Studies of the shellside performance of shell-and-tube heat exchangers

Accurate prediction of shellside pressure drop in a baffled shell-and-tube heat exchanger is very difficult because of the complicated shellside geometry. Ideally, all the shellside fluid should be alternately deflected across the tube bundle as it traverses from inlet to outlet. In practice, up to 60% of the shellside fluid may bypass the tube bundle or leak through the baffles. This short-circuiting of the main flow reduces the efficiency of the exchanger. Of the various shellside methods, it is shown that only the multi-stream methods, which attempt to obtain the shellside flow distribution, predict the pressure drop with any degree of accuracy, the various predictions ranging from -30% to +70%, generally overpredicting. It is shown that the inaccuracies are mainly due to the manner in which baffle leakage is modelled. The present multi-stream methods do not allow for interactions of the various flowstreams, and yet it is shown that three main effects are identified, a) there is a strong interaction between the main cross flow and the baffle leakage streams, enhancing the crossflow pressure drop, b) there is a further short-circuit not considered previously i.e. leakage in the window, and c) the crossflow does not penetrate as far, on average, as previously supposed. Models are developed for each of these three effects, along with a new windowflow pressure drop model, and it is shown that the effect of baffle leakage in the window is the most significant. These models developed to allow for various interactions, lead to an improved multi-stream method, named the "STREAM-INTERACTION" method. The overall method is shown to be consistently more accurate than previous methods, with virtually all the available shellside data being predicted to within ±30% and over 60% being within ±20%. The method is, thus, strongly recommended for use as a design method

Neutrino fluxes from CNO cycle in the Sun in the non stationary case with mixing

The computational analyses is presented of the non stationary case with mixing of the solar model when the neutrino flux $F_{13}$ from the decay of $^{13}N$ is higher than a standard solar model predictsComment: 6 pages, 3figure

Optical fibre digital pulse-position-modulation assuming a Gaussian received pulse shape

The abundance in bandwidth available in the best monomode fibres may be exchanged for improved receiver sensitivity by employing digital PPM. The paper presents a performance and optimisation analysis for a digital PPM coding scheme operating over a fibre channel employing a PIN-BJT receiver and assuming a Gaussian received pulse shape. The authors present original results for a 50 Mbit/s, 1.3 μm wavelength digital PPM system and conclude that, provided the fibre bandwidth is several times that of the data rate, digital PPM can outperform commercially available PIN-BJT binary PCM system

Performance of Di-code Pulse Position Modulation Technique in Diffuse Indoor Wireless Optical Communication Systems

The bandwidth of the indoor optical wireless Pulse Position Modulation (PPM) system can be exploited to improve the receiver sensitivity by using Di-code Pulse Position Modulation (Di-PPM) technique. This paper analysis the receiver sensitivity of indoor optical wireless Di-PPM systems, over a slightly dispersive channel, uses a bandwidth-limited PIN-bipolar (PINBJT) optical receiver, zero guard interval and pre-detection filter based on a simple third-order Butterworth filter, that authorizes the receiver to operate over channels with bandwidth as low as 1.2 times the bit rate. The received pulse shape is a convolution between the impulse response of a diffuse optical wireless link in a ceiling bounce model and rectangular waveform, the operating bit-rate chosen is 100Mbit/s. In this paper the performance analysis is extended in order to include the effects of intersymbol interference (ISI) on error probability and some important errors: wrong slot, erasure and false alarm. A mathematical model is then presented showing how the transfer function of the indoor optical wireless Di-PPM receiver can be calculated. Results are obtained and the pulse shape at input of receiver decision device is illustrated by using MathCAD software

WLAN 802.11e evaluation performance using OPNET

The low cost and easy deployment of Wireless LAN 802.11 standard means it becomes more and more popular, but it has a vital drawback with regard to Quality of Service (QoS). QoS defines the ability of network to introduce consistent services for data transmission, and is evaluated in terms of specific parameters such as jitter, delay, and packet loss. These parameters describe data traffic quality over a network. Service differentiation should be offered to let higher priority multimedia traffic to get a preferred treatment. This deficiency of Wireless LAN 802.11 MAC mechanisms in offering QoS support is a major obstacle in the adaptation of modern multimedia applications in Wireless LAN 802.11 networks. This paper aims to build different scenarios to evaluate QoS characteristics and to examine the effect of enhancement on the QoS. The evaluation, implemented using the OPNET simulator, will contain the different parameters of Wireless LAN 802.11e to see how this enhancement in distributed channel access increases the performance over the Wireless LAN 802.11 standard. The results give a clear picture that the enhanced standard offers a very effective service mechanism to provide QoS support

Climate change and cattle farming

Issues raised by cattle farming in relation to climate change extend beyond discussion of greenhouse gas emissions and global warming. There are profound consequences for water availability, soil degradation, biodiversity and local ecology, as well as in terms of conflict for energy supplies. Although climate change impacts on cattle farming (through effects on water availability, heat stress and flooding, for example), this article focuses on how cattle farming impacts on climate change. It explores the issues in terms of the impact of cattle farming on the environment, and how to measure and reduce climate change impacts at farm level. Managing the complex and conflicting balance of factors required for sustainable food production offers an important role for the veterinary surgeon

Practical implementation of duobinary pulse position modulation using FPGA and visible light communication

Low bandwidth expansion modulation schemes are preferred for free space and optical fibre data transmission, where limited bandwidth is available. One such scheme is duobinary pulse position modulation (DuoPPM), which is the subject of this paper. DuoPPM scheme is not as sensitive to bandwidth expansion issues as digital PPM, with a line rate of twice the data rate. This paper discusses first time practical implementation of DuoPPM coding scheme and its application in free space using visible light LED (30 W) for transmission purposes. Experimental results achieved at the data rate of 14 Mbit/s indicate an error rate that is better than 1 error in 109.The main aim is to analyse the practicality, robustness and limitations of DuoPPM

Utility Independence of Multiattribute Utility Theory is Equivalent to Standard Sequence Invariance of Conjoint Measurement

Utility independence is a central condition in multiattribute utility theory, where attributes of outcomes are aggregated in the context of risk. The aggregation of attributes in the absence of risk is studied in conjoint measurement. In conjoint measurement, standard sequences have been widely used to empirically measure and test utility functions, and to theoretically analyze them. This paper shows that utility independence and standard sequences are closely related: utility independence is equivalent to a standard sequence invariance condition when applied to risk. This simple relation between two widely used conditions in adjacent fields of research is surprising and useful. It facilitates the testing of utility independence because standard sequences are flexible and can avoid cancelation biases that affect direct tests of utility independence. Extensions of our results to nonexpected utility models can now be provided easily. We discuss applications to the measurement of quality-adjusted life-years (QALY) in the health domain