Free Stream Temperature Research Articles

Flame thickness and conditional scalar dissipation rate in a premixed temporal turbulent reacting jet

The flame structure corresponding to lean hydrogen–air premixed flames in intense sheared turbulence in the thin reaction zone regime is quantified from flame thickness and conditional scalar dissipation rate statistics, obtained from recent direct numerical simulation data of premixed temporally-evolving turbulent slot jet flames [1]. It is found that, on average, these sheared turbulent flames are thinner than their corresponding planar laminar flames. Extensive analysis is performed to identify the reason for this counter-intuitive thinning effect. The factors controlling the flame thickness are analyzed through two different routes i.e., the kinematic route, and the transport and chemical kinetics route. The kinematic route is examined by comparing the statistics of the normal strain rate due to fluid motion with the statistics of the normal strain rate due to varying flame displacement speed or self-propagation. It is found that while the fluid normal straining is positive and tends to separate iso-scalar surfaces, the dominating normal strain rate due to self-propagation is negative and tends to bring the iso-scalar surfaces closer resulting in overall thinning of the flame. The transport and chemical kinetics route is examined by studying the non-unity Lewis number effect on the premixed flames. The effects from the kinematic route are found to couple with the transport and chemical kinetics route. In addition, the intermittency of the conditional scalar dissipation rate is also examined. It is found to exhibit a unique non-monotonicity of the exponent of the stretched exponential function, conventionally used to describe probability density function tails of such variables. The non-monotonicity is attributed to the detailed chemical structure of hydrogen-air flames in which heat release occurs close to the unburnt reactants at near free-stream temperatures.

Flow of a monatomic rarefied gas over a circular cylinder: Calculations based on the ab initio potential method

Two-dimensional flows of argon and helium over a circular cylinder are calculated by the Direct Simulation Monte Carlo (DSMC) method in the range of the free stream Mach number Ma∞ from 0.5 to 10 in nearly free molecular, transitional, and nearly continuum flows. In DSMC simulations, inter-particle collisions are calculated with the ab initio (AI) potential method based on the interatomic interaction potentials established in quantum mechanical calculations. It is shown that the AI potential method enables computationally efficient simulations of multidimensional rarefied gas flows without introducing semi-empirical models of collision cross sections. The calculated values of the drag CD and heat flux CQ coefficients of the cylinder for Ar and He at the same values of Ma∞, rarefaction parameter, and surface-to-free-stream temperature ratio are found to be different in less than 1%, ensuring small sensitivity of CD and CQ to the species of a monatomic gas. The simulation results obtained with the AI potential method are systematically compared with results obtained with the hard sphere (HS) molecular model. It is found that the choice of the HS diameter based on the condition of the identical viscosity of the real and HS gases at the free stream temperature ensures calculations of CD and CQ in sub- and supersonic flows at Ma∞⩽2 with errors less than 3% and 6.5%, correspondingly. For hypersonic flows, this choice of the HS diameter is unsatisfactory and results in the errors up to 7% in CD and 28% in CQ. A semi-empirical rule that defines an optimum HS diameter in super- and hypersonic flows is suggested. With the use of this rule, the HS model is capable of predicting CD and CQ with errors less than 1% and 3%, correspondingly, and also provides a good accuracy in calculations of local flow parameters.

Numerical investigation of thermo-sensitive cavitating flows in a wide range of free-stream temperatures and velocities in fluoroketone

The objectives of this paper are to validate an existing numerical modeling framework for fluoroketone and investigate the dynamic evolution of thermo-sensitive cavitating flows. The cavitating flows around a NACA0015 hydrofoil with chord length Co=50.8mm and angle of attack α0=10deg in a wide range of temperatures and velocities in fluoroketone are numerically investigated. Three thermal parameters, including nominal temperature drop ΔT∗, thermodynamic parameter Σ and C-factor, are applied to assess the thermodynamic characteristic of fluoroketone. It is found the thermodynamic effects on cavitating flows for fluoroketone at 373K and nitrogen at 83.06K are similar under the same reference cavitation number and Reynolds number. It indicates thermal parameters C-factor could accurately predict the extent of thermodynamic effects. General agreements are obtained between the numerical results and the experimental measurements, including the pressure distribution and cavity structures. The numerical results show that there are two typical cavitation dynamics in varying temperature fluoroketone under the same free-stream velocity and cavitation number. As the free-stream temperature increases, cavity area increases to the maximum at the transition temperature and then decreases, the dominant frequency significantly increases when the temperature reaches its transition point. Further analysis indicate that the liquid/vapor density ratio D dominates the change of the cavitation dynamics when temperature is below the transition temperature, and the cavity tends to be mushier and longer with the increasing temperature during this temperature range. However, the thermodynamic effects, which could suppress the development of the cavitating flow, dominate the change of the cavitation dynamics when temperature is above the transition temperature. For free-stream velocity U∞=9.6m/s, which has been experimentally investigated in the reference experiment, the transition temperature for thermo-sensitive cavitation is 318K (±2K) and the maximum temperature drop ΔTmax is approximately 0.82K under this condition. For varying free-stream velocity, the increasing velocity could suppress the thermodynamic effects, and hence the transition temperature increases with the increasing velocity under the same flow conditions.

Combined influence of fluctuations in the temperature and stretching velocity of the sheet on MHD flow of Cu-water nanofluid through rotating porous medium with cubic auto-catalysis chemical reaction

This work is devoted to examine the three dimensional flow of Cu-water nanofluid over a vertical oscillatory stretchable sheet having fluctuating temperature in comparison to the constant free stream temperature. The sheet embedded in porous medium is assumed to be rotating in a direction normal to its plane and is considered to be lying in a strong transverse magnetic field. The novel concepts of homogeneous-heterogeneous chemical reaction and thermal radiation are also incorporated while developing unsteady boundary layer equations governing the nanofluid flow. After exploiting appropriate transformations, the equations are reduced to dimensionless form which in turn are solved by employing an explicit finite difference scheme (EFDS). To obtain accurate solutions, stability and convergence conditions are also derived. Further, a comparative analysis is presented for the two cases namely fluctuating nanofluid flow (FNF) and non-fluctuating nanofluid flow (NFNF), and impacts of involved physical parameters on velocity distribution, temperature distribution and concentration fields are explored through graphs, and discussed in detail. The quantities of physical interest like surface shear stress, and heat and mass transfer rates are also computed for pertinent parameters and discussed with the help of tables. An interesting and novel result which we found is that oscillation can be utilized in delaying the boundary layer separation and moreover, in controlling the heat and mass transfer processes.

Nonlinear unsteady streaks engendered by the interaction of free-stream vorticity with a compressible boundary layer

The nonlinear response of a compressible boundary layer to unsteady free-stream vortical fluctuations of the convected-gust type is investigated theoretically and numerically. The free-stream Mach number is assumed to be of $O(1)$ and the effects of compressibility, including aerodynamic heating and heat transfer at the wall, are taken into account. Attention is focused on low-frequency perturbations, which induce strong streamwise-elongated components of the boundary-layer disturbances, known as streaks or Klebanoff modes. The amplitude of the disturbances is intense enough for nonlinear interactions to occur within the boundary layer. The generation and nonlinear evolution of the streaks, which acquire an $O(1)$ magnitude, are described on a self-consistent and first-principle basis using the mathematical framework of the nonlinear unsteady compressible boundary-region equations, which are derived herein for the first time. The free-stream flow is studied by including the boundary-layer displacement effect and the solution is matched asymptotically with the boundary-layer flow. The nonlinear interactions inside the boundary layer drive an unsteady two-dimensional flow of acoustic nature in the outer inviscid region through the displacement effect. A close analogy with the flow over a thin oscillating airfoil is exploited to find analytical solutions. This analogy has been widely employed to investigate steady flows over boundary layers, but is considered herein for the first time for unsteady boundary layers. In the subsonic regime the perturbation is felt from the plate in all directions, while at supersonic speeds the disturbance only propagates within the dihedron defined by the Mach line. Numerical computations are performed for carefully chosen parameters that characterize three practical applications: turbomachinery systems, supersonic flight conditions and wind tunnel experiments. The results show that nonlinearity plays a marked stabilizing role on the velocity and temperature streaks, and this is found to be the case for low-disturbance environments such as flight conditions. Increasing the free-stream Mach number inhibits the kinematic fluctuations but enhances the thermal streaks, relative to the free-stream velocity and temperature respectively, and the overall effect of nonlinearity becomes weaker. An abrupt deviation of the nonlinear solution from the linear one is observed in the case pertaining to a supersonic wind tunnel. Large-amplitude thermal streaks and the strong abrupt stabilizing effect of nonlinearity are two new features of supersonic flows. The present study provides an accurate signature of nonlinear streaks in compressible boundary layers, which is indispensable for the secondary instability analysis of unsteady streaky boundary-layer flows.

Thermal Radiation Effect on the MHD Turbulent Compressible Boundary Layer Flow with Adverse Pressure Gradient, Heat Transfer and Local Suction

The combined effect of magnetic field, thermal radiation and local suction on the steady turbulent compressible boundary layer flow with adverse pressure gradient is numerically studied. The magnetic field is constant and applied transversely to the direction of the flow. The fluid is subjected to a localized suction and is considered as a radiative optically thin gray fluid. The Reynolds Averaged Boundary Layer (RABL) equations with appropriate boundary conditions are transformed using the compressible Falkner Skan transformation. The nonlinear and coupled system of partial differential equations (PDEs) is solved using the Keller box method. For the eddy-kinematic viscosity the Baldwin Lomax turbulent model and for the turbulent Prandtl number the extended Kays Crawford model are used. The numerical results show that the flow field can be controlled by the combined effect of the applied magnetic field, thermal radiation, and localized suction, moving the separation point, xs , downstream towards the plate’s end, and increasing total drag, D . The combined effect of thermal radiation and magnetic field has a cooling effect on the fluid at the wall vicinity. The combined effect has a greater influence in the case of high free-stream temperature.

Unsteady mixed convection dusty fluid flow past a vertical wedge due to small fluctuation in free stream and surface temperature

We examine the unsteady characteristics of the mixed convection boundary layer flow of dusty fluid past a vertical wedge. The free stream and surface temperature are assumed to be fluctuating with small amplitude in time about a steady non-zero mean surface temperature and free stream velocity. The set of governing equations has been solved by two distinct methods, namely, the straightforward finite difference method for the entire frequency range, and the extended series solution for low frequency range and the asymptotic series expansion method for high frequency range. The effects of varying the ratio of the particle density to the gas density, κ, and Richardson’s number, Ri, are discussed in terms of the amplitudes and phase angles of the skin friction and heat transfer and the transient skin friction and heat transfer. The unsteady behaviors of streamlines and isolines of temperature are also observed with the change of these physical parameters as well as of the amplitude of oscillation, ϵ. The results show that the presence of particle into the fluid, buoyancy caused mixed convection and fluctuations of free stream and surface temperature significantly enhance the time-dependent skin friction and heat transfer.

On the Radiative Heat Transfer in a Viscoelastic Boundary Layer Flow Over a Stretching Sheet

Many researchers have studied the radiative heat transfer in a viscoelastic boundary layer flow over a stretching sheet after simplifying the complex nature of the radiative heat flux by expanding the fourth power of temperature (T4) in Taylor series about free-stream temperature (T∞) and neglecting the higher-order terms. Similarity solutions obtained by them are found to be valid only in the asymptotic region. This article suggests a modification in the linearization of T4 by introducing wall temperature (Tw) as well as freestream temperature (T∞) to capture the realistic nature of the temperature distribution in the boundary layer flows from locally nonsimilar solutions.

Evaluation of Computational Modeling of Electron Transpiration Cooling at High Enthalpies

A modeling approach for electron transpiration cooling of high-enthalpy flight is evaluated through comparison to a set of experiments performed in a plasma arc tunnel for air and argon. The comparisons include air and argon flow at high enthalpies (27.9 and , respectively), with a Mach number of 2.5 to 3. The conversion of the reported enthalpies and Mach numbers to freestream temperatures and velocities is discussed. The numerical approach is described, including implementation of a thermionic emission boundary condition and an electric field model. Also described is the implementation of a finite-rate chemistry model for argon ionization. Materials with different electron emission properties are also investigated, including graphite and tungsten. The comparisons include two different geometries with different leading-edge radii. The numerical results produce a wide range of emitted current due to the uncertainties in freestream conditions and emissive material properties, but they still agree well with the experimental measurements.

Thermo-Magneto-Convective Transport around a Square Cylinder in a Square Duct under Strong Axial Magnetic Field

A quasi two-dimensional numerical study is performed to analyze the thermo-magneto-convective transport of liquid metal around a square cylinder in a square duct subjected to a strong externally imposed axial magnetic field. The channel bottom wall is considered heated while the top wall is maintained at the free stream temperature keeping the cylinder adiabatic. The Reynolds and Hartmann numbers are kept in the range 0 4 with Ha = 2160 is also considered to show the development of a Kelvin-Helmholtz-type instability that substantially affects the heat transfer rate.

MHD Mixed Convection Flow of a Micropolar Fluid Past a Wedge Fixed in a Fluctuating Free Stream and Surface Temperature

The flow an heat transfer on the unstea y laminar mixe convection boun ary layer in a micropolar flui past a vertical we ge have been stu ie taking into account the effect of magnetic fiel . We assume that the free stream velocity an surface temperature oscillate in magnitu e but not in the irection of the oncoming flow velocity. The governing equations have been solve numerically by using the straight forwar finite ifference metho . The amplitu es of skin friction an couple stress are foun to be significantly epen ent on the Richar son’s number, Ri, the magnetic parameter, M, an the vortex viscosity parameter, K. We observe that the amplitu es of skin friction an couple stress increases owing to an increase of the Richar son’s number, Ri, while these become lower for the higher value of the magnetic parameter, M, an the vortex viscosity parameter, K. Also the results emonstrate that the effects of the parameters on the amplitu es of heat transfer are rather weak. Dhaka Univ. J. Sci. 64(1): 65-70, 2016 (January)

Modified kinematic viscosity model for 3D-Casson fluid flow within boundary layer formed on a surface at absolute zero

Three-dimensional Casson fluid flow towards a stagnation-point and a surface on which the heat energy falls at lower limit of thermodynamic temperature scale in the presence of cross diffusion is investigated. The flow is induced by a pressure-gradient and impulsive. We modified the space-dependent heat source in such a way that the uniform distribution of internal heat-source, also satisfies the wall and free stream temperatures. Also, the influence of heat energy on plastic dynamic viscosity is investigated by considering the relationship between the intermolecular forces holding the molecules of Casson fluid. The modified kinematic viscosity model suitable for the flow is presented. The governing equations are non-dimensionalized by using suitable similarity transformation, which unravels the behavior of the fluid flow at short-time and long-time period. The effects of pertinent parameters on the flow, heat and mass transfer is discussed with the help of graphs. Local Nusselt and Sherwood numbers are computed and presented through tables. Obtained results are validated and found a favorable agreement. It is observed that the Soret and Dufour parameters have tendency to control the thermal and concentration boundary layers as well as the heat and mass transfer rate.

Evaporation of multicomponent liquid fuel droplets: Influences of component composition in droplet and vapor concentration in free stream ambience

Theoretical studies have been made to explore the coupling influences of droplet constituent compositions and free stream vapor concentrations on evaporation characteristics, namely, mass depletion rate and droplet temperature of a heptane-dodecane droplet (components with widely varying volatility) and methanol-ethanol droplet (components with close volatility) at lower and higher free stream temperatures. In case of heptane-dodecane droplet, temperature and mass depletion histories are controlled by the heavier component (dodecane). In case of methanol-ethanol droplet, the mass depletion history is controlled by the lighter component (methanol), while the droplet temperature history is controlled by the heavier component (ethanol). The presence of fuel vapors in free stream increases the droplet lifetime, and moreover, the influence is relatively more prominent with the presence of heavier component vapor alone at lower free stream temperature. The composition of component vapors for a fixed value of total vapor concentration in free stream has almost negligible influence on droplet lifetime. It is revealed that the well established empirical relation (Sh(1+Bm)0.7=2) for mass evaporation rate of a single component droplet can be used with reasonably fair accuracy in case of multicomponent droplet evaporation with logical modifications of mass transfer number and mass transfer coefficient for multicomponent system.

Unsteady laminar mixed convection boundary layer flow near a vertical wedge due to oscillations in the free-stream and surface temperature

The unsteady laminar boundary layer characteristics of mixed convection flow past a vertical wedge have been investigated numerically. The free-stream velocity and surface temperature are assumed to be oscillating in the magnitude but not in the direction of the oncoming flow velocity. The governing equations have been solved by two distinct methods, namely, the straightforward finite difference method for the entire frequency range, and the extended series solution for low frequency range and the asymptotic series expansion method for high frequency range. The results demonstrate the effects of the Richardson number, Ri, introduced to quantify the influence of mixed convection and the Prandtl number, Pr, on the amplitudes and phase angles of the skin friction and heat transfer. In addition, the effects of these parameters are examined in terms of the transient skin friction and heat transfer.

NACA653218 Airfoil Aerodynamic Properties

In this research we have obtained the drag and lift coefficients, velocity, pressure and path lines contours using CFD which can also be determined by using wind tunnel experimental test. This process is relatively difficult and surely price more than CFD technique cost for the same problem solution. Thus we have gone through analytical method then it can be validated by experimental testing. A CFD procedure is described for determination aerodynamic characteristics of subsonic NACA653218 airfoil. Firstly, the airfoil model shape, boundary conditions and meshes were all formed in GAMBIT® 2.3.16 as a pre-processor. The second step in a CFD model should be to examine the effect of the mesh size on the solution results. In order to save time take case for a grid with around 100000 cells. The third step is validation of the CFD NACA653218 airfoil shape model by different turbulence models with available experimental data for the same model and operation conditions. The temperature of free stream is 288.2 K, which is the same as the environmental temperature. At the given temperature, the density of the air is ρ=1.225kg/m3, the pressure is 101325 Pa and the viscosity is μ=1.7894×10-5 kg/m s. A segregate, implicit solver is utilized (FLUENT® processor) estimate were prepared for angles of attack variety from -5 to 16°. The Spalart-Allmaras turbulence model is more accurate than standard k – e model, RNG k – e model and standard model k–e models. For lift coefficient, it is found maximum error by Spalart-Allmaras model about 12% lower than other turbulence models. For drag coefficient, it is found maximum error by Spalart-Allmaras model about 25% lower than other turbulence models. For pitching moment coefficient, it is found maximum error by Spalart-Allmaras model about 30% lower than other turbulence models.

高温水中の翼形キャビテーションの内部温度

Thermodynamic effect of cavitation is a favorable effect for pumps, in which cavity volume is suppressed and suction performance of the pump is improved. When evaporation occurs in low pressure region inside the pump, local temperature is decreased by the latent heat, the local saturated pressure decreases, then the cavity is suppressed. The thermodynamic effect is known to appear when the main stream temperature closes to critical temperature of the fluid. In order to clarify the basic characteristics of the thermodynamic effect of cavitation, high temperature and high pressure water tunnel is constructed, in which the free stream temperature can be increased to 140°C. In this study, by using high accuracy measuring technique of temperature inside a cavity, temperature distribution inside a supercavitation on a hydrofoil in a hot water at 80°C and the variation of temperature depression inside a supercavitation from room temperature water at 20°C to hot water at 80 °C are measured. And the temperature depression is predicted in more high temperature region by using theoretical prediction. Additionally, through comparison of cavity length between room temperature water and hot water, the thermodynamic effect in hot water is discussed.

Unsteady magnetohydrodynamic thermal and diffusion boundary layer from a horizontal circular cylinder

The unsteady 2-D dynamic, thermal, and diffusion magnetohydrodynamic laminar boundary layer flow over a horizontal cylinder of incompressible and electrical conductivity fluid, in mixed convection in the presence of heat source or sink and chemical reactions. The present magnetic field is homogenous and perpendicular to the body surface. It is assumed that induction of outer magnetic field is a function of longitudinal co-ordinate outer electric field is neglected and magnetic Reynolds number is significantly lower than one, i. e. considered the problem is in approximation without induction. Fluid electrical conductivity is constant. Free stream velocity, temperature, and concentration on the body are functions of longitudinal co-ordinate. The developed governing boundary layer equations and associated boundary conditions are made dimensionless using a suitable similarity transformation and similarity parameters. System of non-dimensionless equations is solved using the implicit finite difference three-diagonal and iteration method. Numerical results are obtained and presented for different Prandtl, Eckart, and Schmidt numbers, and values: magnetic parameter, temperature, and diffusion parameters, buoyancy temperature parameters, thermal parameter, and chemical reaction parameter. Variation of velocity profiles, temperature and diffusion distributions, and many integral and differential characteristics, boundary layer, are evaluated numerically for different values of the magnetic field. Transient effects of velocity, temperature and diffusion are analyzed. A part of obtained results is given in the form of figures and corresponding conclusions.

Unsteady mixed convection of a micropolar fluid past a circular cylinder due to time-dependent free stream velocity and temperature

Unsteady mixed convection boundary-layer flow of an electrically conducting micropolar fluid past a circular cylinder is investigated taking into account the effect of thermal radiation and heat generation or absorption. The reduced non-similar boundary-layer equations are solved using the finite difference method. It is found that the magnitude of the friction factor and the couple stress significantly increases due to the increase of the mixed convection parameter, the conduction-radiation parameter, the surface temperature parameter, the heat absorption parameter and the frequency parameter. However the magnitude of the heat transfer rate decreases with these parameters. The converse characteristics are observed for the Prandtl number. The magnitude of the couple stress and the heat transfer rate is seen to decrease whereas the magnitude of the skin factor increases with increasing the vortex viscosity parameter. The magnetic field parameter reduces the skin factor, couple stress and heat transfer rate. The amplitude of oscillation of the transient skin factor and couple stress gradually increases owing to an increase of $\xi$. But the transient heat transfer rate is found to be oscillating with almost the same amplitude for any value of $\xi$. The amplitude of oscillation of the transient skin factor and couple stress increases with an increase of $S$ and $\xi$ while the amplitude of the transient heat transfer rate increases with increasing Pr and $S$.

液体燃料ロケットタンク破壊時の推進剤ジェット・主流の干渉メカニズムに関する数値解析

Numerical simulations of the jet injected into supersonic cross-flow are carried out. In aborting process in liquid rocket flight, the fuel is transversely injected into the supersonic freestream. Developed jet/freestream interaction structure may trigger unexpected auto-ignition or explosion events; thus the understanding of the flow-field of jet-in-supersonic cross-flow is essential to meet the reliability of rocket. The two-dimensional numerical simulations are carried out in a wide-range of jet and freestream conditions: the jet-to-freestream momentum flux ratio and the pressure ratio. Present result shows recirculation regions are formed both at the upstream and downstream of injector with the characteristic vortex structures. With increasing the jet-to-freestream momentum flux ratio, the vortex structures in the recirculation region are largely developed. It is founded that the highest temperature in the separation region depends on the total temperature in the separation region and furthermore, it is the lower temperature than the total temperature of freestream.

Impact of Deep Gaps on Laminar–Turbulent Transition in Compressible Boundary-Layer Flow

Two-dimensional direct numerical simulations are used to study the impact of deep gaps on laminar–turbulent transition in compressible boundary-layer flow. For these, the gap depth-to-width ratio is always larger than five. They are located on a flat plate without pressure gradient. A steady base flow is used with a Mach number of 0.6, free-stream temperature of 288 K, and free-stream pressure of 1 bar. Subsequently, Tollmien–Schlichting waves are introduced by suction and blowing at the wall, and their growth over the gap is evaluated by factors. The influence of the gap on laminar–turbulent transition is quantified by the difference compared with the factor obtained for a flat plate without gap. A periodic influence of the gap depth on is observed. In the direct numerical simulations, acoustic waves enter the gap and form a standing wave due to reflections, similar as occurring in organ pipes. The feedback of the standing wave on the boundary-layer flow above is essential for the observed variations. In a second case, the influence of a specific gap placed in front of a forward-facing step is studied as well. Here, a reduction of the factor and hence a delay of transition, relative to the flow with step alone, are reached due to the presence of the gap.