1、I n t er n at ion al J o u r n a 1 of H eat i n g ,Ve n t il a t i n g, Air-conditioning and Refrigerating Research HVAC unfortunately, results were only obtained at four Reynolds numbers between 4500 and 8500- These limited data suggested that baffles offer no significant heat-transfer enhancement
2、for a single-row heat exchanger in this Reynolds number range. The length scale was chosen as the equivalent diameter of Kays and London (1984) for all of the studies cited, and none of these studies provided pressure drop data for a heat exchanger with gull-wing baffles. Hu and Jacobi (1993) employ
3、ed the naphthalene sublimation technique to study the local heat-transfer behavior of a single row of unbaffled, annularly finned tubes. The analogy between heat and mass transfer was employed to infer heat-transfer behavior (Nusselt numbers) from experimentally obtained mass transfer data (Shenvood
4、 num- bers). This approach provides a deeper understanding of the flow and heat-transfer interactions. The use of this mass-transfer technique is well established, and an excel- lent review of the method is given by Mendes (1991). The purpose of this paper is to report experiments directed at measur
5、ing the heat- transfer and pressure drop performance of gull-wing baffles as a method for enhancing heat exchanger performance. Local convective data are used to explain the flow and heat-transfer interactions and their impact on heat exchanger performance. Averaged heat-transfer and core pressure d
6、rop data are used, along with a performance evalua- tion criterion, to assess the overall thermal performance of this enhancement method. The results of this study will allow the engineer to evaluate the thermal impact of gull- wing baffles in finned-tube heat exchangers over a wide Reynolds number
7、range and, therefore. to decide whether or not gull-wing baffles are a viable design option. VOLUME 1, NUMBER 4. OCIOBER i 995 259 EXPERIMENT APPARATUS Mass-transfer experiments were conducted in the large, open circuit wind tunnel shown schematically in Figure 2. The apparatus consisted of 4 sectio
8、ns: an inlet con- traction, test section, expansion. and a discharge. A large, 14.9 kW, 3-phase electric motor supplied power to the wind tunnel blower. The contraction ratio at the inlet was approximately 32:1, with test section cross sectional dimensions of 381 mm by 381 mm. Velocity profiles were
9、 mapped at 3 different speed settings using a Pitot-static tube and a micromanometer. These profiles were found to be flat to within 5% for all three settings. The free stream turbulence intensity in the test section was measured using a hot wire anemometer for test section velocities of 1 to 25 m/s
10、. These intensities were found to be less than 1% over the entire range of velocities tested. For the experiments reported in this paper, single rows of five finned tubes, conven- tional or baffled, were placed in the square test section. The frontal area of these tube banks filled the entire wind t
11、unnel cross section. The dimensions of these finned tubes were as follows (see Figure 1): fin diameter ciF = 76.2 mm, tube outside diameter dT = 38.1 mm, fin thickness 6 = 1.02 mm, fin pitch S, = 139.1 m- (fins per meter), and transverse tube spacing S, = 76.2 mm. The free flow gap between the gull-
12、wing baffles, S, was about 18 mm. The total heat-transfer surface area of these tube banks was, A = 2.07 m2, and the minimum free flow areas were A, = 0.062 m2 and 0.030 m2 for the conventional and baffled tube banks, respectively. The hydraulic diameters of the con- ventional and baffled bundles we
13、re, respectively, 9.16 mm and 4.43 mm. Static pres- sure taps were placed upstream and downstream of the heat exchanger core to measure the pressure drop across the bundle. A single naphthalene fin was used in most of the experiments, and this fin was placed in the center of the middle finned tube.
14、The remainder of the core was “unheated or uncoated with naphthalene and simply sup- plied the proper flow conditions. The test tube could be separated into halves, as shown schematically in Figure 3. The assembly was clamped together by tightening the handle on a screw that extended from the bottom
15、 half of the test tube through the top portion. Two opposing naphthalene fins were used in some experiments in order to check the assumption that Figure 2. Schematic of wind tunnel apparatus i) 32 to 1 area contraction, (2) test section, (3) control panel, (4) diffuser, (5) blower, (61 motor, (7) di
16、scharge. ASHRAE TITLE*IJHVAC 1-4 95 0759650 0514143 LbL 260 HVAC. .jU. , t , -. -. =.- o“ t, i -. -. a - - O/ -5 54 55 56 57 58 59 LiBr Concentration (“A by weight) Figure 5. Effect of LiBr concentration on enhancement additive concentration = 50 ppm) ASHRAE TITLE*IJHVAC 1-Y 75 075b50 0518164 996 VO
17、LUME 1, NUMBER 4. OCTOBER 1995 28 1 the additive excess would certainly have a stronger effect on surface tension differen- tials than on bonding in an already additive-saturated solution. This would lend credi- bility to the Marangoni approach. Yet, it has long been known that not all surfactants f
18、unction well as additives (Rush 1991). and ample experimental evidence exists to show that this is indeed the case. Reducing the surface tension is not sufficient for an addi- tive to provide good enhancement (Beutler 1994). Hence, endorsing the Marangoni the- ory without reservations may lead to wr
19、ong conclusions at this time. Consideration of the enhancement versus brine concentration showed relatively con- stant enhancement. This constant enhancement was not expected from the steric hin- drance theory, since as the hydration limit is approached, the additive effectiveness should decrease. S
20、o, this theory seems to fail regarding the effect of concentration. We have offered here evidence that both branched and linear alcohols provide effec- tive enhancement, although the branched structure seems to generally achieve larger enhancement ratios at concentrations of practical interest ( 100
21、0 ppm). This indeed is a trademark of the steric hindrance theory, which advances that only those additives that form relatively weak bonds to Li ions are effective. Branched structures will gener- ally associate more weakly than linear ones, depending of course on the relative posi- tion of the fun
22、ctional group and the length of the additive. In summary, none of the existing theories completely explain the observed phenom- ena. However, certain parameters, such as the diffusion time to the interface, which is clearly one of the most significant variables. has so far been ignored by the propon
23、ents of the two existing theories. Therefore, future work should focus on developing novel theories, and experimental approaches to improve our understanding of the mecha- nisms at play below the solubility limit. ACKNOWLEDGMENT for helpful discussions during this work. REFERENCES ASIIRAE. 1988. i98
24、8 ASHRAE Handbook-Equipment. p. 13.3. Atlanta: ASHRAE. Beutler, A., I. Grieter, A. Wagner. L. Hoffman, S. Schreier and G. Alefeld. 1994. Surfactants and Fluid Properties. In Proceedings of the Muchen Discussion Meeting, Munchen. Germany. Bourne, J.K., and K.V. Eisberg. 1966. U.S. Patent 3,276,217. U
25、niversity Park: The Pennsylvania State University. Available on microfilm from Pattee Library. Bjurstrom, H W. Yao, W. Ji, and F. Setterwall. 1991. Heat-Transfer Additives in Absorption Heat Pumps. In Proceedings of the Environment-Wndig Technology for the Slst Century. Tokyo: JAR. Brining, W.J G.E.
26、H. Joosten, A.A.C.M. Beenackers, and H. Hofman. 1986. Enhancement of Gas-Liquid Mass Transfer by a Dispersed Second Liquid Phase. Chemical Engineering Science The authors wish to thank Mr. C. Sgamboti of United Technologies Research Center 4 l(7): 1873- 1877. Davies. J.T., and E.K. Rideal. 1963. Int
27、erfacial Phenomena. New York: Academic Press. Ji, W M. Bjurstrom. and F. Settenvall. 1993. A Study of the Mechanism for the Effect of Heat Trans- fer Additives in an Absorption System. Journal ofColloid and inteerfacc Science 160: 127- 140. Jung, Sung-Han, C. Sgamboti. and H. Perez-Blanco. 1994. An
28、Experimental Study of the Effect of Some Additives on Falling Film Absorption. International Absorption Heat Pump ConJerence ASE-3 1 :49-55. American Society of Mechanical Engineers. Kashiwagi, T. 1988. Basic Mechanism of Absorption Heat and Mass-transfer Enhancement by lhe Marangoni Effect. Newslet
29、ter ofthe IEA Heat Pump Center 64):2-5. Kim, Kwang J N.S. Berman and B. D. Wood. 1994. Experimental Investigation of Enhanced Heat and Mass-Transfer Mechanisms Using Additives for Vertical Falling Film Absorber. AES-3 1: 41-47. International Absorption Heat Pump Conference. ASME. Rocky Research. 199
30、3. Heat and Mass-transfer Additives for Aqueous Absorption Fluids. In Pro- ceedings of Absorption Experts Reuiew Meeting. Columbus, OH: Batelle Labs. Rush, W.F., J. Wurm. and H. Perez-Blanco. 1991. A Brief Review of the Uses and Effect of Addi- tives for Absorption Enhancement. In Proceedings of Env
31、ironment-Friendly Technologies for the 21st Century1 pp. 183-187. Tokyo: JAR. Yao, W., H. Bjurstrom, and F. Setterwall. 1991. Surface Tension of Lithium Bromide Solutions with Heat-Transfer Additives. J. of Chemical and Erigirieerfrig Data 36: 96-98. ASHRAE TITLE*IJHVAC 1-4 95 0759650 05LBLb5 822 VO
32、L. 1. No. 4 WAC&R RESEARCH OCTOBER 1995 Evaluation of Vortex-Shedding Flow Meters for Monitoring Air Flows in HVAC Applications Mark C. Wolochuk James E. Braun, Ph.D., P.E. Member ASH= Michael W. Plesniak, Ph.D This paper describes the evaluation of vortex-shedding flow meters under highly dis- turb
33、edflow conditions that occur in HVAC ducts. Fiel data were acquiredfrom a typical WAC duct to quantijy the disturbedJow conditions, and to set the boundsfor controlled wind tunnel experiments. Wind tunnel experiments were then used to demonstrate that vortex sheddiWJow meters can be used for measuri
34、ng airJows in the highly disturbed conditions that occur in HVAC ducts, but that care must be taken in their design and application. The results imply that vortex sheddingflow meters should be calibrated in turbulentJows, and that the turbulence length scale at the bluff body must be controlled to e
35、nsure accurate meter calibration. Periodic unsteadiness in the meanjlow caused the vortex shedding to lock-on to a subharmonic of the disturbance frequency for distur- bances at various multiples of the shedding frequency. No lock-on was observed for a forcing frequency at half the shedding frequenc
36、y, indicating that lock-on can be avoided by sizing the bluff body small enough so that its sheddingfrequency is always much greater than any unsteadiness frequency present in theflow. When a long bluff body was spanned across a non-unijorm velocity proJile, singleyrequency shedding cells occurred a
37、long the bluff body. The single-frequency cells broke down with increasing velocity gradient and bluff body aspect ratio. Therefore, to measure an average flow rate. a number of short bluff bodies must be used. INTRODUCTION Motivation and Background With recent concern about improving air quality an
38、d energy efficiency in buildings, accurate monitoring and control of the airflow rate in ducts has become increasingly important. Energy efficiency is improved by delivering the appropriate amount of heated or cooled air to each room in a building, and air quality is improved by carefully moni- tori
39、ng the amount of ventilation air taken into the building. Widespread flow rate sens- ing could also lead to improved performance monitoring and automated fault detection and diagnostics. Vortex-shedding flow meters provide a very promising alternative to Pitot-static probes (Drees et al. 1992) and h
40、eated thermistor sensors for measuring air flow in HVAC ducts that could lead to greater application of flow sensing. When a bluff body is placed in a flow, vortices form and shed alternately from each side, as depicted in Fig- ure 1. The frequency of the shedding is proportional to the mean flow ve
41、locity and bluff body diameter). and the relationship is linear over a wide range of Reynolds numbers. A vortex-shedding flow meter consists of a bluff body and a means of sensing the vor- tex-shedding frequency. &pically, the shedding frequency is determined through spec- tral analysis of pressure
42、measurements made at the surface of the bluff body. Vortex- shedding flow meters offer high accuracy (even at low flow velocities and linearity, and Mark C. Wolochuk is a Research and Development Engineer for Huntair. Portland. Oregon. James E. Braun and Michael W. mesniak are professors at the School of Mechanical Engineering, Purdue University, West Lafayette, indiana. 282
