Modeling of thermal hydraulic instabilities in single heated channel loop during startup transients

A thermal hydraulics computer code was developed to simulate the geysering instability in a natural circulation system starting from subcooled conditions and to assess the impact of the system pressure and channel inlet subcooling on the inception of instability. The formulation of thermal hydraulics is inherently general and accounts for both single-phase liquid flow and nonhomogeneous, nonequilibrium two-phase flow. The computer code is based on momentum integral method where the current practice of basing fluid properties on the system averaged pressure has been relaxed and the local properties are based on local pressures estimated using the shape of steady-state pressure distribution, thereby, improving the predictions while preserving the computation speed, one of the important strength of the integral methods. This is an important modeling feature since the local vapor generation rate depends on local saturation temperature The methodology has been validated with the experiments conducted to investigate the instabilities in a low pressure natural circulation loop at low powers and high inlet subcoolings. The numerical simulations predicted periodic channel flow reversal, which is one of the feature of condensation-induced geysering. Basing local properties on local pressures instead of system average pressure led to decrease in the discrepancy in the prediction of the positive side amplitude from 40% to 6% and in the frequency from −15% to 5%. In addition, it was observed that the start-up instability can be avoided by increasing system pressure or by decreasing channel inlet subcooling. This study showed that the integral method coupled with local pressure variation for the vapor generation model is suitable to predict startup or geysering transients.     

Isostere Adsorptionswärme des Ethans an Zeolithen vom Typ NaX und NaY

Isosteric Heat of Adsorption of n-Ethane on Zeolites NaX and NaY Using a volumetric method isotherms of adsorption of ethane on zeolites NaX and NaY were measured. The Clausius-Clapeyron equation was used to calculate the isosteric heat of adsorption of the paraffin on both molecular-sieves. These enthalpies show a linear dependence of the amount of adsorbed molecules. The zeolite with the higher charge density shows a larger heat of adsorption. The difference in the heat of adsorption is explained by adsorption on different centers in the cavity of both zeolites.     

Formation of structure of the CdTe film, recrystallized on Mo/glass substrate under high temperature and mechanical pressure

Large-grained and 7 μm thick CdTe film has been fabricated on top of Mo coated soda-lime glass substrate. As a new approach the dynamic recrystallization process (DRC) was used to form the structure of films. For the characterization of the structure and composition of the films a scanning electron microscopy (SEM), energy dispersive X-ray microanalysis (EDS) and X-ray diffraction (XRD) methods were used. The changes in the structure of films were studied in accordance with the process temperature, pressure and as-deposited film thickness. Significant changes in the CdTe film were observed after DRC of deposited films at the process temperatures between 450 °C and 550 °C. EDS quantitative analysis showed that during the recrystallization the Mo and CdTe films composition remained stable for all studied samples. The XRD results showed that the increase in the process temperature caused improvement in orientation of the films along direction of (111). The DRC temperature above 550 °C reduced the orientation again. The limits of the temperature and pressure in application of soda-lime glass in DRC were found and discussed.