1、International Journal of HeatingJentilating, Air-conditioning and Refrigerating Research Volume 9, Number 2, April 2003 International Journal of Heating, Ventilating, Air-conditioning and Refrigerating Research Editor Reinhard Radermacher, Ph.D., Professor and Director, Center for Environmental Ener
2、gy Engineering, Department of Mechanical Engineering, University of Maryland, College Park, USA Associate Editors Michael J. Brandemuehl, Ph.D., P.E., Professor, James E. Braun, Ph.D., P.E., Professor, Ray W. Herrick Laboratories, Alberto Cavallini, Ph.D., Professor, Dipartmento di Fisicia Tecnica,
3、University of Padova, Italy Arthur L. Dexter, D.Phil., C.Eng., Professor of Engineering Science, Department of Leon R. Glicksman, Ph.D., Professor, Departments of Architecture and Richard R. Gonzalez, Ph.D., Director, Biophysics and Biomedical Modeling Division, Anthony M. Jacobi, Ph.D., Professor a
4、nd 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, Marne La Valle, France Joint Center for Energy Manageme
5、nt, University of Colorado, Boulder, USA School of Mechanical Engineering, Purdue University, West Lafayette, Indiana, USA Engineering Science, University of Oxford, United Kingdom Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, USA US. Army Research Institute of Environmen
6、tal Medicine, Natick, Massachusetts, USA Mechanical and Industrial Engineering, University of Illinois, Urbana-Champaign, 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
7、 ASHRAE Frank M. Coda, Member ASHRAE W. Stephen Comstock, Associate Member ASHRAE Baw Kurian, Publishing Manager Kristie Blase W. Stephen Comstock Mildred Geshwiler, Special Publications Editor Erin S. Howard, Assistant Editor Christina Helms, Assistant Editor Michshell Phillips, Secretary Publisher
8、 ASHRAE Staff 02003 by the American Society of Heating, Refrigerating and Air- Conditionine Engineers. Inc 1791 Tullie Circle. Atlanta. Georeia 30329. All rights reserved. Periodicals postage paid at Atlanta, Georgia, and additional mailing offices. passages or reproduce illustrations in a review wi
9、th appropriate credit; nor mav anv part ofthis book be reuroduced. stored in a retrieval system. or transmitted in any form or by any means-lectronic, photocopying, recording, or other-without permission in writing from ASHRAE. HVAC Ei (Engineering Information, Inc.) Ei Compendex and Engineering Ind
10、ex; IS1 (Institute for Scientific Information) Web Science and Research Alert; and BSRIA (Building Services Research one example is the MIT design advisor (2) that can be found in the tools section of the web site BuildingEnve- 1opes.org. Substantial effort is needed to develop and make available to
11、ols that meet all of these requirements to fruition. REFERENCES (1) Intrachooto, S., Technological Innovation in Architecture: Effective Practices for Energy Efficient (2) Lehar, M., MS thesis, Mechanical Engineering Department, MIT, 2003. Implementation, Ph.D. thesis, Department of Architecture, MI
12、T, June 2002. 110 HVAC however, there is still much to be understood. This paper presents a review of the literature containing experimental results from falling-film absorption of water vapor on horizontal tubes. The effects of surfactant, absorber geometry including advanced tube surfaces, tube di
13、ameter and spacing, and operating conditions such as liquid$lmflow rate and inlet conditions are systematically considered and the results of different investigations com- pared. Attempting to utilize the results found in the literature for design optimization reveals that the egects of many of thes
14、e important parameters are not fully quantified and so areas and methods for further research are suggested. INTRODUCTION In absorption heat pumps that employ water as the refrigerant and, for example, lithium bro- mide as the absorbent, falling-film absorbers employing internally cooled, horizontal
15、 tubes are widely used. This geometry is also frequently used in absorption cycles that use a volatile absor- bent, such as ammonia-water systems; however, lithium bromide-water systems are more preva- lent in larger installations. The bulk of the research on horizontal tube absorbers utilizes the l
16、ithium bromide-water fluid pair, and the phenomena would be somewhat different if the absor- bent were volatile. Typical absorption heat pump cycles are described in detail by Herold et al. (1996) and in the ASHRAE Handbook-Fundamentals (ASHRAE 1997). It is well known that the performance of the abs
17、orber is the key to the overall system size, performance, and cost; it has been called the “bottleneck” of the system (Beutler et al. 1996b). To achieve maximum heat and mass transfer within the absorber, the design and operating conditions should be such that the falling film is frequently mixed, t
18、he interfacial surface areas are maximized, and the cool- ant-side heat transfer resistance is low. Falling films are favorable due to their potential for high heat and mass transfer rates with low associated pressure drop. In the particular case of falling films over horizontal tubes, the absorbent
19、 solution falling down the tubes experiences frequent mixing due to droplet-mode flow between the tubes, impingement on successive tubes, and rein- itialization of the boundary layer, which enhances absorption rates. On the other hand, some unique challenges arise when using horizontal tubes includi
20、ng solution distribution, surface wet- ting, and the selection of optimum tube spacing, diameter, surface structure, etc. Some useful reviews of falling-film absorption heat and mass transfer have focused on the more general and theoretical aspects of these systems (Grossman 1986; Fujita 1993; Killi
21、on and Garimella 2001); J.D. Kiliion is a graduate research assistant and S. Garimella is an associate professor and director ofthe Advanced Ther- mal Systems Laboratory in the Department of Mechanical Engineering, Iowa State University, Ames, Iowa. 111 112 HVAC the advantage of a full system in the
22、ir case is that the vapor inlet conditions and surfactant circulation within the system are more realistic (Kyung and Herold 2000, 2002). Systems may run in batch mode, where a large amount of solution is prepared to the desired inlet concentration prior to testing by generating water vapor and stor
23、ing it in another container, or in continuous mode, where the vapor generation process occurs at the same time as the testing of the absorption phenomena; see Figure 1 for a typical schematic of a continuous-mode, sin- gle-pressure apparatus. In either case, the pressure within the system must be lo
24、wered by a vac- uum pump. Because of the presence of water vapor, the vacuum system typically includes a cold trap upstream of the pump. System leakage rates must be very low to ensure that non-absorbable air does not enter the system. Even low concentrations of air will retard the absorption proces
25、s, VOLUME 9, NUMBER 2, APRIL 2003 113 as will be discussed later. The ingression of air is, however, inevitable, and periodic use of the vacuum system purges the non-absorbable gases. Vapor generation is typically accomplished with resistance heaters with power output in the range of a few kW depend
26、ing on the size of the absorber. From the generator, liquid with high absorbent concentration is pumped to the top of the absorber and may also be conditioned en route to achieve the desired inlet temperature. The generated vapor usually enters the absorber near the bottom, often by placing the gene
27、rator directly beneath the absorber. But counter-cur- rent flow is not crucial because the absorbent is essentially nonvolatile and, thus, the vapor phase is pure and homogeneous in the absence of non-absorbable gases. It is important, how- ever, that the very low density vapor be given ample flow a
28、rea to reach the absorber to prevent excessive pressure losses. At a nominal pressure of 1 kPa, a pressure gradient of just 100 Pa within the absorber will cause a 1.4OC increase in evaporator temperature due to the higher pres- sure required in the evaporator. Thus, a small pressure drop within the
29、 absorber can lead to a sig- nificant degradation in system performance. The concentrated solution pumped from the generator must be distributed evenly on top of the tube bank. Generally, this is accomplished by an array of holes or nozzles that either drip or spray solution onto the topmost tube. T
30、he design of this distributor must be done with care to ensure even initial distribution; see, for example, Hu and Jacobi (1996). Depending on the solution inlet conditions, adiabatic absorption or genera- tion may occur between the distributor and the topmost tube, which may require compensation in
31、 the analysis of the data, e.g., Atchley et ai. (1998). Often, too, the solution falling off the bot- tommost tube may be collected before it enters the generator to allow measurement of the condi- tions at this point. The coolant circuit presents conflicting requirements. On one hand, a large heat
32、transfer coef- ficient is desirable so that the controlling thermal resistance is on the absorption side, which sug- gests that high coolant flow rates would be appropriate. On the other hand, it is also desirable to determine the heat duty of the coolant, which requires accurate measurements of the
33、 temperature change, which in turn requires a low coolant flow rate. Typically researchers have used one of two approaches to solve this problem: some use inserts in the tube, thick tube walls (to increase coolant velocity), or some other mechanical enhancement to increase the heat transfer coeffi-
34、cient in the tubes while maintaining a flow rate low enough to accurately measure the tempera- ture change. Others have simply used high coolant flow rates and ignored the heat duty on the coolant side, therefore relying only on the absorption side to deduce the heat duty. The coolant circuit is usu
35、ally a counter-current, serpentine configuration where the coolant flows through all the tubes in series, although some researchers have utilized a partially or fully parallel flow arrangement. Other ancillary considerations for a test apparatus include prevention of corrosion and crystal- lization
36、and provision for optical access. Since the test apparatus may be frequently modified, exposure to air is inevitable, and aqueous solutions of LiBr or LiCr are corrosive in the presence of oxygen. The use of stainless steel components or copper-nickel tubes helps reduce the corro- sion problem, but
37、many researchers still add small amounts (around 0.2% to 0.3% by weight) of commercially used corrosion inhibitors such as LiOH, Li2Cr207, or LiMo04. There is very little documentation on the effect of these on system performance, though presumably it is small. Crystallization is a consideration in
38、any lithium bromide-water or other salt-water absorption system, and start-up and shutdown procedures may need to include provisions to avoid it when testing with high absorbent concentrations. For instance, some researchers add water to the sys- tem during the shutdown phase and then purge water du
39、ring the start-up phase to achieve the desired high solution concentrations without crystallization. Finally, the ability to view the fall- ing film has led to numerous insights into the phenomena occurring during the absorption pro- 114 HVAC these instances are identified with NA (not available). I
40、n compiling Table 2, the correlations for thermodynamic properties of aqueous LiBr found in the ASHRAE Hand- book (ASHRAE 1997) as implemented by Klein and Alvarado (2000) in EES, a commercially available software program, were used to convert the data given by the authors into a standard format. In
41、 addition to the solution inlet temperature and concentration, it is also useful to iden- tify the degree of subcooling and the maximum possible concentration change based on the inlet conditions. The degree of subcooling is defined as the difference between the equilibrium tem- perature and the bul
42、k temperature of the inlet solution; in some cases this value is negative, indi- cating that the solution inlet temperature is above the equilibrium temperature, which presumably leads to flash evaporation at the inlet. The equilibrium temperature is evaluated at the absorber pressure and bulk solut
43、ion inlet concentration. The maximum possible concentra- tion change is defined as the difference between the inlet concentration and the concentration in equilibrium with the coolant inlet temperature at the absorber pressure. r is the film flow rate, which is defined as the total mass flow rate of
44、 the incoming absorbent-rich liquid divided by twice the tube length (see section on effect of film flow rate). Other reference lengths such as D, nD, etc., have also been used in the literature. In such cases, they have been converted to the (OD). i 16 HVAC some of these have been reviewed by Ziegl
45、er and Grossman (1996). The surfactant in most experimental stud- ies of LiBr-water is 2-ethyl-1-hexanol (2EH), although some use octano1 (also called 1-octano1 or octyl alcohol). The addition of 2EH to a falling-film absorber enhances the absorption rates by improving the surface wetting and increa
46、sing film instabilities (Cosenza and Vliet 1990) and by increasing the number of droplets (Ziegler and Grossman 1996). 2EH changes the viscosity and surface tension of aqueous LiBr solutions, but the steady-state changes in these properties do not adequately explain the resulting enhancement in abso
47、rption rates (Beutler et al. 1996a). Kulankara and Herold (2000) suggest that the vapor pressure of the surfactant is also important because according to their findings, the most important delivery mechanism of the surfactant to the liquid surface is via the vapor. Also, the effect of surfactants on
48、 the film flow is dependent VOLUME 9, NWER 2, APRIL 2003 117 Table 2. Summary of Operating Conditions Used in the Investigations Reviewed Here * the definition of r used by authors is not clear * absorbent: LiBrZnBrz (2: 1), refrigerant: CH30H on the presence of absorption (Cosenza and Vliet 1990).
49、Moreover, Ziegler et al. (1999) show that the heat transfer coefficient increases as the driving temperature difference increases when 2EH is present. The experimental results of Kyung and Herold (2002) corroborate this finding and the authors explain that this is a result of the higher delivery rate of vapor-borne surfactant to the film surface associated with higher mass and heat fluxes. Although the exact details of how surfactants enhance the heat and mass transfer processes are clearly quite complex and still a subject of current research, results in the literature allow some quantifi
