J. Cent. South Univ. (2013) 20: 2960−2966 DOI: 10.1007/s11771­013­1819­x

Fluid flow characteristics of single inclined circular jet impingement for ultra­fast cooling

WANG Bing­xing(王丙兴), XIE Qian(谢谦), WANG Zhao­dong(王昭东), WANG Guo­dong(王国栋)

State Key Laboratory of Rolling and Automation (Northeastern University), Shenyang 110819, China

© Central South University Press and Springer­Verlag Berlin Heidelberg 2013

Abstract: The fluid flow characteristics of the single bunch inclined jet impingement were investigated with different jet flow velocities, nozzle diameters, jet angles and jet­to­target distances for ultra­fast cooling technology. The results show that the peak pressure varying significantly from nearly 0.5 to above 13.4 kPa locates at the stagnation point with different jet diameters, and the radius of impact pressure affected zone is small promoted from 46 to 81 mm in transverse direction, and 50 to 91 mm in longitude direction when the jet flow velocity changes from 5 to 20 m/s. However, the fluid flow velocity is relatively smaller near the stagnation point, and increases gradually along the radius outwards, then declines. There is an obvious anisotropic characteristic that the flow velocity component along the jet direction is about twice of the contrary one where the jet anlge is 60°, jet diameter is 5 mm, jet length is 8 mm and jet height is 50 mm.

Key words: hot plate; ultra­fast cooling; inclined circular jet; impact pressure; fluid flow velocity

1 Introduction

The ultra­fast cooling (UFC) technology as the core of the new generation thermo­mechanical control process (NG­TMCP) technology is a very hot worldwide research subject which has an important impact on the microstructure and mechanical properties of steel [1−4]. The high accuracy control of technological parameters such as final cooling temperature (FCT), cooling rate (CR) and cooling route are crucial to achieve controlled cooling technology of a variety of plate products [5−6]. On the other hand, the jet impingement cooling technology is used widely in industrial practice for its strong heat and mass transfer ability [7−9], so that the UFC system based on jet impingement cooling technology can obtain high efficient and homogeneous cooling performance [10].

The researchers had made a lot of researches about the jet impinging on flat plate process. AZUMA and HOSHINO [11] divided the wall­jet regions film flow into four forms(stagnation region, boundary layer region with surface velocity equal to the jet flow velocity, and regions of decreasing free surface velocity and hydraulic jump), and they also derived expressions for boundary layer thickness, liquid film thickness and free­surface velocity in the flow region. FITZGERALD and GARIMELLA [12] found velocities and turbulence

levels at pre­impingement and wall­jet regions by using laser­Doppler velocimetry with different nozzle diameters and nozzle to target spacings. In the past two decades, researchers mostly focused on surface flow characteristics as well as heat transfer mechanism about wall­jet regions, well studied flow characteristics were indispensible for the research of surface heat transfer mechanism. KARWA et al [13] performed a study about the surface flow characteristics and heat transfer during quenching a cylindrical stainless steel test specimen. The specimen was heated to an initial temperature of about 900 °C and cooled by water impinging on it. They found three distinct regions on the quenching surface, an expanding circular wetted region surrounding the impinging point, annular transition zone just outside the wetting front and an unwetted region outside this zone. Because of the boiling phenomenon, the flow pattern at high temperature surface was different from normal surface, so numerical simulation methods were used. ABISHEK et al [14] performed the convection and boiling heat transfer distribution on flat plate with different nozzle sizes, over heats, Reynolds numbers and other parameters by establishing vertical shock fluid wall boiling model. GRADECKA et al [15] carried out the investigations about impinging jet with various nozzle sizes and heights on a moving plate. And then through the use of Reynolds and Weber numbers, the power relations were obtained to calculate the radius of the

Foundation item: Project(2010CB630800) supported by the National Basic Research Program of China; Project(N100307003) supported by the Fundamental Research Funds for the Central Universities, China

Received date: 2012−07−09; Accepted date: 2013−09−04 Corresponding author: WANG Bing­xing, Lecturer, PhD; Tel: +86−24−83673172; E­mail: wbxang@126.com

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jump, even under the circumstance that the main parameters of velocity and height were dimensionless. The research results mentioned above are very important to the jet impingement heat transfer technology. However, most of the simulated or experimental parameters are different from the real­time inclined jet impingement cooling process. The fluid dynamic structure of such processes is extremely complex. Thus, the impact characteristics of cooling water on the steel surface by single circular jet were researched, where the jet inlet velocity, jet angle, jet­to­target distance and nozzle diameter were varied. The results will further enrich the inclined jet impingement boiling heat transfer theory of the UFC process.

2 Industry application model

In industry site, the cooling water provided by water supply pump is transported to water distribution pipe, then it will be rectified by damping structure inside the nozzle and impinge on the hot steel surface homogeneously, as shown in Fig. 1. The nozzle arrangement and inclined circular jets impinged on the hot plate surface are shown in Fig. 2. The nozzle header diameter is about 2−10 mm. The cooling water impinging on the hot steel surface can reach the velocity of about 5−20 m/s, while the water pressure is around 0.3−0.5 MPa in the water supply system.

3 Simulation

The research foundation of the water impinging on the hot plate was the fluid flow and heat transfer characteristic of single inclined circular jet. The computational fluid dynamics (CFD) method was used to analyze the fluid flow velocity and the impact pressure by simulating the fluid flow characteristics of the single inclined circular jet, where the water flow followed the principles of conservation of mass, momentum, energy

and thermodynamics. The RNG k−ε double­equation turbulence models [16] were used in this simulation and their tensor form were as follows:

k eff k ( ) ( ) i

i j j

ku k k G t x x x

ρ ρ α µ ρε ∂ ∂ ∂ ∂

+ = + − ∂ ∂ ∂ ∂

(1)

ε eff ( ) ( ) i

i j j

u t x x x

ρε ρε ε α µ ∂ ∂ ∂ ∂

+ = + ∂ ∂ ∂ ∂

2 1ε

k 2ε C

G C k k

ε ε ρ ∗ − (2)

where ρ is the density of the fluid medium; k is the turbulent energy; ui is the jet flow velocity of the coordinate axis xi direction; αk=αε=1.39; μeff is the effective viscosity coefficient and μeff=μ+μt; μt is the eddy viscosity coefficient and μt=ρCμ(k 2 /ε); Cμ= 0.084 5; ε is the turbulence dissipation rate; Gk=

t ; i

j i

j i

j

u u v

x x u x

ρ ∂ ∂

− + ∂ ∂

∂ ∂

0 3 2ε 2ε

(1 / ) ;

1 C C

η η η βη

∗ − = −

+ η =

1/ 2 (2 ) ; ij ij k E E ε

⋅ η0= 4.377; C1ε=1.42; C2ε=1.68; β=0.012

and 1 . 2

j i ij

j i

u u E x x

∂ ∂ = + ∂ ∂ SIMPLE method was chosen to solve the problem,

and the nozzle header surface was set as velocity­inlet boundary condition, and pressure­outlet was set to be the outlet boundary condition.

The simulation model of the single inclined circular jet impingement process is shown in Fig. 3. The jet length was set to be l mm; the jet diameter was set to be d mm; the jet height between the jet outlet and the impinging object surface is set to be h mm; the jet angle between the jet and the impinging object surface was set to be θ° and the inlet jet flow velocity was set to be v m/s. The layout of jet was in the three­dimensional Cartesian coordinate system as shown in Fig. 3.

Fig. 1 Schematic diagram of water supply device and control units

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Fig. 2 Photo of inclined jets impinge on hot plate

Fig. 3 Chart of simulation model

The inlet jet flow velocity, jet diameter, jet angle and jet height were selected as the analog variables. The specific researched contents are given in Table 1.

4 Results and discussion

To analyze the fluid flow characteristics, the pressure distribution through the impact point on the plate surface along the x­axis and z­axis (the longitudinal centerline and the transverse line), and the fluid flow velocity distributions through the point 5 mm above the impact point on the plate surface along the x­axis and z­axis were collected, then these data were discussed.

4.1 Effect of jet flow velocity on fluid flow characteristics Figure 4(a) shows the distribution of transverse

fluid flow velocity, and the velocities distributed symmetrically on both sides of impingement point can be gotten. The velocity around the impingement point is relatively smaller, and the velocity increases gradually with increasing distance, until reaching the peak value at 0.025 m. Then, the fluid flow velocity reduces gradually. From the longitudinal fluid flow velocity distribution curve, as shown in Fig. 4(b), it is known that the fluid flow velocity is asymmetrically distributed. The fluid flow velocity along the jet impinge direction is faster than the value along the opposite direction. Similarly to the transverse fluid flow velocity distribution, the fluid flow velocity also increases around the stagnation point and then decreases as the distance from the stagnation point increases. From the fluid flow velocity trend, it can be concluded that the peak velocity does not locate at the stagnation point, but locates at the edge where the fluid flow pattern changes from free jet region to wall jet region. In the wall jet region, the fluid flow velocity reduces because the fluid flow is affected by the fluid viscosity and the gradually enlarging impact area. Due to the fluid internal shear stress and wall friction, the fluid flow velocity decreases gradually with the increasing radial. Meanwhile, the fluid flow velocity distribution is affected by the inlet jet flow velocity significantly. With the increasing inlet jet flow velocity, the value of the fluid flow velocity increases at the same radial position and the impact area also increases accordingly. From the longitudinal fluid flow velocity trend, it is known that when the inlet jet flow velocities are 5, 10, 15 and 20 m/s, the peak fluid flow velocities are 4.30, 8.46, 12.71 and 16.94 m/s, respectively. There is an obvious anisotropic characteristic that the flow velocity component along the jet direction is about twice of the contrary one. The jet impingement heat transfer capacity is related closely to the wall jet flow velocity distribution. With the increasing inlet jet and wall jet flow velocity, the turbulence degree is strengthened, so the performance of convective heat transfer is improved [17].

The longitudinal and the transverse pressure

Table 1 Research contents of jet impingement No. Simulation condition Analog variable

1 Jet diameter of 5 mm, jet length of 8 mm, jet angle of 60° and jet height of 50 mm Inlet velocities of 5, 10, 15, 20 m/s

2 Jet length of 8 mm, jet angle of 60°, jet height of 50 mm and inlet velocity of 10 m/s Jet diameters of 2.5, 5, 10, 15 mm

3 Jet diameter of 5 mm, jet length of 8 mm, jet height of 50 mm and inlet velocity of 10 m/s Jet angles of 30°, 45°, 60°, 90°

4 Jet diameter of 5 mm, jet length of 8 mm, jet angle of 60° and inlet velocity of 10 m/s Jet heights (h) of 25, 50, 100, 200 mm

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distributions are similar, and this can be seen obviously in Figs. 4(c)−(d). It is interesting to find that the stagnation point locates at the rear of impingement point. There are exchange of the fluid flow and the ambient exchange mass, momentum and energy continuously during the impingement process [18]. The mass momentum and energy lose more and more with increasing radial distance. Finally, the fluid flow energy dissipates to be zero when the viscous deformation process is completed. The same as the fluid flow velocity, the impact pressure area increases with increasing inlet jet flow velocity because the flow with more momentum and energy will exacerbate the disturbance of the impact water and increase the fluid velocity gradient. But the increase of velocity does not prominently broaden the pressure affected zone, the impact pressure is only promoted from 46 to 81 mm in the transverse direction, and 50 to 91 mm in the longitude direction when the jet flow velocity ranges from 5 to 20 m/s. Under the thermal state conditions, the wall on the boundary layer will get thinner and the heat transfer rate will increase accordingly.

4.2 Effect of nozzle diameter on fluid flow characteristics The velocity distribution curve indicates that nozzle

diameter is an important factor for the fluid flow velocity distribution. The fluid flow velocity increases significantly with increasing nozzle diameter, but the gradient declines slightly. Meanwhile, the position of the fluid flow peak velocity expands outward from the impact point. Figure 5(a) shows the transverse fluid flow velocity curve, it can be seen that when the nozzle diameter varies from 2.5 to 15 mm, the distance between fluid flow peak velocity position and the impact point varies from 20.27 to 34.69 mm and with the fluid flow peak velocity varies from 6.32 to 8.01 m/s. The pressure distribution curves in Figs. 5(c)−(d) show that the pressure and area of the impact point increase significantly with increasing nozzle diameter. The impact pressure ranges from 46 to 100 mm in transverse direction, and 49 to 117 mm in longitude direction. The peak pressure varying significantly from nearly 0.5 to above 13.4 kPa loactes at the stagnation point rather than the impact point.

4.3 Effect of jet angle on fluid flow characteristics In Fig. 6, with decreasing jet angle, the jet­to­target

distance becomes larger and the distance between fluid flow peak velocity position and impact point is bigger. The peak velocity in the longitudinal direction increases gradually, and the stagnation point is more and more

Fig. 4 Effect of jet flow velocity on fluid flow characteristics: (a) Transverse fluid flow velocity; (b) Longitudinal fluid flow velocity; (c) Transverse impact pressure; (d) Longitudinal impact pressure

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Fig. 5 Influence of jet diameter on fluid flow characteristics: (a) Transverse fluid flow velocity; (b) Longitudinal fluid flow velocity; (c) Transverse impact pressure; (d) Longitudinal impact pressure

Fig. 6 Influence of jet angle on fluid flow characteristics: (a) Transverse fluid flow velocity; (b) Longitudinal fluid flow velocity; (c) Transverse impact pressure; (d) Longitudinal impact pressure

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Fig. 7 Influence of jet height on fluid flow characteristics: (a) Transverse fluid flow velocity; (b) Longitudinal fluid flow velocity; (c) Transverse impact pressure; (d) Longitudinal impact pressure

close to the jet inlet position and the pressure value is also growing at the same time. When the jet angles are 30°, 45°, 60° and 90°, the distances between the impact point and the stagnation point are 25.18, 11.00, 4.62 and 0.00 mm and the peak impact pressures are 549.79, 1 180.45, 1 912.25 and 2 654.22 Pa, respectively. This is because that the distance between the inlet jet and the steel surface will be shorted when the angle is enlarged, so that the energy and momentum exchange between the fluid flow and the surrounding medium will get smaller.

4.4 Effect of jet height on fluid flow characteristics One of the important parameters influencing the

fluid flow characteristics is the jet­to­target distance，and the effect of jet height on the fluid flow characteristics can be seen in Fig. 7. With the increasing jet height, the fluid flow velocity increases initially and then decreases gradually. When the jet heights are 25, 50, 100 and 200 mm, the peak fluid flow velocities are 8.37, 8.49, 8.47 and 7.86 m/s and the distances between the impact point and the stagnation point are 3.43, 4.59, 7.46 and 16.21 mm, respectively. Meanwhile, the peak value position becomes far from the impact point. The proper jet height of the special jet is from 50 to 100 mm. From

the longitudinal pressure distribution curve in Fig. 7(d), it is known that the higher the jet height is, the further distance between the stagnation point and the jet outlet will be. Similar to the effect of the jet angle, the impact pressure also becomes smaller with increasing jet height. This is because that when the distance between the inlet jets and the steel surface is longer, the energy and momentum exchange between the fluid flow and the surrounding medium will be larger.

5 Conclusions

1) The peak pressure locates at the stagnation point rather than the impact point, and reduces rapidly outward along the radius. The peak pressure varies significantly from nearly 0.5 to 13.4 kPa with different jet diameters. The impact pressure affected zone is not broadened by the jet flow velocity prominently, the impact pressure only promotes from 46 to 81 mm in transverse direction, and 50 to 91 mm in longitude direction where the jet flow velocity ranges from 5 to 20 m/s.

2) Different from the impact pressure, the fluid flow velocity is relatively smaller near the stagnation point. With increasing radius, the fluid flow velocity increases

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gradually, then declines. The fluid flow velocity affected zone is influenced by all parameters mentioned above significantly. However, the peak value is decided by the inlet jet flow velocity only. There is an obvious anisotropic characteristic that the flow velocity component along the jet direction is about twice of the contrary one where the ject anlge is 60°, jet diameter is 5 mm, jet length is 8 mm and jet height is 50 mm.

3) The research results will make contributions to the ultra­fast cooling devices design and the inclined jet impingement cooling heat transfer technology research. In future, research work will be performed including the effects of multi bunch jets impingement arrangement on fluid flow characteristics, and the principles about hydrodynamic behavior, boiling heat transfer, mechanism of heat/fluid coupling function under the condition of anisotropy multi bunch jets of the inclined nozzle (slit or circular) impact on the wall at high temperature.

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(Edited by FANG Jing­hua)