Phase change induced flow

S. Kim, M.M. Mench, Investigation of temperature-driven water transport in polymer electrolyte fuel cell Phase-change-induced flow. J. Electrochem. Soc. 156, B353 (2009)... 

If one desires to know the temperature distribution throughout the ceU, then an overall energy balance and transport equation must be utilized. This is often critical for identification of failure points that are geometry specific or perhaps in understanding phenomena that are due to the temperature gradient (e.g., phase-change-induced flow in fuel cells [4]). 

This article reviews the phase behavior of polymer blends with special emphasis on blends of random copolymers. Thermodynamic issues are considered and then experimental results on miscibility and phase separation are summarized. Section 3 deals with characteristic features of both the liquid-liquid phase separation process and the reverse phenomenon of phase dissolution in blends. This also involves morphology control by definite phase decomposition. In Sect. 4 attention will be focused on flow-induced phase changes in polymer blends. Experimental results and theoretical approaches are outlined. 

Flow imparts both extension and rotation to fluid elements. Thus, polymer molecules will be oriented and stretched under these circumstances and this may result in flow-induced phenomena observed in polymer systems which include phase-changes, crystallization, gelation or fiber formation. More generally, the Gibbs free energy of polymer blends or solutions depends under non-equilibrium conditions not only on temperature, pressure and concentration but also on the conformation of the macromolecules (as an internal variable) and hence, it is sensitive to external forces. 

Before discussing theoretical approaches let us review some experimental results on the influence of flow on the phase behavior of polymer solutions and blends. Pioneering work on shear-induced phase changes in polymer solutions was carried out by Silberberg and Kuhn [108] on a polymer mixture of polystyrene (PS) and ethyl cellulose dissolved in benzene a system which displays UCST behavior. They observed shear-dependent depressions of the critical point of as much as 13 K under steady-state shear at rates up to 270 s Similar results on shear-induced homogenization were reported on a 50/50 blend solution of PS and poly(butadiene) (PB) with dioctyl phthalate (DOP) as a solvent under steady-state Couette flow [109, 110], A semi-dilute solution of the mixture containing 3 wt% of total polymer was prepared. The quiescent... 

Winter et al. [119, 120] studied phase changes in the system PS/PVME under planar extensional as well as shear flow. They developed a lubrieated stagnation flow by the impingement of two rectangular jets in a specially built die having hyperbolic walls. Change of the turbidity of the blend was monitored at constant temperature. It has been found that flow-induced miscibility occurred after a duration of the order of seconds or minutes [119]. Miscibility was observed not only in planar extensional flow, but also near the die walls where the blend was subjected to shear flow. Moreover, the period of time required to induce miscibility was found to decrease with increasing flow rate. The LCST of PS/PVME was elevated in extensional flow as much as 12 K [120]. The shift depends on the extension rate, the strain and the blend composition. Flow-induced miscibility has been also found under shear flow between parallel plates when the samples were sheared near the equilibrium coexistence temperature. However, the effect of shear on polymer miscibility turned out to be less dramatic than the effect of extensional flow. The cloud point increased by 6 K at a shear rate of 2.9 s. ... 

In the following, the discussion will be restricted to flow-induced phase changes in polymer blends under steady-state conditions. Then, a quasi-thermodyamic approach is certainly justified when... 

Previous stopped-flow fluorescence assays investigating matched dNTP incorporation showed that both the fast and the slow fluorescence transitions demonstrated a hyperbolic dependence on dNTP concentra-tion. " " " Similarly, the dNTP dependence of both the fast and the slow fluorescence phases during mismatched dNTP incorporation in stopped-flow has been examined. The observed rate constants for the fast and the slow phases, individually plotted as a function of dNTP concentration, reveal that both phases demonstrate a hyperbolic dependence on dNTP concentration (parameters obtained for k2, K, k o, and d,app as described in Section 8.10.4.2.3 and reported in Table 1). The observed hyperbolic dependence of the fast phase on mismatched dNTP largely indicates that this phase originates from a conformational change induced by mismatched dNTP binding. 

When a liquid is injected into a gaseous environment, it exchanges momentum with the gas and thus induces a flow. This flow is usually turbulent and strongly influences the liquid-gas interactions, such as liquid breakup, phase changes, and mixing. Consequently, turbulence plays a fundamental role in spray phenomena. 

Novel bubble-induced flow designs apply a plethora of mechanisms that help differentiate each specific design from other novel and standard devices. Some changes are structural and include use of different materials and internals. Others include the use of novel methods to excite the bubble interface and induce gas-liquid mass transfer. Novel methods exclude devices that are created to study specific events relating to standard devices. For example, the study by Sotiriadis et al. (2005) using a specially designed bubble column where the phases move downward to specifically study bubble behavior, bubble size, and gas-liquid mass transfer in the downcomer of airhft reactors would fall in the excluded devices. 

When two-phase flow phase change occurs in a channel with a small hydraulic diameter compared with the capillary length, boiling instabilities may arise. As in classical-sized channels, the instabilities can be static or dynamic however, their intensity is higher in microchannels due to the higher rate of volumic generation of vapor which induces crmsiderable pressure drops. 

It has been observed that the crystallization behavior of polymers is modified when the material is strained. This behavior has been found in rubbers and in thermoplastics [14]. In thermoplastics, the effect of strain on crystallization behavior has been studied quite extensively in solutions, in melt, and in solid state. For instance, it has been found that flowing polymer melts can crystallize at temperatures that are substantially above the crystallization temperature of the same material in a quiescent state. This strain-induced crystallization is generally explained in terms of thermodynamic processes. During a phase change, the Gibbs free energy is ... 

---