行政院國家科學委員會專題研究計畫成果報告
具電漿塗佈及外覆線圈之熱傳增強管/及管束池沸騰現象研究 (含 LDV 雷射量測氣泡速度及管排之最佳化)(I)
Satur ated Nucleate Pool Boiling from Plasma Coating with
Wire Coil Enhanced Tubes: The measur ements of bubble velocity using a single-Photo/LDV method as well as the optimization of the tube ar r angements
計畫編號：NSC 89-2212-E-110-015
執行期限：八十八年八月一日至八十九年七月三十一日 主持人：謝曉星教授 國立中山大學機械系
計畫參與人員:楊宗穎 黃國禎 柯重光 Abstr act
Pool nucleate boiling heat transfer
experiments from coated surfaces with porous copper(Cu) and molybdenum(Mo) and spirally wrapped with helical wire on copper surfaces with micro-roughness immersed in saturated R134a and R-600a were conducted. The influence of coating thickness, porosity, wrapped helical angle, and wire pitch on heat transfer and
boiling characteristics including bubble
parameters were studied. The enhanced surface heat transfer coefficients with R-600a as refrigerant found are 2.4 times higher than those of the smooth surface. Furthermore, the heat transfer of the boiling process for the present enhanced geometry was modeled and analyzed. The experimental data for plasma coating and spirally wrapped surfaces were correlated in terms of relevant parameters, respectively to provide a thermal design basis for engineering applications.
Keywords: Nucleate Pool Boiling, Coated and Spirally Wrapped Tubes, R-134a and R- 600a Refrigerants
1 Introduction
Many water chillers of the centrifugal type have evaporators utilizing a flood type of operation whereby the water is circulated through the tube and refrigerant evaporated on the shellside of the tubes which is an area of nucleate boiling. While designing the evaporator of such a system, one must be able to accurately predict the boiling heat transfer coefficients of the refrigerants used. However, the prediction of the heat transfer coefficient is difficult because
the boiling phenomenon is rather complex and is influenced by many variables, such as surface conditions, heater size, geometry, material, and refrigerants, etc.
Various methods of enhancing nucleate boiling heat transfer have been described by Thome [1] which provided a comprehensive
survey, and discussed the fundamental
phenomena of boiling on enhanced surfaces. These surfaces can take a number of forms from simple low integral fins with varying fin profile to more complicated re-entrant cavity type surfaces such as structured and porous coated surfaces. Although a considerable amount of published data exists in the literature on nucleate boiling enhancement, little study has been done on the effects of coating/painting techniques as well as spirally wrapped on these surfaces. Recently, Hsieh and Weng [2] reported a study of nucleate pool boiling from coated surfaces in saturated R-134a and R-407c.
The objective of this work is to develop a surface treatment with coating and wrapped process that will provide high heat transfer rates in nuclear pool boiling and to enlarge the pool boiling date for alternative refrigerants of R-134a and R-600a.
2 Exper imental Setup and Procedure
2.1 Test Facility and Test Section
The experimental apparatus for the study in shown in Fig.1. It consists rectangular container with the dimensions of 300 × 370 × 200mm made from stainless steel, a stainless steel side panel provided with ports for electric wires, a pressure gauge and thermocouples, a vacuum pump, a reflux condenser, auxiliary heaters, and
a test section support. The copper tubes were 20 mm in outer diameter, with an inner diameter of 12 mm. Each cartridge heater was 220 mm long with an actual heated length of 210 mm and 11.95 mm in diameter with a maximum power output 378W and was inserted into the copper tube. The test section included both smooth and treated surface. The dimensional specifics of the surfaces treated and given in Table 1.
The eight test specimens studied had the general characteristics shown in Fig. 1. Four holes of 1.2 mm diameter and 55 mm deep were drilled at each end if the tubes at 90οintervals with the axis within about 1.2 mm of the smooth surface for insertion of wall temperature thermocouples. The tubes with coating surfaces were provided by Metal Industrial Research and Development Center of Taiwan.
Video imaging system was used to record and capture the images of vapor bubble above the heated surface. This system (JVC Model CCD Gr-DVM70) is capable of shutter speeds up to 1/500th of a second and can provide images at a speed of 30 frames per second with an aperture of F8. The lighting provided by one LPL-BROM CINE 500W floodlight was filtered through a diffuser for the video photography. Video images were synchronized and transferred to an IBM 586 PC, where they were digitized and analyzed by image processing software. 2.2 Working Fluid Used
The working fluid enhancement technique and its preparation used in the present experiments were R-600a and R-134a. The properties of R-600a or R-134a supplied by Imperial Chemical Industries Chemicals & Polymers Limited and American Society of Heating, Refrigeration and Air Conditioning Engineers are given in Table 2. During all the tests, the saturation temperature was kept near 18 ℃. It is found that there is a higher latent heat value for R-600a as compared to that of R-134a. So is the surface tension. Using R-600a to replace R-134a seems promising in the near future if the flammability problem of the R-600a can be solved.
2.3 Enhanced Surface Preparation
Two types of Enhancement techniques were used; one is plasma coating and one is spirally wrapping. For plasma coating, a scanning
electron microscope (SEM) image of several typical treated surfaces including smooth surfaces is depicted in Fig. 2.
The substrate material used in the present study is copper. The surfaces of the test copper tubes were first polished by an emery paper. Plasma spraying of Cu(k=401W/mK) and Mo(k=138W/mK) was carried out using PT-F4 plasma gun at 28-52.5 kW power for the purpose of observing any effect that may be produced by differences in thermal conductivity of the substrate material.
2.4 Experimental Procedure
Prepared test sections were cleaned with chlorinol and water and finally, with acetone. The tank was cleaned with acetone before each run. Once the evaporator tube was installed, the system was evacuated to a pressure of about 30 Pa. If no leaks were detected over a 24 hour interval.
The power was given to the pool to degas the test fluid, R-134a and R-600a, at heat flux of
30 kW/m2
for 1.5 hour and 1 hour, respectively. The power supplied to the test section was gradually, and, slowly, reduce to zero. The test pool was maintained close to the saturation temperature with an auxiliary heater for about 40 minutes; then it was switched off to minimize convective effects. The heating power supplied to the test section was slowly and gradually increased to nearly 30 kW/m2. Both increasing and decreasing heat flux data were taken in order to obtain more accurate data and to observe boiling hysteresis.
3 Data Reduction and Uncer tainty Analysis
For each power input, the heat transfer coefficient was calculated on the basis of bulk fluid saturation temperature, tube heat flux, and the average of the four tube wall temperatures. The heat transfer coefficient at each power input
was then calculated following
(
)
[
ATavg Tsat]
Q
h = − when A is the heated area of
the tube.
Using the method of Kline and McClintock [4], uncertainty estimates were made for the heat flux and temperature measurements. The uncertainty in the wall superheat was dominated by the uncertainty in the wall temperature measurements. The value of the four wall temperatures were recorded and compared for
examine variations caused either by nonuniformities in the cartridge heater or by the test tube soldering and assembly procedure. Wall superheat uncertainty can be attributed primarily to thermocouple calibration (±0.1°C) and temperature correction from the thermocouple reading to the reference surface. The maximum variation of the four measured wall temperatures
was ±0.3οC at the maximum heat flux
(≅30kW/m2
). The uncertainty in the saturation temperature was estimated to be less than
±0.1°C.
Substrate conduction heat losses were quantified at different heat flux conditions by solving three-dimensional conduction problems with a finite-difference solver. This loss varied between 10.2% and 0.2% for heat flux conditions between 0.8kW m2 to 30kW m2, respectively. The other primary contributor to heat flux uncertainty was heater surface area. Combining these effects lead to overall uncertainty estimates in heat flux of 11.2% at the lowest heat input.
4 Results and Discussions
A typical result of JSM-6400 SEM examination for the surface images (×1000 and
×5000) of smooth and coated tubes is illustrated in Fig.2(a)-(j), respectively. The reference surface (smooth) has a sparse number of small cavities (<0.5 mµ ) for both image (×1000 and
×5000). Fig.2(c)-(f) illustrates the SEM image
(×1000 and ×5000) with Mo coated tubes from
top and side view. The distribution appears
rather random in these micrographs(×5000).
The microstructures are layed with a total thickness ≅300 mµ and results in a cavity size of about 3 mµ . Again, surface orientation is random, some lying vertically and some
horizontally, which produces a porous
microstructure with a mean pore diameter of 3 mµ and a porosity of 0.053 with Cu coated. Fig.2(g)-(j) indicates a higher mean pore diameter (4 mµ ) and a higher porosity of 0.057 but a lower value of thickness of porous layer (≅100 mµ ). The multi-layered porous structures result in increased nucleate sites above the substrate (base) material as shown in Fig.2(c)-(i) which are believed to provide re-entrant cavities and to have a large variation in pore size and shape. Three smooth tubes and two coated tubes
were also wrapped with 0.1mm dia. copper wire.
The helical angle was defined as
o 1 d / p tan π = θ −
, where d is the smooth tubeo outside diameter and p is the pitch of wire
(è =2ο, p=1.97mm; è =1.5ο, p=1.48mm; and
è =1ο, p=0.99mm)
4.1 Boiling Characteristics
The present heat transfer characteristics were governed by porous layer thickness, the pore diameter, the surface porosity, the contact angle and their complex effect for the tubes with plasma coating; while for spirally wrapped tubes, the helical angle and pitches of the wire and their combined effect have something to do with the heat transfer behavior. Figure 3 compares pool boiling data for the smooth and coated surface as well as helical wire wrapped tubes at identical bulk liquid conditions for R-600a and R-134a, respectively, as shown in Figs. 3(a) and (b). Also included in Fig. 3(a) are the results for water in natural convection ( Junkhan and Bergles [5] ) and nucleate boiling ( Carey [6] ) regimes, respectively for comparisons. Like Hsieh and Weng [2], for coated tubes, the effects due to the contact angle and surface porosity seem the same. It appears, therefore, that the parameters with the most influence are the porous layer thickness and the pore diameter of the treated surface which determines the probability of flooding the reentrant cavities and the degrees of superheat required for bubble growth. The mechanism which described the boiling process from porous structures of the present plasma coating surfaces can be explained as follows; it appears that the heat is conducted to a liquid vapor at the upper surface of the porous structure. This conduction supposedly occurs through the matrix (see Fig.2 for details) formed by the solid portion of these wick and liquid in the porous spaces. On the other hand, for wrapped tubes, since the additional nucleation sites were created and the cross sectional area in the microstructure channel of the coated and smooth surface was reduced, the resulting boiling performances would be substantially improved. This is similar to the results found by Marto et al. [7]. The observed difference in the boiling curves are indications of variations in the surface microstructures between the smooth and enhanced surfaces. Again, like Hsieh and Weng [2], the data show
that the hysteresis phenomenon can be observed for both smooth and enhanced surfaces. However, the hysteresis effect of the coated surfaces seems stronger than that of smooth surface. Moreover, such phenomenon can be clearly noted in R-600a than that in R-134a.
The boiling curves in Fig. 3(a) show that the best heat transfer performance of the enhanced tubes one may obtain. This is because the coated (Cu and Mo) tubes with copper wire wrapped and helical angle θ=1° have more additional nucleation sites and, consequently, result in more vapor bubbles rising on the helical wire sides. Followed by the coated tubes (Cu and Mo) without wrapped wires and then, smooth tubes with wire wrapped. However, little difference in performance is discernible between Cu and Mo. For smooth tubes with helical wire wrapped, the heat transfer performance becomes lower as helical angle increases. Basically, there are at least two competing mechanisms which affect heat transfer performance for tubes with wrapped wires, namely, (a) that the wire on the boiling surface would prevent the bubble from freeing itself from the surface, which would inhibit the boiling, and this effect would become bigger with decreasing wire pitch; and (b) the boiling would be enhanced by refrigerant wetting capacity. Helical angle seems to have some influence on heat transfer performance. Similarly, Fig. 3(b) indicates the same trend as Fig. 3(a) does except R-134a as the refrigerant. Generally, the heat transfer performance in R-600a is bigger than that in R-134a due to a relative higher latent heat value and surface tension force for R-600a. Moreover, the enhanced tube as compared to the smooth surface does demonstrate lower temperature differentials for initiation of vaporization and lower ∆T's for equal heat fluxes. The slope of the boiling curves as shown in Fig.3 are usually bigger than the smooth surface slope indicating a situation where the enhanced surfaces are more effective in thermal transport than the smooth surfaces.
4.2 Heat Transfer Performance and Correlations Generally speaking, as shown in Fig.4(a) for R-600a the best heat transfer performance was found for Cu coated enhanced tube with copper wire wrapped, followed by Mo coated with copper wrapped enhanced tube, Cu coated
without wrapped, Mo coated without wrapped, smooth tube with copper wire wrapped and
helical angle è =1ο, 1.5ο and smooth tube
without copper wire wrapped and the least value
was found for smooth tube with è =2ο
wrapped tube. The same trend was found for R-134a as also shown in Fig.4(b). However, for Cu and Mo coated tubes, the thermal performance for wrapped cases seems no big difference. Also included in Figs.4(a) and (b) are the results from the correlation of Stephan and Abdelsalam [8]. The magnitude for present results seems much higher than those of [8]. However, the slope and trend as q increases appear the same.
Following Hsieh and Weng [2], a correlation for plasma coating tubes (without wrapped) of the present boiling data for both 600a and R-134a was developed. Similarly, the data for smooth tubes with wrapped wire were also
correlated in relevant parameters. Both
correlations are shown in Fig.5. The pressure effect was also included in terms of reduced pressure ratio Pr and a pressure function F(P) (Carey [6]) for completeness. For plasma coating heating surfaces, the conventional Jacob
number (Ja) defined as CPλ∆T/hfg was
correlated in form of Re (=qç /hfgì λå ) where
ç is average pore diameter and å is the
porosity), geometric scale factor ë ( =ç/ä ; ä is porous layer thickness) and the constant heat flux number Ncf. The correlation as shown in Fig.5 has the following form
Ja=0.041[Re]0.282[Pr]0.6F(P)[ë ]0.184[Ncf]0.067[Pr]1.65 (1) F(P)=1.8Pr 0.17 +4Pr 1.2 +10Pr 10
which is applicable to the plasma coating without wire wrapped heating surfaces used in R-134a and R-600a.
While, for the smooth tubes with wire wrapped, the heat transfer coefficient Nu(=hd/k) can be represented as also shown in Fig.5 in terms of Prandtl number (Pr), Reynolds number (Re=qd/ì λhfg), dimensionless pitch of wire (=p/dw; p is pitch and dw is the wire diameter), inverse buoyancy number (=ó /gd (2b ñ -λ ñ )); isv ó the surface tension and db is the bubble departure diameter), and helical angle (in radians)
è which has the following form Nu=29.8[Re]0.687[Pr]0.6[Pr]0.773[p/dw] 0.378 [ó /g 2 b d (ñ -λ ñ )]v 0.621 [è ]-0.263 (2)
and dbwas calculated based on db=0.146â [2ó /g(ñ -λ v
ñ )]0.5(Stephan and Abdelsalam [8] ) where
ο
35
â = for R-134a and R-600a. Furthermore,
in Fig.6, it is found that both correlations can predict 97 % of the data within
±
20 %.4.3 Boiling Visualization and Bubble Parameters Figure 7 is a photograph taken at 900W/m2 for s, s1, s2, and CM tubes in 134a and R-600a refrigerants. It shows the tube in the partially developed nucleate boiling regime except Fig. 7(h). At this stage, no bubble agglomeration occurred. The bubble size seems in R-600a bigger than that in R-134a as compared to Figs. 7(a) – (d) and the corresponding photograph for R-134a. A simple optical method was used to measure bubble parameters; namely, departure diameter and frequency. The bubbles with a measured area on the video-screen are counted easily. To compare with the data reported by Ammerman et al. [9], the same measurement area, 5.5 mm high by 18 mm wide, centered above the tube, was defined within each digitized photo using the image processor.
Figure 8 shows the ratio of number of bubbles (N/N'). N' was calculated (at q=121,000
W/m2
) from Ammerman et al. [9]. The number of bubbles reported (N) are averaged values, the
observed uncertainties being
±
8%. The datahave been taken at a heat flux q=600, 700, 800, 900, and 1,000 W/m2
, respectively and refer to individual, isolated bubbles, not influenced by their neighbours. Naturally, bubble growth after departure and bubble agglomeration were considered as possible sources of error.
In general, the number of bubbles increases as q increases. Different symbols are used to characterize different heating surfaces. The trend for all enhanced surfaces seems the same. However, for the magnitude, there appears two groups; tube Cu1, Cu, CM1 and CM is one group and tubes s, s1, s1.5, and s2 is another group for both R-600a and R-134a. The values of R-134a keep higher than those in R-600a.
Forthermore, it is also in good agreement with those of Ammerman [9]. Taking a closeup examination of Fig. 8, it is found that the present value of N/N' ratio can be up to 6 at
q
≅
1,000W/m2for Cu1 tube and the rate of increase with heat flux for all the tubes considered herein seems the same.
The data in Fig. 9 indicate that for all tube considered, the bubble diameter decreases as the heat flux increases. The departure diameters were generally found to be somewhat higher in R-600a and thus the frequencies to be somewhat lower. The measured and calculated departure diameters based on Zuber [10] are also listed in Table 3. Also listed in Table 3 are data for embryonic bubble radius calculated from Ps(Tw)-Ps(Ts)=2ó (Tw)/rb for comparison and reference. Again, as stated before, it is found that the
corresponding departure diameters and
embryonic bubble in R-600a are bigger than those in R-134a because R-600a has a little higher surface tension. Moreover, for enhanced surfaces, like coated and wrapped surfaces, have a smaller bubble diameters. Among those bubble diameters measured, Cu1 tube has the smallest bubble diameters and thus the highest frequency in both R-600a and R-134a as one would expect. Frequency increases rapidly versus heat flux due in part to the corresponding increases in a active nucleation site. Frequency values are shown versus heat flux in Fig. 9. Finally, the
relationship among frequency f, bubble
diameters (db) and ó g(ñ -λ ñ )/v ñλ 2
was found and it is shown in Fig.11. The traditional fdb
≅
constant is still valid in the present study. Actually, the present results were correlated as a function of ó g(ñ -λ ñ )/v ñλ2
as shown in Fig. 10. The average value of power for above-stated term was found about 0.23 which is very close to that of Zuber [10] (=0.25).
5 Conclusions
Pool boiling plasma coating and wire wrapped tubes for two different refrigerants (R134a and R-600a) at low and moderate heat flux was extensively studied. The results lead to the following conclusions.
1. Boiling heat transfer is enhanced by both thin layer porous matrix coated and wire wrapped on the heating surface, with an enhancement of up to 1.2~2.3 times. The geometric factor such as surface roughness
of coated surface and helical angle of the wrapped tube should be properly chosen to warrant a heat transfer enhancement.
2. The results have generally again confirmed the previous speculated mechanism [8.12] of boiling with porous metallic matrix surface coating. Namely, nucleation takes place within the matrix and in steady boiling, vaporization occurs within the matrix.
3. Correlations for thermal performance were
made for coated and wrapped surface each, respectively.
4. Through boiling visualization, the
photographs qualitatively as well as
quantitatively indicate that the presence of enhanced surface generates more active and stable nucleation sites in the vicinity of the porous matrix and wrapped wire. Bubble departure diameter in R-134a is smaller than that in R-600a. While for R-600a, the bubble frequency is much less than that in R-134a. Both number of bubbles and bubble frequency linearly increase with heat flux.
Reference
[1] Thome, J.R., 1990, Enhanced Boiling Heat Transfer, Hemisphere, New York, pp. 28-63.
[2] Hsieh, S.-S., and Weng, C.-J., 1997, “Nucleate Pool Boiling from Coated Surfaces in Saturated R-134a and R-407c,” International Journal of Heat Mass Transfer, Vol.40, pp. 519-532.
[3] Hsieh, S.-S., and Weng, C.-J., and Chiou, J.-J., 1999, “Nucleate Pool Boiling on Ribbed Surfaces with Micro-Roughness at Low and Moderate Heat Flux,” ASME Journal of Heat Transfer, 121, pp. 376-385. [4] Kline, S.J., and McClintock F. A., 1953,
“Describing Uncertainties in Single Sample Experiments,” Mechanical Engineering, Vol. 75, pp.3-8.
[5] Junkhan, C.H., and Bergles, A.E., 1976, “Heat Transfer Laborating Data Acquisition System,” Heat Transfer Laboratory Report HTL-12, ISU-ERI-Ames-77178, Iowa State University, Ames, Iowa.
[6] Carey, V.P., 1992, Liquid-Vapor Phase
Change Phenomena, Hemisphere
Publishing Corp. Washington D.C., pp. 233.
[7] Marto, P.J., Monlson, J.A., and Maynard, M.D., 1966, “Nucleate Pool Boiling of
Nitrigen with Different Surface
Conditions,” ASME Jounal of Heat Transfer, Vol. 90, pp. 437-444.
[8] Stephen, K., and Abdelsalam, M., 1980, “Heat Transfer Correlation for Natural Convection Boiling,” International Journal of Heat Mass Transfer, Vol. 23, pp. 73-87. [9] Ammerman, C.N., You, S.M., and Hong,
Y.S., 1996, “Identification of Pool Boiling Heat Transfer Mechanisms from a Wire Immersed in Saturated FC-72 Using a Single-Photo / LDA Method,” ASME Journal of Heat Transfer, Vol. 118, pp. 117-123.
[10] Zuber, N., 1963, “Nucleate Boiling, the region of isolated bubble and the similarity with natural convection,” International Journal of Heat Mass Transfer, Vol. 6, pp. 53-78.
Nomenclatur e
Cp = specific heat g = gravitational constant h = heat transfer coefficient hfg = latent heat
k = thermal conductivity P = pressure
Q = heat transfer rate q = heat flux Ra = surface roughness T = temperature ∆T = temperature difference Greek symbols å = porosity
ä = porous layer thickness ç = average pore diameter ì = dynamic viscosity õ = kinematic viscosity ñ = density Subscripts avg = average b = bubble λ = liquid sat = saturation v = vapor
