Experimental study of bubble departure characteristics in forced convective subcooled nucleate boiling

Visualization experiments in upward forced convective nucleate boiling flows were carried out. The bubble growth and collapse have been measured using high-speed photography technique with distilled water under atmospheric pressure. The experiments show that the bubbles depart from nucleating sites shortly after nucleation and slide along the heater surface. The bubbles grow while sliding, attain a maximum size, then lift from the surface sometime during condensation, and quickly vanish in the bulk liquid. Parametric studies show that bubble diameter and departure frequency increases with an increase in heat flux, a decrease in subcooling, and a decrease in mass flux.     

Experimental study of critical thermal load during boiling of binary liquid mixtures

The critical thermal load q _cr is experimentally studied as a function of the composition of an aqueous solution of ethanol, isopropanol, n-butanol, isobutanol, and isoamyl alcohol during pool boiling of saturated and subcooled liquids at atmospheric pressure.     

二元混合工质HFC23/CFC13池核沸腾传热的实验研究

实验测量了二元混合工质HFC23/CFC13池核沸腾传热特性。加热面为紫铜表面,实验测量的压力范围为0.1~0.55 MPa,热流范围为10 kW/m2~300 kW/m2。实验数据同现有经验关联式的计算结果相比较,发现Fu jita andTsutsu i关联式和Thom e and Shak ir关联式对混合工质HFC23/CFC13的传热系数预测较准确,预测值与实验值之差≤±20%。     

Mechanism of convective instability of a binary mixture

Even a very slow background vertical motion can strongly affect convection in a stratified fluid. A previously unknown mechanism of convective instability is demonstrated for a binary mixture in the flow field of such motion.     

Flow boiling heat transfer of refrigerant R21 in microchannel heat sink

Boiling heat transfer in a refrigerant R 21 flow in a microchannel heat sink is studied. A stainless steel heat sink with a length of 120 mm contains ten microchannels with a size of 640×2050 μm at cross-section with a wall roughness of 10 μm. The local heat-transfer coefficient distribution along the heat sink length is obtained. The ranges of parameters are: mass flow from 68 to 172 kg/m²s, heat fluxes from 16 to 152 kW/m², and vapor quality from 0 to 1. The maximum values of the heat transfer coefficient are observed at the inlet of microchannels. The heat transfer coefficients decrease substantially along the length of channels under high heat flux conditions and, on the contrary, change insignificantly under low heat flux condition. A comparison with the well-known models of flow boiling heat transfer is performed and the range of applicability is defined.     

Natural and forced convective heat transfer on slender cylinders

This paper presents an experimental study of free and forced convective heat transfer along vertical slender cylinders. The local heat transfer coefficient is determined from the measurement of the surface temperature distribution performed by quantitative infrared thermography. It is found that the convective heat transfer is strongly dependent on the cylinder curvature and misalignment with the flow. The effect of proximity of two cylinders is emphasized in the case of forced convection. Correlations are proposed for the two types of convection. It is worth noting that circumstances exist where the turbulent heat transfer in free convection can be of the same order of magnitude as for laminar forced convection. The outcome of the study demonstrates the suitability of quantitative infrared thermography to solve complex problems and to provide a deeper understanding of the heat transfer on slender cylinders.     

Instabilities in thin-film binary mixtures

Binary mixtures of ultra-thin films are subject to both height and concentration fluctuations, either of which, under the appropriate conditions, can become unstable. We investigate a simple general thermodynamic model which shows that, due to the constraint imposed upon coupling between these fluctuations, a thin-film binary mixture will be less stable than if either height or concentration fluctuations are considered in isolation. We investigate the height dependence of the stability condition, discuss equilibrium conditions when higher-order contributions to the free energy are negligible, and predict the scaling behaviour.Received: 18 June 2004, Published online: 20 July 2004PACS: 68.55.-a Thin film structure and morphology - 83.80.Tc Polymer blends     

Stripe formation in differentially forced binary systems

We consider pattern formation in periodically forced binary systems. In particular, we focus on systems in which the two species are differentially forced, one being accelerated with respect to the other. Using a continuum model consisting of two isothermal ideal gases which interact via a frictional force we demonstrate analytically that stripes form spontaneously above a critical forcing amplitude. The wavelength of the stripes is found to be close to the wavelength of sound in the limit of small viscosity. The results are confirmed numerically. We suggest that the same mechanism may contribute to the formation of stripes in experiments on horizontally oscillated granular mixtures.     

Cluster-cluster aggregation in binary mixtures

The structure and aggregation kinetics of three-dimensional clusters composed of two different monomeric species at three concentrations are thoroughly investigated by means of extensive, large-scale computer simulations. The aggregating monomers have all the same size and occupy the cells of a cubic lattice. Two bonding schemes are considered: (a) the binary diffusion-limited cluster-cluster aggregation (BDLCA) in which only the monomers of different species stick together, and (b) the invading binary diffusion-limited cluster-cluster aggregation (IBDLCA) in which additionally monomers of one of the two species are allowed to bond. In the two schemes, the mixed aggregates display self-similarity with a fractal dimension d(f) that depends on the relative molar fraction of the two species and on concentration. At a given concentration, when this molar fraction is small, d(f) approaches a value close to the reaction-limited cluster-cluster aggregation of one-component systems, and when the molar fraction is 0.5, d(f) becomes close to the value of the diffusion-limited cluster-cluster aggregation model. The crossover between these two regimes is due to a time-decreasing reaction probability between colliding particles, particularly at small molar fractions. Several dynamical quantities are studied as a function of time. The number of clusters and the weight-average cluster size display a power-law behavior only at small concentrations. The dynamical exponents are obtained for molar fractions above 0.3 but not at or below 0.2, indicating the presence of a critical transition between a gelling to a nongelling system. The cluster-size distribution function presents scaling for molar fractions larger than 0.2.     

Field-driven dynamical demixing of binary mixtures

ABSTRACT

We consider mixtures of two species of spherical colloidal particles that differ in their Stokes coefficients, but are otherwise identical, in the presence of an external field. Since the particle–particle and particle–field interactions are the same for both species, they are completely mixed in the thermodynamic limit in the presence of any static field. Here, we combine Brownian dynamics and dynamic density functional theory of fluids to show that for sufficiently large differences in the Stokes coefficients of the particles (and corresponding differences in their mobilities) dynamical demixing is observed. These demixed states are transient but, under certain conditions, packing effects compromise the relaxation towards the thermodynamic states and the lifetime of the demixed phases increases significantly.     

Refrigerant-oil mixtures boiling in a planar confined space

In the frame of the JOULE 1 R&D programme of the Commission of the European Communities a project has been carried out on enhanced evaporation heat transfer surfaces. A specific investigation has been realised on refrigerant oil mixtures boiling in a planar confined space.An experimental investigation of the boiling phenomenon in the confined space between a 30 mm wide × 120 mm high, heated plate and an opposing, adiabatic plate was carried out. The heated surface was made of a copper-aluminium-nickel alloy (Ra = 1 micrometer) and a saturated R-113/SUNISO 3GS oil mixture at atmospheric pressure was used as the boiling fluid.The maximum heat flux tested was approximately 90% of CHF. The parameters investigated were the gap size (1–5 mm) and the oil concentration (1–7% by weight). It was again observed that confinement does not improve the nucleate boiling performance of pure R-113 in any significant way, whereas the CHF decreases with decreasing gap size. In addition, while the presence of oil was observed to have only a relatively minor effect on low flux nucleate boiling, it caused a serious degradation of the high flux boiling performance. This deterioration increased with increasing oil concentration and was more severe for smaller gap sizes. However, for a given gap size, the CHF increased with increasing oil concentration accompanied by increasingly larger surface superheats.     

单级自动复叠混合工质制冷循环的实验研究

由于压缩机压比的局限,制约着单一制冷剂的压缩回热循环发展,本系统采用单一压缩机和混合工质的自复叠制冷循环系统取得了较好的制冷效果。文中对单级自动复叠制冷系统进行了理论模拟,并设计和搭建了实验台进行分析。通过理论和实验数据的分析得出混合工质组合R600a/R23,并初步得到最优配比。     

Efficiency of vapor compression heat pumps based on non-azeotropic refrigerant mixtures

The work presents the results of cycle computation for vapor compression pumps based on ozone-safe mixed refrigerants. Non-azeotropic binary refrugerants R32/R152a (30/70) and R32/R134a (30/70) were considere as working substances. Properties of non-azeotropic refrigerants were calculated according to the additivity method of thermodynamic functions and method of Lemmon and Jacobsen. Deviations in the values of thermophysical properties obtained with two methods have been determined. It is shown that at the use of nonazeotropic mixture R32/R152a (30/70), energy conversion ratio increases by 2.2–3.6 % compared with the results for R32/R134a (30/70) at temperature difference between the processes of boiling and condensation from 28 to 53 °C.     

Segregation in binary mixtures under gravity

We employ kinetic theory for a binary mixture to study segregation by size and/or mass in a gravitational field. Simple segregation criteria are obtained for spheres and disks that are supported by numerical simulations.     

Ultrasonic absorption in binary liquid mixtures

Ultrasonic absorption in binary liquid mixtures containing benzene, chloroform, cyclohexane and toluene with triethylamine, a rotational isomeric relaxing liquid, as a common component has been studied at a frequency of 7.56 MHz. A pulse techrique has been used for the measurement of absorption with an accuracy of ±5%. Bauer-Sette formula has been used to calculate the absorption of these liquid mixtures at different concentrations. The theoretical values evaluated on the basis of Bauer-Sette theory appear to have good agreement with experimental values. In view of the discrepancy pointed out by Mallikarjuna Rao and Suryanarayana, the mixtures of benzene and ethylacetate have been studied in this context and found the theoretical values coinciding with experimental values.     

Study of non-azeotropic refrigerant mixtures behaviour in gravity assisted heat pipes

In this study, a numerical model has been developed to predict the behaviour of non-azeotropic refrigerant mixtures in smooth as well as textured gravity assisted heat pipes.In addition, experiments were performed on fully instrumented heat pipes using pure as well as non-azeotropic refrigerant mixtures to determine their heat-transfer characteristics. The effects of operating conditions on the transport capability have been measured and analyzed.Numerical results showed that the proposed model favourably predicted the performance of the heat pipes under various conditions and compared well with experimental data.     

Scalar fluctuations in turbulent binary mixtures

Summary  We present an analysis of the temperature and concentration of turbulent fluctuations in a binary mixture. The crossed effects introduce a coupling between temperature and concentration fluctuations that modifies the spectral picture of these passive scalar fluctuations. The author of this paper has agreed to not receive the proofs for correction. This work has been partially supported by the Spanish Ministry of Education and Science under contract n. CLI-95-1867.