The CRC Handbook of Mechanical Engineering. Chapter 4. Heat and Mass Transfer (776127), страница 58
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See Figure 4.8.1a. The configuration is generally chosen to disturb the viscoussublayer rather than to increase the heat transfer surface area. Application of rough surfaces is directedprimarily toward single-phase flow.Extended surfaces are routinely employed in many heat exchangers. See Figure 4.8.1a to d. Work ofspecial interest to enhancement is directed toward improvement of heat transfer coefficients on extendedsurfaces by shaping or perforating the surfaces.Displaced enhancement devices are inserted into the flow channel so as indirectly to improve energytransport at the heated surface.
They are used with forced flow. See Figure 4.8.1e and f.Swirl-flow devices include a number of geometric arrangements or tube inserts for forced flow thatcreate rotating and/or secondary flow: coiled tubes, inlet vortex generators, twisted-tape inserts, andaxial-core inserts with a screw-type winding.Surface-tension devices consist of wicking or grooved surfaces to direct the flow of liquid in boilingand condensing.Additives for liquids include solid particles and gas bubbles in single-phase flows and liquid traceadditives for boiling systems.Additives for gases are liquid droplets or solid particles, either dilute-phase (gas-solid suspensions)or dense-phase (fluidized beds).Mechanical aids involve stirring the fluid by mechanical means or by rotating the surface.
Surface“scraping,” widely used for batch processing of viscous liquids in the chemical process industry, isapplied to the flow of such diverse fluids as high-viscosity plastics and air. Equipment with rotating heatexchanger ducts is found in commercial practice.Surface vibration at either low or high frequency has been used primarily to improve single-phaseheat transfer.© 1999 by CRC Press LLC4-241Heat and Mass TransferabcdefFIGURE 4.8.1 Enhanced tubes for augmentation of single-phase heat transfer. (a) Corrugated or spirally indentedtube with internal protuberances. (b) Integral external fins. (c) Integral internal fins.
(d) Deep spirally fluted tube.(e) Static mixer inserts. (f) Wire-wound insert.Fluid vibration is the practical type of vibration enhancement because of the mass of most heatexchangers. The vibrations range from pulsations of about 1 Hz to ultrasound.
Single-phase fluids areof primary concern.Electrostatic fields (DC or AC) are applied in many different ways to dielectric fluids. Generallyspeaking, electrostatic fields can be directed to cause greater bulk mixing or fluid or disruption of fluidflow in the vicinity of the heat transfer surface, which enhances heat transfer.Injection is utilized by supplying gas to a stagnant or flowing liquid through a porous heat transfersurface or by injecting similar fluid upstream of the heat transfer section. Surface degassing of liquidscan produce enhancement similar to gas injection.
Only single-phase flow is of interest.Suction involves vapor removal, in nucleate or film boiling, or fluid withdrawal, in single-phase flow,through a porous heated surface.Two or more of the above techniques may be utilized simultaneously to produce an enhancement thatis larger than either of the techniques operating separately. This is termed compound enhancement.It should be emphasized that one of the motivations for studying enhanced heat transfer is to assessthe effect of an inherent condition on heat transfer.
Some practical examples include roughness producedby standard manufacturing, degassing of liquids with high gas content, surface vibration resulting fromrotating machinery or flow oscillations, fluid vibration resulting from pumping pulsation, and electricalfields present in electrical equipment.© 1999 by CRC Press LLC4-242Section 4The surfaces in Figure 4.8.1 have been used for both single-phase and two-phase heat transferenhancement.
The emphasis is on effective and cost-competitive (proved or potential) techniques thathave made the transition from the laboratory to commercial heat exchangers.Single-Phase Free ConvectionWith the exception of the familiar technique of providing extended surfaces, the passive techniques havelittle to offer in the way of enhanced heat transfer for free convection. This is because the velocities areusually too low to cause flow separation or secondary flow.The restarting of thermal boundary layers in interrupted extended surfaces increases heat transfer soas to more than compensate for the lost area.Mechanically aided heat transfer is a standard technique in the chemical and food industries whenviscous liquids are involved.
The predominant geometry for surface vibration has been the horizontalcylinder, vibrated either horizontally or vertically. Heat transfer coefficients can be increased tenfold forboth low-frequency/high-amplitude and high-frequency/low-amplitude situations. It is, of course, equallyeffective and more practical to provide steady forced flow. Furthermore, the mechanical designer isconcerned that such intense vibrations could result in equipment failures.Since it is usually difficult to apply surface vibrations to practical equipment, an alternative techniqueis utilized whereby vibrations are applied to the fluid and focused toward the heated surface.
With propertransducer design, it is also possible to improve heat transfer to simple heaters immersed in gases orliquids by several hundred percent.Electric fields are particularly effective in increasing heat transfer coefficients in free convection.Dielectrophoretic or electrophoretic (especially with ionization of gases) forces cause greater bulk mixingin the vicinity of the heat transfer surface. Heat transfer coefficients may be improved by as much as afactor of 40 with electrostatic fields up to 100,000 V. Again, the equivalent effect could be produced atlower capital cost and without the voltage hazard by simply providing forced convection with a bloweror fan.Single-Phase Forced ConvectionThe present discussion emphasizes enhancement of heat transfer inside ducts that are primarily of circularcross section.
Typical data for turbulence promoters inserted inside tubes are shown in Figure 4.8.2. Asshown in Figure 4.8.2a, the promoters produce a sizable elevation in the Nusselt number, or heat transfercoefficient, at constant Reynolds number, or velocity. However, as shown in Figure 4.8.2b, there is anaccompanying large increase in the friction factor.Surface roughness has been used extensively to enhance forced convection heat transfer.
Integralroughness may be produced by the traditional manufacturing processes of machining, forming, casting,or welding. Various inserts can also provide surface protuberances. In view of the infinite number ofpossible geometric variations, it is not surprising that, even after more than 300 studies, no completelysatisfactory unified treatment is available.In general, the maximum enhancement of laminar flow with many of the techniques is the same orderof magnitude, and seems to be independent of the wall boundary condition. The enhancement with somerough tubes, corrugated tubes, inner-fin tubes, various static mixers, and twisted-type inserts is about200%.
The improvements in heat transfer coefficient with turbulent flow in rough tubes (based on nominalsurface area) are as much as 250%. Analogy solutions for sand-grain-type roughness and for squarerepeated-rib roughness have been proposed. A statistical correlation is also available for heat transfercoefficient and friction factor.The following correlations are recommended for tubes with transverse or helical repeated ribs (Figure4.8.1a) with turbulent flow (Ravigururajan and Bergles, 1985):177-0.210.212ì0.29-0.024 ù üNu Di ,a Nu Di ,s = í1 + é2.64 Re 0.036 (e Di )p Di )a 90) (Pr )((ýûú þî ëê(© 1999 by CRC Press LLC)(4.8.1)4-243Heat and Mass TransferFIGURE 4.8.2 Typical data for turbulence promoters inserted inside tubes: (a) heat transfer data, (b) friction data.(From Bergles, 1969. With permission.)ì é( 0.37- 0.157 p Di )0.67- 0.06 p Di - 0.49 a 90 )( -1.66´10-6 Re Di -0.33a 90 )fa fs = í1 + ê29.1Re (Di´ (e Di )´ ( p Di )î ë4.59 + 4.11´10´ (a 90)(-6Re D - 0.15 p Dii) ´ æ1 + 2.94 ö sinbùèüïýïþúûn ø(4.8.2)15 16 16 15where the subscript a refers to the enhanced tube and the subscript s refers to the smooth tube.
Thespecial symbols are given as follows: e = protuberance height; p = repeated-rib pitch; a = spiral anglefor helical ribs, °; n = number of sharp corners facing the flow; and b = contact angle of rib profile, °.Also,(Nu s = 0.125 f Re Di Pr 1 + 12.7(0.125 f )0.5)Pr 0.667 - 1and(fs = 1.82 log10 Re Di - 1.64)-2**Much work has been done to obtain the enhanced heat transfer of parallel angled ribs in shortrectangular channels, simulating the interior of gas turbine blades.
Jets are frequently used for heating,cooling, and drying in a variety of industrial applications. A number of studies have reported thatroughness elements of the transverse-repeated-rib type mitigate the deterioration in heat transfer downstream of stagnation.Extended surfaces can be considered “old technology” as far as most applications are concerned. Thereal interest now is in increasing heat transfer coefficients on the extended surface.
Compact heatexchangers of the plate-fin or tube-and-center variety use several enhancement techniques: offset stripfins, louvered fins, perforated fins, or corrugated fins. Coefficients are several hundred percent above the*The Fanning friction factor is used throughout this section.© 1999 by CRC Press LLC4-244Section 4smooth-tube values; however, the pressure drop is also substantially increased, and there may be vibrationand noise problems.For the case of offset strip fins the following correlations are recommended for calculating the j andf characteristics (Manglik and Bergles, 1990)[][]jh = 0.6522 Re h-0.5403 a -0.1541 d 0.1499 g -0.0678 ´ 1 + 5.269 ´ 10 -5 Re1h.340 a 0.504 d 0.456 g -1.055fh = 9.6243Re h-0.7422 a -0.1856 d 0.3053 g -0.2659 ´ 1 + 7.669 ´ 10 -8 Re h4.429 a 0.920 d 3.767 g 0.2360.10.1(4.8.3)(4.8.4)where jH (the heat transfer j-factor NuH/ReHPr1/3), and fh, and Reh are based on the hydraulic diametergiven byDh = 4shl [2(sl + hl + th) + ts](4.8.5)Special symbols are a = aspect ratio s/h, d = ratio t/l, g = ratio t/s, s = lateral spacing of strip fin, h =strip fin height, l = length of one offset module of strip fins, and t = fin thickness.These equations are based on experimental data for 18 different offset strip-fin geometries, and theyrepresent the data continuously in the laminar, transition, and turbulent flow regions.Internally finned circular tubes are available in aluminum and copper (or copper alloys).
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