Enhancing Heat Transfer Rates by Inducing Liquid-Liquid Phase Separation: Applications and Modeling

Abstract

This paper focuses on heat transfer enhancement during spinodal decomposition, and it provides an updated review as well as a discussion of future developments. The analysis is mainly based on the work of two research groups at Tel-Aviv University (Israel) and at University of Brescia (Italy). We review the theory of spinodal decomposition of liquid-liquid binary mixtures and we discuss the diffuse interface (DI) approach. While mass and momentum equations in the DI approach have been developed and discussed in other works, we also look into the energy equation, which has been only recently investigated. Direct visualizations of both static and flowing mixture during decomposition are provided. Visualizations of the decomposition in a quiescent fluid have been previously reported, while flowing conditions have been analyzed only recently. Interestingly enough, the morphology is rather different during flowing conditions, where the decomposition exhibits a nucleationlike morphology and not the typical bicontinuous structure observed during spinodal decomposition of a quiescent fluid. Enhancement of heat transfer performances is shown in channels (sizes of 0.8 and 2 mm) using an upper critical solution temperature (UCST) mixture. Although different conditions are analyzed, the results show a consistent enhancement of the heat transfer. The paper reports also some new experimental work on the heat transfer for a lower critical solution temperature (LCST) mixture that can be actually used in cooling applications. A coarse-grained model that could be potentially used for the sizing of large-scale equipment is discussed in term of a possible future development that needs to be further investigated and validated.

Investigation of Binary Liquids in Unstable States—An Experimental Approach

“…At the initial, so-called diffusion stage of spinodal decomposition, a decrease in the heat flux density is more likely. At the next, convective stage, its intensification is more probable [25,26]. The scale of the effect systematically decreased with increasing the heating rate; see Figure 7a,b, which can be explained by a decrease in the thickness of the heated layer l~(a•t) 0.5 that forms the corresponding response signal.…”

“…Taking into account the diversity of phase diagrams of partially compatible solu tions, the proposed approach applied to other systems may undergo only partial trans formations. Judging by the observed unexpectedly significant enhancement in hea transfer at the convective stage of decay, the discussed objects can find application in the problems of removing high-density heat fluxes from local areas of heat-generating sur faces [25][26][27][28][29][30], for example, in miniature chemical reactors and microelectronics.…”

The Effect of Conductive Heat Transfer on the Morphology Formation in Polymer Solutions Undergoing Thermally Induced Phase Separation

“…The temperature drops suddenly to induce phase separation by bringing the system into the two-phase region of the phase diagram where the system phase separates into polymer-rich and polymer lean phases [ 1 , 7 , 19 , 20 , 21 ]. In the final step, the medium is frozen via crystallization, gelation, or vitrification, and the solvent may be extracted to obtain the desired morphology [ 2 , 22 , 23 , 24 , 25 ]. Spinodal decomposition is categorized into three principal stages of early, intermediate, and late stages based on the time evolution of the sinusoidal waves that specify the one-dimensional spatial concentration fluctuations [ 1 , 2 , 8 , 26 , 27 ].…”

“…In the majority of theoretical investigations of the TIPS process, the process is presumed to be isothermal, which implies that the system is quenched to a constant temperature and is maintained at that temperature with no variations in space and time [ 30 , 49 , 50 , 51 , 52 , 53 ]. Several attempts have been made to study the TIPS process considering a temperature gradient [ 1 , 2 , 22 , 31 , 50 , 54 , 55 , 56 ]. Caneba and Soong [ 16 , 31 ] were the first researchers to provide a noteworthy description of the spinodal decomposition through experimental and modelling studies of the coupled spinodal decomposition and heat transfer in one dimension for a critical quench of polymethyl methacrylate (PMMA)/sulfolane solution using the thermal inversion process.…”

“…The development of smaller droplets on the quench side and larger pores on the insulated side was confirmed. Ullmann et al [ 22 ] investigated the influence of heat transfer during phase separation through spinodal decomposition in the deep quenched critical mixtures. They confirmed heat transfer enhancement in systems with an UCST through coupling mass, momentum, and energy conservation equations.…”