Hong Sun Ryou

An Experimental Study of the Effect of the Aspect Ratio on the Critical Velocity in Longitudinal Ventilation Tunnel Fires

In this study, the critical velocity was investigated with various aspect ratios (0.5, 0.667, 1.0, 1.5, 2.0) of the tunnel cross-section in tunnel fires. Reduced-scale experiments were carried out under the Froude scaling using ethanol pool fire. Temperatures were measured under the ceiling and vertical direction along the center of the tunnel. The effects of aspect ratio on critical velocity were analyzed from smoke movement. It has been found that the critical velocity increases with aspect ratio, where the aspect ratio is height/ width for a tunnel of rectangular cross-section. This study used new dimensionless velocity and dimensionless heat release rate parameters considering aspect ratio of the tunnel. The critical velocity varied with the one-third power of the heat release rate for under-medium fires. The results of this study are compared with other small-scale and full-scale experiments. A good agreement has been obtained.

An experimental and numerical study on fire suppression using a water mist in an enclosure

Abstract The present study investigates the fire suppression characteristics using a water mist fire suppression system. The fire extinguishing time and the temperature fields in the enclosed compartment of 4.0 m ×4.0 m ×2.3 m are measured by K-type thermocouples for various fire sources. The oxygen concentrations are also obtained by the gas analyzer. It is found that there are two different regimes in the temporal variations of the smoke layer temperature. One is the initial sudden cooling regime, and the other is the gradual cooling one. A critical cooling time is defined as the time during which the sudden cooling persists. Furthermore, the numerical simulations are extensively performed using Fire Dynamics Simulator (FDS, Ver. 3.0) code and the predictions are compared with experimental data. The predicted results are in good agreement with experimental data in temperature fields.

Tunnel fires: experiments on critical velocity and burning rate in pool fire during longitudinal ventilation

In this study, the 1/20 reduced-scale experiments using Froude scaling are conducted to investigate the effect of longitudinal ventilation velocity on the burning rate in tunnel fires. The methanol pool fires, acetone pool fires, and n-heptane pool fires with a heat release rate ranging from 2.02 to ~6.15 kW, 2.76 to ~11.04 kW, and 2.23 to ~15.6 kW, respectively, are used. A load cell is used to measure the mass loss rate of the burning fuel and the temperature distributions are measured by K-type thermocouples in order to investigate smoke movement. The ventilation velocity in the reduced-scale tunnel is controlled by the wind tunnel through an inverter. In the case of a methanol pool fire, the increase in ventilation velocity reduces the burning rate. On the contrary, for acetone and n-heptane pool fires, the increase in ventilation velocity leads to the enhanced burning rate of fuels. The reason is that, for acetone and n-heptane pool fires, the oxygen supply effect prevails rather than the cooling effects as the ventilation velocity increases. However, for a methanol pool fire the cooling effect outweighs the effects of oxygen supply. The relationship between the critical velocity (Vc, m/s) and the heat release rate (Q, MW), for a full tunnel, has been found to be:[ILLEGIBLE].

An Experimental Study on the Effect of Slope on the Critical Velocity in Tunnel Fires

The present article deals with the experimental study to investigate the effects of tunnel slope on the critical velocity in the tunnel fires. The 1/20 reduced-scale model tunnel is adopted on the basis of Froude scaling law. The methanol, acetone, and n-heptane pool fires are used as fire sources with the heat release rate ranging from 1.11 to 1.85 kW, 3.13 to 5.21 kW, and 9.16 to 15.6 kW, respectively. The angle of tunnel slope is varied as five different degrees 0, 2, 4, 6, and 88. A load cell is used to measure the mass loss rate of the burning fuel. The location of smoke layer is determined on the basis of temperature variation measured at the tunnel ceiling by K-type thermocouples. The present experiments observe the effects of the tunnel slope, fire size, and ventilation velocity on the back-layering distance. The results showed that critical velocity increases with tunnel slope due to the stack effect. The correlation between the critical velocity and the angle of tunnel slope was obtained for the pool fires whose heat release rate was varied with the ventilation velocity.The present article deals with the experimental study to investigate the effects of tunnel slope on the critical velocity in the tunnel fires. The 1/20 reduced-scale model tunnel is adopted on the basis of Froude scaling law. The methanol, acetone, and n-heptane pool fires are used as fire sources with the heat release rate ranging from 1.11 to 1.85 kW, 3.13 to 5.21 kW, and 9.16 to 15.6 kW, respectively. The angle of tunnel slope is varied as five different degrees 0, 2, 4, 6, and 8°. A load cell is used to measure the mass loss rate of the burning fuel. The location of smoke layer is determined on the basis of temperature variation measured at the tunnel ceiling by K-type thermocouples. The present experiments observe the effects of the tunnel slope, fire size, and ventilation velocity on the back-layering distance. The results showed that critical velocity increases with tunnel slope due to the stack effect. The correlation between the critical velocity and the angle of tunnel slope was obtained for the pool fires whose heat release rate was varied with the ventilation velocity.

Development of a new spray/wall interaction model

Abstract This paper deals with the development and testing of a new spray/wall impingement model, which is based on the energy conservation law and experimental considerations. A new formula for the viscous dissipated energy of the film is derived in the present investigation to determine the viscous dissipation process of the film in the energy conservation law. In addition, the tangential behavior of droplets after impingement is determined by the newly proposed model, which incorporates both the kinematic parameters of the impinging droplets and the fluid properties. The new model consists of three representative regimes such as rebound, deposition and splash from experimental considerations. To assess the new model, the numerical calculation for several experimental conditions are carried out for the non-evaporative impinging sprays on a flat wall. The numerical results using the new model are compared with the experimental data and the results of the previous impingement models. The results show that the new model generally predicts the splash behavior better than the previous models, and it performs for prediction of local droplet velocities and size effectively, relative to the previous models. Therefore, it can be concluded that the new model is acceptable for predicting the non-evaporative sprays impinging on the wall.

An Experimental and Numerical Study on Thermal Performance of a Regenerator system with Ceramic Honeycomb

The aim of this paper is to perform the experiment and the numerical simulation for investigating the heat transfer in a regenerator system with ceramic honeycomb and to suggest a useful correlation for optimization of the regenerator system. For achieving this, the effects of some parameters were investigated, e. g., switching time, cell size and length of honeycomb on the mean temperature efficiency. The measured temperatures by R-type thermocouples were compared with the predictions by means of the commercial package, STAR-CD. A useful correlation for thermal efficiency was newly proposed as a function of the normalized switching time, defined in terms of switching time, cell size and length of honeycomb. The results showed that the thermal efficiency is above 90% and the normalized heat exchange rate is higher than 80% when the normalized switching time is less than 1000.

Development and Application of a New Spray Impingement Model Considering Film Formation in a Diesel Engine

The present article presents an extension to the computational model for spray/ wall interaction and liquid film processes that has been dealt with in the earlier studies (Lee and Ryou, 2000a). The extensions incorporate film spread due to impingement forces and dynamic motion induced by film inertia to predict the dynamic characteristics of wall films effectively. The film model includes the impingement pressure of droplets, tangential momentum transfer due to the impinging droplets on the film surface and the gas shear force at the film surface. Validation of the spray/ wall interaction model and the film model was carried out for non-evaporative diesel sprays against several sources of experimental data. The computational model for spray/wall interactions was in good agreement with experimental data for both spray radius and height. The film model in the present work was better than the previous static film model, indicating that the dynamic effects of film motion should be considered for wall films. On the overall the present film model was acceptable for prediction of the film radius and thickness.

Construction of healthy arteries using computed tomography and virtual histology intravascular ultrasound

Vessel geometry for numerical analysis is generally obtained by computed tomography (CT) or magnetic resonance imaging (MRI) and intravascular ultrasound (IVUS). Most medical imaging is obtained from patients for hemodynamic analysis due to the properties of vascular disease and the difficulties in angiography. To predict the site where plaque occurs and understand the progression of the lesion, however, it is necessary to take into consideration not only the diseased artery, but also the blood flow characteristics of healthy artery. In order to simulate healthy vessels prior to lesion formation, we performed CT and virtual histology intravascular ultrasound (VH-IVUS) on three actual patients and this data was used to develop criteria for healthy vessel construction, a method that virtually removes all intravascular plaque. The lumen of a vessel generated by CT and the lumen from VH-IVUS were compared, and the cross-sectional areas of plaque components (fibrous, fibrofatty, dense calcium, and necrotic) and the lumen from VH-IVUS were analyzed. Geometric differences in the healthy vessel and diseased vessel were analyzed, and flow characteristics of the healthy vessel and diseased vessel were compared through computational fluid dynamics simulation. Low average wall shear stress (AWSS) was distributed in the site where plaque was removed from the healthy vessel, and a high oscillatory shear index (OSI) was observed in the region proximal to the site where plaque previously existed. Low AWSS and high OSI are widely accepted indicators of plaque formation or the direction of plaque progression. A numerical model that effectively predicts lesion forming sites was also generated based on the healthy vessel construction method presented in this study.

The Effect of Water Mist on Burning Rates of Pool Fire

The present study aims to investigate the interactions between water mists and fires and show the influence of water mist on burning rates of pool fires. Experiments are extensively performed for various sizes of fire source with different discharge rates of water. From the experimental observations, it is noted that the fuel burning rate is substantially changed when the water mists are interacted with fires and its difference with and without water mist increases with the discharge rate of water and decreases as the fire intensity increases. In addition, the interactions of water mists with pool fires are simulated by the FDS (Fire Dynamics Simulator; Ver. 3.0) program with the measured burning rates as input data in the present study. Considering the influence of water mists on burning rates, better predictions are yielded compared with experimental data for the suppression time and the mean ceiling temperature.

Comparison of Spray/Wall Impingement Models with Experimental Data

The aim of the study is the comparison of earlier published submodels for the spray impingement and the suggestion of a modie ed impingement model. The modie ed model was devised to give better predictions for the upward dispersion of droplets after impingement in diesel engines. The present paper includes two parts, one of which is to analyze the overall structure of impinging sprays and the other to do the internal structure for which the main parameters are the local velocities and mean diameter of droplets. The submodels were investigated to compare the results with the modie ed model. Numerical results using four different models were compared with several experimental cases for the nonevaporative impinging sprays. As a result, it is found that model 3 and model 4 are acceptable for the predictions of the droplet dispersion normal to the wall.