Effect of the storage tank thermal insulation on the thermal performance of an integrated collector storage solar water heater (ICSSWH)

The thermal behavior of an integrated collector storage solar water heater (ICSSWH) is numerically studied using CFD simulations. Based on the good agreement between the numerical results and the experimental data from literature, we propose a geometrical change allowing limiting the main disadvantage of this solar system which is its high night losses due to the non-insulated storage tank surface. A second 3D CFD model of an ICSSWH in which the storage tank is partially insulated is developed and three values of this tank thermal insulated fraction are studied. Numerical results show that the partially insulated tank based ICSSWH presents lower thermal losses during the night and this night thermal losses coefficient is reduced from 14.6 to 11.64 W K^?1 for the tank thermal insulation fraction τ = 1/4. Similarly, the modified system presents the advantage of its lower thermal losses even during the day. Regarding the thermal production, it is seen that the modified system presents higher water temperature at night and that for all the tank thermal insulation fractions. Concerning the operation of this modified system during the day, the water temperature is lower during the day and that up to 16 h but the water temperature which achieves 324 K for the storage tank thermal insulation fraction τ = 1/8 still sufficiently high to satisfy a family hot water needs.     

Carbonate-salt-based composite materials for medium- and high-temperature thermal energy storage

This paper discusses composite materials based on inorganic salts for medium- and high-temperature thermal energy storage application. The composites consist of a phase change material （PCM）, a ceramic material, and a high thermal conductivity material. The ceramic material forms a microstructural skeleton for encapsulation of the PCM and structural stability of the composites; the high thermal conductivity material enhances the overall thermal conductivity of the composites. Using a eutectic salt of lithium and sodium carbonates as the PCM, magnesium oxide as the ceramic skeleton, and either graphite flakes or carbon nanotubes as the thermal conductivity enhancer, we produced composites with good physical and chemical stability and high thermal conductivity. We found that the wettability of the molten salt on the ceramic and carbon materials significantly affects the microstructure of the composites.     

Heating of a medium as a result of Joule energy dissipation

It is well known that when a magnetic field is present and electric currents flow through a gas, terms over and above those present in the case of ordinary thermal conductivity appear in the heat flux density vector. If the gas is dense enough and the magnetic field not over large, then the anisotropy caused by the magnetic field may be neglected. However, for a sufficiently large electric current a term proportional to the temperature and to the current density vector remains in the heat flux density vector. This effect explains, for example, the asymmetry of heat fluxes in the electrodes of a continuously operated electromagnetic accelerator (1).We shall consider this situation in relation to the example of a fully ionized quasineutral gas with identical electron and ion temperatures.     

Combined thermal radiation and natural convection in a cavity containing a discrete heater: Effects of nature of heating and heater aspect ratio

This paper investigates the interaction between thermal radiation and natural convection in an air filled square cavity with a discrete heater placed inside. The vertical walls of the cavity are cooled while the horizontal ones are insulated. Two types of the heater, viz., isothermal and heat generating are considered. The governing coupled nonlinear partial differential equations were solved using a finite volume method on a uniformly staggered grid system. The effects of the pertinent parameters such as the Rayleigh number, aspect ratio of the discrete heater and surface emissivity on the heat and fluid flow characteristics are investigated in detail. In general it is found that the overall heat transfer rate is enhanced with an increase of the surface emissivity and the Rayleigh number for both isothermal and heat generating heaters. Buoyancy induced flow gets augmented for the isothermal solid body whereas it is weakened for the heat generating body in the presence of radiation exchange.     

Self-similar axisymmetric thermal in a variable-density medium

A model of a turbulent axisymmetric thermal rising in a medium whose density varies with height is proposed. An analytic solution describing the self-similar stage of ascent of the thermal is constructed. The effect of the density stratification of the atmosphere on the structure of the thermal during its motion is analyzed, and relations describing the characteristic points of the thermal and the smearing of its leading edge are obtained.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 4, pp. 45–53, July–August, 1991.     

Horizontal thermal convection in a porous medium

Linearised instability and nonlinear stability bounds for thermal convection in a fluid-filled porous finite box are derived. A nonuniform temperature field in the steady state is generated by maintaining the vertical walls at different temperatures. The linearised instability threshold is shown to be well above the global stability boundary, which is strongly suggested by the lack of symmetry in the perturbed system. The numerical results are evaluated utilising a newly developed Legendre-polynomial-based spectral method.     

Shell-and-tube type latent heat thermal energy storage: numerical analysis and comparison with experiments

The melting process of industrial grade paraffin wax inside a shell-and-tube storage is analyzed by means of numerical simulation and experimental results. For this purpose, the enthalpy porosity method is extended by a continuous liquid fraction function. The extended method is tested using results gained from a gallium melt test inside a rectangular enclosure.     

Effective thermal conductivity of a medium with ellipsoidal particles

The principal components of the effective thermal conductivity tensor, characterizing stationary heat macrotransfer in a dense medium with dispersed ellipsoidal particles of a different material are calculated by a method suggested in [1]. The case of equally oriented ellipsoids and of isotropically distributed ones are considered as examples.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 1, pp. 107–111, January–February, 1974.The author is grateful to Yu. A. Buevich for his interest.     

Orthotropic semi-infinite medium with a crack under thermal shock

A cracked orthotropic semi-infinite plate under thermal shock is investigated. The thermal stresses are generated due to sudden cooling of the boundary by ramp function temperature change. The superposition technique is used to solve the problem. The crack problem is formulated by applying the thermal stresses obtained from the uncracked plate with opposite sign to be the only external loads on the crack surfaces as the crack surface tractions. The Fourier transform technique is used to solve the problem leading to a singular equation of the Cauchy type. The singular integral equation is solved numerically using the expansion method. The influence of the material orthotropy on the stress intensity factors is shown by comparing the results obtained for different orthotropic materials and isotropic materials in the case of plane stress. The numerical results of the stress intensity factors are demonstrated as a function of time, crack length, location of the crack and the duration of the cooling rate.     

Experiments on thermal characteristics of a natural circulation loop with latent heat energy storage under cyclic pulsed heat load

 An experimental investigation has been made of thermal characteristics of a rectangular, annular single-phase natural circulation loop with the inner tube filled with a solid–liquid phase change material (PCM) under cyclic pulsating heat load. A rectangular, annular loop of 150 cm in height and 75 cm in width was constructed with an annular gap of 0.6 cm, within which water was filled. The inner tube of the annular loop was filled with a PCM (n-Eicosene) or air. Under the cyclic pulsating heat load, temperature field within the water-filled annular loop with PCM- or air-filled inner tube was found to evolve into a steady periodic variation for the range of parameters considered. The water temperature and/or its fluctuating amplitude along the heated or cooled sections of the loop with the PCM-filled inner tube were found to be markedly lower than those measured in the loop with the air-filled inner tube under the identical conditions. On the other hand, along the insulated sections of the loop a somewhat minute difference in temporal variations of the water temperatures exists between the loops with PCM- and air-filled inner tube. In addition, at the outer wall along the cooled section, a time-periodic variation of temperature was detected in synchronizing with the pulsating heat load. Parametric effects of varying amplitude and time-period of the pulsating heat input, as well as of varying the inlet coolant temperature of the cooling jacket were investigated. Received on 30 June 2000 The authors are grateful for the support for this study from National Science Council of Republic of China in Taiwan under the Project Nos. NSC87-2212-E006-054 and NSC88-2212-E006-022.     

钢筋混凝土柱抗弯承载力的概率模型与校准分析

为了克服传统确定性抗弯承载力模型和校准方法无法合理考虑不确定性所存在的缺陷,分别建立了钢筋混凝土(RC)柱的概率抗弯承载力模型与概率校准方法。首先,基于RC柱正截面受弯承载力的基本假定,结合偏心受压RC柱的截面内力平衡条件,分别建立了大(小)偏心受压RC柱的确定性抗弯承载力模型;然后,综合考虑固有不确定性和认知不确定性的影响,分别建立了大(小)偏心受压RC柱概率抗弯承载力模型的解析表达式,进而结合贝叶斯理论和MCMC法确定了概率模型参数的后验分布信息,从而建立了RC柱的概率抗弯承载力模型;最后,基于概率抗弯承载力模型所确定的概率密度函数、置信区间和置信水平,提出了传统确定性抗弯承载力模型的概率校准方法。研究结果表明,所建立的概率抗弯承载力模型不仅可以合理描述RC柱抗弯承载力的概率分布特性,而且可以校准传统确定性抗弯承载力模型的计算精度和置信水平。     

Multi-walled carbon nanotubes added to Na2CO3/MgO composites for thermal energy storage

Na2CO3/MgO composites with added multi-walled carbon nanotubes （MWCNTs） were prepared and tested as phase change materials （PCMs） for thermal energy storage. Na2CO3/MgO composite PCMs were prepared and their chemical compatibility and thermal stability were studied. MWCNTs introduced with Na2CO3/MgO composite PCMs were also investigated and scanning electron microscopy （SEM） characterization was used to demonstrate the uniform dispersion of MWCNTs in Na2CO3/MgO composite PCMs. The composites with added MWCNTs still display good thermal stability with mass losses lower than 5%. Introducing MWCNTs into composite Na2CO3/MgO PCMs by material formation/calcination signifi.cantly enhances the thermal conductivity of the composite PCMs. The thermal conductivity of the composite PCMs was found to increase with an increase in the weight fraction of the added MWCNTs and an increase in the testing temperature. This study may present a promising way to prepare high temperature phase change materials with superior properties such as improved thermal stability.