Backflow Length Research Articles

Study of temperature distribution and smoke movement by mobile ventilation during emergency rescue in tunnel fires

Mobile ventilation unit (MVU), as a convenient and maneuverable ventilation method, can provide forced airflow for emergency ventilation to reduce tunnel fire risk. Compared with the conventional natural ventilation and mechanical ventilation, this mobile ventilation method plays a crucial role in the current tunnel fire emergency rescue. To explore the effects of MVU on the temperature or smoke spread in tunnel fires, Computational Fluid Dynamics simulation, validated by the previous experimental data, is applied in the current study. A series of tests are conducted with fire heat release rate of 1–30 MW and MVU fan flow being up to 3 × 105 m3/h. The results show the maximum tunnel ceiling temperature presents exponential attenuation with increasing fan flow and its relational expression with fan flow and fire size is proposed. Furthermore, the temperature decay distribution between upstream and downstream area is obviously different and a new coefficient γ is imported to represent the difference by connecting the fan flow and fire size. Moreover, the smoke backflow length theoretical equation in the upstream area is obtained, which can provide effective guidance to tunnel fire emergency rescue with MVU.

A Biphasic Fluid-Structure Interaction Model of Backflow During Infusion Into Agarose Gel.

The efficacy of convection-enhanced delivery as a technique to treat disorders of the central nervous system is limited by backflow, in which the infused fluid flows backwards along surface of the catheter rather than towards the targeted area. In order to improve treatment protocols, finite element models of backflow have been developed to understand the underlying physics. Garcia et al. (Journal of Computational and Nonlinear Dynamics, 8:011017, 2013) presented a finite element model that accounted for the flow in the annular gap that develops between the tissue and the outer surface of the catheter by using a layer of biphasic elements with a formula for the axial hydraulic conductivity to represent annular Poiseuille flow. In this study, we present a generalization of that model using fluid-FSI and biphasic-FSI elements that are recently available in FEBio. We demonstrate that our model of a 0.98-mm-radius catheter is able to reproduce experimental backflow lengths and maximum fluid pressures for infusions into a brain tissue surrogate and that it agrees well with the previous model by Garcia et al. (2013). The model predicts that the backflow length and the total amount of flow into the hemispherical region forward of the catheter tip is comparable for two different catheter sizes, albeit at a higher fluid pressure for the smaller catheter. This biphasic-FSI model has the potential to be extended to a stepped catheter geometry, which has been shown in experiments to be successful in controlling backflow.

Numerical study on upstream smoke propagation and induced airflow velocity in a tilted channel under natural ventilation

A series of fire simulations were executed to explore the upstream smoke propagation characteristics and induced airflow velocity in tilted channels under natural ventilation. The channel slope ranging from horizontal to 58% (30°) and five heat release rates were used. Simulative results indicate that the airflow introduced by stack effect can enable the upstream smoke spread to reach sub-critical, critical and super-critical states without forced longitudinal ventilation. Meanwhile, taking the effect of fire heat release rate and height difference between the fire source and the upper portal into account, a piecewise function is developed to estimate the induced airflow velocity. It is found that when Ld∗·Q∗0.2 is less than or equal to 1.05, the airflow velocity increases exponentially with the variety of dimensionless elevation difference, but slowly rises logarithmically when Ld∗·Q∗0.2 is greater than 1.05. Besides, for scenarios in the sub-critical state, the relationship between the induced airflow velocity and the upstream smoke backflow length is revealed, which demonstrates that the dimensionless reverse flow length is in inverse proportion to the 1.5 power of dimensionless airflow velocity. For scenarios in the critical and super-critical states, there is still smoke backflow upstream of the fire source at the early period of a fire. And the disappearing time of smoke backflow reduces exponentially with the increase of heat release rate and elevation difference. Based on dimensional analysis and simulative data, a novel predictive model for the backflow disappearing time is proposed.

Backflow modeling in nanofluid infusion and analysis of its effects on heat induced damage during magnetic hyperthermia

• Simulate the backflow behavior for nanofluid infusion inside bio-tissue under a geometric nonlinear model. • Investigate the bio-tissue deformation and its effect on the change of material property during nanofluid infusion. • Investigate the interstitial pressure and the local stress distribution inside bio-tissue due to the nanofluid infusion. • Effect of backflow on treatment temperature distribution and heat induced damage for malignant tissue. Although targeted magnetic hyperthermia has been proven to be an effective tumor ablation technique, its use in clinical applications is still scarce particularly due to the difficulty in imposing a desired nanofluid distribution in the therapeutic area. In addition to the inherent difficulty of imposing a distribution with few injection shots, during the nanofluid infusion, the tissue deformation can cause the nanofluid deviation from the targeted injection area and the backflow along the needle can deliver the injected nanofluid to the outer surface of the tissue. Both phenomena can result in an irregular distribution for nanofluid inside bio-tissue. This study develops a poroelastic model considering geometrically nonlinear behavior in order to evaluate the effect of syringe needle size and infusion rate on the backflow. A 26 gauge needle for syringe is used as a typical example to further investigate the nanofluid transport and the change of solid matrix material properties under different infusion rates after comparing the infusion results for several sizes of needle. Finally, the resulting nanofluid concentration distribution obtained with the proposed model is used to simulate the temperature distribution and the cancerous cell damage. The results demonstrate that the infusion pressure and its resulting tissue deformation are the fundamental reasons for obtaining an irregular solution distribution. Tissue deformation induces the increase of porosity and permeability for biomaterials around the tip, and enhances the fluidity of nanofluids inside the tissue. The results also indicate that the increase in backflow length can improve the uniformity of the nanofluid distribution after diffusion and, consequently, the treatment effect. However, it also increases the risk of MNP leakage from the targeted area to the tumor surface, so it is important to keep the backflow rate limited during the injection process.

Assessment of needle tip geometry during infusions into a brain phantom gel

Convection-enhanced delivery (CED) is a promising method to deliver therapeutic drugs directly into the brain that has shown limited efficacy, mainly attributed to backflow, in which the infused drug flows back along the needle track rather than forward into tissue. This study evaluates the effect of sharp and blunt needle tips on backflow length under different flow rates via CED. Infusions were performed in a transparent 0.6% (w/v) brain phantom agarose hydrogel. Backflow length was significantly higher using sharp-tip needles for higher flow rates. No significant differences were observed between tip shapes for lower flow rates. In conclusion, sharp-tip needles present limitations for higher flow rates, which are needed to deliver more drug during shortest times.

Three-dimensional nonlinear finite element model to estimate backflow during flow-controlled infusions into the brain.

Convection-enhanced delivery is a technique to bypass the blood-brain barrier and deliver therapeutic drugs into the brain tissue. However, animal investigations and preliminary clinical trials have reported reduced efficacy to transport the infused drug in specific zones, attributed mainly to backflow, in which an annular gap is formed outside the catheter and the fluid preferentially flows toward the surface of the brain rather than through the tissue in front of the cannula tip. In this study, a three-dimensional human brain finite element model of backflow was developed to study the influence of anatomical structures during flow-controlled infusions. Predictions of backflow length were compared under the influence of ventricular pressure and the distance between the cannula and the ventricles. Simulations with zero relative ventricle pressure displayed similar backflow length predictions for larger cannula-ventricle distances. In addition, infusions near the ventricles revealed smaller backflow length and the liquid was observed to escape to the longitudinal fissure and ventricular cavities. Simulations with larger cannula-ventricle distances and nonzero relative ventricular pressure showed an increase of fluid flow through the tissue and away from the ventricles. These results reveal the importance of considering both the subject-specific anatomical details and the nonlinear effects in models focused on analyzing current and potential treatment options associated with convection-enhanced delivery optimization for future clinical trials.

Finite Element Model to Reproduce the Effect of Pre-Stress and Needle Insertion Velocity During Infusions into Brain Phantom Gel

Convection-enhanced delivery (CED) is a technique to bypass the blood-brain barrier and deliver therapeutic agents into the brain. However, animal studies and preliminary clinical trials have reported reduced efficacy to transport drugs in specific regions, attributed mainly to backflow, in which an annular zone is formed outside the catheter and the fluid preferentially flows toward the surface of the brain rather than through the tissue toward the targeted area. In this study, a finite element model of backflow was updated by implementing the pre-stress generated during needle insertion, which allows considering the effect of needle insertion velocity during CED infusions in agarose gel. The nonlinear mechanical properties of the agarose solutions were obtained by fitting experimental data from stress-relaxation tests. Additional experimental measurements of backflow lengths were used to adjust the pre-stress model. The developed model was able to reproduce changes of backflow length under different insertions velocities and flow rates. These findings reveal the relevance of considering the pre-stress in the tissue located around the needle surface during CED infusions into the brain.

Smoke control in tunnel with a transverse ventilation system: An experimental study

We carried out a series of experiments in a small-scale tunnel to study the confinement of smoke flow between two exhaust vents located on either side of a buoyant source placed at ground level within a ventilated tunnel. The objective of our study is to quantify the influence of the shape and the position (with respect to the tunnel axis) of the dampers, including the specific case of full-width dampers, on the performance of the transverse ventilation system. To that purpose, we investigated the extent of the backflow length downstream of the dampers, the “confinement velocity” (suppressing the back-layering flow downwind of the vents) and the stability of the smoke stratification. The results show that the best performances are achieved when the width of the dampers is the same as the tunnel width: this minimises the extent of the back-layering flow (for given extraction flow rates) and ensures the stability of the smoke stratification. The worst conditions are observed with square-shaped dampers located on the side of the ceiling, this configuration disturbs the stratification of the smoke layer before the confinement conditions are reached. Subsequently, we evaluated the effect of solid barriers -placed on the tunnel ceiling downstream of same three dampers typologies-on the propagation of the smoke. The results show that these barriers improve significantly the efficiency of system with square-shaped dampers located on the side of the ceiling: they reduce the confinement velocity and enhance the stability of the smoke stratification.

Numerical Simulation for Mine Oblique Lane Fire Based on PDF Non-Premixed Combustion

ABSTRACT In the mine fire period, the smoke plume temperature, critical air velocity, back-flow layer and other fire parameters in the lanes are subjected to environmental factors such as lane dip angle and airflow conditions, coupled with uncertain evolution process. In view of this, a numerical simulation is performed for exploring the dynamic evolution law of fire possibly occurred in the mine. The ANSYS Fluent PDF non-premixed combustion model is applied to reveal the fire development in the lanes with different dip angles under different air velocity. The temperature distribution in the lanes and the evolution law of the fire back-flow layer are also analyzed. Here introduces the dimensionless number-Richardson number (Ri value) to determine the critical value of the development of the fire back-flow layer. The results show that there is a regular relationship of the spatiotemporal evolution properties of fire in the inclined lane with the two factors: dip angle, airflow velocity, According to this law, the calculation model of smoke back-flow layer length with respect to air velocity and inclination angle is established, and the critical air velocity prediction model under the condition of large inclination angle is revised. Based on the critical air velocity and critical angle, the critical value of the Richardson number is determined to be 91.6. When Ri is greater than 91.6, the fire back-flow layer appears, otherwise, it disappears. The findings provide certain clues to identifying the fire in the underground tunnel and predicting the fire development and smoke spread.

Smoke movement characteristics under stack effect in a mine laneway fire

Results from a series of simulations with regard to a mine laneway fire are presented. Large Eddy Simulation with Fire Dynamics Simulator was carried out for a fire located in a horizontal laneway, with investigating the smoke movement under the influence of stack effect formed inside an inclined laneway. The length and angle of the inclined laneway were systematically changed to show the influence of stack effect on the smoke characteristics, involving smoke temperature, smoke spread process and smoke backflow. Results show that as the stack effect is proportional to both length and angle of the inclined laneway, increasing either length or angle of the inclined laneway will contribute to decreasing the backflow length in the horizontal laneway and thus leading to more smoke flowing into the inclined laneway. A non-dimensional number Ri was proposed to demonstrate the coupled influence of length and angle of the inclined laneway on smoke movement, which can well distinguish whether there is a smoke backflow upstream of the fire or not. The critical value of Ri was found to be 100. For cases without backflow at the steady period of smoke movement, the time when the backflow disappears decreases with the increasing vertical height of the inclined laneway, and the data can be correlated well by an exponential expression. For all cases, the mass flow rate of smoke flowing into the inclined laneway increases with the increasing vertical height of the inclined laneway exponentially.

Smoke development in full-scale sloped long and large curved tunnel fires under natural ventilation

A full-scale experiment was conducted to research the smoke development of a sloped long and large curved tunnel in the natural ventilated underground space under three different fire powers. The tests measured the evolution of the longitudinal smoke temperature rise, the fire plume flow characteristics near the fire source and the maximum smoke temperature during the fire growth, stable and decay stages. The results show the asymmetrical temperature distribution, the variation of the vertical position of the maximum temperature rise along the tunnel and the variation of the backflow length due to varying interactions of the fire wind pressure and the natural wind pressure during the complete fire process. The smoke development characteristics during each fire stage are described based on the measured data.

The performance of the vapor chamber based on the plant leaf

The performance of a novel vapor chamber based on the leaf vein system is tested in this study. In this vapor chamber, the conceptual structure, being composed of leaf-vein-like fractal network and micro fin-pins and used to simulate the transportation principle of plant leaf, is made on the inner surface of the top copper plate to form the wick of the condenser. To the inner surface of the bottom, a copper plate sintered with a layer of porous wick is used as the evaporator. Because the conceptual structure is not restricted by the shape, the vapor chamber is made into circular and rectangular shape respectively. In the circular vapor chamber, the supporting columns sintered with copper powder are used to strength the vapor chamber. However, in the rectangular vapor chamber, the wick of the condenser contacts directly with the wick of the evaporator so as to strength the vapor chamber and shorten the back flow length of the liquid condensed on the groove surface. In the circular vapor chamber, the fractal angle of the leaf-vein-like fractal network is 30–70°, and the vapor chamber is packaged with eight bolts for convenience. In the measurement, the vapor chamber has small thermal resistance when the fractal angle is 40° and 50°, and in the rectangular vapor chamber, the thermal resistance of the vapor chamber is about 0.06°C/W.

Backflow length predictions during flow-controlled infusions using a nonlinear biphasic finite element model.

A previously proposed finite element model that considers geometric and material nonlinearities and the free boundary problems that occur at the catheter tip and in the annular zone around the lateral surface of the catheter was revised and was used to fit a power-law formula to predict backflow length during infusions into brain tissue. Compared to a closed-form solution based on linear elasticity, the power-law formula for compliant materials predicted a substantial lower influence of the shear modulus and catheter radius on the backflow length, whereas the corresponding influence for stiffer materials was more consistent with the closed-form solution. The finite element model predicted decreases of the backflow length for reduction of the shear modulus for highly compliant materials (shear modulus less than 500 Pa) due to the increased area of infusion and the high fluid fraction near the infusion cavity that greatly increased the surface area available for fluid transfer and reduced the hydraulic resistance toward the tissue. These results show the importance of taking into account the material and geometrical nonlinearities that arise near the infusion surface as well as the change of hydraulic conductivity with strain for a proper characterization of backflow length during flow-controlled infusions into the brain.

Description and Validation of a Finite Element Model of Backflow During Infusion Into a Brain Tissue Phantom

An axisymmetric biphasic finite element model is proposed to simulate the backflow that develops around the external boundary of the catheter during flow-controlled infusions. The model includes both material and geometric nonlinearities and special treatments for the nonlinear boundary conditions used to represent the forward flow from the catheter tip and the axial backflow that occurs in the annular gap that develops as the porous medium detaches from the catheter. Specifically, a layer of elements with high hydraulic conductivity and low Young’s modulus was used to represent the nonlinear boundary condition for the forward flow, and another layer of elements with axial hydraulic conductivity consistent with Poiseuille flow was used to represent the backflow. Validation of the model was performed by modifying the elastic properties of the latter layer to fit published experimental values for the backflow length and maximum fluid pressure obtained during infusions into agarose gels undertaken with a 0.98-mm-radius catheter. Next, the finite element model predictions showed good agreement with independent experimental data obtained for 0.5-mm-radius and 0.33-mm-radius catheters. Compared to analytical models developed by others, this finite element model predicts a smaller backflow length, a larger fluid pressure, and a substantially larger percentage of forward flow. This latter difference can be explained by the important axial flow in the tissue that is not considered in the analytical models. These results may provide valuable guidelines to optimize protocols during future clinical studies. The model can be extended to describe infusions in brain tissue and in patient-specific geometries.

Numerical study of nanofluid infusion in deformable tissues for hyperthermia cancer treatments

Direct infusion by means of needles is one of the widely used methods for the delivery of nanoparticles in tumors for hyperthermia cancer treatments. During an infusion process, infusion-induced deformation can substantially affect the dispersion of the nanoparticles injected in a biological tissue. In this study, a poroelastic model is developed to investigate fluid transport and flow-induced tissue deformation in a tumor during an infusion process. A surface tracking technique is employed to predict the shape of nanofluid spreading after injection. The model is then used to simulate the formation of backflow and the change of tissue porosity due to the deformation. Specifically, we quantify the influence of the backflow on the spreading shape of the nanofluid and its dependence on injection parameters such as infusion rates, needle diameters, and tumor elastic properties. It is found that backflow is an important factor causing an irregular distribution of the nanofluid injected in a tumor. A higher infusion rate, larger needle diameter, and lower elastic modulus yield a longer backflow length and cause a more irregular spreading shape of the nanofluid. The infusion-induced tissue deformation also leads to a pore swelling and an increase of the porosity in the vicinity of the needle tip and the needle outer surface. It is anticipated that the increased pore size may facilitate the particle penetration in a tumor. To achieve a controlled heat generation, the injection parameters should be selected judiciously with the consideration of tumor sizes, tumor properties, and thresholds at which tumors break under the infusion pressure.

Prediction of soot and carbon monoxide production in a ventilated tunnel fire by using a computer simulation

Numerical study is performed by using a large eddy simulation to give a quantitative description of the toxic products (CO, soot), temperature stratification and heat fluxes from a circular pool fire in a longitudinally ventilated tunnel. The turbulent combustion process is modelled by an eddy break-up concept by using two sequential, semi-global steps to CO prediction. The numerical model solves three-dimensional, time-dependent Navier–Stokes equations, coupled with submodels for soot formation and thermal radiation transfer. The smoke movement is modelled as an unsteady process, from the time of ignition until convergence to a quasi-steady state. The computed, time-averaged temperatures are compared with experimental data from a full-scale ventilated tunnel, and a relatively good agreement is attained. Below a critical ventilation velocity, the back layer flow carries the CO and soot production towards the entry, and an inverse dependence of the backflow length to ventilation velocity is predicted. For the tunnel hydraulic height of 3.3 m considered here, the heat-release rate (HRR) approaches a maximum of approximately 35 MW, beyond which the critical ventilation velocity is unchanged even though the theoretical HRR continues to increase. In the case where HRR reaches to a maximum, the high-temperature level ( T>500 °C) in the smoke layer and the strong heat flux (>20 kW/m 2) pose significant threat to people or combustible objects exposed downstream the fire source, potentially inducing flashover within tunnel. This study highlights the utility of field modelling for the analysis of fires and smoke (CO and soot) movement in tunnels.

4232 旋回逆流の振動特性に及ぼす入口曲管の影響(G05-14 管内流・旋回流,G05 流体工学)

The present paper describes the experimental study of effects of a bent inlet on the swirling backflow oscillation. The backflow is generated by using a 4-bladed swirler with zero stagger. The tests were carried out by particle image velocimetry (PIV) in the region between bend and swirler. The backflow length fluctuations measurements show spread to the axial direction with increasing swirling speed, and its unsteadiness become stronger. Results of spectrum analyses of backflow length fluctuations show three oscillation patterns. At low swirling speed, the backflow oscillation has one dominant frequency. With increase of swirling speed, dominant frequency of the backflow oscillation shift lower frequency and it has two peaks on spectrum analysis.