ORIGINAL

Ultra fast cooling of hot steel plate by air atomized spraywith salt solution

Soumya S. Mohapatra • Satya V. Ravikumar • Jay M. Jha •

Akhilendra K. Singh • Chandrima Bhattacharya •

Surjya K. Pal • Sudipto Chakraborty

Received: 31 October 2012 / Accepted: 7 November 2013

� Springer-Verlag Berlin Heidelberg 2013

Abstract In the present study, the applicability of air

atomized spray with the salt added water has been studied

for ultra fast cooling (UFC) of a 6 mm thick AISI-304 hot

steel plate. The investigation includes the effect of salt

(NaCl and MgSO4) concentration and spray mass flux on

the cooling rate. The initial temperature of the steel plate

before the commencement of cooling is kept at 900 �C or

above, which is usually observed as the ‘‘finish rolling

temperature’’ in the hot strip mill of a steel plant. The heat

transfer analysis shows that air atomized spray with the

MgSO4 salt produces 1.5 times higher cooling rate than

atomized spray with the pure water, whereas air atomized

spray with NaCl produces only 1.2 times higher cooling

rate. In transition boiling regime, the salt deposition occurs

which causes enhancement in heat transfer rate by con-

duction. Moreover, surface tension is the governing

parameter behind the vapour film instability and this length

scale increases with increase in surface tension of coolant.

Overall, the achieved cooling rates produced by both types

of salt added air atomized spray are found to be in the UFC

regime.

List of symbols

Fa Air flow rate (N m3/h)

Fw Water flow rate (m3/s)

S1 Section 1 (mm)

S2 Section 2 (mm)

S3 Section 3 (mm)

Sc Salt concentration (mol/l)

T Temperature (�C)

T1 Temperature measured by thermocouple 1 (�C)

T2 Temperature measured by thermocouple 2 (�C)

T3 Temperature measured by thermocouple 3 (�C)

t Time (s)

X Direction along the length of the plate (mm)

Y Direction along the thickness of the plate (mm)

1 Introduction

The modern day requirements of high tensile strength

steel have placed a great emphasis on the rapid cooling

methods for the hot plate immediately after rolling. The

desired mechanical properties of steel are achieved during

cooling on the Run-Out table (ROT) of a hot strip mill.

During the cooling process, the thermo-metallurgical

phase transformation of steel takes place and that governs

the final mechanical properties. However, the slow

quenching rate generated by conventional laminar cooling

system, which is generally fitted in the ROT, cannot

generate the desired high tensile strength steel. According

to Lucas et al. [1], the rapid cooling rate has been the

main focus for the development of ultra fast cooling

(UFC) technologies.

UFC technique, depending upon the final microstructure

of dual phase steel, was investigated by Bin et al. [2]. Any

cooling process, for which the product of cooling rate (�C/

s) and plate thickness (mm) yields a number greater than

800, is called UFC technique.

S. S. Mohapatra � S. V. Ravikumar � J. M. Jha �A. K. Singh � C. Bhattacharya � S. Chakraborty (&)

Department of Chemical Engineering, Indian Institute

of Technology, Kharagpur 721032, India

e-mail: sc@che.iitkgp.ernet.in

S. K. Pal

Department of Mechanical Engineering, Indian Institute

of Technology, Kharagpur 721302, India

123

Heat Mass Transfer

DOI 10.1007/s00231-013-1260-6

For rapid cooling operations, amongst all the liquid

coolants, water is the most economical and easily available

component. When water is used to cool a hot substrate at

high initial surface temperature, the Leidenfrost effect

plays an important role. Oliveria and Sousa [3] reported

that due to the aforesaid effect, a vapour layer covers the

hot surface and simultaneously reduces the heat transfer

rate.

Air atomized spray is found to be a promising cooling

system among different cooling methods reported by Mo-

hapatra et al. [4]. Moreover, the study made by Puschmann

and Specht [5] point out that the main advantage of air

atomized spray cooling is that a high volumetric flow of air

sweeps away the partially evaporated drops from the hot

surface. This phenomenon prevents the formation of a

stable vapour film at high surface temperature, but is

observed only at low mass flux. However, the investigation

made by Al-Ahmadi and Yao [6] depicts that cooling

occurs in transition boiling regime in the case of high-mass

flux atomized spray cooling also. As a result, in the

aforesaid case, high cooling rate is observed when it starts

from a high surface temperature.

In case of air atomized spray with high mass flux, the

droplet evaporation time plays an important role as repor-

ted by Bhattacharya et al. [7]. The study made by Qio and

Chandra [8, 9] revealed that droplet life time can be

decreased by making the droplet thin when it impinges the

hot plate, and creating high contact surface for heat transfer

by using surfactant added water. They noticed that cooling

enhancement was limited within the nucleate boiling

regime. Cui et al. [10] examined the effect of dissolving

gas and salt in boiling of water droplet. They concluded

that gas had little effect on boiling, and the dissolved salt in

water prevented the coalescence of vapour bubble inside

the droplet. In the follow up research by King et al. [11], it

was observed that dissolved salt decreased the droplet

vaporization rate due to the boiling point elevation.

Cui et al. [12, 13] experimentally studied salt added

water-spray cooling in the temperature range of

100–240 �C, and found that the dissolved salt in water

droplet deposits on the hot plate during evaporation. The

deposited salt penetrates the vapour film and touches the

water droplet. Therefore, the heat transfer occurs in con-

duction mode from the hot plate to the droplet and as a

consequence, the cooling in transition boiling regime of a

conventional spray cooling improved. The research with

dissolved salt in water has so far been limited to spray

cooling, and was conducted at a very low substrate tem-

perature (*240 �C) and low mass flux (\5 kg/m2 s) of

water.

The literature asserts that the dissolved salt in water

enhances the cooling in transition boiling regime due to the

deposition of salt on the hot surface. Therefore, the authors

expect an improvement of cooling rate in the transition

boiling regime (900–600 �C) of an air atomized spray with

salt-water. The reasons are—prevention of vapour bubble

coalescence inside the droplet, the effect of deposition of

salt on the cooling surface and the effect of superposed air

flow and foaming.

The degree of film boiling depends on the extent of

vapour bubble generation and its coalescence. Since the

dissolved salt in water droplet prevents the vapour bubble

coalescence, the film boiling effect is expected to be a

minimum in this case. Additionally due to salt deposition,

the vapour film in the transition boiling regime collapses,

and as a result an enhancement in cooling is expected.

Therefore, in the current research, salt added air-atomized

spray cooling has been studied at a high initial surface

temperature (*900 �C), which is generally found to be in

the transition boiling regime, and high mass flux of water

to examine its applicability in achieving UFC rate for a hot

steel plate.

2 Experimental configuration

Experimental setup has been initially fabricated to conduct

the required number of experiments according to the

Design of Experiment (DOE) methodology followed in

the current work. After the experimentations, the surface

heat flux and the surface temperatures have been calcu-

lated for the interpretation of the heat transfer data. In

addition to the above, to understand the heat transfer

mechanism during cooling, the spray behaviours have also

been investigated.

2.1 Experimental procedure

Systematic experiments have been conducted on a 100

mm 9 100 mm AISI 304 steel plate of 6 mm thickness.

For the measurement of temperature during experimenta-

tion, three K-type thermocouples having diameter 3 mm

each (T1, T2 and T3) were inserted in the plate. The exact

locations of the thermocouples within the steel plate have

been shown in Fig. 1. By following the methodology

proposed by Li and Wells [14], in the current work ther-

mocouples were placed parallel to the quenching surface to

avoid the error induced due to the thermocouple hole

diameter. The temperature data were recorded at a sam-

pling rate of 10 data per second during experimentation

with the help of a data acquisition system (NI-USB-6210,

from National Instrument, USA).

The steel plate was initially heated to a temperature of

1,050 �C. After attaining the desired temperature

(1,050 �C) and proper thermal soaking in the muffle fur-

nace, the compressor and pump were switched on for air

Heat Mass Transfer

123

and water spray respectively. The water temperature for

each experiment was maintained at 25 �C. The hot steel

plate was then taken out from the muffle furnace and put

on the cooling pad. The spray was showered from a full

cone air-assisted atomizer (Lechler 170.801) at a fixed

nozzle to plate distance (60 mm), and the atomizer was

kept vertically downward to get the maximum gravity

effect. The time–temperature data was recorded continu-

ously and saved in a file. The experiments were conducted

with various water flow rates (6.67 9 10-5–

16.67 9 10-5 m3/s) and at different salt concentrations

(0–0.4 mol/l) of sodium chloride (NaCl) and magnesium

sulphate (MgSO4). The solution was prepared with the

proper ratio of water at 25 �C and salt. During the

experimentation, the air flow rate was maintained at

40 N m3/h. A complete schematic diagram of the experi-

mental setup is shown in Fig. 2.

2.2 Estimation of surface heat flux

In the current work, the prediction of surface heat flux and

calculation of surface temperature have been done from the

measured temperature inside the plate, by using an inverse

heat conduction software (INTEMP) developed by Trujillo

and Busby [15]. The ‘‘INTEMP’’ is widely used by the

scientific community for the inverse heat conduction

problem; therefore, in the current research also, INTEMP

software has been used. INTEMP solves both linear and

nonlinear inverse heat conduction problems with unknown

surface heat flux, as claimed by Busby and Trujillo [16].

The plate used in the current experiment was 6 mm

thick and square in shape. Therefore, the heat transfer

within the plate can be modelled as a two dimensional

transient heat conduction problem with unknown surface

heat flux. Hence, in INTEMP, a 2-D planar thermal model

Fig. 1 Sketch of thermocouple

installation locations

Heat Mass Transfer

123

of 100 mm length and 6 mm thickness corresponding to

the actual steel plate has been considered. In INTEMP, for

the prediction of surface heat flux and surface tempera-

tures, the plate geometry has been discretised into 3,340

quadratic element with four nodes per element. The dis-

cretisation has been performed by using GAMBIT soft-

ware. In meshing, 0.06 mm increment in the direction of

X-axis and 0.03 mm in Y-axis have been contemplated,

and as a result a total of 3,528 nodes have been found.

For boundary conditions, all plate surfaces were

assumed to be adiabatic, except the top impinging surface.

The top non-adiabatic surface has been divided into three

flux sections. The first section extends from x = 0 to

35 mm (S1), the second section from x = 35 to 65 mm

(S2), whereas the third section is from x = 65 to 100 mm

(S3).

In the current calculation, the measured thermocouple

data were given as the input at the nodes, which belong to

the location of the thermocouples inserted inside the metal

plate. The nodes corresponding to the locations where the

temperature measurements occur are numbered as—1,715

at position (20, 3); 1,765 at (50, 3) and 1,797 at (70, 3)

respectively. Based on the above information, INTEMP

predicts the transient surface heat flux in all the sections

Fig. 2 Schematic diagram of the experimental setup

Fig. 3 Computational domain

of the steel plate for INTEMP

Fig. 4 Measured temperature and predicted temperature by INTEMP

with time

Heat Mass Transfer

123

and also calculates the transient surface temperatures at

each node. Figure 3 shows the entire computational

domain.

To check the accuracy of the nodal temperature pre-

diction by INTEMP, the predicted temperature for node

number 1,765 has been compared with the measured

temperature (T2). These temperatures have been compared,

because both the temperatures correspond to the same

location of the steel plate. From the comparison (Fig. 4), it

is seen that there is a close match (below 0.05 % error)

between the values estimated by the INTEMP software and

that measured by thermocouples.

2.3 Characterization of salt solution

The dissolved salt in water changes the surface tension

depending upon the nature and concentration of the salts.

Hence, the aqueous solution was characterized for the

aforesaid physical property of water.

Surface tension is an important parameter which influ-

ences the droplet contact angle with the solid surface. The

effect of salt concentration on the surface tension is shown

in Fig. 5. All the measurements shown in the said figure are

the average values of three repeated measurements taken

by using a surface tension meter (Model No: 232, Testing

Instruments, Kolkata, India).

In Fig. 5, it can be seen that the surface tension of water

marginally increases with an increasing NaCl concentra-

tion, and this is quite consistent with the information

reported in the literature. Sodium chloride dissolves per-

fectly in water, and as a result its surface tension at 25 �C

increases by adding NaCl, which in turn induces attractive

forces on the surface of the water molecule. However, the

surface tension of water decreases up to an MgSO4 salt

concentration of 0.05 mol/l. The same trend line was also

observed by the Cui et al. [12].

2.4 Spray characterization

The parameters which influence the air atomized spray are

droplet diameter, spray area and impingement density. The

details about each parameter has been described below. By

using the data sheet provided by Lechler Company

(GmbH), the variation of volumetric mean diameter of the

droplet with the water flow rate is as shown in Fig. 6. At a

constant air flow rate of 40 Nm3/h, it can be seen that the

droplet diameter increases with an increase in water flow

rate. The maximum droplet diameter of 170 lm was found

at a water flow rate of 16.67 9 10-5 m3/s, whereas the

minimum diameter of 88 lm was obtained at

6.67 9 10-5 m3/s.

Fig. 5 Variation of surface tension with salt concentrationFig. 6 The variation of droplet diameter with water flow rate

Fig. 7 Photograph taken at the moment spray impinges on the hot

plate

Heat Mass Transfer

123

To observe the spray impinging area, photographs were

taken by a digital camera during experiments. Figure 7

shows one such photograph taken during the experiment

with salt added (NaCl concentration 0.1 mol/l) air-atom-

ized spray. The nozzle height (60 mm) and the spray angle

(60�) data have been used to calculate the spray area. From

Fig. 7, it was noticed that a dark circle having radius equal

to the radius of the spray area appeared. The forced con-

vection cooling in the direct spray impinging area lowers

the surface temperature, compared to the surrounding red

hot zone leading to this dark circle. From the dark circle, it

was apparent that thermocouples T3 and T2 were in the

direct spray impinging area, whereas thermocouple T1 lies

outside.

The local variation of the spray impingement density

with the different water flow rates is shown in Fig. 8. All

impingement density measurements were taken by using an

indigenously designed and fabricated Patternator. Here, it

is found that the impingement density varies within the

radial distance of ±22 mm from the spray axis. The same

observation was also found from the photograph taken

during cooling. It was also noticed that the local

impingement density decreases with an increase in distance

from the spray axis, and the maximum value has been

achieved at the spray centre. The same trend line was also

observed by Alam et al. [17]. Moreover, it was also

observed that the local impingement density increases with

an increase in water flow rate. The maximum impingement

density was observed at a water flow rate of

16.67 9 10-5 m3/s and the minimum was found at

6.67 9 10-5 m3/s. The average impingement density of

air-atomized spray increases with an increase in water flow

rate and this is shown in Fig. 9. In the current research, the

average impingement density varies in the range of

82–255 kg/m2 s in the said water flow rate range. In the

present work, since the water impingement density depends

on the flow rate, the cooling rates and the surface heat flux

have been represented as a function of flow rate instead of

water impingement density.

2.5 Experimental uncertainty

In the present case, the measured variables are temperature

(Ti, inside temperature) and impingement density. The

main sources of uncertainty in temperature measurements

are temperature fluctuations and uncertainty in the loca-

tions of the thermocouples. All measurements were taken

thrice and their average was considered for the final cal-

culation. From the average data, the maximum uncertainty

is found to be 4 %.

3 Results and discussion

In the current investigation, the effect of water flow rate

and salt concentration on the heat transfer characteristics of

an air atomized spray has been studied for both types of

salts (NaCl and MgSO4), and from the experimental data,

relevant mathematical correlations have been developed.

Four sets of experiments have been designed which is

presented in Table 1.

3.1 Experiment with NaCl

3.1.1 Cooling curve

As per the DOE, a total of 20 experiments were performed,

and the time–temperature data were recorded for each of

these experiments. From these time–temperature histories,

by using INTEMP software, the surface heat flux and

surface temperature have been estimated. Here, only one

Fig. 8 Variation of water impingement density with water flow rate Fig. 9 Variation of average impingement density with water flow

rate

Heat Mass Transfer

123

such result is given as a sample case to understand the

cooling behavior of the steel plate during the experiment.

The remaining results have, however, been used to analyze

the effect of salt concentration and water flow rate on the

cooling rate, as illustrated later.

Figure 10 shows the temperature acquired by the ther-

mocouples at different locations of the plate during cool-

ing. The experimental conditions at which the experiments

were conducted are—water flow rate of 13.33 9 10-5 m3/s

and sodium chloride (NaCl) concentration of 0.1 mol/l.

Before the atomized spray was turned on, the three ther-

mocouples show the same temperature, which confirms

proper thermal soaking inside the muffle furnace. Fur-

thermore, Fig. 10 also shows that up to initial 37 s, the heat

loss is due to natural convection and radiation, because

some time is required to bring out the hot plate from the

muffle furnace and place it properly on the cooling pad.

After this initial time period, the heat loss significantly

increases with an increase in time due to forced convection

and boiling heat transfer caused by the atomized spray, and

consequently the temperature falls sharply. It was also

observed that there are 1.5 and 4 s delay in cooling for

thermocouples T3 and T1 respectively. The reason being

that, the thermocouple T1 is not under the direct spray

impingement section, whereas T3 is the second nearest

thermocouple from the spray centre.

The experimental data corresponding to Fig. 10 has

been used for the estimation of surface heat flux. The

transient variation of the estimated surface heat flux is

shown in Fig. 11. The point at which atomized spray

cooling starts in Fig. 10 has been considered as the initial

time (time = 0) for Fig. 11. Here it was observed that the

surface heat flux increases with an increase in time up to

2.65 s, and thereafter decreases. This could be due to the

change in heat transfer mechanism. For better under-

standing of the heat transfer mechanism, the boiling curve

Table 1 Design of experiment

Sl. No. Name of salt Name of variables Max. level Min. level Levels

1 NaCl Water flow rate (m3/s) 16.67 9 10-5 6.67 9 10-5 6.67 9 10-5, 10 9 10-5, 13.33 9 10-5, 16.67 9 10-5

2 NaCl Salt concentration (mol/l) 0.4 0 0, 0.1, 0.2, 0.3, and 0.4

3 MgSO4 Water flow rate (m3/s) 16.67 9 10-5 6.67 9 10-5 6.67 9 10-5, 10 9 10-5, 13.33 9 10-5, 16.67 9 10-5

4 MgSO4 Salt concentration (mol/l) 0.4 0 0, 0.1, 0.2, 0.3, and 0.4

Fig. 10 Temperature-time history for NaCl added air atomized spray

cooling

Fig. 11 Estimated surface heat flux and calculated surface temper-

ature with time for NaCl

Fig. 12 Boiling curve for air atomized spray with salt (NaCl)

Heat Mass Transfer

123

of the salt added air-atomized spray cooling has been

shown in Fig. 12.

For the boiling curve of air-atomized spray with salt, the

surface heat flux has been plotted as a function of surface

temperature (Fig. 12). Here, it was seen that transition

boiling regime cooling occurs in the temperature range of

900–525 �C, and the surface heat flux reaches the maxi-

mum value (CHF) of 1.9 MW/m2 at T = 525 �C. There-

after, the surface heat flux decreases with a decrease in

surface temperature, because the boiling regime changes

from transition to nucleate. Hence, in Fig. 11, the surface

heat flux is directly proportional to the time from

t = 0–2.65 s, due to the early onset of transition boiling

regime and thereafter, the surface heat flux decreases with

time due to the cooling in nucleate boiling regime. The film

boiling mechanism which weakens the heat transfer rate

has not been found in the experimental study of air-atom-

ized spray cooling. This is due to the fact that in air-

atomized spray cooling the finer droplets of water with

higher momentum touch the hot surface, gets deformed

increasing the contact area, the conduction heat transfer

takes place through the contact area, the water temperature

inside the droplet rises and it starts evaporating. Before the

droplet evaporates completely, they are swept away from

the plate surface by high velocity superposed air flow.

Hence, the formation of vapour film is prevented as there is

no occurrence of water pool on the hot surface unlike the

pure water jet impingement cooling. The same has also

been observed in the authors’ earlier studies with air-

atomized spray cooling system [18–20].

3.1.2 Transient variation of surface heat flux

The variation of surface heat flux with time at different salt

concentrations is shown in Fig. 13. Here, it was observed

that all the curves show a similar trend that initially

increases with an increase in time, and then decreases. The

increasing nature corresponds to the cooling in transition

boiling regime, whereas the decreasing characteristic of the

surface heat flux is due to the cooling in nucleate boiling

regime.

Figure 13 shows that the surface heat flux at any time

increases with an increase in salt concentration up to

0.1 mol/l, and thereafter shows a decreasing trend with an

increase in cooling in both the boiling regimes. In the

nucleate boiling regime, up to a salt concentration of

0.1 mol/l, the cooling enhancement could be due to the

prevention of the vapour bubble coalescence, which always

promotes the nucleate boiling.

In the transition boiling regime, up to a salt concentra-

tion of 0.1 mol/l, the surface heat flux mainly increases due

to the effect of salt deposition on the hot plate during the

droplet evaporation. As suggested in the literature, this

deposited salt breaks through the vapour layer insulating

the hot plate, and touches the water droplet during cooling.

Therefore, the heat transfer takes place by conduction

mode from the hot plate to the water droplet through the

deposited salt, and enhances the surface heat flux in the

transition boiling regime. However, the improvement in

surface heat flux with an increase in salt concentration in

the transition boiling regime is lesser compared to the

nucleate boiling regime. This could be due to insufficient

amount of salt deposition, which might not be adequate to

significantly enhance the surface heat flux.

In the case of salt concentration higher than 0.1 mol/l,

the effect produced due to the rise in surface tension of

resulting droplet dominates over the salt deposition, and

vapor bubble coalescence to enhance the heat transfer.

Moreover, surface tension is the governing parameter

behind the vapour film wave length of instability and this

wave length scale increases with increase in surface tension

of coolant. This instability length scale is a function of

surface tension, density of liquid and gas, and expressed as

the following equation [21]:

k ¼ rgðql � qgÞ

!1=2

ð1Þ

where r is surface tension, ql is density of liquid, qg is

density of gas, and g is acceleration due to gravity.

Therefore, the maximum surface heat flux has been

observed at a salt concentration of 0.1 mol/l.

3.1.3 Effect of water and salt concentration on average

surface heat flux and cooling rate

The steel strip/plate in the temperature range of

900–600 �C is cooled on the ROT, where the cooling rate

Fig. 13 Varition of surface heat flux with time at different concen-

trations of salt (NaCl)

Heat Mass Transfer

123

is an important parameter to control its mechanical/met-

allurgical properties. So, in the present work the average

surface heat flux and the cooling rate have been estimated

for the surface temperature drop of 900–600 �C. Figure 14

depicts that for all levels of water flow rates, the average

heat flux increases with an increase in salt concentration,

and reaches the maximum value at a salt concentration of

0.1 mol/l. The maximum average heat flux is obtained at a

salt concentration of 0.1 mol/l and water flow rate of

16.67 9 10-5 m3/s. Moreover, it is also noticed that the

average surface heat flux increases with an increase in

water flow rate at a constant salt concentration. This is due

to the fact that at higher water flow rate the impingement

density is also high, and this enhances the critical heat flux

(CHF) and nucleate boiling heat flux.

By using Design Expert Software version 6.0, a math-

ematical correlation (Eq. 2) has been developed involving

the surface heat flux, water flow rate and the surfactant

concentration. The determination coefficient for this cor-

relation is 0.99 which shows a proper fitting of this cor-

relation to the current experimental data.

qS ¼ 1:68þ 0:31� FW þ 0:075� SC � 0:034� F2w

� 0:49� S2C ð2Þ

The variation of average surface cooling rate with the water

flow rate and the salt concentration is shown in Fig. 15.

The figure shows that at constant water flow rate, cooling

rate gradually increases with an increase in salt concen-

tration, and reaches the maximum value at a salt concen-

tration of 0.1 mol/l. Thereafter, the cooling rate decreases

with an increase in salt concentration. The same trend is

observed for all levels of water flow rates.

The maximum cooling rate (178 �C/s) was observed at a

water flow rate of 16.67 9 10-5 m3/s, and the achieved

cooling rate was found to be in the ultra fast domain of an

AISI-304 steel plate. In addition to the above, by using

Design Expert Software version 6.0, a correlation which

relates cooling rate with water flow rate (Fw) and salt

concentration (Sc) is given Eq. 3. The determination

coefficient (R2) of the developed correlation is 0.99.

Z ¼ 158:28þ 21:33� FW þ 0:33� SC � 10:97� F2W

� 23:97� S2C � 4:5� FW � Sc

ð3Þ

3.1.4 Effect of salt (NaCl) concentration on critical heat

flux (CHF) and critical surface temperature (TCHF)

Figure 16 shows the estimated CHF and critical surface

temperature (TCHF) as a function of salt concentration. The

CHF increases from 1.4 to 2.58 MW/m2, while the salt

concentration increases from 0 to 0.1 mol/l. The deposited

salt on the cooling surface increases the surface roughness

of the hot substrate during cooling, and this process is the

main reason for the variation in CHF. After a concentration

Fig. 14 Variations of average surface heat flux with salt (NaCl)

concentration

Fig. 15 Cooling rate variation with salt (NaCl) concentration

Fig. 16 The variation of CHF and TCHF with salt (NaCl)

concentration

Heat Mass Transfer

123

of 0.1 mol/l, the CHF shows a decreasing nature with an

increase in salt concentration. In addition to the above, it is

also observed that the critical surface temperature vary

from 575 to 605 �C when the salt concentration changes

from 0 to 0.1 mol/l.

3.2 Experiment with MgSO4 salt

3.2.1 Cooling curves

Figure 17 shows the temperature measured by the ther-

mocouples at different positions of the steel plate during

atomized spray cooling with MgSO4 added water. The

experiments were performed at water flow rate of

13.3 9 10-5 m3/s, magnesium sulphate (MgSO4) concen-

tration of 0.1 mol/l and an air flow rate of 40 N m3/h.

Before the atomized spray is turned on, the three thermo-

couples exhibit the same temperature, which indicates the

uniformity of temperature at different locations before air

atomized spray cooling.

Figure 18 shows the variation of estimated surface

heat flux and surface temperature with time. The surface

heat flux increases with an increase in time up to 2 s

and thereafter shows a decreasing trend. This is because

of the two different boiling regimes (Fig. 19) at two

varying range of surface temperatures. Here, it is

observed that the transition boiling regime cooling

occurs in the temperature range of 900–570 �C, which

corresponds to the time period of 0–2 s in Fig. 18.

Furthermore, it is also found that below 570 �C, the

nucleate boiling initiates.

3.2.2 Transient variation of heat flux

To observe the transient variation of surface heat flux at

different salt concentrations, experiments were conducted

at a water flow rate of 13.33 9 10-5 m3/s and air flow rate

of 40 N m3/h. The variation of estimated surface heat flux

Fig. 17 Temperature-time history for MgSO4 added air atomized

spray cooling

Fig. 18 Estimated surface heat flux and surface temperature with

time for MgSO4

Fig. 19 Boiling curve for air atomized spray with salt (MgSO4)

Fig. 20 Varition of surface heat flux with time for different

concentrations of salt (MgSO4)

Heat Mass Transfer

123

with time is shown in Fig. 20. Here, it was noticed that

initially surface heat flux increases with an increase in time

due to the early initiation of transition boiling, and there-

after decreases with time because of cooling in the nucleate

boiling regime.

In the nucleate boiling regimes, the surface heat flux is

enhanced due to the decrease in surface tension of the

resulting salt added water, production of foaming during

cooling, and the prevention of vapor bubble coalescence

inside the evaporating droplets. In Sect. 2.3, it was dis-

cussed that the surface tension of water decreases with an

increase in salt concentration up to 0.05 mol/l. As a result,

droplet contact angle with the solid surface decreases with

an increase in salt concentration. Due to the said phe-

nomenon, high contact area is produced during droplet

evaporation, and thereby droplets evaporate at a faster rate

from the hot plate.

The dissolved salt in water prevents vapor bubble

coalescence inside the evaporating droplets, and this

process minimizes the film boiling affect. Foaming is also

observed during air-atomized spray cooling with salt, and

this could be another reason for the surface heat flux

enhancement. Due to the foaming, the heat transfer rate

enhances. The above mentioned effect on heat transfer

becomes stronger with an increase in salt concentration up

to 0.05 mol/l. However, above a salt concentration of

0.05 mol/l, all the said positive effects enhance the sur-

face heat flux, except the effect produced due to a

decrease in surface tension. This is because above

0.05 mol/l of salt concentration, the surface tension of the

resulting solution starts increasing with an increase in salt

concentration.

In the transition boiling regimes, the surface heat flux

enhances because of the dominating effects of salt depo-

sition on the hot plate, and the prevention of vapor bubble

coalescence inside the evaporating droplet. In the previous

section, it was discussed that deposition of salt on the hot

plate increases the surface heat flux by minimizing the

negative effect produced due to the unstable vapour film

during cooling.

Beyond salt concentration of 0.05 mol/l, the surface

tension increases with an increase in salt concentration, and

as a result droplet contact angle increases. But, the effect is

not significant, and therefore an increasing trend of surface

heat flux with the salt concentration has been noticed in the

transition boiling regime.

In case of MgSO4 added air atomized spray cooling, the

effect due to salt deposition was the controlling factor.

Moreover, the chances of salt deposition increase with an

increase in salt concentration. As a result, the surface heat

flux shows an increasing trend with an increase in salt

concentration.

3.2.3 Effect of water flow rate and salt concentration

on both average surface heat flux and cooling rate

The average surface heat flux and surface temperature for

900–600 �C drop are shown in Figs. 21 and 22 respectively.

Figure 21 depicts that the average surface heat flux increases

with an increase in salt concentration at a constant water flow

rate. The maximum variation in surface heat flux is observed

when the concentration changes from 0 to 0.1 mol/l. The

average surface heat flux increases with an increase in water

flow rate due to the increase in water impingement density.

By using Design Expert Software version 6.0, a correlation

describing surface heat flux as a function of salt concentration

and water flow rates has been shown in Eq. 4. The determi-

nation coefficient (R2) for the aforesaid correlation is 0.98.

qS ¼ 2:03þ 0:21� Fw þ 0:54� SC þ 0:04� F2a � 0:36

� S2c � 0:075� Fa � Sc ð4Þ

Fig. 21 Variations of average surface heat flux with salt (MgSO4)

concentration

Fig. 22 Variation of cooling rate with salt (MgSO4) concentration

Heat Mass Transfer

123

Similarly, the average surface cooling rates at different

water flow rates and salt concentrations are given in

Fig. 22. The maximum cooling rate of 242 �C/s has been

achieved at a water flow rate of 16.67 9 10-5 m3/s and salt

concentration of 0.4 mol/l, and moreover this achieved

cooling rate was found to be in the UFC regime of a 6 mm

thick AISI-304 steel plate. The developed correlation by

using Design Expert Software version 6.0 (R2 = 0.99) is

shown in Eq. 5.

Z ¼ 198þ 20:83� Fa þ 40:83� S c þ 10:64� F2a

� 21:38� S2c � 4:75� Fa � Sc ð5Þ

3.2.4 Effect of salt (MgSO4) concentration on critical

surface heat flux (CHF) and critical surface

temperature (TCHF)

High value of critical surface heat flux and corresponding

temperature have been of great practical interest in many

applications related to metal quenching especially in

cooling operations such as UFC of a hot steel plate.

Therefore, the effect of salt concentration on critical sur-

face heat flux and critical surface temperature was studied

in great detail and is as shown in Fig. 23. The CHF

increases with an increase in salt concentration. The same

trend was observed by Cui et al. [13] for salt added con-

ventional spray cooling. The maximum CHF has been

observed at a salt concentration of 0.4 mol/l, and the

obtained value is almost twice that obtained for air-atom-

ized spray cooling with pure water. This could be due to

the salt deposition, which increase the surface roughness

value and thereby CHF. In addition to the above, it is also

found that the critical surface temperature changes from

550 to 610 �C with an increase in salt concentration from 0

to 0.4 mol/l. This is due to the fact that at higher salt

concentrations, the Leidenfrost temperature transits to a

higher value according to Huang and Carey [22], and as a

result the critical surface temperature also increases to a

higher value with an increase in salt concentration.

3.3 Comparative study (NaCl and MgSO4)

3.3.1 Transient surface heat flux

To compare the effect of two different types of salts during

air atomized spray cooling illustrated earlier, experiments

were conducted at water flow rate of 16.67 9 10-5 m3/s,

air flow rate of 40 N m3/h and with pure water and salt

added water (Sc = 0.1 mol/l for both NaCl and MgSO4).

The variation of surface heat flux with time for different

type of salt is shown in Fig. 24.

The initial surface heat flux (at t = 0) achieved for air-

atomized spray with pure water and both types of saline

water are 0.6, 0.7 and 2.3 MW/m2 respectively. This dif-

ference between salt added air-atomized spray and pure

water is mainly due to the occurrence of cooling in dif-

ferent boiling regimes at high initial surface temperatures.

But, the difference between two types of salt added air-

atomized spray is mainly due to the occurrence of foaming

in case of MgSO4 added air atomized spray.

Another reason could be the insufficient amount of salt

deposition at the beginning of cooling. However, it is

expected that the deposition becomes significant with an

increase in time, and subsequently prove to be an important

factor in the cooling enhancement. The literature reveals

that the chances of film rupture increase with an increase in

the possibility of salt deposition and moreover, the depo-

sition intensity depends on the rate of diffusion of a dis-

solved salt in water. When the droplet evaporates from the

hot surface, the salt concentration increases in the

remaining liquid. This causes a local increment of ionic

strength at the droplet and hot surface interface, promoting

Fig. 23 The variation of CHF and TCHF with salt (MgSO4)

concentration

Fig. 24 Comparative study among the transient effect of different

types of salts

Heat Mass Transfer

123

deposition of salt on the hot surface. The lower the rate of

diffusion of salt from the region of high concentration to

the bulk liquid, the higher is the formation and stability of

the salt deposition.

According to the Cui et al. [12], the mass diffusion

coefficient of MgSO4 is much lower in water than that of

NaCl and therefore, a higher amount of salt is likely to

deposit in case of MgSO4 added atomized spray cooling.

As a result, the surface heat flux obtained for MgSO4 added

water atomized spray in transition boiling regime is higher

than that with NaCl. In addition to the above, significant

difference in critical surface heat flux has also been

observed. Among the three cooling process, the maximum

CHF has been found in case of air-atomized spray with the

salt (MgSO4), whereas air-atomized spray with pure water

produces minimum critical surface heat flux. Furthermore,

the critical surface heat flux in case of air-atomized spray

with salt (MgSO4) was observed much earlier than the

Fig. 25 Comparison of the average surface heat flux of two different

types of saltFig. 26 Comparison of the average cooling rate of two different

types of salt

Fig. 27 Photographs during air

atomized spray cooling at

different conditions

Heat Mass Transfer

123

other two types of air- atomized spray (with pure water and

with NaCl added water). Therefore, salt (MgSO4) added air

atomized spray cooling was found to be one of the efficient

cooling methods for fast quenching operation.

3.3.2 Average surface heat flux

Figure 25 shows the achieved average heat flux at a water

flow rate of 16. 67 9 10-5 m3/s for both NaCl and MgSO4

added air-atomized spray. In case of MgSO4 added air-

atomized spray, the average surface heat flux increases

with an increase in salt concentration, and shows the

maximum value of 2.4 MW/m2 at a salt concentration of

0.4 mol/l. But, in case of NaCl added air-atomized spray,

the maximum average surface heat flux (2 MW/m2) has

been observed at a salt concentration of 0.1 mol/l, and

thereafter it shows a decreasing trend with an increase in

salt concentration. The maximum observed average surface

heat flux in case of MgSO4 added air-atomized spray is

20 % higher than that with NaCl added water. Similarly,

the maximum average cooling rate in case of MgSO4 added

air atomized spray cooling is 35 % higher than that

obtained in case of NaCl (Fig. 26).

3.3.3 Visual comparison

Figure 27 shows the photographs taken after the different

types of air atomized spray (air atomized spray with the salt

and without salt) impinge the hot surface. The first pho-

tograph was taken after 0.5 s from the time of impinge-

ment, and the subsequent photographs were taken at a time

interval of 1 s. It was observed that in case of air atomized

spray with the MgSO4 and NaCl, at t = 0.5 s, a black dark

circle appears around the spray centre, and this indicates a

significantly colder region which is also an evidence of fast

cooling. However, in case of pure water, the dark circle

appears after 1.5 s from the starting of the spray. The

successive photographs also show that in case of MgSO4,

the dark circle moves at a faster rate than the other two

cases.

4 Conclusion

The experimental studies of UFC of a 6 mm thick hot static

AISI-304 steel plate by salt added air atomized spray has

been successfully conducted. The results depict that salt

added air atomized spray enhances the surface heat flux in

nucleate boiling regime, because of the suppression of the

vapor bubble coalescence inside an evaporating droplet and

also due to the occurrence of foaming. Moreover, the

surface heat flux in transition boiling regime is also

enhanced, and this could be due to the deposition of salt on

the hot surface during cooling. Therefore, the obtained

cooling rate (242 �C/s) in case of MgSO4 added air

atomized spray cooling and (178 �C/s) for the case of NaCl

were found to be in the UFC regime of a 6 mm thick AISI-

304 steel plate.

In case of NaCl added air atomized spray cooling, the

transient surface heat flux as well as the average surface

heat flux increases with an increase in salt concentration up

to 0.1 mol/l, and thereafter it shows a decreasing trend.

Therefore, the maximum average cooling rate of (178 �C/s)

has been observed at a salt concentration of 0.1 mol/l.

However, in case of MgSO4 added air atomized spray

cooling, the transient and average surface heat flux

increases with an increase in salt concentration and the

maximum cooling rate of 242 �C/s was observed at a salt

concentration of 0.4 mol/l.

In case of NaCl added air atomized spray, the critical

surface heat flux increases from 1.4 to 2.58 MW/m2 with

an increase in salt concentration from 0 to 0.1 mol/l, but in

case of MgSO4, the critical surface heat flux shows an

increasing trend with an increase in salt concentration up to

0.4 mol/l.

References

1. Lucas A, Simon P, Bourdon G, Herman JC, Riche P, Neutjens J,

Harlet P (2004) Metallurgical aspect of ultra fast cooling in front

of down-coiler. Steel Res Int 75:139–146

2. Bin H, Hua LX, Guo-dong W, Guang-fu S (2005) Development

of cooling process technique in hot strip mill. J Iron Steel Res Int

12:12–16

3. Oliveria MSA, Sousa ACM (2001) Neural network analysis of

experimental data for air/water spray cooling. J Mater Process

Technol 113:439–445

4. Mohapatra SS, Chakraborty S, Pal SK (2012) Experimental

studies on different cooling process to achieve ultra fast cooling

rate for hot steel plate. Exp Heat Transf 25:111–126. doi:10.1080/

08916152.2011.582567

5. Puschmann F, Specht E (2004) Transient measurement of heat

transfer in metal quenching with atomized sprays. Exp Thermal

Fluid Sci 28:607–615. doi:10.1016/j.expthermflusci.2003.09.004

6. Al-Ahmadi HM, Yao SC (2008) Spray cooling of high temper-

ature metals using high mass flux industrial nozzle. Exp Heat

Transf 21:38–54. doi:10.1080/08916150701647827

7. Bhattacharya P, Samanta AN, Chakraborty S (2009) Spray

evaporative cooling to achieve ultra fast cooling in run out table.

Int J Therm Sci 48:1741–1747. doi:10.1016/j.ijthermalsci.2009.

01.015

8. Qio YM, Chandra SM (1997) Experiments on adding a surfactant

to water drops boiling on a hot surface. Math Phys Eng Sci

453:673–689

9. Qio YM, Chandra SM (1998) Enhancement of spray cooling by

addition of surfactant water spray on a hot surface. J Heat Transf

304:63–71. doi:10.1115/1.2830070

10. Cui Q, Chandra S, McCahan S (2001) The effect of dissolving

gases or solids in water droplets boiling on a hot surface. J Heat

Transf 123:719–728. doi:10.1115/1.1376394

Heat Mass Transfer

123

11. King MD, Yang JC, Chien WS, Grosshandler WL (1997)

Evaporation of a small water droplet containing an additive. In:

Proceedings of the 32nd national heat transfer conference, vol 4,

pp 45–57

12. Cui Q, Chandra S, McCahan S (2003) The effect of dissolving

salts in water spray used for quenching a hot surface: part-1

boiling of single droplets. J Heat Transf 125:326–332. doi:10.

1115/1.1532010

13. Cui Q, Chandra S, McCahan S (2003) The effect of dissolving

salts in water spray used for quenching a hot surface: part-2 spray

cooling. J Heat Transf 125:333–338. doi:10.1115/1.1532011

14. Li D, Wells MA (2004) Effect of surface thermocouple instal-

lation on the discrepancy of the measured thermal history and

predicted surface heat flux during a quench operation. Metall

Mater Trans B 36:343–354

15. Trujillo DM, Busby HR (2003) INTEMP—inverse heat transfer

analysis—user’s manual. TRUCOMP CO. FOUNTAIN VAL-

LEY, Canada, pp 1–47

16. Busby HR, Trujillo DM (1985) Numerical solution to a two-

dimensional inverse heat conduction problem. Int J Numer Meth

Eng 21:349–359

17. Alam U, Krol J, Specht E, Schmidt J (2008) Enhancement and

local regulation of metal quenching using atomized sprays.

J ASTM Int 5:1–10. doi:10.1520/JAI101805

18. Ravikumar SV, Jha JM, Sarkar I, Mohapatra SS, Pal SK, Cha-

kraborty S (2013) Achievement of ultrafast cooling rate in a hot

steel plate by air-atomized spray with different surfactant addi-

tives. Exp Thermal Fluid Sci 50:79–89

19. Ravikumar SV, Jha JM, Mohapatra SS, Sinha A, Pal SK, Cha-

kraborty S (2013) Experimental study of the effect of spray

inclination on ultrafast cooling of a hot steel plate. Heat Mass

Transf 49:1509–1522

20. Ravikumar SV, Jha JM, Mohapatra SS, Pal SK, Chakraborty S

(2013) Influence of ultrafast cooling on microstructure and

mechanical properties of steel. Steel Res Int. doi:10.1002/srin.

201200346

21. Lamb H (1993) Hydrodynamics. Cambridge University Press,

United Kingdom

22. Huang C, Carey V (2007) The effect of dissolved salt on the

leidenfrost transition. Int J Heat Mass Transf 50:269–282. doi:10.

1016/j.ijheatmasstransfer.2006.06.031

Heat Mass Transfer

123