Mohammed S. Imbabi

Physical and numerical modelling of a solar chimney-based ventilation system for buildings

Abstract This paper describes an experimental and numerical study to analyse the thermal performance of a bio-climatic building prototype in Nigeria. The roof performs as a solar chimney, generating an air flow through the living space of the building to provide cooling. Experimental tests on a 1:12 small-scale model of the prototype are outlined, and the results, bith qualitative and quatitative, are used to validate a two-dimensional flow simulation model, in which the steady state conservation equations of mass, momentum and thermal energy are solved using a finite volume formulation. The experimental and numerical results, expressed in terms of temperature and velocity fields, for two different window geometries are critically evaluated and compared with good agreement.

Analytical investigation of the steady-state behaviour of dynamic and diffusive building envelopes

Abstract A one-dimensional model describing the steady-state dynamic and diffusive behaviour of a three-layer building envelope element is developed with the objective of elucidating the physics of simultaneous heat and vapour transport through dynamically insulated building envelopes. The equations are simple to program on a spreadsheet, enabling architects to build tools which will allow them to design “breathing” envelope constructions. The variables at the designers disposal are air and vapour permeability, thermal conductivity and thickness of the layers comprising the envelope. Users wishing to consider more complicated constructions will find that they can readily extend, by inspection, the equations from the three layers presented here to any number of layers. Whilst mass transfer has been discussed in terms of water vapour transport, the equations are very general and can be applied to the transport of any gas through a permeable wall.

The building envelope as an air filter

Recent research suggests that fine-particulate air pollution increases the incidence of lung disease and premature death. In this paper, single fibre filter theory was used to predict the theoretical particulate collection efficiency of air permeable walls (dynamic insulation). The relationship between particle diameter and filtration efficiency for dynamic insulation, as a function of flow rate, is examined and compared to that for a conventional filter. Factors such as filter penetration as a function of flow rate, filter thickness, and packing density for a range of particle diameters are also presented. The findings suggest that, in addition to reducing heat loss through the building fabric, dynamic insulation can act as a high performance air filter in naturally ventilated buildings, thus providing a viable and attractive alternative to mechanical air-conditioning in congested urban environments.

Edge effects in the failure of elastic/viscoelastic joints subjected to surface tractions

Abstract The stress and displacement solutions are obtained for an elastic/viscoelastic joint subjected to a surface traction in the vicinity of an interface corner using elastic–viscoelastic correspondence principles and existing corresponding solutions for elastic/elastic joints. The intensity of the resulting stress singularity is determined by a combination of asymptotic analysis and the finite element method. A quasi-static assumption is used to investigate the effects of sliding and rolling contact loads near the interface corner on failure initiation. The results suggest the interface may experience stress reversal as the contact load (normal or shear) moves from one side of an interface corner to the other, leading to the possibility of fatigue failure. Further a relaxation or an increase of the interfacial stresses occurs depending on whether the edge load near the interface corner is on the elastic or viscoelastic side of the joint. The implications of the results in predicting the deformation and failure of asphalt concrete used in highway bridges are discussed.

Sequestering CO2 by Mineralization into Useful Nesquehonite-Based Products

The precipitation of magnesium hydroxy-carbonate hydrates has been suggested as a route to sequester CO2 into solids. We report the development of self-cementing compositions based on nesquehonite, MgCO3·3H2O, that are made from CO2-containing gas streams, the CO2 being separated from other gases by its high solubility in alkaline water, while magnesium is typically provided by waste desalination brines. Precipitation conditions are adjusted to optimize the formation of nesquehonite and the crystalline solid can readily be washed free of chloride. Products can be prepared to achieve self-cementation following two routes: (i) thermal activation of the nesquehonite then rehydration of the precursor or (ii) direct curing of a slurry of nesquehonite. The products thus obtained contain ~ 30 wt% CO2 and could form the basis for a new generation of lightweight, thermally insulating boards, blocks and panels, with sufficient strength for general construction.

Dynamic deformation signatures in reinforced concrete slabs for condition monitoring

Abstract We examine the use of ordinary steel and concrete strain gauges to monitor the dynamic response of reinforced concrete (RC) slabs excited in non-destructive vibrational testing. Plots of measured dynamic strains in the embedded steel reinforcement bars and on the concrete surface, as well as of mid-span deflection, are presented at different levels of load-induced damage. The plots show unique strain and deflection signatures that vary with the internal state of the slab, and could be used for condition monitoring and residual strength identification. It is feasible that the techniques outlined could be used in AI-based evaluation tools for RC slabs and other RC structures.

Effects of geometry on the deformation of asphaltic plug joints subjected to surface tractions

Asphaltic plug joints are widely used for accommodating structural movement in motorway bridges. In spite of their simplicity and low cost, these joints suffer premature failure, of which debonding at the asphaltic plug/pavement interface is an important mode. In this paper, fundamental studies of the debonding are carried out by means of detailed stress analysis. The asphaltic plug is assumed to be viscoelastic while the pavement is assumed to be linear elastic. Stress solutions are obtained using the finite element method for a range of joint angles between the asphaltic plug and the pavement. The traffic loads are simulated using surface tractions applied in the vicinity of an interface corner on the wearing surface. The results show that the interface may experience stress reversal as the surface traction moves from one side of the interface corner to the other. Further, the magnitudes of the interfacial stresses decrease as the joint angle is increased from the current value of 0° to about 45°, thereby reducing the likelihood of debonding.

The influence of variable gypsum and water content on the strength and hydration of a belite-calcium sulphoaluminate cement

ABSTRACT The relative impact and interaction of two components, gypsum and water, in high ye’elimite containing calcium sulphoaluminate-belite cement pastes at an age of 7 days was studied to find the optimum conditions for compressive strength. Using statistical analysis of 149 compressive strength results having a range of water and gypsum contents, the statistical significance of the variables was investigated. X-ray diffraction analysis demonstrated the different hydration processes occurring within samples based on water and gypsum contents, while mercury intrusion porosimetry comparison confirmed the importance of pore structure towards determining ultimate paste strength. Thermodynamic modelling was also used to contribute to the understanding of the microstructural development. The compressive strengths of cement pastes with varying gypsum and water content were shown to be influenced by both factors, with a non-linear dependence on gypsum content observed.

Dynamic insulation in multistorey buildings

Dynamic insulation permits the movement of air and moisture through the external walls of a building to reduce heat loss and achieve high indoor air quality. The present paper details a pilot study carried out to examine the influence of fire safety requirements and external wind on the performance of naturally ventilated multistorey buildings in which the external envelope is dynamically insulated. The theoretical foundation is outlined for a spreadsheet model used to simulate prototype 3-, 4-, 5- and 10-storey buildings all sharing the same rectangular floor plan, with fresh air drawn into the building through the envelope by depressurisation using a fan-driven, ducted extract system. From the analysis, the effects of wall porosity, depressurisation level, extract system deployment, occupant density and distribution, and building orientation have been quantified, confirming the practical feasibility of such a system.

Scale modelling of the built environment

Abstract Dimensional analysis was applied to the design of a small-scale model building. The model was used to simulate the thermal response of a building and two-stage heating, ventilation, and airconditioning (HVAC) system, operating under automatic control. Temperature variation, in the course of a series of heating and cooling cycles, was monitored by computer. The observed thermal response of the model was found to be fairly realistic. The modelling technique is currently being refined, and will include the effects of solar gains, humidity, and air movement. The method should offer an inexpensive and reliable means for evaluating the impact of new building materials and technologies on the built environment.