Natural convection in square enclosures differentially heated at sides using alumina-water nanofluids with temperature-dependent physical properties

Laminar natural convection of Al2O3 + H2O nanofluids inside square cavities differentially heated at sides is studied numerically. A computational code based on the SIMPLE-C algorithm is used for the solution of the system of the mass, momentum and energy transfer governing equations. Assuming that the nanofluid behaves like a single-phase fluid, these equations are the same as those valid for a pure fluid, provided that the thermophysical properties appearing in them are the nanofluid effective properties. The thermal conductivity and dynamic viscosity of the nanofluid are calculated by means of a couple of empirical equations based on a wide variety of experimental data reported in the literature. The other effective properties are evaluated by the conventional mixing theory. Simulations are performed for different values of the nanoparticle volume fraction in the range 0-0.06, the diameter of the suspended nanoparticles in the range 25-100 nm, the temperature of the cooled sidewall in the range 293-313 K, the temperature of the heated sidewall in the range 298-343 K, and the Rayleigh number of the base fluid in the range 103-107. All computations are executed in the hypothesis of temperature-dependent effective properties. The main result obtained is the existence of an optimal particle loading for maximum heat transfer, that is found to increase as the size of the suspended nanoparticles is decreased, and the nanofluid average temperature is increased

Definition of parameters useful to describe dynamic thermal behavior of hollow bricks

Dynamic thermal behavior of hollow bricks is attracting much interest nowadays as there is much concern on energy performance of building envelope. In fact, high thermal inertia of outer walls provides mitigation of the daily heat wave, which reduces the cooling peak load and the related energy demand. Different approaches have been used to study dynamic thermal behavior within the papers available on unsteady heat transfer through hollow bricks. Actually, the usually employed methods for calculation of unsteady heat transfer through walls are based on the hypothesis that such walls are composed by homogeneous layers, so they are not suitable for many common building components. In this framework, a study on the dynamic thermal performance of hollow bricks is brought forth in the present paper. A critical review of available data from the literature is provided. Literature data are used to propose a parameter useful to predict dynamic thermal behavior. A finite-volume method is used to solve two-dimensional unsteady thermal fields in some standard bricks with different imposed temperature solicitations, with a numerical code developed by the authors. New results are used to check the effectiveness of the proposed parameters

Natural convection of water near 4°C in a bottom-cooled enclosure

A study of natural convection in water-filled square enclosures whose bottom wall is cooled at 0°C, whereas the top wall is partially or entirely heated at a temperature ranging between 10°C and 30°C is performed numerically through a computational code based on the SIMPLE-C algorithm, assuming temperature-dependent physical properties, for cavity widths in the range 1 cm-10 cm, with the main aim to point out the basic heat and momentum transfer features

Dynamic thermal features of insulated blocks: Actual behavior and myths

The latest updates in the European directive on energy performance of buildings have introduced the fundamental “nearly zero-energy building (NZEB)” concept. Thus, a special focus needs to be addressed to the thermal performance of building envelopes, especially concerning the role played by thermal inertia in the energy requirements for cooling applications. In fact, a high thermal inertia of the outer walls results in a mitigation of the daily heat wave, which reduces the cooling peak load and the related energy demand. The common assumption that high mass means high thermal inertia typically leads to the use of high-mass blocks. Numerical and experimental studies on thermal inertia of hollow envelope components have not confirmed this general assumption, even though no systematic analysis is readily available in the open literature. Yet, the usually employed methods for the calculation of unsteady heat transfer through walls are based on the hypothesis that such walls are composed of homogeneous layers. In this framework, a study of the dynamic thermal performance of insulated blocks is brought forth in the present paper. A finite-volume method is used to solve the two-dimensional equation of conduction heat transfer, using a triangular-pulse temperature excitation to analyze the heat flux response. The effects of both the type of clay and the insulating filler are investigated and discussed at length. The results obtained show that the wall front mass is not the basic independent variable, since clay and insulating filler thermal diffusivities are more important controlling parameters

Deconstructing the interconnectedness of community: An exploratory study on skill shortages, labour migration, and mining booms in Western Australia

In this exploratory study the experiences of employers, migrant workers, and stakeholders are captured in the context of the mining boom in the Mid West region, Western Australia. Framed in a community psychology paradigm, the findings identified four overarching themes which contribute to and perpetuate the skill shortage and xploitation of migrant workers. These findings illuminate the lack of awareness of the interconnectedness of the phenomenon, and the functioning of power embedded in these relationships

A Study of Attitudinal Responses of Selected School District Personnel and Related Others Regarding Six Performance Contracting Hypotheses in Seventeen Selected Performance Contracting Projects in the United States for the Year 1970-71

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Thermal inertia of hollow wall blocks: actual behavior and myths

In the context of growing requirements to save energy in buildings and high objectives for Net Zero Energy Buildings (NZEBs) in Europe, strong emphasis is placed on the thermal performance of building envelopes, and in particular on thermal inertia to save cooling energy. High thermal inertia of outer walls leads to a mitigation of the daily heat wave, reducing cooling peak load and energy demand. Moreover, building envelopes with high heat capacity act as heat storages, increasing the effectiveness of natural ventilation for thermal comfort through a night-day energy shifting. Even though there are some papers available in the open literature on dynamic heat transfer through hollow bricks, yet common calculation methods are applicable to homogeneous layers only. That is the case of ISO 13786 regulation "Thermal performance of building components - Dynamic thermal characteristics - Calculation methods", for example. On the other hand, hollow blocks are very commonly used in building envelopes. Thus, available methods are not suitable for prediction of dynamic thermal performances. On the other hand, the widely common assumption that high mass means high thermal inertia leads to the use of higher mass blocks or bricks. Yet, numerical and experimental studies on thermal inertia of hollow envelope-components have not confirmed this general assumption, even though no systematic analysis has been found in the open literature. In this framework, numerical simulations of the thermal performance of hollow bricks have been done with a specifically-developed finite-difference computational code. Three common basic shapes with different void fraction and thermal properties have been analyzed with a triangular pulse solicitation, in order to highlight the relevance of front mass and other parameters on the thermal inertia, measured through heat wave delay. Results show that wall front mass is often misleading as thickness, number of cavities and clay thermal diffusivity are more important

Effect of thermal diffusivity of insulating materials on room free-float temperature with façade external insulation

External insulation of building façade is widely used to reduce heating energy demand in buildings. Usually, its design concerns only thermal transmittance, while transient thermal behaviour is commonly addressed only as dumping factor or time lag of outdoor heat wave. During summer, in many mild climates, outdoor daily mean temperature is close to comfort temperature. Yet, even though mean heat transfer through building envelope is null, heating during daytime may lead to positive cooling loads or discomfort temperatures in non-conditioned rooms. In residential buildings internal loads are usually very low, so the most relevant loads are heat transfer though outer facades. Moreover, where there is no cooling, wall dumping factor is not meaningful to evaluate the thermal performance of wall insulation, as it is referred to constant indoor temperature. In this framework, a model of a room with a single outer wall has been developed to study the effect of insulating material on free-float temperature. Transient heat transfer through the envelope as well as through inner walls is considered to model indoor air temperature. Different localities in Italy and commonly used insulating materials are considered