An experimental study on heat transfer process of multiple mist impinging jets

Baosteel Technical ResearchVolume 5 ,Number 4 ,December 2011 ,Page 49An experimental study on heat transfer process of multiple mist impinging jetsLU Huafei)，XIANG Shunhua?) and MA Xinjian2)1) Auto Steel Division, Research Institute, Baoshan Iron & Steel Co, Ltd, Shanghai 201900, China2) Equipment Division, Research Institute , Baoshan Iron & Steel Co., Ltd, Shanghai 201900, ChinaAbstract: Mist jet impingement cooling is an enhanced heat transfer method widely used after the continuous galvanizingprocess. The key of a successful design and operation of the mist jet impingement cooling system lies in mastering heattransfer coefficients. The heat transfer coefficients of high temperature steel plates cooled with multiple mist impinging jetswere experimentally investigated ，and the effects of gas and water flow rates on heat transfer coefficients were studied. Thetest results ilustrate that the gas flow rate has lttle effect on the mist heat transfer rate. It is also found that the water flowrate has a great impact on the heat transfer coefficient. When the water flow rate ranges from 0. 96m'/h to 1.59 m'/h,anincrease in the rate will produce a higher heat transfer coefficient with a maximum of 5650 W/(m2●K). Compared withthe conventional gas jet cooling, the heat transfer coefficient of the mist jet cooling will be much higher , which caneffectively strengthen the after pot cooling.Key words :multiple mist jet ;jet impingement heat transfer ; after-pot coolingdoi :10.3969/j. issn. 1674 - 3458.2011.04.009required in the industrial application, multiple mistimpinging jets are usually used. Therefore, it is1 Introductionnecessary to investigate the effects of nozzleIn the continuous hot-dip galvanizing line, airconfiguration , such as nozzle pitch ,jet interacting，etc.impinging jets are usually used to achieve the coolingon the heat transfer process. At present, only a fewof galvanized steel strip'12]. In the last few decades , inlaboratory investigations concerning multiple mist jetorder to produce high strength or thick galvannealedheat transfer are available. Shimada M, et al.sheets ，the mist jets impingement cooling technique hasexamined the impingement heat transfer of multipleattracted more and more attention'. This coolingmist jets using a silver specimen. It was found that in ahigh temperature stage, the average heat transfertechnique features a high cooling rate ,a short coolingcoefficient of multiple mist jets is the sum of the heattime, a controllable target temperature and automatictransfer coefficient of air jet and that of water jet. Anddrying of steel sheets after cooling.The key of a successful design and operation of thethe heat transfer of water jet is proportional to thevelocity of water droplets on the impingement surface.mist jet impingement cooling apparatus lies inMitsutsuka M,et al.' " conducted such an experimentmastering mist nozzles’atomization characteristics andwith a steel plate specimen. The test results show thatheat transfer performance. In the past , the atomizationheat transfer coefficient h depends only on the watercharacteristics of the mist nozzle have been extensivelyflux density W and the heat transfer surface temperaturestudied4-. The atomization characteristics, such asT,. When T, is greater than 150C,h is proportional todroplet size and the spray angle ,have provides a usefulwater flux density W with an exponent of 0. 726. Inguidance to the selection of the nozzle. Compared toaddition, air flow only works on the atomizationthe investigations of atomization characteristics, theprocess and its effects on the heat transfer process canstudies on mist jet impingement heat transfer processbe neglected.are still lacking. Liu Z H,et al. (8) employed the high-Obviously, ,there is lttle agreement on the influencepressure compressed air to atomize the pure water andof air velocity on heat transfer. To clarify theseexamined the effects of spray condition on the criticalphenomena , some experiments with a wider range ofheat flux. A simple quantitative expressionwasair velocity should be conducted. Furthermore , mostdeduced to determine the quantitative relationshipmist heat transfer coefficients reported valid only forbetween critical heat flux and spray condition. Su L M,the corresponding test conditions and no empiricalet al. 9 tested a single air-nitrogen mist flow generatedformula for the cooling rate is available because of theby liquid nitrogen atomization and inspected the effectscomplexity of mist jet impingement heat transfer.of Reynolds number , gas/liquid ratio and jet height onTherefore,it is required to make a snecial experimentthe heat transfer coefficient. The above experimentsto study mi中国煤化工with a newlyaimed to study heat transfer of a single mist jet.developed crYHCNMHGHowever, as a large volume of cooling medium isIn the prescn wun,d uaisicii toulg method wasCorresponding author:LIU Huafcei; E-mail:liubuafei@ basteel. com50Baosteel Technical Research, Vol. 5, No.4, Dec. 2011used to study the heat transfer characterstics whenpure water were employed as working medium.high-temperature steel plates were cooled with multipleFig. 1 shows the schematic diagram of the experi-mist impingement jets. The cooling and boiling curves，mental apparatus, which consists of gas and waterwith experimental conditions including gas flow rate ，supplies ,heating furmace , mist atomization chamber andwater flow rate and surface superheat , were measured.measurement system.Ihe results of mist heattransfer coefficients wouldHigh velocity air needed to atomize the water jet wasprovide an instructive guidance for the design of thesupplied from a double-stage turbine centrifugal blowermist jet after-pot cooling apparatus.(flow rate :5000 m*/h, static pressure :9050 Pa) to thenozzles through a regulator, an orifice flowmeter, a2 Experiment apparatus and procedureplenum chamber and a test section with array of jenozzles. The pure water were supplied to the nozzles2.1 Apparatusvia a centrifugal pump ( lift :20 m; flow rate :20 m'/h;In heat transfer experiments，newly-developed cross-type : IS80-65-125) through a regulator , a rotameter andtype nozzles were used to produce mist jets. Air anda water collecting pipe.20UI413中中NI81. Air fan 2. Orifice plate 3. Merury thermometer 4. Pessre gage 5. Water cllcting pipe 6. Plenum 7. U-tube manometer 8. Water tank 9. Water pump10. Rotameter 11. Rail 12. Fog cllctor 13. Test plate 14. Furnace lid 15. Resistant electronic elements 16. Eletric fumace 17. Lifer and transport arrier18. Thermocouple wires 19. Data acquisition 20. RegulatorFig. 1 Schematic diagram of experimental apparatusThe structure of the air-atomized nozzle is shown inA detailed drawing of the heat transfer plate isFig.2. It is of air/water cos-flow type. It has theshown in Fig.3. The test plate was made of SUS 304following characteristics: the impingement dirction ofstainless steel with a length of 630 mm,a width ofair is normnal to that of water, and air jet and water jet630 mm and a thickness of 25 mm. To measure thcollides outside the nozzle. When the nozzle works , thetemperature data of the test plate during the mistair jet ejecting from the gas hole is vertical to steel strip,cooling , a series of holes were drilled in it to mount theand the air jet crashes the water jet into water droplets atthermocouples. The measurement locations are shownthe exit of the nozzle. Its detailed spray characteristicsin Fig. 3. The thermocouples were installed in thehave been reported in Reference[5]. There are 23 cross-following locations: 2 mm below from the upperflow type nozzles with a staggered arrangement. Thesurface , half of the thickness from the upper surfacepitch of adjacent nozzles is 120 mm and the total widthand the inner surface. The wire diameter of theof the chamber is 600 mm . When pure water entered thethermocouples is 0.6 mm to ensure quick response tocollecting pipe close to the air chamber , 23 water jetstemperature variation. The hot junctions of thewill be atomized by the respective air jets and the 23thermocouples were securely spot-welded to the steelmist jets will be generated.plate. Bach measurement hole was flld with refractory120 mmx4seals to prevent cooling water from entering the gaps.The steel plate was heated by a resistance furmacewith a power of 35 W. The heating space in the furmaceis 650 mm long,650 mm wide and 30 mm high and.......suitable for heating of the sample plate. Steel plates can❼❼199be heated up中国煤化工20 - 30 min.η↑198The air ter1HCNMHGby a mercury19999thermometerunimum scale:0.1C) on the air pipe and the air pressure by aFig.2 Configuration of cross-1low type nozzlepressure gage ( Range:0 - 9 000 Pa; Accuracy:1% ).52Basteel Technical Research, Vol.5, No.4, Dec. 2011velocity in Shimada's test is 196 m/s to 319 m/s,flow rate.which is much higher than that of the presentexperiment, ranging from 55 m/s to 75 m/s.6.0E+03 rExperimental conditions in Mitsutsuka ’s test are0-0.96 mS/hsimilar to this test. Therefore, it can be deduced thatmist jets at a higher velocity help damage the unstablevapor cushion covering the hot surface and lead to heattransfer enhancement. But if the gas jet velocity is low4.0E+03enough , most of the kinetic energy will be consumed inthe atomization process and has litle effect on heatdftransfer.2.0E+031.4E+0628路469 mm1.2E+080881.0E+06g。100200 3000050号8.0E+05Tsp/K示。Fig.6 Effect of the water flow rate on heat fux6.0E+05Fig. 7 ilustrates the variation trend of the critical4.0E+05heat flux (CHF) against the water flow rate when therate is 2700 m'/h. It is shown that the increase of the2.0E+05water flow rate gives a rise of CHF. Similar to that ofthe heat transfer coefficient , the range of the effective00 200 300 400 500influence by the water flow rate is 0. 96 m'/h to1.59 m'/h.Fig.5 Efect of superheat on heat fnux at different air flowrates1.8E+06Fig.6 ilustrates the effect of water flow on heatflux. Three boiling curves are presented in the figure1.6E+06when the air flow rate is 2700 m'/h. It can be seenofrom Fig. 6, within a superheat range of 450 -550 K,an air film forms quickly on the surface due to its high1.2E+06temperature to prevent water droplet from impinging它1.0E+06onto the surface in the initial cooling. Due to the lackof water droplet impingement in such a film boiling害8.0E+05 .regime ,the water flow rate has a slight effct on heatflux and heat transfer coefficient is as low as about1000 W/(m2. K). When the superheat is graduallyreduced from about 400 K to 150 K ,a transition boilingregime develops and the heat coefficient increases from2.0E+O51.0 1.52.0 2.s5the minimum ( Leidenforst point) up to the maximumQw/ (m2:h)of5650 W/(m2●K). With a further reduction ofsuperheat to 50 - 140 K, water droplets can contactFig. 7 Effect of the water flow rate on critical heat fuxdirectly with the hot surface and lead to the nucleate(CHF)boiling regime. The heat transfer coefficient in theUsing the cros-flow type nozzle, it is possible tonucleate boiling regime decreases slightly and remainsswitch from mist cooling to gas jet cooling by cuttinga high value when the superheat is reduced. As shownoff the pure water. Fig. 8 ilustrates the comparisonin Fig. 6,the water flow rate is the goveming parameterbetween the heat transfer coefficient of gas jets whenin this regime. When the water flow rate is raised fromthe air flow is 2700 m'/h and that of mist jets when0.96 m'/h to 1. 59 m'/h, the heat transfer coefficientthe water flow is 2.33 m/h. As is shown in Fig. 8 ,thein the nucleate regime will increase by about 1. 2heat transfer中国煤化工ooling generallytimes. A further increase in water flow from 1. 59 m'/hdoes notFerature of steelto 2.23 m'/h only causes a slight increase of the heatplates ,rangit:TYHCN MH Gto 300 W/(m2transfer coefficient. It indicates that the optimum design●K). However , the heat transfer coefficient of the mistof the pump for atomization can be achieved if its flowjet cooling strongly depends on the surfacerate is in the range of effective influence by the watertemperature ,ranging from 800 W/(m2●K) to 5 650