Study Of Heat And Mass Transfer Mechanisms And Application Of Supercritical Binary Fluid

A single critical point also exists in a binary fluid, which is between the critical points corresponding to the two components. The thermo-physical properties of a binary fluid also vary drastically under the state close to the critical point. There are more and more cases and occasions in the industry that would be related to supercritical binary fluid, such as air separation, energy industry, and cryogenic engineering. Thus the understanding of the heat and mass transfer mechanisms in the supercritical binary fluid can contribute to the improvement of the air separation process, the enhancement of the heat transfer in the liquefication of natural gas or petroleum gas, the performance improvement of the cryocooler with binary working fluid, and the optimization of the chemical extraction process. Except for the Piston effect alike to the supercritical pure fluid, two more thermo-physical phenomena also occur in supercritical binary fluid, i.e., Soret effect and Dufour effect. Piston effect in supercritical fluid is a special heat transfer process resulted from compression on the fluid by the thermoacoustic wave, which is caused by the intensive thermal expansion in the fluid under the thermal perturbation. Soret effect refers to the mass migration in binary fluid driven by temperature gradient, and Dufour effect is an associated heat transportation of the mass migration. The aim of the present study is to investigate the heat and mass transfer mechanisms at different timescale in supercritical binary fluid under the continuous working of these three effects through numerical simulation and experimental methods.A complete mathematical and physical model is established by considering all the related heat and mass transfer processes for a supercritical binary fluid. The governing equations are derived, especially; special treatments are introduced in the energy conservation equation. The numerical results for a 1D model indicate that the Piston effect occurs first at acoustic timescale after a thermal perturbation being imposed on a supercritical binary fluid, which is in complete equilibrium initially. The bulk fluid is heated up evenly very fast and temperature gradients are established close to both boundaries. But the thermal boundary layers are very thin and the Soret effect and Dufour effect occurring in them display no obvious effects. At diffusion timescale, the gradually thickened thermal boundary layers result in the occurrences of apparent Soret effect and Dufour effect. The direction of thermal diffusion of a composition is determined by the sign of its thermal diffusion ratio kT, and the direction of actual diffusion depends on the local gradient ratio γ, i.e., the ratio of concentration gradient to temperature gradient. There is a balance gradient ratio for a specific binary fluid, which can be obtained by γb = |-kT/T|.The Soret effects close to both boundaries with opposite directions drive a counter composition convection throughout the entire fluid domain, which leads to a considerable Dufour effect enhancing the heat transfer in the supercritical binary fluid. In difference binary fluids, the mechanisms of these thermomechanical effects and the interactions between them are all the same, the only differences are the specific amplitude of the effects and the traveling speed of thermoacoustic wave in the fluids. Therefore, these thermo-physical phenomena also occur in the conventional fluid, but the effects cannot be displayed because of the extremely weak amplitudes in conventional binary fluid.In the case of supercritical binary fluid in a closed chamber under gravity field, natural convection will be generated under a bottom heating, and such natural convection in supercritical nitrogen/argon binary fluid(0.9/0.1 in molar fraction) is studied in the present study by numerical method and visualization experiments by means of laser holographic interferometry. It is observed in both numerical and experimental results that the thermal boundary layer at the bottom is gradually thickened and finally loses its stability and then thermal plumes are generated because of the buoyancy caused by the density difference between the thermal boundary layer and fluid bulk. Under the working of Piston effect, thermal plumes also appear near the top wall with constant temperature in numerical results. It is further find that the natural convection in supercritical binary fluid is developed faster and stronger than the case of supercritical pure fluid, as the Soret effect and Dufour effect enhance the heat and mass transfer. Thereafter, the plumes in the numerical study develop and large scale circulations in the fluid domain are formed. However, the natural convection is limited under a finite height in in the experiments in the case of slight bottom heating and no thermal plumes appear on the top of the cavity as there are apparent adiabatic temperature gradient(ATG) and density stratification along the height of the closed chamber. But in both cases, there are no obvious gradients on any parameters except velocity in the fluid bulk because of the intensive mixing of convection. Piston effect is directly observed in the experiments and the working effect of Dufour effect is obtained by the comparison between the experimental results of supercritical nitrogen/argon(0.9/0.1) binary fluid and supercritical nitrogen under the same dimensionless reduced temperature ε = 0.026, which also testifies the existence of Soret effect.The natural convection develops to be a stable one after its formation. Then the heat transfer characteristics of the stable natural convection of supercritical fluid(both pure and binary) are experimentally studied in a long closed vertical cylinder with an aspect ratio of 27. It is found that the heat transfer performance of supercritical fluid is much better that the case of gases. The scaling law of the natural convection of supercritical helium keeps unchanged as Nu ∝ CRa1/3 in a large Ra range(109 ~1017) of this study. When the cylinder is filled with supercritical nitrogen/argon binary fluid, no heat transfer enhancement is observed compared with pure fluids as the convection is too strong to display the Soret effect and Dufour effect in the present Ra range 5.0 × 1010 ~ 2.0 ×1014. Therefore, the supercritical binary fluid in this kind of situation can be treated as a Pseudo-pure fluid. According to the heat transfer characteristics, the natural convection of supercritical nitrogen/argon binary fluid can be divided into three regimes: Laminar thermal boundary layer regime, Transition regime, and Ultimate regime, and the two critical Ras between them are Racrit1 ≈ 4.0×1013 and Racrit2 ≈ 1.14×1014, respectively. The heat transfer characteristics in the first and the last regimes can be scaled as Nu = 0.135Ra0.22 and Nu = 0.0027Ra1/3, respectively, according to the experimental results. When natural convection experiences the near-critical temperature range, an inflection occurs on the variation trend of Nu versus Ra because of the peculiar change on the thermo-physical properties of fluid, which will also occur in the natural convection of any supercritical pure and mixed fluid. If the inflection further occurs in the Transition regime, then a transitional bifurcation phenomenon occurs, i.e., a Ra will correspond to two different Nus. Furthermore, a valid numerical model for the evaluation of the natural convection heat transfer performance of supercritical fluid in a long vertical cylinder is confirmed by the comparison of numerical results with experimental results.If the cold source in the above natural convection model can make the fluid temperature lower than its critical temperature, the natural convection model then converts to a cryogenic thermosyphon. The converting process, i.e., the supercritical startup process of the cryogenic thermosyphon, is investigated. Then the heat transfer characteristics of the cryogenic thermosyphon working with liquid-vapor phase change are studied. The results show that the thermal resistances of the cryogenic thermosyphon with any working fluids all decrease firstly and then tend to be constant with an increase in heat transfer rate. The working temperature range of the cryogenic thermosyphon is successfully extended to 64.0 ~ 150.0 K by adopting nitrogen/argon binary fluid as working fluid, but its heat transfer performance is slightly deteriorated by the concentration layer between the wall and binary working fluid bulk. The heat transfer performance is also calculated by proper empirical correlations chosen according to the heat transfer characteristics under each situation, and the uncertainties are all within ±20%. Furthermore, dry-out limit, outer boiling limit, inner boiling limit, and critical limit are observed, and the critical limit is unique to cryogenic thermosyphon. According to their mechanisms, their corresponding heat transfer rates are evaluated by proper methods, and the results show satisfactory consistence with the experimental results. The effects of filling ratio of working fluid and cooling condition on the heat transfer characteristics are also investigated, and a final conclusion is reached that the optimal filling ratio is 1.0 for this cryogenic thermosyphon and the impovement on the cooling condition can considerably enhance the heat tranfer performance of the cryogenic thermosyphom.