Ahmad Sharifian

The Effect of the Width and Number of Gaps on the Characteristics of Swirl Flow Induced Naturally inside Split Channel Using Hot Air Inlet

Split channels are commonly employed in research laboratories to generate fire whirls. The purpose of this computational study is to investigate the effects of the width and number of side gaps on the performance of the split channel when hot air enters at the base of the channel instead of using a fire. Three cylindrical channels are modeled. The first channel includes two identical slots, the second channel has only one gap with a width double that of the first channel, and the third channel comprises one gap identical to that of the first channel. Comparisons between the results of the first and the second channel provide the assessment of the impacts of the number of gaps on the performance of the split channel. Moreover, the comparison between the results of the second and third channels shows the effect of changing the width of the gap. The combined (overall) effects of the number and the width of the gaps on the characteristics of the flow within the channel are also evaluated by comparing the results of the first and the third channel. The results show that increasing the number of the slots without increasing the total area of the gaps leads to an increase in centerline vorticity within the channel. In addition, the inlet velocity of hot air from the base and entrainment ratio decreases with the increase of the number of the side slots. Increasing the width of the gap without changing the number of the slots reduces the entrainment velocity, the inlet velocity, but the entrainment mass flow rate and centerline vorticity increase. The entrainment mass flow rate centerline vorticity and entrainment ratio increases by the increase in the number of identical gaps and the increase of gap width whereas the entrainment velocity and the inlet velocity decrease.

Double-layered Metal Mesh Screens to Contain or Exclude Thermal Radiation from Bush Fires

A computational technique is used to establish a view factor correlation for the target-object — screen — bushfire arrangement for aligned, double-layered screens consisting of woven wire, square-celled elements. The results indicate that the average view factor between an infinite fire and a target object of the same size as the square cell is related to the porosity of screens. The spacing of two layers only affects the view factor if two screen layers are very close. The results also show that it is possible to block up to about 98% of the direct radiant heat flux by using a low porosity (25%) double-layered screen.

A novel ember shower simulator for assessing performance of low porosity screens at high wind speeds against firebrand attacks

Previous studies have shown the effectiveness of low porosity and double-layer screens against radiant heat flux from fires. The performance of screens against firebrand attack is also required to be assessed prior to making a decision on their possible application in wildland fire prone areas. The available laboratory-scale devices simulate firebrands at wind speeds less than those of severe firebrand attacks. A relatively low cost, laboratory-scale fire ember shower simulator has been designed and manufactured in-house. The simulator is able to assess the performance of low porosity screens at high wind speeds without any reverse flow and possible consequent safety hazards and offers reasonable control over the size and mass of the generated firebrands.

Impacts of Room Temperature on the Performance of a Portable Propane Air Conditioner

The performance of a portable propane air conditioner system, in which the temperatures of the air passing over the condenser and evaporator are equal, has been experimentally investigated under different room temperatures and refrigerant charge levels. The research has been carried out in a range of room temperatures from 20°C to 35°C and in undercharge, standard charge and overcharge conditions. The results show that, at higher room temperatures, the refrigerant temperature in all parts of the system, the density of the refrigerant at the inlet and outlet of the condenser, mass of the refrigerant in the compressor, the mass flow rate of the refrigerant and the cooling capacity of the system in either the undercharge or full charge condition, the specific cooling capacity of the undercharge system, the useful work of the compressor, and the maximum pressure of the refrigerant increase. The increase in room temperature decreases the density of the refrigerant at the inlet and outlet of the capillary tube, the mass of the refrigerant in the capillary tube, the refrigerant subcooling at the inlet of the capillary tube, the maximum velocity of the refrigerant and the coefficient of performance. In addition, the increase in room temperature at overcharge condition causes an increase in the mass flow rate, cooling capacity and specific cooling capacity to a maximum value followed by their decrease. The most important difference between a portable air conditioner and a nonportable system is the increase in cooling capacity with an increase in room temperature in full charge condition.

Experimental Measurement of Direct Thermal Radiation Through Single-Layer Square-Cell Plain Woven Screens

Australian bushfires have killed many people and caused severe damage; like as any other type of disaster. Previous studies have identified direct flame contact and radiant heat as the main cause of fatalities.The role of screens in limiting the radiation exposure on an object by fire on the other side of the screen is well-known, but still it has not been experimentally determined. A screen between a radiation source and an object divides the radiant heat flux into two parts. The first part is direct radiant heat flux that passes directly through the screen without any interaction. The second part is indirect radiation heat flux, which includes both emitted and reflected radiant heat fluxes by the heated screen. This experimental study deals with the direct radiant heat flux, which depends on the screen porosity and is independent of the material composition or the surface quality of the screen. The experimental results of four square cell,plain woven screens with porosities ranging from 41% to 66% show that the passing ratios of direct radiant heat flux through screens are less than those suggested by their porosity. Four empirical equations have been developed to determine the passing ratio of the direct radiation through screens, and tunnel vision angles of the screens.

Wind tunnel experiments on effects of woven wire screens and buffer zones in mitigating risks associated with firebrand showers

Abstract The benefits of using wire screens to mitigate wildfire spread are well known and many standards mandate their use in wildland fire prone areas. However, some recent studies show that firebrands can penetrate through screens. The penetrated firebrands are smaller in size, have less potential to ignite fuel beds and have a shorter burn-out time. This leads to the idea that a combination of a buffer zone and screens might be able to serve as a wildfire barrier. This study aims to explore the effectiveness of such an approach for two screens with a relatively short buffer zone. The results show that the combination is very effective in reducing firebrand intensity but falls short of full protection. The study suggests the combination of screens and a wider buffer zone is a promising approach to mitigate the spread of wildfires.

Effective factors on the performance of woven wire screens against leaf firebrand attacks

Firebrand spotting is the dominant wildfire propagation mechanism. The use of wire screens to prevent firebrand entry into structures is recommended or mandated by many standards. The existing standards address only one feature of screens which is opening size. This experimental study aims to explore several other factors that may influence the performance of wire screens and may need to be incorporated in codes and standards development process for structures located in wildfire-prone areas. The results demonstrate a previously unreported shattering mechanism in the case of Eucalyptus populnea (from the Myrtle family) leaf firebrands under a moderate wind speed of 14.5 m s−1. The results also show that screen porosity, screen type and wind speed would be appropriate to be addressed in developing standards, but there is no need for concern about screen orientation as long as the screen is placed perpendicular to the wind direction during the experiment.

Optimizing the Coupling of a Firebrand Generator to a Horizontal Wind Tunnel

Australia is considered as the most fire-prone country in the world. Spotting ignition by lofted firebrands is the main mechanism of fire spread. Many experimental studies have been conducted to evaluate the effect of the firebrand attacks on structures and to identify possible solutions. The experimental facility consists of a firebrand generator coupled to a wind tunnel. The wind speed in the firebrand generator is relatively low, in order to assure a quality continuous flow of glowing firebrands. On the contrary, the wind speed in the wind tunnel is high to duplicate actual firebrand attacks. Previous works show a highly turbulent region above the entrance of firebrands to the wind tunnel which is formed because of the velocity difference and penetration of firebrand entrance hose into the wind tunnel. The penetration is required to provide a uniform firebrand distribution along the height of the test section. In this computational work, the influence of the height of the entrance hose, its orientation respect to the tunnel and the distance between the coupling port and the test section are analyzed. The optimized results are presented and discussed for a variety of wind speeds within the wind tunnel and the firebrand generator.

Direct Radiation From Wildfires Through Square Woven Screens

The positioning of a metal mesh between a property and a wildfire not only reduces the threat of ember attack, but can also be effective in reducing the radiation heat flux from the fire to the property. A computational technique is used to establish a view factor correlation for the object-screen-wildfire arrangement for screens consisting of woven wire, square-celled elements. The view factor between the object and fire is calculated separately for each square-celled element of the screen and the results are added. The results indicate that the average view factor between an infinite fire and an object of the same size as the square cell is only related to the porosity of screen and is independent of distance of the object from the screen (when the object is in the proximity of the mesh). The results also show the view factor is less than screen porosity, particularly at lower porosities.