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 Micha
2、el J. Brandemuehl, Ph.D., P.E., Professor, Joint Center for Energy Management, University of Colorado, Boulder, USA James E. Braun, Ph.D., P.E., Associate Professor, Ray W. Herrick Laboratories, School of Mechanical Engineering, Purdue University, West Lafayette, Indiana, USA Alberto Cavallini, Ph.D
3、., Professor, Dipartmento di Fisicia Tecnica, University of Padova, Italy Arthur L. Dexter, D.Phil., C.Eng., Reader in Engineering Science, Department of Leon R. Glicksman, Ph.D., Professor, Departments of Architecture and Richard R. Gonzalez, Ph.D., Director, Biophysics and Biomedical Modeling Divi
4、sion, Anthony M. Jacobi, Ph.D., Professor and Associate Director ACRC, Department of Keith E. Starner, P.E., Engineering Consultant, York, Pennsylvania, USA Jean-Christophe Visier, Ph.D., Head, Centre Scientifique et Technique du Btiment, Energy Management Automatic Controller Division, Mame La Vall
5、e, France Engineering Science, University of Oxford, United Kingdom Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, USA U.S. Army Research Institute of Environmental Medicine, Natick, Massachusetts, USA Mechanical and Industrial Engineering, University of Illinois, Urbana-C
6、hampaign, USA Policy Committee Editorial Assistant Stephen W. Ivesdal, Chair, Member ASHRAE P. Ole Fanger, Fellow/Life Member ASHRAE Ken-Ichi Kimura, Fellow ASHRAE John W. Mitchell, Fellow ASHRAE Frank M. Coda, Member ASHRAE W. Stephen Comck, Associate Member ASHME Kristie Blase W. Stephen Comstock
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9、HRAE. Abstracts-Abstracted and indexed by ASHRAE Abstract Center; Ei (Engineering Information, Inc.) Ei Compendex and Engineering Index; IS1 (Institute for Scientific Information) Web Science and Research Alert; and BSRIA (Building Services Research informa- tion on the corresponding experimental pa
10、rameters is given in Table 2. Bryan and Seyed- Yagoobi (1 997, 2000) showed that enhancements were highly dependent on the quality, flow regime, heat flux, mass flux, and strength of the EHD forces relative to the flow momentum. The occurrence of the maximum enhancements shown in Table 1 for the dif
11、ferent researchers was primarily due to the electrode designs, the geometry of the heat transfer surface, and the operating conditions. However, both Bryan and Seyed-Yagoobi (1997, 2000) and Noms et al. (1997) showed that in many cases the EHD forces can drastically reduce the rate of heat transfer.
12、 In convective boiling with the electrode designs considered in these studies, the EHD force is primarily perpendicular to the bulk fluid motion, and its influence is strongly dependent on the variables mentioned previously and on the fluid electrical properties. The EHD force, which acts primarily
13、perpendicular to the fluid flow, can also result in increased flow resistance, thereby increasing the pressure drop. Bryan and Seyed-Yagoobi (2000) developed a simple theory to determine the mean radial EHD pressure, which included the characteristics of two-phase flow. Their analysis showed that th
14、e amount of heat transfer enhancement and the pressure-drop pen- alty depended on the size of the mean radial EHD pressure relative to the flow axial momentum flux rate. The ranges of EHD-enhanced heat transfer and pressure-drop penalty in convective boiling obtained by these researchers as well as
15、by Cotton et al. (2000) are shown in the results. EHD James E. Bryan is a professor in the Department of Mechanical and Aerospace Engineering, University of Missouri, Columbia. Jamal Seyed-Yagoobi is chair and professor in the Deparhnent of Mechanical, Materials, and Aerospace Engineering, Illinois
16、Institute of Technology, Chicago. 337 338 HVAC not constant along test section CElectric field based on radius; local electric field is different because of geometrical shape of surface dPower estimated from power ratio presented by researchers eEstimated from Reynolds number presented by researcher
17、s; based on cross-sectional area of microchannel fTest section heated by hot water gTest section heated by resistive electrical beater Table 2. Experimental Parameters Investigated by Various Researchers G X 4“ DO, DE, TSah range, range, range, Reference Tube mm L,m Electrode mm OC kg/(m2-s) kgkg kW
18、/m2 Smooth 10 3.75 Perforated tube 5 30 33and66 Oto0.9 4 Yabe et al. (1 992) Smooth 9.4 1.22 Cylindrical 3 25 50to400 O to0.5b 5 to20 Singh et al. (1994) Smooth 9.4 1.22 Cylindrical 3 25 50 to 400 O to 0.5b 5 to 20 Singh et al. (1995) Singh (1995) Microfin 12.7 0.3 Helical 9.8 25 50to 150 Ot00.8 5 t
19、o 30 Ohadi et al. (1995) 50and Oto0.6b 25 Microfin 12.7a 0.11 Cylindrical 9.5a 25 Salehi et al. (1996) Microfin 12.7 0.3 Helical 9.8 25 50to200 Ot00.8 5, 10 Bryan and 0.1, loo and O to 0.6 4 to 153 bMeasured at inlet to test section and different from test CHeat flux is averaged and is not constant
20、along test section. 300 Seyed-Yagoobi Smooth 15.9 0.2,0.3, Cylindrical 1.6 5,25 (1997) 0.5 ?4nnulus resulting from tube and cylindrical elec- trode is separated into 12 microchannels, each with a hydraulic diameter on order of 1 mm. section quality. enhancements are affected by the mass flux, qualit
21、y, flow regime, heat flux, operating tempera- ture, geometrical characteristics of the heat transfer surface and electrode, applied electric field, and fluid properties. In this paper, the effects of the EHD force on convective boiling are described based on the knowledge to date. Experimental resul
22、ts are presented to further explain the role of the heat transfer surface geometry and the refrigerant when the EHD force is applied. Additional results are provided to illustrate the transient effects that are much different from the steady-state effects. Finally, the applicability of the EHD pheno
23、mena to convective boiling is briefly discussed. VOLUME 8, NUMBER 4, OCTOBER 2002 339 EHD is an interdisciplinary phenomenon dealing with the interaction between electric fields and flow fields in a dielectric fluid medium. This interaction can result in electrically induced fluid motion and interfa
24、cial instabilities, which are caused by an electric body force. The electric body force density acting on the molecules of a fluid in the presence of an electric field consists of three terms (Melcher 1981): 2. 2.12 fe = p,E-E 2 The three terms in Equation (1) stand for two primary force densities a
25、cting on the fluid. The first term represents the force acting on the free charges in the presence of an electric field and is known as the Coulomb force. The second and third terms represent the polarization forces induced in the fluid. The electric body force density components defined in Equation
26、 (1) are responsible for the induced forces on the dielectric fluid medium. Two examples are A charged body in a nonuniform or uniform electric field will move along the electric field lines and impart momentum to the surrounding fluid. This Coulomb force is expressed by the first term in Equation (
27、1). If an interface exists, such as between a liquid and vapor in a nonuniform electric field, an attraction force is created that draws the fluid of higher permittivity (liquid) toward the region of higher electric field strength. This dielectrophoretic force results from the application of a nonun
28、iform electric field in the presence of a permittivity gradient. This force is described by the second term in Equation (1). All three components of the EHD force density can be significant, or one can dominate over the others. In convective boiling, all three forces will be present; however, the se
29、cond term of Equation (1) is expected to dominate as the quality and heat flux increase. EXPERIMENTAL METHODS The experimental apparatus, shown in Figure 1, is a closed loop that refngerant is pumped through with a positive-displacement gear pump instead of a compressor. R-134a and R-404A refngerant
30、s were used in all experiments. The experimental apparatus is described in more detail by Bryan (1 998). The four test sections of 10,20, 30, and 50 cm in length, shown in Figure 1, were connected in series and heat was provided to each by a recirculating hot-water loop. The different test sec- tion
31、s were used to allow measurement of the local heat transfer coefficients and heat fluxes. Each test section, as shown in Figure 2, consisted of a copper boiling tube surrounded by an acrylic tube inside diameter (ID) 38.1 mm that the hot water flowed through. The boiling tubes were either a smooth t
32、ube ID = 14.1 mm, outside diameter (OD) = 15.9 mm or a microfin tube (inside back-wall diameter = 14.6 mm, OD = 15.9 mm) and were oriented in a horizontal config- uration for all experiments. T-type thermocouples were soldered to the surface of the boiling tubes at the top, middle, and bottom in eac
33、h test section. In the 10 and 20 cm sections, the sur- face thermocouples were soldered to the tube surface at the center of the test section. In the 30 cm section, they were soldered in two stations at 10 cm intervals, and in the 50 cm section in three stations at 12.5 cm intervals. The surface the
34、rmocouples allowed measurement of the local heat transfer coefficient in each test section. The inlet and outlet water temperatures of each test section were measured with thermistors so that in all experiments the temperature difference across each test section could be accurately measured and main
35、tained to 1 K or less. The pres- sure drop across each test section was measured with a variable-reluctance pressure transducer. 340 HVAC Kandlikar 1990; Torikoshi and Ebisu 1993; Wattelet 1994; Wattelet et al. 1994) to veri the quality of the data acquired in this work. The electrode did influence
36、the heat transfer and pressure drop without the application of EHD. The heat transfer was enhanced from a few percent up to about 25%, with the high enhancements occurring at mass fluxes above 250 kg/(m2.s). The pressure drop increased about 20 to 25% because of the presence of the electrode and was
37、 effectively independent of the mass flux between 100 and 400 kg/(m2-s). For complete details, refer to Bryan (1998). RESULTS AND DISCUSSION EHD Enhanced Convective Boiling-Qualitative Discussions The qualitative discussions given here are supported by the recent theoretical and experimen- tal work
38、of Bryan and Seyed-Yagoobi (1997,2000). The EHD force in convective boiling is pri- marily perpendicular to the bulk fluid motion, especially with the electrode design considered in this study and other previous studies. The influence of this force is strongly dependent on qual- ity, flow regime, he
39、at flux, mass flux, fluid properties, electrode and surface geometry, and the applied voltage. The saturated liquid begins to change phase as it enters the evaporator tube. The fluid transi- tions from single-phase liquid through the different two-phase flow regimes. Nucleate boiling and convection
40、are the primary modes of heat transfer to the fluid. The contribution of these two heat transfer mechanisms varies depending on the fluid properties, the mass flux, the quality, and the heat flux. Because the flow is in a horizontal configuration, the mass flux will influence the distribution of the
41、 liquid and vapor in the tube. The distribution of the liquid and vapor will substantially influence the local heat transfer. At low mass fluxes, the flow will be wavy- stratified. As the mass flux increases, the amount of stratification will decrease and the liquid VOLUME 8, NUMBER 4, OCTOBER 2002
42、343 will become more evenly distributed around the tube wall as the flow transitions to the annular regime. The pressure drop will also be significantly influenced by the two-phase flow. The inter- action between the two phases will drastically increase the frictional losses in the flow. The momentu
43、m of the flow over a given tube length will increase as more energy is added to the flow. The increases in both the flow friction and flow momentum will significantly increase the pressure drop. When an electric field is applied in the convective boiling process, the EHD force density components, as
44、 defined in Equation (i), produce some interesting behavior. In the bubbly and plug flow regimes, the electric force generates secondary motions in the liquid. The polariza- tion component of the electric force also acts on the bubble at the liquid-vapor interface to move it around (because of the n
45、onuniformity in the electric field distribution caused by the presence of a bubble) and break it apart. At this point, the EHD force density components are enhancing the heat transfer; because their influence is significant, they are also generating a substantial increase in the pressure drop. Howev
46、er, the heat flux can play a significant role in the amount of EHD enhancement, and potentially lead to heat transfer suppression. The bub- bles are held around the tube surface by the polarization forces. Because the bubbles are of lower permittivity than the surrounding liquid, they will be driven
47、 to the region of lowest elec- tric field strength, which is the tube wall. If the heat flux is high enough, the significant bubble nucleation will start to increase the resistance to heat transfer at the wall, leading to heat trans- fer suppression. When the flow transitions to the annular regime,
48、the role of the EHD force density compo- nents change. The Coulomb and polarization forces still generate secondary motions in the liq- uid, but in this regime there is less liquid to act on because more energy has been added to the flow. The polarization forces start to play a more significant role
49、 at this point in the flow. The amount of heat transfer enhancement or suppression as well as pressure drop will depend on the strength of the EHD forces relative to the flow momentum. If the polarization forces only thin the liquid layer at the tube surface, heat transfer can be significantly enhanced. However, sup- pression will occur when the polarization forces hold the vapor bubbles on or near the tube wall as they form and start to remove the thin liquid layer from the tube surface. Again, the heat flux will also play a ro
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