The engineering cognizance of the need to increase the thermal performance of heat exchangers, thereby effecting energy, material, and cost savings as well as a con-sequential mitigation
Trang 11026 HEAT TRANSFER IN ELECTRONIC EQUIPMENT
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45
[1026],(80)
Lines: 2223 to 2266
———
0.0pt PgVar
———
Custom Page (3.0pt) PgEnds: TEX [1026],(80)
Classes of Interstitial Materials, Proc NSF/DITAC Workshop, Monash University,
Mel-bourne, Victoria, Australia, pp 103–115
Schl¨under, E U., Krotzsch, P., and Hennecke, F W (1970) Gesetzm¨abkerten der W¨arme-und
Stoff¨ubertragung bei der Prallstr¨omung aus Rung-und Schlitzd¨usen, Chem Ing Tech., 42,
333–338
Sergent, J., and Krum, A (1994) Thermal Management Handbook for Electronic Assemblies,
McGraw-Hill, New York
SemiconductorIndustry Association (1997) National Technology Roadmap for
Semiconduc-tors: Technology Needs, SemiconductorIndustry Association, Washington, DC.
Simons, R E., Antonetti, V W., Nakayama, W., and Oktay, S (1997) Heat Transfer in
Elec-tronic Packages, in MicroelecElec-tronics Packaging Handbook, 2nd ed Part I, Tummala, R R.,
Rymaszewski, E J., and Kloptenstein, A G., eds., pp 1-315 to 1-403, Chapman and Hall, New York, N.Y
Sitharamayya, S., and Raju, K S (1969) Heat Transfer between an Axisymmetric Jet and a
Plate Hold Normal to the Flow, Can J Chem Eng., 47, 365–368.
Solbrekken, G L., and Chiu, C.-P (1998) Single Temperature Calibration Method for Die Level Temperature Sensors Using a Single Temperature Technique, IEEE Trans Compo-nents and Packaging Tech., 23(1), 40–46
Song, S., Lee, S., and Au, V (1994) Closed Form Equation for Thermal Constriction/
Spreading Resistances with Variable Resistance Boundary Condition, Proc 1994 IEPS
Conference, pp 111–121.
Sparrow, E M., and Bahrami, P A (1980) Experiments on Natural Convection from Vertical
Parallel Plates with EitherOpen orClosed Edges, J Heat Transfer, 102, 221–227.
Starner, K E., and McManus, H N (1963) An Experimental Investigation of Free Convection
Heat Transfer from Rectangular Fin Arrays, J Heat Mass Transfer, 85, 273–278.
Stevens, J., and Webb, B W (1989) Local Heat Transfer Coefficients under an Axisymmetric,
Single-Phase Liquid Jet, Proc National Heat Transfer Conference, Philadelphia, pp 113–
119
Swartz, E T., and Pohl, R O (1989) Thermal Boundary Resistance, Rev Modern Phys., 61(3).
Tien, C L., and Chen, G (1994) Challenges in Microscale Conductive and Radiative Heat
Transfer, J Heat Transfer, 116, 799–807.
Van de Pol, D W., and Tierney, J K (1973) Free Convection Nusselt Number for Vertical
U-Shaped Channels, J Heat Transfer, 95, 542–543.
Wadsworth, D C., and Mudawar, I (1990) Cooling of a Multichip Electronic Module by
Means of Confined Two Dimensional Jets of Dielectric Liquid, J Heat Transfer, 112, 891–
898
Wang, X S., Dagan, Z., and Jiji, L M (1990) Heat Transfer between a Laminar Free-Surface
Impinging Jet and a Composite Disk, Proc 9th International Heat Transfer Conference,
Vol 4, Hemisphere Publishing, New York, p 137
Watwe, A A., Bar-Cohen, A., and McNeil, A (1997) Combined Pressure and Subcooling
Effects on Pool Boiling from a PPGA Chip Package, J Electron Packag., 119(2), 95–105.
Welling, J R., and Wooldridge, C B (1965) Free Convection Heat Transfer Coefficients from
RectangularVertical Fins, J Heat Transfer, 87, 439–444.
Witzman, S (1998) Thermal Modeling of Air Cooled Components Mounted on Printed Circuit
Boards, in Advances in Thermal Modeling of Electronic Components and Systems, Vol 4,
A Bar-Cohen and A D Kraus, eds., ASME, New York, pp 179–250
Trang 2REFERENCES 1027
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45
[1027],(81)
Lines: 2266 to 2287
———
———
Custom Page (3.0pt)
* PgEnds: PageBreak
[1027],(81)
Womac, D H (1989) Single-Phase Axisymmetric Liquid Jet Impingement Cooling of Dis-crete Heat Sources, M.S thesis, Department of Mechanical Engineering, Purdue University, West Lafayette, IN
Womac, D J., Aharoni, G., Ramadhyani, S., and Incropera, F P (1990) Single Phase Liquid
Jet Impingement Cooling of Small Heat Sources, in Heat Transfer 1990 (Proc 9th
Interna-tional Heat Transfer Conference), Vol 4, Hemisphere Publishing, New York, pp 149–154.
Wroblewski, D., and Joshi, Y (1992) Transient Natural Convection from a Leadless Chip
Carrier in a Liquid Filled Enclosure: A Numerical Study, J Electron Packag., 114, 271–
279
Xu, Y., Luo, X., and Chung, D D L (2000) Sodium Silicate Based Thermal Interface Material
forHigh Thermal Contact Conductance, J Electron Packag., 122, 128–131.
You, S M., Simon, T W., and Bar-Cohen, A (1990) Experiments on Boiling Incipience with a Highly-Wetting Dielectric Fluid: Effects of Pressure, Subcooling and Dissolved Gas
Content, Proc 9th International Heat Transfer Conference, Jerusalem, Israel, Vol 3, pp.
337–342
Yovanovich, M M (1990) Personal communication
Yovanovich, M M., and Antonetti, V W (1988) Application of Thermal Contact Resistance
Theory to Electronic Packages, in Advances in Thermal Modeling of Electronic
Compo-nents and Systems, Vol 1, A D Kraus, and A Bar-Cohen, eds., Hemisphere Publishing,
New York
Zeng, S., Chen, C.-H., Mikkelsen, J C., and Santiago, J G (2000) Fabrication and
Charac-terization of Electrokinetic Micro Pumps, Proc Itherm, May 23–26, pp 31–36.
Zorbil, V., Stulc, P., and Polase, F (1988) Enhancement Cooling of Boards with Integrated
Circuits by Heat Pipes, Proc 3rd International Heat Pipe Symposium, Tsukuba, Japan, pp.
273–279
Zuber, N (1959) Atomic Energy Commission Technical Information Service, Report
AECU-4439, Atomic Energy Commission, Washington, DC.
Trang 31 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45
[First Page]
[1029],(1)
Lines: 0 to 91
———
6.25609pt PgVar
———
Normal Page
* PgEnds: PageBreak
[1029],(1)
CHAPTER 14 Heat Transfer Enhancement
R M MANGLIK Thermal-Fluids and Thermal Processing Laboratory Department of Mechanical, Industrial and Nuclear Engineering University of Cincinnati, Cincinnati, Ohio
14.1 Introduction 14.1.1 Classification of enhancement techniques 14.1.2 Performance evaluation criteria
14.2 Treated surfaces 14.2.1 Boiling 14.2.2 Condensing 14.3 Rough surfaces 14.3.1 Single-phase flow 14.3.2 Boiling
14.3.3 Condensing 14.4 Extended surfaces 14.4.1 Single-phase flow 14.4.2 Boiling
14.4.3 Condensing 14.5 Displaced enhancement devices 14.5.1 Single-phase flow 14.5.2 Boiling
14.5.3 Condensing 14.6 Swirl flow devices 14.6.1 Single-phase flow 14.6.2 Boiling
14.6.3 Condensing 14.7 Coiled tubes 14.7.1 Single-phase flow 14.7.2 Boiling
14.7.3 Condensing 14.8 Additives forliquids 14.8.1 Single-phase flow 14.8.2 Boiling
14.9 Active techniques 14.10 Compound enhancement Nomenclature
References
Trang 41030 HEAT TRANSFER ENHANCEMENT
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45
[1030],(2)
Lines: 91 to 108
——— 0.0pt PgVar
———
Normal Page PgEnds: TEX [1030],(2)
14.1 INTRODUCTION
The conversion, utilization, and recovery of energy in every industrial, commercial, and domestic application involve a heat exchange process Some common examples are steam generation and condensation in power and cogeneration plants; sensible heating and cooling of viscous media in thermal processing of chemical, pharma-ceutical, and agricultural products; refrigerant evaporation and condensation in air conditioning and refrigeration; gas flow heating in manufacturing and waste-heat re-covery; air and liquid cooling of engine and turbomachinery systems; and cooling of electrical machines and electronic devices Improved heat exchange, over and above that in the usual or standard practice, can significantly improve the thermal efficiency
in such applications as well as the economics of theirdesign and operation
The engineering cognizance of the need to increase the thermal performance of heat exchangers, thereby effecting energy, material, and cost savings as well as a con-sequential mitigation of environmental degradation had led to the development and
use of many heat transfer enhancement techniques These methods have in the past been referred to variously as augmentation and intensification, among otherterms.
There is an enormous database of technical literature on the subject, now estimated at over 8000 technical papers and reports, which has been disseminated periodically in numerous bibliographic reports (Bergles et al., 1983, 1991, 1995; Jensen and Shome, 1994), reviews (Webb and Bergles, 1983a; Webb, 1987; Shatto and Peterson, 1996;
Bergles, 1998, 1999; Manglik and Bergles, 2002a), and monographs and edited texts (Thome, 1990; Webb, 1994; Manglik and Kraus, 1996; Kakac¸ et al., 1999) This information growth, represented by the typical yearly distribution of technical pub-lications depicted in Fig 14.1, urgently warrants that a digital library of enhanced heat and mass transfer be established (Bhatnagar and Manglik, 2002) This literature documents the extensive research effort devoted to establishing the conditions under which enhancement techniques will improve the heat or mass transfer in various ap-plications An effort that began more than 140 years ago, when the first attempt to enhance heat transfer coefficients in condensing steam was reported in the classical study by J P Joule (1861), continues to be a major research and development activity (Manglik et al., 2000; Bergles, 2000)
Enhancement techniques essentially reduce the thermal resistance in a conven-tional heat exchangerby promoting higherconvective heat transfercoefficient with
or without surface area increases (as represented by fins or extended surfaces) As a result, the size of a heat exchanger can be reduced, or the heat duty of an existing exchanger can be increased, or the pumping power requirements can be reduced, or the exchanger’s operating approach temperature difference can be decreased The latter is particularly useful in thermal processing of biochemical, food, plastic, and pharmaceutical media, to avoid thermal degradation of the end product On the other hand, heat exchange systems in spacecraft, electronic devices, and medical applica-tions, for example, may rely primarily on enhanced thermal performance for their successful operation The commercialization of enhancement techniques, where the technology has been transferred from the research laboratory to full-scale industrial use of those that are more effective and workable (Bergles et al., 1991; Fletcher and
Trang 5INTRODUCTION 1031
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45
[1031],(3)
Lines: 108 to 139
———
1.097pt PgVar
———
Normal Page PgEnds: TEX
[1031],(3)
Figure 14.1 Yearly growth of the published technical literature on heat transfer enhancement (citations up to mid-1995) (From Bergles et al., 1995.)
Andrews, 1994), has also led to a largernumberof patents (Webb et al., 1983; Bergles
et al., 1991; Webb et al., 1993, 1994)
In this chapter we give an overview of the state of the art in heat transfer enhance-ment in various applications and provide a review of the pertinent literature The use of many different techniques, their thermal–hydraulic performance (with lists of predictive correlations for the heat transfer and pressure drop, or reference to design databases, whereveravailable), and some methodologies fordesign optimization and overall performance evaluation of heat exchangers are outlined
14.1.1 Classification of Enhancement Techniques
Sixteen different enhancement techniques have been identified by Bergles et al
(1983, 1991, 1995), which can be classified broadly as passive and active techniques.
A list of the various methods ordevices undereach of these two categories in given in Table 14.1 The primary distinguishing feature is that unlike active methods, passive techniques do not require direct input of external power They generally use surface
Trang 61032 HEAT TRANSFER ENHANCEMENT
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45
[1032],(4)
Lines: 139 to 153
———
2.78607pt PgVar
———
Short Page PgEnds: TEX [1032],(4)
TABLE 14.1 Classification of Various Heat Transfer Enhancement Techniques
Passive Techniques Active Techniques Treated surfaces Mechanical aids Rough surfaces Surface vibration Extended surfaces Fluid vibration Displaced enhancement devices Electrostatic fields Swirl flow devices Injection
Surface tension devices Jet impingement Additives forliquids
Additives forgases
Compound Enhancement Two ormore passive and/oractive techniques that are employed together
or geometrical modifications to the flow channel, or incorporate an insert, material, oradditional device Except forextended surfaces, which increase the effective heat transfer surface area, these passive schemes promote higher heat transfer coefficients
by disturbing or altering the existing flow behavior This, however, is accompanied by
an increase in the pressure drop In the case of active techniques, the addition of ex-ternal power essentially facilitates the desired flow modification and the concomitant improvement in the rate of heat transfer The use of two or more techniques (passive
and/oractive) in conjunction constitutes compound enhancement.
The effectiveness of any of these methods is strongly dependent on the mode of heat transfer (single-phase free or forced convection, pool boiling, forced convection boiling or condensation, and convective mass transfer), and type and process appli-cation of the heat exchanger In considering their specific appliappli-cations, a descriptive characterization of each of the 16 techniques is useful in assessing their potential
The descriptions of passive techniques, as given by Bergles (1998), are as follows:
1 Treated surfaces are heat transfer surfaces that have a fine-scale alteration to
theirfinish orcoating The alteration could be continuous ordiscontinuous, where the roughness is much smaller than what affects single-phase heat transfer, and they are used primarily for boiling and condensing duties
2 Rough surfaces are generally surface modifications that promote turbulence in
the flow field, primarily in single-phase flows, and do not increase the heat transfer surface area Their geometric features range from random sand-grain roughness to discrete three-dimensional surface protuberances
3 Extended surfaces, more commonly referred to as finned surfaces, provide an
effective heat transfer surface area enlargement Plain fins have been used routinely
Trang 7INTRODUCTION 1033
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45
[1033],(5)
Lines: 153 to 183
———
3.94pt PgVar
———
Short Page PgEnds: TEX [1033],(5)
in many heat exchangers The newer developments, however, have led to modified finned surfaces that also tend to improve the heat transfer coefficients by disturbing the flow field in addition to increasing the surface area
4 Displaced enhancement devices are inserts that are used primarily in confined
forced convection, and they improve energy transport indirectly at the heat exchange surface by “displacing” the fluid from the heated or cooled surface of the duct with bulk fluid from the core flow
5 Swirl flow devices produce and superimpose swirl or secondary recirculation
on the axial flow in a channel They include helical strip or cored screw-type tube inserts, twisted ducts, and various forms of altered (tangential to axial direction) flow arrangements, and they can be used for single-phase as well as two-phase flows
6 Coiled tubes are what the name suggests, and they lead to relatively more
compact heat exchangers The tube curvature due to coiling produces secondary flows
or Dean vortices, which promote higher heat transfer coefficients in single-phase flows as well as in most regions of boiling
7 Surface tension devices consist of wicking or grooved surfaces, which direct
and improve the flow of liquid to boiling surfaces and from condensing surfaces
8 Additives for liquids include the addition of solid particles, soluble trace
ad-ditives, and gas bubbles in single-phase flows, and trace adad-ditives, which usually depress the surface tension of the liquid, for boiling systems
9 Additives for gases include liquid droplets or solid particles, which are
intro-duced in single-phase gas flows in eithera dilute phase (gas–solid suspensions) or dense phase (fluidized beds)
Descriptions for the various active techniques have been given as follows:
1 Mechanical aids are those that stirthe fluid by mechanical means orby rotating
the surface The more prominent examples include rotating tube heat exchangers and scraped-surface heat and mass exchangers
2 Surface vibration has been applied primarily, at either low or high frequency,
in single-phase flows to obtain higherconvective heat transfercoefficients
3 Fluid vibration or fluid pulsation, with vibrations ranging from 1.0 Hz to
ultra-sound (∼1.0 MHz), used primarily in single-phase flows, is considered to be perhaps
the most practical type of vibration enhancement technique
4 Electrostatic fields, which could be in the form of electric or magnetic fields,
ora combination of the two, from dc orac sources, can be applied in heat exchange systems involving dielectric fluids Depending on the application, they can promote greater bulk fluid mixing and induce forced convection (corona “wind”) or electro-magnetic pumping to enhance heat transfer
5 Injection, used only in single-phase flow, pertains to the method of injecting the
same or a different fluid into the main bulk fluid either through a porous heat transfer interface or upstream of the heat transfer section
Trang 81034 HEAT TRANSFER ENHANCEMENT
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45
[1034],(6)
Lines: 183 to 205
———
-2.0pt PgVar
———
Normal Page PgEnds: TEX [1034],(6)
6 Suction involves either vapor removal through a porous heated surface in
nucle-ate or film boiling, or fluid withdrawal through a porous henucle-ated surface in single-phase flow
7 Jet impingement involves the direction of heating or cooling fluid
perpendicu-larly or obliquely to the heat transfer surface Single or multiple jets (in clusters or staged axially along the flow channel) may be used in both single-phase and boiling applications
As acknowledged by Bergles et al (1983, 1996), there are some difficulties in classifying a few techniques, and the somewhat arbitrarily fuzzy distinctions between them should be recognized A good example to illustrate this is the classification of some of the newer structured surfaces used in boiling (Bergles, 2000; Webb, 1994;
Thome, 1990) as treated, rough, or extended surfaces Perhaps a future subcatego-rization and/or recategosubcatego-rization of the enhancement techniques might be needed to sort out such issues in the ever-expanding database
Furthermore, as mentioned earlier, any two or more of these techniques (passive and/oractive) may be employed simultaneously to obtain enhancement in heat trans-ferthat is greaterthan that produced by only one technique itself This
simultane-ous utilization is termed compound enhancement Some promising applications, for
example, are in heat or mass exchangers where one technique may preexist; this is particularly so when the existing enhancement is from an active method
14.1.2 Performance Evaluation Criteria
The issue of quantifying and evaluating the performance of enhancement devices, with a very broad and universal set of criteria, is complex and difficult As Bergles (1998) has pointed out in his survey of the subject: “It seems impossible to establish a generally applicable selection criterion .” Besides the relative thermal–hydraulic performance improvements brought about by the enhancement devices, there are many factors that may have to be considered They include economic (engineer-ing development, capital, installation, operat(engineer-ing, maintenance, and other such costs), manufacturability (machining, forming, bonding, and other production processes), reliability (material compatibility, integrity, and long-term performance), and safety, among others The assessment of these factors, as well as the enhanced convection performance, is usually application driven The discussion in this chapter, however, will be restricted only to the convective thermal–hydraulic behavior, for which several criteria have been developed to quantify the relative enhancement in different applica-tions (Bergles, 1998; Webb, 1994; Kays and London, 1984; Bergles et al., 1974a,b)
The enhanced heat transfer obtained in forced convection is always accompanied
by an increase in the pressure drop This is illustrated in Fig 14.2 for typical cases of single-phase flows in (a) enhanced plate fin cores, and (b) tubes with structured sur-face roughness As recommended by Marner et al (1983), the results for the enhanced geometries are based on an equivalent parallel-plate duct (same plate separation) in the first case, and the “envelope” or“empty” tube diameterin the second case This allows direct comparison of enhancement data with the “normal” or “smooth tube”
Trang 9INTRODUCTION 1035
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45
[1035],(7)
Lines: 205 to 212
———
0.097pt PgVar
———
Normal Page PgEnds: TEX [1035],(7)
Figure 14.2 Typical thermal–hydraulic performance of enhanced surfaces: (a) enhanced plate fin geometries; (continued)
performance It may be noted that in many enhanced duct geometries, particularly with inserts and extended surfaces, the hydraulic diameter is different from their “par-ent” flow channel The improved heat transfer coefficients due to enhancement are evident from Fig 14.2, as are the concomitant friction factors Thus, the problem is essentially one of finding the optimum ornet gain afterconsidering the overall energy use of the heat exchanger
In most practical applications of enhancement techniques, the following
perfor-mance objectives, along with a set of operating constraints and conditions, are usually
considered for optimizing the use of a heat exchanger:
Trang 101036 HEAT TRANSFER ENHANCEMENT
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45
[1036],(8)
Lines: 212 to 227
———
———
Normal Page PgEnds: TEX [1036],(8)
Figure 14.2 (Continued) (b) tubes with different internal roughness.
1 Increase the heat duty of an existing heat exchanger without altering the pump-ing power (or pressure drop) or flow rate requirements
2 Reduce the approach temperature difference between the two heat-exchanging fluid streams for a specified heat load and size of exchanger
3 Reduce the size or heat transfer surface area requirements for a specified heat duty and pressure drop
4 Reduce the process stream’s pumping power requirements for a given heat load and exchanger surface area