Rapid stabilization of thawing soils: field experience and application

Thawing soils can severely restrict vehicle travel on unpaved surfaces. However, a variety of materials and construction techniques can be used to stabilize thawing soils to reduce immobilization problems. The US Engineer Research and Development Center's Army Cold Regions Research and Engineering Laboratory (CRREL) and the Wisconsin National Guard evaluated several stabilization techniques in a field demonstration project during spring thaw at Fort McCoy, Wisconsin, in 1995. Additional tests on chemical stabilizing techniques were conducted at CRREL's Frost Effects Research Facility. The results of these test programs were reduced to a decision matrix for stabilizing thawing ground, and used during the deployment of US troops in Bosnia during January and February of 1996. The soil frost and moisture conditions expected during this time frame were predicted using MIDFROCAL (MIDwest FROst CALculator). This paper is an overview of the stabilization techniques evaluated and their recommended application based on the expected soil frost conditions and traffic requirements. Although the experiments were performed with military vehicles in mind, the techniques are suitable for many civilian applications such as forestry, construction, mining, and oil exploration.     

Effect of wall heat transfer on shock-tube test temperature at long times

When performing chemical kinetics experiments behind reflected shock waves at conditions of lower temperature (<1,000 K), longer test times on the order of 10–20 ms may be required. The integrity of the test temperature during such experiments may be in question, because heat loss to the tube walls may play a larger role than is generally seen in shock-tube kinetics experiments that are over within a millisecond or two. A series of detailed calculations was performed to estimate the effect of longer test times on the temperature uniformity of the post-shock test gas. Assuming the main mode of heat transfer is conduction between the high-temperature gas and the colder shock-tube walls, a comprehensive set of calculations covering a range of conditions including test temperatures between 800 and 1,800 K, pressures between 1 and 50 atm, driven-tube inner diameters between 3 and 16.2 cm, and test gases of N₂ and Ar was performed. Based on the results, heat loss to the tube walls does not significantly reduce the area-averaged temperature behind the reflected shock wave for test conditions that are likely to be used in shock-tube studies for test times up to 20 ms (and higher), provided the shock-tube inner diameter is sufficiently large (>8cm). Smaller diameters on the order of 3 cm or less can experience significant temperature loss near the reflected-shock region. Although the area-averaged gas temperature decreases due to the heat loss, the main core region remains spatially uniform so that the zone of temperature change is limited to only the thermal layer adjacent to the walls. Although the heat conduction model assumes the gas and wall to behave as solid bodies, resulting in a core gas temperature that remains constant at the initial temperature, a two-zone gas model that accounts for density loss from the core to the colder thermal layer indicates that the core temperature and gas pressure both decrease slightly with time. A full CFD solution of the shock-tube flow field and heat transfer at long test times was also performed for one typical condition (800 K, 1 atm, Ar), the results of which indicate that the simpler analytical conduction model is realistic but somewhat conservative in that it over predicts the mean temperature loss by a few Kelvins. This paper presents the first comprehensive study on the effects of long test times on the average test gas temperature behind the reflected shock wave for conditions representative of chemical kinetics experiments.     

Solvabilitity of a model problem of heat and mass transfer in thawing snow

A model problem of the motion of water and air in thawing snow is examined using the Masket-Leverett equations of two-phase filtration. The theorem of existence of a self-similar solution is proved. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 49, No. 4, pp. 13–23, July–August, 2008.     

Effect of temperature dependent viscosity on revolving axi-symmetric ferrofluid flow with heat transfer

The prime objective of the present study is to examine the effect of temperature dependent viscosity μ(T) on the revolving axi-symmetric laminar boundary layer flow of an incompressible, electrically non-conducting ferrofluid in the presence of a stationary plate subjected to a magnetic field and maintained at a uniform temperature. To serve this purpose, the non-linear coupled partial differential equations are firstly converted into the ordinary differential equations using well-known similarity transformations. The popular finite difference method is employed to discretize the non-linear coupled differential equations. These discretized equations are then solved using the Newton method in MATLAB, for which an initial guess is made with the help of the Flex PDE Solver. Along with the velocity profiles, the effects of temperature dependent viscosity are also examined on the skin friction, the heat transfer, and the boundary layer displacement thickness. The obtained results are presented numerically as well as graphically.     

Simultaneous droplet impingement dynamics and heat transfer on nano-structured surfaces

This study examines the hydrodynamics and temperature characteristics of distilled deionized water droplets impinging on smooth and nano-structured surfaces using high speed (HS) and infrared (IR) imaging at We = 23.6 and Re = 1593, both based on initial drop impingement parameters. Results for a smooth and nano-structured surface for a range of surface temperatures are compared. Droplet impact velocity, transient spreading diameter and dynamic contact angle are measured. The near surface average droplet fluid temperatures are evaluated for conditions of evaporative cooling and boiling. Also included are surface temperature results using a gold layered IR opaque surface on silicon. Four stages of the impingement process are identified: impact, boiling, near constant surface diameter evaporation, and final dry-out. For the boiling conditions there is initial nucleation followed by severe boiling, then near constant diameter evaporation resulting in shrinking of the droplet height. When a critical contact angle is reached during evaporation the droplet rapidly retracts to a smaller diameter reducing the contact area with the surface. This continues as a sequence of retractions until final dry out. The basic trends are the same for all surfaces, but the nano-structured surface has a lower dissipated energy during impact and enhances the heat transfer for evaporative cooling with a 20% shorter time to achieve final dry out.     

Effect of temperature gradient on Marangoni condensation heat transfer for ethanol–water mixtures

This paper experimentally studied the effect of macroscopic temperature gradient on Marangoni condensation of ethanol–water vapor mixtures under a wide range of concentrations. For each concentration, the experiment was performed at different velocities and pressures. An oblique copper block was employed to create surface temperature gradient. The results indicated that local heat flux was varied along transversal condensation surface, which was caused by surface temperature gradient. This difference in heat flux might be attributed to the variation of condensate thickness on condensation surface. In addition, a mean heat transfer coefficient was derived along transversal condensation surface. For low ethanol concentration (0.5%, 1%), the coefficient kept a high value over a relative wide range of vapor-to-surface temperature difference (<10 K) and could be augmented as much as 15% as compared with literature under similar experimental condition. Moreover, the mean heat transfer coefficient generally increased with increasing velocity or pressure for all concentrations of the ethanol–water mixtures.     

Effect of magnetic field on 3D flow and heat transfer during solidification from a melt

We present the effect of a magnetic field on three-dimensional fluid flow and heat transfer during solidification from a melt in a cubic enclosure. The walls of the enclosure are considered perfectly electrically conducting and the magnetic field is applied separately in three directions. The finite-volume method with enthalpy formulation is used to solve the mathematical model in the solid and liquid phases. The results obtained by our computer code are compared with the numerical and experimental data found in the literature. For Gr = 5 × 10⁵ and Ha = 0, 25, 50, 75, and 100 (where Gr and Ha are the Grashof and Hartmann numbers, respectively), the effects of magnetic field on flow and thermal fields, and on solid/liquid interface shape are presented and discussed. The interface is localized with and without magnetic field. The results show a strong dependence between the interface shape and the intensity and orientation of magnetic field. When the magnetic field is applied along the X-direction, the magnetic stability diagrams (V_max–Ha) and (Nu_avg–Ha) show the strongest stabilization of the flow field and heat transfer.     

Effect of porosity and the inlet heat transfer fluid temperature variation on the performance of cool thermal energy storage system

This paper discusses the results of numerical and experimental study of an encapsulated cool thermal energy storage system. The storage system is a cylindrical storage tank filled with phase change material encapsulated in spherical container, placed in a refrigeration loop. A simulation program was developed to evaluate the temperature histories of the heat transfer fluid and the phase change material at any axial location during the charging period. The present analysis aims at studying the influence of the inlet heat transfer fluid temperature and porosity on system performance. An experimental setup was designed and constructed to conduct the experiments. The results of the model were validated by comparison with experimental results of temperature profiles for different inlet heat transfer fluid temperatures and porosity. The results are in good agreement with the experimental results. The results reported are much useful for designing cool thermal energy storage systems.     

Effect of acoustic field on heat transfer in a sound assisted fluidized bed of fine powders

The fluidized beds are widely used in a variety of industries where heat transfer properties of the fluidized system become important for successful operation. Fluidized are preferred in heat recovery processes because of their unique ability of rapid heat transfer and uniform temperature. Fine powders handling and processing technologies have received widespread attention due to increased use of fine powders in the manufacture of drugs, cosmetics, plastics, catalysts, energetics and other advanced materials. A better understanding of fluidization behavior of fine powders is of great importance in applications involving heat transfer, mass transfer, mixing, transporting and modifying surface properties etc. The difficulty in putting the fine powders in suspension with the fluidizing gas is related to the cohesive structure and to the physical forces between the primary particles. The sound waves agitate bubbling and this results in improving solids mixing in the fluidized bed. The improved solids mixing results in uniform and smooth fluidization, which leads to better heat transfer rates in the fluidized bed.     

Boiling heat transfer region with independence of the wall temperature

 Extensive measurements of the intensive cooling of hot-rolled wires with temperatures between 1000 ^°C and 1100 ^°C are analysed. The analysis proves the existence of a convection-controlled boiling region, which has been previously observed by few authors in the case of high mass fluxes and high liquid subcooling. This region is characterised by an independence of the heat flux of the surface temperature. The heat flux depends essentially on the Reynolds number, the main influence parameter of the single phase convection, and on the liquid subcooling. Received on 13 September 1999     

Effect of longitudinal surface curvature on heat transfer

The effect of longitudinal surface curvature on heat transfer has been analysed for laminar forced convection by the method of matched asymptotic expansions. Utilizing the classical solution of boundary layer equations as the first order approximations, the second order perturbation for the velocity and temperature field has been calculated by a similarity analysis. The analysis permits the wall temperature to vary as a power function of distance from the stagnation point. Numerical solutions have been obtained for the resulting coupled ordinary differential equations. The results for the variation in the second order temperature profile and the second order wall temperature gradient due to surface curvature parameter, Prandtl number, wall temperature distribution parameter, and pressure gradient parameters are presented graphically. The variation in a typical temperature profile due to curvature, and percentage variations from the first order theory due to longitudinal surface curvature are also presented graphically.     

Effect of high-amplitude flow pulsations on heat transfer

We present results of an experimental study of the effect of flow pulsations on turbulent heat transfer in the transverse direction in propagation of sound waves. A significant increase in heat-transfer intensity was recorded at mean-square pressure pulsation amplitude p=168–180 dB at frequency f=100–150 Hz. The dependence of heat-transfer intensity on acoustic field characteristics is obtained.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 2, pp. 169–172, March–April, 1975.     

Effect of acoustic cavitation on boiling heat transfer

Boiling heat transfer on a horizontal circular copper tube in an acoustical field is investigated experimentally and the relation between the liquid cavitation, the boiling and the micro bubble radii are analyzed theoretically. The results show that cavitation bubbles have an important influence on the nucleation, growth and collapse of vapor embryo within cavities on the heat transfer surface and that the enhancement of boiling heat transfer by acoustic cavitation mainly depends on whether the vapor embryo is activated by the cavitation bubbles to initiate boiling.     

Velocity field and heat transfer in a vortex flow exchanger

This study deals with the flow of newtonian and non-newtonian fluids, inside a cylindrical flat cavity, provided with a tangential injection device, model of heat exchangers with spiral flow.The dynamic analysis which requires essentially a laser Doppler velocimeter focusses on the determination of the velocity field and brings out the existence of secondary flows which are important for heat transfer.In turbulent regime, the law of exchange agrees with the Chilton-Colburn analogy. On the other hand, for laminar flow, it becomes necessary to include the variations with temperature of the consistency (K); the Nusselt number (Nu) not only depends on the flow rate, but also on the density of transferred heat flux.

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Geschwindigkeitsfeld und Wärmeübertragung in einem Wärmeüberträger mit spiralförmiger Strömung

Zusammenfassung Diese Studie betrifft die Strömung von Newton'schen oder Nicht-Newton'schen Flüssigkeiten innerhalb eines flachen zylindrischen Hohlraumes, der mit einem tangentialen Einspritzungssystem versehen ist und als Modell für Wärmeaustauscher mit spiralförmiger Strömung dient.Die dynamische Analyse, die das Laser-Dopplerverfahren (Laser-Velocimetrie) erfordert, kennzeichnet das Geschwindigkeitsfeld und beweist die Existenz von Sekundärströmungen, die für die Wärmeübertragung wichtig sind.Für den turbulenten Betrieb folgt das Gesetz des Wärmeaustausches der Chilton-Colburn Analogie. Was den laminaren Betrieb angeht, wird es erforderlich, die Variationen der BeschaffenheitK mit der Temperatur zu berücksichtigen. Die Nusseltsche Zahl hängt natürlich vom dynamischen Betrieb ebenso wie von der übertragenen Leistung ab.

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Nomenclature Nu=[/S T]D _h/ Nusselt number - Re=V ₀ D _h/ Reynolds number - Re _g=V ₀ ^2–n D _n ^h /K generalized Reynolds number - Pr=C _p / Prandtl number - Pr _g=C _p[V ₀/D _h] ^n–1 K/ generalized Prandtl number - H height of cylinders (m) - H dimension of the entrance device (m) - S ₁ tested section - S ₂ tested section - shear stress (Pa) - K consistency (Pa s ⁿ ) - shear rate (s^–1) - n power law index - a, b constants in the consistency formulaK=a exp [–b t] - a, b constants in the rheological indexn=a exp [b T] - T, T _e,T _p temperature, inlet temp., wall temp. - p pressure (Pa) - p pressure drop (Pa) - V velocity vector (components:V ₁,V ₂,V ₃) (m/s) - V ₀ mean velocity (m/s) - X ₁,X ₂,X ₃ cylindrical coordinate system - C _f friction factor - R ₁,R ₂ radius (m) - heat flux (W) - thermal conductivity (W/(m °C)) - C _p specific heat of fluid (J/(kg C°)) - dynamic viscosity (Pa s) - Q _v volumic flow rate (m³/s) - =/S heat flux density (W/m²) - S exchange area (m²)