FREE CONVECTIVE DRAFT INDUCED BY THERMAL AND CONCENTRATION GRADIENTS INSIDE AN ISOTHERMAL,VERTICAL CYLINDER†

Laminar, free convective flow through a vertical cylinder induced by the thermal and concentration buoyancy forces is investigated. The numerical studies involve development of a steady-state, two-dimensional heat and mass transfer model for the moist air core of the vertical tube. The stream function-vorticity method is employed to simplify the governing, coupled conservation equations which were then numerically solved by the successive over-relaxation (SOR) and alternating direction implicit ((ADI) methods.

A graphical correlation was found between dimensionless flow rate and dimensionless tube length as a function of the buoyancy force ratio N = Gr/Grc. Excellent agreement was obtained for the dimensionless flow rale results with those of Davis and Perona¹⁶ and Kageyama and Izumi¹³ for the case when only the-thermal buoyancy force is considered.

The combined buoyancy force from thermal and species diffusion provides larger local Nusselt Nu and local Sherwood Sh numbers relative to the case when just one buoyancy force is accounted for. Both local Nu and local Sh are seen to asymptotically approach a constant value as flow develops.     

MELTING OF ICE AROUND A HORIZONTAL ISOTHERMAL CYLINDRICAL HEAT SOURCE

Processes during melting from a horizontal cylindrical heat source of uniform surface temperature embedded in ice have been studied experimentally. The volume of the melt and its shape were photographed at different times for various constant temperatures of the heat source. At early times and under all conditions, the melt occupied a cylindrical annulus. At later times free convective motion caused pear-shaped melt contours which pointed downward when the temperatures of the heat source were below 7°C and upward when the temperatures were above 8°C. Instabilities in cellular natural convection motion resulted in waviness of the interface. The location and magnitude of these ripples were found to depend on the temperature of the heat source and the melt layer thickness. Shadowgraph techniques were used to determine local heat transfer coefficients at the heat source surface.     

Specific intentions–general realities: on the relation between urban forms and political aspirations in Berlin during the twentieth century

Berlin as a capital city seems to be a very specific case: no other capital city experienced political changes as extreme as Berlin in the twentieth century. All the different political systems tried to impose their political aspirations on the urban fabric, planning large government complexes with specific urban images and highly political overtones. Therefore, the planning history of Berlin is often considered as unique – each phase of planning belonging closely to the political body that installed it – and each phase of planning being part of a special Berlin planning history.

Within this article Berlin's planning history is interpreted in a different way. First, there are planning ideas, which are far more constant than the political systems and which cannot be attributed to only one system. Secondly, for each planning idea which had been developed for special political purposes in Berlin, some international examples can be found which root it in an international urban debate rather than in a regional tradition. The result is quite astonishing, since for nearly every plan international examples with often contradictory political ideas can be found. This means that one can no longer interpret Berlin's planning history as a special case, explainable mostly by local traditions and political intentions. In fact, urban design for government districts in Berlin is more closely connected to international trends and the discipline of urban design. Thus, in addition, the relationship between urban forms and political aspirations must be considered as being more arbitrary than conventionally seen.     

Calculation of the inversion channel resistance for n-MOSFETs using a classical and semi-quantum-mechanical model

Two models are presented to calculate the inversion channel carrier distribution of an n-type MOSFET. The classical approach solves Poisson's equation and applies Fermi-Dirac statistics in conjunction with Gauss's law to determine the carrier distribution. The semi-quantum-mechanical model solves Schrodinger's equation decoupled from Poisson's equation and the Fermi-Dirac statistics by assuming a constant electrostatic field and quantized energy levels. The results of both models are used to determine the channel resistance taking transversal field initiated mobility reductions into account. A comparison is made to a practical device and the results discussed.     

Dilatation Behavior of Two Differently Heat-Treated Commercial Magnesia-Partially-Stabilized Zirconias

The dilatation behavior of two grades of partially stabilized zirconia (PSZ) was determined. ZN 40 is a material which contains 40% to 50% t -ZrO₂ particles, and has a reproducible dilatometric behavior up to 900°C. ZN 50 is a Mg-PSZ containing 20% to 30% of the monoclinic phase. The cyclic dilatation curves show an unstable hysteresis effect which arises from the tetragonal-monoclinic phase transformation.     

Viscosity-Drift Technique for Determination of Critical Liquid-Liquid Immiscibility Temperature of Glasses

The time dependence of the low-temperature viscosity was used to determine the occurrence of phase separation in glass systems and to measure the critical immiscibility temperature. The technique is fast and easily performed and can be applied to materials with very low immiscibility gaps. Immiscibility temperatures for several technically important glass compositions are given.     

NMR Study of Adsorbate Self-Diffusion in Porous Glasses

The NMR pulsed field gradient technique was used to study molecular transport in porous glasses. The adsorbent materials were produced by leaching phase-separated sodium-boron-silica glasses of different composition, then heat-treating. The pore diameters of the glass samples produced were within the interval 0.8 to 50 nm. Water, methanol, dodecane, and do-decene were used as adsorbates. The coefficients of adsorbate self-diffusion were found to decrease with decreasing pore diameter. In comparison with the neat liquids, a reduction in the adsorbate mobility (up to two orders of magnitude) was observed. For the larger pores, that decrease may be attributed to the tortuosity of the adsorbent, whereas the low diffusivities in the porous glasses with small pores are a consequence of the stabilizing effect of the rigid adsorbent framework.     

Evaporative Cooling and Heat Transfer Augmentation Related to Reduced Condenser Temperatures

In recent articles evaporative cooling has proved to be the utmost air-side heat transfer augmentation. This kind of augmentation, when applied to steam condensers in power plants or to refrigerants in air conditioner systems, results in a lowering of the condensing temperature, even below ambient levels. This yields a remarkable increase of thermodynamic efficiencies of both processes and, therefore, a reduction of energy consumption. The combined heat and mass transfer processes in an evaporatively cooled device are very complex, due to the vast numbers of parameters which must be considered. Previous studies have assumed it impossible to perform any analysis unless many simplifying assumptions are made. The present article indicates that analysis can be performed where none of the assumptions originally used by Merkel are made. It is demonstrated by experiments that plate-fin tube and bare tube condenser performances can be predicted reliably. The insensitivity of the analysis to geometric design eliminates the need for extensive model tests and for production of correction and design factors for the design of evaporative condensers.     

Evaluation of a CPU Scheduling Mechanism for Multimedia Systems

Multimedia applications handling audio and video data have to obey time characteristics of these media types. Besides a basic functionality to express time relations, correctness with respect to time constraints requires mechanisms which lead to favoured processing of multimedia operations. CPU scheduling techniques based on the experience from real-time operating systems offer a solution and provide multimedia applications with the ability to meet time-related quality of service requirements. This paper discusses mechanisms to express time in multimedia systems and describes an implementation of a CPU scheduler designed to run under IBM's UNIX derivate AIX. The evaluation of the implementation based on measurements shows that the scheduler is able to support the time requirements of multimedia applications and that such mechanisms are indeed necessary since otherwise deadline violations occur.     

FAST S-PARAMETER COMPUTATION OF A MICROSTRIP INTERDIGITAL FILTER USING TLM,PRONY'S AND DIGITAL FILTERING TECHNIQUES

A spectral estimation technique based on Prony's method has been used to analyse a five section microstrip interdigital filter with the TLM method. The time response of the filter is expressed as the sum of only five damped sinusoids, yielding an analytical expression for the time response. Furthermore, since an analytical Fourier transform exists for this expression, the frequency response is obtained without applying the fast Fourier or discrete Fourier transform. This typically reduces the time required for a TLM analysis by a factor of five. The approximation agrees very well with a full TLM time response. It is also shown that the numerical noise due to single precision computation of the absorbing boundary algorithm can be eliminated by digital filtering techniques. The effect of the via holes used for grounding the quarter-wave resonators of the filter has been included by discretizing them along with the rest of the circuit. Scattering parameters for the filter compare well with measurements published by Swanson.