Micro capillary pumped loop system for a cooling high power device

This work discusses the operation of a capillary-driven two-phase loop, configured on a micro capillary pumped loop (MCPL) system without an external power supply but capable of automatic heat transmission. The MCPL device, fabricated using MEMS (microelectricomechanical system) technology, was tested and yielded the following results: first, the proposed design of a new MCPL system with a water reservoir operating at low pressures is feasible and requires no additional power supply and instead relies on automatic heat transmission. Second, the issue of depriming in a MCPL was effectively controlled, the endurance of MCPL for the depriming problem can be executed by yielding input heat fluxes of 185.2 W/cm² at an evaporator temperature of 165 °C, thus revealing that this model provides excellent cooling performance. Third, the effective operation range was determined and its successful operation was confirmed for MCPL. The ease of starting up increased with the temperature of the reservoir. Finally, two-phase tension that originated in the groove structures in the evaporator and condenser was confirmed to control the movement of the fluids throughout the system and verified to be effective in improving cooling efficiency.     

硅微结构惯性传感器的研制现状及应用前景

本文从介绍硅微结构惯性传感器的研制背景入手，分析了微米／纳米技术近年来的进展对发展硅微传感器的推动作用，重点介绍了美国主要公司有关硅微结构惯性仪表的研制现状，最后结合市场预测，提出笔者对我国发展硅微惯性传感器的几点浅见。     

Heat transfer parameter models and correlations for cooling applications

 Cooling technology is widely used in a variety of engineering applications ranging from cooling of electronic devices to food cooling. It makes this industry play an important role in the economies. In practice there is an urgent need to design a cooling system with the optimum processing conditions and to provide energy saving along with the high quality products. In this respect, accurate determination of the heat transfer parameters for the cooling operation is essential. In this paper, recently developed models and correlations, e.g., thermal diffusivity and heat transfer coefficient models, and Nusselt–Reynolds and Fourier–Reynolds correlations to determine the cooling process and heat transfer parameters are presented with an illustrative example. Another important point is that the cooling parameters are incorporated into the heat transfer parameters and correlations. It is believed that these models and correlations are of great significance to refrigeration industry. Received on 27 October 1998     

Performance of textile and building materials for a particular evaporative cooling purpose

High energy demand associated to the massive use of air conditioning systems requires careful consideration of passive cooling strategies, with evaporative cooling being recognized as a useful possibility for that purpose. One important factor that influences the performance of evaporative cooling systems is the media material that supports water evaporation process. In this work evaporative cooling capabilities of different building and textile materials were experimentally determined. The major purpose of the study was to select an evaporative cooling material to be used in a more complex passive cooling unit under research development. A test tunnel was constructed for this particular work and the behavior of several samples was analyzed. Results show that among the studied materials a polyester spacer fabric with honeycomb structure presents best performance.     

Effects of surface radiation on natural convection in a rayleigh-benard square enclosure: steady and unsteady conditions

Coupled laminar natural convection with radiation in air-filled square enclosure heated from below and cooled from above is studied numerically for a wide variety of radiative boundary conditions at the sidewalls. A numerical model based on the finite difference method was used for the solution of mass, momentum and energy equations. The surface-to-surface method was used to calculate the radiative heat transfer. Simulations were performed for two values of the emissivities of the active and insulated walls (ɛ₁=0.05 or 0.85, ɛ₂=0.05 or 0.85) and Rayleigh numbers ranging from 10³ to 2.3×10⁶ . The influence of those parameters on the flow and temperature patterns and heat transfer rates are analyzed and discussed for different steady-state solutions. The existing ranges of these solutions are reported for the four different cases considered. It is founded that, for a fixed Ra, the global heat transfer across the enclosure depends only on the magnitude of the emissivity of the active walls. The oscillatory behavior, characterizing the unsteady-state solutions during the transitions from bicellular flows to the unicellular flow are observed and discussed.     

Integrated microthermoelectric cooler for microfluidic channels

Precise fluid temperature control in microfluidic channels is a requirement for many lab-on-a-chip and microreactor devices, especially in biotechnology where most processes are highly temperature sensitive. We demonstrate the concept of a microthermoelectric cooler integrated into a microfluidic channel in order to give rapid and localised fluid cooling. The key aspect of this concept is the use of a second imbedded microfluidic channel that is used as a miniature heat sink. An analytical thermal model has been derived that couples thermoelectric effects with fluid heat-transfer rates from both the hot and cold connections. Using this model, the effect on cooling performance of varying the thermal resistance between the hot and cold connections and the fluid has been quantified, as well as the effect of substrate thermal conductivity. If the substrate thermal conductivity is too high, heat leakage renders the thermoelectric cooler ineffective. The optimum electrical current for cooling has been shown to be a function of the thermal resistance of the heat sink. For thermoelectric coolers there is competition between temperature reduction and cooling power. Using this fact, based on the final fluid temperature required, we have calculated the maximum flow rate that will achieve this. Finally, a prototype integrated microthermoelectric cooler has been fabricated and tested.     

Interaction of surface radiation with combined conduction and convection from a discretely heated L-corner

Prominent results pertaining to the problem of multi-mode heat transfer from an L-corner equipped with three identical flush-mounted discrete heat sources in its left leg are given here. The heat generated in the heat sources is conducted along the two legs of the device before being dissipated by combined convection and radiation into air that is considered to be the cooling agent. The governing equations for temperature distribution along the L-corner are obtained by making appropriate energy balance between the heat generated, conducted, convected and radiated. The non-linear partial differential equations thus obtained are converted into algebraic form using a finite-difference formulation. The resulting equations are solved simultaneously by Gauss–Seidel iterative solver. A computer code is specifically written to solve the problem. The computational domain is discretised using 101 grids along the left leg, with 15 grids taken per heat source, and 21 grids along the bottom leg. The effects of surface emissivity, convection heat transfer coefficient, thermal conductivity and aspect ratio on local temperature distribution, peak device temperature and relative contributions of convection and radiation to heat dissipation from the L-corner are studied in detail. The point that one cannot overlook radiation in problems of this class has been clearly elucidated.     

Multiphysics Simulation Combining Large-Eddy Simulation,Wall Heat Conduction and Radiative Energy Transfer to Predict Wall Temperature Induced by a Confined Premixed Swirling Flame

A multi-physics simulation combining large-eddy simulation, conjugate heat transfer and radiative heat transfer is used to predict the wall temperature field of a confined premixed swirling flame operating under atmospheric pressure. The combustion model accounts for the effect of enthalpy defect on the flame structure whose stabilization is here sensitive to the wall heat losses. The conjugate heat transfer is accounted for by solving the heat conduction within the combustor walls and with the Hybrid-Cell Neumann-Dirichlet coupling method, enabling to dynamically adapt the coupling period. The latter coupling procedure is enhanced to determine statistics (mean, RMS, \(\ldots \)) in a permanent regime accurately and efficiently thanks to an acceleration technique which is derived and validated. The exact radiative heat transfer equation is solved with an advanced Monte Carlo method with a local control of the statistical error. The coupled simulation is carried out with or without accounting for radiation. Excellent results for the wall temperature are achieved by the fully coupled simulation which are then further analyzed in terms of radiative effects, global energy budget and fluctuations of wall heat flux and temperature.     

Transformational-zone method of calculation of complex heat exchange during flow of optically active medium inside tube of diffuse grey surface

The paper presents analytical and experimental investigations of influence of radiative heat transfer on complex heat exchange during flow of optically active gas inside a pipe of diffusegrey properties. It was assumed that the pipe is heated from the outside by a constant heat flux and gas flowing inside is both absorbing and emitting and of small optical density. The influence of length and radiative properties of the pipe surface and of the gas temperature distribution on the wall and in the gas were analysed. The influence of radiative energy transfer on overall heat transfer coefficient was estimated. Mathematical model of radiative convective heat exchange in a system of one-dimensional temperature field, based on zone division method of Hottel and surface transformation, was verified numerically and experimentally. The results of numerical calculations were compared with experimental results obtained during carbone dioxide (CO₂) flow inside electrically heated ceramic tube. The set of nonlinear differential equations was solved by Runge-Kutta method with Hamming modification and with the use of separable-kernel method.     

Theoretical analysis for the transient behaviour of radiative cooling of cavities in superconducting LINAC cryomodule

For the superconducting linear accelerator program, three cryomodules each houses eight superconducting cavities were successfully developed at IUAC, New Delhi. In each cryomodule, the cold mass at 4.2 K is surrounded by the liquid nitrogen cooled thermal shield maintained at 100 K. Three stages of cooling namely, radiation cooling followed by liquid nitrogen pre-cooling and finally liquid helium (LHe) cooling, are followed to reduce the temperature of cold mass from 300 to 4.2 K. The cold mass at 4.2 K consists of cavities, LHe vessel and the support structure. The temperature of cavity and helium vessel reaches to 210–220 K in 40 h of time by the natural radiation from the thermal shield. The radiative cooling rates for the cavities, helium vessel and support structure are found to be 3.0, 4.0 and 2.0 K/h respectively. A detailed analytical calculation has been done to understand the transient cool-down phenomenon for each component and compared with the experimental measured values. The experimental values are in agreement with the analytical data within 5 % variation considering the correction factor of radiation funneling. This paper presents the role of different thermal parameters like shield temperature, conduction load and radiation funneling area in the transient radiative cool-down behaviour of different components.     

面向双碳目标的建筑能源系统再认识

建筑领域是实现双碳目标的关键部门,在双碳目标指引下建筑能源系统需要做出革新.为此,本研究对建筑能源系统的发展任务进行了深入探讨,提出了面向双碳目标的建筑能源系统发展方向:传统的建筑能源系统以满足建筑自身冷、热、电等基本能源需求为主;双碳目标下,建筑能源系统需要从建筑节能向建筑低碳的新目标转变,需要在降低建筑本体能源需求、全面电气化、提升建筑能源系统能效水平、实现灵活可调并成为具有柔性调节能力的能源系统可调负载等方面做出变革,需要从单纯能源系统的消费者转变为集能源生产、消费、调蓄于一体的复合体.以构建低碳建筑能源系统为目标,对建筑能源系统的研究趋势进行了展望:需要进一步认识建筑能源、建筑环境营造的需求从而更好地理解建筑能源系统的基本要求,需要建筑与交通、电力等领域进一步融合,需要从单体建筑向区域建筑、城市等多个尺度上以建筑为载体构建城乡新型能源系统.本研究可为建筑能源系统如何实现自身角色转变、加快实现双碳目标下的能源系统变革提供有益参考.     

Calculation of reflection of a blast wave by a plane

Some results are given of calculation of the reflection of a blast wave by a rigid flat surface. A model of the explosion with a simple energy dissipation mechanism is considered, radiation being taken into account in the approximation of radiative heat conduction. The pressure distribution on the surface and the flow pattern in the region of propagation of the incident and reflected shock waves are obtained.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 179–182, September–October, 1982.We thank L. A. Chudov for helpful discussions of the work.     

Turbulent Heat Transfer and Large Coherent Structures in Trailing-edge Cutback Film Cooling

Film cooling is a key technology for improving the thermal efficiency and power output of gas turbines. The trailing-edge section of high-pressure turbine blades can be efficiently cooled by ejecting a film over a cutback on the pressure side of the blade. In this paper, results of Large–Eddy Simulations (LES) are presented that match an existing experimental setup. Altogether, eight simulations with the blowing ratio M varying as the only parameter were performed over a range from M?=?0.35 to 1.4. Reasonably good agreement between LES and experiments were obtained for flow field statistics and adiabatic film-cooling effectiveness η _aw. Within a limited range of blowing ratios, an increase in the blowing ratio results in a counter-intuitive decrease of the cooling effectiveness. The present work suggests a mechanism that can explain this behavior. The visualization and analysis of large coherent structures showed that there exists dominant clockwise-rotating structures that can give rise to a combined upstream- and wall-directed turbulent heat flux. This turbulent heat flux represents the main contribution of the total heat flux and causes a significantly intensified thermal mixing process, which in turn results in the counter-intuitive decrease of the cooling effectiveness.     

A general mathematical modelling for heat and mass transfer in unsaturated porous media: an application to free evaporative cooling

The present paper develops a general mathematical model with some improvements in mass, momentum and energy equations, which introduce more transport mechanisms to simulate simultaneous transfer of heat and mass in the porous media unsaturated with liquid. Numerical calculation results in two-dimension are obtained for the vertical packed bed with its right opening surface exposing to atmospherical environment. The calculating data can demonstrate the cooling effect of the water evaporation for the bed if it is used as a cooling wall of building for room air-conditioning in the hot and dry climate.     

A radiation and convection fluxmeter for high temperature applications

Heat flux is an essential parameter for the diagnostic of thermal systems. In high temperature industrial environment, there are difficulties in measuring incident radiation heat flux as well as in differentiating between the convective and radiative components of heat flux on the heat transfer surface. A new method for heat flux measurement is being developed using a porous sensing element. The gas stream flowing through the porous element is used to measure the heat received by the sensor surface exposed to the hot gas environment. A numerical model of sensor with appropriate boundary condition has been developed in order to perform analysis of possible options regarding its design. The analysis includes: geometry of element, physical parameters of gas and solid and gas flow rate through the porous element. For the optimal selection of parameters, an experimental set-up was designed, including the sensor element with respective cooling and monitoring systems and a high temperature radiation source. The experimental set-up was used to obtain calibration curves for a number of sensors. The linear dependency of the heat flux and respective temperature difference of the gas were verified. The accuracy analysis of the sensor reading has proved high linearity of the calibration curve and accuracy of ±5%.     

Influence of PM fouling on effectiveness of heat exchanges in a diesel engine with fin-type EGR coolers of different sizes

The particulate matter (PM) fouling in exhaust gas recirculation (EGR) coolers degrades the heat transfer performance considerably, which in turn has a significant influence on the design of the EGR cooler. To investigate the effects of PM fouling on the heat exchange effectiveness in the fin-type EGR coolers of six different sizes, engine dynamometer experiments were performed. The experimental results show that the heat exchange effectiveness varies with the size of the fin-type EGR cooler.     

Transient two-dimensional radiative and conductive heat transfer in an axisymmetric medium

This work considers transient conductive and radiative heat transfer in a two-dimensional, cylindrical, scattering medium heated or cooled by internal heat source or boundary surface. A finite difference scheme is employed for handling the energy storage and the heat diffusion by conduction, while a discrete-ordinate method is used to analyze the radiative heat transfer. The effects of various parameters, including the conduction-radiation parameter, the scattering albedo and the emissivity of the boundary surfaces, are investigated. Received on 30 April 1997     

Unsteady MHD free convective heat and mass transfer from a vertical porous plate with Hall current,thermal radiation and chemical reaction effects

An analysis is presented to investigate the effects of chemical reaction, thermal radiation and heat generation or absorption on unsteady free convective heat and mass transfer along an infinite vertical porous plate in the presence of a transverse magnetic field and Hall current. The governing partial differential equations are formulated and transformed by using a similarity transformation into a system of ordinary differential equations. The resulting equations are solved numerically using a fourth‐order Runge–Kutta scheme along with the shooting method. The Rosseland approximation is used to describe the radiative heat flux in the energy equation. Numerical results for the velocity, temperature and concentration distributions are shown graphically for different parametric values. The effects of parameters on the local friction coefficients, the Nusselt number and Sherwood numbers are depicted in tabulated form. Copyright © 2009 John Wiley & Sons, Ltd.     

Heat exchange between a selectively emitting liquid and a laminar gas flow in the presence of an external source of radiation

An investigation is conducted in the solution of a number of practical problems of the radiative and combined heat exchange in nonuniform systems having widely different physical properties. The processes of thermal interaction between the ocean and the atmosphere have been treated in the paper [1], the effect of thermal radiation on the melting and solidification of semitransparent crystals has been investigated in [2], the flow of a selectively emitting gas around the lateral surface of an object evaporating under the action of radiative heating has been discussed in [3], and heat transfer from a jet to the molten mass of glass in a glassmaking furnace tank has been investigated in [4]. The radiative and combined heat exchange between a selectively emitting liquid and a transparent heat-conducting laminar gas flow in the case of a specified external thermal radiation field is discussed in this paper. The energy conservation equations are set up taking into account the heat transfer by radiation, convection, and molecular thermal conduction. A differential approximation is used in calculating the values of the radiation fluxes. A system of fundamental computational equations is solved by the method of finite differences and iterations and by the Runge-Kutta method. The results of the calculations are presented in the form of graphs.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 3, pp. 116–122, May–June, 1976.