Roof Cavity Research Articles

Numerical simulation of natural ventilation of a factory roof cavity

The study targets the reduction of roof solar heat gain through the use of natural ventilation in a cavity of a factory roof. In the laboratory experiment [1], the average air velocity reached 0.25 m/s. A simulation program was developed to calculate the heat and air flow attained in the experiment. An airflow passage was divided into sections to trace the pattern of the air temperature rise. When the cavity was divided into 20 sections, it was enough to trace the temperature rise pattern, and hence to calculate buoyancy for natural ventilation. Then the simulated air velocities, temperatures and heat transportations were compared with the experimental results. The molecular viscosity and thermal conductivity of the air were modified to adjust the simulation results to the experimental results in a wide range of experimental conditions. When they were multiplied with a magnitude of 30 equally, the least root mean square of the ratio of deviations of the heat transportation was obtained. This simulation could predict the heat transportation as a result of natural ventilation with a root mean square of the deviation of 0.25 in a short calculation time.

Numerical Simulation of Natural Ventilation of a Factory Roof Cavity

Numerical Simulation of Natural Ventilation of a Factory Roof Cavity

A laboratory experiment on natural ventilation through a roof cavity for reduction of solar heat gain

The study targets the reduction of roof solar heat gain through the use of natural ventilation in a roof cavity. This study is mainly concerned with factory buildings. Experimental outcomes were obtained from an inclined cavity model which was heated on the upper surface to mimic solar radiation on a roof. The dimensions of the cavity were 4882 mm× 400 mm × 78 mm. The two opposing smallest sides were allotted as the inlet and outlet, and narrowed to simulate resistance of the air flow in practical applications. Temperature and velocity measuring facilities were prepared in the experimental model. A number of measurements were carried out by varying the combinations of different heat production, inclination angles, and opening ratios. It was found that resistance to heat and air flow in the cavity was strongly influenced by the opening size. When the Reynolds number was examined, it showed that the flow belonged to the laminar region. The average velocity reached to 0.25 m/s at the highest in the examined cases. In other words, the cavity air was turned over 184 times in an hour. Natural ventilation in the roof cavity seemed to be effectively applicable to solar incidence discharges in factory buildings.

Conjugate natural convection in the roof cavity of heavy construction building during summer

This paper presents the results of a study of conjugate natural convection inside a building attic in the shape of a rectangular enclosure bounded by realistic walls made from composite construction materials under summer day boundary conditions. The effects of cavity aspect ratio, Rayleigh number ( Ra), and orientation of the external surfaces on the flow and heat transfer characteristics were the main focus of the investigation. The problem was formulated in terms of the vorticity-stream function procedure, and the governing equations for steady, laminar, two-dimensional conjugate natural convection heat transfer were solved by employing the Alternating Direction Implicit (ADI) control volume method along with under-relaxation factors for temperature, vorticity, and stream functions. For Ra ranging from 10 3 to 10 10, steady state results of the streamline and temperature contours in addition to local and mean Nusselt number at all surfaces of the cavity were obtained. The results show that the values of Ra and the aspect ratio have significant effect on the temperature and stream function contours within the enclosure. Another important finding of the study is that heat flux into the room increases with the increase of both the aspect ratio and Rayleigh number.

Life under a sandstone overhang: the ecology of the eastern cave bat Vespadelus troughtoni in northern New South Wales.

We studied the eastern cave bat (Vespadelus troughtoni) in northern New South Wales to provide ecological information on roosts and foraging. Radio-tracking of five bats was supplemented with opportunistic visits to roosts over five years. One male was radio-tracked and its roost was within a corrugated iron roof cavity of a dairy. Maternity roosts were located in the overhangs of large sandstone caves usually containing a dome at the rear. Searches of nearby cliff-lines found that small caves, crevices and overhangs were not used as day-roosts. Nor did any radio-tagged bat roost in tree hollows. Colony size of one tight roosting cluster was estimated as 240 individuals. Radio-tagged bats frequently switched roosts, while opportunistic visits over a five-year period often found caves unoccupied. Most movements between roosts were within 1.5 km, although one female moved about 3.75 km between roosts. One radiotagged female shifted roost to beneath the corrugated iron of a farm-shed that supported at least 50 bats, predominantly females with young. Observations of foraging radio-tagged and lighttagged bats were frequent along a stream lined with trees, but surrounded by cleared paddocks. Foraging was observed in the air space above the creek, interspersed with occasional rapid flights across paddocks (> 500 m). We suggest that a key requirement for V. troughtoni in the rural landscape is the presence of native vegetation in close proximity to roosts, although extensive forested areas may not be required. Education of local communities about the sensitivity of these bats at cave roosts would be an important contribution to their conservation.

An evaluation of highly insulated cold zinc roofs in a moderate humid region—part I: hygrothermal performance

In order to investigate the hygrothermal performance and durability of well-insulated cold zinc roofs, a total of four cold zinc roofs with a low U-factor [ U<0.25 W/(m 2·K)] were constructed in a test building at the KU Leuven, Belgium. This paper presents the hygrothermal performances measured in the four roofs during an exposure period of 28 months. The zinc roofs with a specific design and careful workmanship exhibited a satisfactory thermal performance. The moisture content in the wood deck generally stayed below 20% by weight. However, the moisture situation at the underside of zinc sheeting was bad in all four roofs studied. We found that the strong undercooling of the zinc sheeting was the main cause for the bad moisture situation observed. Even a combination of a low air exfiltration and a good roof cavity ventilation rate did not eliminate or reduce the condensation at the underside of zinc sheeting.

An evaluation of highly insulated cold zinc roofs in a moderate humid region—part II: corrosion behavior of zinc sheeting

In a previous paper [Zheng et al., Hygrothermal Performance, Part I; Const Building Mater, Vol. 18, Issue 1, pp. 49–59], the hygrothermal performances of four well-insulated cold zinc roofs were evaluated. This paper investigates the corrosion behavior of the zinc sheeting used in the roofs. The main corrosion products identified are Zn 5(CO 3) 2(OH) 6 and ZnO. Little corrosion occurred at the upside of the zinc sheeting. However, serious pitting corrosion developed at the underside. The maximum pit depths measured in the collected samples with an area of 0.35 m 2 were between 167 and 273 μm. The traditional measures to minimize condensation deposit, i.e. low air exfiltration and good roof cavity ventilation, clearly did not work effectively in reducing the underside corrosion of the zinc sheeting. The results show that new techniques and roofing systems should be developed and implemented.

Analytical study of summer/winter houses

In this present communication, an analytical model has been evolved for passive cooling and heating of a building. An analytical expression for room temperature has been derived for different roof treatments, namely, air flow through the cavity of the roof, water flowing over the roof and air and water both flowing in the roof cavity and over the roof, respectively, in the same direction. The effect of ventilation and infiltration is also included. It is found that the roof treatment with both air and water flow and a large isothermal/air mass within the room gives the best load levelling and room thermal comfort temperature.

On transport in turbulent wakes: methane wakes

An approximate treatment is given for turbulent diffusion of buoyant fluids in the wake of a falling body. The treatment is developed in detail for the case when a body falls through a uniform region of one fluid, across an interface, into a uniform region of a second fluid which is denser than and freely miscible with the first. This problem has relevance to the ignition of methane in coal mines, where a stone might fall from the top of a roof cavity filled with methane and draw down behind it an inflammable column into the air below: hence the name methane wake. A model theory is developed from very simple mixing assumptions which have already proved extremely useful in the investigation of buoyant plumes. The resulting equations can be solved without difficulty for the case of forced convection when buoyancy forces are negligible, and this solution is used to show that once the body has passed through the interface, upper fluid is transported down in the wake through only apart of the distance between interface and body. Thus there is a clearly defined mixture front in the wake separating regions of relatively low and relatively high concentration of the upper fluid. A numerical solution is necessary in the case where buoyancy forces are significant, and this mixed convection solution is developed to show that in suitable circumstances the region above the mixture front may divide because of buoyancy, and the upper parts return towards the interface. Some preliminary experiments are described and the results are shown to be in moderate agreement with the theoretical predictions.