Transfer in Two-Phase Flow

Two-phase flow often occurs in electrochemical reactors, usually in a gas/liquid or liquid/liquid form. 

On applying phase transfer agents, electrochemical reactions in liquid/ liquid systems can attain industrial importance. Overall mass transfer limiting currents have been found to be the linear sum of three effects  

Mass transfer enhancement resulting from the extraction of the reacting species from the dispersed phase in intimate contact with the electrode surface k  

---

Heat transfer in two-phase flow can be studied most conveniently by adopting a two-part classification heat transfer without phase change, and heat transfer with phase change. In this section, the non-phase-change problems will be discussed the phase-change problem will be dealt with in Section III. 

Some empirical equations for heat transfer in two-phase flow... 

When the air-content is 10%, and the fluid inlet velocity is the only parametric conditions, the distance of the radial heat transfer in two-phase flow is increase and then decreases, so there is a peak value. The simulation results show when the inlet velocity is 50 m/s, there is a biggest heat transfer distance value, which is 1.8 m. The smallest one is 1.1 m under the speed of 5 m/s. The average temperature of the wall inlet velocity of two-phase fluid can be calculated through the scatter plot, and the outlet temperature of the outlets in this two-phase flow can be determined by this way, which is 21.21 °C. When the fluid inlet velocity is 5 m / s, the average outlet temperature of the two-phase fluid is 18.24 °C, higher than the inlet 3.24 °C, and it is the minimum temperature rise of the same group. From the temperature rise we can drawn that under the same working conditions, when the fluid inlet velocity is 50 m / s, the effect of heat transfer fluid-solid is the best. 

Pohorecki, R. (2007) Effectiveness of interfacial area for mass transfer in two-phase flow in microreactors. Chem. Eng. ScL, 62 (22), 6495-6498. 

Limited experimental data for liquid-soUd mass transfer are available. Oliver and Hoon [47] measured heat transfer in two-phase flow in capillaries using very viscous liquids and hence with thick lubricating layers. The best data set is by Horvath et al. [48], who measured the hydrolysis of N-benzoylarginine ethyl ester in a 1.2 m long tube coated with the immobilized enzyme trypsin. The intrinsic rate of this reaction was high enough that mass transfer from the liquid was limiting. The experimental data are reported as Sh versus Is/dh with the Reynolds number Re as a parameter and Sh versus Re with the aspect ratio Is/dh as a parameter. 

Pohorecld [59] investigated the effectiveness of the interfadal area for mass transfer in two-phase flow in an MSR. He developed criteria to avoid saturation of the liquid wall film in the case of physical absorption or liquid of the film for absorption with instantaneous chemical reaction ... 

In reviews of mass transfer in two-phase flow in microchannels, Kreutzer and coworkers [40, 41] noted that the Taylor flow regime is typically associated with the best gas-liquid mass transport because of the no-slip condition at the channel walls. The velocity gradient within the liquid slug promotes a chaotic mixing effect. The resultant continual refreshing of the two-phase interface provides a high driving force... 

Multiphase catalytic reactors are employed in nearly 80% of industrial processes with annual global sales of about 1.5 trillion, contributing around 35% of the world s GDP [17]. Microreactors for multiphase reactions are classified based on the contact principles of gas and liquid phases continuous-phase contacting and dispersed-phase contacting [18]. In the former type, the two phases are kept in continuous contact with each other by creating an interface. In the latter case, one fluid phase is dispersed into another fluid phase. In addition, micro trickle bed operation is reported following the path of classical chemical engineering. The study of mass and heat transfer in two-phase flow in micro trickle bed reactors still remains as a less... 

The pressure drop due to acceleration is important in two-phase flow because the gas is normally flowing much faster than the liquid, and therefore as it expands the liquid phase will accelerate with consequent transfer of energy. For flow in a vertical direction, an additional term — AZ y must be added to the right hand side of equation 5.5 to account for the hydrostatic pressure attributable to the liquid in the pipe, and this may be calculated approximately provided that the liquid hold-up is known. 

The results of experimental and theoretical investigations related to smdy of drag and heat transfer in two-phase gas-liquid flow are presented in Chap. 5. 

Understanding the differences in two-phase flow characteristics between conventional size channels and micro-channels is also important for designing mini- or micro-heat exchangers, since the flow characteristics will affect the phase change heat transfer. 

On the top of the horizontal tube = 49.2 mm, 0 = 0°) the heat transfer coefficient in two-phase flow is less than that for single-phase water flow. 

Ghajar et al. (2004) studied heat transfer of two-phase flow in a horizontal tube of d = 25.4 mm and reported that the effect of superficial gas velocity on heat transfer depended on flow pattern and showed its own distinguished trend (Fig. 5.44). 

Hetsroni G, Hu BG, Yi JH, Mosyak A, Yarin LP, Ziskind G (1998b) Heat transfer in intermittent air-water flows Part II. Int J Multiphase Flow 24(2) 189-212 Hetsroni G, Mewes D, Enke C, Gurevich M, Mosyak A, RozenbUt R (2003b) Heat transfer of two-phase flow in inclined tubes. Int J Multiphase Flow 29 173-194 Hetsroni G, Mosyak A, Segal Z, Pogrebnyak E (2003a) Two-phase flow patterns in parallel microchannels. Int J Multiphase Elow 29 341-360... 

The detail experimental study of flow boiling heat transfer in two-phase heat sinks was performed by Qu and Mudawar (2003b). It was shown that the saturated flow boiling heat transfer coefficient in a micro-channel heat sink is a strong function of mass velocity and depends only weakly on the heat flux. This result, as well as the results by Lee and Lee (2001b), indicates that the dominant mechanism for water micro-channel heat sinks is forced convective boiling but not nucleate boiling. 

Figure 6.31 compares the measured heat transfer coefficient by Lee and Mudawar (2005b) in two-phase flow of R-134a to predictions based on previous studies. The predictive accuracy of a correlation was measured by the mean absoiute error, defined as... 

Prodanovic V, Fraser D, Salcudean M (2002) On transition from partial to fuUy developed subcooled flow boiling. Int J Heat Mass Transfer 45 4727-4738 Qu W, Mudawar I (2003a) Measurement and prediction of pressure drop in two-phase micro-channel heat sinks. Int J Heat Mass Transfer 46 2737-2753 Qu W, Mudawar I (2003b) Flow boiling heat transfer in two-phase micro-channel heat sink. 1 Experimental investigation and assessment of correlation methods. Int J Heat Mass Transfer 46 2755-2771... 

Peles YP, Yaiin LP, Hetsroni G (1998) Heat transfer of two phase flow in a heated capillary, Heat Transfer 1998. In Proceedings of the 11th International Heat Transfer Conference, Kyongju, Korea, 23-28 August 1998, vol 2... 

Commonly used nondimensional groups. The commonly used nondimen-sional groups in boiling heat transfer and two-phase flow are summarized as follows. Some are used more frequently than others, but all represent the boiling mechanisms in some fashion. 

In summary, we agree with the conclusions of Mayinger (1981) in his scaling and modeling criteria in two-phase flow and boiling heat transfer ... 

Borishansky, V. M., and I. I. Paleev, Eds., 1964, Convective Heat Transfer in Two-Phase and One-Phase Flows, transl. from Russian-Israeli Program for Scientific Translation, U.S. Dept, of Commerce, Washington, DC, 1969. (4)... 

Chyu, M. C., 1989, Formation and Heat Transfer Mechanism of Vapor Mass during Nucleate Boiling, in Thermal, Non-equilibrium in Two-Phase Flow, pp. 157-181, ENEA, Rome. (2)... 

Delhaye, J. M., 1986, Recent Advances in Two-Phase Flow Instrumentation, Proc. 8th Int. Heat Transfer Conf, San Francisco, vol. 1, p. 215, Hemisphere, New York. (3)... 

---