Thermal-Mechanical Effects and Near-Critical Fluid Dynamic Behaviors in Micro-Scale

Near-Critical Background and General Applications

In recent years, supercritical fluid and near-critical fluid have been utilized in micro-/nano-chemical synthesis, microscale fluid flow and heat transfer devices, chemical extraction and separation, biological synthesis and many other fields. The applications of critical fluids have been welcomed in many advanced strategic technologies of 21st century, such as novel chemicals, energy technology, biology and other fields (Kuang et al., 2004; Kumar et al., 2011; Zhao et al., 2011). For example, the utilization of supercritical H₂O and CO₂, by using their special density and diffusivity characteristics, has been found very effective for chemical extraction and preparation processes, which have become matured ‘green’ technology nowadays (Wang et al., 2011). Also, based on the preferable thermal and transport properties of supercritical/near-critical fluids, new heat transportation designs have been proposed and tested, which have shown potential in new generation of renewable energy technologies and energy conversion systems (Zhang et al., 2007; Zhang and Yamaguchi, 2008; Chen and Zhang, 2011). For example, for new generation of nuclear power plant, supercritical fluids based system has become one of the promising choices in recent framework for the global development (Dimmic et al., 2002). The application fields of critical fluids also include solar conversion, heat pump design, cooling and drying, micro-scale cooling, micro-devices and heat transfer designs. Such system design and performance analysis have drawn the attention of both engineers and scientists around the world and have become one major direction of future development in fluid and energy technology fields.

At the same time, the fluid flow and heat transfer engineering field has been more focused on micro-scales in recent years, such as for molecular level understanding of human life, chemical engineering, medicine, energy and resources engineering (Ameel et al., 1997). Several representative projects of the supercritical/near-critical fluids in microscale engineering can be found in Figure 1, where the micro-/nano-scale chemical synthesis, chemical mixing and reaction, novel energy conversion and heat transfer heating/cooling applications are shown. The scientific analysis and design of those processes have to consider the microscale effects for critical fluid flow and heat transfer under designed conditions. There are a lot of question to be asked before one can try to see into the new world of supercritical/near-critical world! For instance, if the nanoparticle is to be synthesized in a microscale chamber, which is one typical situation of nanoparticle synthesis, the dynamics of fluid confinement and local heat transfer (equilibrium or irreversible procedures) of critical fluids should also be considered, besides the generally known microscale temperature and pressure control (Kuang et al., 2004; Wang et al., 2011; Zhao et al., 2011).

For supercritical/near-critical fluid, the challenges for system control and stability formation are much different from traditional kinds of operation systems. This is due to the microscale effect. The near-critical synthesis of nanoparticles in micro-chamber can provide not only much better products for customers but it also yields much critical conditions of formation (Beckman, 2004; Sierra-Pallares, et al., 2011). The near-critical fluids have very large changes of properties when they go across (or near) the critical point. The specific heat tends to infinity while the thermal diffusivity goes to zero, which effect will lead to special thermal convective structures and operation time scales. Therefore, the applications and their optimizations need more deep understanding into the fundamentals of supercritical/near-critical fluid dynamics and transport characteristics. In recent years, many new findings and breakthroughs have been made by scientists (Zappoli, 2003; Carles, 2010).

Figure 1.

Applications of near-critical mini-/micro-channel flow and heat transfer: (a) nanoparticle synthesis inside near-critical CO₂ fluid; (b) Microscale T-channel mixing and reaction inside near-critical H₂O fluid; (c) solar conversion using trans-critical CO₂ fluid natural circulation; (d) Electronic cooling tests with supercritical CO₂ jet flow

Wang et al., 2011; Blood et al., 2004; Chen and Zhang, 2014; Ducoulombier et al., 2011.

Characteristics of Near-Critical Microchannel Flows

In real designs, near-critical fluid flow and heat transfer would largely be affected by its thermal status of existence and operation parameters. For example, when Wang and his colleagues (2011) tried to use a channel of several hundred micrometers for the preparation of micro/nano-particles (sized from 0.5 μm to 5.0 μm), they utilized a precise control of the operation temperate and pressure in each procedure, thus to give good synthesis route, rate and product quality. The detailed changes and evolutions of such processes inside microscale for near-critical fluids are quite complicated and sensitive as for the control of production or fluid manipulations, which brings new challenges in the design of heat and mass transportation processes in both theoretical analysis and real applications.

Near-critical fluid microchannel flows should also take the above mentioned effects into consideration. The entrance effects, geometry and surface effects are similar with general sized channel. However, for supercritical/near-critical fluids, there are special points to be clarified: (1) rarefied gas effect is not important as the density of critical fluids are still comparable large (similar to the density of liquid), and a calculation of Knudson number would easily be helpful; (2) the surface tension of near-critical fluids would be zero when the condition goes above the critical point; therefore the supercritical fluid flow would have high diffusivity and solubility as a host fluid and also have low flow friction and pressure loss. Thus, it affects the energy consumption and also stability (Chen et al., 2010; Zhang et al., 2010); (3) the nonlinear changes of near-critical fluid properties is the major difference from normal fluids; as the properties would change dramatically when it goes near the critical point, some new and different behaviors may become possible in microscale.