Home – Home – Jornals – Thermophysics and Aeromechanics 2010 number 1

Here is the review of experimental and theoretical results on the mechanism of heat transfer in nanofluids. A wide scope of problems related to the technology of nanofluid production, experimental equipment, and features of measurement methods is considered. Experimental data on heat conductivity of nanofluids with different concentrations, sizes, and material of nanoparticles are presented. Results on forced and free convection in laminar, and turbulent flows are analyzed. The available models of physical mechanisms of heat transfer intensification and suppression in nanofluids are presented. There are significant divergences in data of different researchers; possible reasons for this divergence are analyzed.

Effects of the upstream conditions and the degree of the wall roughness on the mean velocity profiles and some integral flow parameters in two dimensional zero-pressure-gradient boundary layer were characterized experimentally. The results were analyzed utilizing conventional and recent scaling flow parameters for 245< Re_{θ ≤ 11·10³, where Reθ is the Reynolds number based on the free stream velocity and the momentum thickness (θ). Good correlation of the quantity as a function of the roughness parameter k^{+ was obtained for sand roughness of 1.7 < k+ ≤ 172, revealing a universality of the roughness effect, where =
= ( − )/uτ and k+ = kuτ /ν. The mean flow structure of the outer flow was observed not to be influenced by the degree of the wall roughness, i. e., the outer flow of either the smooth or the rough surfaces scales similarly with the various scaling parameters regardless the degree of the wall roughness. However, it made flow confined to the wall region away from the classical universality, allowing similarity hypothesis not to be identical in the wall region at least for the current range of the Reynolds number.}}

Results of experimental investigations of the evolution of a vortex system formed in a supersonic flow past a streamwise-aligned external dihedral right angle owing to a difference in pressures on the upper and side faces of the corner are analyzed. The experiments are performed in a T-313 wind tunnel based at ITAM SB RAS at Mach numbers М_∞ = 2.27, 3, and 4, and angles of attack α = −4° ÷ +20°. It is shown that the size of the vortex system influence zones is almost independent of the free-stream Mach number in the examined range of the angles of attack, and the relative values of flow rarefaction on the model surface under the primary vortex core smoothly tend to their minimum values.

The computational method for aerodynamic design of aircraft is applied more universally than before, in which the design of an airfoil is a hot problem. The forward problem is discussed by most relative papers, but inverse method is more useful in practical designs. In this paper, the inverse design of 2D airfoil was investigated. A finite element method based on the variational principle was used for carrying out. Through the simulation, it was shown that the method was fit for the design.

The results of designing and numerical gas-dynamic modeling a supersonic three-dimensional inlet of a new type are considered. A ramp of external compression of this inlet is the V-shaped body forming an initial plane oblique shock wave and a subsequent isentropic compression wave. The inlet incorporates an entrance section of internal compression, where also a plane oblique shock wave and a subsequent isentropic compression wave are formed by a cowl. The designed three-dimensional inlet has small inclination angles of compression surfaces, which ensures its low wave drag. According to the estimates of inlet efficiency in terms of the compression ratio and the total pressure recovery factor, it is close to the optimal two-dimensional shocked inlet of external compression considered by Oswatisch as well as Petrov and Ukhov. The flow in the inlet was computed with the use of the Euler and Navier ⎯ Stokes codes provided by the commercial package FLUENT. The flow in the inlet throat in the design regime computed under the inviscid flow approximation is uniform. The most substantial effect of the flow viscosity in this regime manifests itself in the interaction of the shock wave from the cowl with the boundary layer on the V-shaped compression body in the inlet internal duct. According to computed data, the boundary layer separation does not occur in this case; however, due to viscosity effects, reflected shock waves are formed here which results in significant deviations of flow structure as compared to the computed inviscid flow.

In the present paper, we introduce an experimental technique for evaluating the equivalent aerodynamic diameter d_{ae of non-spherical aerosol particles upon their sedimentation in a gravity field within the Reynolds-number range from 0.1 to 6.0. A description of the sedimentometer is given, and aerodynamic diameters dae of plexiglass powder particles measured on this sedimentometer are reported.}

Results of experimental study of the nonstationary temperature field on the surface of a complex shape channel at a jump-like change in heat release with time in the wall of packing with one-way heat supply are presented. Measurements were carried out in the air flow between two plates with triangular corrugations directed at 90° relative each other. Measurements were carried out by thermocouples with the thickness of less than 10 microns. The effect of Reynolds number and supplied power on the temperature change over the heat exchanger surface with time caused by a jump-like change in supplied electric power at a turbulent air flow is analyzed. Investigations have revealed typical features of temperature evolution over the perimeter of the heated cell. Experimental data on time dependence of temperature can be approximated well by the exponential function.

In the present paper, we consider the heat transfer of non-Newtonian liquid flows moving in convergent-divergent channels with common boundary. The case of flows moving in opposite directions was addressed. At the common boundary, continuity conditions for temperature and heat flow were adopted. A mathematical model, a calculation algorithm, and simulation data are reported. Plots of numerical data characterizing the channel flows are presented. A comparative analysis of heat transfer in smooth and convergent-divergent channels is given.

The gas-liquid upward flow was studied in a rectangular minichannel of 1.75×3.8 mm and length of 0.7 m. The experiments were carried out within the range of the gas superficial velocity from 0.1 to 10 m/s and the liquid superficial velocity from 0.07 to 0.7 m/s for the co-current H₂O/CO₂ flow under the conditions of saturation. The method for the two-beam laser scanning of structure and determination of statistic characteristics of the two-phase flow was worked through. The slug-bubble, slug, transitional, churn, and annular flows were distinguished. The statistics characteristics of liquid and gas phases motion in a minichannel were obtained for the first time including the velocities of phase motion.

Experiments on water cavitation in ultrasonic field have been carried out. Low frequency fluctuations of the intensity of laser beam run through the cavitation area have been studied. Experiments have proved presence of low-frequency random fluctuations with frequency dependence of power spectra where the exponent α ranged within 0.8 ≤ α ≤ 1.2. From experimental realizations, large-scale low frequency pulsations characterized by scale invariance, which duration is distributed according to the power law, have been distinguished. The results are explained on the basis of mathematical model for the rise of scale invariant fluctuations with power spectrum in the system of two nonlinear stochastic differential equations describing interaction of heterogeneous phase transitions. Distribution of extreme low frequency emissions obtained from numerical solutions to stochastic equations takes the power series form. Correlations of dynamic scaling between critical indicators determining frequency dependence of pulsations power spectra α and distribution functions of extreme low frequency pulsation amplitudes β have been determined. It is shown that both in the experiments on acoustic cavitation of water and in the theoretical model of interacting phase transitions critical indicators are bound with the correlation α + β = 2. Spectra of fluctuation power are determined in the experiments simpler and more accurately than the function of extreme amplitudes distribution. In case when only one frequency dependence of fluctuation capacity spectra is known correlations between indicators serve to obtain information on the distribution of large scale emissions and to estimate critical amplitudes.

Boiling-up kinetics of superheated distilled water and sodium chloride solution in a glass cell at atmospheric pressure and low superheating of 15−35 °C has been studied far from the boundary of attainable superheating in the area of heterogeneous nucleation. Temperature dependences of average waiting time of superheated liquids boiling-up have been studied experimentally under natural conditions and in the ultrasonic field Waiting time of boiling-up at these temperatures reaches 1000 s, and average time is 600 s. Empirical distribution functions have been found with the use of the waiting time samples obtained by the method of order statistics. Omega-square goodness-of-fit test has shown that they disagree with exponential distribution describing stationary random process of supercritical embryo generation separating the system to macroscopic phases. Thus, it is shown that this random process is not stationary, consequently, nucleation rate to be depending on time.

Altitude and temperature dependences of inhomogeneous substance heat capacity have been studied in the gravitational field of the Earth in the vicinity of critical point on the basis of the fluctuation theory of phase transitions and the theory of gravitational effect. The obtained data prove non-monotonous field and temperature dependences of heat capacity of spatially inhomogeneous substance that is supported by experimental research of heat capacity in macro and confined systems under terrestrial conditions and at space flight microgravitation.

Thermal conductivity and thermal diffusivity coefficients of liquid indium have been determined in the range of temperatures from 470 to 1275 K by the laser flash method. Errors of heat transfer coefficients are ±(3.5−5) %. Approximating equations and tables of reference data have been developed for temperature dependence of properties. Measurement results have been compared with the data available in the literature. Temperature dependence of Lorentz number has been calculated up to 1000 K.

Based on the classical concept of atomic motion and the Goodman and Wachman lattice theory, we have developed a computer program to model the equilibrium and non-equilibrium scattering of helium atoms by 3D tungsten crystal lattice with allowance for adsorption surface coating. Within the concept of energy accommodation coefficient, we have calculated the rate of molecular heat transfer of helium to clean tungsten surface or to tungsten surface partially covered with an adsorbate. The calculations were performed for various surface temperatures. The calculated dependences were compared to test data obtained in experiments with surfaces controlled in terms of their chemical composition. Within the developed approach, the simulations proved capable of providing an adequate description to experimental data obtained for the equilibrium energy accommodation coefficient on the clean surface, and also for the non-equilibrium energy accommodation coefficient for the surface partially covered with adsorbate.

A new statement and the numerical solution of the problem of the peat layer firing as a result of the effect of the surface fire is given on the basis of a mathematical model of a porous reacting medium. The original reagent smoldering at moderate temperatures (Т_{1 ≤ 750 K) is found to be determined by the processes of heat and mass exchange with the forest fire source, peat drying and pyrolysis, reaction of the carbon oxide oxidation, thermophysical characteristics, and the peat height as well as by the thickness of the water layer under the peat layer.}