Heat and mass transfer enhancing

When a process is produced under the divergent or convergent action of two different forces, the ratio between them represents a dimensionless number. The heat and mass transfer enhanced by the supplementary action of a pulsating field (vibration of apparatus, pulsation of one (or two) phase flow(s), ultrasound action etc.) has been experimented and applied in some cases [6.25-6.27]. Then, the new... 

A. E. Bergles, M. K. Jensen, and B. Shome, Bibliography on Enhancement of Convective Heat and Mass Transfer, Heat Transfer Lab Report HTL-23, Rensselaer Polytechnic Institute, Troy, NY, 1995. Also, The Literature on Enhancement of Convective Heat and Mass Transfer, Enhanced Heat Transfer (4) 1-6,1996. 

Zhou, T. and Liu, H.T., Effects of Heat and Mass Transfer Enhancements on PEM Fuel Cell Performances, I2th International Heat Transfer Conference, France, August 18-23, 2002. 

The historical aspects of heat and mass transfer enhancement, or intensification, are of interest for many reasons. We can examine some processes that were intensified some decades before the phrase process intensification became common in the process engineering (particularly chemical) literature. Some used electric fields, others employed centrifugal forces. The use of rotation to intensily heat and mass transfer has, as we wiU see, become one of the most spectacular tools in the armoury of the plant engineer in several unit operations, ranging from reactors to separators. However, it was in the area of heat transfer - in particular two-phase operation - that rotation was first exploited in industrial plants. The rotating boiler is an interesting starting point, and rotation forms the essence of PI within this chapter. 

Electrostatic fields, the application of which is sometimes known as electrohydrodynamics (EHD), are of growing interest in the inventory of tools for heat and mass transfer enhancement. Normally limited in their application to dielectric fluids, such as refrigerants and transformer oil, the high voltages (and low currents) associated with such fields are reflected in increased activity close to the heat transfer surface. The technique has been successfully used to enhance boiling and condensation - see Allen et al. (1995) for an historical review. 

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. 

When a liquid is dispersed into droplets the surface area is increased, which enhances the rates of heat and mass transfer. For a particular liquid dispersed at constant concentration in air the MIE varies with approximately the cube of surface average droplet diameter, hence the MIE decreases by a factor of about 8 when the surface average diameter D is halved (A-5-1.4.4). Ease of ignition is greatly enhanced for finely divided mists with D less than about 20 /rm, whose MIE approaches that of the vapor. Below 10 /rm a high flash point liquid mist (tetrahydronaphthalene) was found to behave like vapor while above about 40/rm the droplets tended to burn individually [ 142]. Since liquid mists must partially evaporate and mix with air before they ignite, the ease with which a liquid evaporates also affects MIE (Eigure 5-1.4.4). 

In many instances, two or more miscible liquids must be mixed to give a product of a desired specification, such as, for example, in the blending of petroleum products of different viscosities. This is the simplest type of mixing as it involves neither heat nor mass transfer, nor indeed a chemical reaction. Even such simple operations can however pose problems when the two liquids have vastly different viscosities. Another example is the use of mechanical agitation to enhance the rates of heat and mass transfer between the wall of a vessel, or a coil, and the liquid. Additional complications arise in the case of highly viscous Newtonian and non-Newtonian liquids. 

Many other, less obvious physical consequences of miniaturization are a result of the scaling behavior of the governing physical laws, which are usually assumed to be the common macroscopic descriptions of flow, heat and mass transfer [3,107]. There are, however, a few cases where the usual continuum descriptions cease to be valid, which are discussed in Chapter 2. When the size of reaction channels or other generic micro-reactor components decreases, the surface-to-volume ratio increases and the mean distance of the specific fluid volume to the reactor walls or to the domain of a second fluid is reduced. As a consequence, the exchange of heat and matter either with the channel walls or with a second fluid is enhanced. 

The studies mentioned above referred to the general advantages of micro flow processing in terms of enhanced heat and mass transfer [85] (see a more detailed description in [42]). 

A more recent review by Fahidy (FI) concerns the chemical engineering approach to electrochemical processes, such as fluidized-bed reactors, bipolar particulate reactors, pulsed electrochemical reactors, gas-phase electrochemical reactors, electrocrystallization and electrodissolution, and the enhancement of heat and mass transfer in electric fields. In this review, the author also discusses dimensionless mass-transfer equations applied in cell design. Such equations are reviewed in greater detail in Section VI. 

Park KJ, Jung D (2007) Enhancement of nucleate boiling heat transfer using carbon nanotubes. International Journal of Heat and Mass Transfer 50 4499 1502. 

Steady-state operation was quickly achieved under SCF conditions and the SCF-FT process has a marked effect on the hydrocarbon product distribution with a shift to higher carbon number products owing to enhanced heat and mass transfer from the catalyst surface. In addition, an obvious difference in the olefin content was observed where the 1-olefin content in the SCF phase was always higher than in the gas phase. Based on the experimental observations, a mechanistic explanation is provided for the difference of the reaction behavior under supercritical and gas-phase environments. 

There is conflicting evidence regarding the extent to which imposed vibrations increase particle to fluid heat and mass transfer rates (G2), with some authors even claiming that transfer rates are decreased. For sinusoidal velocity variations superimposed on steady relative motion, enhancement of transfer depends on a scale ratio A/d and a velocity ratio Af /Uj (G3). These quantities are rather like the scale and intensity of turbulence (see Chapter 10). For Af /Uj < l/2n, the vibrations do not cause reversal in the relative motion and the enhancement of mass transfer has been correlated (G3) by... 

One of the most important factors to consider in designing a fermenter is the provision for adequate mixing of its contents. The main objectives of mixing in fermentation are to disperse the air bubbles, to suspend the microorganisms (or animal and plant tissues), and to enhance heat and mass transfer in the medium. 

A high-intensity heat pump, called Rotex, has been developed taking advantage of the enhanced heat and mass transfer performance of rotating discs (44). This single device carries out the processes of evaporation, condensation, absorption, and heat transfer to a working fluid. 

Heat and mass transfer, especially mass transfer, in multiphase systems are problems commonly encountered in processing units in the chemical, petrochemical, and many other process industries. Because transfer rates significantly affect the efficiencies and technical-economic indexes of the processes, the enhancement of transfer has been a continuing topic of interest in chemical engineering since the late 1930s. A vast number of theoretical and experimental investigations have been carried out in the search for new methods of enhancing transfer between phases. 

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