D0I:10.13374/i.issn1001-053x.1989.06.021 北京科技大学学报 第11卷第6期 Vol.11 No.6 1989年11月 Journal of University of Science and Technology Beijing No.1989 Investigation of Heat Transfer in the Spray Cooling of Continuous Casting' Cai Kaike（蔡开科)"，Yang Jichen(杨吉春) ABSTRACT:The heat transfer coefficient between spray water droplets and hot surface is measured in the laboratory.The effect of spray water flow rate, water pressure,spray distance of nozzle from the surface of strand,spray water temperature on the heat transfer is made a detail studied.And meanwhile,the effect of the strand surface FeO scale on the heat transfer is also investigated. According to the experimental results,the influence of above factors on the heat transfer coefficient has been discussed and a experience formula between the heat transfer coefficient and spray water flow rate is given out. KEY WORDS:continuous casting,spray cooling,heat transfer There are a lot of parameters which influence this heat transfer process.Most of the investigrtors indicated that the effect of water flux on heat transfer is the greatest and the relationship between heat transfer coefficient and water flux is nonlinear t1-51. The distribution of water flux is affected by the type of spray nozzles.water pressure,spray distances and so on.The heat transfer coefficient is also influe- nced by water temperature.Some measurements indicated that if water tempera- ture rises from 20C to 60C,the heat transfer coefficient will decrease about 14%1.The formation of FeO on the hot strand surface is very important for heat transfert7. This paper deals with the heat transfer between atomised droplets and high temperature surface.Experiments have been carried out with nozzles and an equipment to determine spray density and heat transfer coefficient. Manuscript Received April 12,1989 ··Dcpt.of Metallurgy ...Baotou Institut of Iron and Stecl 510
第 卷第 期 北 京 科 技 大 学 学 报 。 。 年 月 。 犷 蔡开 科 ’ ， 。 夕 杨 吉春 … 户 · ， ， ， 。 ， ， ， ， 尸 。 一 。 ， ， · · ， ‘ 。 。 · 二 ， ， 一 。 · 幻 DOI ：10．13374／j ．issn1001－053x．1989．06．021
1 Spray Characteristic of Nozzles 、 The ideal spray characteristics of nozzles are provided with suitable distribu- tion of water flux,suitable angle of impingement,suitable atomizing of water droplets and enough velocity of water droplets. The results of experiment measurement for spray characteristic of nozzles are shown as following. 1.1 Distribution of Water flux The distribution of water flux represents the spray shape on the strand sur- face and spray density on the impacting face. Fig.1 shows the measured results of full cone nozzle.From Fig.1 can see: (1)Peak flux density of nozzle(w)is in the center of spray zone.(2)For the constant water pressure(Pconst),increasing the spray distance tends to lower the peak flux and to decrease water flux.(3)For the constant distance(s= const),increasing the water pressure makes the central peak flux steeper and increases the central water flux. ,1m. 21 ●D,GMPa ,/m5 e0.4MPa ·120mm o0.2MPa ●160mm -含里120Tm 0200mm Pw0.4MPa 12096724824024487296120 12096724824024487296120 Spray radius,mm Fig.1 Spray profile of full cone nozzle 30 -s:120mm ●0.2MPa 00.4 MPa o0.6MPa 识 20 KouenbaJd 10 200 400 600 800 Droplet diameter,gm Fig,2 Spray state comparision 511
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It is evident that the water pressure and spray distance are important para- meters to influence on water flux distribution of nozzles. 1.2 Atomizing of Water Droplets The diameter of water droplets represents the atomization degree of spray water.As shown in Fig.2,the droplets size varied between 200-4004m. 1,3 Mater Droplets Velocity At present,to get accurate measurement of water droplet velocity is still difficult.Generally,water droplets velocity impacting against strand surface ean be estimated from the theoretical calculationt. 1=vep〔-0.33(g）&〕 where: vo is initial droplety velocity at the exit of nozzlesp is water density;p.is the air density:d is the water droplets size;s is the spray distance. The Fig.3 shows the relation between spray pressure and terminal droplets velocity v.Droplet velocity increases in proportion with spray water pressure. Thus droplets have enough kinetic energy to penetrate into the vapor film,leading to contact with the surface and even increase heat exchange capacity. 2 Heat Transfer Characteristic of Spray Cooling The heat removed from hot surface by pulverised water droplets can be defi- ned as follow: Q=h(T.-T) where:Q is the heat flux,T.is the surface temperature of cooling water. Based on the body without internal heat source during cooling and the boundary condition of convection,continuity of heat flux can be formulated as follows: -207=Qx=h(T,-T) dx When the surface temperaturc T.,the temperature of cooling water T.,the heat conductivity of steel and temperature gradient of body surface(0T/0x)x are all known,it is possible to calculate heat transfer coefficient h.Hence,both surface temperatures T.and the lemperature profile of the body must be first determined. The formula to calculate the surface temperature can be derived from the differential cquation of heat conduction by using the finite difference method. 512
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2.1 Effect of the Surface Temperature on Heat Transfer Fig.4 shows that in the same cooling condition,surface temperature change of the stainless steel and the carbon stce!are similar to the high temperature zone(>950C).The cooling rate respectively are 16.8C/s and 16.5C/s,which means that their heat conduction capacity is essentially same. 40- 120mm 0160mm oCarbon steel 0200mm 1200 ●Stainless steel E 30 1000 800 20 600 400 10 00 2 4 10 20 30 90 Water pressure,MPa Time,s Fig,3 Effect of spray pressure on Fig.4 Temperature profile for spray droplet speed cooling 350 90.1MP3,5160元 35 ·0.iNPe,5200mm 300 800℃ 25 2.0 900℃ 200 20 1.5 15 100 1.0 1000℃ 0.5 5 10 15 20 100 400' 600 800 1G00 Surface temperature,C Water flux,1/m2.s Fig,5 Effect of surface temperature Fig.6 Effect of water flux on heat on heat flux trasfer coefficient In thethe lower temperature zone (<950C),the cooling rate of stainless steel and carbon steel are 17.6C/s and 18.5C/s respectively,which means that the heat conduction capacity of carbon steel is better than that of stainless steel. The relationship between the surface heat flux and surface temperature is shown in Fig.5. For the surface tempcrature T:<300C,water droplets are wetting out the 513
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hot surface of body forming the transient heat transfer of half boiling and as the surface temperature increases,the surface heat flux increases too. For the surface temperature T.>300C,water droplets are non-wetting out hot surface.As the surface temperature increases,a steaming film is formed preventing water droplet from contact with hot surface,so that surface heat flux decreases gradually. 2.2 Effect of Water Flux on Heat Transfer As shown in Fig.6,for the constant surface temperature,heat transfer coeffi- cient increases with water flux increasing and the effect of water flux on the h in the low temperature zone is stronger than that in the high temperature zone. Based on the experiments,the relation between heat transfer coefficent and water flux can be found to have the following formules. 800°Ch=0.6w030 kW/m2.°C w=3-101/m2s 900°Ch=0.59w039 kW/m2.°C w=3-101/m2s 1000°Ch=0.42w005 kW/m2.°C w=3-101/m*.s The comparision with results of other investigators is shown in Fig.7. 2.3 Effect of Spray Water Temperature and Surface State on Heat Transfer The effect of spray water temperature on heat transfer is small.An incre- ase of spray water temperature will decrease slightly heat transfer coefficient. Fig.8 shows that surface temperature of carbon steel with Ar protection decreased faster than without any protection.It shows that the effect of FeO formed during heating on heat transfer is very great.For the heating carbon steel probe without Ar protection 3.15g Feo was formed for four experiments,the average value is 1.12kg/m2.For the heating carbon steel probe with Ar prote- ction 0.25g FeO was formed for four experiments,the average value is 0.1 kg/m2. 3厂1-M1zkr(1D 2-Ishigaro(7) 3-Sasaki(4) 4'5-Alberny(8) 6-Ours o Without Ar 1200 P 1000 -●Ar protection, 王 800 600 400 200 20 40 60 10 20 30 40 Time,s Water flux,1/m2..s Fig.7 Comparision of experlmental results Fig,8 Cooling comparision 514
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3 Conclusions The secondary cooling of continuous casting is a complicated heat transfer process. The spray characteristic is a basic parameters in order to evaluate and select suitable nozzle. The heat transfer coefficient between the water droplets and the high temperature surface decreases gradually with increasing surface temperature.It depends mostly upon the spray flux received by the high temperature.Increasc P of spray flux leads to an increase of heat transfer coefficient.Experimental formula obtained between h and w can be applied as reference. An increase of spray water temperature decrease slightly heat transfer coeffi- cient. The heat transfer coefficient with Feo on the hot surface is about 15% lower than that without it. REFERENCES 1 Brimacombe J K,et al.Steelmaking Proc.,1980;63:235 2 Lambert N,et al.J Iron and Steel Inst.,1970;208:917 3 Mizikar E A.Iron and Sfeel Engineer.,1970;47:53 4 Sasaki et al.Tetsu-to-Hagane.1979;65:90 5 Muller H,ct al.Arch Eisenhuttenwes,1973;44:589 6 Wolf M,et al.On Control of Residual Elements in Electric Fur- nace.Personal Communication,1988 7 Ishigaro M,et al.Tetsu-to-Hagane 1974;60:464 8 Alberny R,et al.Revue de Metallurgie,1978;75:353 9 Araki K,et al,Trans Iron and Steel Inst.Japan,1980;20:462 515
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