CD、DVD信号源对AV及Hi-Fi系统的重要性

很多消费者舍得花一万元以上买放大器或音箱、或者买万多元的投影机,但投资在信号源的部分却只有三两千元。本人三十年来认识的发烧人很多,圈中一致认为对信号源的投资是要重视的对象,否则AV及Hi-Fi系统的投资会失误,投影机也不能有理想的发挥。我们发烧界的Hi-Fi理论认为越是接近音源投资越大,这个理论是发烧公认的。所以近日我把所有常见的蓝光DVD机进行测试后,最后我会把个人认为可以有理想效果的3台蓝光DVD机留在AV房占为己有。     

Optimizing headspace sampling temperature and time for analysis of volatile oxidation products in fish oil

Headspace-gas chromatography (HS-GC), based on adsorption to Tenax GR®, thermal desorption and GC, has been used for analysis of volatiles in fish oil. To optimize sampling conditions, the effect of heating the fish oil at various temperatures and times was evaluated from anisidine values (AV) and HS-GC. AV indicated sample degradations at 90°C but only small alterations between 60 and 75°C. HS-GC showed increasing response with temperature and time. Purging at 75°C for 45 min was selected as the preferred sampling condition for oxidized fish oil.     

Pyrolysis of Natural Gas: Effects of Process Variables and Reactor Materials on the Product Gas Composition

High‐temperature pyrolysis of natural gas is the basis of the standard method for the manufacture of acetylene. The study of methane pyrolysis was designed to find optimum process conditions that would produce high yields of acetylene with minimal carbon formation. High temperatures and short residence times enhanced the selectivity for acetylene, while hydrogen dilution was found to suppress the generation of carbonous products. Carbon formation on reactor surfaces over time may be mainly responsible for the misalignment of predicted and measured product gas compositions, as the mechanisms reported do not consider the surface chemistry. In essence, the pyrolysis system favors the highest possible temperature and shortest possible residence time, suggesting that the selection of reactor materials is the key for pyrolysis process optimization. The operating temperature is likely dictated by the physical properties of the reactor materials rather than the selection of optimal pyrolysis conditions.     

The pyrolysis of natural gas: A study of carbon deposition and the suitability of reactor materials

This study of methane pyrolysis was designed to look at carbon deposition on the internal reactor and wafer surface during CH₄ pyrolysis. The rate of carbon deposition on the internal reactor surfaces could be reduced with: lower methane/oxygen ratios, shorter residence times, and lower temperatures. The type of carbon formed appeared to have a significant effect on the pyrolysis process. Pyrolytic carbon with a lower order structure produces a higher selectivity for carbon formation compared to carbon with a higher order structure. Form a process perspective, there are two obvious means of addressing this: deposited carbon could be regularly removed; and/or pyrolysis conditions are selected that produce carbon with a higher order structure. From the results, it is very clear that any development of a commercial process for natural gas pyrolysis in ceramic reactor systems would have to carefully address the selection of reactor material. © 2018 American Institute of Chemical Engineers AIChE J, 65: 1035–1046, 2019     

An improved method for surface modification of porous water purification membranes

The surfaces of polysulfone and polyethersulfone ultrafiltration membranes were coated with polydopamine, yielding hydrophilic membranes that, under constant transmembrane pressure fouling conditions, have previously shown enhanced flux relative to unmodified membranes. When evaluated under constant permeate flux fouling, however, modified membranes exhibited higher transmembrane pressures than their unmodified analogs. This increased transmembrane pressure in the coated membranes was ascribed to the decrease in membrane permeance resulting from applying the polydopamine coating. The membrane permeance could be tuned by varying polydopamine deposition time and, even at the shortest deposition times studied here, a few minutes, a substantial increase in membrane hydrophilicity could be achieved. Therefore, polydopamine was deposited on a membrane of relatively high permeance until the pure water permeance of the modified membrane matched that of a membrane having lower native permeance, permitting a comparison of the fouling performance of a modified and unmodified membrane with the same pure water permeance. This approach was repeated, using a single, high permeance membrane as the base membrane for modification, to produce a family of modified membranes having the same initial pure water permeances as lower permeance, unmodified membranes. When unmodified and modified membranes of the same initial permeance were compared at constant flux fouling conditions, the modified membranes consistently exhibited lower transmembrane pressures and similar organic rejections to the unmodified membranes. Because many porous water purification membranes are operated at constant flux in industrial settings, an interesting methodology for membrane surface modification may be to surface-modify a membrane of high permeance until the desired permeance is achieved, rather than by surface modification of a membrane that natively has the desired water transport characteristics, since the surface modification procedures almost invariably lead to lower pure water permeance.     

Single,twin and triple buried heating pipes: on potential savings in heat losses and costs

In order to make district heating systems competitive in areas with single family houses or in other areas with low heat demands it is necessary to reduce the heat losses from the pipes. In recent years the twin pipe has become popular in the Nordic countries. In the article we describe how the heat loss and the heat loss coefficients can be calculated. We introduce the triple pipe with three media pipes (two supply pipes and one return pipe). The temperature dependency and the ageing of polyurethane insulation are briefly discussed. A comparison is made for different 80 mm distribution pipes and for different service pipes with respect to heat losses and to resources, i.e. materials needed for the casing and polyurethane insulation and the gravel in the excavations. For the distribution pipe we found that an egg‐shaped twin pipe can reduce the heat loss by 37% and the investments by 12% compared with a pair of single pipes. For the service pipes we found that the triple pipe reduces the heat loss by 45% compared with a common pair of single pipes and by 24% compared with circular twin pipes. The reduction in investment index is 21%. The article also addresses the question of the heat exchange between the two media pipes in a twin pipe. Copyright © 2005 John Wiley & Sons, Ltd.     

RVGEN: a tool for generation of random variates

A number of applications in simulation and software testing require random number generation, both uniform and non-uniform. Although libraries are available for random number generation, there is no user-friendly tool to enable the user to use and build random number generators. This paper presents the RVGEN tool, developed at TRDDC. Using the tool, the software developer can design a random number generator specific to a particular pattern. The tool takes specifications at a high level and also partly in C++, and generates C++ code for a number of user-friendly functions. These include setting and getting of random variate parameters, GUIs for inputting parameters, validation of parameters, parameter input and output from a file, generation of random variate class declarations, generation of seeds for multiple streams, and testing of the random variate generator. The tool provides a number of classical, general-purpose and empiric distributions by default.     

A matrix geometric representation for the queue length distribution of multitype semi-Markovian queues

In this paper we study a broad class of semi-Markovian queues introduced by Sengupta. This class contains many classical queues such as the GI/M/1 queue, SM/MAP/1 queue and others, as well as queues with correlated inter-arrival and service times. Queues belonging to this class are characterized by a set of matrices of size $m$ and Sengupta showed that its waiting time distribution can be represented as a phase-type distribution of order $m$. For the special case of the SM/MAP/1 queue without correlated service and inter-arrival times the queue length distribution was also shown to be phase-type of order $m$, but no derivation for the queue length was provided in the general case.This paper introduces an order $m^2$ phase-type representation $(\kappa ,K)$ for the queue length distribution in the general case and proves that the order $m^2$ of the distribution cannot be further reduced in general. A matrix geometric representation $(\kappa ,K,\nu )$ is also established for the number of type $\tau ?\{1,\dots ,m\}$ customers in the system, where a customer is of type $\tau$ if the phase in which it completes service belongs to $\tau$. We derive these results in both discrete and continuous time and also discuss the numerical procedure to compute $(\kappa ,K,\nu )$. When the arrivals have a Markovian structure, the numerical procedure is reduced to solving a Quasi–Birth–Death (for the discrete time case) or fluid queue (for the continuous time case).Finally, by combining a result of Sengupta and Ozawa, we provide a simple formula to compute the order $m$ phase-type representation of the waiting time in a MAP/MAP/1 queue without correlated service and inter-arrival times, using the $R$ matrix of a Quasi–Birth–Death Markov chain.