Jul 7, 1999 - pressure jet atomizers at higher mass flux G (water flow rate per unit area) up to 50 kg m-2 s-1, and higher water pressure typically between 0.3 ...
ILASS-Europe'99
Toulouse 5-7 July 1999
Transient High Pressure Water Spray Cooling of a Rotating Steel Plate at High Temperature R Sharief, GG Nasr University College of Manchester (Stockport College), Department of Engineering Technologies, Stockport SK1 3EQ, England JR Jeong, DD James, IR Widger, AJ Yule Department of Mechanical Engineering, UMIST, PO Box 88 Manchester M60 1QD, England ABSTRACT Transient high-pressure water spray cooling of a high temperature surface is common practice in several industrial applications. In the production of steel, copper, aluminium and other similar alloys consistent mechanical and metallurgical properties are essential requirements and these are normally achieved by accurate surface temperature control during the process operation. A new transient cooling methodology has been developed to investigate spray-cooling characteristics of steel relevant to the inter-stand cooling process in a hot rolling mill. The transient cooling apparatus consists of a 250 mm diameter steel disk which is rotating up to 1800 rpm and heated by propane-oxygen flames. The thermocouple signals are measured via a commutator, with slip rings, attached to the shaft. A commissioning test was successfully performed under conditions with and without spray.
NOTATION A m q& w = q& total − q& nospray
Evaporative heat flux (i.e. that due to the spray)
q& total q& nospray
Total heat flux during spraying Heat flux from disk with no spray
T ∆T
Average disk temperature at any time Temperature drop of disk at time t, compared with disk temperature without spray
Area of impacting spray on the test piece Mass of the test piece corresponded to the total steel annulus ( ρV )
INTRODUCTION In a global economy that is becoming steadily more competitive, continuous improvements to the cost, quality and grade of the steels produced in modern steel-making processes are indispensable to meet changing market demands and customer needs. In these processes, water sprays are used during continuous casting, for hot and cold rolling of steel sheets. In the casting process, liquid steel must be cast into shapes so that it can be rolled. This is done by continuous casting machines that mould the liquid steel into different sized shapes called slabs, blooms and billets. After the shell leaves the mould, it moves between successive rolls in the spray zones. Here water is sprayed along the machine to help cool and solidify the steel. Rolling mills are part of the production process from primary iron and steel. The products are strips and sheets. The hot slabs are prepared for rolling by heating in re-heating furnaces to rolling temperature (about 1503 K). During hot rolling, water cooling of the strip is undertaken both between the stands of the finishing mill and on the run-out table. Inter-stand cooling has an influence on a number of parameters such as the process speed and scale formation during rolling. Accurate temperature control requires a cooling system that combines the required cooling rate with the heat transfer capabilities of the water spray system, production speed and line length. Rapid and uniform cooling rates are needed to ensure that each stage of manufacture achieves the correct uniform microstructure and mechanical properties. The application of controlled spray cooling with uniform spraying patterns is crucial for maintaining and enhancing product quality. The specific water-cooling methods currently in use in steel-making industries for the above applications are: (a) relatively low pressure dilute sprays, positioned between mill rollers, to spray on both slabs and rollers in continuous casting and (b) laminar water cooling curtains, with separate water sprays for fume suppression, positioned between rolling stands in the hot strip mill: so-called inter-stand cooling. Both methods can provide a non-ideal heat transfer process (e.g. inconsistent material properties, overheating rollers and excessive water consumption). Experience has shown that in many industrial applications of spray cooling
1
there is a need for a higher rate of heat transfer from heated surfaces. This has usually been addressed by using pressure jet atomizers at higher mass flux G (water flow rate per unit area) up to 50 kg m-2 s-1, and higher water pressure typically between 0.3 MPa and 12 MPa [1, 2]. However, quantitative information regarding the spray parameters and their effects on cooling is not readily available at these high pressure and mass fluxes. There is a lack of information on the fundamental mechanisms of heat transfer to individual droplets or an array of droplets impacting under different surface conditions (material properties, surface roughness, surface temperature, etc). Most of the experimental data available are for dilute sprays, rather than dense sprays, using low-pressure water sprays or two-fluid atomization which have more restricted industrial applications [3]. There is a growing concern in industry with obtaining systematic information on the effects of the spray parameters at high water pressure on the heat transfer characteristics of hot surfaces particularly in the film boiling regime. It is worth noting that water atomization involving a high velocity dense spray impacting on hot surfaces has a number of other applications, including in diesel engines and in metal powder production [4]. The prime objective of this study is to develop a transient cooling test apparatus which allows the heat transfer method of using high pressure (up to 12 MPa) water sprays during the cooling processes of hot rolling steel sheets to be evaluated under various operating conditions, particularly for the case of a moving steel surface.
EXPERIMENTAL APPARATUS AND PROCEDURE Overview The transient cooling apparatus was designed by the authors and built at UMIST and was based upon the concept of a rotating disk described in reference [5]. In the current cooling inter-stand cooling process of the iron and steel making industry, the strip is cool down rapidly by continuous laminar streams of water (a water curtain). The maximum strip steed and temperature reaches up to 18 m s-1 and 1123 K at the inter-standing cooling process, respectively. However from the point of view of this project the objective has been to design and carry out critical commissioning of the apparatus, using the experience gained from steady state tests [2]. This test apparatus should meet the specifications of; (a) achieving high and uniform initial surface temperatures (1300 K), (b) obtaining the required strip speeds (up to 18 m s-1) and spray angles (up to 45o), (c) simultaneously spraying bottom and top and (d) accurately measuring the temperature drop as the surface passes through the spray. Some of the apparatus used in the steady state cooling test, such as the pump and atomizers, were also used for the transient test apparatus.
Test Apparatus The transient cooling test apparatus consists of a main frame constructed of tubular steel box section, a heating system, a spray system, a rotation disk and a data acquisition system. Fig 1 shows a schematic diagram of the transient cooling apparatus. The main frame was designed to satisfy several conditions including (a) the requirement to support the shaft with the rotating disk, slip ring, motor and burners, (b) the strength to endure the impact force of a spray, and (c) the possibility of being tilted to study the effects of heat transfer characteristics for different spray angles (e.g. 15o, 30o and 45o) relative to the hot surface. The spray angles are related directly to the conditions of the inter-stand cooling process where the angle of the steel strip in the hot rolling mill is changed between 18-45o with respect to the horizontal by a "looper", which is installed between stands in order to keep proper strip tension. The angles of both the atomizers and also the disk could be varied independently. Fig 2 shows the instrumentated mild steel test segment of the rotating disk. The test segment was tapered 10o and screwed into place so that it would not come loose due to centrifugal force when the disk was rotated. The test segment consisted of two pieces for convenience when welding thermocouples to the bottom of their holes by spot welding. There were nine holes on three planes to measure temperature differences between the thermocouples in order to derive heat fluxes in the metal. The centres of each set of holes were offset at around 45o with respect to the vertical to minimise temperature distortion caused by having all three holes in a vertical line. Thin bare K-type thermocouple wires (0.067 mm diameter) and spot welding method were selected for this work because of its suitable temperature range, response time, cheapness and convenience of use. Before welding, both conductors were twisted together to form about two turns. The junction was welded, by using a spot welder, directly on to the steel of the disk at the bottom of the hole prepared for the thermocouple. Direct contact between the junction and the metal minimises the time lag. To prevent the thermocouple wires from touching any other part of the rotating disk, ceramic double bore insulators of 2.2 mm outer diameter were used. The heated disk (or annulus) with its test segment is attached to the shaft by a supporting flange and rotated by a
2
M
water
M
atomizer
slip ring pump valve
data acquisition system
surge cylinder
gas (C3H8)
flow meter
oxygen
meter pressure gauge
burner rotating disk burner
by pass
canopy
frame
atomizer
drainage
Fig 1. Schematic diagram of transient test apparatus motor of 3000 rpm maximum speed. The motor is controlled up to 1800 rpm by a controller and a tachometer was used to measure the speed of the rotating disk. To protect the supporting flange of the disk and minimise heat transfer to the shaft, insulating materials (10 mm thickness "Kaowool Board" and "Kaowool 1260 Grade Paper" by Thermal Ceramics Ltd.) are placed on both sides of the supporting flange.
97 90
R 90 R 125 10
R 102
5°
φ 2.3
6
9
8
Six propane-oxygen "surface mix surface tube" (SMST) burners specially manufactured by "Nordsea Gas Technology Ltd." are used to heat the rotating disk up to 1300 K. Three burners are mounted on top of the disk and three underneath it. The intensity of the heat can be controlled by adjusting the propane and oxygen flows and by moving the burners nearer to or further from the disk.
10 2.05 7 6
5
4
2 3
1
12
unit : mm
Fig 2. Detail of test segment
Water is supplied by the high-pressure pump (manufactured by "Speck Triplex") via a header tank to either one or two full cone spray atomizers, depending upon whether both or just one surface of the disk is sprayed. Two protective canopies are mounted to protect the motor, slip ring and shaft from residual spray. A full cone atomizer manufactured by "Spraying System Co.", TG 0.5 (0.61 mm diameter), was used in this test. Here "TG" and "0.5" are manufacturer’s codes which relate to tip type and capacity size, respectively. A rectangular crosssection sprays were impacted on the disk by using the orifice in the metal canopy with dimensions 35 mm, in the direction of the disk radius, and 10 mm width. Details of the performance of the atomizer are given in [6] and [7]. To measure the rapid temperature drop in the transient cooling test under rotating conditions, a slip ring commutator manufactured by "I.D.M Electronics Ltd." and a data acquisition system were used. The data acquisition system consisted of a PC and an ADC and multiplexer fed by eight preamplifiers (manufactured by Bell and Howell). The maximum sampling data rate per channel was 16 kHz. The output of the thermocouples
3
was amplified up to ± 10 V and converted to temperatures by a calibration data file in the computer. This file was created with the aid of a "Temperature Reference Unit" manufactured by CP Instruments.
Test Procedure The commissioning tests were performed with a vertical spray impinging on to the rotating test piece from the top and the temperature time histories were recorded by the data acquisition system. The water sprayed downwards on the rotating disk at a distance of 240 mm from the atomizer exit and the water supply pressures were 0.69 and 1.38 MPa. Rather than analysing transient heat fluxes, as the test segment moved under the spray, the heat transfer was calculated from the decay of the averaged disk temperature obtained by averaging all the thermocouples at a given time. Relatively low temperatures and rotational speeds were used in the initial tests carried out in this work. The burners were switched on and the test piece was heated up without the water spray while the disk was rotated at 100 rpm. When the target temperature (up to 580 K) was achieved, the motor was set up for the target speed (600 rpm), then the spray was started as soon as the burners were switched off. However, the spray was allowed to impact on the test segment, only after achieving a stable spraying condition: A metal plate was used to cover the rectangular slot until the spray reached a steady state and then it was then rapidly removed. The time histories of temperatures were recorded at the sampling rate of 2048 samples per channel per minute.
RESULTS AND DISCUSSION The commissioning tests were carried out without spray and with sprays for two conditions at the rotating speed of 600 rpm. The spray conditions at the surface were, for 0.69 MPa, mass flux = 0.23 kg m-2 s-1, volume median drop diameter = 55.25 µm and mean drop velocity = 7.05 m s-1, and for 1.38 MPa, mass flux = 0.45 kg m-2 s-1, volume median drop diameter = 47.74 µm and mean drop velocity = 12.09 m s-1 [6, 7]. Temperatures fluctuated mainly because of electrical noise from the amplifiers of the data acquisition system. The averaged temperatures of the thermocouples were plotted as shown in Fig 3 and, as expected in the test with higher water supply pressure, the temperature decreased more rapidly than in those without spray and with the lower water supply pressure. Also, as the cooling time increased the temperature decreased more rapidly. 600
500
T (K)
400 Rotating speed: 600 600rpm rpm Without Without spray spray 240 mm, 0.69 1A240 (TG0.5, 0.69 (TG 0.5MPa 0.69MPa) MPa) 240 mm, 1.38 2A240 (TG0.5, 1.38 (TG 0.5MPa 1.38MPa) MPa)
300
200
100
0 0
5
10
15
20 25 Coolingtime time (s) Cooling
30
35
40
45
Fig 3. Temperature-time histories The "evaporative heat flux" (i.e. that due to the spray), q& w = (q&total − q& nospray ) , was evaluated with the assumption that the cooling rate with no spray was due to conduction from the disk to its support, radiation and convection to the air, and that these quantities were similar when spraying. The "evaporative heat flux" was thus calculated using:
4
q& w =
mc s ∆T
dt
(1)
A
where m (kg) is mass of the test piece which should corresponded to the total steel annulus, ∆T (K) is assumed to be the temperature difference with and without the spray at the same cooling time t and A (m2) is the area of impacting spray on the test piece (estimated as 349.3 mm2). Fig 4 shows the heat flux calculated in this way and it is seen to increase with cooling time. This can be explained by the fact that the temperature range belonged to transitional boiling regime so that the rate of heat transfer increases with reducing surface temperature. This is 1A240 (TG0.5, 0.69 240 240mm, mm, 0.69 0.69MPa MPaMPa) 2A240 (TG0.5, 1.38 240 1.38 240mm, mm, 1.38MPa MPaMPa)
1.2E+07
-2
Evaporative heat flux, qww(W (Wmm -2) )
1.4E+07
1.0E+07
•
8.0E+06 6.0E+06 4.0E+06 2.0E+06 0.0E+00 0
5
10
15
20 25 30 35 40 Cooling time time (s) (s) Cooling Fig 4. Variation of evaporative heat flux as a function of cooling time for two spray conditions
45
shown more clearly in Fig 5 which is the heat flux versus average disk temperature, derived after smoothing Fig 4 and obtaining simultaneous values of q& w and T from Figs 3 and 4. It is seen in Fig 5 that, at the higher water supply pressure, the heat flux is approximately 20% higher than for the lower pressure case. It is up to an order of magnitude higher than the evaporative heat flux measured in steady state tests of the authors with no surface movement (The highest heat flux from the steady state tests reached up to 1.38 106 W m-2 at surface temperature
240 mm, 0.69 MPa 240 mm, 1.38 MPa Steady state (240 mm, 2.07 MPa, orifice diameter 1.19 mm)
1.2E+07 1.0E+07
•
Evaporative heat flux, q w (W m-2-2)) (Wm
1.4E+07
8.0E+06 6.0E+06 4.0E+06 2.0E+06 0.0E+00 300
350
400
450 500 Average temperature (K)
Fig 5. Heat flux versus disk temperature for two spraying conditions
5
550
600
around 462 K with the mass flux of 3.32 kg m-2 s-1). Such high heat fluxes were unexpected. It is possible that because of the relatively dry moving surface that enters the spray, super-cooling could occur. In addition it is possible that significant heat transfer occurs to the water outside the spray impaction area, i.e. the value of A in equation (1) is underestimated. Fig 6 sketches this situation and indicates that significant heat transfer to the water may occur outside the main impaction zone, particularly at the steel surface after it has passed under the spray. It was intended when the rig was originally designed, to use instantaneous temperature differences between the thermocouples in order to derive time dependent heat fluxes in the metal and this will be explored in future work.
principal heat flux canopy rectangular slot
velocity
heat flux
spray
steel footprint over A Fig 6. Cross-section of impaction zone
CONCLUSIONS An experimental method for measuring spray cooling of moving hot steel surfaces, relevant to the cooling processes of hot rolling of steel sheets, has been developed which uses a preheated rotating disk. A commissioning test has been performed and analysed at low temperature and rotational speed and the result indicated that the method for exploration of transient cooling was successful. The test apparatus is now being extended to higher temperatures and speeds.
REFERENCES 1. Nasr G.G., Yule A.J. and James D.D., The fluid dynamics and heat transfer mechanisms of high pressure water sprays impinging upon a heated surface: a review, Proceeding 12th Conference ILASS-Europe, University Lund, pp. 77-84, 1996 2. Nasr G.G., Sharief R., Yule A.J., James D.D., Widger I.R. and Jeong J.R., Steady state high pressure spray cooling of high temperature steel surfaces, 14th ILASS Europe '98, Manchester, England, pp. 451-456, 1998 3. Choi K.J. and Kang B.S., Parametric studies of droplet wall direct heat transfer in spray cooling process, ASME FED-Vol. 178/HDT-Vol. 270, Fluid Mechanics and Heat Transfer in Sprays, pp. 161-165, 1993 4. Yule A.J. and Dunkley J.J., Atomisation of melts, Oxford University Press, 1994 5. Blackwell J, The fluid dynamics and heat transfer mechanisms of high pressure water sprays impinging upon a hot surface: transient cooling test rig design, Final year project 1996/97, Faculty of Engineering and the Built Environment, Stockport College, 1997 6. Nasr G.G., Sharief R., James D.D., Jeong J.R., Widger I.R. and Yule A.J., Studies of high pressure water sprays from full-cone atomizers, Proc. ILASS-Europe'99, Toulouse, 1999 7. Yule A.J., Jeong J.R., James D.D., Nasr G.G. and Sharief R., The performance characteristics of solid cone spray pressure swirl atomizers, Report TFD 04/99, UMIST Dept. Mech. Eng., Thermofluid Division Report, submitted to Atomization and Spray, February 1999
6
