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Boiling Heat Transfer on Wire-Mesh-Wrapped Extended Tube Surfaces Ritunesh Kumar* Department of Mechanical Engineering, IIT Delhi, Hauz Khas, New Delhi, India-110016
Akhilesh Gupta Department of Mechanical and Industrial Engineering, IIT Roorkee, Uttranchal, India-247667
Nirupam Rohatgi Department of Mechanical Engineering, MNIT Jaipur, Rajasthan, India-302017
This article discusses one of the most promising techniques for enhancing boiling heat transfer by wrapping a wire mesh around a plane surface. Experimental investigations have been carried out to study boiling heat transfer for a plain stainless steel tube and stainless steel tubes wrapped with different sizes of stainless steel wire mesh, for saturated distilled water at atmospheric pressure in cross-flow configurations. The mass flux was varied from 0 to 8.27 kg/(m2 s), and the tubes were heated resistively with a heat flux ranging from 12 to 55 kW/m2. The results show that the tubes wrapped with wire mesh provide better performance than the plain tube at low heat flux (up to 30 kW/m2), whereas at high heat flux, the order of their performance reverses. The increase in boiling heat transfer is due to the enhanced retention time of the bubbles over tube surface and the increase in turbulence created around the tube surface as a result of the wire-mesh wrapping. The effects of input parameters such as the heat flux and mass flux on the boiling heat transfer coefficient for plain and wire-mesh-wrapped tube surfaces were also investigated in the present work. 1. Introduction The boiling heat transfer process has been the subject of intense research during the past few decades. Heat transfer by boiling is the most effective heat-transfer method because boiling heat transfer is many times greater than that of ordinary heattransfer processes. It provides the possibility of transferring enormous amount of heat using much smaller heat-transfer surfaces. Therefore, boiling heat transfer finds extensive application in a variety of industries, e.g., metallurgical processing, refrigeration and air conditioning, thermal and nuclear power generation, cryogenics, and electronic component cooling. Even though boiling is associated with a high heat-transfer rate, efforts are being made to find better ways to utilize the available energy more efficiently. Webb1 divided the techniques of enhancing heat transfer into active and passive techniques. An active technique requires external power to enhance heat transfer, e.g., mechanical aids, electrostatic fields in polar fluids, etc.; on the other hand, a passive technique requires special kinds of surface treatments, e.g., coated surfaces, finned surfaces, and wire-meshwrapped surfaces. Some researchers2-6 have experimentally verified benefit of using microcoated surfaces for the enhancement of boiling heat transfer. Sami and Song7 and Jung et al.8 used microfinned surfaces to enhance boiling heat transfer. Finned and coated surfaces are costly and are prone to scaling, whereas wire-mesh-wrapped surfaces provide the benefits of easy installation and cheaper and simpler maintenance. Liu et al.9 experimentally investigated the effects of wire-meshwrapped surfaces for methanol and HFE-7100. They concluded that the application of a mesh increases the boiling heat transfer coefficient in the low-heat-flux region only when a fine mesh is employed. Alam et al.10 experimentally investigated the effects of fine wire meshes wrapped on plain tubes on boiling heat transfer for water and methanol as the boiling fluids. They * To whom correspondence should be addressed. E-mail: ritunesh@ rediffmail.com.
concluded that wire-mesh-wrapped surfaces perform better than plain surfaces because of an increase in the number of active nucleation sites. Most of the above authors worked on different types of refrigerants, except for Alam et al.,10 who used water as the working fluid. However, heat and mass flux ranges in which this technique (fine mesh wrapping) is useful are not mentioned in their article. In the present work, experiments have been carried out with water as the boiling fluid to determine the heat and mass flux ranges in which the above technique is useful, as most energy-producing devices use water as the working fluid. 2. Experimental Apparatus A schematic diagram of the experimental setup is shown in Figure 1. The stainless steel test vessel, measuring 300 mm × 200 mm × 425 mm, is the main component of experimental setup. To ensure a uniform flow at the entrance of test vessel, a 2-mm-thick aluminum sheet with 1-mm-diameter holes uniformly drilled at a pitch of 1 cm × 1 cm throughout the surface was placed at the bottom of the test vessel. To facilitate flow visualization, three sides of the test vessel were fitted with sight glasses. The diameter of each of the two side glasses was 152 mm, and that of the front glass was 203 mm. A Bakelite sheet (250 mm × 250 mm × 25 mm) was fitted on the fourth side of test vessel. Two sample tubes were embedded in the Bakelite sheet at equal distances from the center in the same horizontal plane to provide a uniform flow around the sample tubes. The fluid flowed from the test vessel to a receiver tank through a 50-mm-diameter pipe. Two thermocouples were fitted at the center of test vessel to measure the bulk fluid temperature. A 2-kW preheater (controlled by variable transformer) was attached at the lower end of the test section to ensure that the water entered the test vessel at its saturation temperature. The receiver tank measuring 450 mm × 450 mm × 870 mm was made of a 3-mm-thick stainless steel sheet. Two heating elements, each of 3-kW (controlled by a variable transformer),
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Figure 3. Sample tubes used for boiling heat transfer. Figure 1. Schematic diagram of the experimental setup.
thermocouple (28 BWG, T-type) was inserted in each groove. The average temperature of the inside tube surface was measured using these thermocouples (Figure 3). The wrapping of the wire mesh is the most important aspect of this technique; careful precautions should be taken to minimize the chances that the mesh overlap or fit loosely, which would lead to an increase in contact resistance. A strip of paper was cut so that it could be wrapped tightly on the tube. This template of paper was further used to cut the strip of wire mesh. The mesh strip was then wrapped tightly on the tube, and its ends were brazed at a few locations at the two ends of the tube. 3. Experimental Procedure
Figure 2. Experimental setup.
were fitted at the bottom of the receiver tank to heat the water at the start of the experiment and to make up for heat lost to the environment during the experiment. Steam collected over the top portion of the receiver tank was condensed with the help of a condensing unit (a plate and tube type of heat exchanger). Cold water was passed through the condensing unit, and its supply was regulated by a gate valve. A 1-hp pump was used to circulate water from the receiver tank to the test vessel, and its flow was regulated by another gate valve. The flow rate was measured with an orifice meter fitted between the pump and the preheater. All components were well insulated to reduce heat losses. A photograph of the experimental setup is shown in Figure 2. Two sample tubes made of stainless steel (AISI 304), 19.05-mm o.d. and 17.27-mm i.d., with an effective heated length of 190 mm were fitted in the Bakelite sheet. One of the tubes was kept bare, and the other was wrapped with stainless steel wire mesh for comparative studies of the plain and meshwrapped tubes. The two tubes were connected in series to ensure equal heating of the tubes. The sample tubes are shown in Figure 3. The hollow tubes were heated resistively by passing a high alternating current through a step-down transformer of 15-kVA capacity. A digital ammeter and voltmeter were used to measure the power supplied to the tubes. The sensitivities of the ammeter and voltmeter were 10 mV and 1 mA, respectively. Porcelain tubes, having groves at an equal angle of 60° across their periphery, were embedded inside the sample tubes. One
The experiment was planned to study boiling heat transfer on plain and wire-mesh-wrapped tubes. Hence, of the two tubes connected in series in the test vessel, one was kept bare, and the other was wrapped with wire mesh. Before actual data collection, the experimental setup was charged with distilled water and run for about 8-10 h daily for 1 week with both tubes heated resistively by passing a constant current through them. This was done to eliminate the effects of aging. To ensure air removal from the setup, air vents were kept open initially, and when steam of sufficient quantity started exiting from the vents, the air vents were closed. The heat flux and flow rates were adjusted to the desired levels, and a constant pressure was maintained in the test vessel by regulating the supply of cold water in the condenser coil located in the top portion of the receiver unit. The preheater was adjusted such that saturated water entered the test vessel. Readings from the thermocouples, voltmeter, ammeter, and orifice meter were recorded at steady state. A similar procedure was adopted for each set of constant heat flux and mass flux values. Readings from the ammeter and the voltmeter were used to calculate the heat flux on the outer surface of the tube. The average outer surface temperature of the sample tube was calculated from the inner surface temperTable 1. Operating Parameters of the Present Investigations parameter working fluid inlet temperature system pressure heat flux, q mass flux, G tube geometry material outer diameter inner diameter wire-mesh geometry material (d, p)
value distilled water saturation temperature of water atmospheric pressure 12-55 kW/m2 0-8.27 kg/(m2 s) SS (AISI 304) 19.05 mm 17.27 mm stainless steel (0.05 mm, 0.075 mm), (0.15 mm, 0.22 mm)
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Figure 6. Bubble formation on plain and wire-mesh-wrapped tubes.
Figure 4. Comparison of pool boiling at G ) 0.00 kg/(m2 s) on a plain tube.
Figure 7. Boiling heat transfer on a mesh- [(d, p) ) (0.05, 0.075)] wrapped tube.
Figure 5. Boiling heat transfer on a plain tube.
ature of the tube. The average heat flux divided by the difference between the average wall temperature and the bulk fluid temperature (degree of superheat) is the boiling heat transfer coefficient on the tube surface. The operating parameters of the present investigations are listed in Table 1. 4. Results and Discussions The experimental results of nucleate boiling heat transfer for a plain tube surface under pool boiling conditions are compared with the results of other researchers in Figure 4. It can be seen that there is a large variation in the experimental results of these authors.11-16 The slope of the nucleate boiling curve in the present case is the same as that obtained by Forster and Zuber,11 and the relationship between the nucleate boiling heat transfer coefficient and the heat flux is given by
h ) 0.930q0.418
(1)
The effects of the heat and mass fluxes on the boiling heat transfer coefficient for the plain surface are shown in Figure 5. The boiling heat transfer coefficient increases with increasing heat flux and increasing mass flux. Boiling heat transfer on the plain surface is governed by two types of nucleus (bubble) generation. The first type of bubbles are generated from higherenergy molecular groups resulting from thermal fluctuations of the liquid molecules, and the second type of bubbles are
generated because of the reduction in local pressure resulting in turbulence such as in accelerated flow. The enhancement of the boiling heat transfer coefficient by the heat flux is governed by the first type of bubble generation. The increased heat flux causes some new spots (with higher-energy molecular groups) to become active sites for nucleation. The enhancement of the boiling heat transfer coefficient by the mass flux is governed by the second type of bubble generation. Sufficient nucleation sites around the tube surface are present at high heat flux; therefore, increasing the turbulence by increasing the mass flux does not enhance the boiling heat transfer significantly. This is reflected in the convergence of the boiling curve at high values of the heat and mass fluxes. However, the effect of the mass flux on boiling heat transfer is significant at low heat flux values. The application of a wire mesh increases the bubble generation rate by providing artificial nucleation sites in the form of small cavities on the tube surfaces; these cavities are also responsible for providing higher contact angles on the tube surface. Higher contact angles provides better chances of liquid trapping inside the cavity by capillary effect, as shown in Figure 6. It was observed that bubble generation in the case of wiremesh-wrapped tubes started at lower heat flux values than was the case for the plain tube, and the retention time of the bubbles was also greater in the case of the wire-mesh-wrapped tubes. The effects of the heat and mass fluxes on the boiling heat transfer coefficient around tubes wrapped with wire meshes with (d, p) ) (0.05, 0.075) and (d, p) ) (0.15, 0.225) are shown in Figures 7 and 8, respectively. The boiling heat transfer coefficient increases with increasing heat flux and increasing mass flux. A comparison of Figures 5, 7, and 8 shows that the rate of change of the boiling heat transfer coefficient with heat flux or
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Figure 8. Boiling heat transfer on a mesh- [(d, p) ) (0.15, 0.22)] wrapped tube.
Figure 9. Comparison of boiling heat transfer under pool boiling at G ) 0.00 kg/(m2 s).
Figure 10. Comparison of boiling heat transfer under flow boiling at G ) 4.94 kg/(m2 s).
mass flux is higher in the case of the plain tube surface than in the case of the wire-mesh-wrapped tubes. An examination of Figures 9-11 shows that wire-meshwrapped tube surfaces perform better than plain tube surface in the low-heat-flux region (10-30 kW/m2), because of the increase in the number of nucleation sites. The number of nucleation sites increases because of the wire-mesh wrapping around the tubes. At high heat flux values, the lower perfor-
Figure 11. Comparison of boiling heat transfer under flow boiling at G ) 6.94 kg/(m2 s)).
Figure 12. Comparison of boiling heat transfer under pool boiling at G ) 0.00 kg/(m2 s).
Figure 13. Comparison of boiling heat transfer under flow boiling at G ) 4.94 kg/(m2 s).
mance of wire-mesh-wrapped tubes can be attributed to the difficulty bubbles experience in detaching from the tube surface. Further, tube wrapped with the wire mesh with (d, p) ) (0.15, 0.22) provides better performance than those wrapped with the mesh with (d, p) ) (0.05, 0.075) in the low-heat-flux region, whereas in the high-heat-flux region, the order of their performance reverses. Figures 12 and 13 show the variations of the boiling heat transfer coefficient with the degree of superheat. It can be seen
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that meshed surfaces provide better performance than plain tube surfaces for the degree of superheat up to 7-10 K. Liu et al.9 obtained similar results for methanol. A detailed analysis of the present experimental investigation is available in Kumar.17
where tw,i is the temperature of the inside surface of the tube measured by the thermocouple and tw,o can be calculated from eq A-8. The contact resistance between the tube surface and the wire mesh is neglected as the wire mesh is very fine and tightly wrapped on the tube surface.
5. Conclusions A comparison of boiling heat transfer on a plain tube and tubes wrapped with wire mesh of different grades under pool and cross-flow boiling for saturated distilled water at atmospheric pressure has been made. The boiling heat transfer coefficient in the case of wire-mesh-wrapped tubes in the lowheat-flux region (10-30 kW/m2) was found to be higher than that for plain tube. Further, at low heat flux values, the tube wrapped with a coarse wire mesh [(d, p) ) (0.15 mm, 0.22 mm)] had a higher boiling heat transfer coefficient than the tube wrapped with a finer wire mesh [(d, p) ) (0.05 mm, 0.075 mm)], whereas in the case of the high-heat-flux region, the order of their performance reversed. Acknowledgment The authors gratefully acknowledge the financial support provided by the Indian Institute of Technology Roorkee for this work. Appendix At steady-state conditions, the heat transfer across the radial direction inside the tube with volume heat generation is given by the equation
d2t 1 dt Q + + )0 dr2 r dr kw
(A-1)
where t is the temperature of the surface, r is the radial distance, Q is the heat generation per unit volume, and kw is the thermal conductivity of the tube material The general solution for eq A-1 above is given by
t)-
()
Q r2 + C1 log(r) + C2 kw 4
(A-2)
The boundary conditions are
(drdt)
rw,i
)0
(A-3)
t ) tw,i at r ) rw,i
(A-4)
t ) tw,o at r ) rw,o
(A-5)
C1 can be calculated from eq A-3 as 2
C1 )
Q rw,i kw 2
(A-6)
Using eqs A-2 and A-4-A-6, tw,o can be written as
tw,o - tw,i ) -
tw,o - tw,i ) -
(
)
( ) ( )]
2 2 2 rw,o Q rw,o - rw,i Q rw,i + log kw 4 kw 2 rw,i
[
dw,o Q (dw,o2 - dw,i2) - 2dw,i2 log 16kw dw,i
(A-7)
Abbreviations d ) diameter of the wire (mm) G ) mass flux (kg/(m2 s)) h ) heat-transfer coefficient, (kW/m2K1) p ) pitch of the wire (mm) q ) heat flux, kW/m2 t ) temperature (°C) ∆T ) degree of superheat (K)
Literature Cited (1) Webb, R. L. Principles of Enhanced Heat Transfer; Wiley: New York, 1994. (2) Hsieh, S. S.; Weng, C. J. Nucleate Pool Boiling from Coated Surfaces in Saturated R-134a and R-407c. Int. J. Heat Mass Transfer 1997, 40, 519. (3) Chang, J. Y.; You, S. M. Boiling Heat Transfer Phenomena from Microporous and Porous Surfaces in Saturated FC-72. Int. J. Heat Mass Transfer 1997, 40, 4437. (4) Chang, J. Y.; You, S. M. Enhanced Boiling Heat Transfer from Microporous Surfaces: Effect of a Coating Composition and Methodology. Int. J. Heat Mass Transfer 1997, 40, 4449. (5) Vemuri, S.; Kim, J. K. Pool Boiling of Saturated FC-72 on Nanoporous Surface. Int. Commun. Heat Mass Transfer 2005, 32, 27. (6) Hsieh, S. S.; Yang, T. Y. Nucleate Pool Boiling from Coated and Spirally Wrapped Tubes in Saturated R-134a and R-600a at Low and Moderate Heat Flux. J. Heat Transfer 2001, 123, 257. (7) Sami, S. M.; Song, B. Heat Transfer and Pressure Drop Characteristics of HFC Quaternary Refrigerant Mixtures Inside Horizontal Enhanced Surface Tubing. Appl. Therm. Eng. 1996, 16, 461. (8) Jung, D.; An, K.; Park, J. Nucleate Boiling Heat Transfer Coefficient of HCFC22, HFC134a, HFC125, and HFC32 on Various Enhanced Tubes. Int. J. Ref. 2004, 27, 202. (9) Liu, J. W.; Lee, D. J.; Su, A. Boiling of Methanol and HFE-7100 on Heated Surface Covered with a Layer of Mesh. Int. J. Heat Mass Transfer 2001, 44, 241. (10) Alam, S. S.; Husain, S.; Khan I. A. Heat Transfer to Boiling Pure Liquid from Enhanced Surfaces; Hazira Project ONGC: Surat, India, 1994; p 26. (11) Forster, H. K.; Zuber, N. Bubble Dynamics and Boiling Heat Transfer. AIChE J. 1955, 1, 531. (12) Voloshko, A. A. Free Convection Boiling of Freons. Heat Transfer-SoV. Res. 1972, 4, 60. (13) McNelly, M. J. A Correlation of Rates of Heat Transfer to Nucleate Boiling Liquids. J. Imp. Coll. Chem. Eng. Soc. 1953, 7, 18. (14) Rohsenow, W. M. A Method of Correlating Heat Transfer Data for Surface Boiling of Liquids. Trans. ASME 1952, 74, 969. (15) Kutateladze, S. S. Fundamental Heat Transfer; Arnold: London, 1963. (16) Gupta, A. Investigation of Pool Boiling Heat Transfer from Tube Bundle in Cross Flow. Ph.D. Thesis, IIT Roorkee, Uttranchal, India, 1992. (17) Kumar, R. Boiling Heat Transfer from an Extended Tube Surfaces. M.Tech. Thesis, IIT Roorkee, Uttranchal, India, 2002.
ReceiVed for reView December 3, 2005 ReVised manuscript receiVed August 16, 2006 Accepted August 29, 2006
(A-8) IE0513494
