1、 International Journal of Heating, Ventilating, Air-conditioning and Refrigerating Research Editor Reinhard Radermacher, Ph.D., Professor and Director, Center for Environmental Energy Engineering, Department of Mechanical Engineering, University of Maryland, College Park, USA Associate Editors James
2、 E. Braun, Ph.D., P.E., Professor, Ray W. Herrick Laboratories, Alberto Cavallini, Ph.D., Professor, Dipartmento di Fisicia Tecnica, University of Padova, Italy Qingyan wan) Chen, Ph.D., Professor of Mechanical Engineering, School of Mechanical Engineering, Purdue University, West Lafayette, Indiana
3、, USA Arthur L. Dexter, D.Phil., C.Eng., Professor of Engineering Science, Department of Engineering Science, University of Oxford, United Kingdom Srinivas Garimeia, Ph.D., Associate Professor and Director, Advanced Thermal Systems Laboratory, Department of Mechanical Engineering, Iowa State Univers
4、ity, Ames, Iowa, USA Leon R. Glicksman, Ph.D., Professor, Departments of Architecture and Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, USA Anthony M. Jacobi, Ph.D., Professor and Co-Director ACRC, Department of Mechanical and industrial Engineering, University of Illinoi
5、s, Urbana-Champaign, USA Bjarne W. Olesen, Ph.D., Professor, International Centre for Indoor Environment and Energy Technical University of Denmark Nils Koppels All, Lyngby, Denmark Jeffrey D. Spitler, W.D., P.E., Professor, School of Mechanical and Aerospace Engineering, Oklahoma State University,
6、Stillwater, Oklahoma, USA School of Mechanical Engineering, Purdue University, West Lafayette, Indiana, USA Editorial Assistant Lori Puente, CEEE OfficelMechanical Engineering, University of Maryland (301-405-5439) Policy Committee Special Publications Staff Daryl Boyce, Chair, Member ASHRAE P. Ole
7、Fanger, FeIlow/Life Member ASHRAE Curtis O. Pedersen, Fellow ASHRAE Reinhard Radermacher, Member ASHRAE Jeff Littleton, Associate Member ASHRAE W. Stephen Comstock, Associate Member ASHRAE Mildred Geshwiler, Editor Erin S. Howard, Associate Editor Christina Helms, Associate Editor Michshell Phillips
8、, Secretary W. Stephen Comstock Publisher 02005 by the American Society of Heatina, Refrigerating and Air- passages or reproduce illustrations in a review with appropriate credit; Conditiohg Engineers, Inc., i791 Tullie Circle, Atlanta, Georgia 30329. All rights reserved. Periodicals postage paid at
9、 Atlanta, Georgia, and additional mailing offices. KVAC Ei (Engineering Information, inc.) Ei Compendex and Engineering Index; IS1 (Institute for Scientific information) Web Science and Research Alert; and BSRiA (Building Services Research Class 3 is a more loosely controlled environment that houses
10、 workstations, PCs, and portables; and Class 4 is for point-of-sales equipment with virtu- ally no environmental control. Second, this publication also outlined locations for temperature and humidity measurements within a data center to verifi that environmental conditions are acceptable. Third, rec
11、ommendations are provided on the arrangement of server racks within the data center, as well as airflow patterns within the racks themselves. Finally, a reporting format is generated for server manufacturers to follow in providing environmental information for their servers. Roger R. Schmidt is a di
12、stinguished engineer in the Systems and Technology Group, IBM Corporation, Poughkeepsie, NY. 339 340 HVAC accepted September IO, 2004 Part II of this article will be published in Volume II, Issue 4, October 2005 The influence of refrigeration lubricants on pool andjlow boiling of refrigerant-oil mix
13、tures is a complex subject, and a consistent relationship cannot be identijed. Instead, the influence varies greatly depending on oil concentration, operating parameters, and application. This paper intends topresent a comprehensive summary of the various studies that have been conducted in this are
14、a and tries to identifi some general relationships. In addition, the research methods and correlations presented on each of these subjects are summarized. Finally, technical recommen- dations regarding the lubricant influences on these aspects are provided in this paper. INTRODUCTION The most pronou
15、nced properties of the refrigerant-oil mixture are its high viscosity and the preferential evaporation of the pure refrigerant in comparison to the lubricant. Due to the prefer- ential evaporation of the refrigerant, the mixture bubbleldew-point temperatures increase with local oil concentration, si
16、milar to a zeotropic refrigerant mixture. The lubricant exists only in the liquid phase and the concentration of the lubricant in the liquid phase varies greatly as a function of quality. To study a refrigerant-oil mixture actually means to study the liquid phase of the mix- ture and to study the lu
17、bricant influence as a local behavior. In addition, due to the preferential evaporation of the refrigerant, the lubricant presence causes local oil accumulation. For exam- ple, oil-rich layers may form at the solid-liquid interface and liquid-vapor interfaces. The behav- ior of local oil accumulatio
18、n is especially important for pool boiling and partially miscible refrigerant-oil mixtures. A good methodology to evaluate the influence of lubricants should be applicable to any occasion. However, when local oil accumulation occurs, the lubricant influ- ence tends to depend on the special interacti
19、on of a refrigerant-oil pair. Due to its large viscosity and mass transfer resistance effect, the lubricant tends to decrease the heat transfer and increase the pressure drop of rehgerant in most cases, especially at high lubricant fraction. However, the opposite phenomena have been observed in some
20、 cases at low oil concentrations, generally around 2%-3%. This leads to the fact that the lubricant influence on refrigerant heat transfer and pressure drop is a complex subject, and no consistent agreement has been reached to date. Thome (1996, 1998) conducted a state-of-the-art review on the boili
21、ng of refrigerant-oil mix- tures. The author summarized that there are two typical methods to study the lubricant influence. Bo Shen and Eckhard A. Groll are at Purdue University, Ray W. Herrick Laboratories, West Lafayette, Ind. 341 342 HVAC one tube was called a W-40-fpi tube; one was called a W-S
22、C tube, which had an enhanced condensation tube surface with “Y-tipped” fins; and the last tube was called a Tu-b tube, which had an enhanced pool boiling surface. The authors reported that the “Y-tipped sur- face structure tended to hinder the foaming from wetting the gap between two adjacent fins.
23、 Thus, the oil presence was less beneficial to this kind of tube surface. Contrary to all other tube sur- faces, the evaporation of refrigerant-oil mixtures on the Tu-b surface decreased with increased heat flux, although the evaporation of the pure refrigerant still increased with heat flux on this
24、 tube surface. The authors attributed this to the occurrence of local dryout at high heat flux, which might be due to the oil presence. In Moeykens and Pate (1996a), investigation of spray evapora- 344 HVAC (2) the foam promotes secondary pool boiling; and (3) the foam may be useful in removing the
25、oil from the heated surface. According to this theory, there is an exceptional occasion during which it is possible that the oil-rich film reduces the liquid-gas interface surface tension, as discussed by Mitrovic (1998). This is possible when the oil contains some surface-active component. It appea
26、rs that this representative theory puts emphasis on the liquid-gas interface. This theory appears only to be a hypothesis and it may be problematic since Kedzierski (1999) implied that not all of the pool boiling enhancement could be related to the foaming or the surface additive component. The othe
27、r representative theory to account for the oil influence on the refngerant pool boiling was put forward by Kedzierski. Kedzierski attributed the pool boiling mechanism to the varia- tions of bubble size and bubble number. Kedzierski (2000b) attempted to explain the pool boil- ing mechanism of refrig
28、erant-oil mixtures by introducing the concept of an oil excess (oil rich) layer at the heated surface. Kedzierski stated three possible reasons for lubricant enhancing pool boiling: 1. The oil excess layer is able to reduce the solid-liquid interaction. Then the solid-liquid oil-rich layer leads to
29、a reduction in bubble size and an increase in bubble frequency. 2. The high oil viscosity induces a thicker thermal boundary layer at the heated surface. The thicker thermal boundary layer can increase the site density to activate the bubbles. 3. The oil partial miscibility might contribute to the e
30、nhanced boiling. When a partial miscible refiigerant-oil mixture boils at the temperature close to the critical solution temperature, there are two liquid films enveloping the bubble, an oil-rich film and a refrigerant-rich film. The interface of the two films has a large curvature gradient, which l
31、eads to a great film pressure gradient. The superheated liquid may be moved to the bubble side by the pressure gradient. As a result, the bubble superheat increases, and, thus, the nucleate boiling is enhanced. In summary, whether a lubricant enhances the nucleate boiling or not depends on the balan
32、ce of the reduced bubble size and the increased bubble number. Due to the preferential evaporation of the refrigerant, an oil excess layer exists during the pool boiling. In the test of Kedzierski (2002b), the existence of a lubricant excess layer was detected using a fluorescent measurement techniq
33、ue, which utilized the distinctive fluorescent properties of the lubricant and the refrigerant to obtain the thickness of the oil excess layer. The experimen- tal results proved that the excess layer thickness decreased with increasing heat flux because the larger heat flux activates larger bubble s
34、ite density, so that the leaving bubbles remove more VOLUME 11, NUMBER 3, JULY 2005 345 lubricant. The author stated that the balance of the oil removal and deposition determines the lubricant excess layer thickness. Based on the concept of the oil excess layer at the solid-liquid interface, Kedzier
35、ski (2001) pointed out that there are three key factors to impact the pool boiling. They are the lubricant mass fraction, the difference between the oil viscosity and the refrigerant viscosity, and the dif- ference between the refrigerant-oil saturation temperature and the critical solution temperat
36、ure. Kedzierski (2003) developed a semi-empirical correlation for predicting the heat transfer of refrigerant-oil pool boiling, based on three assumptions: The oil excess layer on the heated surface is assumed to be totally composed of the lubricant. The lubricant in the oil excess layer is assumed
37、to be removed by the bubbles, and the removal rate is proportional to the excess layer thickness and the bubble diameter. In addi- tion, the mixture mass that leaves the heated surface is assumed as the refrigerant vapor in the bubble. Thus, the same amount of mixture mass needs to flow back to the
38、heated surface to maintain the mass balance. In the thermal boundary layer on the heated surface, the temperature distribution in the oil excess layer is assumed to be linear, and the temperature distribution outside the oil excess layer is assumed to be exponential. The proposed refrigeranthbricant
39、 mixture nucleate boiling correlation is shown in Equation 1 : where h, is the mixture boiling coefficient, ob is the bulk oil mass fraction, p0,l is the lubricant density, hfg is the latent heat of vaporization, AT, = T, - T, is the wall superheat degree, T, is the wall temperature, T, is the satur
40、ation temperature of the mixture, ko,l is the lubricant liquid ther- mal conductivity, (3 is the liquid-vapor surface tension of the refrigerant, and Yb is the detach- ment bubble radius, which is determined by solving the oil mass balance; h is the thermal boundary constant, which was obtained from
41、 the experimental data of the R-123/oil mixture as a function of the detachment bubble radius Yb and the ratio of the refrigerant-lubricant heat flux to the pure refrigerant heat flux at the same superheat degree; I, is the thickness of the oil excess layer. Kedzierski (2002b) proposed a new physica
42、l quantity for the refrigerant-oil nucleate boiling, which is the excess surface density, . r represents the mass of lubricant per unit surface area minus the lubricant of the bulk mixture in the same volume; Pb is the mixture bulk density. Kedzierski was able to measure this quantity with his new f
43、luorescent measurement technique. Plotting the measured r against led to a constant, which was expressed as follows: 346 HVAC Hahne and Noworyta (1984) for R-11 mixed with four lubricants; and Jensen and Jackman (1984) for R-1 l/oil and R-l13/oil mixtures. However, all of these correlations can only
44、 be used for a particular refrigerant-oil pair. LUBRICANT INFLUENCE ON FLOW BOILING reviewed within this paper with respect to the lubricant influence on flow boiling. Factors Affecting Refrigerant-Oil Flow Boiling Effect of Flow Pattern. The lubricant can impact the flow boiling through a change of
45、 the flow pattern. The increased mixture viscosity and surface tension promotes the early formation of annular flow. Due to foaming, the presence of bubbles increases the volume occupied by the fluid. The increased fluid volume is more effective to wet the heat transfer surface. As known, microfin t
46、ubes and large mass fluxes also promote annular flow. These two factors in addition to the presence of oil lead to extra complexities. Manwell and Bergles (1994) observed the flow pattern of R-12/300-SUS oil in a smooth tube. The authors proved that the refrigerant-oil mixture reached annular flow a
47、t a lower mass flux in comparison to the pure refrigerant. Wongwises et al. (2002) studied the lubricant influence on the flow pattern transition of R-l34a/PAG oil in a smooth tube. The authors reported basically the same results as observed by Manwell and Bergles (1 994). Since microfin tubes activ
48、ate annular flow automatically, the presence of oil may reduce its effect to promote annular flow. Manwell and Bergles (1 994) reported that the microfin tube and the presence of oil both promoted annular or semi-annular flow, compared to pure refrigerant in a smooth tube. However, the microfin tube
49、 suppresses the foaming. Ha and Bergles (1993) found that the oil presence increases the wetted surface only at very low mass fluxes in a micro- fin tube. In their tests, the microfin tube was observed to suppress the foaming behavior as well. Yoshida et al. (1991) and Yoshida and Matsunaga (1991) observed the enhanced flow boiling of R-22/3GS oil in both the smooth tube and microfin tubes. The authors implied that the foaming behavior was not as effective to increase the wetted surface in the microfin tube as in the smooth tube. In comparison to th
