The Heat Transfer Characteristics in Stepped Hole of Traditional Metal Materials Induced by Stepped Film Holes

With the increase of temperature requirement, the application of metal matrix materials is gradually reduced due to its poor temperature resistance. In order to improve the application of traditional metal based materials, new cooling technology must be developed to meet the application of metal based materials. In this paper, the cooling flow field of cylindrical hole and two kinds of stepped film holes based on metal materials is simulated by large eddy method(LES), the development of vortex structure and flow characteristics in the mixing area of coolant flow and mainstream are analyzed and studied. The distribution is strongly effected by stronger thermal convection compared with the new temperature resistant material. The results show that four kinds of vortices form downstream the film hole, namely, horseshoes vortex, shear layer vortex, hairpin vortex and the counter rotate vortex pair (CVP). The CVRP formed on the stepped plane which strongly influences the flow and heat transfer of downstream, which is more evident in metal based materials, especially resulted by its isotropic features. Compared with the cylindrical film hole, the structure of step plane efficiently decreases the coolant flow velocity which results in a decrease of the CVP intensity so that the cooling film is adherent to the wall. The thermal conductivity of metal base material is strong, which has a great influence on the temperature distribution inside the wall. Velocity pulsation tightly influenced by CVP, in where the CVP intensity is strong, velocity pulsation is also strong so that the coolant flow strongly mixes with the mainstream. The flow velocity and velocity pulsation decrease with the increasing of the area ratio of stepped hole. The influence of geometric parameters on the heat transfer performance is mainly due to the high heat transfer performance of the metal matrix material.

Mathematical and Physical Simulation of the Cross-Flow Velocity Pulsation Effect on the Flame Structure during the Diffusion Mode of Methane Combustion

The paper presents the results of calculation and experimental studies of the diffusion combustion of methane in the air cross-flow. We developed a mathematical model for describing a diffusion air-methane flame, the model being based on solving a system of averaged Navier --- Stokes equations in an unsteady setting. To calculate the combustion processes, we used the flamelet models and eddy dissipation concept (EDC) model. The mathematical model was supplemented by a detailed kinetic mechanism consisting of 325 elementary reactions involving 53 substances. Furthermore, we carried out calculations and comparative analysis of the flame characteristics using various turbulence models: k − ε, k − ω SST and Transition SST. The study introduces a diagram of the experimental setup for physical modeling of methane combustion in the air cross-flow, and presents the results of the calculation and experimental study of the cross-flow velocity pulsation effect on the flame structure, as well as the efficiency of methane combustion in the diffusion mode. We obtained data on temperature and concentration fields at pulsation frequencies of 0--100 Hz. Findings of research show that for the case under consideration, stable combustion occurs at pulsation frequencies of 0--90 Hz. The maximum observed flame lift-off is 3.2 times the diameter of the burner nozzle

Oscillating Flows in Circular Pipes

Pulsation flows in pipes heated externally produces oscillating temperature field. This type of unsteady flow happens in heat exchangers. Simulating this type of flows is complex in engineering. In this present study the field variables like velocity and temperature are calculated by numerical control volume scheme. Velocity pulsation is applied at inlet of pipe to produce oscillations. Simulation variables like lengths, diameter and thickness of the pipe are considered as parameters for this study. Also additionsl structural constraints has been added to see how it influences effective thermal stresses.

On the theory of the passive impurity distribution in the turbulent air flow

The statistical model of passive impurity transfer in surface boundary layers of the turbulent atmosphere in the presence of wind is offered. Analytical expressions of the normalized concentration of impurity for arbitrary correlation tensor of the second rank of velocity pulsation when the emission source is located at a certain height over the Earth's surface are obtained. The effective coefficient of turbulent diffusion contains coefficient of molecular diffusion, longitudinal and transverse turbulent diffusion coefficients. Numerical calculations were carried out using experimental data of ground(-based) observations. The isolines describing distribution of the passive impurities at calm case are depicted at different values of a wind speed and at certain distances from a source. Dynamics of globules formation with various concentration of impurity transferred by wind is constructed. They have specific characteristic spatial scales and lifetimes.Â

The Optimal Formation Condition of the Axis-Symmetry Cyclic Vortex Rings

The cyclic vortex rings, which are generated cyclically by a pulsating jet, appear to be an effective method for particles and fluids transport. In order to identify the formation conditions of cyclic vortex rings that are optimal for transport, the effect of the formation conditions of cyclic vortex rings, such as the velocity pulsation amplitude, period and acceleration of jet pulsation conditions, and conditions of including intermittent period, on the axis symmetry characteristics of behavior of vortex rings is investigated experimentally. The results indicate that the axis symmetry characteristics of cyclic vortex rings begin to loss by the following four factors, the influence of separated vortex rings, the influence of trailing vortex rings, the influence of trailing vortex rings formed in the previous cycle, and the influence of the cyclic vortex ring formed in the previous cycle. The behavior of cyclic vortex rings can be classified into three types, and is determined by the combination of loss-inducing factors and their individual strengths. We also discuss the formation conditions of cyclic vortex rings that are optimal for transport.

Fractal Approach to Bubble Rising Dynamics in Non-Newtonian Fluids

The Laser Doppler anemometry was employed to determine quantitatively the liquid velocity induced by the successive rising of single bubble in non-Newtonian carboxymethylcellulose (CMC) aqueous solutions under various experimental conditions of mass concentration solutions, measures heights and gas flow rate. The features of liquid motion in the region of bubble rising channel were investigated by analysis the liquid velocity pulsation using fractal theory. The results show that the liquid motion in the channel zone of bubble rise has a special feature of double fraction, and shows strong positive persistence characteristics for a small delay, but the positive persistence characteristics begins to reduce obviously with the increase of the delay, and even presents the anti-persistence for some measured points.

A Review of Fire Interactions and Mass Fires

The character of a wildland fire can change dramatically in the presence of another nearby fire. Understanding and predicting the changes in behavior due to fire-fire interactions cannot only be life-saving to those on the ground, but also be used to better control a prescribed fire to meet objectives. In discontinuous fuel types, such interactions may elicit fire spread where none otherwise existed. Fire-fire interactions occur naturally when spot fires start ahead of the main fire and when separate fire events converge in one location. Interactions can be created intentionally during prescribed fires by using spatial ignition patterns. Mass fires are among the most extreme examples of interactive behavior. This paper presents a review of the detailed effects of fire-fire interaction in terms of merging or coalescence criteria, burning rates, flame dimensions, flame temperature, indraft velocity, pulsation, and convection column dynamics. Though relevant in many situations, these changes in fire behavior have yet to be included in any operational-fire models or decision support systems.

Numerical simulation of the flow and heat transfer around a cylinder with a pulsating approaching flow at a low Reynolds number

Two-dimensional numerical simulations of flow and heat transfer around a cylinder at a Reynolds number Re= 100 have been performed in order to investigate the effect of imposed inlet velocity pulsation on the heat transfer and flow fields. First the code is validated against existing results from the literature and then several external frequencies are examined. The numerical results confirm the existence of a vortex shedding lock-on regime where the wake behaves in a very ordered manner (completely periodic). Outside the lock-on region the flow is quasiperiodic. The length and centre of the mean recirculating zone downstream of the cylinder are also affected by the external pulsation. Regarding heat transfer, the results indicate that by imposing an external velocity pulsation, the root mean square (r.m.s.) of the local Nusselt number Nu increases, but the mean value increases only in the area downstream of the separation point. The mechanism responsible for this is identified: hot fluid is engulfed by stronger vortices (compared with the steady approaching flow case) shed from the upper and lower side of the cylinder and returned close to the downstream stagnation point. This mechanism also explains the observed variation in Nu with time. In the front part of the cylinder, the Nu varies almost sinusoidally and closely follows the imposed external velocity pulsation. The results indicate also that there is a range of external frequencies where the time and spatially averaged Nu number is maximized.

An Experimental Study of the Mixing by an Acoustically Pulsed Axisymmetrical Air-Jet

The mixing by an acoustically pulsed axisymmetrical air-jet, flowing into the atmosphere, has been studied by means of velocity and temperature profile measurements. The strength of the velocity pulsation imparted to the jet flow and of the associated toroidal vortices were also measured. The entrainment rate was increased by up to two times, with the majority of the extra entrainment occurring over the first five diameters downstream of the jet orifice, where toroidal vortices are formed and attain their greatest strength. The jet response depends on Strouhal number and appears to be optimum at about 0.25. The response starts to saturate at the limit of pulsation strength used.