Investigation of heat transfer on 2024 aluminum alloy thin sheets by water spray quenching

Experimental Thermal and Fluid Science 72 (2016) 249–257

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Investigation of heat transfer on 2024 aluminum alloy thin sheets by water spray quenching Ruichao Guo, Jianjun Wu ⇑, Weiping Liu, Zengkun Zhang, Mingzhi Wang, Shaochang Guo School of Mechanical Engineering, Northwestern Polytechnical University, Xi’an 710072, PR China

a r t i c l e

i n f o

Article history: Received 2 September 2015 Received in revised form 6 November 2015 Accepted 6 November 2015 Available online 2 December 2015 Keywords: Spray quenching Water temperature Nozzle pressure Aluminum alloy Heat transfer

a b s t r a c t In this paper, the hot 2024 aluminum alloy thin sheets at 495 °C were quenched by two spray nozzles with different pressures and water temperatures. The heat fluxes were determined using an inverse heat conduction method, and time–temperature curves were smoothed by B-spline approximation. The results indicate that there exists a film boiling regime and Leidenfrost point when water temperature is 70 °C. The critical heat flux first increases and then decreases with increasing nozzle pressure. The critical heat flux decreases when water temperature increases from 25 °C to 60 °C. However, the critical heat flux increases when water temperature varies from 60 °C to 70 °C. Ó 2015 Elsevier Inc. All rights reserved.

1. Introduction With the development of the aviation industry and in order to satisfy large size aircraft parts weight requirements, more and more metal honeycomb bonding composites have been applied in the design of aircraft structures, such as aircraft radome, spoiler, aileron, rudder, side wall and hatch door. The 2024 aluminum alloy thin sheets (0.30–0.50 mm thick), due to their good forming ability and machinability [1,2], are widely used as preferred materials for aircraft metal honeycomb panels. In order to satisfy the required mechanical properties of structures, the 2024 aluminum alloy thin sheets are usually heated above the solution temperature (approximately 500 °C), and then cooled in a cold medium [3]. The cooling process is generally carried out either by putting the components into water, oil or other liquids, or by spraying them with liquids. The spraying quenching, compared with other quenching techniques, has the higher heat fluxes and heat transfer efficiency. Therefore it is widely used in steel and aluminum alloy quenching [4,5]. The spray quenching of aluminum alloys were widely discussed in the literature, however, mainly about aluminum alloy castings and plates. Mascarenhas and Mudawar [6] studied quenching 2024 aluminum alloy cylinder by full-cone pressure sprays. The results showed that the increasing nozzle pressure drop or the decreasing orifice-to-surface distance caused the transitions to ⇑ Corresponding author at: Northwestern Polytechnical University, 127 West Youyi Road, Xi’an, Shaanxi 710072, PR China. Tel.: +86 02988493101. E-mail address: [email protected] (J. Wu). http://dx.doi.org/10.1016/j.expthermflusci.2015.11.014 0894-1777/Ó 2015 Elsevier Inc. All rights reserved.

the lower temperature boiling regimes to occur at higher surface temperatures and also it could accelerate the exit from poor film boiling regime to more efficient transition boiling regime. Cheung et al. [7] investigated the metal/mold heat transfer coefficient during solidification about 6101 aluminum casting. Finite difference method was used to obtained time-dependent temperature distribution and then the heat transfer coefficient was based on the inverse heat conduction problem. Xu et al. [8] focused on the influence of spray pressure and surface roughness on heat transfer about 6082 aluminum alloy. They found that the both heat flux and heat transfer coefficient increased with increasing of spray pressure. However, the influence of surface roughness was insignificant when surface temperature was higher than 170 °C. Zhang et al. [9] studied the influence of latent heat and thermocouples distance on heat transfer coefficient about A356 aluminum casting. Inverse methods were used and the results showed that thermocouples should be placed less than 2 mm from interface. Golovko et al. [10] investigated the impact of spraying distance, inclination angle and flow rate on the heat transfer coefficients about EN AW-6082 aluminum alloy by lumped heat capacitance method. They found that neither spraying distance nor inclination angle had significant influence on heat transfer coefficient. Mascarenhas and Mudawar [11] examined the influence of spray pressure, orifice-to-surface distance and thermal properties on temperature response during spray quenching for thick-walled aluminum alloy 2024 tube. The results showed that the cooling effectiveness improved with the increasing spray pressure and the decreasing orifice-to-surface distance.

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Nomenclature B c CHF d d32 D d0 DN F h HTC IHCP k m n N PN q Q Q

matrix of coefficient specific heat capacity, J/(kg °C) critical heat flux, W/m2 thickness of sheets, mm Sauter mean diameter, mm the interval nozzle orifice diameter, mm nozzle distance, mm the force required to rupture a liquid in tension, N heat transfer coefficient, W/(m2 K) heat transfer coefficient, W/(m2 K) inverse heat conduction problem thermal conductivity, W/(m °C) an integer which defines the length of interval D an integer which defines the number of time–temperature data the normalized uniform B-spline elements nozzle pressure, bar heat flux, W/m2 flow rate, L/min matrix with dimensions of ðm þ 3Þ  ðm þ 3Þ

According to the above references, research on the aluminum alloys is not systematic. Due to different experimental conditions, the obtained results are inconsistent. The heat transfer coefficient (HTC) or heat flux is the most decisive factor in influencing the quenching results, compared with other data such as material property, process parameter and other conditions [12,13]. In spray quenching, the heat transfer is a complicated process which depends on many factors such as water temperature and nozzle pressure. Therefore, in this paper, to fully understand the influence of quenching conditions on heat transfer during water spray quenching, a spray quenching apparatus is built to conduct water quenching experiments for 2024 aluminum thin sheets. The surface heat fluxes of thin sheets are inversely determined from the smoothing time–temperature curves by B-spline approximation. 2. Experimental procedure 2.1. Spray quenching system Schematic diagram and photo of the water spray quenching system are shown in Fig. 1. The system has the following sections: the tank zone, the water spray quenching zone, the furnace zone and the data acquisition system zone. At first, the water is adjusted inside the tank varying from the room temperature to 95 °C by temperature controller, resistive heater and temperature sensor. Then, water is pumped by pump which is located above the tank. A bottom valve, located in the tank, is used to avoid water backflow. Nozzle pressure, varying from 1 bar to 5 bar, can be regulated by the inverter. Two commercial full-cone spray nozzles (Spraying Systems, Co., Ltd., 1/4HH-SS6.5) are located on both sides of the quenching tank and the distance between the nozzles is 140 mm, and in this study the spray angle is 65° and nozzle orifice diameter is 2.4 mm. Locations of nozzles and thermocouple are shown in Fig. 2. The Sauter mean diameter d32 of droplets, obtained from manufacturer, is approximately 0.3 mm. The volumetric spray flux Q 00SP is assumed uniform over the spherical surface area and it can be defined as [14]:

Q 00sp ¼

Q 2pD2N ð1  cos ðh=2ÞÞ

ð1Þ

Q 00sp si t T T Ts Tw DT x y(t)

volumetric spray flux, m3/(s m2) time fraction of discrete temperature data, s time, s temperature, °C temperature vector surface temperature of specimens, °C water temperature, °C difference between surface and water temperature temperatures, °C coordinate defined in Fig. 4 B-spline smoothed temperature, °C

Greek symbols a thermal diffusivity, m2/s k smoothing parameter h spray angle, ° q density, kg/m3 r surface tension, N/m s control point vector x a constant for scaling the interval

The volumetric spray flux is obtained by the data in Table 1. The influence of water temperature and nozzle pressure will be investigated, and characteristics of water spray quenching are listed in Table 1. 2.2. Heat transfer process The method of investigation is based on the measurement carried out at the middle of two sheets. A CHROMEGAÒ – ALOMEGAÒ thermocouple (CO2-K, OMEGA, USA) is sandwiched between two sheets, and the thermocouple is around 0.013 mm in diameter. High temperature air set cement (OMEGABONDÒ 400) is applied to thermocouples to ensure good contact between surface of specimens and thermocouples (shown in Fig. 2b). The aluminum sheets are 60 mm  60 mm  0.508 mm in dimensions which are made of 2024-O aluminum alloy from Kaiser Aluminum, Co., USA. Chemical limits of 2024 aluminum alloy is illustrated in Table 2. The specimens are heated to 495 °C by resistive heaters and retained for 30 min in furnace, and then quickly transferred to the spraying position. Two valves are simultaneously opened on water circuit, thus, two surfaces of specimens are symmetrically quenched. Output signals are recorded by a high DC data acquisition system (OM-DAQ-USB-2401, USA) at high frequencies of 100 Hz when specimens are transferred to the spraying position. 2.3. Uncertainty analysis The dimensions of the specimens are determined by a water jet cutting machine which has an repeated accuracy of ±0.050 mm according to the manufacturer. The CO2-K thermocouples have the uncertainty of ±0.75% full scale. The accuracy of temperature measurement is approximately ±1.2 °C for K-type thermocouples. The nozzle pressure is obtained with an error of ±0.05 bar by pressure controller. And water temperature has an uncertainty of ±0.05 °C. The percentage of conductivity and heat capacity error is about 5%. The precision of the heated temperature of specimens is estimated as ±1 °C based on performance data of digital temperature controller in furnace. The uncertainty of surface temperature and heat flux are calculated by a root summary square method of

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partial derivatives. This method is based on Moffat’ theory [15]. Main uncertainties in this experiment are listed in Table 3.

The careful selection of k is of considerable importance, it can help stabilize the estimation procedure. In this work, a typical smoothing curve with six values of k (k ¼ 106 ; 105 ; 104 ; 103 ;

3. Inverse determination of the heat flux 3.1. B-spline approximation Once the mid-plane temperature of the specimen during spray quenching is recorded, the raw data should be smoothed since the temperature data are influenced by truly random noise. In this study, B-spline approximation is used to obtain the smoothing curves. A uniform B-spline of degree 3 in the interval D = [t0, tm] can be presented as: m1 X

yðtÞ ¼

yi ðtÞ

06i6m

i¼0

yi ðtÞ ¼

ð2Þ

3 X

si3 Nj;3 ðxðt  ti ÞÞ ti 6 t < tiþ1

j¼0

where tiþ1  t i ¼ 1=x, x is a constant for scaling the interval between equally-spaced knot points, and si are the control points we should find. Nj;3 are the normalized uniform B-spline of degree 3 elements and more details can be found in Ref. [16]. Thus, the B-spline y(t) in Eq. (2) can be rewritten as:

yðtÞ ¼ Bs

ð3Þ

102 ; 101 ) are used to compared the results obtained by seeking the solution in Eq. (7), as is shown in Fig. 3. From Fig. 3, it can be seen that all curves can describe the variation trend of temperature data. When smoothing parameter k is 102 and 101 (as shown in the beginning of spray quenching), curves tend to depart excessively from experimental data, and they are likely to smooth away meaningful features of curves. However, curves included with truly random noise at k of 106 and 105 (as shown in the detail with enlarged scale Fig. 3). When k is 103 and 104, the two curves have the similar trends when temperature varies from 455 °C to 25 °C. But there has some difference for the quenching time ranging from 0.8 s to 3 s, a relatively more smoothed curve (k ¼ 103 ) is better for the computation since the heat flux reaches zero when surface temperature is equal to water temperature. Therefore, the smoothing parameter of 103 is used in this study. 3.2. Solution procedure of inverse problem In the present case, the geometry of the thin sheets is simple enough. Thus, we can treat the problem as one-dimensional, as shown in Fig. 4. The one-dimensional Fourier heat conduction equation is as follows:

where

2

3

N 0;3 ðu0 Þ N1;3 ðu0 Þ N2;3 ðu0 Þ N3;3 ðu0 Þ

6 6 6 6 6 B¼6 6 6 6 6 4

N0;3 ðu1 Þ N1;3 ðu1 Þ N2;3 ðu1 Þ N3;3 ðu1 Þ .. .. .. .. . . . . N 0;3 ðui Þ

N1;3 ðui Þ .. .

N 2;3 ðui Þ .. .

N3;3 ðui Þ .. .

..

.

7 7 7 7 7 7 7 7 7 7 5

ð4Þ

N 0;3 ðum1 Þ N1;3 ðum1 Þ N2;3 ðum1 Þ N3;3 ðum1 Þ

s ¼ ½s3 ; s2 ; s1 ; s0 ; . . . ; sm1 T

ð5Þ

Given a set of discrete time–temperature data ðsi ; T i Þ, i ¼ 1; . . . ; n, noise reduction is therefore done by fitting the smoothing function y(t) with the objective function [17,18]:

k

@2T @T þ qv ¼ qc @x2 @t

ð10Þ

where k is a smoothing parameter, k > 0 and it controls the amount of smoothing, the careful choice of k can stabilize the smoothing procedure. The optimal solution is given as a solution of linear algebraic equation:

where k is the thermal conductivity of sheet, k ¼ kðTÞ; c is the specific heat capacity, c ¼ cðTÞ. Functions kðTÞ and cðTÞ, used in inverse heat conduction problem, can improve the calculation accuracy compared the results when the parameters are constant, and thermal properties of 2024 aluminum are shown in Table 4. q is the density; t is the time; Tðx; tÞ is the object temperature; qv is the latent heat of phase transformation, in such case qv ¼ 0 for the metallurgical properties of quenching 2024 aluminum alloy [3]. For symmetry reason, the heat flux at x = 0 is:

ð7Þ

ð11Þ

Z Min J ¼ k

tm

n X ðy ðtÞÞ dt þ ðyðsi Þ  T i Þ2 2

ð2Þ

t0

ð6Þ

i¼1

  kQ þ B0T B0 s ¼ B0T T

Here Q 2 Rðmþ3Þðmþ3Þ and T 2 Rn are defined by:

Z Q¼

tm

t0

!T 2 ! 2 d B d B dt

2

T ¼ ½T 1 ; T 2 ; . . . ; T n T

dt

2

dt

 @T  ¼0 @x x¼0 and

ð8Þ ð9Þ

And B0 2 Rnðmþ3Þ is obtained by putting time–temperature data into Eq. (2).

Tðx; 0Þ ¼ T 1

ð12Þ

Tð0; tÞ ¼ yðtÞ

ð13Þ

where T 1 is the initial temperature, and y(t) is the smoothing B-spline time–temperature curve in Eq. (2). The heat transfer condition between the outer surface of sheet and the liquid temperature belongs to the third boundary condition

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Fig. 1. (a) Schematic diagram of experiment; (b) spray quenching system.

  ¼ hðT s  T w Þ k@T @x x¼d  @T  ¼ hðT s  T w Þ k

ð14Þ

@x x¼d

where T w is the temperature of water (assumed uniform); T S is surface temperature; d is the thickness of sheet; h is the heat transfer coefficient (HTC). An analytical method is derived to obtain the surface temperature and heat flux based on the temperature of a thermocouple positioned at the middle of two sheets, the general solution of inverse heat conduction problem (IHCP) was given by Burggraf [20].

Tðx; tÞ ¼ yðtÞ þ

 2 n n 1 X 1 x d y n ð2nÞ! a dt n¼1

qðtÞ ¼ k @T @x 1 X x2n1 ¼ k ð2n1Þ! n¼1

n 1 d y

an dtn

ð15Þ

ð16Þ

where a is thermal diffusivity, a ¼ k=ðqcÞ. Taler [21] and Ciofalo et al. [22] showed that truncating Eqs. (15) and (16) after second derivative yields approximate solutions of acceptable accurate, therefore, in this paper, only the first and second derivatives are considered. For this reason, the B-spline smoothing curve of degree

3 is considered for numerical computation, thus, substituting Eq. (3) into Eqs. (15) and (16), the simplified IHCP is: 2

1 x2 dB 1 x4 d B sþ s 2 a dt 24 a2 dt 2 2 1 dB x3 1 d B sk s qðtÞ ¼ kx a dt 6 a2 dt 2

Tðx; tÞ ¼ Bs þ

ð17Þ ð18Þ

4. Results and discussion 4.1. Time–temperature curves and heat flux Fig. 5 shows spraying quenching curves obtained at different spraying pressures (Fig. 5a) and different water temperatures (Fig. 5b), respectively. It is to be noted that all the curves shown in the figures are smoothed and filtered by B-spline approximation. It may be seen from the two figures that the initial temperatures are lower than the ideal temperature and also they are not fairly uniform because the transferred time is not consistent when specimens are transported in air from the furnace to the spraying position. Comparing Fig. 5a and b shows that for all the curves at different pressures and temperatures, those curves have similar

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Fig. 2. (a) Schematic diagram of nozzles and thermocouple locations; (b) schematic of the specimen with a thermocouple (unit: mm).

Table 1 Characteristics of spray quenching examined in present study. Spray angle h (°) Nozzle orifice diameter d0 (mm) Nozzle distance DN (mm) Water temperature T W (°C) Sauter mean diameter d32 (mm)

Table 3 Summary of uncertainties. 65 2.4 70 25, 40, 60, 65, 70 0.3

Volumetric spray flux Q 00sp (m3 =ðs m2 Þ)

Nozzle pressure P N (bar)

Flow rate Q (L/min)

2

4.0

1:38  102

3

4.8

1:66  102

4

5.5

1:90  102

5

6.0

2:07  102

trend, and as soon as the sprays hit the sheets, surface temperatures drop remarkably. It takes about 0.5 s for the surface temperatures to drop from 450 °C to 100 °C, then the gradient decreases slowly. The whole spray quenching stage is approximately 3 s. Generally, in spray quenching process of aluminum alloys, heat transfer consists of four regimes: film boiling, transition boiling,

Parameters

Uncertainty

Nozzle pressure Heated temperature of specimens Dimensions of specimens CO2-K thermocouples Temperature measurement The conductivity and specific heat capacity Water temperature Surface temperature Heat flux

±0.05 bar ±1 °C ±0.050 mm ±0.75% ±1.2 °C ±5% ±0.05 °C ±1.17% ±3.02%

nucleate boiling and single convective cooling, as shown in Fig. 6. During the film boiling, a large amount of bubbles are formed around the surface and heat transferred between the hot surface and liquid is prevented by the vapor film. This vapor film leads to the poor heat transfer effectiveness during the film boiling. Vapor film collapses at Leidenfrost point with decreasing surface temperature. Then, quenching enters the transition boiling, and in this regime liquid comes into direct contact with hot surface. The critical heat flux point (maximum heat flux point) marks the

Table 2 2024 aluminum alloy limits. Chemistry

Si

Fe

Cu

Mn

Mg

Cr

Zn

Ti

V

Zr

Other

Min Max

0.00 0.50

0.00 0.50

3.80 4.90

0.30 0.90

1.20 1.80

0.00 0.10

0.00 0.25

0.00 0.15

0.00 0.05

0.00 0.05

Remainder

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Fig. 3. Temperature data and smoothing curves obtained by B-spline approximation (PN = 2 bar, Tw = 25 °C).

Fig. 5. Spray quenching curves for 2024 aluminum alloy thin sheets with DN = 70 mm and (a) variations of nozzle pressure at Tw = 25 °C; (b) variations of water temperature at PN = 3 bar.

Fig. 4. Sketch of one-dimensional model.

Table 4 Thermal properties of 2024 aluminum at different temperatures [19]. T (°C)

c ðJ=kg  CÞ

k=ðW m  CÞ

q ðkg=m3 Þ

20 100 200 300 400 500

881 927 1047 1130 1210 1300

164 182 194 202 210 220

2780

temperature at which liquid gives a fully contact with hot surface. In the third regime (nucleate boiling), effectiveness of heat transfer is very high due to bubbles growth, and also nucleation absorbs a lot of heat from hot surface. The heat transfer subsides in the single convective cooling since heat is transferred directly into the liquid. When the water temperature is less than 70 °C, typical spray quenching curve (water temperature is 25 °C, nozzle pressure is 3 bar) for the variations of heat flux with surface temperature is shown in Fig. 7a. It can be seen from the figure that, as soon as the spray water impinges onto the sheets, the heat fluxes immediately jump to a relatively high value and then the heat flux increases with the surface temperature decreasing. It reaches the critical heat flux (1.57 MW/m2) when surface temperature is 275 °C. Between 275 °C and around 100 °C, the heat flux rapidly decreases with decreasing surface temperature and finally it accesses the single convective cooling regime, in which heat flux has significantly lower value.

Fig. 6. A typical boiling curve.

However, compared with Fig. 7a, it is shown that there exists a film boiling and the Leidenfrost point when the water temperature is 70 °C (Fig. 7b). This temperature is higher than the findings in Refs. [23,24] where water temperature is 60 °C and has a similar agreement with Kokado’s [25] conclusion which indicates that no film boiling occurs when water temperature is lower than 68 °C. The relatively lower heated temperature in this experiment may explain the reason behind this phenomenon. There needs much more heat to heat the water to vaporization temperature when quenching water of 60 °C hits the surface. However, compared with the heated temperature used in references (about 900 °C), it is harder for this heated temperature (about 470 °C) to form thin vapor film. Therefore, higher water temperature (about 70 °C) is needed for film boiling occurrence in this experiment conditions. From this figure we can find that the Leidenfrost point is about 420 °C, and also it indicates that in the single convective cooling stage, the heat flux has a rapidly drop. This phenomenon is different with Fig. 7a, which shows that the heat flux decreases slowly when surface temperature is less than 100 °C. It may be attributed

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255

450 °C to 25 °C. But when the nozzle pressure is 3 bar, the critical heat flux occurs at surface temperature 250 °C, and this is slightly different from others when the nozzle pressures are 2 bar, 4 bar and 5 bar. Also in Fig. 8 we can see that, in the single convective cooling stage, the heat flux decreases slowly with the decrease of surface temperature when nozzle pressure is 3 bar and 4 bar. When nozzle pressure varies from 2 bar to 5 bar, the critical heat flux firstly increases with increasing nozzle pressure and reaches maximum value at 3 bar, then the critical heat flux decreases with the increase in nozzle pressure. When nozzle pressure varies from 2 bar to 3 bar, the changes of the critical heat flux is larger than that when nozzle pressure varies from 3 bar to 5 bar. It is to be noted that the increase of nozzle pressure does not improve the critical heat flux (CHF). Xu et al. [8] indicates that there exists the surface tension of quenching water and the entrapped air pressure of specimen surface during spray quenching. At critical heat flux point (CHF) where the vapor film breaks down entirely, the water is directly contacted with heated surface. The contact area increases with the increasing nozzle pressure and improves the heat transfer efficiency since heat transfer depends on the contact area between droplets and heated surface. However, cavitation theory indicates that the force required to rupture a liquid in tension is proportional to surface tension as [26]:

F / r3=2 Fig. 7. Variations of heat flux with surface temperature when (a) water temperature less than 70 °C; (b) water temperature at 70 °C.

to the nozzle pressure used in this experiment. It is known that heat transfer is transferred through natural convection of water in single convection cooling regime. In this regime, more droplets that have higher water temperature will escape out of the quenching surface, this trend is remarkable when relatively higher nozzle pressure is used. Thus, the heat flux under Tw = 70 °C is much higher than Tw = 25 °C in the single convective cooling regime. However, further investigation is needed to verify this phenomenon whether this may be attributed to a complex relationship at higher water temperature or to measurement error. 4.2. Effect of spray pressure In this section, the nozzle pressure is from 2 bar to 5 bar (other parameters are consistent, water temperature is 25 °C and nozzle distance is 70 mm) and its influence on heat transfer is discussed. The effect of spray pressure on heat flux for 2024 aluminum alloy thin sheets is shown in Fig. 8. It can be seen from Fig. 8 that heat transfer regimes are transition boiling, nucleate boiling and single convective cooling stage when surface temperature decreases from

ð19Þ

where F is the force required to rupture a liquid in tension and r is the surface tension. Therefore because it is difficult for droplets to be fractured into smaller ones when the surface tension is larger, the increase of nozzle pressure which increases surface tension and entrapped air pressure will also inhibit the heat transfer efficiency reversely. When the nozzle pressure is low, CHF increases with the nozzle pressure increasing due to smaller surface tension and entrapped air pressure, but with nozzle pressure continues to increase, the improving and inhibiting of heat transfer ability will reach balance. In other words, it will reach the maximum value, and below this value CHF will decrease with nozzle pressure going to increase because the increase of surface tension and entrapped air pressure is larger than contact area increases. So just as shown in Fig. 8, when nozzle pressure is 3 bar, CHF reaches the maximum value and when nozzle pressure varies from 3 bar to 5 bar, CHF decreases with the increasing nozzle pressure. The rationale behind the changing trend for the surface temperature at which critical heat flux occurs is the result of combined action of nozzle pressure and surface tension. It is known that surface tension is smaller and more droplets are contacted with heated surface with nozzle pressure increases at lower nozzle pressure. So the vapor film is broken through easier. This promotes the transition boiling and the CHF is occurred at a lower temperature. However, the surface tension is larger when more and more droplets are contacted with heated surface. According to Weissenborn and Pugh [27], bubble coalescence is prevented with the increasing of surface tension. Therefore nucleate boiling is enhanced which leads to the CHF occurs at a higher temperature. 4.3. Effect of water temperature

Fig. 8. Effect of nozzle pressure on heat flux for 2024 aluminum alloy thin sheets.

The obtained heat flux vs. surface temperature plots during spray quenching of 2024 aluminum alloy thin sheets when water temperature varies from 25 °C to 70 °C are shown in Fig. 9. The nozzle pressure is 3 bar and nozzle distance is 70 mm. From Fig. 9 we may see that, contrast to the boiling curves at relatively lower water temperature, a film boiling regime occurs when water temperature is 70 °C. Xu and Gadala [24] indicates that compared with water at

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3. When the water temperature is 25 °C, due to the surface tension of water and entrapped air pressure of specimen surface during spray quenching, the critical heat flux first increases and then decreases with increasing nozzle pressure. 4. Heat transfer behavior is also affected by water temperature, the critical heat flux decreases when water temperature increases from 25 °C to 60 °C, however, the critical heat flux increases when water temperature varies from 60 °C to 70 °C.

Acknowledgements

Fig. 9. Effect of water temperature on heat flux for 2024 aluminum alloy thin sheets.

lower temperature, as the sprays impinge onto the hot surface, it is easily to reach the boiling temperature for water of higher temperature which keeps heat flux at relatively lower value. Compared the four boiling curves at different water temperatures, it shows that there are some differences at single convective cooling regime. When water temperature is 25 °C, 40 °C and 60 °C, it has similar trend and the heat flux decreases slowly, but the heat flux has a rapidly decreasing when water temperature is 65 °C and 70 °C. Fig. 9 also shows that the maximum value of critical heat flux is 1.57 MW/m2 when water temperature is 25 °C, and the minimum value of critical heat flux is 1.1 MW/m2 at 60 °C. The critical heat flux first decreases with increasing water temperature and then increases with the increasing of water temperature. This is because that the surface tension of cooling sheets and entrapped air pressure of specimen surface hinders heat transfer during spray quenching. Water temperature also affects liquid surface tension and entrapped air pressure. When water temperature increases, the distance of surface water molecules increases, thus reduces the surface tension. However, entrapped air pressure increases with water temperature increasing. When water temperature varies from 25 °C to 60 °C, reducing surface tension is less than increasing entrapped air pressure, and therefore the critical heat flux decreases. With water temperature continues to increase, the influence of reducing surface tension is larger than the effect of increasing air pressure, giving rise to the increase in critical heat flux. 5. Conclusions This study examined the problem of spray quenching 2024 aluminum alloy thin sheets. Quench curves were smoothed and filtered by B-spline approximation and heat flux were obtained using an inverse heat conduction method. These quench curves were used to assess the influence of nozzle pressure and water temperature of the thin sheets on heat transfer. The major findings from this study are as follows: 1. B-spline approximation is a feasible and effective method for determination of smoothed and filtered quench curves, and when smoothing parameter is 103, it tends to neither depart excessively from experimental points nor smooth away physically meaningful features of curves. 2. At nozzle pressure of 3 bar, when water temperature is less than 65 °C heat transfer in spray quenching of 2024 aluminum alloy thin sheets consists of three regimes, namely transition boiling, nucleate boiling and single convective cooling. However, there exists a film boiling regime when water temperature is 70 °C.

The authors wish to thank Chuanmei Xie from College of Chemistry and Chemical Engineering, Shannxi University of Science and Technology for helpful discussions. The authors would like to thank the National Science Foundation of China (Grant No. 51075332). References [1] J.T. Maximov, G.V. Duncheva, A.P. Anchev, M.D. Ichkova, Modeling of strain hardening and creep behavior of 2024-T3 aluminum alloy at room and high temperatures, Comput. Mater. Sci. 83 (2014) 381–393. [2] J. Zhang, Y.L. Deng, W. Yang, S.S. Hu, X.M. Zhang, Design of the multi-stage quenching process for 7050 aluminium alloy, Mater. Des. 56 (2014) 334–344. [3] L. Zhang, X. Feng, et al., FEM simulation and experimental study on the quenching residual stress of aluminum alloy 2024, J. Eng. Manuf. 227 (7) (2013) 954–964. [4] Z. Zhang, P.X. Jiang, X.L. Ouyang, J.N. Chen, D.M. Christopher, Experimental investigation of spray cooling on smooth and micro-structured surfaces, Int. J. Heat Mass Transfer 76 (2014) 366–375. [5] H.B. Xu, C.Q. Si, S.Q. Shao, C.Q. Tian, Experimental investigation on heat transfer of spray cooling with isobutane (R600a), Int. J. Therm. Sci. 86 (2014) 21–27. [6] N. Mascarenhas, I. Mudawar, Analytical and computational methodology for modeling spray quenching of solid alloy cylinders, Int. J. Heat Mass Transfer 53 (25–26) (2010) 5871–5883. [7] N. Cheung, N.S. Santos, J.M.V. Quaresma, G.S. Dulikravich, A. Gracia, Interfacial heat transfer coefficients and solidification of an aluminum alloy in a rotary continuous caster, Int. J. Heat Mass Transfer 52 (1–2) (2009) 451–459. [8] R. Xu, L.X. Li, L.Q. Zhang, B.W. Zhu, X. Liu, X.B. Bu, Influence of pressure and surface roughness on heat transfer coefficient during water spray quenching of 6082 aluminum alloy, J. Mater. Process. Technol. 214 (12) (2014) 2877–2883. [9] L.Q. Zhang, C. Reilly, L.X. Li, S. Cockcroft, Development of an inverse heat conduction model and its application to determination of heat transfer coefficient during casting solidification, Heat Mass Transfer 50 (7) (2014) 945–955. [10] O. Golovko, I. Frolov, D. Rodman, F. Nurnberger, O. Grydin, M. Schaper, Spray cooling of extruded EN AW-6082 aluminum alloy sheets: spatial heat transfer coefficients, Forsch. Ingenieurwes. 78 (3) (2014) 131–137. [11] N. Mascarenhas, I. Mudawar, Methodology for predicting spray quenching of thick-walled metal alloy tubes, Int. J. Heat Mass Transfer 55 (11) (2012) 2953– 2964. [12] X.W. Yang, J.C. Zhu, Z.S. Nong, Z.H. Lai, D. He, FEM simulation of quenching in A357 aluminum alloy cylindrical bars and reduction of quench residual stress through cold stretching process, Comput. Mater. Sci. 69 (2013) 396–413. [13] A. Sugianto, M. Narazaki, M. Kogawara, A. Shirayori, A comparative study on determination method of heat transfer coefficient using inverse heat transfer and iterative modification, J. Mater. Process. Technol. 209 (10) (2009) 4627– 4632. [14] I. Mudawar, K.A. Estes, Optimizing and predicting CHF in spray cooling of a square surface, J. Heat Transfer 118 (1996) 672–679. [15] R.J. Moffat, Describing the uncertainties in experimental results, Exp. Thermal Fluid Sci. 1 (1988) 3–17. [16] H. Kano, H. Fujioka, Optimal smoothing and interpolating splines with constraints, Appl. Math. Comput. 218 (2011) 1831–1844. [17] C.H. Reinsch, Smoothing by spline functions I, Numer. Math. 10 (1967) 177– 183. [18] C.H. Reinsch, Smoothing by spline functions II, Numer. Math. 16 (1971) 451– 454. [19] N. Zhao, Y.Q. Yang, M. Han, X. Luo, G.H. Feng, R.J. Zhang, Finite element analysis of pressure on 2024 aluminum alloy created during restricting expansion deformation heat treatment, Trans. Nonferr. Met. Soc. China 22 (9) (2012) 2226–2232. [20] O.R. Burggraf, An exact solution of the inverse problem in heat conduction theory and applications, J. Heat Transfer 86 (3) (1964) 373–382. [21] J. Taler, Theory of transient experimental techniques for surface heat transfer, Int. J. Heat Mass Transfer 39 (17) (1996) 3733–3748. [22] M. Ciofalo, I.D. Piazza, V. Brucato, Investigation of the cooling of hot walls by liquid water sprays, Int. J. Heat Mass Transfer 42 (7) (1999) 1157–1175.

R. Guo et al. / Experimental Thermal and Fluid Science 72 (2016) 249–257 [23] Z.H. Liu, J. Wang, Study on film boiling heat transfer for water jet impinging on high temperature flat plate, Int. J. Heat Mass Transfer 44 (13) (2001) 2475– 2481. [24] F.C. Xu, M.S. Gadala, Heat transfer behavior in the impingement zone under circular water jet, Int. J. Heat Mass Transfer 49 (21–22) (2006) 3785–3799. [25] J.I. Kokado, N. Hatta, H. Takuda, N. Harada, An analysis of film boiling phenomena of subcooled water spreading radially on a hot steel plate, Arch. Eisenhuttenwes. 55 (3) (1984) 113–118.

257

[26] C.E. Brennen, Cavitation and Bubble Dynamics, Oxford University Press, Oxford, 1995. [27] P.K. Weissenborn, R.J. Pugh, Surface tension of aqueous solutions of electrolytes: relationship with ion hydration, oxygen solubility, and bubble coalescence, J. Colloid Interface Sci. 184 (2) (1996) 550–563.