in previous section that the presence of lubricant may affect the flow pattern, thereby affecting the heat transfer performance. In this regard, Hu et al.
[47]developed a flow pattern map for R-410A/oil mixture which was originally developed by Wojtan et al. [48,49] using the mixture properties to replace the refrigerant properties. They found that the presence of oil promotes the transition from ‘‘Slug’’
to ‘‘Intermittent’’, while it delays the transition from ‘‘SlugþSW (stratified wavy)’’ to ‘‘Slug’’, ‘‘Intermittent’’ to ‘‘Annular’’, from Annular’’ to ‘‘Dryout’’ and from ‘‘Dryout’’ to ‘‘Mist’’. Using the information, they developed a correlation that can correlated their measurements for the 4.18 mm horizontal smooth tube with a deviation of 730%, and it agrees with 96% of their experimental data in the 6.34 mm horizontal smooth tube, within a deviation of 720%. However, it should be noted that the correlation is, again, applicable to their own database. Some important effects, like surface tension, locally oil-rich tear flow pattern, and the like are still missing in the correlation. In summary, there is no well accepted correlations available that can describe the influence of lubricant on the convective boiling heat transfer performance up to now.