Luminescent properties and energy transfer processes of co-doped Yb–Er poly-crystalline YAG matrix

A poly-crystalline YAG matrix was obtained through the precipitation method; this matrix was single and co-doped with ytterbium and erbium ions, i.e., Yb:YAG, Er:YAG and Yb,Er:YAG. It is found that the measured luminescent properties are similar to those reported for a mono-crystal YAG matrix. In addition, by studying the energy transfer processes in co-doped samples, it is shown that at high erbium concentrations the red emission is enhanced through an up-conversion process that takes place from the ⁴I_13/2 to the ⁴F_9/2 state of erbium ions. This enhanced red emission becomes comparable in intensity to the observed green emission and occurs by pumping at 800 nm through a back energy transfer process.     

Pool boiling heat transfer of carbon dioxide on a horizontal tube

Carbon dioxide is again becoming an important refrigerant. While the thermophysical properties are well known there is a lack of data on its heat transfer characteristics.

In this study, heat transfer coefficients for nucleate boiling of carbon dioxide are determined using a standard apparatus for the investigation of pool boiling based on a set-up from Karlsruhe [D. Gorenflo, J. Goetz, K. Bier. Vorschlag für eine Standard-Apparatur zur Messung des Wärmeübergangs beim Blasensieden. Wärme-und Stoffübertragung 16 (1982), 69–78; J. Goetz, Entwicklung und Erprobung einer Normapparatur zur Messung des Wärmeübergangs beim Blasensieden. Dissertation Universität Karlsruhe (1980).] and built at our institute. Electrically heated horizontal cylinders with an outer diameter of 16 mm and a length of 100 mm are used as heating elements. Measurements with constant heat flux are performed for different wall materials and surface roughnesses. The heat transfer is investigated within the pressure range of 0.53≤ p ≤1.43 MPa (0.072≤ p/p_c ≤0.190) and a temperature range of −56≤ t ≤−30 °C, respectively. Heat fluxes of up to 80,000 W m⁻² are applied.

The influences of wall material and roughness on the heat transfer coefficient are evaluated separately. The obtained coefficients are compared to generally accepted correlations and to experimental results of other authors, who used similar configurations with copper tubes and carbon dioxide. These are the only previous experimental data, which could be found. Results for copper, stainless steel and aluminium as wall materials are presented.     

Electromagnetic control of convective heat transfer during electron beam evaporation: model experiments

The present paper aims to demonstrate that melt-flow during electron beam evaporation can be effectively controlled by using external magnetic fields to considerably reduce the convective heat transfer. We discuss the various effects of a static magnetic field, a static field combined with an applied electrical current, and a rotating magnetic field. We perform model experiments using GaInSn in eutectic composition as a test liquid. The liquid metal is heated locally at its free surface by an electric resistance heater. The results of the measurements are compared to prediction of numerical simulations.     

A continuous/discrete simulation of controlled atmosphere (CA) cool storage systems: evaluation of plant performance/design and product quality evolution

In this paper, a controlled atmosphere storage model, developed earlier, is validated using an industrial CA cool storage facility with stepwise product loading strategy. The proposed combined continuous and discrete CA cool storage simulation model was able to represent the step response transients and continuous events, with accuracy less than the observed variability or at best with accuracy level as set by the measurement instrument. Moreover, the handling of discrete/continuous events enabled the implementation of practical operational procedures and to investigate their implication on plant performance/design and directly on the product quality. The model is a valid working model for design and optimization of CA cool storage facilities both with respect to plant performance and final quality of the stored product.     

Heat transfer characteristics of flat plate finned-tube heat exchangers with large fin pitch

The objective of this study is to provide experimental data that can be used in the optimal design of flat plate finned-tube heat exchangers with large fin pitch. In this study, 22 heat exchangers were tested with a variation of fin pitch, number of tube row, and tube alignment. The air-side heat transfer coefficient decreased with a reduction of the fin pitch and an increase of the number of tube row. The reduction in the heat transfer coefficient of the four-row heat exchanger coil was approximately 10% as the fin pitch decreased from 15.0 to 7.5 mm over the Reynolds number range of 500–900 that was calculated based on the tube diameter. For all fin pitches, the heat transfer coefficient decreased as the number of tube row increased from 1 to 4. The staggered tube alignment improved heat transfer performance more than 10% compared to the inline tube alignment. A heat transfer correlation was developed from the measured data for flat plate finned-tubes with large fin pitch. The correlation yielded good predictions of the measured data with mean deviations of 3.8 and 6.2% for the inline and staggered tube alignment, respectively.     

A bending specimen for constant energy release rate under controlled displacement conditions

A bending specimen that gives a constant energy release rate (independent on the crack length) under controlled displacement conditions is developed. It is assumed that engineering beam theory is applicable, i.e. that among other things, small deformations and linear elastic isotropic material can be assumed.A specific geometry is analyzed numerically by use of the finite element method and experimentally by manufacturing a specimen with the actual geometry in PMMA (polymethylmetacrylate) and then determining the specimen stiffness for different crack lengths.It is found that the geometry considered gives a constant energy release rate under controlled displacement conditions.     

双线高速线材轧机速度控制系统

通过双线高速线材轧机两种常规速度控制系统模型的分析，重点介绍了湘潭第二高速线材厂的双线速度控制系统，其与常规的速度控制系统比较。其有单／双线轧制控制模式的无扰切换、精轧机速度波动小、调整灵活和便于检修等优点。     

Heat transfer coefficient measurements for mixed gas working fluids at cryogenic temperatures

The use of mixed gas working fluids has become common in Joule-Thomson type cryocoolers for a variety of applications. However, there is very little data or theory currently available regarding the heat transfer coefficient associated with these multi-component, multi-phase mixtures at cryogenic temperatures. This paper describes an experimental test facility and procedure that has been used to make careful measurements of the horizontal, flow boiling heat transfer coefficient for several hydrocarbon mixtures that are nominally optimal for small, Joule-Thomson cryocoolers in the 80 K to 120 K operating range. Data are presented over a range of temperatures from 100 K to room temperature and for several pressures and mass flow rates. The results indicate that quality and mass flux are the most important parameters governing the heat transfer coefficient among those that were varied. The experiment is verified by carrying out tests using single-phase, pure nitrogen gas and comparing the results with the Dittus-Boelter equation. The experimental uncertainty of the measurements is estimated from 1st principles; additionally, the repeatability of the experimental measurements was investigated by replicating tests at a nominal set of operating conditions and composition on separate days. The measurements presented here are intended to aid in the design of small, mixed-gas Joule-Thomson cryocoolers.