NUMERICAL INVESTIGATION OF HEAT TRANSFER UNDER CONFINED IMPINGING TURBULENT SLOT JETS

This work numerically in vestigates confined impinging turbulent slot jets. Eight turbulence models, including one standard and seven low-Reynolds-number k-epsilon models, are employed and tested to predict the heat transfer performance of multiple impinging jets. Validation results indicate that the prediction by each turbulence model depends on grid distribution and numerical scheme used in spatial discretization. In addition, spent fluid exits are set between impinging jets to reduce the cross-flow effect in degradation of the heat transfer of downstream impinging jets. The overall heat transfer performance can be enhanced by proper spent fluid removal.     

The Flood and Sediment Characteristics of the Lower Yellow River in China

Abstract

The Lower Yellow River is well known for its high sediment content and disastrous floods. For thousands of years, roaring floods have run through China's populated Northern Plain and caused countless damages. Understanding the basic characteristics of floods andsedimentation and their relationship is crucial to the development of a comprehensive flood protection plan for the Lower Yellow River By illustrating the erosion/deposition patterns under different flood and sediment inputs, this article explains these characteristics and discusses principles for controlling sediment distributions in this complex river system.     

Probability-Based Optimization of Inter-Sensing Duration and Power Control in Cognitive Radio

Probability-based strategies are proposed in this letter to determine the optimal inter-sensing duration and power control for cognitive radio (CR). With utilization of the statistics of licensed band occupancy, appropriate inter-sensing duration is determined to capture the recurrence of spectrum opportunity in time when the licensed signal is detected, or to achieve the maximum spectrum efficiency under a certain level of interference with licensed communication when the licensed signal is declared absent. Transmit power is varied dynamically according to the non-interfering probability at each sample so as to increase the transmission rate and decrease the interference power.     

Evaluation of current criteria in predicting the separation necessary to prevent seismic pounding between nonlinear hysteretic structural systems

This study evaluates the accuracy of four different criteria to calculate the separation necessary to prevent seismic pounding between nonlinear hysteretic structural systems. All of the criteria considered in this paper make use of the same basic equation of the Double Difference Combination rule, but they adopt different procedures to estimate the correlation between displacement responses of nonlinear hysteretic systems. The seismic excitation was modeled as a nonstationary random process, and adjacent structures were modeled as 5%-damped bilinear hysteretic SDOF systems having a 5% post-yielding stiffness ratio. Results obtained through Monte Carlo simulations indicate that none of the criteria evaluated in this study is completely satisfactory in the sense that none of them can consistently provide “exact” or “somewhat conservative” results.     

Experimental study and computer simulation of a two-stage continuous polymerization process for the production of methyl methacrylate

The two major problems encountered in industrial liquid-phase addition polymerization are: the heat released by highly exothermic reactions and the great increase in viscosity with conversion. The high rate or heat generation, coupled with the low thermal diffusivity of the reacting system, often lead to thermal runaway. Even with the process kept under marginal control, large temperature variations broaden the product molecular-weight distribution. Temperature control is particularly difficult in the Trommsdorff region, where reaction rate rapidly increases as temperature rises and viscosity builds up. A two-stage process is developed in this work to attack these problems and to achieve continuous operation of poly(methyl methacrylate) bulk polymerization. This process utilizes a continuous stirred-tank reactor (CSTR) as a first-stage prepolymerizer and a spray tower as the second-stage finishing reactor. Use of a CSTR offers good temperature control and product uniformity during the early stages of reaction and eases delivery of the reacting system to the second stage at the desired conversion and molecular-weight level. Spraying the partially polymerized mixture into the tower as fine droplets prior to the onset of gel effect eliminates the problems of transporting, agitating, and mixing a reacting system with a rapidly increasing viscosity. Heat of reaction is efficiently removed by a countercurrent stream of nitrogen in the tower, in direct contact with the falling droplets. The high surface-to-volume ratio of these small droplets facilitates heat transfer, and the problem of heat buildup can be efficiently controlled. Products from the bottom of the tower can then be melt-processed by conventional methods, such as extrusion. Experiments performed in the laboratory have demonstrated the feasibility of this proposed concept. Process optimization was in no way achieved due to serious space and equipment limitations. The process was thus further examined by computer simulation and model parameter sensitivity study. A practical design was recommended based on the model predictions.     

A VISCOSITY CONSTITUTIVE EQUATION FOR PMMA—MMA SOLUTIONS

A viscosity constitutive equation has been developed to describe PMMA—MMA solutions over a wide range of concentration and temperature. The approach used here combines both free-volume and coil-coil interaction considerations. The final expression successfully correlates not only our experimental data on several solutions but also literature melt data. This equation provides an essential step in modeling tubular polymerization reactors, where the residence time distribution depends strongly on the velocity profile development in the tube, and thus the rheological properties of the reacting PMMA — MMA mixture at different conversion levels and temperatures.     

Control of free-radical emulsion polymerization of methyl methacrylate by oxygen injection. II. Experimental study

Oxygen injection has been proposed and experimentally demonstrated as an effective control measure for limiting the rate of heat release and altering the rate of polymerization in emulsion processes. A detailed mathematical model has been previously developed to describe the system behavior with and without oxygen injection. A simple lab scale apparatus was constructed and run extensively. Only trace quantities of oxygen are needed to inhibit the reaction completely. The facile response makes this method attractive for fast temperature control. However, because of the rapid penetration of dissolved oxygen into the polymer particles, growing radical chains are terminated prematurely, lowering product molecular weights. To minimize this detrimental effect, pulsed oxygen control is used and extensive experimental work was performed to determine the effects of controller set points on molecular weight. Moderate oxygen flows and moderate set point temperatures are found to give the optimal response without significant lowering of the final molecular weight. Injected quantities agree well with the order-of-magnitude sparging calculations needed to completely stop initiation. However, complete agreement between model simulations and experimental results was not reached due to an unmeasured loss of monomer from the reactor. Chain transfer to monomer is found to be important in modeling the polymer molecular weight. It has, however, a negligible effect on the reaction rate.     

Adaptive dual‐radio spectrum‐sensing scheme in cognitive radio networks

In this paper, a novel spectrum‐sensing scheme, called adaptive dual‐radio spectrum‐sensing scheme (ADRSS), is proposed for cognitive radio networks. In ADRSS, each secondary user (SU) is equipped with a dual radio. During the data transmission, with the received signal‐to‐noise ratio of primary user (PU) signal, the SU transmitter (SUT) and the SU receiver (SUR) are selected adaptively to sense one channel by one radio while communicating with each other by the other one. The sensing results of the SUR are sent to the SUT through feedback channels (e.g., ACK). After that, with the sensing results from the SUT or the SUR, the SUT can decide whether the channel switching should be carried out. The theoretical analysis and simulation results indicate that the normalized channel efficiency, defined as the expected ratio of time duration without interference to PUs in data transmission to the whole frame length, can be improved while satisfying the interference constraint to PUs. After that, an enhanced ADRSS is designed by integrating ADRSS with cooperative spectrum sensing, and the performance of ADRSS under imperfect feedback channel is also discussed. Copyright © 2011 John Wiley & Sons, Ltd.