The CRC Handbook of Mechanical Engineering. Chapter 4. Heat and Mass Transfer (776127), страница 60
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With permission.)only a high nucleate boiling heat transfer coefficient, but the fact that boiling can take place at very lowtemperature differences.These structured boiling surfaces, developed for refrigeration and process applications, have beenused as “heat sinks” for immersion-cooled microelectronic chips.The behavior of tube bundles is often different with structured-surface tubes. The enhanced nucleateboiling dominates, and the convective boiling enhancement, found in plain tube bundles, does not occur.Active enhancement techniques include heated surface rotation, surface wiping, surface vibration,fluid vibration, electrostatic fields, and suction at the heated surface.
Although active techniques areeffective in reducing the wall superheat and/or increasing the critical heat flux, the practical applicationsare very limited, largely because of the difficulty of reliably providing the mechanical or electrical effect.Compound enhancement, which involves two or more techniques applied simultaneously, has alsobeen studied. Electrohydrodynamic enhancement was applied to a finned tube bundle, resulting in nearlya 200% increase in the average boiling heat transfer coefficient of the bundle, with a small powerconsumption for the field.Convective Boiling/EvaporationThe structured surfaces described in the previous section are generally not used for in-tube vaporization,because of the difficulty of manufacture.
One notable exception is the high-flux surface in a verticalthermosiphon reboiler. The considerable increase in the low-quality, nucleate boiling coefficient isdesirable, but it is also important that more vapor is generated to promote circulation.Helical repeated ribs and helically coiled wire inserts have been used to increase vaporization coefficients and the dry-out heat flux in once-through boilers.Numerous tubes with internal fins, either integral or attached, are available for refrigerant evaporators.Original configurations were tightly packed, copper, offset strip fin inserts soldered to the copper tubeor aluminum, star-shaped inserts secured by drawing the tube over the insert. Examples are shown inFigure 4.8.5.
Average heat transfer coefficients (based on surface area of smooth tube of the samediameter) for typical evaporator conditions are increased by as much as 200%. A cross-sectional viewof a typical “microfin” tube is included in Figure 4.8.5. The average evaporation boiling coefficient is© 1999 by CRC Press LLC4-248Section 4FIGURE 4.8.5 Inner-fin tubes for refrigerant evaporators: (a) Strip-fin inserts, (b) Star-shaped inserts, (c) Microfin.increased 30 to 80%.
The pressure drop penalties are less; that is, lower percentage increases in pressuredrop are frequently observed.Twisted-tape inserts are generally used to increase the burnout heat flux for subcooled boiling at highimposed heat fluxes 107 – 108 W/m2, as might be encountered in the cooling of fusion reactor components.Increases in burnout heat flux of up to 200% were obtained at near atmospheric pressure.Vapor-Space CondensationAs discussed elsewhere, condensation can be either filmwise or dropwise. In a sense, dropwise condensation is enhancement of the normally occurring film condensation by surface treatment. The only realapplication is for steam condensers, because nonwetting coatings are not available for most other workingfluids. Even after much study, little progress has been made in developing permanently hydrophobiccoatings for practical steam condensers.
The enhancement of dropwise condensation is pointless, becausethe heat transfer coefficients are already so high.Surface extensions are widely employed for enhancement of condensation. The integral low fin tubing(Figure 4.8.4a), used for kettle boilers, is also used for horizontal tube condensers. With proper spacingof the fins to provide adequate condensate drainage, the average coefficients can be several times thoseof a plain tube with the same base diameter. These fins are normally used with refrigerants and otherorganic fluids that have low condensing coefficients, but which drain effectively, because of low surfacetension.The fin profile can be altered according to mathematical analysis to take full advantage of the Gregorigeffect, whereby condensation occurs mainly at the tops of convex ridges.
Surface tension forces thenpull the condensate into concave grooves, where it runs off. The average heat transfer coefficient isgreater than that for an axially uniform film thickness. The initial application was for condensation ofsteam on vertical tubes used for reboilers and in desalination. According to numerical solutions, theoptimum geometry is characterized by a sharp fin tip, gradually changing curvature of the fin surfacefrom tip to root, wide grooves between fins to collect condensate, and periodic condensate strippers.Figure 4.8.6 schematically presents the configuration.FIGURE 4.8.6 Recommended flute profile and schematic of condensate strippers.© 1999 by CRC Press LLCHeat and Mass Transfer4-249Recent interest has centered on three-dimensional surfaces for horizontal-tube condensers. The considerable improvement relative to low fins or other two-dimensional profiles is apparently due tomultidimensional drainage at the fin tips.
Other three-dimensional shapes include circular pin fins, squarepins, and small metal particles that are bonded randomly to the surface.Convective CondensationThis final section on enhancement of the various modes of heat transfer focuses on in-tube condensation.The applications include horizontal kettle-type reboilers, moisture separator reheaters for nuclear powerplants, and air-conditioner condensers.Internally grooved or knurled tubes, deep spirally fluted tubes, random roughness, conventional innerfin tubes have been shown to be effective for condensation of steam and other fluids.The microfin tubes mentioned earlier have also been applied successfully to in-tube condensing.
Asin the case of evaporation, the substantial heat transfer improvement is achieved at the expense of alesser percentage increase in pressure drop. By testing a wide variety of tubes, it has been possible tosuggest some guidelines for the geometry, e.g., more fins, longer fins, and sharper tips; however, generalcorrelations are not yet available.
Fortunately for heat-pump operation, the tube that performs best forevaporation also performs best for condensation.Twisted-tape inserts result in rather modest increases in heat transfer coefficient for complete condensation of either steam or refrigerant. The pressure drop increases are large because of the large wettedsurface. Coiled tubular condensers provide a modest improvement in average heat transfer coefficient.ReferencesBergles, A.E. 1969.
Survey and evaluation of techniques to augment convective heat and mass transfer,in Progress in Heat and Mass Transfer, Vol. 1, Pergamon, Oxford, England.Bergles, A.E. 1985. Techniques to augment heat transfer, in Handbook of Heat Transfer Applications,W.M. Rohsenow, J.P. Hartnett, and E.N. Ganic, Eds., McGraw-Hill, New York, 3-1–3-80.Bergles, A.E. 1988.
Some perspectives on enhanced heat transfer — second generation heat transfertechnology, J. Heat Transfer, 110, 1082–1096.Bergles, A.E. 1997. Heat transfer enhancement — the encouragement and accommodation of high heatfluxes. J. Heat Transfer, 119, 8–19.Bergles, A.E., Blumenkrantz, A.R., and Taborek, J. 1974. Performance evaluation criteria for enhancedheat transfer surfaces, in Heat Transfer 1974, The Japan Society of Mechanical Engineers, Tokyo,Vol. II, 234–238.Bergles, A.E., Jensen, M.K., Somerscales, E.F.C., and Manglik, R.M.
1991. Literature Review of HeatTransfer Enhancement Technology for Heat Exchangers in Gas-Fired Applications, Gas ResearchInstitute Report, GR191-0146.Carnavos, T.C. 1979. Heat transfer performance of internally finned tubes in turbulent flow, in Advancesin Advanced Heat Transfer, ASME, New York, 61–67.Manglik, R.M. and Bergles, A.E. 1990. The thermal-hydraulic design of the rectangular offset-strip-fincompact heat exchanger, in Compact Heat Exchangers, Hemisphere Publishing, New York,123–149.Pate, M.B., Ayub, Z.H., and Kohler, J.
1990. Heat exchangers for the air-conditioning and refrigerationindustry: state-of-the-art design and technology, in Compact Heat Exchangers, Hemisphere Publishing, New York, 567–590.Ravigururajan, S. and Bergles, A.E. 1985. General Correlations for Pressure Drop and Heat Transferfor Single-Phase Turbulent Flow in Internally Ribbed Tubes, in Augmentation of Heat Transferin Energy Systems, HTD-Vol. 52, ASME, New York, 9–20.Thome, J.R.
1990. Enhanced Boiling Heat Transfer, Hemisphere Publishing, New York.Webb, R.L. 1994. Principles of Enhanced Heat Transfer, John Wiley & Sons, New York.© 1999 by CRC Press LLC4-250Section 4Further InformationThis section gives some indication as to why heat transfer enhancement is one of the fastest growingareas of heat transfer. Many techniques are available for improvement of the various modes of heattransfer.
Fundamental understanding of the transport mechanism is growing, but, more importantly,design correlations are being established. Many effective and cost-competitive enhancement techniqueshave made the transition from the laboratory to commercial heat exchangers.Broad reviews of developments in enhanced heat transfer are available (Bergles, 1985; Bergles, 1988;Thome, 1990; Webb, 1994, Bergles, 1997). Also, several journals, especially Heat Transfer Engineering,Enhanced Heat Transfer, and International Journal of Heating, Ventilating, Air-Conditioning and Refrigerating Research, feature this technology.Cooling TowersAnthony F.
MillsIntroductionIn a wet cooling tower, water is evaporated into air with the objective of cooling the water stream. Bothnatural- and mechanical-draft towers are popular, and examples are shown in Figure 4.8.7. Large naturaldraft cooling towers are used in power plants for cooling the water supply to the condenser. Smallermechanical-draft towers are preferred for oil refineries and other process industries, as well as for centralair-conditioning systems and refrigeration plant. Figure 4.8.7a shows a natural draft counterflow unit inwhich the water flows as thin films down over a suitable packing, and air flows upward. In a naturaldraft tower the air flows upward due to the buoyancy of the warm, moist air leaving the top of thepacking. In a mechanical-draft tower, the flow is forced or induced by a fan.
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