Optimum distribution of concrete in a double hollow concrete slab

In this paper we have investigated the optimum distribution of concrete thickness (keeping the total thickness constant) in a double hollow concret slab for achieving the best load levelling of thermal flux; it is seen that for a given total thickness of concrete, this is achieved when the thicknesses of the outer and inner layers of the concrete are least (consistent with structural considerations).     

Periodic heat flux through a three layered slab

This communication presents a derivation of an explicit expression for the periodic variation of thermal flux through a composite slab (insulation-concrete-insulation or concrete-insulation-concrete), with one face exposed to solar radiation and atmospheric air and the other face in contact with room air at constant temperature. It is seen that for maximum or minimum load levelling of the periodic heat flux the thicknesses of the outer layers of the composite slab are equal.     

Numerical modeling of conjugate heat transfer through concrete hollow bricks

The purpose of this study is to determine the thermal conductance of concrete hollow bricks, which is necessary for the evaluation of the energy efficiency of a building. The three varieties of hollow concrete bricks that are often used to build walls in Morocco are the subject of this study. A computational model created using the finite volume method is used to evaluate the conjugate heat transfer through concrete hollow bricks. According to the results, the use of hollow brick type A_h3 reduces the heat flux by approximately 86% compared with type A_h1. It is undeniable that hollow bricks type A_h3 with a thermal conductivity of 1 W/m K can improve the thermal characteristics of building walls.     

Periodic heat transfer through inhomogeneous media part 3: Hollow sphere

The paper presents the exact analytical solutions for periodic radial heat conduction through an inhomogeneous hollow sphere for a certain class of thermal-conductivity profile. The exact analytical solutions for some of these profiles, (including linear and quadratic) have been compared with those obtained by considering the spherical medium to be made up of a number of homogeneous layers with different thermal conductivities, varying from layer to layer, and using the layered-structure (or matrix-multiplication) method. The numerical results arrived at by the layered-structure method converge rapidly (with increasing number of layers considered) to the values obtained from the exact analytical solutions. It strengthens the confidence in applying the layered-structure method to the case of periodic heat conduction through an inhomogeneous hollow cylinder. Considering the inhomogeneous conducting medium to be made up of a number of spherical layers with a linear profile of thermal conductivity has been shown to be a more effective alternative method of accounting for any type of inhomogeniety; and it saves computation time, as the rate of convergence is much higher than for the homogeneous-layered structure method. The numerical results have been presented in the form of elements of a 2 × 2 matrix, relating the sinusoidal steady-state temperature and heat flux of the two surfaces of the hollow sphere.     

Periodic heat transfer through inhomogeneous media part 2: Hollow cylinder

The paper presents exact analytical solutions for periodic radial heat conduction through an inhomogeneous hollow circular cylinder for a certain class of thermal conductivity profile. The exact analytical solutions for some of these profiles (including linear and quadratic) are compared with those obtained by considering the cylindrical medium to be made up of a number of homogeneous layers with different thermal conductivities, varying from layer to layer, and using the layered-structure (or matrix-multiplication) method. The numerical results arrived at by the layered-structure method converge rapidly (with increasing number of layers considered) to the values obtained from the exact analytical solutions. This gives confidence in the application of the layered-structure method to periodic heat conduction through an inhomogeneous hollow cylinder. Assuming the inhomogeneous hollow cylinder to be made up of a number of cylindrical layers with a linear profile of thermal conductivity has also been shown to be a more effective alternative method of considering any type of inhomogeniety; it saves computation time, as the rate of converegence is much higher than for the homogeneous-layer structure method. Numerical results are presented in the form of elements of a 2 × 2 matrix, relating the sinusoidal steady-state temperature and the heat flux on the two sides of the cylinder.     

Heat,air and moisture transfer through hollow porous blocks

The combined heat, air and moisture transfer in building hollow elements is of paramount importance in the construction area for accurate energy consumption prediction, thermal comfort evaluation, moisture growth risk assessment and material deterioration analysis. In this way, a mathematical model considering the combined two-dimensional heat, air and moisture transport through unsaturated building hollow bricks is presented. In the brick porous domain, the differential governing equations are based on driving potentials of temperature, moist air pressure and water vapor pressure gradients, while, in the air domain, a lumped approach is considered for modeling the heat and mass transfer through the brick cavity. The discretized algebraic equations are solved using the MTDMA (MultiTriDiagonal-Matrix Algorithm) for the three driving potentials. Comparisons in terms of heat and vapor fluxes at the internal boundary are presented for hollow, massive and insulating brick blocks. Despite most of building energy simulation codes disregard the moisture effect and the transport multidimensional nature, results show those hypotheses may cause great discrepancy on the prediction of hygrothermal building performance.     

Heating and cooling of a building by double hollow concrete slab

This paper presents the thermal performance, in the heating and cooling of a building, of a double hollow concrete slab, one of whose faces is exposed to solar radiation and ambient air while the other is in contact with room air at constant temperature. A blackwened network of pipes is laid on the top surface and glazed sutiably. the flow rate of water / air through pipes is kept constant. It is seen that there is a time difference of 10-12 h between the maximum/ minimum of the thermal flux extering the room and the solair temperature for any flow rate. the heat flux inside the room is reduced appreciably for higher infiltration when there is no water flow to heat the building. the effect of a water film on the performance of the wall/roof has also been discussed and found to be more effective for the reduction of the heat flux coming into the building.     

Periodic heat transfer through inhomogeneous media. Part 1. Slab

The paper presents exact analytical solutions of one-dimensional periodic heat conduction through an inhomogeneous slab for a certain class of thermal conductivity profiles (including linear and exponential). The exact analytical solutions for some of these profiles have been compared with those obtained by considering the slab to be made up of a number of homogeneous layers with different thermal conductivities varying from layer to layer and using the layered structure (or matrix multiplication) method. The numerical results arrived at by the layered-structure method converge rapidly (with increasing number of layers considered) to the values obtained from the exact analytical solutions. This gives confidence in the application of the layered-structure method to periodic heat conduction through inhomogeneous slabs. The numerical results have been presented in the form of elements of a 2 × 2 matrix, relating the sinusoidal steady-state temperature and heat flux on the two sides of the slab.     

Enhanced reduction of heat flux through a roof by a water film and air gap combination

In this paper we have investigated the reduction of heat flux through a roof using a water film with air gap in the structure. It is seen that by using this combination, the heat flux coming through the roof can be reduced by 75 per cent. the reduction in heat flux is found to increase with decreasing humidity.     

Optimum tip gap and orientation of multi-piezofan for heat transfer enhancement of finned heat sink in microelectronic cooling

Piezoelectric fans can be manipulated to generate airflow for cooling microelectronic devices. Their outstanding features include noise-free operation, low power consumption and suitability for confined spaces. This paper presents experimental optimization of tip gap and orientation angle of three piezoelectric fans (multi-piezofan) to maximize the heat removal performance of finned heat sink for microelectronic cooling. Design of experiments (DOE) approach is used for the optimization, and a three dimensional simulation using FLUENT 6.3.2 is carried out to better understand the flow induced by the multi-piezofan and the resulting heat transfer from the heat sink surface. For the optimization, the Central Composite Design (CCD) of response surface methodology (RSM) is exploited from the Design Expert software. In the numerical model, the flow induced by the piezofan is treated as incompressible and turbulent; the turbulence is taken care by the shear stress transport (SST) k–ω model. The experimental results are found to be in good agreement with the predictions. Out of 13 experimental trials determined by CCD, the optimum tip gap and fan orientation are found to be δ = 0.17 and 90° respectively. At this condition, an enhancement in convective heat transfer coefficient exceeding 88% is achieved, compared to natural convection.     

Coupled heat and mass transfer in a counter flow hollow fiber membrane module for air humidification

Hollow fiber membrane based air humidification offers great advantages over the traditional methods because the liquid water droplets are prevented from mixing with the process air, while water vapor can permeate through the membranes effectively. The novelty in this research is that the coupled heat and moisture transport in a hollow fiber membrane module for air humidification is investigated, both numerically and experimentally. The air stream and the water stream flow in a counter flow arrangement. It is found that the membranes play a key role in humidification performances. For sensible heat transfer, both the liquid side and the membrane side resistance can be neglected, while the total heat transfer coefficients are determined by the air side heat transfer coefficients. In contrast, in mass transfer, only the liquid side resistance can be neglected, while the total mass transfer coefficients are co-determined by membrane properties and the air side convective mass transfer coefficients.     

Periodic homogenization for heat,air, and moisture transfer of porous building materials

ABSTRACT

In this work, a macroscopic model of hygrothermal transfers in porous building materials was developed, using periodic homogenization, where the air infiltration was added to the classical mass and energy balance equations written at the microscopic scale. The corresponding infiltration, hygric, and thermal input parameters were carefully identified. Numerical calculations of thermal and diffusion tensors were performed on a representative concrete elementary cell. Further, the diffusion tensor was compared to the equivalent experimental results available in the literature, and its sensitivity to the water content variations and porosity has been evaluated on the concerned elementary cell.     

太阳能空气隙膜蒸馏系统实验研究

空气隙膜蒸馏是一种新的膜分离技术,它可以使用太阳能等低位热能作驱动力,用于海水、苦咸水的淡化处理,在淡化水制备方面具有极强的竞争力.文章介绍的空气隙膜蒸馏实验装置,采用太阳集热装置来加热进液,通过膜蒸馏获得纯净水.结合实验数据,文章给出了料液进口温度、流量、空气隙厚度等参数对于膜通量的影响和空气隙膜蒸馏系统热容腔内对流换热传热系数的经验公式.     

Turbulent heat and mass transfer through air curtains devices for the confinement of heat inside tunnels

The influence of flow instabilities on the efficiency of a twin-jet air curtain formed by a cold stream parallel to a hot stream was studied. Two cases were considered: the Reynolds number and mean flow velocity of both streams were set to 1000 and 3 m/s, respectively. The results obtained from 3D LES simulations were compared with experimental data and earlier LES numerical results using the FDS code. Energy spectra exhibit decay laws with − 5/3 and − 3 slopes. Instabilities with a characteristic Strouhal number of about 0.4 were detected. Kelvin-Helmoltz type instabilities play a more significant role than the Görtler type instabilities present at the wall for applications of plane impinging jets to ambience separation.     

Conjugate heat and mass transfer in a cross-flow hollow fiber membrane contactor for liquid desiccant air dehumidification

The fluid flow and conjugate heat and mass transfer in a cross-flow hollow fiber membrane contactor are investigated. The shell-and-tube like contactor is used for liquid desiccant air dehumidification, where numerous fibers are packed into the shell and air flows across the fiber bank. To overcome the difficulties in the direct modeling of the whole contactor, a representative cell, which comprises of a single fiber, a liquid solution inside the fiber, and an air stream across the fiber, is selected as the calculation domain. The air stream in the cell is surrounded by an assumed outer free surface. The equations governing the fluid flow and heat and mass transfer in the two cross-flow streams are solved together with the heat and mass diffusion equations in the membrane. The friction factor and the Nusselt and Sherwood numbers on the air and stream sides are then calculated and experimentally validated.     

Analytical modelling of heat transfer from a single slab in freezing

The present study deals with the experimental and theoretical analysis of the transient heat transfer from an individual slab body to the medium during freezing. In this respect, a new model was developed to determine the heat transfer coefficient and this heat transfer coefficient was used in the computation of the dimensionless theoretical temperature distribution. On the other hand, experimental work was performed to measure the centre temperatures of the individual slab products (viz. dried figs) during freezing at the median temperature of -22°C. In the comparison of the theoretical and experimental temperature distributions, very good agreement was found.     

Conjugate conduction-natural convection heat transfer in a hollow building block

Conjugate heat transfer across a hollow block is investigated numerically. Conduction heat transfer in the block material and natural convection in the cavity are considered. Results show that increasing the number of cavities while keeping the block width constant decreases the heat loss (increases the R-value) significantly. A maximum number of six cavities can fit the building block without compromising the strength. With this number of cavities, no insulation would be needed to fill the cavities as a result of the reduced effect of natural convection. This study may provide guidelines for engineers toward better design and selection of building materials for higher thermal resistance.     

Effect of side walls on heat transfer through a vertical air layer in laminar natural convection

The paper presents the results of numerical investigation of local and average heat transfer of a vertically bounded air layer, one of the side boundaries of which has finite thickness and thermal conductivity. The effect of the ratio of the wall to air thermal resistances on heat transfer is discussed. The investigation was carried out over the following range of parameters: Pr = 0.72; Gr < 10⁷; 1 h 10; 0 α 0.14.     

Numerical and Experimental Modelling of Gas Flow and Heat Transfer in the Air Gap of an Electric Machine

This work deals with the cooling of high-speed electric machines, such as motors and generators, through an air gap. It consists of numerical and experimental modeling of gas flow and heat transfer in an annular channel. Velocity and temperature profiles are modeled in the air gap of a high-speed test machine. Friction and heat transfer coefficients are presented in a large velocity range. The goals are reached acceptably using numerical and experimental research. The velocity field by the numerical method does not match in every respect the estimated flow mode. The absence of secondary Taylor vortices is evident when using time averaged numerical simulation.