Simulation by solutal convection of a thermal plume in a confined stratified environment: application to displacement ventilation

This paper describes the experimental study, by solutal simulation, of a thermal plume in a confined stratified environment, a situation encountered in displacement ventilation systems. The criteria enabling similarity to be established between the thermal plume in air and the solutal plume in a hydraulic model are discussed. Density stratification is detected by a planar laser-induced fluorescence (PLIF) technique. Criteria for defining the interface height and thickness are determined After validation of these criteria in the fully developed region of the plume in a confined stratified environment, a formulation of the stratification height in the region close to the source has been established.     

Numerical computation of a round laminar plume in a stratified environment

The round laminar plume issued in a stratified environment has been studied under the boundary layer approximation. The differential equations governing the problem have been solved numerically using the finite difference method developed by Patankar-Spalding. The numerical solutions are in good agreement with the exact ones for the unstratified case. The method gives results for the stratified ambient case for which no similarity solution exists.     

Analysis of ignition by a plane laminar thermal plume

A theoretical investigation of ignition of a premixed combustible gas by a line heat source is described. Laminar flow was examined by numerical solution of the nonsimilar boundary layer equations, for an ideal gas with a one-step reaction and neglecting radiation. A deflagration wave is always present in the plume for these assumptions. However, the flame makes a rapid transition away from the source over a relatively small range of reaction parameters. This transition state provides a practical estimate of limiting ignition conditions-assuming that flames far from the source are not observed due to flow disturbances, transition to turbulence, or quenching by surrounding surfaces. Computations of limiting ignition conditions using this criterion indicate that while plume ignition approximates ignition by finite-sized sources, as the source size becomes small, there are interesting differences between the two cases due to effects of distributed heat addition and surface friction when the source size is finite. Streamwise gradients were appreciable in some circumstances; therefore, further analysis of this problem is suggested-relaxing the boundary layer approximations.     

Molecular dynamics simulation of thermal and thermomechanical phenomena in picosecond laser material interaction

In recent years, short pulsed laser materials interaction has attracted considerable attention owing to the rapid development of short pulsed lasers and their potential applications in laser-material processing. In this work, molecular dynamics (MD) simulations are conducted to study the thermal and thermomechanical phenomena induced by picosecond laser heating. The generation and propagation of the stress wave are calculated and depicted in detail. Results of the MD simulation are compared with those obtained with an analytical solution. In addition, a temperature wave as a result of the coupling between temperature and the strain rate is observed, which propagates at the same speed as the stress wave.     

Mapping out the regulatory environment and its interaction with land and property markets

The purpose of this paper is to explain how the regulatory environment created by planning and building regulations interacts with land and property markets. Since this regulatory environment operates as a form of intervention within property markets in general and the development process in particular, it is essential first to understand the nature, structure and operation of land and property markets. These are covered in the first section. The second section identifies the aims and components of the planning system and building regulations. From this basis, four broad types of policy intervention are reviewed in the third section. These cover policies that seek to shape, regulate or stimulate market activity, and those that aim to build state or market capacity to produce desired outcomes. The final section uses the private residential development process as a case study to explore the extent to which state intervention can influence producer–consumer relationships in one important market sector. The paper argues that effective intervention in land and property markets requires the creation of a broad range of policy tools and their appropriate deployment to suit particular market circumstances.     

Experimental investigation of a thermoelectric refrigeration system employing a phase change material integrated with thermal diode (thermosyphons)

This paper presents results of tests carried out to investigate the potential application of phase change materials (PCMs) integrated with thermosyphons in a thermoelectric refrigeration system. The work involved design, fabrication and test of a 150 W thermoelectric refrigeration system. The system was first fabricated and tested using a conventional heat sink system (bonded fin heat sink system) at the cold heat sink. In order to improve the performance and the storage capability, the system was reconstructed and tested using an encapsulated PCM as a cold sink. Results of tests of the latter system showed an improved performance compared with the former system. However to improve the storage capability, in particular during off-power periods, it was found necessary to integrate the PCM with a thermal diode, which would allow heat flow in one direction only. Results of tests carried out on the new system showed considerable improvement in the storage capability of the thermoelectric refrigeration system compared with the previous ones. Overall the system suits operation with renewable energies, e.g., solar energy.     

Preparation, characterization, and thermal properties of microencapsulated phase change material for thermal energy storage

This study is focused on the preparation, characterization, and determination of thermal properties of microencapsulated docosane with polymethylmethacrylate (PMMA) as phase change material for thermal energy storage. Microencapsulation of docosane has been carried out by emulsion polymerization. The microencapsulated phase change material (MEPCM) was characterized using scanning electron microscopy (SEM) and Fourier transform infrared (FT-IR) spectroscopy. Thermal properties and thermal stability of MEPCM were measured by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). DSC analysis indicated that the docosane in the microcapsules melts at 41.0 °C and crystallizes at 40.6 °C. It has latent heats of 54.6 and −48.7 J/g for melting and crystallization, respectively. TGA showed that the MEPCM degraded in three distinguishable steps and had good chemical stability. Accelerated thermal cycling tests also indicated that the MEPCM had good thermal reliability. Based on all these results, it can be concluded that the microencapsulated docosane as MEPCMs have good potential for thermal energy storage purposes such as solar space heating applications.     

Experimental characterization of thermal radiation properties of dispersed media

As dispersed materials generally are semi-transparent media which absorb, emit and scatter thermal radiation, the predictive modeling of thermal processes involving such kind of materials requires the knowledge of a number of radiative properties to feed the models. These properties cannot be directly measured but are identified from a set of experimental data of radiative flux collected from a sample submitted to appropriate experimental conditions. This paper focuses on identification methodology for thermal radiation properties of dispersed media such as fibers, foams, pigmented coatings, ceramics. After a brief introductive overview of the subject, the parameter identification methodology and two experimental facilities used for radiative properties determination are firstly described. As the identification process involves a solution model for the Radiative Transfer Equation inside the sample, some attention is then paid to the development of RTE solution models well matched to this specific purpose. Two examples of application are described before concluding on the advantages, limitations and remaining difficulties connected to this new and promising metrology of thermal radiative properties of dispersed media.     

谷电利用复合石蜡蓄热材料的制备及供暖墙体构造实验

针对石蜡作为相变蓄热材料导热性能差、供热能力不足的问题,向石蜡中添加膨胀石墨(EG)制备导热性能增强的复合相变材料(PCM),探讨EG与石蜡的配比对复合PCM热性能的影响.测试结果表明,当EG质量分数达到12％时,复合PCM的导热系数提升至纯石蜡的12倍,相变潜热从纯石蜡的190.8 J/g减小至152.1 J/g,相...     

Impingement heat transfer of a single thermal plume on the upper wall

We present numerical calculations of the generation, growth and impingement of a thermal plume in a two-dimensional buoyancy induced flow. Numerical values are obtained for the aspect ratios H/W=1/4, 3/8, 1/2, the Grashof numbers Gr=10⁴, 10⁵, and the Prandtl number Pr=170. Impingement heat transfer on the upper wall is evaluated at various times. Numerical results show that before a thermal plume impinges on the upper heated wall, the thermal conduction layer, which is the stable stratification, near the upper wall becomes thinner and the local heat transfer peaks. The local Nusselt number approaches the steady condition after the impingement of a thermal plume. Additionally, under certain conditions the stream function takes a symmetrical form of two ellipses.     

Preparation of form-stable carbonates/magnesium oxide-flake graphite composite thermal storage material and its thermal conductivity

以混合碳酸盐为相变材料,以氧化镁为陶瓷基骨架材料,以鳞片石墨为导热增强剂,通过混合烧结法制备出中高温复合蓄热材料。基于XRD和SEM表征分析可知,添加鳞片石墨后复合材料具有较好的化学稳定性,而且由于鳞片石墨的原因复合材料形成较多的孔隙结构。通过分析添加鳞片石墨后复合材料的热物性可知,随着鳞片石墨含量的增加,复合材料的熔点基本不变,而其热导率不断提高。鳞片石墨含量为25%的混合碳酸盐/氧化镁复合材料在250℃和560℃时的热导率分别达到3.88 W/（m·K）和2.52 W/（m·K）。基于微观结构和界面层理论对复合材料的导热增强机制进行了分析与讨论。     

Observation of preferred instability modes in a mechanically excited thermal plume using Schlieren visualizations

An experimental study of the instability modes of a laminar axisymmetric thermal plume under symmetrical controlled disturbances of varicose type was carried out. The Schlieren technique was used to investigate the considered flow. The range of excitation frequencies over which the studied plume became unstable was determined. The size of the corresponding induced large-scale structures was analyzed in function of the disturbance frequency. The results obtained throughout this work confirmed the ability of a plume to behave like a frequencies filter.     

Transient free convective flow through a vertical cylinder filled with a porous material

This article aims to explore the impressive impact of emerging parameters on transient fully evolved free convective flow inside a vertical cylinder containing a porous material. The mathematical formulation of the model related to the considered physical circumstance is presented under compatible boundary conditions. Closed‐form solutions are received for the velocity field, the temperature distribution, mass flux, skin friction, and the Nusselt number in terms of Bessel functions and modified Bessel functions of the first kind. Impressive effects of parameters such as the Darcy number $Da$, Prandtl number $\text{Pr}$, viscosity ratio $M$, and also time $t$ on both the velocity and temperature distribution have been explored employing graphs and tables. It is irradiated by analysis that flow erection, heat transfer rate, skin friction, and mass flux are admirably impacted by the Prandtl number, the Darcy number, viscosity ratio parameter, and time. It is found that both the velocity and temperature field profiles rise with the rising value of time and ultimately attain their steady state. Moreover, the Prandtl number and the viscosity ratio parameter reduce the velocity profiles, while the reverse phenomenon occurs with the Darcy number.     

Electrical and thermal characterization of a PV-CPC hybrid

Long term evaluation of an asymmetric CPC PV-thermal hybrid built for high latitudes, MaReCo (MaximumReflectorCollector), is performed in Lund, lat 55.7°, and this paper discusses output estimates and characteristics of the system. The output estimates are calculated using the MINSUN simulation program. To get the input for MINSUN, measurements were performed on two MaReCo prototypes. These measurements show that the front reflector collects most of the irradiation in the summer, and the back reflector in the spring and fall. Two different reflector materials were used, anodized aluminium and aluminium laminated steel. The steel based reflector was selected for its rigidness. The output estimates show no difference in yearly output between the two reflector materials, both back reflectors deliver 168 kW h/(m² cell area) of electricity compared to 136 kW h/m² cell area for cells without reflectors. The cells facing the front reflector deliver 205 kW h/(m² cell area) of electricity. The estimated output of thermal energy was 145 kW h/(m² glazed area) at 50 °C. The estimates show that the optimal placement of the photovoltaic cells is facing the front reflector, but having cells on both sides is in most cases the best option.     

Experimental and numerical investigation of thermal energy storage with a finned tube

A latent heat thermal energy storage system using a phase change material (PCM) is an efficient way of storing or releasing a large amount of heat during melting or solidification. It has been determined that the shell‐and‐tube type heat exchanger is the most promising device as a latent heat system that requires high efficiency for a minimum volume. In this type of heat exchanger, the PCM fills the annular shell space around the finned tube while the heat transfer fluid flows within the tube. One of the methods used for increasing the rate of energy storage is to increase the heat transfer surface area by employing finned surfaces. In this study, energy storage by phase change around a radially finned tube is investigated numerically and experimentally. The solution of the system consists of the solving governing equations for the heat transfer fluid (HTF), pipe wall and phase change material. Numerical simulations are performed to investigate the effect of several fin parameters (fin spacing and fin diameter) and flow parameter (Re number and inlet temperature of HTF) and compare with experimental results. The effect of each variable on energy storage and amount of solidification are presented graphically. Copyright © 2005 John Wiley & Sons, Ltd.     

Importance of fracture criterion and crack tip material characterization in probabilistic fracture mechanics analysis of an RPV under a pressurized thermal shock

Using a probabilistic fracture mechanics code, the importance of the choice of fracture criterion and the material fracture resistance characterization at the crack tip is elucidated in the failure probability analysis of an reactor pressure vessel. The paper describes the procedure to evaluate the crack extension based on R6, where an increase in fracture resistance by ductile crack extension is considered. Two standard J–resistance curves are prepared for applying the elasto-plastic fracture criterion by R6 tearing.Case studies concerning the effect of the tearing fracture criterion were carried out using a severe pressurized thermal Shock transient. Results are discussed with respect to the fracture criterion, the material J–resistance curve, the algorithm for evaluating the crack tip fracture toughness and the initial crack geometry.The introduction of the elasto-plastic fracture criterion significantly contributes to removal of over-conservatism in applying a linear elastic fracture criterion. It was also confirmed that the algorithm of the re-evaluation method for crack tip fracture toughness has to be correctly applied.     

Performance of a liquid cooling-based battery thermal management system with a composite phase change material

This article reports a recent study on a liquid cooling-based battery thermal management system (BTMS) with a composite phase change material (CPCM). Both copper foam and expanded graphite were considered as the structural materials for the CPCM. The thermal behaviour of a lithium-ion battery was experimental investigated first under different charge/discharge rates. A two-dimensional model was then developed to examine the performance of the BTMS. For the copper foam-based CPCM modelling, an enthalpy-porosity approach was applied. The numerical modelling aimed to study the impacts of CPCM types and inlet velocity of heat transfer fluid on both the maximum battery temperature and temperature distribution under different current rates. Dimensional analyses of the results were performed, leading to the establishment of relationships of the Nusselt numbers and dimensionless temperature against the Fourier and Stefan numbers.     

Experimental study of thermal behaviors in a rectangular channel with baffle of pores

This experimental study attempts to explore the local heat transfer in rectangular channel with baffles, and analyzes the experimental results of baffles with different heights and pores in the event of five Reynolds numbers and three heating quantities. Apart from increasing the perturbation of flow field, the channel's flow field with baffles, which is similar to a backward-facing step flow field, is very helpful to heat transfer. To obtain an optimized baffle and increase the perturbation of flow field, this experiment employed baffles with five heights (H = 10–50 mm) and different numbers of pores (N = 1–3), as well as heat flux: Q = 40–100 l/min, Reynolds number: 702–1752, and heating quantity: q_in = 90–750 W/m². In addition to measurement of overall temperature distribution, emphasis is also placed on analysis of local heat transfer coefficient. Furthermore, heat transfer distribution of channel can be applied to explain how the baffles of pores have an influence upon backward-facing step flow field, shear layer, recirculation region, reattachment region and redeveloped boundary layer. Finally, some empirical formulas derived form experimental results may provide a reference for future design.     

Experimental study of forced and free convective heat transfer in the thermal entry region of horizontal concentric annuli

Forced and free convective heat transfer for thermally developing and thermally fully developed laminar air flow inside horizontal concentric annuli in the thermal entrance length has been experimentally investigated. The experimental setup consists of a stainless steel annulus having a radius ratio of 2 and an inner tube with a heated length of 900 mm subjected to a constant wall heat flux boundary condition and an adiabatic outer annulus. The investigation covers Reynolds number range from 200 to 1000, the Grashof number was ranged from 6.2 × 10⁵ to 1.2 × 10⁷. The entrance sections used were long tube with length of 2520 mm (L/D_h = 63) and short tube with length of 504 mm (L/D_h = 12.6). The surface temperature distribution along the inner tube surface, and the local Nusselt number distribution versus dimensionless axial distance Z_t were presented and discussed. It is inferred that the free convection effects tended to decrease the heat transfer at low Re number while to increase the heat transfer for high Re number. This investigation reveals that the Nusselt number values were considerably greater than the corresponding values for fully developed combined convection over a significant portion of the annulus. The average heat transfer results were correlated in terms of the relevant dimensionless variables with an empirical correlation. The local Nusselt number results were compared with available literature and show similar trend and satisfactory agreement.