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1.
In the present case, the conjugate heat transfer involving a turbulent plane offset jet is considered. The bottom wall of
the solid block is maintained at an isothermal temperature higher than the jet inlet temperature. The parameters considered
are the offset ratio (OR), the conductivity ratio (K), the solid slab thickness (S) and the Prandtl number (Pr). The Reynolds number considered is 15,000 because the flow becomes fully turbulent and then it becomes independent of the
Reynolds number. The ranges of parameters considered are: OR = 3, 7 and 11, K = 1–1,000, S = 1–10 and Pr = 0.01–100. High Reynolds number two-equation model (k–ε) has been used for turbulence modeling. Results for the solid–fluid interface temperature, local Nusselt number, local
heat flux, average Nusselt number and average heat transfer have been presented and discussed. 相似文献
2.
H. H. Liakos E. P. Keramida M. A. Founti N. C. Markatos 《Heat and Mass Transfer》2002,38(4-5):425-432
Impinging jet combusting flows on granite plates are studied. A mathematical model for calculating heat release in turbulent
impinging premixed flames is developed. The combustion including radiative heat transfer and local extinction effects, and
flow characteristics are modeled using a finite volume computational approach. Two different eddy viscosity turbulence models,
namely the standard k–ɛ and the RNG k–ɛ model with and without radiation (discrete transfer model) are assessed. The heat released predictions are compared with
experimental data and the agreement is satisfactory only when both radiative heat transfer and local extinction modeling are
taken into account. The results indicate that the main effect of radiation is the decrease of temperature values near the
jet stagnation point and along the plate surface. Radiation increases temperature gradients and affects predicted turbulence
levels independently of the closure model used. Also, the RNG k–ɛ predicts higher temperatures close the solid plate, with and without radiative heat transfer.
Received on 13 November 2000 / Published online: 29 November 2001 相似文献
3.
This study is motivated by understanding the connections between the vortical structures in impinging jets and the wall heat transfer. Of particular interest are: (1) examining how the stage of evolution of vortex pairing in the jet might influence the wall heat transfer, and (2) establishing correlations between the vortex characteristics and the Nusselt number (Nu) distribution. To this end, CFD simulations are conducted of three simplified model problems involving the interaction of isolated axisymmetric vortex rings with a flat, constant-temperature, heated wall. The cases represent three scenarios of vortex-wall interaction: before (Case I), during (Case II) and after (Case III) pairing. The results show that when two vortices concurrently interact with the wall and undergo pairing (Case II), a significant instantaneous enhancement in Nu is attained in comparison to that associated with a single vortex interacting with the wall (Cases I and III). However, Case II also leads to the largest subsequent decay in Nu enhancement due to the formation of a particularly strong secondary vortex. In all cases, a deterioration in Nu, relative to unsteady diffusion, is observed simultaneously with the enhancement. Notwithstanding this deterioration, the net effect of vortex-wall interaction on the heat transfer remains positive with Case II producing the highest heat transfer rate. An analysis is conducted to establish the connection between the instantaneous maximum and minimum Nu, the circulation and the radial and the wall-normal location of the core-centers of the vortices, the thermal boundary layer thickness, the boundary layer separation location and the wall shear stress. 相似文献
4.
5.
The free convective flow and heat transfer, within the framework of Boussinesq approximation, in an anisotropic fluid filled
porous rectangular enclosure subjected to end-to-end temperature difference have been investigated using Brinkman extended
non-Darcy flow model. The studies involve simultaneous consideration of hydrodynamic and thermal anisotropy. The flow and
temperature fields in general are governed by, Ra, the Rayleigh number, AR, the aspect ratio of the slab, K*, the permeability ratio and k*, the thermal conductivity ratio, and Da, Darcy number. Numerical solutions employing the successive accelerated replacement (SAR) scheme have been obtained for 100 ≤
Ra ≤ 1000, 0.5 ≤ AR ≤ 5, 0.5 ≤ K* ≤ 5, 0.5 ≤ k* ≤ 5, and 0 ≤ Da ≤ 0.1. It has been found that [`(Nu)]{\overline {Nu}}, average Nusselt number increases with increase in K* and decreases as k* increases. However, the magnitude of the change in [`(Nu)]{\overline {Nu}} depends on the parameter Da, characterizing the Brinkman extended non-Darcy flow. 相似文献
6.
Kalabin et al. (Numer. Heat Transfer A 47, 621-631, 2005) studied the unsteady natural convection for the sinusoidal oscillating wall temperature on one side wall
and constant average temperature on the opposing side wall. The present article is on the unsteady natural convective heat
transfer in an inclined porous cavity with similar temperature boundary conditions as those of Kalabin et al. The inclined
angle of the cavity is varied from 0° to 80°. The flow field is modeled with the Brinkman-extended Darcy model. The combined effects
of inclination angle of the enclosure and oscillation frequency of wall temperature are studied for Ra* = 103, Da = 10−3, , and Pr=1. Some results are also obtained with the Darcy–Brinkman–Forchheimer model and Darcy’s law and are compared with the present
Brinkman-extended Darcy model. The maximal heat transfer rate is attained at the oscillating frequency f = 46.7π and the inclined angle . 相似文献
7.
S. A. Nada 《Heat and Mass Transfer》2009,45(8):1083-1097
The present article reports on heat transfer characteristics associated with multiple laminar impinging air jet cooling a
hot flat plat at different orientations. The work aims to study the interactions of the effects of cross flow, buoyancy induced
flow, orientation of the hot surface with respect to gravity, Reynolds numbers and Rayleigh numbers on heat transfer characteristics.
Experiments have been carried out for different values of jet Reynolds number, Rayleigh number and cross flow strength and
at different orientations of the air jet with respect to the target hot plate. In general, the effective cooling of the plate
has been observed to be increased with increasing Reynolds number and Rayleigh number. The results concluded that the hot
surface orientation is important for optimum performance in practical applications. It was found that for Re ≥ 400 and Ra ≥ 10,000 (these ranges give 0.0142 ≤ Ri ≤ 1.59 the Nusselt number is independent on the hot surface orientation. However, for Re ≤ 300 and Ra ≥ 100,000 (these ranges give 1.59 ≤ Ri ≤ 42.85): (i) the Nusselt number for horizontal orientation with hot surface facing down is less that that of vertical orientation
and that of horizontal orientation with hot surface facing up, and (ii) the Nusselt number of vertical orientation is approximately
the same as that of horizontal orientation with hot surface facing up. For all surfaces orientations and for the entire ranges
of Re and Ra, it was found that increasing the cross flow strength decreases the effective cooling of the surface. 相似文献
8.
Ikue Itamoto Hideshi Ishida Masahiko Higashiyama Daigo Miki Genta Kawahara 《Heat and Mass Transfer》2009,46(2):153-165
In this study surface-averaged and extremal properties of heat transfer and shear stress on the upper wall surface of Rayleigh–Bénard
convection are numerically examined. The Prandtl number was raised up to 103, and the Rayleigh number was changed between 104 and 107. As a result, average Nusselt number Nu and shear rate τ/Pr depends on Pr, Ra, and the entire numerical results are distributed between two correlation equations corresponding to small and large Pr. The small and large Pr equations are closely related to steady and unsteady flow regimes, respectively. Nevertheless, a single relation τ/Pr ~ Nu
3.0 exists to explain the entire results. Similarly the change of local maximal properties Nu
max and τ
max/Pr depends on Pr, Ra, and these values are also distributed between two correlation equations corresponding to small and large Pr cases. Despite such complicated dependence we can obtain a correlation equation as a form of τ
max/Pr ~ Nu
max2.6, which has not been obtained theoretically. 相似文献
9.
The present paper deals with the prediction of three-dimensional fluid flow and heat transfer in rib-roughened ducts of square
cross-section, which are either stationary, or rotate in orthogonal mode. The main objective is to assess how a recently developed
variant of a cubic non-linear k−ε model (proposed by Craft et al. Flow Turbul Combust 63:59–80, 1999) can predict three-dimensional flow and heat transfer characteristics through stationary and rotating ribbed ducts. The present
paper discusses turbulent air flow and heat transfer through two different configurations, namely: (I) a stationary square
duct with “in-line” normal and (II) a square duct with normal ribs in a “staggered” arrangement under stationary and rotating
conditions, with the axis of rotation normal to the flow direction and parallel to the ribs. In this paper the flow and thermal
predictions of the linear k−ε model (EVM) are also included, as a set of baseline predictions. The mean flow predictions show that both linear and non-linear
k−ε models can successfully reproduce most of the measured data for stream-wise and cross-stream velocity components. Moreover,
the non-linear model is able to produce better results for the turbulent stresses. The heat transfer predictions show that
both EVM and NLEVM2, the more recent variant of the non-linear k−ε, with the algebraic length-scale correction term, overestimate the measured Nusselt numbers for both geometries examined.
While the EVM with the differential length-scale correction term underestimates heat transfer levels, the Nusselt number predictions
with the NLEVM2 and the ‘NYP’ term are in close agreements with the measured data. Comparisons with our earlier work, Iacovides
and Raisee (Int J Heat Fluid Flow, 20:320–328, 1999), show that the NLEVM2 thermal predictions are of similar quality to those of a second-moment closure. 相似文献
10.
Grant England Peter A. M. Kalt Graham J. Nathan Richard M. Kelso 《Experiments in fluids》2010,48(1):69-80
The triangular oscillating jet nozzle generates a triangular jet partially confined within an axi-symmetric chamber to produce
a large scale flow oscillation that has application in thermal processes. Particle image velocimetry and oscillation frequency
measurements were conducted to investigate the influence of the jet fluid to ambient fluid density ratio on the resulting
oscillating flow. The investigation was conducted with a jet momentum flux of 0.06 kg m s−2 (Re = 7.3−47.2 × 103) and density ratios ranging from 0.2 to 5.0. The initial spread and decay of the emerging jet was found to depend upon the
density ratio but in a more complex way than does an unconfined jet. Both the spread and decay are strongly influenced by
the instantaneous angle of jet deflection, with greater deflection leading to increased spreading and decay of the jet. Decreasing
the density ratio below unity results in a rapid decrease in the deflection angle, while increasing the density ratio above
unity results in an increase in the deflection angle, albeit with less sensitivity. The frequency of oscillation was also
shown to depend on the density ratio with an increase in the density ratio causing a decrease in the dominant oscillation
frequency. 相似文献
11.
Natural convection in a partially filled porous square cavity is numerically investigated using SIMPLEC method. The Brinkman-Forchheimer
extended model was used to govern the flow in the porous medium region. At the porous-fluid interface, the flow boundary condition
imposed is a shear stress jump, which includes both the viscous and inertial effects, together with a continuity of normal
stress. The thermal boundary condition is continuity of temperature and heat flux. The results are presented with flow configurations
and isotherms, local and average Nusselt number along the cold wall for different Darcy numbers from 10−1 to 10−6, porosity values from 0.2 to 0.8, Rayleigh numbers from 103 to 107, and the ratio of porous layer thickness to cavity height from 0 to 0.50. The flow pattern inside the cavity is affected
with these parameters and hence the local and global heat transfer. A modified Darcy–Rayleigh number is proposed for the heat
convection intensity in porous/fluid filled domains. When its value is less than unit, global heat transfer keeps unchanged.
The interfacial stress jump coefficients β
1 and β
2 were varied from −1 to +1, and their effects on the local and average Nusselt numbers, velocity and temperature profiles
in the mid-width of the cavity are investigated. 相似文献
12.
A mixedness-reactedness flamelet combustion model coupled with a comprehensive radiation heat transfer model based on the
discrete transfer method of solution of the radiative transport equation is applied for the simulation of a 3 MW non-swirling
turbulent non-premixed natural gas flame in the experimental furnace at the International Flame Research Foundation. In the
calculation, turbulence is represented by the standard k − ε and a differential Reynolds-stress model. Predictions are compared with measurements of mean gas velocity, temperature, major
species concentrations and incident radiation wall flux. The radiative mixedness-reactedness flamelet combustion model, irrespective
of the model for turbulence, is able to reproduce the basic structure of the experimental flame, which is stabilised downstream
of the burner nozzle. In the near burner region, encompassing the non-reacting lift-off zone, good quality predictions are
obtained using both the turbulence models, whereas further downstream, within the combusting zone of the jet, the Reynolds-stress
turbulence model generates better predictions at and about the furnace axis. The nitric oxide (NO) formation via the thermal-
and prompt-NO routes was also calculated and compared with in-flame and flue-gas NO data. The measured NO level at the furnace
exit is well reproduced in the calculation, however discrepancies exist near the burner where NO concentrations around the
furnace axis are overpredicted. 相似文献
13.
The purpose of this paper is to focus on the experimentally obtained results of impinging jet applications by the help of
two different analysis methods. Circular round pipes (D = 7.9, 10.8, 13.8 and 23.1 mm) have been used as the impinging jets. The heat transfer is calculated with Nusselt number
(Nu). The variable parameters are the dimensionless jet-to-impingement plate distance (z/D), Reynolds number (Re) and dimensionless temperature measurement points on the heated surface (x/L, y/L). Some important analysis methods such as artificial neural network (ANN), statistical regression, and uncertainty analysis
are applied to the obtained data. It is shown that the ANN application is not simply a classification analysis; it is actually
an application of the convergence of functions. As a result, by considering random data, 4.57% convergence level is obtained
regarding the pipe diameter. The software STATISTICA 5.0 is used to estimate new empirical correlations nonlinearly. The smallest
regression coefficient for the correlations is 0.87, while the highest value is 0.99. The result of the uncertainty analyses
showed that the total uncertainties are in the agreeable range; 8% for Nu, and 2.89% for Re.
Dr. Nevin Celik is a Post Doctoral Fellowship in University of Minnesota since August 2007. 相似文献
14.
A SIMPLE-C algorithm and Jones-Launder k-ε two-equation turbulence model are used to simulate a two-dimensional jet impinging obliquely on a flat surface. Both the continuity and momentum equations for the unsteady state are cast into suitable finite difference equations. The pressure, velocity, turbulent kinetic energy and turbulent energy dissipation rate distributions are solved and show good agreement with various experimental data. The calculations show that the flow field structure of the jet impinging obliquely on a flat surface is strongly affected by the oblique impingement angle. The maximum pressure zone of the obliquely impinging jet flow field moves towards the left as the oblique impingement angle is decreased. 相似文献
15.
This paper reports a numerical investigation on the prediction of the thermal and hydrodynamic flow fields of multi micro
jet impingement cooling of three dimensional turbine vanes. A three dimensional vane is modeled with an in-line array of impinging
jets of diameters 0.5 and 0.25 mm. The numerical model consists of the steady, Reynolds-Averaged Navier–Stokes equations and
the Kω SST Turbulence model. The governing equations are solved using a finite volume method. The crossflow mass velocity (G
c
) to jet mass velocity (G
j
) ratio, and the average and local heat transfer distributions are analyzed with varying mass velocity and jet-to-target spacing.
It is found out that a significant decrease in crossflow ratio occurs with the smaller diameters. Due to the lower crossflow
and higher exit velocities of the smaller jets, the penetration into the crossflow is much higher. Moreover, at a constant
mass flow, the use of micro-jets enhanced the overall average heat transfer coefficient by 63%, while at a fixed pressure
drop across the vane instead of the mass flow, the smaller diameters will still yield an enhancement of 34.3% in the overall
average heat transfer coefficient. 相似文献
16.
The characteristics of unsteady entrance heat transfer in the combined entrance heat transfer region of laminar pipe flows resulting from time-varying inlet temperature are numerically investigated. Three non-dimensional parameters,Nu 0, a*, andf are identified in the study. Also, their effects on the non-dimensional duct wall temperature, fluid bulk temperature, and duct wall heat flux are discussed in great detail. Comparisons are made with the zero thermal capacity wall solution. 相似文献
17.
The effects of the air variable properties (density, viscosity and thermal conductivity) on the buoyancy-driven flows established
in open square cavities are investigated, as well as the influence of the stated boundary conditions at open edges and the
employed differencing scheme. Two-dimensional, laminar, transitional and turbulent simulations are obtained, considering both
uniform wall temperature and uniform heat flux heating conditions. In transitional and turbulent cases, the low-Reynolds k − ω turbulence model is employed. The average Nusselt number and the dimensionless mass-flow rate have been obtained for
a wide and not yet covered range of the Rayleigh number varying from 103 to 1016. The results obtained taking into account variable properties effects are compared with those calculated assuming constant
properties and the Boussinesq approximation. For uniform heat flux heating, a correlation for the critical heating parameter
above which the burnout phenomenon can be obtained is presented, not reported in previous works. The effects of variable properties
on the flow patterns are analyzed. 相似文献
18.
The flow of a two-dimensional plane turbulent jet impinging on a porous screen has been studied experimentally. It is shown
how the overall flow structure depends on the porosity of the surface. For low screen porosity (β < 0.41, say), transverse wall jets can be formed on both sides of the screen and in extreme cases the axial momentum flux some way downstream of the screen falls to zero, so that
the screen has the same drag as would a solid wall. For high screen porosity (β > 0.57, say) the axial volume flux is largely preserved through the screen, but the dominant eddy structures present in the
upstream jet are largely destroyed, so that entrainment rates downstream of the screen can be very low. The relatively small,
intermediate range of porosities (0.41 < β < 0.57, where β is the screen open area ratio) is associated with dramatic changes in flow pattern and recirculating regions can exist on
the upstream side of the screen. These flows, although all geometrically very simple, provide a serious challenge for computational
modelling.
Received: 25 May 2000 / Accepted: 22 February 2001 相似文献
19.
The flow boiling heat transfer coefficients of R-134a/R-290/R-600a (91%:4.068%:4.932% by mass) refrigerant mixture are experimentally
arrived in two tubes of diameter 9.52 and 12.7 mm. The tests are conducted to target the varied heat flux condition and stratified
flow pattern found in evaporators of refrigerators and deep freezers. The varied heat flux condition is imposed on the refrigerant
using a coaxial counter-current heat exchanger test section. The experiments are performed for mass flow rates of the refrigerant
mixture between 3 and 5 g s−1 and entry temperature between −8.59 and 5.33°C which are bubble temperatures corresponding to a pressure of 3.2 and 5 bar.
The influences of heat flux, mass flow rate, pressure, flow pattern, tube diameter on the heat transfer coefficient are discussed.
The profound effects of nucleate boiling prevailing even at higher vapor qualities in evaporators are highlighted. The heat
transfer coefficient of the refrigerant mixture is also compared with that of R-134a. 相似文献
20.
Heat transfer characteristics of premixed butane/air flame jet impinging on an inclined flat surface 总被引:1,自引:0,他引:1
A series of experiments were carried out to determine the heat transfer characteristics of a round, premixed butane/air flame
jet impinging upwards on an inclined flat plate, at different angles of incidence. The flame was fixed with an equivalence
ratio of 1.0, a Reynolds number of 2500 and a plate-to-nozzle distance of 5d, while the inclination angles chosen for investigation were 57°, 67°, 80° and 90°. It was found that the location of the
maximum heat flux point would be shifted away from the geometrical impingement point by reducing the angle of incidence. Decreasing
the angle of incidence also enhanced the maximum local heat flux, while reduced the average heat transfer. The present study
presented the effect of angle of incidence on the heat transfer characteristics of an impinging butane/air flame jet, which
had been rarely reported in previous similar studies.
Received on 11 October 2000
The authors wish to thank The Hong Kong Polytechnic University for the financial support of the present study. 相似文献