Heat Transfer Handbook part 106 pptx

K] Teflon in pits Teflon spots on surface Smooth surface Pitted surface Water = 1 atm boiling on stainless steel p Figure 14.8 Enhanced boiling heat transfer characteristics of water at

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potential startup problems (Thome, 1990; Webb, 1994; Bergles, 1998) Furthermore, there are limited data which indicate that the tube bundle performance in a ther-mosyphon reboiler may be different from that of a single tube (Yilmaz et al., 1981)

A laterstudy (Jensen et al., 1992), however, suggests that single-tube results may be used to predict tube bundle behavior

Several attempts have been made to model the nucleate boiling mechanisms on structured surfaces and to develop predictive correlations These analyses have tried

to scale and correlate the effects of geometrical properties of the structured surface, or

2

2

5 4 4

3

3

6 7 8 9

10

1

⌬Tsat[K]

K]

Teflon in pits

Teflon spots

on surface

Smooth surface

Pitted surface

Water ( = 1 atm) boiling on stainless steel

p

Figure 14.8 Enhanced boiling heat transfer characteristics of water at 1 atm from a stainless steel surface coated with Teflon spots (From Young and Hummel, 1965.)

1048 HEAT TRANSFER ENHANCEMENT

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the characteristics of nucleating cavities and liquid-replenishing intercavity channels,

on the heat transfer Extended discussions of several different models can be found in Nakayama et al (1980a,b), Webb (1994), Chien and Webb (1998), Yabe et al (1999), and Kim and Choi (2001), among others

Forless wetting orrelatively higher-surface-tension fluids, coatings of nonwetting material (e.g., Teflon) on either the heated surface or its pits and cavities have been found to improve stable nucleation and reduce the required wall superheat (Griffith and Wallis, 1960; Young and Hummel, 1965; Gaertner, 1967; Vachon et al., 1969)

Young and Hummel (1965) sprayed a smooth as well as a “pitted” stainless steel sur-face with Teflon to create spots of the no-wetting material on the heated sursur-face and

in the pits This was found to promote nucleate boiling in water, with relatively low wall superheat and three- to four-times-higher heat transfer coefficients, as shown

in Fig 14.8 In a more recent study of boiling of alcohols (methanol, ethanol, and isopropanol) at atmospheric and subatmospheric pressures on a horizontal brass tube coated with polytetrafluoroethylene, Vijaya Vittala et al (2001) have reported a sig-nificant enhancement in the heat transfer A typical set of theirdata forethanol is presented in Fig 14.9 Surfaces coated with thin films of low-thermal-conductivity materials have also been shown to enhance pool boiling heat transfer under heater surface temperature-controlled experimental conditions (Zhukov et al., 1975) This

6

5 7 8 9

104 2

5 4

3

6 7 8 9

104

103

q⬙ [W/m ]2

PTFE coated tube, = 97.60 kN/mp 2

Plain tube, = 97.60 kN/mp 2

PTFE coated tube, = 37.67 kN/mp 2

Plain tube, = 37.67 kN/mp 2

Experimental data (Vijaya Vittala et al., 2001)

Figure 14.9 Enhancement in the boiling heat transfercoefficient forethanol at different subatmospheric pressures and heat fluxes from PTFE-coated tubes

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essentially breaks up film boiling by reducing the fluid-surface interface temperature

to promote a more effective transition or nucleate boiling This has also been found

to be the case with scale oroxide coatings, which destabilize film boiling and signif-icantly reduce the quench times of a heated surface (Bergles and Thompson, 1970)

Vapor space condensation heat transfer can be enhanced primarily by treated surfaces that promote dropwise condensation The intent here is to prevent surface wetting and break up the condensate film into droplets, as depicted in Fig 14.10, which lends to better drainage and more effective vapor renewal at the cold heat transfer interface This technique had been found to enhance the heat transfer by factors

of 10 to 100 in comparison with that in filmwise condensation (Bergles, 1998)

Nonwetting coatings of several different materials have been used effectively in different investigations (Hannemann, 1977; Tanasawa, 1978; Griffith, 1985) This includes coating the condensing surface with a thin film of an inorganic compound, a thin-film plating of a noble metal, ora film of an organic polymer(Rose et al., 1999;

Das et al., 2000) Of these, organic coatings have been used considerably in steam systems, and Marto et al (1986) and Holden et al (1987) have given a comprehensive evaluation of their performance in promoting dropwise condensation However, this technique is useful in promoting dropwise condensation in steam (or other relatively high-surface-tension fluid) condensers, as there seem to be no nonwetting (or “Freon-phobic”) substances that would induce dropwise condensation in refrigerants and otherlow-surface-tension fluids (Iltscheff, 1971)

Glicksman et al (1973) have shown that by strategically placing strips of Teflon

or other nonwetting material in a helical or axial arrangement around the circum-ference of horizontal tubes, the average condensation heat transfer coefficients of steam on horizontal tubes can be improved by 20 to 50% In such an arrangement, the condensate flow is interrupted near the leading edge of the nonwetting strip, and

Figure 14.10 Examples of dropwise condensation of steam: (a) on a plain surface (from Hampson and Ozisik, 1952); (b) on a SAM-coated corrugated tube (from Das et al., 2000).

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the condensate film that reforms at its downstream edge is much thinner Cary and Mikic (1973) suggest further that Marangoni convection (surface tension–gradient or interfacial temperature–gradient, driven flow) at the liquid–vapor–strip interface may also be a contributing factor in enhancing the heat transfer There are, however, several practical issues relating to the application and surface integrity of nonwetting material coating, as discussed by Tanasawa (1978), Marto et al (1986), and Das et al (2000)

There have been a few attempts to develop theoretical models for dropwise con-densation as well In this, the studies reported by Le Fevre and Rose (1966), Rose (1988), and Tanaka (1975a,b) are particularly noteworthy, and their brief assessment

is provided by Rose et al (1999) The application of a hydrophobic coating of self-assembled monolayers, formed by a chemisorption of alkylthiols on metallic surfaces,

to promote dropwise condensation has been investigated by Das et al (2000) Experi-ments on steam condensation on coated corrugated tubes with gold and copper–nickel alloy surfaces under atmospheric (101 kPa) and subatmospheric (10 kPa) pressure conditions with wall subcooling of about 16°C and 6°C, respectively, showed that condensation heat transfer coefficients increased by factors of 2.3 to 3.6 compared

to those for uncoated tubes The characteristic condensation behavior is illustrated

photographically in Fig 14.10b Although technically, this study would fall under

the compound enhancement category, the noteworthy and novel technique to effect dropwise condensation is particularly relevant here

In a different scheme to enhance film condensation, Notaro (1979) has described

a patent for a tube surface that is intermittently coated with small-diameter metal particles The objective of this scheme is that condensation would occuroverthe strategically coated surface, and liquid film would then drain along the uncoated, bare tube surface With a typical configuration (a 6-m vertical tube with 0.5-mm-diameter particle coatings interspaced over 50% of the tube surface), up to 17 times highersteam condensation coefficients than that in normal film condensation have been reported (Notaro, 1979; Webb, 1984)

One of the earliest and perhaps simplest and yet highly effective techniques is the use of surface roughness in turbulent single-phase flows (Nikuradse, 1933; Dippery and Sabersky, 1963; Webb et al., 1971); in laminar flows, small-scale surface rough-ness tends to have little effect It essentially disturbs the viscous sublayer in the near-wall turbulent flow structure to promote higher momentum and heat transport (Niku-radse, 1933) Much of the early work focused on “naturally” occurring roughness in commercial tubes However, as pointed out by Bergles (1998), because such natural roughness is not well defined, artificial or structured roughness is now commonly employed in most applications Structured roughness can be integral to the surface,

or the protuberances can be introduced in the form of wire-coil-type inserts The former can be produced by machining (e.g., knurling, threading, grooving), forming,

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casting, or welding, and the resulting surface proturberances or grooves can be two-dimensional or discrete three-two-dimensional in their geometrical arrangement (Webb, 1994; Bergles, 1998) As a result, almost an infinite number of geometric variations are possible for structured roughness, and this is also reflected in the more than 700 studies, and still counting (Bergles et al., 1995; Bhatnagar and Manglik, 2002) that have been reported in the literature

Rough surfaces have been employed to enhance heat transfer in single-phase flows both inside tubes and outside tubes, rods, and tube bundles A collection of studies and patents that deal with their thermal–hydraulic performance as well as fabrica-tion and manufacturing methods is given by Webb et al (1983), Ravigururajan and Bergles (1986), Webb (1994), Bergles et al (1995), and Bergles (1998) Some of the two-dimensional structured roughness used inside tubes includes repeated or trans-verse ribs, helical ribs, and wire coil inserts, in a variety of profiles, as illustrated in Fig 14.11 Of these, tubes with grooves are often referred to as corrugated, roped, indented, fluted, orconvoluted tubes, among othernames, and are illustrated in Fig

14.12a They provide an external rough surface as well and have been used in

double-pipe and shell-and-tube bundles to enhance annulus- orshell-side heat transfer Two representative tubes with three-dimensional roughness, composed of serrated ribs,

dimples, and cross-rifled surfaces, are also depicted in Fig 14.12b and c Besides

these, knurling (to form diamond knurls), discrete slotting, rolling, and particle depo-sition have been investigated as possible candidates for three-dimensional roughness (Durant et al., 1965; Groehn and Scholz, 1976; Achenbach, 1977; Fenner and Ragi, 1979; Menze et al., 1994; Webb, 1994)

Over the last five decades, a fairly extensive set of experimental data for heat transfer coefficients and friction factors in rough tubes has been obtained (Ravigu-rurajan and Bergles, 1986; Esen et al., 1994; Bergles, 1998) Several attempts have also been made to develop predictive equations for the turbulent flow regime One

of the first such efforts was the momentum heat transfer analogy solution proposed

by Dippery and Sabersky (1963) for sand-grain roughness Subsequently, Webb et al

(1971, 1972) devised another analogy-based correlation for heat transfer coefficients fordifferent fluids (0.7 ≤ Pr ≤ 38) and tubes with transverse repeated-rib roughness

(α = 90°, 0.01 ≤ e/d ≤ 0.04, 10 ≤ p/e ≤ 15; Fig 14.11) Withers (1980a,b),

sim-ilarly, extended this correlating technique to commercial single- and multiple-helix internally ridged tubes A detailed discussion of these solutions and their prediction efficacy is given by Webb (1994) In a more recent experimental study on turbulent flows of water and oil in spirally corrugated tubes, Dong et al (2001) have developed

a new set of analogy-based friction factor and Nusselt number correlations Adopting

an empirical approach, combined with a statistical analysis of a fairly large database for heat transfer coefficients and friction factors for the various types of roughness shown in Fig 14.11, Ravigururajan and Bergles (1996) have proposed the following Nusselt number and Fanning friction factor correlations:

Nu= Nuo



1+



d

0.212 p d

90

0.29

(14.10)

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f = fo



1+



29.1Re a1  e

d

a2 p d

a3 α 90

a4

1+ 2.94 sinβn

 15/16 16/15

(14.11a) where

d



− 0.49 α

90



d



90



d

 (14.11b)

Figure 14.11 Different type of structured roughness and their profile shapes considered by Ravigururajan and Bergles (1986, 1996)

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and the respective smooth tube Nuoandf operformances are given by

Nuo= Re· Pr(f o /2)

1+ 12.7√f o /2(Pr2/3 − 1) (14.12)

f o = (1.58 ln Re − 3.28)−2 (14.13) The prediction reliability and accuracy of eqs (14.10) and (14.11) have been shown to

be very good compared with more than 1800 experimental data points, representing the full range of roughness types and profiles depicted in Fig 14.11 (Ravigurura-jan and Bergles, 1986; Bergles, 1998), and some typical predictions are graphed in

Fig 14.2b.

A novel technique to provide variable-roughness “on demand” inside tubes has recently been proposed and tested by Champagne and Bergles (2001) The concept involves using a wire-coil insert made of a shape-memory alloy (SMA) that alters its geometry in response to changes in temperature (Bergles and Champagne, 1999)

With a fixed roughness height (e/d), the wire-coil insert changes from a compressed

shape, which occupies a small fraction of the tube length, to an expanded shape that has the desired or “trained” roughness pitch (p/d) and helix pitch (α/90°) upon being

heated In essence, the SMA insert can be designed to provide the required roughness

to meet “active” changes in process conditions, which are usually reflected by the variations in the tube wall temperature and enhancement objectives A schematic rendering of this concept is given in Fig 14.13 In a proof-of-concept experimental

Figure 14.12 Tubes with two- and three-dimensional structured roughness: (a) corrugated tubes (courtesy of Wolverine Tube, Inc.); (b) serrated ribs on outer surface and staggered dimples on inner surface (courtesy of Sumitomo Light Metal Industries); and (c) Cross-rifled

inner surface (from Nakamura and Tanaka, 1973)

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test, Champagne and Bergles (2001) have further shown that depending on the flow rate, a typical SMA (NiTi) wire coil insert can produce 30 to 64% increases in the heat transfer coefficients in single-phase turbulent flows

Flows in annuli with an inner rough surface, commonly encountered in a double-pipe heat exchangerwith an innertube that is convoluted orcorrugated, orhas some other structured roughness on its outer surface, have also been investigated in the literature (Kemeny and Cyphers, 1961; Brauer, 1961; Dalle Donne and Meyer, 1977;

Hudina, 1979; Garimella and Christensen, 1995a,b; Salim et al., 1999; Kang and Christensen, 2000) It an early study with helical grooves and helical protuberances, the experimental data of Kemeny and Cyphers (1961) suggest enhancement factors of 1.25 to 2.0 for different flow rates and roughness (Bergles, 1998) Surface protrusions had a relatively higher performance than grooves Some of the more recent work with water and propylene glycol flows in annuli with fluted or corrugated tubes (Garimella and Christensen, 1995a,b; Salim et al., 1999; Kang and Christensen, 2000) indicate

up to fourtimes higherNusselt numbers, with up to 10 times higherfriction factors in the turbulent flow regime Two different sets of correlations have also been devised

by Garimella and Christensen (1995a,b), and Salim et al (1999) that are based on their own respective data No attempt appears to have been made to consolidate and compare the various data sets available in the literature For high-viscosity fluids (e.g., propylene glycol) and/or large temperature-dependent viscosity variations in the flow field, Kang and Christensen (2000) suggest incorporating a Sieder and Tate (1936) type of viscosity-ratio correction factor in the predictive equation

The heat transfer enhancement in annuli with three-dimensional diamond-knurls type of roughness on the innerheated tube’s outersurface has been investigated by Durant et al (1965) At fixed pumping power conditions, the heat transfer coeffi-cients forthe knurled annuli were found to be up to 75% higherthan those forthe equivalent smooth annuli Dalle Donne (1978) provides turbulent flow data for three-dimensional roughness made up of rows of staggered studs The Stanton numbers were found to increase by factors of 3 and 4; the associated friction factors were 8

to 12 times higher For their application in gas-cooled reactors, the grouping

St3/f

is often used as a figure of merit to interpret the performance data This translates into enhancement ratios of up to 2.3 for the three-dimensional roughness Similarly, annuli data have been “transformed” for application to the rod-bundle geometry (Hall, 1962; Lewis, 1974; Hudina, 1979; Webb, 1994) Some work has also been reported

Figure 14.13 Conceptual representation of “roughness on demand” provided by a shape-memory alloy wire coil insert (From Champagne and Bergles, 2001.)

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on crossflow over tube bundles that have roughness on the outer surface, particularly for gas-cooled reactors and shell-and-tube heat exchangers (Bergles, 1998) Much

of this work is based on the extended experimental investigations with cross flows over a single rough cylinder The case of pyramid-shaped roughness elements has been considered by Achenbach (1977) and Zhukauskas et al (1978), with air and water flows, respectively, and up to 150% increase in the Nusselt number has been reported

Structured roughness, usually in the form of ribs, in both two-dimensional and discrete three-dimensional arrangements, has also been applied to plate-type and rect-angularchannels (Han, 1988; Han et al., 1991; Liou and Hwang, 1992; Prakash and Zerkle, 1995; Ekkad and Han, 1997; Olsson and Sund´en, 1998; Sund´en, 1999; Gao and Sund´en, 2001) Much of this work is directed toward compact heat exchang-ers and gas turbine cooling and regeneration systems A variety of arrangements forrectangularribs attached to two opposite heated walls of rectangular(orplate) channels have been considered (Han et al., 1991; Zhang et al., 1994; Olsson and Sund´en, 1998) With ribs angled at 60°, some examples of the geometrical schemes investigated experimentally by Olsson and Sund´en (1998) and Sund´en (1999) are depicted in Fig 14.14 Their relative enhanced performance, as represented by the

volume goodness factor orheat transfercoefficient versus pumping powerexpended

per unit heat transfer area [the development of this figure of merit is given in Kays and London (1984) and Shah and London (1978)], is also presented in this figure Of the five configurations shown here, the last one, with short rib segments, essentially constitutes three-dimensional structured roughness An interesting variation in the form of wedge- and delta-shaped ribs, which tend to produce longitudinal vortices in the near-wall region of the flow field, have been suggested by Han et al (1993)

Finally, as noted by Bergles (1998), surface roughness is usually not considered

in applications with free or natural convection In this thermal-fluid-transport regime, the buoyancy-induced fluid flow velocities are generally very low and are not easily disrupted by small-scale surface roughness so as to produce flow separation and recirculation (the primary mechanisms for roughness-induced enhancement) Bergles

et al (1979) reviewed the limited data available in the literature up to that time, which are for air, water, and oil with machined or formed roughness on the surface exposed to free convection Where up to 100% increases in heat transfer have been reported in an air system, it is questionable if radiation effects have been accounted foradequately; in liquids, on the otherhand, the enhancement has been found to be considerably small

14.3.2 Boiling

The application of rough-structured surfaces in pool boiling situations has been

clas-sified under treated surfaces (Bergles, 1998), and their performance was discussed

earlier Their use in forced convection or flow boiling is primarily addressed here

The earliest works on the effects of surface roughness in forced-convection boiling were the investigation of improvements in subcooled CHF by knurling or threading the heated surface (Durant and Mishak, 1959; Durant et al., 1965) When compared

(b) variation of heat transfer coefficient with specific pumping power (volume goodness factor performance).