LES of a Round Impinging Jet: Investigation of the Link Between Nusselt Secondary Peak and Near-Wall Vortical Structures

2016 ◽  
Author(s):  
Pierre Aillaud ◽  
Florent Duchaine ◽  
Laurent Gicquel
Keyword(s):  
Heat Transfer ◽  
Axial Velocity ◽  
Large Scale ◽  
Local Heat Transfer ◽  
Impinging Jet ◽  
Secondary Peak ◽  
Wall Heat Transfer ◽  
Mixing Processes ◽  
Vortical Structures ◽  
The Mean

In an attempt to improve our understanding of the fundamental flow problem that is an impinging jet, a wall-resolved Large Eddy Simulation (LES) is produced to investigate large-scale unsteady flow features, mixing processes near the wall and heat transfer. The simulation focuses on a single unconfined round jet normally impinging on a flat plate at a Reynolds number (based on the pipe diameter and bulk velocity) of Re = 23 000 and for a nozzle to plate distance of H = 2D. This configuration is known to lead to a double peak in the Nusselt distribution. Evaluation of the high order statistics, such as Skewness and Kurtosis of the temporal evolution of axial velocity and wall heat flux, provides first-ever insights into the effect of the vortical structures on the mean wall heat transfer. Heat transfer statistics such as probability density functions (PDF) confirm the ability of LES to reproduce the strong intermittent thermal events responsible for the increase of the mean wall heat transfer radial distribution. Axial velocity and temperature temporal distributions are analysed simultaneously to gain further insight into the mixing process near the wall. In particular, the probabilities of the different cold/hot fluid ejection/injection events prove that the strong intermittent thermal events are linked to a change in the mixing behavior induced by the passage of the large-scale vortical structures. These structures are found to preferentially produce a cold fluid flux towards the wall leading to the local heat transfer enhancement usually identified by the secondary peak.

Fuzzy logic and bed-to-wall heat transfer in a large-scale CFBC

2018 ◽  
Vol 28 (1) ◽  
pp. 254-266 ◽  
Author(s):  
Jaroslaw Krzywanski ◽  
Marta Wesolowska ◽  
Artur Blaszczuk ◽  
Anna Majchrzak ◽  
Maciej Komorowski ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Fuzzy Logic ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Large Scale ◽  
Weighted Average ◽  
Local Heat Transfer ◽  
Wall Heat Transfer ◽  
Transfer Coefficients ◽  
Content Type

Purpose The purpose of this paper is to first present the key features of the fuzzy logic (FL) approach as a cost-effective technique in simulations of complex systems and then demonstrate the formulation and application of the method. Design/methodology/approach The FL approach is used as an alternative method of data handling, considering the complexity of analytical and numerical procedures and high costs of empirical experiments. The distance from gas distributor, the temperature and the voidage of the bed, flue gas velocity and the load of the boiler are the input parameters, whereas the overall heat transfer coefficient for the membrane walls constitutes the output. Five overlapping sigmoid and constant linguistic terms are used to describe the input and the output data, respectively. The Takagi–Sugeno inference engine and the weighted average defuzzification methods are applied to determine the fuzzy and crisp output value, respectively. Findings The performed FL model allows predicting the bed-to-wall heat transfer coefficient in a large-scale 670 t/h circulating fluidized bed (CFB) boiler. The local heat transfer coefficients evaluated using the developed model are in very good agreement with the data obtained in complementary investigations. Originality/value The performed model constitutes an easy-to-use and functional tool. The new approach can be helpful for further research on the bed-to-wall heat transfer coefficient in the CFB units.

Characterization of the Surface Curvature Effect Using LES for a Single Round Impinging Jet

2017 ◽  
Author(s):  
Pierre Aillaud ◽  
Florent Duchaine ◽  
Laurent Gicquel ◽  
Sheddia Didorally
Keyword(s):  
Heat Transfer ◽  
Flat Plate ◽  
Temporal Distribution ◽  
Impinging Jet ◽  
Ambient Air ◽  
Surface Curvature ◽  
Flow Topology ◽  
Reference Case ◽  
Vortical Structures ◽  
Eddy Simulation

In this paper, wall resolved Large Eddy Simulation is used to study the effect of the surface curvature for two impinging jet configurations. The reference case is a single round jet impinging on a flat plate at a Reynolds number (based on the bulk velocity Ub and the pipe diameter D) Re = 23 000 and for a nozzle to plate distance H = 2D. The results on this configuration have been previously analyzed and validated against experimental results. This paper compares for the same operating point, the flat plate impingement to an impinging jet on a concave hemispherical surface with a relative curvature d/D = 0.089 where d is the concave surface diameter. Mean and Root Mean Square (RMS) quantities are compared to highlight differences and similarities between the two cases. In addition high order statistic such as Skewness of the temporal distribution of wall heat flux is analyzed. Probability density functions (PDF) are also built to further characterize the effect of surface curvature. It is shown that the surface curvature has a destabilizing effect on the vortical structures present in such a flow leading to a modification of the wall heat transfer compared to the flat plate case. The flow topology in the concave case is dominated by a large toroidal stationary vortex. This vortex generates a natural confinement that causes the increase of the mean temperature of the ambient air around the jet. The main effect is the reduction of the capacity of the vortical structures to enhance heat transfer. Finally, the confinement effect combined with the destabilization due to the concave curvature lead to an alleviation of the secondary peak in the Nusselt distribution and a reduction of the heat transfer at the wall.

Investigation of Endwall Heat Transfer in Staggered Pin Fin Arrays

2021 ◽  
Vol 143 (2) ◽  
Author(s):  
Marcel Otto ◽  
Gaurav Gupta ◽  
Patrick K. Tran ◽  
Shinjan Ghosh ◽  
Jayanta S. Kapat
Keyword(s):  
Heat Transfer ◽  
Maximum Velocity ◽  
Local Heat Transfer ◽  
Horseshoe Vortex ◽  
Local Heat ◽  
Channel Length ◽  
Stereoscopic Particle Image Velocimetry ◽  
Thermochromic Liquid Crystal ◽  
Transfer Coefficients ◽  
Vortical Structures

Abstract Arrays of staggered pin fins are a typical geometry found in the trailing edge region of modern airfoils. If coolant is supplied by bleeding from the mid-section of the airfoil instead of provided through the root, the channel length is insufficiently long to reach a fully developed flow which is commonly found from the fifth row downstream. This present study focuses on the developing section (four rows) of a staggered array with a height-to-diameter ratio of 2 and a spanwise and streamwise spacing of 2.5, respectively. Measurements are conducted at Reynolds numbers of 10,000 and 30,000 based on the maximum velocity and pin diameter. Stereoscopic particle image velocimetry (PIV) is used to describe the flow field and turbulence characteristics in the wake of the first and third row pin. It is found that the dominating vortical structures depend highly on the Reynolds number. A transient thermochromic liquid crystal (TLC) technique is used to obtain local heat transfer coefficients on the endwall which are then discussed in the context with the vortical structures. The structure of the horseshoe vortex and the transient wake shedding behaves differently in the first and third row. The interaction of both vortex systems affects directly the endwall heat transfer. The results are supplemented by a thorough discussion of TLC and PIV uncertainty.

Experimental Studies and Correlations of Radially Outward and Inward Air-Flow Heat Transfer in a Rotating Square Duct

1996 ◽  
Vol 118 (1) ◽  
pp. 23-30 ◽  
Author(s):  
C. R. Kuo ◽  
G. J. Hwang
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Experimental Studies ◽  
Leading Edge ◽  
Wall Heat Transfer ◽  
Square Duct ◽  
Reinforced Plastic ◽  
Flow Heat ◽  
The Mean ◽  
Heated Length

Experiments were conducted to investigate the convective heat transfer of radially outward and inward air flows in a uniformly heated rotating square duct. The interior duct surfaces, constructed by fiberglass-reinforced plastic, were plated with separated film heaters for distinguishing the local wall heat transfer rate. The duct hydraulic diameter, the actively heated length, and the mean rotation radius are 4, 120, and 180 mm, respectively. In the experiments, the parameters were the throughflow Reynolds number, Re = 1,000∼15,000; the rotation number, Ro = 0∼0.32; and the rotational buoyancy parameter, Ra* = 0∼0.5. For the outward flow the Coriolis-induced cross-stream secondary flow strongly enhanced the heat transfer on the leading edge. But for the radially inward flow the trend was reversed. When the throughflow Reynolds number was increased, the rotating-buoyancy decreased, then increased the heat transfer for the outward flow; however, the rotating-buoyancy always increased the heat transfer for the inward flow. The heat transfer data are correlated for the outward and inward flows for the ranges of parameters under study.

Large Eddy Simulation of Round Impinging Jets With Large Stand-Off Distance

2014 ◽  
Author(s):  
Mehrdad Shademan ◽  
Vesselina Roussinova ◽  
Ron Barron ◽  
Ram Balachandar
Keyword(s):  
Large Eddy Simulation ◽  
Flow Characteristics ◽  
Impinging Jet ◽  
Impinging Jets ◽  
Vortical Structures ◽  
Nozzle Height ◽  
Eddy Simulation ◽  
Large Eddy ◽  
The Mean ◽  
Good Agreement

Large Eddy Simulation (LES) has been carried out to study the flow of a turbulent impinging jet with large nozzle height-to-diameter ratio. The dynamic Smagorinsky model was used to simulate the subgrid-scale stresses. The jet exit Reynolds number is 28,000. The study presents a detailed evaluation of the flow characteristics of an impinging jet with nozzle height of 20 diameters above the plate. Results of the mean normalized centerline velocity and wall shear stress show good agreement with previous experiments. Analysis of the flow field shows that vortical structures generated due to the Kelvin-Helmholtz instabilities in the shear flow close to the nozzle undergo break down or merging when moving towards the plate. Unlike impinging jets with small stand-off distance where the ring-like vortices keep their interconnected shape upon reaching the plate, no sign of interconnection was observed on the plate for this large stand-off distance. A large deflection of the jet axis was observed for this type of impinging jet when compared to the cases with small nozzle height-to-diameter ratios.

Experimental Measurement of the Vortex Development Downstream of a Lobed Forced Mixer

1992 ◽  
Vol 114 (1) ◽  
pp. 63-71 ◽  
Author(s):  
W. A. Eckerle ◽  
H. Sheibani ◽  
J. Awad
Keyword(s):  
Secondary Flow ◽  
Turbulent Mixing ◽  
Large Scale ◽  
Vortex Breakdown ◽  
Velocity Ratio ◽  
Mixing Process ◽  
Mixing Processes ◽  
Design Variables ◽  
Secondary Motion ◽  
The Mean

An experimental study was conducted to investigate the mixing processes downstream of a forced mixer. A forced mixer generates large-scale, axial (stirring) vorticity, which causes the primary and secondary flow to mix rapidly with low loss. These devices have been successfully used in the past where enhanced mixing of two streams was a requirement. Unfortunately, details of the mixing process associated with these lobed forced mixers are not well understood. Performance sensitivity to design variables has not been documented. An experiment was set up to investigate the mixing processes downstream of a mixer. Air flow was independently supplied to each side of the forced mixer by separate centrifugal blowers. Pressures were measured at the entrance to the lobes with a pitot-static probe to document the characteristics of the approaching boundary layer. Interior mean and fluctuating velocities were nonintrusively measured using a two-component laser-Doppler velocimetry (LDV) system for velocity ratios of 1:1 and 2:1. The wake structure is shown to display a three-step process where initially secondary flow was generated by the mixer lobes, the secondary flow created counterrotating vortices with a diameter on the order of the convolute width, and then the vortices broke down resulting in a significant increase in turbulent mixing. The results show that the mean secondary motion induced by the lobes effectively circulated the flow passing through the lobes. This motion, however, did not homogeneously mix the two streams. Turbulent mixing in the third step of the mixing process appears to be an important element in the enhanced mixing that has been observed with forced mixers. The length required for the flow to reach this third step is a function of the velocity ratio across the mixer. The results of this investigation indicate that both the mean secondary motion and the turbulent mixing occurring after vortex breakdown need to be considered for prediction of forced mixer performance.

Heat Transfer Characteristics of an Inclined Impinging Jet on a Curved Surface in Crossflow

2014 ◽  
Vol 136 (8) ◽  
Author(s):  
X. L. Wang ◽  
H. B. Yan ◽  
T. J. Lu ◽  
S. J. Song ◽  
T. Kim
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Flat Plate ◽  
Local Heat Transfer ◽  
Impinging Jet ◽  
Local Heat ◽  
Curved Surface ◽  
Heat Transfer Characteristics ◽  
Potential Core ◽  
Transfer Characteristics

This study reports on heat transfer characteristics on a curved surface subject to an inclined circular impinging jet whose impinging angle varies from a normal position θ = 0 deg to θ = 45 deg at a fixed jet Reynolds number of Rej = 20,000. Three curved surfaces having a diameter ratio (D/Dj) of 5.0, 10.0, and infinity (i.e., a flat plate) were selected, each positioned systematically inside and outside the potential core of jet flow where Dj is the circular jet diameter. Present results clarify similar and dissimilar local heat transfer characteristics on a target surface due to the convexity. The role of the potential core is identified to cause the transitional response of the stagnation heat transfer to the inclination of the circular jet. The inclination and convexity are demonstrated to thicken the boundary layer, reducing the local heat transfer (second peaks) as opposed to the enhanced local heat transfer on a flat plate resulting from the increased local Reynolds number.

An Analysis of the Effect of Entrainment Temperature on Jet Impingement Heat Transfer

1984 ◽  
Vol 106 (4) ◽  
pp. 804-810 ◽  
Author(s):  
S. A. Striegl ◽  
T. E. Diller
Keyword(s):  
Heat Transfer ◽  
Analytical Solutions ◽  
Jet Impingement ◽  
Local Heat Transfer ◽  
Impinging Jet ◽  
Local Heat ◽  
Impinging Jets ◽  
Recirculation Region ◽  
Plate Temperature ◽  
Single Plane

An analytical model is developed to determine the effect of the temperature of entrained fluid (entrainment temperature) on the local heat transfer to a single, plane, turbulent impinging jet. Solutions of the momentum and energy equations for a single impinging jet are accomplished using similarity and series analyses. Solutions of the energy equation are obtained for the two limiting cases of entrainment temperatures equal to the plate temperature and the initial jet temperature. The analytical solutions are superposed to obtain the solution for all intermediate entrainment temperatures. The constants in the turbulence model are determined by comparing the analytical solutions to experimentally determined local heat transfer rates for single impinging jets issuing into an environment with a controlled entrainment temperature. When the single jet model is applied to jet arrays it predicts that the entrainment in the recirculation region between the jets can significantly affect the heat transfer. Comparison of the model to heat transfer measurements performed for jet arrays shows that the model successfully predicts the local heat transfer in jet arrays.

Sign in / Sign up

Export Citation Format

Share Document