A literature review on the improvement strategies of passive design for the roofing system of the modern house in a hot and humid climate region

Increase of indoor temperature compared with outdoor temperature is a major concern in modern house design. Occupants suffer from this uncomfortable condition because of overheating indoor temperature. Poor passive design causes heat to be trapped, which influences the rise in indoor temperature. The upper part, which covers the area of the roof, is the most critical part of the house that is exposed to heat caused by high solar radiation and high emissivity levels. During daytime, the roof accumulates heat, which increases the indoor temperature and affects the comfort level of the occupants. To maintain the indoor temperature within the comfort level, most house designs usually depend on mechanical means by using fans or air conditioning systems. The dependence on a mechanical ventilation system could lead to additional costs for its installation, operation, and maintenance. Thus, this study concentrates on reviews on passive design and suggests recommendations for future developments. New proposals or strategies are proposed to improve the current passive design through ventilated and cool roof systems. It is possible to achieve the comfort level inside a house throughout the day by reducing the transmitted heat into the indoor environment and eliminating the internal hot air. These recommendations could become attractive strategies in providing a comfortable indoor temperature to the occupants as well as in minimizing energy consumption.

CFD analysis and experimental comparison of novel roof tile shapes

In tiled pitched roofs,a ventilated layer reduces the heat transfer between tiles and roof structure by means of natural and forced convection,thereby also reducing the cooling energy requirement.This effect could be enhanced by increasing the air permeability between the tiles by means of novel tile shapes,as proposed by the HEROTILE European project(UFE14 CCA/TT/000939),of which this work presents the preliminary analysis supporting the new tile designs.Using an experimental rig,the air pressure difference and the volumetric flow rate between tiles have been measured for an existing Portoghese tile design over a range of pressures.Then,in older to understand the air flows under different conditions,a three-dimensional computational fluid dynamics(CFD)model has been implemented to recreate the full geometry of the rig.The model was calibrated against the aforementioned experimental results,and ran with boundary conditions simulating different wind directions.Even in the low velocities typical of average local wind patterns,the fluid dynamic problem remains complex because of the geometry of the gaps between the tiles.analytical relationship between pressure drop and flow rate,taking into account the open area.The results have shown how the wind direction affects the air permeability and,therefore,important insights have been gathered for the design of novel tiles.

CFD simulation of wind and thermal-induced ventilation flow of a roof cavity

The hygrothermal performance of a ventilated roof cavity is greatly affected by the airflow passing through it.This ventilation flow is mainly driven by the wind pressure difference between openings and the thermal-induced buoyancy.However,the wind effect is not well understood as it is often neglected in previous studies.The present study investigates the properties of such airflows,including the flow pattern,flow regime,and flow rate,using a CFD method.The target building is a large-span commercial building with a low-pitched roof.To study the wind-induced airflows,the onset atmospheric boundary layer wind flow was modelled,and the results were compared with the site-measured data recorded in the literature.To study the thermal-induced buoyancy effects,a roof cavity model found in the literature with experimental data was adopted.The findings show that the flow pattern in the roof cavity varied with the airflow driven factors.The flow separation at the windward eave inlet of the thermally induced flows are more pronounced compared with those of the wind-induced flows.Furthermore,the wind-induced airflows can generate around two times more ventilation flow rate through the roof cavity compared to the thermal-induced airflow.The findings indicate that wind-induced ventilation flows are the dominant factor of the roof cavity ventilation in a large-span,low-pitched building.

Mechanical and natural ventilation systems in a greenhouse designed using computational fluid dynamics

The aim of this study was to analyse air exchange and temperature distribution in a greenhouse with combined mechanical and natural ventilation and to design more efficient mechanical ventilation systems.For this purpose,a computational fluid dynamics(CFD)model of the greenhouse was used.Three configurations were considered:Configuration 1(mechanical ventilation and closed roof ventilators),Configurations 2 and 3(mechanical ventilation and roof ventilators open 30%and 100%,respectively).After validation,the CFD model was used to improve the design of the greenhouse mechanical ventilation system in each of the three configurations analyzed.Four greenhouse lengths,28 m,50 m,75 m and 100 m,were used in the simulations.Compared to fan ventilation only,roof ventilation improved the climate of fan-ventilated greenhouses in terms of the air exchange rate(22%)and climate uniformity because the internal air was mixed better than with mechanical ventilation only.As the greenhouse length increased,more advantages were achieved with natural ventilation compared to mechanical ventilation.For most configurations,there was a strong linear correlation between temperature gradient and greenhouse length.The greenhouse whose regression line had the steepest slope was the one with closed roof ventilators.Increasing the fan capacity produced a general reduction in temperature,but the effect was less intense for the greenhouses with open roof ventilators.Compared to box inlet ventilators,an enlarged continuous inlet in the wall opposite the fans increased overall system performance because it eliminated backflow recirculation zones,which are prone to produce high temperatures.