The CRC Handbook of Mechanical Engineering. Chapter 4. Heat and Mass Transfer (776127), страница 59
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Correlations(for heat transfer coefficient and friction factor) are available for laminar flow, for both straight andspiral continuous fins.Turbulent flow in tubes with straight or helical fins (Figure 4.8.1c) was correlated by (Carnavos, 1979)éA ùNu h = 0.023Pr 0.4 Re h0.8 ê c úë Aci ûfh = 0.046Re-0.2h0.1é Ac ùêúêë Ac,i úûé As,i ùúêë As û0.5(sec a)3(4.8.6)0.5(sec a)0.75(4.8.7)where Ac,i is based on the maximum inside (envelope) flow area, As,i is based on the maximum inside(envelope) surface area, and a the spiral angle for helical fins, °.A numerical analysis of turbulent flow in tubes with idealized straight fins was reported.
The necessaryconstant for the turbulence model was obtained from experimental data for air. Further improvementsin numerical techniques are expected, so that a wider range of geometries and fluids can be handledwithout resort to extensive experimental programs.Many proprietary surface configurations have been produced by deforming the basic tube. The“convoluted,” “corrugated,” “spiral,” or “spirally fluted” tubes (Figure 4.8.1a) have multiple-start spiralcorrugations, which add area, along the tube length.
A systematic survey of the single-tube performanceof condenser tubes indicates up to 400% increase in the nominal inside heat transfer coefficient (basedon diameter of a smooth tube of the same maximum inside diameter); however, pressure drops on thewater side are about 20 times higher.Displaced enhancement devices are typically in the form of inserts, within elements arranged topromote transverse mixing (static mixers, Figure 4.8.1e).
They are used primarily for viscous liquids,to promote either heat transfer or mass transfer. Displaced promoters are also used to enhance the radiantheat transfer in high-temperature applications. In the flue-tube of a hot-gas-fired hot water heater, thereis a trade-off between radiation and convection.
Another type of displaced insert generates vortices,which enhance the downstream flow. Delta-wing and rectangular wing promoters, both co-rotating and© 1999 by CRC Press LLC4-245Heat and Mass Transfercounterrotating, have been studied. Wire-loop inserts (Figure 4.8.1f) have also been used for enhancementof laminar and turbulent flow.Twisted-tape inserts have been widely used to improve heat transfer in both laminar and turbulentflow. Correlations are available for laminar flow, for both uniform heat flux and uniform wall temperatureconditions.
Turbulent flow in tubes with twisted-tape inserts has also been correlated. Several studieshave considered the heat transfer enhancement of a decaying swirl flow, generated, say, by a shorttwisted-tape insert.Performance Evaluation Criteria for Single-Phase Forced Convection in TubesNumerous, and sometimes conflicting, factors enter into the ultimate decision to use an enhancementtechnique: heat duty increase or area reduction that can be obtained, initial cost, pumping power oroperating cost, maintenance cost (especially cleaning), safety, and reliability, among others.
These factorsare difficult to quantitize, and a generally acceptable selection criterion may not exist. It is possible,however, to suggest some performance criteria for preliminary design guidance. As an example, considerthe basic geometry and the pumping power fixed, with the objective of increasing the heat transfer. Thefollowing ratio is then of interestæh öR3 = ç a ÷è hs ø D , L, N ,P,Tiin , DT(Nu Pr )(Nu Pr )0.4=a0.4s=qaqs(4.8.8)where P = pumping power, Tin = inlet bulk temperature of fluid, and DT = average wall-fluid temperaturedifference.With the pumping power (neglecting entrance and exit losses) given asP = NVAc 4 f ( L D) rV 2 2(4.8.9)fs = 0.046 Re 0s .2(4.8.10)andAc,a fa Re 3a =0.046Ac,s Re 2s .8(4.8.11)The calculation best proceeds by picking Re Di ,a , and reading Nu Di ,a / Pr 0.4 and fa.
Re Di ,s is then obtainedfrom Equation (4.8.11) and Nu Di ,s / Pr 0.4 obtained from a conventional, empty-tube correlation. Thedesired ratio of Equation (4.8.8) is then obtained. Typical results are presented in Figure 4.8.3 for arepeated-rib roughness (Bergles et al., 1974).Active and Compound Techniques for Single-Phase Forced ConvectionUnder active techniques, mechanically aided heat transfer in the form of surface scraping can increaseforced convection heat transfer. Surface vibration has been demonstrated to improve heat transfer toboth laminar and turbulent duct flow of liquids. Fluid vibration has been extensively studied for bothair (loudspeakers and sirens) and liquids (flow interrupters, pulsators, and ultrasonic transducers). Pulsations are relatively simple to apply to low-velocity liquid flows, and improvements of several hundredpercent can be realized.Some very impressive enhancements have been recorded with electrical fields, particularly in thelaminar-flow region.
Improvements of at least 100% were obtained when voltages in the 10-kV rangewere applied to transformer oil. It is found that even with intense electrostatic fields, the heat transferenhancement disappears as turbulent flow is approached in a circular tube with a concentric innerelectrode.© 1999 by CRC Press LLC4-246Section 4FIGURE 4.8.3 Constant pumping power performance criterion applied to repeated rib roughness.Compound techniques are a slowly emerging area of enhancement that holds promise for practicalapplications, since heat transfer coefficients can usually be increased above each of the several techniquesacting along.
Some examples that have been studied are as follows: rough tube wall with twisted-tapeinserts, rough cylinder with acoustic vibrations, internally finned tube with twisted-tape inserts, finnedtubes in fluidized beds, externally finned tubes subjected to vibrations, rib-roughened passage beingrotated, gas-solid suspension with an electrical field, fluidized bed with pulsations of air, and a ribroughened channel with longitudinal vortex generation.Pool BoilingSelected passive and active enhancement techniques have been shown to be effective for pool boilingand flow boiling/evaporation.
Most techniques apply to nucleate boiling; however, some techniques areapplicable to transition and film boiling.It should be noted that phase-change heat transfer coefficients are relatively high. The main thermalresistance in a two-fluid heat exchanger often lies on the non-phase-change side. (Fouling of either sidecan, of course, represent the dominant thermal resistance.) For this reason, the emphasis is often onenhancement of single-phase flow. On the other hand, the overall thermal resistance may then be reducedto the point where significant improvement in the overall performance can be achieved by enhancingthe two-phase flow.
Two-phase enhancement would also be important in double-phase-change (boiling/condensing) heat exchangers.As discussed elsewhere, surface material and finish have a strong effect on nucleate and transitionpool boiling. However, reliable control of nucleation on plain surfaces is not easily accomplished.Accordingly, since the earliest days of boiling research, there have been attempts to relocate the boilingcurve through use of relatively gross modification of the surface. For many years, this was accomplishedsimply by area increase in the form of low helical fins.
The subsequent tendency was to structure surfacesto improve the nucleate boiling characteristics by a fundamental change in the boiling process. Manyof these advanced surfaces are being used in commercial shell-and-tube boilers.Several manufacturing processes have been employed: machining, forming, layering, and coating. InFigure 4.8.4a standard low-fin tubing is shown. Figure 4.8.4c depicts a tunnel-and-pore arrangementproduced by rolling, upsetting, and brushing. An alternative modification of the low fins is shown inFigure 4.8.4d, where the rolled fins have been split and rolled to a T shape.
Further modification of theinternal, Figure 4.8.4e, or external, Figure 4.8.4f, surface is possible. Knurling and rolling are involvedin producing the surface shown in Figure 4.8.4g. The earliest example of a commercial structured surface,shown in Figure 4.8.4b is the porous metallic matrix produced by sintering or brazing small particles.Wall superheat reductions of up to a factor of ten are common with these surfaces. The advantage is not© 1999 by CRC Press LLCHeat and Mass Transfer4-247FIGURE 4.8.4 Examples of commercial structured boiling surfaces. (From Pate, M.B. et al., in Compact HeatExchangers, Hemisphere Publishing, New York, 1990.
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