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Tsinghua Science and Technology

Issue 2 • Date April 2002

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Displaying Results 1 - 22 of 22
  • [Front cover]

    Publication Year: 2002 , Page(s): c1
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    Freely Available from IEEE
  • Contents

    Publication Year: 2002 , Page(s): 1
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    Freely Available from IEEE
  • Nonlinear bubbling and micro-convection at a submerged orifice

    Publication Year: 2002 , Page(s): 97 - 108
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (4349 KB)  

    The present paper describes the nonlinear behavior of bubble formation from a single submerged orifice and induced liquid motion (micro-convection) surrounding the bubble. The experimental data reveals that departing periods of successive bubbles evolve multiple periods from single to triple periods when the gas flow rate is increased and that the micro-convection evolves bifurcation phenomena similar to the so-called “period doubling” in chaos dynamics. The photographic observation using high-speed video movies and data analysis indicate that the nonlinear features come from the deformation of the bubble and also the interaction between consecutive bubbles. A new comprehensive theoretical model is developed for describing the instantaneous bubble behaviors during formation and ascendance processes and for predicting the departing periods and sizes of successive bubbles for constant flow rate conditions. Owing to the estimation of instantaneous interactions between successive bubbles and the incorporation of the wake effect of previous bubbles, the present model describes the evolution process and mechanisms of bubble departing periods corresponding to different gas flow rate regimes. The theoretical results are in good agreement with experimental results. View full abstract»

    Open Access
  • What is going on in a critical thermosyphon?

    Publication Year: 2002 , Page(s): 109 - 115
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    Critically filled thermosyphons for heat transport differ in size and temperature distribution from critically filled testcells for property research. Consequently, the thermodynamic states within a thermosyphon are distributed more heterogeneously. In a glass thermosyphon, flows and phase distributions were observed. A break-down of the heat transport mechanism was found as a consequence of flooding the cooling region with liquid. Observed and explained — by use of a density/pressure diagram — were the effects of heating, of pressure and of fluid quantity on the distribution of the phases, the appearance of pulsations and the propagation of a heat pulse near the critical state. View full abstract»

    Open Access
  • Recent developments in cooling using blowing

    Publication Year: 2002 , Page(s): 116 - 134
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    Influence of the blowing through a porous flat plate and a circular cylinder has been studied. Experimental and numerical results agree and show a thickening of the boundary layers, both dynamic and thermal. The thermal protection effectiveness has been quantified for the two geometries, and comparisons between discrete injection and blowing, in the case of the flat plate, were carried out, showing a stronger impact of the latter. Finally, the blowing influence on the primary instability properties results in a decrease of the vortex shedding frequency and a modification of the shear layer. View full abstract»

    Open Access
  • Transient behaviour of a heat pipe with extracapillary circulation

    Publication Year: 2002 , Page(s): 135 - 145
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    Heat pipe devices, for their typical working mode, are particularly suitable for zero gravity applications, and have also been considered for applications in space satellites with nuclear generators because of the absence of active systems for the coolant circulation. The present work reports the results of experimental tests carried out on a heat pipe facility designed to investigate the thermal-hydraulic performance of a water heat pipe. The device layout, configuration and geometry, simulate a heat pipe working mode utilizable in space applications under zero gravity conditions. The evaporating section, completely lined (covered) with wicks (sintered stainless steel), and nearly plane shaped, is housed in a cylindrical container. The obtained results show that the system can approach steady-state conditions, at a pressure of 4 bar and with a heat flux transferred of about 150 W /cm2, supporting an electric power step of about 1.8 kW. View full abstract»

    Open Access
  • Heat transfer characteristics of wavy-wall channels in micropolar fluids

    Publication Year: 2002 , Page(s): 146 - 154
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    Forced convection flow through a sinusoidally curved converging-diverging channel in micropolar fluids has been investigated numerically. A simple coordinate transformation is employed to transform the complex wavy-wall channel to a parallel-plate channel, and the cubic spline alternating-direction implicit method is then used to solve the flow patterns and heat transfer characteristics. The effects of the wavy geometry, vortex viscosity parameter and Reynolds number on skin-friction coefficient and Nusselt number have been examined in detail. Results show that the flow through a sinusoidally curved converging-diverging channel forms a strong forward flow and a reticular vortex within each wave for larger Reynolds number and wavy amplitudes. The heat transfer rate of a micropolar fluid is smaller than that of a Newtonian fluid, but the skin friction of a micropolar fluid is larger than that of a Newtonian fluid. Moreover, both Reynolds number and wavy amplitude tend to enhance the total heat transfer rate, irrespective of whether the fluids are Newtonian fluids or micropolar fluids. View full abstract»

    Open Access
  • Using boiling for treating waste activated sludge

    Publication Year: 2002 , Page(s): 155 - 159
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    In this work we investigated the feasibility of using short time, low superheat boiling to treat biological sludge. The treated sludge exhibited reduced filterability and enhanced settleability. The boiling treatment released a large amount of extra-cellular polymers (ECPs) from the solid phase and reduced the microbial density levels of the total coliform bacteria and the heterotrophic bacteria. A diluted sludge is preferable for its high degree of organic hydrolysis and sufficient reduction in microbial density levels. View full abstract»

    Open Access
  • Bubble interaction in thermal boundary layers

    Publication Year: 2002 , Page(s): 160 - 164
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (332 KB)  

    The boiling process is characterized by strong nonlinear interactions between bubbles in the thermal boundary layer. The present paper uses a heat balance approach to derive the interaction length between bubbles based on boiling heat transfer mechanisms and bubble dynamics. The effects of some important parameters on the interaction length are then analyzed. Interestingly, the whole range of fluxes from low heat flux nucleate boiling to high heat flux boiling, departure from nucleate boiling (DNB), critical heat flux (CHF), and transition boiling, can be well understood by the concept of interaction length. The present paper also uses the concept of interaction length to present a unified description of ONB and CHF phenomena through a better understanding of nucleate boiling. View full abstract»

    Open Access
  • Cryogenic thermophysical studies for clinical medicine

    Publication Year: 2002 , Page(s): 165 - 170
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (407 KB)  

    Cryogenic technology has been widely used in clinical medicine and in pharmaceutics, so thermophysical studies are extremely important to solve problems during freezing and thawing. This paper reports some recent research in clinical medicine, including cryo-injury, cryosurgery, and cryopreservation of some important cells and tissues. Microscopic images of the freezing process with a cryomicroscope system show that the dendritic ice growth is affected by the solution concentration, the cooling rate, and the number of embryos. An enthalpy method is used for the freeze-thaw analysis of the cryosurgery with a program developed to predict the temperature profile and the interface motion, which compares well with experimental results. A very rapid cooling technique is developed by quenching the samples into subcooled liquid nitrogen for vitrification of cells and tissues. An analytical method developed to prevent the fracture of arteries during freezing has been verified by the electronic microscopic investigation. View full abstract»

    Open Access
  • Similarity solution for convection heat transfer due to Marangoni flow over a flat surface

    Publication Year: 2002 , Page(s): 171 - 176
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    Marangoni convection occurs along any liquid-vapor interface that has a surface tension gradient. The surface tension gradient can result from either temperature or concentration gradients along the surface. Marangoni convection is of importance in crystal growth melts and during boiling as it influences the flow around the vapor bubbles. The influence of Marangoni induced convection is more obvious under microgravity but also occurs in earth gravity. This paper presents a similarity solution for Marangoni induced flow for both the velocity and temperature profiles, assuming developing boundary-layer flow along a surface with various imposed temperature profiles. The surface velocity, the total flow rate, and the heat transfer characteristics are given for various temperature profiles and various Prandtl numbers. Since the predicted boundary layer thickness would be much less than the diameter of vapor bubbles during nucleate boiling, the bubble surface curvature effects can be neglected and this analysis can also be used as a first estimate of the effect of Marangoni flow around a vapor bubble. View full abstract»

    Open Access
  • Heat transfer in flue gas with vapor condensation

    Publication Year: 2002 , Page(s): 177 - 181
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    This paper combines the film model with Nusselt's condensation theory to analyze the effects of water vapor condensation on the heat transfer performance of flue gas flowing through a vertical tube. The analysis compares the condensation and convective heat transfer rates. For the concentration range investigated, the water vapor condensation transfers more energy than the flue gas convection, but the convective heat transfer can not be neglected. The heat transfer intensification due to the condensation increased as the water vapor fraction increased. The theoretical results compared well with experimental data. View full abstract»

    Open Access
  • Shear force distribution and heat transfer in laminar boundary layer flows for power law fluid

    Publication Year: 2002 , Page(s): 182 - 186
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    Analytical and numerical solutions are presented for the momentum and energy laminar boundary layer equations in power law fluids utilizing a similarity transformation and the shooting technique. The results indicated that for power law exponents 0 < n ≤ 1, the skin friction σ decreases with increasing n, and the dimensionless shear force decreases with increasing dimensionless velocity t. When Pr = 1, the velocity distribution in the viscous boundary layer is the same as the temperature distribution in the thermal boundary layer and δ = δT. For Pr > 1, the increase of the viscous diffusion exceeds that of thermal diffusion with increasing Pr, i.e., δT(t) < δ(t). The thermal diffusion ratio increases with increasing n(O < n ≤ 1). View full abstract»

    Open Access
  • Explicit analytical solutions of coupled fluid flow transfer equation in heterogeneous porous media

    Publication Year: 2002 , Page(s): 187 - 189
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    Explicit analytical solutions are presented for the coupled fluid flow transfer equation in heterogeneous porous media. These analytical solutions are useful for their description of actual flow fields and as benchmark solutions to check the rapidly developing numerical calculations and to study various computational methods such as the discrete approximations of the governing equations and grid generation methods. In addition, some novel mathematical methods are used in the analyses. View full abstract»

    Open Access
  • Exergy analysis of methanol-IGCC Polygeneration technology based on coal gasification

    Publication Year: 2002 , Page(s): 190 - 193
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (319 KB)  

    Polygeneration is a key strategy for making ultra-clean energy products highly competitive with conventional energy systems. A polygeneration system based on coal gasification was analyzed using the exergy method to calculate the system thermal efficiency. The results show that the polygeneration system has less pollutants and higher efficiency than the separate systems. View full abstract»

    Open Access
  • Discontinuous operation of geothermal heat exchangers

    Publication Year: 2002 , Page(s): 194 - 197
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    Ground-source heat pump (GSHP) systems for HV AC have aroused more and more interest in China in recent years because of their higher energy efficiency compared with conventional systems. The design and performance simulation of the geothermal heat exchangers is vital to the success of this technology. In GSHP systems, the load of the geothermal heat exchanger varies greatly and is usually discontinuous even during a heating or cooling season. This paper outlines a heat transfer model for geothermal heat exchangers. The model was used to study the influence of the discontinuous operation of the heat pumps on the performance of the geothermal heat exchangers. A simple and practical approach is presented for sizing the geothermal heat exchangers. View full abstract»

    Open Access
  • Experimental viscosity measurements for copper oxide nanoparticle suspensions

    Publication Year: 2002 , Page(s): 198 - 201
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (258 KB)  

    The viscosity of water with copper oxide nanoparticle suspensions was measured using capillary viscometers. The mass fractions of copper oxide nanoparticles in the experiment, w, varied between 0.02 and 0. 10, and the temperature range was 30°C to 80°C. The experimental results show that the temperature was the major factor affecting the viscosity of the nanoparticle suspensions, while the effect of the mass fraction on the viscosity was not so obvious as that of the temperature for the mass fractions chosen in the experiment. The effect of the capillary tube size on the viscosity was also found to be relatively important at higher mass fractions. View full abstract»

    Open Access
  • Forced convection heat transfer in plate channels filled with packed beds or sintered porous media

    Publication Year: 2002 , Page(s): 202 - 208
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (492 KB)  

    In the present work, forced convection heat transfer in plate channels filled with metallic or non-metallic particles (packed beds) or sintered porous media is simulated numerically using a thermal non-equilibrium model. The numerical simulation results are compared with experimental data. The difference between convection heat transfer in packed beds and in sintered porous media and the effects of the boundary condition assumptions are investigated. The results show that the numerical simulation of convection heat transfer of air or water in packed beds using the local thermal non-equilibrium model and the variable porosity model agrees well with the experimental data. The convection heat transfer coefficient in sintered porous media is much higher than that in packed beds. In the numerical simulation of convection heat transfer in sintered porous media, the boundary conditions on the wall should be that the particle temperatures are equal to the fluid temperature. View full abstract»

    Open Access
  • Boiling heat transfer on porous surfaces with vapor channels

    Publication Year: 2002 , Page(s): 209 - 212
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (299 KB)  

    Boiling heat transfer on porous coated surfaces with vapor channels was investigated experimentally to determine the effects of the size and density of the vapor channels on the boiling heat transfer. Observations showed that bubbles escaping from the channels enhanced the heat transfer. Three regimes were identified: liquid flooding, bubbles in the channel and the bottom drying out region. The maximum heat transfer occurred for an optimum vapor channel density and the boiling heat transfer performance was increased if the channels were open to the bottom of the porous coating. View full abstract»

    Open Access
  • Introdution to institute of engineering thermophysics, department of thermal engineering, Tsinghua University

    Publication Year: 2002 , Page(s): 213
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    Open Access
  • Tsinghua Develops World's Thinnest Ultrasonic Motor

    Publication Year: 2002 , Page(s): 201
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    Open Access
  • THSCAN large container stationary inspection system in service

    Publication Year: 2002 , Page(s): 208
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    Open Access

Aims & Scope

Tsinghua Science and Technology (Tsinghua Sci Technol) aims to highlight scientific achievements in computer science, electronic engineering, and other IT fields. Contributions all over the world are welcome.

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