Results 1 - 10 of 12105
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[en] This paper introduces two methods for solving two bottlelike problems regarding the shape memory alloy (SMA) application as actuators. These methods are ‘rotating output,’ which aims to solve the problem of the low working frequency caused by the demand for cool time, and ‘accumulated shifting,’ which solves the problem of difficult-to-obtain output displacements in a large scale. We also introduce a hybrid linear actuator that applies the two methods and achieves both a strong force and an accurate large output displacement while working at a high frequency based on the SMA wires and DC motors. A prototype of this actuator was fabricated and tested to verify the two methods. This hybrid actuator system dynamic model, which was composed of the constitutive model of the SMA, the electrical and heat transfer behavior of the SMA wires and the dynamics of the linear actuation system, was established and discussed. Our study aims to illuminate the application of an SMA in actuators with the proposed methods with regard to its two main problems. An actuator with a high power-weight ratio and the capability to work at a high frequency, as well as accurate linear step displacements in a large scale, is also presented. (paper)
[en] This article solves the two dimensional inverse problem of estimating the unknown heat flux at a pin fin base by the conjugate gradient method. In the estimating processes, no prior information on the functional form of the unknown quantity is required. The accuracy of the inverse analysis is examined by simulated exact and inexact measurements of temperature at interior locations of the pin fin. The numerical results show that good estimations on the heat flux can be obtained for all the test cases considered in this study. Furthermore such a technique can be applied to determine the heat flux acting on an internal surface, where a direct measurement is not feasible
[en] Several low-dimensional systems show a crossover from diffusive to ballistic heat transport when system size is decreased. Although there is some phenomenological understanding of this crossover phenomenon at the coarse-grained level, a microscopic picture that consistently describes both the ballistic and the diffusive transport regimes has been lacking. In this work we derive a scaling form for the thermal current in a class of one dimensional systems attached to heat baths at boundaries and rigorously show that the crossover occurs when the characteristic length scale of the system competes with the system size. (paper)
[en] In this paper, a new I-D/2-D model is proposed for closed-loop thermosyphons with vertical heat transfer sections. This model improves the results of traditional 1-D models for cases where: (i) mixed-convection effects are important in the heated and cooled sections of the loop; and (ii) heat losses (or gains) from the insulated portions of the loop are significant. This is achieved by iteratively coupling local results of 2-D numerical simulations of mixed-convection flows, performed in the heated and cooled sections, and a 1-D analysis. The proposed 1-D/2-D model is validated by comparing its results with those of a complementary experimental study. The results include predictions and measurements of the average velocity in the loop, local wall temperatures in the heated section of the loop, and bulk temperatures of the fluid. The agreement between the model predictions and the experimental results is shown to be very good. (author)
[en] A detailed study of the three models commonly used to characterize cell radiosensitivity (multitarget, multihit and linear quadratic) was carried out, starting from the derivative of the inactivation probability function with respect to the dose, interpreted as a probability density. It is shown that distribution parameters such as mean inactivation dose, variance etc.. can easily be computed from the usual parameters D0 and n, λ and k, α and β. The behaviour and limits of each model are discussed and shape and scale parameters are singled out to improve the analysis of distribution characteristics. Data reported in the literature, relative to C3H10T1/2 cells irradiated with low LET radiation, were analysed and compared using the approach described. (author)
[en] The on-line performance monitoring of a shell-and-tube type heat exchanger using steam and water, which is one of the key components of distributed energy supply systems, has been developed on the basis of a theoretical model. In this on-line performance monitoring, output process variables and performance of the heat exchanger in response to the variation in measured input process variables are first estimated by using its steady-state model. These estimated values are then compared with the measured ones. Its effectiveness has been confirmed by the demonstration experiments conducted not only in the steady and transient states but also under the condition of air accumulation in the shell as one of the common failures of this type of heat exchanger. The results confirm the sufficient accuracy of estimating the output process variables and heat exchange performance in a wide operating range, and the capability of detecting the deterioration of the heat exchange performance due to the air accumulation in a short period
[en] Highlights: • Optimal controlled variable of counter flow heat exchanger is constructed. • T-Q diagram analysis inducing entropy angle and thermal capacity angle is proposed. • Effect of irreversibility on terminal temperature difference is identified. • Relation between effectiveness, transfer and remanent irreversibility is revealed. - Abstract: To optimize the controlled variable of counter flow heat exchanger, T-Q diagram inducing entropy angle and thermal capacity angle is used to analyze heat exchange process. The results show that selecting stream outlet temperature as controlled variable is incapable of perceiving overall variation of thermal capacity flow rates. The change of heat exchanger effectiveness isn’t completely consistent with heat transfer irreversibility, and cannot reflect the effect of remanent (flow-imbalance) irreversibility. The terminal temperature difference imposed by heat transfer irreversibility is the same at both ends. However, the remanent irreversibility makes the terminal temperature difference of one end deviate from the other. Based on maximizing the heat exchange amount and minimizing the irreversible loss, a new controlled variable named as heat exchanger comprehensive effectiveness is constructed, which is easy to be measured and calculated. It can reflect the effect of heat exchanger effectiveness, remanent irreversibility, and heat transfer irreversibility simultaneously.
[en] Improved methods of performing steady one and two dimensional boiler thermal analyses have been developed. These new methods are applicable to computer analyses resulting in shorter execution times. In the case two dimensional analyses, these two methods also result in reduced program storage requirements. (author)
[en] This letter explains briefly why so-called thermal waves are not truly waves—despite being described mathematically as such—because of their intrinsic diffusive character. This topic should be of interest to those students and teachers dealing with heat transport in the presence of non-stationary harmonic heat sources, photothermal phenomena and techniques, and other related subjects. (letters and comments)
[en] Highlights: • Three parameters are used to evaluate the startup performance. • The startup is faster and overshoot is larger when the distribution is more uneven. • Heating on one evaporator with same Q with the other makes transition time longer. • Heating on one evaporator with same Q with the other makes the overshoot smaller. • Transition time is about twice as much as peak time when peak time exists. - Abstract: Loop thermosyphon with multiple evaporators is a promising device in multi-source heat transfer. The startup performance is very important for its thermal control ability. In this paper, the effect of heating power distribution on the startup of a loop thermosyphon with dual evaporators is investigated experimentally. The startup time and stationarity under different power distributions are analyzed utilizing three parameters: peak time, transition time and temperature (pressure) overshoot. The results show that the startup process is faster and the overshoot of pressure and temperature is larger when the distribution is more uneven; Heating on one evaporator with the same heating power with the other evaporator makes the startup process longer while it makes the overshoot smaller or even disappear; The transition time is approximately twice as much as the peak time when the peak time exists.