AUGMENTATION OF HEAT TRANSFER, SINGLE PHASE

With the exception of the familiar technique of providing extended surfaces, the passive techniques have little to offer in the way of augmented heat transfer for free convection. This is because the velocities are usually too low to cause flow separation or secondary flow. The limited data for free convection from machined or formed rough surfaces, with water, and oil, indicate that increases in heat transfer coefficient up to 100%can be obtained with air, but that the increases with liquids are very small.

Design procedures for single fins and fin arrays are well established (see Extended Surface Heat Transfer); however, little testing or analysis has been directed at interrupted extended surfaces. The restarting of thermal boundary layers is expected to increase coefficients, so as to more than compensate for the lost area. The effectiveness of this concept has been demonstrated in the wire-loop fins used for base-board hot water heaters or “convectors”. This problem is also of interest in electronic cooling, where the “heat sinks” are often discontinuous fins, and in natural draft cooling of finned tube banks, necessitated by loss of fan power.

Mechanically aided heat transfer is a standard technique in the chemical and food industries when viscous liquids are involved. (See also Agitated Vessel Heat Transfer.)

Surface vibration has been extensively studied in the laboratory. The predominant geometry has been the horizontal cylinder vibrated either horizontally or vertically. Heat transfer coefficients can be increased 10-fold for both low-frequency/high-amplitude and high-frequency/low-amplitude situations. Although the improvements can be dramatic, it must be recognized that natural convection is inherently an inefficient mode of heat transfer. Since at maximum enhancement, the average velocity of the surface over a cycle is less that 1 m/s, it is more practical to provide steady forced flow. The mechanical designer is also concerned that such intense vibrations could result in equipment failures.

Rotating surfaces may also be used near the prime heat transfer surface to aid the free convection flow, thereby augmenting the heat transfer. Since it is usually difficult to apply surface vibrations to practical equipment, an alternative technique is utilized whereby vibrations are applied to the fluid and focused toward the heated surface. Generators employed range from flow interrupters to piezoelectric transducers, thus covering the range from pulsation of 1 Hz to ultrasound of 106 Hz. Much research has been reported on the effects of acoustic vibrations on heat transfer to gases from horizontal cylinders. Increases in average coefficients are observed only above an intensity of about 140 db, which is far in excess of human ear tolerance. The maximum increases reported are usually 100 to 200%. With proper ultrasonic transducer design, it is also possible to improve heat transfer to simple heaters immersed in liquids by several hundred percent. Cavitation is generally the dominant enhancement mechanism. In general, with liquids, there is considerable difficulty in designing systems to transmit vibrational energy to large surfaces.

Electric fields can be utilized to increase heat transfer coefficients in free convection. The configuration may be a heated wire in a concentric tube maintained at a high voltage relative to the wire, or a fine wire electrode may be utilized with a horizontal plate. Dielectrophoretic or electrophoretic (especially with ionization of gases) forces cause greater bulk mixing in the vicinity of the heat transfer surface. Heat transfer coefficients may be improved by as mush as a factor of 40.

Recent activity has centered on the application of corona discharge cooling to practical problems. Cooling of cutting tools by point electrodes has been proposed. Also, parallel-wire electrodes have been used to improve the heat dissipation of a standard, horizontal finned tube; heat transfer coefficients can be increased by several hundred percent when sufficient electrical power is supplied. It would appear, however, that the equivalent effect could be produced at lower capital cost and without the hazards of 10,000-100,000 V by simply providing forced convection with a blower or fan.

Gas injection into a liquid through a porous heated plate, which has been used to simulate nucleate boiling, can be regarded as an augmentation technique. Although coefficients can be increased several hundred percent, the practical applications of injection would appear to be rather limited, because of the difficulty of supplying and removing the gas.

The present discussion emphasizes augmentation of heat transfer inside ducts that are primarily of circular cross section. The book by Zukauskas and Kalinin (1990) can be consulted for information on heat transfer in external flow, including augmented tube banks.

Surface roughness has been used extensively to augment forced convection heat transfer. Integral roughness 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 of possible geometric variations, it is not surprising that, even after more than 300 studies, no completely satisfactory unified treatment is available.

In general, the maximum augmentation of laminar flow with many of the techniques is the same order of magnitude, and seems to be independent of the wall boundary condition [Joshi and Bergles (1980)]. The augmentation with some rough tubes, corrugated tubes, inner-fin tubes, various static mixers, and twisted-type inserts is about 200 percent. Spikes and ripples have been used to enhance nominally laminar flow of air in parallel-plate channels of large aspect ratios (~ plate heat exchangers). Most plate heat exchangers utilize corrugated surfaces, for structural reasons as well as augmentation. It is generally agreed that the heat transfer and pressure drop characteristics of commercial corrugated surfaces used in plate exchangers are quite similar. The improvements in heat transfer coefficient with turbulent flow in rough tubes (based on nominal surface area) are as much as 250%. Analogy solutions for sand-grain type roughness and for square repeated-rib roughness have been proposed. A statistical correlation is also available for heat transfer coefficient and friction factor.

Much work has been done to obtain the augmented heat transfer of parallel angled ribs in short rectangular 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 that roughness 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. The real interest is in increasing heat transfer coefficients on the extended surface. Compact Heat Exchangers of the plate-fin or tube-and-center variety use several augmentation techniques: offset strip fins, louvered fins, perforated fins, or corrugated fins. Coefficients are several hundred percent above the smooth-tube values; however, the pressure drop is also substantially increased, and there may be vibration and noise problems. For more details on the current status of air-side (external) heat transfer in finned-tube heat exchangers, the review of Webb (1994) should be consulted.

Internally finned circular tubes are available in aluminum and copper (or copper alloys). Correlations (for heat transfer coefficient and friction factor) are available for laminar and turbulent flow, for both straight and spiral continuous fins.

A numerical analysis of turbulent flow in tubes with idealized straight fins was reported. The necessary constant for the turbulence model was obtained from experimental data for air. Further improvements in numerical techniques are expected, so that a wider range of geometries and fluids can be handled without 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 have multiple-start spiral corrugations, which add areas, along the tube length. A systematic survey of the single-tube performance of condenser tubes indicates up to 400% increase in the nominal inside heat transfer coefficient (based on diameter of a smooth tube of the same maximum inside diameter); however, pressure drops on the water side are about 20 times higher.

Displaced enhancement devices are typically in the form of inserts, within elements arranged to promote transverse mixing (static mixers) (Figure 2e). They are used primarily for viscous liquids, to promote either heat transfer or mass transfer. There are no broad-based correlations available, because of the many geometric arrangements and the strong influence of fluid properties and heating conditions. In general, the higher the heat transfer coefficient, the higher the pressure drop. Similar inserts or packings have been used for turbulent flow; however, this application is recommended only for short sections with high heat fluxes, since the pressure drop is so high.

Displaced promoters are also used to enhance the radiant heat transfer in high-temperature applications. In the flue-tube of a hot gas-fired hot water heater, there is 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 counter-rotating, have been studied.

Wire-loop inserts (Figure 2f) have also been used for augmentation of laminar and turbulent flow.

Heat transfer coefficients can be substantially higher in coiled tubes than in straight tubes because of the secondary flow generated by the curvature.

Twisted-tape inserts have been widely used to improve heat transfer in both laminar and turbulent flow. Correlations are available for laminar flow, for both uniform heat flux and uniform wall temperature conditions. Turbulent flow in tubes with twisted-tape inserts has also been correlated. Several studies have considered the heat transfer augmentation of a decaying swirl flow, generated, say, by a short twisted-tape insert.

Modest improvements in heat transfer are observed when gas bubbles or solid particles are added to liquids.

Gas-side heat transfer can also be augmented by adding a small volumetric fraction of solid particles. The particles are carried along with the stream and separated for reuse, in the case of a once-through system, or circulated, in the case of a closed system. The enhancements of up to four times the pure gas heat transfer coefficients are attributed to thinning of the viscous sublayer and higher thermal conductivity in that layer. It does not appear that any practical applications of dilute-phase gas-solid heat transfer are currently being considered.

Fluidized Beds are used in many industrial applications. Heat transfer coefficients to tubes within a bed can be enhanced by a factor of 20, compared to pure gas flow at the same flow rate.

When liquid droplets are added to a flowing gas stream, heat transfer is enhanced by sensible heating of the two-phase mixture, evaporation of the liquid, and disturbance of the boundary layer. When spray cooling was applied to a compact heat exchanger core, the maximum improvement of 40% was attributed to formation of a partial liquid film and sensible heating of that film. In general, the large flow rate of liquid required tends to limit practical applications of this technique.

Under active techniques, mechanically aided transfer in the form of surface-scraping can increase forced convection heat transfer. Unfortunately, the necessary hardware is not particularly compatible with most heat exchangers. Surface-scraping has been used with air flow on flat plates.

The other aspect of this technique is rotating surfaces. Moderate increases in heat transfer coefficients have been reported for laminar flow in a straight tube rotating about its own axis, a straight tube rotating around a parallel axis, a rotating circular tube, and the rotating, curved, circular tube. Maximum improvements of 350% were recorded for laminar flow, but for turbulent flow the maximum increase was only 25%. In general, these are examples of naturally occurring phenomena that result in enhancement: cooling windings of rotating electrical machinery, cooling of gas turbine rotor blades, and so in. (See Rotating Duct Systems, Heat Transfer In.)

The passive augmentation techniques may be compromised by use in fouling situations, because augmented heat transfer often means augmented mass transfer. However, some surfaces display “antifouling” behavior. [See Bergles and Somerscales (1995)].

Surface vibration has been demonstrated to improve heat transfer to both laminar and turbulent duct flow of liquids. The largest improvements (up to 200%) are observed with laminar or transitional flow utilizing a concentric-tube heat exchanger with the inner tube vibrated transversely or a rectangular channel with a flexible, vibrating side. The complexity of the vibrational equipment and the relatively large power expenditure would seem to rule out this technique fir practical application.

Fluid vibration has been extensively studied for both air (loudspeakers and sirens) and liquids (flow interruptors, pulsators, and ultrasonic transducers). The gas results are not encouraging, as intensities above 120 db are required, and the effect is largely one of triggering fury turbulent flow at transitional Reynolds numbers. On the other hand, pulsating jets have significant augmentation as compared to steady impinging jets.

Pulsations are relatively simple to apply to low-velocity liquid flows, and improvements of several hundred percent can be realized. Turbulence triggering and cavitation appear to be important enhancement mechanisms. Application of high-frequency vibrations is difficult, and only modest improvements in heat transfer coefficient are recorded.

Of course, heat transfer rates in pulse combustion tailpipes and in other oscillating turbulent flows have been found to be significantly higher than those of steady, turbulent flow.

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 10-kV range were applied to transformer oil. Although it is desirable to take advantage of any naturally occurring electrical fields in electrical equipment, it would appear to be quite difficult to introduce this augmentation in practical equipment.

It is found that even with intense electrostatic fields, the heat transfer augmentation disappears as turbulent flow is approached in a circular tube with a concentric inner electrode. There is little effect of corona wind, even at low air velocities, with the exception of tests with three electrodes under a finned tube. In this case, a 60% increase in heat transfer coefficient was noted. A comprehensive survey of the subject is given by Ohadi (1991).

Single-phase heat transfer can be enhanced by injecting gas into a liquid through a porous heated surface. Up to five-fold increases in local heat transfer coefficient have been observed by injecting similar fluid into a turbulent tube flow. In pipe flow, this is done by having injectors spaced along the pipe.

Large increases in heat transfer coefficient are predicted for laminar and turbulent flow with surface suction. The general characteristics of the latter predictions were confirmed by experiments. However, suction is difficult to incorporate into practical heat exchange equipment.

Compound techniques are slowly emerging area of enhancement that hold 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:

One may consider the use of augmentation techniques to satisfy any of the following thermal-hydraulic objectives: (1) to reduce prime surface area, (2) to increase heat transfer capacity, (3) to reduce the approach temperature difference for the process streams, or (4) to reduce pumping power. Having defined a basic objective, the designer will establish the parameters that are fixed and the basic constraints that must be satisfied. Through manipulation of the data or correlations for heat transfer coefficients and friction factors, performance ratios can be calculated, for example, the ratio of the prime surface area of the enhanced heat exchanger to that of the normal or reference exchanger at constant pumping power. Design objectives and performance ratios are discussed in detail in Nelson and Bergles (1986).

Augmentation usually will not be attractive unless the augmented heat exchanger offers a cost advantage relative to the use of conventional heat transfer configurations. Additional factors entering into the ultimate decision to use an augmentation technique are materials limitations, fouling potential, safety, and reliability. The status and prospects for commercial development of augmentation techniques are discussed by Bergles (1988).