P. LI, H. CHEN

Open Access JAMP

Table 6. Effects of slot gap on the steel strip vibration.

H(mm) 0.8 0.9 1.0 1.1 1.2

T (10−5s) 6.02 6.19 6.25 6.44 6.96

Pm (kPa) 16.75 20.03 22.79 24.59 26.37

P’(kPa) 5.49 7.84 11.03 12.91 15.73

f’ = 1/T(Hz) 16611.3 16155.1 15037.6 14619.9 14367.8

F’(N) 128.25 183.14 257.66 301.58 399.79

Dmax (μm) 0.614 0.927 1.51 1.87 2.56

steel strip.

For different impingement distances, air knife slot gaps,

and jet pressures, the mean (in time) pressure Pm and its

standard deviation P’ on the surface of steel strip are listed

in Tables 4-6, as well as the pressure fluctuation period

T. With the aid of the Equations (1) and (2), the amplitude

of the harmonic force F’ can be obtained. Listed in Ta-

bles 4-6 is also maximum displacement of the strip steel.

The effects of impingement distance are shown in Ta-

ble 4. With the increase of the impingement distance S,

the jet fluctuation frequency f’ = 1/T increases, the mean

pressure decreases, standard deviation of pressure in-

creases, the amplitude of the harmonic force increases,

and the maximum displacement of strip vibration in-

creases. Therefore, the steel strip vibrates more seriously

when the air knife is farther away (within the range of 8 -

12 mm) from the steel strip.

The effects of jet pressure are shown in Table 5. With

the increase of the jet pressure P0, jet fluctuation fre-

quency increases, average pressure increases, the stan-

dard deviation of pressure increases, the amplitude of the

harmonic force increases, and the maximum displace-

ment of strip vibration decreases. Although the average

pressure and the standard deviation of pressure increase

with the increasing of the jet pressure, normalized pres-

sure fluctuation P’/Pm decreases. Therefore, the steel strip

vibrates more seriously when the jet pressure is lowered

(within the range of 20 - 60 kPa).

The effects of the air knife slot gap are shown in Table

6. With the increase of the slot gap H, jet fluctuation

frequency decreases, average pressure increases, standard

deviation of pressure increases, the amplitude of the

harmonic force increases, and the maximum displace-

ment of strip vibration increases. Therefore, the steel

strip vibrates more seriously when the air knife slot gap

is opened wider.

4. Conclusions

Vibration of the steel strip in the continuous hot-dip gal-

vanization process can be attributed to the turbulent

fluctuation of the jet flow field. In this paper, the turbu-

lent jet flow field is first numerically obtained using the

CFD method and the influence of the turbulent jet flow

on the steel strip is simplified as a harmonic line force.

Then, vibration modes of the steel strip with different

pretension forces, strip lengths and thicknesses are ana-

lyzed using the finite element method. Our results indi-

cate that both frequency and magnitude of steel strips

vibration increases with the decrease of the pretension

force, the strip length and thickness.

Effects of impingement distances, air knife slot gaps,

and jet pressure, on vibration of the steel strip are also

analyzed. It is demonstrated that the steel strip vibrates

more seriously when the distance of the air knife to the

steel strip increases, the jet pressure decreases, and the

air knife slot gap in creases.

Our results will help gain insights into the formation

mechanism of strip vibration in continuous hot-dip gal-

vanizing processes, and lay the oretical foundation for the

further reduction of zinc ripples and increase of galvani-

zation quality.

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http://dx.doi.org/10.3901/JME.2011.09.060