Under active techniques, mechanically aided heat transfer in the form of surface-scraping can increase forced convection hat transfer. Surface vibration has been demonstrated to improve heat transfer to both laminar and turbulent duct flow of liquids. Fluid vibration has been extensively studied for both air (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 hundred percent can be realized.
A novel, active enhancement concept for a variable roughness, heat exchanger tube is reported by Champagne and Bergles [10]. Shape-memory-alloy wire coils ride along a support structure in close proximity to the inner-tube (or outer-tube) wall of a single- phase heat exchanger. At low wall temperture (efficient heat transfer), the coils are close together. An excessive tube-wall temperature (low heat transfer coefficient) results in extension of the coils. It has been demonstrated that coil extension does produce a substantial heat transfer enhancement. Since the extended coils, at the present time, must be reset by manual or mechanical means (owing to limitations in material processing and training techniques of shape-memory alloys), this is classified as an active enhancement technique.
Some very impressive enhancements have been recorded with electrical fields, particularly in the laminar flow region. Improvements of at least 100% were obtained when voltages in the lO-kV range were applied to transformer oil. Itis found that even with intense electrostatic fields, the heat transfer enhancement disappears as turbulent flow is approached in a circular tube with a concentric inner electrode.
Compound techniques are slowly emerging area of enhancement that holds promise for practical applications, since heat transfer coefficients can usually be increased above each of the several techniques acting alone. Some examples that have been studied are as follows: rough tube wall with twisted-tape inserts, rough cylinder with acoustic vibrations, internally finned tube with twisted-tape insert, finned tubes in fluidized beds, externally finned tubes subjected to vibrations, rib-roughened passage being rotated, gas- solid suspension with an electrical field, fluidized bed with pulsations of air, and a rib- roughened channel with longitudinal vortex generation.
4. Pool Boiling
Selected passive and active enhancement techniques have been shown to be effective for pool boiling and flow boiling/evaporation. Most techniques apply to nucleate boiling;
however, some techniques are applicable to transition and film boiling.
Itshould be noted that phase-change heat transfer coefficients are relatively high.
The main thermal resistance in a two-fluid heat exchanger often lies on the non-phase- change side. (Fouling of either side can, of course, represent the dominant thermal resistance.) For this reason, the emphasis is often on enhancement of single-phase flow. On the other hand, the overall thermal resistance may then be reduced to the point where significant improvement in the overall performance can be achieved by enhancing the 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 transition pool 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 boiling curve through use of relatively gross modification of the surface. For many years, this was accomplished simply by area increase in the form of low helical fins. The subsequent tendency was to structure surfaces to improve the nucleate boiling characteristics by a fundamental change in the boiling process. Many of these advanced surfaces are being used in commercial shell- and-tube boilers.
Several manufacturing processes have been employed: machining, forming, layering, and coating. In Fig. 4(a), standard low-fin tubing is shown. Figure 4(c) depicts a tunnel-and-pore arrangement produced by rolling, upsetting, and brushing. An alternative modification of the low fins is shown in Fig. 4(d), where the rolled fins have been split and rolled to a T shape. Further modification of the internal, Fig. 4(e), or external, Fig. 4(f), surface is possible. Knurling and rolling are involved in producing the surface shown in Fig. 4(g). The earliest example of a commercial structured surface, shown in Fig. 4(b) 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 only high nucleate boiling heat transfer coefficients, but the fact that boiling can take place at very low temperature differences.
The structured boiling surfaces developed for refrigeration and process applications have been used as "heat sinks" for immersion-cooled microelectronic chips.
The behavior of tube bundles is often different with structured-surface tubes. The enhanced nucleate boiling dominates, and the convective boiling enhancement, found in plain tube bundles, does not occur.
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Figure4. Examples of commercial structured boiling surfaces (Pate et al. [IIJ).
Active enhancement techniques include heated surface rotation, surface wiping, surface vibration, fluid vibration, electrostatic fields, and suction at the heated surface.
Although active techniques are effective in reducing the wall superheat and/or increasing the critical heat flux, the practical applications are 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 also been studied. Electrohydrodynamic enhancement was applied to a finned tube bundle, resulting in nearly a 200 percent increase in the average boiling heat transfer coefficient of the bundle, with a small power consumption for the field.
5. Convective Boiling/Evaporation
The structured surfaces described in the previous section are generally not used for intube vaporization, due to the difficulty of manufacture. One notable exception is the High Flux surface in a vertical thermosiphon reboiler. The considerable increase in the low quality, nucleate boiling coefficient is desirable, 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 dryout 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 tube or aluminum, star-shaped inserts secured by drawing the tube over the insert. Examples are shown in Fig. 6. Average heat transfer coefficients (based on surface area of smooth tube of the same diameter) for typical evaporator conditions are increased by as much as 200 percent.
A cross-sectional view of a typical micro-fin tube, now widely used, is included in Fig. 5. The average evaporation boiling coefficient is increased 30-80 percent. The pressure drop penalties are usually less; that is, lower percentage increases in pressure drop are frequently observed. Twisted-tape inserts are generally used to increase the burnout heat flux for subcooled boiling at high imposed heat fluxes (l08 - 108W/m2), as might be encountered in the cooling of fusion reactor components. Increases in burnout heat flux of up to 200 percent have been reported at near-atmospheric pressure, but the enhancement drops off rapidly as the pressure is increased above 2 Mpa.
Since pressure drop is important to the stability of parallel cooling channels, recent work has emphasized both burnout and pressure drop [12].
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Figure5. Inner-fin tubes for refrigerant evaporators: (a) Strip-fin insert; (b) Star-shaped inserts;
(c) Micro-fin.