New analytical model for local heat flux density in the mold in continuous casting of steel

a b s t r a c t In this article, a model representing local heat flux density in the mold in continuous casting of steel is presented. This model is a modification for empirical model of Schwerdtfeger. A new parameter is defined as LHTC z (longitudinal heat transfer coefficient). The multiplication of LHTC z to total increase of the cooling water temperature (DT T w ) leads to a new model for local heat flux density. Since DT T w is a measurable parameter, the present model can be used in on-line controlling of continuous casting process. Results of the present model were compared with the experimental data and a good agreement was seen. Also it was shown that present model can be used as a boundary condition in solidification numerical modeling

ICNMM2008-62090 FREE CONVECTION OF NANOFLUID IN VERTICAL ANNULI

ABSTRACT This paper presents the numerical study of internal free convection in a vertical annulus. To predict the characteristics of nanofluids in heat transfer regime, thermal conductivity, density, and viscosity of clear fluid need to be modified. Numerous models have been disclosed to calculate these properties but recent models for viscosity and thermal conductivity which are in excellent agreement with experiments have been used. These models are thermal conductivity model of Jang and Choi and viscosity correlation of Nguyen et al. for 36nm Al 2 O 3 particles. Inner and outer vertical walls are in constant temperature while horizontal walls are adiabatic. The continuity, Navier-Stokes and energy equation were solved numerically. Effect of nanofluids on buoyancy-driven heat transfer is investigated as a function of geometrical and physical parameters and various particle concentrations for aspect ratio of 0.2<H/L<5, Grashof number of 10 3 <Gr<10 5 and concentration of 0<φ <0.12. Finally, one correlation is developed to demonstrate the effect of using atom-size particles on Nusselt number. INTRODUCTION There are several ways to influence the heat transfer in a given problem. Modifying geometry, boundary conditions, and flow regime are parameters that can alter this energy transportation. Since traditional heat transfer fluids such as water, oil, and ethylene glycol mixtures have poor heat transfer characteristics one way to enhance heat transfer is to increase thermal conductivity of the fluid. The thermal conductivity of solid is typically higher than that of liquids. For instance, thermal conductivity of copper is 700 times greater than that of water and 3000 times greater than that of engine oil

Numerical investigation of molecular nano-array in potential-energy profile for a single dsDNA

A Rigorous numerical investigation on dsDNA translocation in quasi-2-dimensional nano-array filter is performed using Molecular Dynamics (MD) method. Various dsDNA molecules with different sizes are chosen in order to model Ogston sieving in a nano-array filter. The radius of gyration of dsDNA molecule is less than the characteristic length of the shallow region in nano-array. The dsDNA molecule is assumed to be in the 0.05M NaCl electrolyte. MD shows acceptable results for potential-energy profile for nano-array filter. According to the MD outcomes, the dsDNA electrophoretic mobility decreases almost linearly with dsDNA size and show the same trend as Ogston sieving for gel electrophoresis. In addition, different shapes for nano-array filter are studied for a unique dsDNA molecule. It is concluded that steeping the nano-array wall can cause the retardation of dsDNA translocation and decreases dsDNA electrophoretic mobility

Rigorous study of molecular dynamics of a single dsDNA confined in a nanochannel: Introduction of a critical mobility behaviour

The essential and effective characteristics of a double-stranded DNA (dsDNA) confined in a nanochannel is revisited by employing the rigorous full numerical approach of Molecular Dynamics (MD). The deformation of dsDNA and wall-biomolecule interaction which is critical in highly confined regime has been precisely imposed in numerical simulations. The numerical approach has been justified against available theoretical outcomes. A new and general expression for DNA electrophoretic mobility versus DNA length is extracted from numerical simulation which is out of reach of experimental methods due to practical shortcomings. The newly derived expression suggests an essential correction in the previously proposed expression for the critical case of small DNA molecules and reveals an astonishingly unbeknown trend of small DNA's mobility. Sub-molecular phenomenon of dsDNA melting under the condition of large external force is also studied. Assuming strong electric field exertion, the MD approach aptly demonstrates the elaborate melting phenomenon for dsDNA in sub-molecular scale

Airborne particle dispersion to an operating room environment during sliding and hinged door opening

Background: Operating rooms (ORs) are usually over-pressurized in order to prevent the penetration of contaminated air and the consequent risk of surgical site infection. However, a door-opening can result in the rapid disappearance of pressure and contaminants can then easily penetrate into the surgical zone. Therefore, a broad knowledge and understanding of OR ventilation systems and their protective potential is essential for optimizing the surgical environment. Objectives: This study investigated the air quality and level of airborne particles during a single and multiple door-opening cycles in an operating room supplied by a turbulent-mixing ventilation system. Methods: The exploration was carried out numerically using computational fluid dynamics. Model validation was performed to ensure the validity of the achieved results. The OR was initially over-pressurized by approximately 15 Pa, relative to the adjacent corridors. Both sliding and hinged doors were simulated and compared. Results: Penetration of bacteria carrying particles from the corridors to the OR can be successfully restricted by using a positive-pressure system. However, the results clearly indicate that frequent door opening can interfere with airflow ventilation systems, alter the pressure gradient, and increase the infection risk for the patient undergoing surgical intervention. Door-opening disturbs the airflow field and could result in containment failure. Keywords: Door-opening, Operating room, Bacteria-carrying particles, Particle transport, Computational fluid dynamic

CFD simulations of a semi-transverse ventilation system in a long tunnel

In the present work, a semi-transverse ventilation system in a long tunnel with a length of 4.9 km, as a complex case study, is numerically studied by performing a set of three-dimensional steady incompressible computational fluid dynamics (CFD) simulations. The ventilation system consisted of a ceiling duct connected to two axial fans at the ending portals, and a series of jet fans in the main tunnel for supporting airflow in the desired direction. To focus on what can and cannot be achieved in commissioning tests, the ventilation system’s performance in various scenarios is numerically evaluated with two different tunnel states; empty tunnel and complete traffic congestion with 1176 stationary vehicles – which is almost impossible to evaluate during a commissioning test. By considering two hypothetical locations for the extraction zone from the main tunnel (in a distance of 450 and 1000 m from one portal), it is shown that the required number of jet fans in a traffic condition drops from 57 for the first extraction location to 43 (25% decrease) when the ventilation system extracts from the second zone. We show that if only the close axial fan to the extraction zone is activated, the required number of jet fans reduces by 56% and 72% for the first and second extraction locations, respectively. This finding can provide a cheaper and easier controlling scenario for emergency ventilation

NUMERICAL INVESTIGATION OF IRRIGANT PENETRATION INTO DENTINAL MICROTUBULES

ABSTRACT Root canal irrigation is an important procedure in endodontic treatment. After mechanical preparation of root canal, NaOCl, which is the most common antibacterial irrigant, is inserted by special needles. This work helps to remove bacteria and debris and dissolves the organic tissues in the root canal. In the vicinity of the main root canal, there are a large number of microchannels attached to its wall named "dentinal tubules". The success of irragation depends on the penetration of irrigant in these tubules, which results in killing the bacteria and preventing complexities after root canal therapy. There is rather limited earlier research on modeling of dentinal tubules. Nevertheless, it has been shown that the flow rate, insertion depth and needle types affect the flow pattern in the root canal. The concentration difference between inserted irrigant and the liquid filling the tubules is the main driving force for penetration. Diffusion of irrigant, however, is a time dependent process and should be analyzed as an unsteady problem. In prior studies, the geometry was considered as cylinders with a constant diameter of 2.5μm and the effect of tapering was neglected. In reality the diameter varies from about 2.5μm near the pulp to about 1.5μm at the distance of 1 mm from the pulp. In the present study, a more detailed and exact model of dentinal tubules geometry was considered. The computational fluid dynamics (CFD) is used for the modeling of flow and diffusion of irrigant as a function of time. The unsteady and 3D continuity and Navier-Stokes equations as well as a scalar transport equation are solved and the flow field and the concentration of antibacterial irrigant were evaluated. The simulation results were compared to the earlier works. It was shown that the use of the correct detailed geometry of tubules led to noticeable differences compared to those found for the idealized model

A simple mathematical model of retinal reattachment after scleral buckling

Rhegmatogenous retinal detachment (RRD) is a dangerous pathological condition that can lead to blindness and requires surgical treatment. Scleral buckling is a surgical technique that has been in use for many years to repair RRD. It consists in the application of a piece of silicone on the outer surface of the sclera, that pushes the eye wall inwards and modifies its curvature in correspondence of the retinal tear. It is observed that this facilitates retinal reattachment. Various authors speculated that basic principles of fluid mechanics can be invoked to explain the reattachment process, though a convincing explanation of the mechanics underlying the process is still elusive. In this study, we propose an idealized two-dimensional model of a detached retina, surrounded by liquefied vitreous and study its dynamics secondary to eye movements. This is done using an immersed boundary numerical code. The retinal flaps are modeled as slender one-dimensional elastic bodies, one extremity of which is clamped to the retinal wall. For simplicity we model the retina as a rigid, flat wall. We account for the presence of scleral buckling by inserting a wall bump underneath the detached filaments. We show that the dynamics of the detached filaments is very complicated and that the presence of a buckle significantly contributes to reduce the time averaged distance between the detached filaments and the wall, thus facilitating reattachment. The mechanisms involved are inherently associated with the dynamics of the filament