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[en] In this research, a series of experiments have been performed to study the thermal resistance of an oscillating heat pipe equipped with cooling tower. The effects of filling ratio and input heating power on the thermal resistance of the heat pipe and temperatures of different sections of evaporator and condenser of the heat pipe are investigated and discussed. All tests are taken for input heating power and filling ratio in the ranges of 20–200 W and 10–60%, respectively. A correlation for the thermal resistance is presented, which the effects of input heating power and filling ratio are taken into account in this correlation. The results showed that the heat pipe with filling ratio of 40% and input heating power of 160 W has the minimum value of thermal resistance among all cases considered in this research. Moreover, the thermal resistance decreases about 86% as the input heating power increases in the range of 20–120 W, while this reduction is only 23% by increasing the input heating power in the range of 160–200 W.
[en] In 1994, De Mey and De Vos (MV) proposed an analysis of an array of two endoreversible heat engines connected by a thermal bridge. Their objective was to show that the well-known Curzon-Ahlborn (CA) efficiency, , is not a universal value for endoreversible engines with a Newtonian heat transfer law. MV found that the efficiency of such an array performing at maximum power is given by . However, we show that the CA formula also is present in the MV array when it performs at maximum efficiency. In the present work we made both thermodynamic and thermoeconomic analyses of the MV model and we show an equivalent array formed by three uncoupled endoreversible engines operating simultaneously between the same thermal reservoirs. We extend all the previous analyses to the case of a chemical MV-type array. In all cases three objective functions were used: maximum power, maximum ecological function and maximum efficiency.
[en] A novel methodology to analyze non-Gaussian probability distribution functions (PDFs) of intermittent turbulent transport in global full-f gyrokinetic simulations is presented. In this work, the auto-regressive integrated moving average (ARIMA) model is applied to time series data of intermittent turbulent heat transport to separate noise and oscillatory trends, allowing for the extraction of non-Gaussian features of the PDFs. It was shown that non-Gaussian tails of the PDFs from first principles based gyrokinetic simulations agree with an analytical estimation based on a two fluid model. (paper)
[en] This paper explores an opportunity for Generation IV reactors that use molten salt as a heat transfer medium to provide and be paid for frequency-keeping, regulating reserve, spinning reserve, and automatic generator control while maximizing the total volume of electricity sold. To achieve this, a small amount of thermal energy storage is integrated with a fast response turbine generator operating at 95 - 105% of nominal capacity. Over time, the total volume of electricity sold is very close to 100% of the capacity of the facility. This facility would also be capable of operation at 110% of nominal maximum capacity for as long as thermal energy is available from storage. The costs and benefits of this opportunity are assessed in this paper. The potential uplift in revenue is in the order of CAD20M per annum for a nominal 1,000 MWe plant. The solution discussed can be adapted and applied to other Generation IV designs that have thermal storage capability at moderate cost. (author)
[en] Highlights: • 1-D finite difference-based fully transient thermal model developed for PV panels. • The model uses realistic boundary conditions and physical heat transfer mechanisms. • Model validated against PV panel’s datasheet information and field experiments. • Model compared to existing analytical, empirical and numerical thermal models. • The developed model performs better than existing models, in all the test criteria. - Abstract: A precise estimate of PV panels temperature is crucial for accurately assessing their electrical performance. Therefore, in this study, one of the main aims has been to significantly improve the prediction accuracy of the PV cell temperature, by using realistic boundary conditions. Unlike previous thermal models in the literature, which usually focus on its mere application, a detailed step by step development and numerical implementation of the complete model has also been provided in great details in this work. The developed model is transient, so it can fully simulate the thermal performance of any PV panel under time-varying field conditions. Once the model is defined for a specific PV panel, the only external inputs it needs are the total incident solar irradiation, wind speed and the ambient temperature. The model has been adequately validated through PV panel’s datasheet provided information, literature data and against a versatile set of experimental data under various weather conditions. After thorough validations, the developed model was compared to various other widely used empirical, analytical and numerical thermal models from the literature. The comparison shows that by using realistic boundary conditions, the developed thermal model has far better prediction accuracy than other models from the literature. The methodology presented in this study is completely generic. That is, though it has been implemented and validated here for a silicon-based PV module the approach may be used to model any free-standing plane PV surface, with appropriate modifications to layer thicknesses and material properties. A range of weather conditions may also be accommodated.
[en] Extended surfaces represent one of practical approaches to enhance heat transfer. Based on the laws of conductive and convective heat transfer, an increase in the area across which the object is in contact with the fluid can increase heat transfer. Due to its special structure, porous media can be seen as suitable alternatives for extended surface applications. On this basis, this research investigates the effect of connection type of sintered porous fins on heat transfer and pressure drop in the fluid flow. Connection model of four- and six-contact sintered balls of constant dimensions was evaluated by means of CFD simulation in this research. To describe the problem further, surface analysis on the reference cube is presented. The results indicate that the six-contact model has more porosity than the four-contact in reference cube by 29.45%. It was further found that the six-contact model tends to increase convective heat transfer by 33%. Results of surface analysis show that the main reasons for the difference in heat transfer between the four- and six-contact models are porosity and the angle at which balls are arranged with another.
[en] In the present work the course of temperature air change is investigated at its presence essential fluctuation. In connection with that the greatest climate change has started since 60th years, has considered the question of course temperature air change for the period since 60 to 90 years on the West Pamir. (author)
[en] Traditionally, time delay in overdamped Brownian ratchet systems reduces the rectified transport. Strikingly, in our delayed feedback ratchets, which are alternatively switched on and off in dependence of the state of the system, time delay can have significant positive-effects that the average velocity of the coupled ratchets are improved with the presence of the delayed time. Moreover, the anomalous transport can arise and then the negative mobility phenomenon appears by changing the bias force. Meanwhile, the bias force can facilitate Stokes efficiency of delayed feedback ratchets in the anomalous transport region. Remarkably, it is interesting to find that the coupled ratchets can acquire a series of resonant steps that are induced by frequency locking. More importantly, the optimal delay time can also facilitate Stokes efficiency. The theoretical results may provide a new operating technique in which micro- and nano-motor performance could be improved by the state or information of the delayed feedback coupled ratchets. (paper: classical statistical mechanics, equilibrium and non-equilibrium)
[en] In this study, the impingement of a conductive droplet on a hot wall under an electric field in the Leidenfrost regime is simulated. Apart from electrostatic equations, the governing equations are conservation equations of mass, momentum, and energy in the incompressible case. The level set method is used for interface tracking. For the appropriate application of discontinuities at the interface, the ghost fluid method is adopted. First, a sessile droplet on a superheated surface under an electric field is simulated. Simulation results are validated against the experiments. Under an electric field, an increase in the heat flux dissipated from the surface is observed for a sessile droplet. In the next step, droplet impact on a hot surface in the range of low Weber numbers in the presence of an electric field is simulated. According to the results, the droplet spreading radius and contact time increase with electric field strength. In addition, applying an electric field increases the heat transfer rate and total heat removal from the surface. If the potential difference between the droplet and the surface exceeds a specific value, the Leidenfrost state is suppressed. The threshold potential difference for Leidenfrost suppression decreases with Weber number and increases with surface superheat.
[en] This article scrutinizes the features of viscous dissipation in the stagnation point flow past through a linearly stretched Riga wall by implementing Cattaneo-Christov heat flux model. Viscous dissipation is carried out in Cattaneo-Christov diffusion analysis for the first time in this letter. As a result of Cattaneo-Christov model, some extra terms of viscous dissipation are appeared in the energy equation. These extra terms of viscous dissipation are missing in the literature. On the utilization of suitable transformations, the equations governing the problem are reduced under the boundary layer approximation into the non-linear and dimensionless ordinary differential equations. Convergent approach is utilized to solve the dimensionless governing equations. The solution thus acquired is used to highlight the effects of emerging parameters on velocity distribution and fluidʼs temperature through the graphs. Features of the drag force (or skin friction co-efficient) are graphically interpreted. It is noticed that the presence of modified Hartman number helps to reduce the fluidʼs temperature but enhances the velocity profile. Further an enlargement in the value of thermal time relaxation parameter helps to decrease the temperature distribution. (paper)