Heat Transfer Enhancement Techniques

Passive heat-transfer enhancement techniques, retrofitted, 13 267 Passive mixers, in microfluidics, 26 966, 967 Passive noise detectors, 11 673 Passive nondestructive tests, 17 416, 425 Passive reactors, 17 555 Passive sensing materials, 22 706-707 Passive smart textiles, 24 625 Passive solar collection, silica aerogel application, 1 761-762 Pasta products, 26 278 Paste-extrusion process, 18 301-302 Paste forming, ceramics, 5 651 Paste inks, 14 315-316... 

Merrill, T. L., Setoguchi, T., and Perez-Bianco, H. (1995) Passive Heat Transfer Enhancement Techniques Applied to Compact Bubble Absorber Design, Journal of Enhanced Heat Transfer, Vol. 

Enhancement has been used for many years to improve heat transfer, particularly for viscous liquids. The principle involved is to disturb the viscous sub-layer near to the heat transfer surface thereby reducing the resistance to heat flow. Since the problem of fouling is very concerned with the transport of foulants across the viscous sub-layer it is to be expected that any attempt to reduce the resistance to heat flow will also affect the propensity towards fouling. Hewitt et al [1994] have given some background to the use of heat transfer enhancement techniques. 

Over the past 60 years, a great deal of applied and theoretical research has been carried out on both polymer and surfactant DRAs because of their potential useful applications and the influence of the additives on both turbulent structure and rheology. Important results include the identification of maximum drag reduction asymptotes (MDRAs) both Virk s MDRA for polymer solutions [Virk et al., 1970 see Eq. (2.5)] and Zakin et al. s MDRA for surfactant solutions [Zakin et al., 1996 see Eq. (2.6)], relating solution nanostructures and rheological properties to macroscopic DR phenomena hypotheses on the influence of DRAs on turbulent structures, mechanisms for turbulent drag reduction, developing heat transfer enhancement techniques, and so on. 

Reay, D. A. Heat Recovery Systems CHP 11 (1) (1991) 1-4. Heat transfer enhancement - a review of techniques and their possible impact on energy efficiency in the U.K. 

Enhancement of nucleate boiling heat transfer in large diameter ehannels is often used to augment flow boiling heat transfer. Enhancement teehniques can be elassified either as passive (without external power) or as active (with external power or external additives). Table 1 gives a short list of the various techniques used in the area of boiling heat transfer. From proeess industry perspective the passive techniques are more important than active techniques because of the benefit of simplicity. 

The effectiveness of different techniques to enhance the heat transfer ability of DR surfactant solutions depends on the surfactant solution, its concentration and temperature, and details of the circulating system. Minimum added costs and maximum heat transfer enhancement are the goals. ... 

Enhanced heat transfer in the industrial applications, such as electronics cooling, is often required. One of the most common methods of heat transfer enhancement is the use of enhanced surfaces, e.g. fins. Moreover, for a constant size and heat exchange rate, a lower temperature gradient shows a more efficient heat transfer. Enhanced heat transfer techniques can be classified as active and passive. 

Finally, it is evident that heat transfer enhancement is well established and is used routinely in the power industry, process industry, and heating, ventilation, and air-conditioning. Many techniques are available for improvement of the various modes of heat transfer. Fundamental understanding of the transport mechanism is growing but, more importantly,... 

Substantial energy cost savings across the whole of the process industries can result if the performance of heat exchangers is improved. One technique which is already used to improve heat exchanger performance (and indeed process heat transfer in general) is enhancement. In fact heat transfer enhancement is the most common way of increasing the heat transfer in equipment, typified by fins on most air-cooled equipment. Of particular relevance is the fact that enhancement, be it of heat and/or mass transfer, is critical to many other PI items of plant. 

Convection heat transfer is dependent largely on the relative velocity between the warm gas and the drying surface. Interest in pulse combustion heat sources anticipates that high frequency reversals of gas flow direction relative to wet material in dispersed-particle dryers can maintain higher gas velocities around the particles for longer periods than possible ia simple cocurrent dryers. This technique is thus expected to enhance heat- and mass-transfer performance. This is apart from the concept that mechanical stresses iaduced ia material by rapid directional reversals of gas flow promote particle deagglomeration, dispersion, and Hquid stream breakup iato fine droplets. Commercial appHcations are needed to confirm the economic value of pulse combustion for drying. 

The use of electromagnetic techniques represents the final method for process intensification of heat transfer that will be dealt with here. Developments in the fuel cell sector hold out the promise of cheap electric power based on surplus hydrogen in chemical plants. The liberalization of the energy sector has also cut electricity prices and enhanced the attractiveness of using electricity in connection with chemical reactors. The precise regulation possible with electrical processes and their clean, environmentally friendly nature are further inducements for their application. 

When extended surfaces such as fins are used to enhance natural convection heat transfer between a solid and a fluid, the flow rate of the fluid in the vicinity of the solid adjusts itself to incorporate the changes in buoyancy and friction. It is obvious that this enhancement technique will work to advantage only when the increase in btroyancy is greater than the additional friction introduced. One does not need to be concerned with pressure drop or pumping power when studying natural convection since no pumps or blowers are used in (his case. Therefore, an enhancement technique in natural convection is evaluated on heat transfer performance alone. 

Boiling heat transfer can also be enhanced by other techniques such as mechanical agiraiion and surface vibration. These techniques ate not practical, however, because of the complications involved. 

To improve the overall heat transfer coefficient and thus the heat transfer in this heat exchanger, we must use some enhancement techniques on the oil side, such as a finned surface. 

Table 1 Enhancement techniques in boiling heat transfer...

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