Enhancement of heat transfer

Thus, the enhancement of heat transfer may be connected to the decrease in the surface tension value at low surfactant concentration. In such a system of coordinates, the effect of the surface tension on excess heat transfer (/z — /zw)/ (/ max — w) may be presented as the linear fit of the value C/Cq. On the other hand, the decrease in heat transfer at higher surfactant concentration may be related to the increased viscosity. Unfortunately, we did not find surfactant viscosity data in the other studies. However, we can assume that the effect of viscosity on heat transfer at surfactant boiling becomes negligible at low concentration of surfactant only. The surface tension of a rapidly extending interface in surfactant solution may be different from the static value, because the surfactant component cannot diffuse to the absorber layer promptly. This may result in an interfacial flow driven by the surface tension gradi-... 

Additive of surfactant leads to enhancement of heat transfer compared to water boiling in the same gap size however, this effect decreases with decreasing channel size. For the same gap size CHF in surfactant solutions is significantly lower than that in water. At high values of heat flux some foaming patches began to occur this process increased with decrease in gap size and led to decrease in CHF, Hetsroni et al. (2007). 

The heat transfer coefficient h increases with increasing superficial liquid velocity 7ls- Enhancement of heat transfer is more pronounced for low values of superficial liquid velocity. An increase in superficial gas velocity [/gs to the same value of the superficial liquid velocity led to a decrease in the heat transfer coefficient. The Nusselt number may be calculated as ... 

The second approach assigns thermal resistance to a gaseous boundary layer at the heat transfer surface. The enhancement of heat transfer found in fluidized beds is then attributed to the scouring action of solid particles on the gas film, decreasing the effective film thickness. The early works of Leva et al. (1949), Dow and Jacob (1951), and Levenspiel and Walton (1954) utilized this approach. Models following this approach generally attempt to correlate a heat transfer Nusselt number in terms of the fluid Prandtl number and a modified Reynolds number with either the particle diameter or the tube diameter as the characteristic length scale. Examples are ... 

Enhancement of Heat Transfer and Mass Transfer Processes... 

The enhancement of heat transfer inside a circular duct is often achieved by inserting a thin, metal tape in such a way that the tape is twisted about its longitudinal axis, as indicated in Fig. 5.47. Swirl flow is created in this manner. The width of the tape is usually the same as the internal diameter of the duct. The tape twist ratio XL is defined as H/d. When XL approaches infinity, the circular duct with the twisted tape becomes two semicircular straight ducts separated by the tape. 

Koch [4] employed suspended rings and disks as inserts, as well as tubes packed with Raschig rings and round balls. The disks give maximum enhancement with moderate increases in friction factors, as indicated in Figs. 11.26 and 11.28 (curve d). Enhancement of heat transfer with rings and round balls is quite comparable to that with disks, but rings and balls increase the friction factor by more than 1600 percent (curves c and d). For further comments on packed tubes, see the chapter on heat transfer in fluidized and packed beds. 

Fluidized beds represent the other end of the spectrum in terms of solids loading (this subject is covered in the chapter on heat transfer in fluidized and packed beds). The very considerable enhancement of heat transfer coefficients, up to a factor of 20 compared to pure gas flow at the same flow rate, has led to applications in such areas as flue gas heat recovery. 

Rotating heat exchanger surfaces occur naturally in rotating electrical machinery, gas turbine rotor blades, and other industrial systems. However, rotation may be deliberately applied to provide active enhancement of heat transfer. 

A. E. Bergles, Enhancement of Heat Transfer, in Heat Transfer 1978, Proceedings of the 6th International Heat Transfer Conference, vol. 6, pp. 89-108, Hemisphere, Washington, DC, 1978. 

I. I. Gogonin and A. R. Dorokhov, Enhancement of Heat Transfer in Horizontal Shell-and-Tube Condensers, Heat Transfer-Soviet Research, 3, pp. 119-126,1981. 

Viskanta, R. "Enhancement of Heat Transfer in Industrial Combustion Systems Problems and Future Challenges." Proceedings of ASME/JSME Thermal Engineering Joint Conference, edited by J. R. Lloyd and Y. Kurosaki, Vol. 5, 161-73, New York ASME, 1991. 

The enhancement of heat transfer at a isothermal tube wall due to temperature-dependent viscosity has been correlated by Kwant et al. [1973] as follows ... 

The linkage between the enhancement of heat transfer at boiling of dilute polymer solutions and the elastic properties of the system is confirmed by the existence of the optimal concentration corresponding to (Figure 7.2.14). Similar optimal concentration was established in addition of polymers to water to suppress turbulence - the phenomenon that also owes its origin to elasticity of macromolecules. Therefore, it is possible to expect that the factors favoring the chain flexibility and increase in the molecular mass, should lead to strengthening of the effect. 

In highly diluted solutions the change in Newtonian viscosity due to polymer is insignificant, and though the correlation between heat transfer enhancement and increase in viscosity has been noticed, it cannot be the reason for observed changes of a. In hydrodynamics, the effect of turbulence suppression by small polymeric additives is known, but it also cannot be considered for such a reason because laminarization of the boimdary layer leads to reduction of the intensity of convective heat transfer. Nevertheless, the phenomenon of the decrease of hydrodynamic resistance and enhancement of heat transfer in boiling dilute solutions have a common nature. The latter effect was connected... 

The linkage between the enhancement of heat transfer at boiling of dilute polymer solutions and the elastic properties of the system is confirmed by the existence of the optimal concentration corresponding to (Figure 7.2.14). Similar optimal concentration was es-... 

Enhancement of heat transfer can lead to an extra energy need. This may be due to an increased resistance to flow, giving a greater pressure drop. Some techniques do not incur an obvious penalty of this type while others, more specifically active enhancement methods, by definition require an external energy input. Both are considered here. 

Laohalertdecha, S., Naphon, R and Wongwises, S. (2007). A review of electrohydrodynamic enhancement of heat transfer. Renewable and Sustainable Energy Reviews, Vol. 11, pp. 858-876. 

Li TX, Lee JH, Wang RZ, Kang YT. Enhancement of heat transfer for thermal energy storage application using stearic acid nanocomposite with multi-walled carbon nanotubes. Energy 2010 55 752-61. 

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