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71.
In this study, experimental and three-dimensional numerical studies were performed to investigate the effects of plate spacing and temperature difference on natural convection between isothermally heated upward-facing lower horizontal plate and externally insulated horizontal upper plate. Air is used as the heat transfer medium. Rayleigh number varied in the 1108–2.339 × 105 range. Several numerical simulations for three-dimensional steady laminar and turbulent flows heat transfer were carried out using a commercial CFD code Fluent 6.2. Results have shown that there is a good agreement between the numerical and present experimental results as well as with available data in literature. 相似文献
72.
Hadi Nozari Gizem Karaca Onur Tuncer Arif Karabeyoglu 《International Journal of Hydrogen Energy》2017,42(21):14775-14785
Due to its high hydrogen density and extensive experience base, ammonia (NH3) has been gaining special attention as a potential green energy carrier. This study focuses on premixed ammonia–hydrogen–air flames under standard temperature and pressure conditions using an inert silicon-carbide (SiC) porous block as a practical and effective medium for flame stabilization. Combustion experiments conducted using a lab scale burner resulted in stable combustion and high combustion efficiencies at very high ammonia concentration levels over a wide range of equivalence ratios. Noticeable power output densities have also been achieved. Preliminary results of NOx emission measurements indicate NOx concentrations as low as 35 ppm under rich conditions. The remarkable capability of this specific burner to operate efficiently and cleanly at high ammonia concentration levels, which can easily be achieved by partial cracking of NH3, is believed to be a key accomplishment in the development of ammonia fired power generation systems. 相似文献
73.
Abstract—This article proposes a new robust adaptive hybrid controller for three-phase pulse-width modulated rectifiers to improve control performance. This hybridization is implemented by combining the second-order sliding mode with the adaptive gain super-twisting control law in the DC bus voltage control loop and state feedback adaptive control in the d-q-axis current control loops. The control objectives are fourfold: (1) driving the DC bus voltage to a reference signal without using a priori knowledge of the plant parameters, (2) forcing the d-q-axis current errors to zero, (3) assuring a satisfactory power factor correction in relation to the AC source, and (4) eliminating the chattering effect. In the closed-loop control system, three-phase source currents and the DC bus voltage are supposed to be available for feedback. As a result, no a priori knowledge of the plant parameters is necessary in the design of the proposed controller. In addition, the proposed controller does not have the disadvantages of singularity, over-parameterization, and chattering phenomenon. Results of experimental studies prove that the proposed control system guarantees the tracking of reference signals with high performance despite all the parameter and external disturbance uncertainties. 相似文献
74.
Onur Toker 《国际强度与非线性控制杂志
》2010,20(6):623-638
》2010,20(6):623-638
In this paper, a Kharitonov‐like theorem is proved for testing robust stability independent of delay of interval quasipolynomials, p(s)+∑eqk(s), where p and qk's are interval polynomials with uncertain coefficients. It is shown that the robust stability test of the quasipolynomial basically reduces to the stability test of a set of Kharitonov‐like vertex quasipolynomials, where stability is interpreted as stability independent of delay. As discovered in (IEEE Trans. Autom. Control 2008; 53 :1219–1234), the well‐known vertex‐type robust stability result reported in (IMA J. Math. Contr. Info. 1988; 5 :117–123) (See also (IEEE Trans. Circ. Syst. 1990; 37 (7):969–972; Proc. 34th IEEE Conf. Decision Contr., New Orleans, LA, December 1995; 392–394) does contain a flaw. An alternative approach is proposed in (IEEE Trans. Autom. Control 2008; 53 :1219–1234), and both frequency sweeping and vertex type robust stability tests are developed for quasipolynomials with polytopic coefficient uncertainties. Under a specific assumption, it is shown in (IEEE Trans. Autom. Control 2008; 53 :1219–1234) that robust stability independent of delay of an interval quasipolynomial can be reduced to stability independent of delay of a set of Kharitonov‐like vertex quasipolynomials. In this paper, we show that the assumption made in (IEEE Trans. Autom. Control 2008; 53 :1219–1234) is redundant, and the Kharitonov‐like result reported in (IEEE Trans. Autom. Control 2008; 53 :1219–1234) is true without any additional assumption, and can be applied to all quasipolynomials. The key idea used in (IEEE Trans. Autom. Control 2008; 53 :1219–1234) was the equivalence of Hurwitz stability and ?‐o‐stability for interval polynomials with constant term never equal to zero. This simple observation implies that the well‐known Kharitonov theorem for Hurwitz stability can be applied for ?‐o‐stability, provided that the constant term of the interval polynomial never vanishes. However, this line of approach is based on a specific assumption, which we call the CNF‐assumption. In this paper, we follow a different approach: First, robust ?‐o‐stability problem is studied in a more general framework, including the cases where degree drop is allowed, and the constant term as well as other higher‐orders terms can vanish. Then, generalized Kharitonov‐like theorems are proved for ?‐o‐stability, and inspired by the techniques used in (IEEE Trans. Autom. Control 2008; 53 :1219–1234), it is shown that robust stability independent of delay of an interval quasipolynomial can be reduced to stability independent of delay of a set of Kharitonov‐like vertex quasipolynomials, even if the assumption adopted in (IEEE Trans. Autom. Control 2008; 53 :1219–1234) is not satisfied. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
75.
Abstract: In this study, an automatic three-dimensional computer-aided detection system for colonic polyps was developed. Computer-aided detection for computed tomography colonography aims at facilitating the detection of colonic polyps. First, the colon regions of whole computed tomography images were carefully segmented to reduce computational burden and prevent false positive detection. In this process, the colon regions were extracted by using a cellular neural network and then the regions of interest were determined. In order to improve the segmentation performance of the study, weights in the cellular neural network were calculated by three heuristic optimization techniques, namely genetic algorithm, differential evaluation and artificial immune system. Afterwards, a three-dimensional polyp template model was constructed to detect polyps on the segmented regions of interest. At the end of the template matching process, the volumes geometrically similar to the template were emhanced. 相似文献
76.
Onur Ouz Ümit Ayglü Erdal Panay&#x;rc&#x; 《Transactions on Emerging Telecommunications Technologies》2008,19(1):67-75
In this paper a new transmit diversity technique is proposed for wireless communications over frequency selective fading channels. The proposed technique utilises orthogonal frequency division multiplexing (OFDM) to transform a frequency selective fading channel into multiple flat fading subchannels on which first space–frequency coding and then additional space–time coding are applied resulting in a space–time–frequency OFDM (STF‐OFDM) scheme. This scheme provides a quasi‐orthogonal 4 × 4 transmission matrix but relax the constraint on channel fading parameters to be constant over four time or frequency slots into two time and two frequency slots. The pairwise error probability of the new scheme is evaluated and new code design criteria are obtained to improve the error performance. 64‐, 128‐ and 256‐state 4‐PSK trellis codes for STF transmission scheme are generated based on these criteria and their frame error performances in an OFDM system are evaluated by computer simulations. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
77.
Onur G Guleryuz 《IEEE transactions on image processing》2007,16(12):3020-3034
We consider the scenario where additive, independent, and identically distributed (i.i.d) noise in an image is removed using an overcomplete set of linear transforms and thresholding. Rather than the standard approach, where one obtains the denoised signal by ad hoc averaging of the denoised estimates provided by denoising with each of the transforms, we formulate the optimal combination as a conditional linear estimation problem and solve it for optimal estimates. Our approach is independent of the utilized transforms and the thresholding scheme, and as we illustrate using oracle-based denoisers, it extends established work by exploiting a separate degree of freedom that is, in general, not reachable using previous techniques. Our derivation of the optimal estimates specifically relies on the assumption that the utilized transforms provide sparse decompositions. At the same time, our work is robust as it does not require any assumptions about image statistics beyond sparsity. Unlike existing work, which tries to devise ever more sophisticated transforms and thresholding algorithms to deal with the myriad types of image singularities, our work uses basic tools to obtain very high performance on singularities by taking better advantage of the sparsity that surrounds them. With well-established transforms, we obtain results that are competitive with state-of-the-art methods. 相似文献
78.
Most of the published literature on robust design is basically concerned with a single response. However, the reality is that common industrial problems usually involve several quality characteristics, which are often correlated. Traditional approaches to multidimensional quality do not offer much information on how much better or worse a process is when finding optimal settings. Köksoy and Fan [Engineering Optimization 44 (8): 935–945] pointed out that the upside-down normal loss function provides a more reasonable risk assessment to the losses of being off-target in product engineering research. However, they only consider the single-response case. This article generalizes their idea to more than one response under possible correlations and co-movement effects of responses on the process loss. The response surface methodology has been adapted, estimating the expected multivariate upside-down normal loss function of a multidimensional system to find the optimal control factor settings of a given problem. The procedure and its merits are illustrated through an example. 相似文献
79.
Andre L. Magdaleno;Gabriel A. Cerrón-Calle;Alexsandro J. dos Santos;Marcos R. V. Lanza;Onur G. Apul;Sergi Garcia-Segura; 《Small (Weinheim an der Bergstrasse, Germany)》2024,20(3):2304547
The electrogeneration of hydrogen peroxide (H2O2) via the oxygen reduction reaction is a crucial process for advanced water treatment technologies. While significant effort is being devoted to developing highly reactive materials, gas provision systems used in these processes are receiving less attention. Here, using oxygen nanobubbles to improve the gas efficiency of the electrogeneration of H2O2 is proposed. Aeration with nanobubbles is compared to aeration with macrobubbles under an identical experimental set-up, with nanobubbles showing a much higher gas–liquid volumetric mass transfer coefficient (KLa) of 2.6 × 10−2 min−1 compared to 2.7 × 10−4 min−1 for macrobubbles. Consequently, nanobubbles exhibit a much higher gas efficiency using 60% of O2 delivered to the system compared to 0.19% for macrobubbles. Further, it is observed that the electrogeneration of H2O2 using carbon felt electrodes is enhanced using nanobubbles. Under the same dissolved oxygen levels, nanobubbles boost the reaction yield to 84%, while macrobubbles yield only 53.8%. To the authors’ knowledge, this is the first study to investigate the use of nanobubbles in electrochemical reactions and demonstrate their ability to enhance gas efficiency and electrocatalytic response. These findings have important implications for developing more efficient chemical and electrochemical processes operating under gas-starving systems. 相似文献
80.