Ice-slurry production using direct contact heat transfer

An ice slurry generation system was developed using direct contact heat transfer between water and the coolant, Fluroinert FC-84. The location of the coolant nozzle is an important design consideration to avoid clogging due to freezing of water. An ice fraction of up to about 40 percent was obtained with the nozzle located at the bottom of the ice slurry tank and the jet directed upwards into the water. Two simplified model were developed to extract the heat transfer coefficient between the coolant drops and the water. The first model requires as input the average drop diameter and the residence time while the second model uses the measured drop diameter distribution. The estimated heat transfer coefficients are much smaller than those computed using single-sphere correlations.     

Constructal multi-scale structure for maximal heat transfer density

Summary.  This paper presents a new concept for generating the multi-scale structure of a finite-size flow system that has maximum heat transfer density–maximum heat transfer rate installed in a fixed volume. Laminar forced convection and parallel isothermal blades fill the volume. The spacings between adjacent blades of progressively smaller scales are optimized based on constructal theory: the goal is maximum heat transfer density. The smaller blades are installed in the fresh-fluid regions that sandwich the tips of the boundary layers of longer blades. The overall pressure difference is constrained. As the number of length scales increases, the flow rate decreases and the volume averaged heat transfer density increases. There exists a smallest (cutoff) length scale below which heat transfer surfaces are no longer lined by distinct (slender) boundary layers. Multi-scale flow structures for maximum heat transfer rate density can be developed in an analogous fashion for natural convection. The constructal multi-scale algorithms are deduced from principles, unlike in fractal geometry where algorithms are assumed. Received January 3, 2003 Published online: June 12, 2003     

Modeling of piezoelectric transformers using finite‐element technique

Abstract

In this work, the modeling of piezoelectric transformers using the finite‐element technique is presented. A 3‐D finite element method solver, which employs 20‐node brick‐element formulation, is developed. Using the solver, the characteristics of piezoelectric transformers under different operating frequencies can be simulated. Also, the solver is capable of accounting for the effects of the electrical loadings attached to the output electrodes of piezoelectric transformers. The modeling results for two different types of piezoelectric transformers, the Rosen‐modal‐type and the unipoled‐disk‐type, are presented. For the Rosen‐modal‐type devices, the simulated voltage gains and the phase differences are validated with our measured results. Also, the simulated results of the unipoled‐disk‐type transformers agree with the measured results found in previously published literature. The effects of electrical loadings on these piezoelectric transformers are also discussed.     

Finite element simulation of piezoelectric transformers

Piezoelectric transformers are nothing but ultrasonic resonators with two pairs of electrodes provided on the surface of a piezoelectric substrate in which electrical energy is carried in the mechanical form. The input and output electrodes are arranged to provide the impedance transformation, which results in the voltage transformation. As they are operated at a resonance, the electrical equivalent circuit approach has traditionally been developed in a rather empirical way and has been used for analysis and design. The present paper deals with the analysis of the piezoelectric transformers based on the three-dimensional finite element modelling. The PIEZO3D code that we have developed is modified to include the external loading conditions. The finite element approach is now available for a wide variety of the electrical boundary conditions. The equivalent circuit of lumped parameters can also be derived from the finite element method (FEM) solution if required. The simulation of the present transformers is made for the low intensity operation and compared with the experimental results. Demonstration is made for basic Rosen-type transformers in which the longitudinal mode of a plate plays an important role; in which the equivalent circuit of lumped constants has been used. However, there are many modes of vibration associated with the plate, the effect of which cannot always be ignored. In the experiment, the double resonances are sometimes observed in the vicinity of the operating frequency. The simulation demonstrates that this is due to the coupling of the longitudinal mode with the flexural mode. Thus, the simulation provides an invaluable guideline to the transformer design     

Analysis of rosen piezoelectric transformers with a varying cross-section

We study the effects of a varying cross-section on the performance of Rosen piezoelectric transformers operating with length extensional modes of rods. A theoretical analysis is performed using an extended version of a one-dimensional model developed in a previous paper. Numerical results based on the theoretical analysis are presented.     

Features of nonstationary periodic contact heat transfer

Results are presented of investigations of nonstationary periodic contact heat transfer performed on special experimental installations and under full-scale tests. The regularity of thermal hysteresis is here disclosed experimentally.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 60, No. 1, pp. 151–161, January, 1991.     

Dependence of unsteady heat transfer on heat flux density

With the aid of an interferometer, the dependence of heat transfer coefficient in unsteady heat transfer on the rate of change of thermal stress at the heat transfer surface has been determined.     

压电陶瓷变压器研究和发展现状

概述了压电陶瓷变压器的原理和分类，介绍了各种振动模式的压电陶瓷变压器，并重点叙述了两种电极设计精巧的压电陶瓷变压器，对体型和膜型压电陶瓷变压器的研究进展现状进行了分析。     

Contact heat transfer and external mass transfer in contact drying

The heat transfer, compounded by a phase transformation, between the heater surface and the adjoining dried material is considered here as well as the mass transfer between the free surface of this material and surrounding medium (air or cloth) during the first stage and the second stage of the drying process.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 19, No. 3, pp. 482–490, September, 1970.     

Investigation of contact heat transfer at floor surfaces

A solution is given for the problem of contact heat transfer at the surface of a two-layer floor in the nonstationary regime.     

Heat transfer enhancement in distribution transformers using TiO2 nanoparticles

A transformer is an electrical device that transfers electrical energy between windings by electromagnetic induction while producing a considerable amount of heat in circuits. The heat produced in windings is removed by an appropriate heat transfer fluid such as liquid dielectrics. The cooling and insulating of a liquid dielectric depend on the properties of the oil filling the transformer. One of the approaches to enhance the thermal and dielectric properties of transformer oil is employing an appropriate nanoparticle in a transformer.In this paper, a three-phase distribution transformer is simulated three-dimensionally in order to study the heat transfer efficiency for pure oil (single phase) and nanofluid (TiO₂ nanoparticles- transformer oil). For both models, the electromagnetic field in solid sections and heat transfer in fluid and solid sections of the transformer are simultaneously investigated. The simulation results show that the presence of TiO₂ nanoparticles in the transformer oil increases the heat transfer coefficient, i.e. adding 1% (vol/vol) of TiO₂ nanoparticles to the transformer oil increases the Nusselt number from 2.17 to 2.49, while the maximum temperature of transformer components decreases from 47.20?°C to 43.05?°C.     

Influence of nonideal plate contact on heat transfer in compact heat exchangers

Conclusion The experimental data obtained here and the corresponding analysis indicate the need to take account of thermal resistance in the contact zone of plates with the cooling system of the heat exchanger. This applies, above all, to heat exchangers of highly conducting material, in which the heating is from the unribbed plate, or multilayer heat exchangers. In these cases, stricter requirements must be imposed to ensure high-quality thermal contact of the plates with the cooling system.Translated from Inzhenerno-fizicheskii Zhurnal, Vol. 60, No. 5, pp. 776–782, May, 1991.     

The transfer of heat between moving bodies in contact

We present a solution for the problem of the distribution of temperature and that of the intensity of heat flows in bodies that are in contact in the case of steady-state heat transfer and great velocities of motion.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 16, No. 3, pp. 484–488, March, 1969.     

Solution of the inverse contact problem of heat transfer in a rectangular plate

An approximate method of solving an inverse nonlinear contact heat-transfer problem in a rectangular plate by local one-dimensional difference schemes is examined.     

Calculation of the temperatures and the heat balance during contact heat transfer in a system of bodies

A method is proposed for calculating the mean-integral contact temperatures and the heat balance during complex heat transfer between four bodies in contact.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 21, No. 6, pp. 1068–1073, December, 1971.     

Freezing due to direct contact heat transfer including sublimation

From the energy saving viewpoint, the utilization of LNG cold is very important in the refrigeration industry concerning the low temperature region. In this paper, as a basic study of the freezing due to direct contact, including evaporation, solid–liquid direct contact heat transfer associated with sublimation has been investigated using the dry ice in water experimentally and theoretically. Based on a two-layers-model composed of CO₂ vapor and the bulk water around the dry ice, the velocity and temperature fields in two layers was calculated numerically and the calculated results for the freezing condition of the bulk water were compared with the experimental results.     

Multimodal electromechanical model of piezoelectric transformers by Hamilton's principle

This work deals with a general energetic approach to establish an accurate electromechanical model of a piezoelectric transformer (PT). Hamilton's principle is used to obtain the equations of motion for free vibrations. The modal characteristics (mass, stiffness, primary and secondary electromechanical conversion factors) are also deduced. Then, to illustrate this general electromechanical method, the variational principle is applied to both homogeneous and nonhomogeneous Rosen-type PT models. A comparison of modal parameters, mechanical displacements, and electrical potentials are presented for both models. Finally, the validity of the electrodynamical model of nonhomogeneous Rosen-type PT is confirmed by a numerical comparison based on a finite elements method and an experimental identification.     

复合材料夹芯结构非线性热传导分析

针对复合材料夹芯结构温度场分布的特点, 提出一种用于复合材料夹芯结构热传导精细分析的有限单元模型。这种单元模型为三维六面体模型, 单元模型厚度方向的插值函数在芯层和面板交界节点处温度值是连续的, 但温度变化率是不连续的, 而芯层内部节点处沿厚度方向温度值和温度变化率都是连续的。在考虑材料热传导参数随温度变化的情况下, 基于这种有限单元模型建立的复合材料夹芯结构瞬态温度场分析有限元方程为非线性方程。在求解此瞬态热传导非线性有限元方程时, 改进了常用的动力学平衡方程的解法, 改进后的动力学平衡方程解法避免了迭代运算, 提高了求解非线性动态平衡方程的效率。数值算例结果显示了该分析模型的有效性和可靠性。      

Direct contact heat transfer during evaporation of immiscible liquid mixtures

A study of the heat transfer between a boiling refrigerant in direct contact with hot water was made, the two fluids being immiscible. The refrigerant and water were mixed in an injector, boiling taking place in a vertical tube. The location of the point where boiling terminated was determined at various system conditions. Volumetric heat transfer coefficients in the system can be up to 40 times higher than those for conventional systems.In order to illustrate system phenomena, a simple model was developed taking into account the heat transfer around a small droplet of refrigerant. The theoretical results agree with experimental values.     

Mathematical modeling of moving contact lines in heat transfer applications

We provide an overview of research on the mathematical modeling of apparent contact lines in non-isothermal systems conducted over the past several decades and report a number of recent developments in the field. The latter involve developing mathematical models of evaporating liquid droplets that account not only for liquid flow and evaporation, but also for unsteady heat conduction in the substrate. The droplet is placed on a flat heated solid substrate and is assumed to be in contact with a saturated vapor. Furthermore, we discuss a careful comparison between mathematical models and experimental work that involves simultaneous measurement of shapes of evaporating droplets and temperature profiles in the solid substrate. The latter is accomplished using thermochromic liquid crystals. Applications to new research areas, such as studies of the effect of evaporation on fingering instabilities in gravity-driven liquid films, are also discussed.