Evaporative two-phase flow

The investigations of fluid flow in micro-channels may be divided in two groups (1) single-phase flow, and (2) evaporative two-phase flow. The first was intensively investigated beginning from the pioneer work by Tukermann and Pease (1981). Two-phase flow is much less understood. 

Evaporative two-phase flow in a heated micro-channel resembles a two-phase slug flow with distinct domains of liquid and vapor. These domains are divided by the infinitely thin evaporating front, which propagates relatively to the fluid with a velocity u f equal (numerically) to the linear rate of liquid evaporation. In the frame of reference associated with micro-channel walls, the velocity of the evaporation front is... 

Because of a vivid interest in spray combustion, quite a few papers deal with the effect of finely dispersed particles or droplets on the turbulence characteristics of jets and sprays. The interaction of particle dynamics and turbulence in these flows has prompted very fundamental stochastic approaches involving filtering and averaging techniques, probability density functions, and quadrature-based moment methods (Fevrier et al, 2005 Fox, 2012 Labourasse et al, 2007) which are beyond the scope of this chapter. Riber et al (2009) and Senoner et al (2009) obtained LES results for recirculating and evaporating two-phase flows, respectively, by both Euler-Lagrange and Euler—Euler methods and compared them mutually and with experimental data. 

Morris SJS (2003) The evaporating meniscus in the channel. J Fluid Mech 494 297-317 Peles YP, Yarin LP, Hetsroni G (2000) Thermohydrodynamic characteristics of two-phase flow in a heated capillary. Int J Multiphase Flow 26 1063-1093 Peles YP, Yarin LP, Hetsroni G (2001) Steady and unsteady flow in a heated capfllary. Int J Multiphase Flow 27 577-598... 

Wallis GB (1969) One-dimensional two-phase flow. McGraw-HUl, New York Wayner PC, Kao YK, LaCroix LV (1976) The interline heat transfer coefiicient of an evaporating wetting film. Int 1 Heat Mass Transfer 19 487-492 Weisberg A, Bau HH, Zemel IN (1992) Analysis of micro-channels for integrated cooling. Int 1 Heat Mass Transfer 35 2465-2472... 

Chapter 9 consists of the following in Sect. 9.2 the physical model of two-phase flow with evaporating meniscus is described. The calculation of the parameters distribution along the micro-channel is presented in Sect. 9.3. The stationary flow regimes are considered in Sect. 9.4. The data from the experimental facility and results related to two-phase flow in a heated capillary are described in Sect. 9.5. 

Two-phase flows in micro-channels with an evaporating meniscus, which separates the liquid and vapor regions, have been considered by Khrustalev and Faghri (1996) and Peles et al. (1998, 2000). In the latter a quasi-one-dimensional model was used to analyze the thermohydrodynamic characteristics of the flow in a heated capillary, with a distinct interface. This model takes into account the multi-stage character of the process, as well as the effect of capillary, friction and gravity forces on the flow development. The theoretical and experimental studies of the steady forced flow in a micro-channel with evaporating meniscus were carried out by Peles et al. (2001). These studies revealed the effect of a number of dimensionless parameters such as the Peclet and Jacob numbers, dimensionless heat transfer flux, etc., on the velocity, temperature and pressure distributions in the liquid and vapor regions. The structure of flow in heated micro-channels is determined by a number of factors the physical properties of fluid, its velocity, heat flux on... 

Wyllie, G., 1965, Evaporation and Surface Structure of Liquid, Proc. Roy. Soc. A197.383. (2) Yadigaroglu, G., 1993, Instabilities in Two-Phase Flow, in Workshop on Multiphase Flow and Heat Transfer Bases, Modeling and Applications, chapter 12, University of California, Santa Barbara, CA. (4)... 

Chisholm, D., Pressure gradient due to friction during the flow of evaporating two-phase mixtures in smooth tubes and channels, International Journal of Heat and Mass Transfer, 16, pp. 347-58 (1973). 

Figure 14.3. The nature of two-phase flow in an evaporator tube...

O. Ghapuis, M. Prat, M. Quintard, et al. Two-phase flow and evaporation in model fibrous media Application to the gas diffusion layer of PEM fuel cells. Journal of Power Sources 178 (2008) 258-268. 

Further development of statistical closures, especially in the algebraic form, is strongly recommended for description of two-phase flows. Of interest is inclusion of evaporation in the modeling strategies. It is expected that optimum closures will remain at the level of single-point, one-time, second-order moment. Consideration of differential transport equations for such moments appears to be computationally excessive so algebraic closures are expected to be more widely utilized. These closures portray the simplicity of zero-order schemes, yet preserve (some of) the capabilities of second-order closures. In more complicated flows, it... 

We note in passing that two-phase flow leads to circumstances where for a given phase there are source or sink terms. For example, consider situation wherein water droplets are evaporating in a moist-air flow. It is possible to write mass-conservation equations for the liquid and the vapor phases. The conversion of liquid to vapor (and vice-versa) causes... 

By dissolving the compressible media in a liquid, a so-called gas-saturated solution may be formed. By expansion of such a solution in an expansion unit (e.g., a nozzle) the compressed medium is evaporated and the solution is cooled. Owing to the cooling caused by evaporation and/or the Joule-Thompson effect the temperature of the two-phase flow after the expansion nozzle is lowered. At a certain point, the crystallization temperature of the substance to be solidified is reached, and solid particles are formed and cooled further. 

The first measure is to use the evaporative cooling or controlled depressurisation to keep the reaction mass under control. The distillation system must be designed for such a purpose and has to function, even in the case of failure of utilities. A backup cooling system, dumping of the reaction mass, or quenching could also be used. Alternatively, a pressure relief system may be used, but this must be designed for two-phase flow that may occur, and a catch pot must be installed in order to avoid any dispersion of the reaction mass outside the equipment. Of course, all these measures must be designed for such a purpose and must be ready to work immediately after the failure occurs. The use of thermal characteristics of the scenario for the choice of technical measures is presented in detail in Chapter 10. 

Carey VP. Two-phase flow in small-scale ribbed and finned passages for compact evaporators and condensers. Nucl Eng Design 1993 141 249-268. 

Optimization of the coiled-tube heat exchanger is quite complex. There are numerous variables, such as tube and shell flow velocities, tube diameter, tube pitch, and layer spacing. Other considerations include single-phase and two-phase flow, condensation on either the tube or shell side, and boiling or evaporation on either the tube or shell side. Additional complications come into play when multicomponent streams are present, as in natural gas liquefaction, since mass transfer accompanies the heat transfer in the two-phase region. 

Kandlikar, S.G., (2001), Two-phase flow patterns, pressure drop and heat transfer during boiling in minichannels and microchannels flow passages of compact evaporators. Keynote Lecture presented at the Engineering foundation Conference on Compact Heat Exchangers, Davos, Switzerland, July 1-6. 

Kuznetsov, V.V., Safonov, S.A., Shamirzaev, A.S., Houghton, P.A. and Sunder, S.D. (1999) Two-Phase Flow Patterns and Local Evaporating Heat Transfer in a Vertical Rectangular Channel, Proc. Int. Conf. Compact Heat Exch. andEnh. Tech. Proc. Ind., Banff, Canada, pp. 311-320. 

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