NUMERICAL STUDY OF THE INFLUENCE OF DYNAMIC AND THERMAL EXIT CONDITIONS ON AXISYMMETRIC LAMINAR BUOYANT JET

We propose in our article numerical solutions of the Navier-Stokes equations governing the buoyant round laminar jet. The purpose of this work is to study the influence of the exit conditions at the nozzle exit on the dynamic and thermal parameters of the vertical jet flow. Two emission cases have been considered: Velocity and temperature are uniform or parabolic. The numerical code developed uses a finite difference scheme. The numerical results have been compared with those given by Martynenko, who considered in his analysis only two integration constraints: the momentum at the nozzle exit and the conservation of the energy transported by the jet flow. The obtained results are in good agreement with those proposed in the literature in the plume zone, in which the emission conditions are ignored, and the jet flow is governed mainly by the buoyancy forces. We propose in this paper a correlation to predict the axial virtual origin Xp of the plume zone. Experiments were conducted to validate the numerical model using a nonintrusive method, namely, laser doppler velocimetry.     

柴油机喷嘴喷孔内气液两相湍流场三维数值模拟

柴油机喷嘴内部空穴流动是影响喷雾特性极为重要的因素。在开发了喷嘴流动区域三维实体参数化生成软件的基础上,对完全发展了的空穴流动建立起三维空穴两相流动数学模型,提出将计算区域的出口边界延伸至气缸内部,以减小出口对喷孔内部求解区域的影响。对多孔垂直喷嘴进行了喷孔内部空穴两相湍流流动的三维数值模拟,得出喷嘴内部燃油流动的压力降基本发生在喷孔入口处,喷孔入口锐边过渡,使得该处必定出现空穴现象,且会延伸至喷孔出口,增加液流紊乱,引起燃油在喷孔出口的初次雾化。     

Effects of fuel inlet boundary condition on aromatic species formation in coflow diffusion flames

In a number of previous numerical studies, the fuel inlet velocity boundary conditions (BC) of coflow diffusion flames were specified at the exit of the fuel nozzle with a parabolic velocity profile. Such choices were based on the assumption that the flow inside the vertical fuel tube is fully developed and the buoyancy has negligible impact on the fuel flow at the nozzle exit. These assumptions, however, might not hold in practical experiments. This study demonstrates it is necessary to account for the effect of inlet BC location to accurately predict the nozzle exit velocity profile as well as the velocity, temperature profiles downstream, which are prerequisites for meaningful polycyclic aromatic hydrocarbon (PAH) and soot prediction in coflow diffusion flames. In particular, laboratory-scale laminar coflow diffusion flames at atmospheric pressure have been studied computationally with a focus on the effects of the fuel inlet velocity profile on PAH formation. Two sets of simulations were conducted which differ in the location specified for the fuel inlet boundary. In the first case, the fuel inlet boundary was specified at the nozzle exit while in the second case it was specified at a distance of 7 cm upstream of the nozzle exit. Parabolic velocity profiles were specified for both cases. In each set of simulations, flames with three different fuels (methane, ethylene and propane) were tested. Detailed high-temperature reaction mechanisms accounting for the formation of aromatic species were employed. The results showed that the fuel inlet BC location notably influence the predicted flow/temperature field and the resultant PAH concentration. Moreover, the effects become more notable with lower fuel stream velocities. It was also found that for propane with a density larger than air, recirculation zones were formed near the nozzle exit which exerted an additional influence on the flow development and temperature field as well as PAH formation. In addition, the effects of nozzle heating on flow development and PAH formation were also investigated.     

Fluid flow and heat transfer characteristics of cone orifice jet (effects of cone angle)

The use of a jet from an orifice nozzle with a saddle‐backed‐shape velocity profile and a contracted flow at the nozzle exit may improve the heat transfer characteristics on an impingement plate because of its larger centerline velocity. However, it requires more power to operate than a common nozzle because of its higher flow resistance. We therefore initially considered the use of a cone orifice nozzle to obtain better heat transfer performance as well as to decrease the flow resistance. We examined the effects of the cone angle α on the cone orifice free jet flow and heat transfer characteristics of the impinging jet. We compared two nozzles: a pipe nozzle and a quadrant nozzle. The first one provides a velocity profile of a fully developed turbulent pipe flow, and the second has a uniform velocity profile at the nozzle exit. We observed a significant enhancement of the heat transfer characteristics of the cone orifice jets at Re=1.5×10⁴. Using the cone orifice impinging jets enhanced the heat transfer rates as compared to the quadrant jet, even when the jets were supplied with the same operational power as the pipe jet. For instance, a maximum enhancement up to approximately 22% at r/d_o?0.5 is observed for α=15^°. In addition, an increase of approximately 7% is attained as compared to when the pipe jet was used. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20243     

Mixed Thermocapillary and Forced Convection Heat Transfer Around a Hemispherical Bubble in a Miniature Channel—A 3D Numerical Study

It has been established that for certain conditions, such as microgravity boiling, thermocapillary Marangoni flow has associated with it a significant enhancement of heat transfer. Typically, this phenomenon was investigated for the idealized case of an isolated and stationary bubble resting atop a heated solid that is immersed in a semi-infinite quiescent fluid or within a two-dimensional cavity. This article presents a three-dimensional numerical study that investigates the influence of thermal Marangoni convection on the fluid dynamics and heat transfer around a bubble during laminar flow of water in a minichannel. This mixed thermocapillary and forced convection problem is investigated for channel liquid inlet velocity of 0.01 m/s to 0.03 m/s and Marangoni numbers in the range of 10 to 300 under microgravity conditions. Three-dimensional effects become particularly important on the side and rear regions of the bubble. The thermocapillary forces accelerate the flow along almost the entire bubble interface. The hot core fluid from the heated bottom wall region is forced inward and propelled upward into the thermocapillary jet above the bubble. It can be quantified that the influence of thermocapillary flow on heat transfer enhancement shows an average increase by 40% at the downstream of the bubble and by 60% at the front and rear regions. This heat transfer enhancement depends mainly on the temperature differential as the driving potential for thermocapillary flow and bulk liquid velocity.     

Conjugate Mixed Convection Heat Transfer in Plane Laminar Wall Jet Flow

A numerical investigation of heat transfer from a uniformly heated slab of finite thickness by plane laminar wall jet flow under combined forced and natural convection, i.e., mixed convection, is presented. The problem has been solved for two classical cases such as Pr ? 1 and Pr ? 1. The effects of the Grashof number (Gr), Reynolds number (Re), Prandtl number (Pr), and thermal conductivity ratio (R_k) between the slab and fluid medium are investigated on the heat transfer characteristics, i.e., local Nusselt number, interface temperature, and average Nusselt number.     

INTERNAL HEAT TRANSFER AUGMENTATION IN A CHANNEL USING AN ALTERNATE SET OF POROUS CAVITY-BLOCK OBSTACLES

A numerical investigation for forced convection in a constant-temperature parallel plate channel with porous cavity and block alternately emplaced on the bottom plate is presented in this work. The Brinkman-Forchheimer-extended Darcy model, which accounts for the effects of impermeable boundary and inertia, is used to characterize the flow field inside the porous region. Solutions of the coupled governing equations are tarried out through the stream function-voracity analysis. The characteristics of fluid flow and forced convection heat transfer have been obtained by the examinations of various governing parameters, such as the Reynolds number, Darcy number, inertial parameter, Prandtl number, and two geometric parameters. Several interesting phenomena such as the heat transfer augmentation in the channel were presented and discussed. The results of this investigation indicate that the size of recirculation caused by porous block will have a profound effect on the flow and     

Effect of Annular Slit Geometry on Characteristics of Spiral Jet

A spiral flow using an annular slit connected to a conical cylinder does not need special device to generate a tangential velocity component of the flow and differs from swirling flows. Pressurized fluid is supplied to an annular chamber and injected into the convergent nozzle through the annular slit. The annular jet develops into the spiral flow. In the present study, a spiral jet discharged out of nozzle exit was obtained by using a convergent nozzle and an annular slit set in nozzle inlet, and the effect of annular slit geometry on characteristics of the spiral jet was investigated by using a Laser Doppler Velocimeter (LDV) experimentally. Furthermore, velocity distributions of the spiral jet were compared with those of a normal jet.     

Jet impingement onto a conical cavity: Effects of annular nozzle outer angle and jet velocity on heat transfer and skin friction

Jet impingement onto a conical cavity results in complicated flow structure in the region of the cavity. Depending on the nozzle geometric configurations and jet velocities, enhancement in the heat transfer rates from the cavity surface is possible. In the present study, annular nozzle and jet impingement onto a conical cavity are considered and heat transfer rates from the cavity surfaces are examined for various jet velocities, two outer angles of the annular nozzle, and two cavity depths. A numerical scheme adopting the control volume approach is used to simulate the flow situation and predict the heat transfer rates. It is found that increasing jet velocity at the nozzle exit modifies the flow structure in the cavity while altering the heat transfer rates and skin friction; in which case, increasing nozzle outer angle and jet velocity enhances the heat transfer rates and skin friction.     

Jet characteristics from a submerged combustion system

The characteristics of a combustor operating under submerged conditions are affected by the two phase interaction of exhaust gas jet from the combustor with surrounding liquid. The characteristics of combustion gases are simulated with air and helium to represent combustor operation under different conditions. The exhaust gas signatures under submerged conditions are examined using different nozzle exit cross-sections (circular, square, triangular and elliptical with aspect ratio of 1.5 and 2.5) for their effect on sound pressure levels and pressure fluctuations in the combustion chamber. High-speed cinematography is used to examine the two-phase region and the associated instabilities by the gas jet. Dynamic pressure sensor is used to study the effect of submerged jet on the pressure fluctuations in the upstream gas chamber. The sound pressure level from the elliptical nozzle is found to be lower than the circular, square and triangular nozzles. The frequency of jet instabilities is observed to increase with increase in gas jet momentum but independent of nozzle exit cross-section. The pressure fluctuation in the gas chamber is closely coupled with two phase instabilities downstream of the jet region. At lower jet momentum bubbling regime is present but it transitions to more jet like behavior with increase in the jet momentum, representing deep water and shallow water propulsion applications, respectively. These studies provide valuable fundamental information for range of applications in energy systems extending from underwater propulsion, evaporator, heater, desalination and waste water treatment.     

流线形喷嘴时射流泵流场的数值模拟

为分析流线形喷嘴时射流泵的水力特性,采用紊流数值模拟方法,对流线形喷嘴时射流泵流场进行了三维计算。结果表明,随着流量比的增大,工作液流核区衰减得越慢,在喉管入口段工作液提升被吸液的区域有所减小;流量比越大喉管内紊动能最大值出现的位置会靠后且其数值会降低,两股液体混掺作用会变弱;喉管内压力随流量比的增大而逐渐降低且负压区范围有所扩大,最低负压发生在喉管壁处。同一流量比下,流线形喷嘴时射流泵的压力比略微高于圆锥形喷嘴时,但差别很小。流线形喷嘴时射流泵流场的计算成果,可为研究射流泵水力特性提供参考。     

Buoyancy and wall conduction effects on forced convection of micropolar fluid flow along a vertical slender hollow circular cylinder

This paper presents a numerical analysis of the flow and heat transfer characteristics of forced convection in a micropolar fluid flowing along a vertical slender hollow circular cylinder with wall conduction and buoyancy effects. The non-linear formulation governing equations and their associated boundary conditions are solved using the cubic spline collocation method and the finite difference scheme with a local non-similar transformation. This study investigates the effects of the conjugate heat transfer parameter, the Richardson number, the micropolar parameter, and the Prandtl number on the flow and the thermal fields. The effect of wall conduction on the thermal and the flow fields are found to be more pronounced in a system with a greater buoyancy effect or Prandtl number but is less sensitive with a greater micropolar material parameter. Compared to the case of pure forced convection, buoyancy effect is found to result in a lower interfacial temperature but higher the local heat transfer rate and the skin friction factor. Finally, compared to Newtonian fluid, an increase in the interfacial temperature, a reduction in the skin friction factor, and a reduction in the local heat transfer rate are identified in the current micropolar fluid case.     

A Study of Under-expanded Moist Air Jet Impinging on a Flat Plate

When a gas expands through a convergent nozzle in which the ratio of the ambient to the stagnation pressures is higher than that of the critical one, the issuing jet from the nozzle is under-expanded. If a flat plate is placed normal to the jet at a certain distance from the nozzle, a detached shock wave is formed at a region between the nozzle exit and the plate. In general, supersonic moist air jet technologies with non-equilibrium condensation are very often applied to industrial manufacturing processes. In spite of the importance in major characteristics of the supersonic moist air jets impinging to a solid body, its qualitative characteristics are not known satisfactorily. In the present study, the effect of the non-equilibrium condensation on the under-expanded air jet impinging on a vertical flat plate is investigated numerically in the case with non-equilibrium condensation, frequency of oscillation for the flow field becomes larger than that without the non-equilibrium condensation, and amplitudes of static pressure become small compared with those of dry air. Furthermore, the numerical results are compared with experimental ones.     

Numerical simulation of double diffusive mixed convection in an open enclosure with different cylinder locations

This article reports numerical simulation of the double diffusive mixed convection around a cylinder in an open enclosure with an inlet and exit ports. The temperature and mass concentration of the cylinder are higher than those of the inlet flow and the cylinder can be at three different locations (lower, middle and upper) in the enclosure. The inlet flow with low temperature and mass concentration is located at the lower-left wall of the enclosure and the exit is at the upper-right wall. Other walls are assumed to be adiabatic. Effects of Lewis number Le, buoyancy ratio Br, and cylinder locations on the double diffusive mixed convection are investigated at Richardson number Ri = 1.0 and 0.01 while Prandtl number Pr is kept at 0.7. Streamlines, isotherms, isoconcentrations, and the average and local Sherwood number at different parameters are reported to characterize the double diffusive mixed convection phenomena in the open enclosure.     

Effect of tabs on impinging heat transfer

The present work experimentally investigates the effect of vortex generators, in the form of small tabs projecting normally into the flow at the nozzle exit, on the fluid flow and heat transfer characteristics of an axisymmetric impinging air jet in the subcritical Reynolds number range. With this comes the expectation of a large eddy structure variation and the possibility of active control. Local heat transfer and static pressure were measured on a target plate for a round air jet issuing from a circular nozzle with rectangular tabs whose numbers and lengths changed at a constant nozzle‐to‐plate gap (L/d = 8) and jet Reynolds number (Re = 34,000). The main results are the following: When two tabs were set at the exit of the circular nozzle, Cpw and Nu profiles flatten in the direction of the tab setting. In the case of three tabs, however, among both Cpw and Nu profiles a concentric profile is found, as well as in the case without any tabs. © 2001 Scripta Technica, Heat Trans Asian Res, 30(7): 561–570, 2001     

Numerical simulation of supercritical carbon dioxide flows across critical point

Numerical study of supercritical carbon-dioxide flows across the critical point is presented. The present numerical method is based on the preconditioning method developed by Yamamoto and mathematical models of thermophysical properties for carbon dioxide programmed in the program package for thermophysical properties of fluids, developed by Kyushu University. First, the two-dimensional natural convection of carbon dioxide between two parallel plates is calculated while changing the bulk pressure. The calculated thermophysical properties of the carbon-dioxide flow under supercritical pressure are compared with those in a gas condition. Next, the natural convection of carbon dioxide in an O-shaped cyclic channel is calculated, and the effect of the density difference induced by the phase change to the flow is investigated. For application to high-speed flows, supercritical carbon dioxide flows through a nozzle with free-jet expansion (known as the process of rapid expansion of supercritical solutions) process are calculated. The calculated shock distance to the Mach disk generated in the free jet is compared with experiments and the density variations in the nozzle while changing the inlet temperature are numerically predicted.     

Prediction of the flow structure in a turbulent rectangular free jet

A numerical analysis is conducted for turbulent flow of a rectangular free jet with an aspect ratio of 2:1. The computations were performed using two standard two-equation turbulence models (the k–ε and the k–ω models). Two inflow boundary conditions were evaluated with each model: a uniform inlet velocity profile and a profiled inlet velocity fitted to experimental data. The results show that the k–ε model with the profiled inlet velocity succeeded in predicting the main features of the flow, including the vena contracta and the saddle-shaped velocity profiles in the near-field region, and the rate of velocity decay in the far-field region.     

COMBINED FORCED AND BUOYANCY-INDUCED CONVECTIVE HEAT TRANSFER IN A PARTIALLY CLOSED VERTICAL CHANNEL

This paper reports a numerical study of mixed convection flow over a horizontal surface in a partially closed adiabatic symmetric vertical channel. For low Rayleigh numbers, the forced flow enhances the buoyant convective heat transfer rates. For higher Rayleigh numbers, complex interactions arise between the buoyancy-induced flow and forced convection. The shearing between the buoyancy-dominated wall layer and the forced flow over it induces a stagnation region and a pair of weak recirculating cells in the middle of the channel above the horizontal surface. This effectively reduces heat transfer rates. Nusselt number data are presented for a wide range of Rayleigh and Reynolds numbers. Finally, the effect of the inlet height of forced flow on the transport is discussed.     

Mixed convection in wall plumesConvection mixte dans les panaches parietauxErzwungene und freie konvektion bei wandauftriebsströmungenCмeшaннaя кoнвeкция в cвoБoднoкoнвeктивныч cтpyяч нa cтeнкe

An analysis is presented for mixed convection effects in a wall plume flow. The results apply downstream from the source. The method of matched asymptotic expansions has been used to obtain a consistent solution by simultaneously including both the effects of mixed convection and of higher order boundary layer corrections. Several subtle and interesting features of this flow arise and are explained. The unexpected role played by Prandtl number is clearly illustrated. For example, the effect of an aiding imposed forced flow is to reduce surface shear stress, for Pr = 0.7, and to increase it for Pr = 6.7. Further, it has been shown that such a Prandtl number effect on the surface shear stress is dependent on the kind of heating condition imposed at the surface. Results of numerical calculations are presented for two values of Prandtl number, 0.7 and 6.7, as characteristic of air and water, respectively. The overall effect of mixed convection on the downstream decay of surface temperature and on the maximum value of tangential velocity is seen to be greater in air than in water.     

Study of heat transfer for a pair of rectangular jets impinging on an inclined surface

Critical design parameters in jet impingement heat transfer like nozzle hydraulic diameter, jet angle and velocity, physical properties of the fluid, and nozzle-to-target plane spacing are the subject. This paper identifies the dominant fluid-thermal characteristics of a pair of rectangular air jets impinging on an inclined surface. Heat transfer modes and flow characteristics are studied with eight different Reynolds numbers ranging from 500 to 20 000. Local and average Nusselt numbers are evaluated with two different boundary conditions on three specified lines located on the inclined surface. The correlation between stagnation Nusselt number and Reynolds number is presented. Turbulent intensity and wall y⁺ distributions are compared on three lines parallel to the incline. The effect of jet impingement angle on local and average Nusselt number is also documented. Finally, a correlation between the average Nusselt number, nozzle exit Reynolds number and the jet angle is documented.