Wärmeübertragung und Druckabfall in keramischen Schaumstrukturen bei erzwungener Strömung

Zusammenfassung Der konvektive Wärmeübergangskoeffizient und der Druckabfall in keramischen Schaumstrukturen werden gemessen. Es werden Cordierite-Schäume mit 10, 20, 30 und 50 Poren pro inch (PPI) bei Leerrohrgeschwindigkeiten (Luft) von 0,5 bis 14 m/s untersucht. Der konvektive Wärmeübergang zwischen Fluid und Feststoff wird in diesem Geschwindigkeitsbereich mit dem AnsatzNu=¢ ·Re ^m · Pr ^1/3 gut beschrieben. Die zur Bestimmung der Parameter ¢ undm erarbeitete Meßmethode wird erläutert. Die damit erhaltenen Daten werden im Hinblick auf die Verwendung von keramischen Schäumen als Katalysatorträger diskutiert und mit Literaturdaten von herkömmlichen Parallelkanal-Monolithen verglichen.

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Heat transfer and pressure drop in ceramic foams in forced convective flow

The convective heat transfer coefficient and pressure drop in ceramic foam structures are measured. Cordierite foams with 10, 20, 30 und 50 pores per inch (PPI) at empty tube velocities (of air) at 0.5 to 14 m/s are investigated. The convective heat transfer between fluid and substrate is described well within this velocity range by means of the assumptionNu=¢ ·Re ^m · Pr ^1/3. The experimental method developed to obtain the values of ¢ andm is described. The data thus gained are discussed in view of an application of ceramic foams as catalyst supports and are compared with data of conventional parallel channel monoliths mentioned in the literature.

     

Optimization of thermal conditions during crystal growth in a multi‐zone resistance furnace

The thermal condition is one the of most important control parameters for crystal growth. In this paper we present an effective numerical method in detail for optimizing thermal conditions in multi‐zone crystal growth facilities, especially for crystal growth by the float zone (FZ) technique. A furnace function Ω is introduced to integrate the character of a growth furnace into a linear equation system. The desired power distribution can be therefore approached by solving the linear equation system iteratively. An expert systemlike algorithm has been developed in order to obtain a more suitable solution for practical applications. This method was used to investigate thermal parameters for experiments of SiGe/GeSi single crystal growth by the FZ technique. It is an individual program which can be combined with any commercial finite element/finite volume (FE/FV) program such as FIDAP^TM.     

Compensation of CdTe by Doping With Gallium

Semi‐insulating CdTe single crystals doped with Ga were grown from the vapour phase by the modified Markov technique MMT. The study of the resistivity map in the cross‐sections cut a long the growth direction has been performed. The compensation phenomenon is analysed in the framework of the three levels Fermi‐statistic model. It is shown that a semi‐insulation behaviour throughout the ingot is due to the compensation of shallow impurities by the deep level. From the low ‐temperature photolum inescence spectra it was concluded that shallow donors (Ga_Cd) are partly compensated by (Ga_Cd‐V_Cd)^‐and (Ga_Cd‐Cd_Te) complexes and by residual acceptors (Na_Cd, Cu_Cd) . The microscopic structure of (Ga_Cd‐Cd_Te) complexis proposed based on the value of its local phonon mode and the growth conditions. A native defect like Te_Cd which has a deep level near the middle‐band‐gap is suppose to give a stable compensation and a tolerance for variation in shallow impurity concentrations.     

Über verschränkte Produktordnungen

This paper, which is dedicated to Emmy Noether on the occasion of the centenary of her birthday, is concerned with the arithmetics of crossed products. In particular, the definition of a crossed product order is tightened and it is shown that such orders are “one-headed,” i.e., that the “idealizer of radical” method of embedding always leads to the same hereditary order.     

On Barbilian domains over commutative rings

W. LEISSNER proved in [2] that an arbitrarily given affine BARBILIAN PLANE must be isomorphic to a plane affine geometry over a Z-ring R and moreover did he establish the converse theorem among other results in [3]. One of the fundamental notions in this axiomatic approach of ring geometry is that of a BARBILIAN DOMAIN (BARBILIANBEREICH). The aim of our note is to present sufficient conditions in case of commutative rings R which guarantee that R admits exactly one BARBILIAN DOMAIN. If for instance R is an euclidean ring, then R admits exactly one BARBILIAN DOMAIN (P.M.COHN [1], corollary to Theorem 3 of our note).The author is indebted to Professor LEISSNER for several helpful discussions during the preparation of this note.     

Influence of particle aspect ratio on the midinfrared extinction spectra of wavelength-sized ice crystals

We have used the T-matrix method and the discrete dipole approximation to compute the midinfrared extinction cross-sections (4500-800 cm(-1)) of randomly oriented circular ice cylinders for aspect ratios extending up to 10 for oblate and down to 1/6 for prolate particle shapes. Equal-volume sphere diameters ranged from 0.1 to 10 microm for both particle classes. A high degree of particle asphericity provokes a strong distortion of the spectral habitus compared to the extinction spectrum of compactly shaped ice crystals with an aspect ratio around 1. The magnitude and the sign (increase or diminution) of the shape-related changes in both the absorption and the scattering cross-sections crucially depend on the particle size and the values for the real and imaginary part of the complex refractive index. When increasing the particle asphericity for a given equal-volume sphere diameter, the values for the overall extinction cross-sections may change in opposite directions for different parts of the spectrum. We have applied our calculations to the analysis of recent expansion cooling experiments on the formation of cirrus clouds, performed in the large coolable aerosol and cloud chamber AIDA of Forschungszentrum Karlsruhe at a temperature of 210 K. Depending on the nature of the seed particles and the temperature and relative humidity characteristics during the expansion, ice crystals of various shapes and aspect ratios could be produced. For a particular expansion experiment, using Illite mineral dust particles coated with a layer of secondary organic matter as seed aerosol, we have clearly detected the spectral signatures characteristic of strongly aspherical ice crystal habits in the recorded infrared extinction spectra. We demonstrate that the number size distributions and total number concentrations of the ice particles that were generated in this expansion run can only be accurately derived from the recorded infrared spectra when employing aspect ratios as high as 10 in the retrieval approach. Remarkably, the measured spectra could also be accurately fitted when employing an aspect ratio of 1 in the retrieval. The so-deduced ice particle number concentrations, however, exceeded the true values, determined with an optical particle counter, by more than 1 order of magnitude. Thus, the shape-induced spectral changes between the extinction spectra of platelike ice crystals of aspect ratio 10 and compactly shaped particles of aspect ratio 1 can be efficiently balanced by deforming the true number size distribution of the ice cloud. As a result of this severe size/shape ambiguity in the spectral analysis, we consider it indispensable to cross-check the infrared retrieval results of wavelength-sized ice particles with independent reference measurements of either the number size distribution or the particle morphology.