Characteristics of CNG Bubbles in Diesel Flow under the Influence of the Magnetic Field

Transcription

1 Joural of Applied Fluid Mechaics, Vol. 10, No., pp , Available olie at ISSN , EISSN DOI: /acadpub.jafm Characteristics of CNG Bubbles i Diesel Flow uder the Ifluece of the Magetic Field H. A. Abdul Wahhab 1, A. R. A. Aziz 1, H. H. Al-Kayiem 2 ad M. S. Nasif 2 1 Cetre for Automotive Research ad Electric Mobility, Mechaical Egieerig Departmet, Uiversiti Tekologi PETRONAS, Seri Iskadar, Perak, 3210, Malaysia 2 Mechaical Egieerig Departmet, Uiversiti Tekologi PETRONAS, Srei Iskadar, Perak, 3210, Malaysia Correspodig Author Rashid@utp.edu.my (Received September 3, 201; accepted September 7, 2017) ABSTRACT This paper coducts a aalytic study of the hydrodyamics of a small CNG bubbles i lamiar horizotal Diesel flow uder the ifluece of the magetic field. Ivestigatio based o experimets was carried out to idetify the effects caused by varyig the magetic field itesity o the trajectory, the formatio of bubbles ad their shape ad velocity. Differet images at differet positios were captured through a high speed camera, image processig techique ad dowstream from the CNG bubbles ijectio poit delivered iformatio o bubble velocity, bubbles size, spatial locatio ad gas area fractio as a fuctio of chagig magetic field itesity. The outcomes cofirmed that CNG bubbles uder magetic field grow up vertically to have a bigger elliptical shape i the Diesel phase with the twofold of diameter. Also, it has bee oted that the CNG bubbles velocity decreased as the magetic field stregtheed. Keywords: Fuel techology; Liquid-gas fuel mixer; Magetic field; Two phase hydro-magetic flow. NOMENCLATURE Ab Af B db Di bubble area area of image frame magetic field itesity, Gauss bubble diameter pipe diameter Gy Gz Ub α the y coordiate of the th shape with area A the z coordiate of the th shape with area A total umber of bubbles average bubble velocity gas area fractio 1. INTRODUCTION Carryig out the aalysis about the flow dyamics of liquid ad gas uder the ifluece of both magetic alog with electric fields has tured ito a sigificat factor i the cotrol as well as desig of several electric-apparatus systems such as magetohydrodyamic problems (MHD), gas ad oil facilities, processig uclear reactors ad MHD power statios Ki (2010). The latest pla of precombiig of CNG with Diesel was proposed i order to raise the amout of air i the cylider to a greater extet, i the coditio where a combiatio of air ad CNG is added iside the egie, it ca form a heavy mixture Abdul Wahhab et al. (2015). This mixture eeds the ivestigatio regardig the behavior of gas bubbles formed i the liquid fuels ad the methods to maage their size prior to their arrival iside the ijectio system uder the effect of exteral forces like magetic field, i which the patter of horizotal flow of bubbles is characterized by the presece of bubbles dispersed i a costat liquid phase, havig a very small size tha the diameter of the cotaiig pipe. As compared to the vertical flow of the bubbles, the abovemetioed flow directio has maaged to get ot as much of attetio. Because of buoyacy, a additioal difficulty arises i this case whe the dispersed bubbles move to the top of the pipe ad results i extremely o-symmetric volume fractio dispersio i the pipe cross-sectio Ekambara et al. (2008). Sussma ad Smereka (1997) discussed that a example of complex flow simulatio is the flow of two fluids with high desity ad viscosity ratios, such as bubbly alog with droplets flow. They stated that the mai cocer is the behavior of gas bubble ad its deformatio i a viscous liquid. The mechaism of the movemet of the bubbles ad the

2 actio of coalescece ad bubble breakup is due to high desity ad viscosity ratios as well as topology chages. Hece, the ispectio of the shape of the bubble domai i sporadic flows is very importat to ehace the expectacy of the flow structure, flow patter trasitio, as well as defiitio of a wide rage of fluid properties. Bubble s shape alog with velocities preset i viscous liquids was deduced by Bhaga ad Weber (1981) through coductig a experimet, whereas Bruer et al. (1980) ad Bruer et al. (1983) suggested that the flow patter may be cotrolled by electrohydrodyamic forces ad several flow patter trasitio mechaisms were preseted. So far, Ishimoto et al. (1995) carried out a experimet to study the effect o the bubbly flow because of ouiform magetic field, ad made a compariso of the outcomes of the umerical as well as experimetal outcomes carried out by Hat ad Buckmaster (197), i which a outstadig acceptace was accomplished. O the other had, may measurig techiques have bee used to aalyze the bubbly flow properties such as, ray absorptio method, quick closig valve method, ultrasoud method, electrical method, image processig techiques ad magetic resoace method Lugga (1999), Scheer (199), Fossa (1998), Dih ad Choi (1999) techiques ca be divided ito itrusive ad o-itrusive techiques accordig to whether flow fields are disturbed or ot by the measurig equipmet. Jarrahi et al. (2011) have used PIV techique to measure the velocity fields i the secodary flow ad aalyzed the variatio i axial vorticity ad trasverse strai for differet coditios that cotributed to the mixig evaluatio uder differet flow coditios. Table 1 Properties of Diesel fuel ad CNG Parameter Diesel Fuel CNG Desity (kg/m 3 )(20 o C) Dyamic Viscosity (Pa.s) (20 o C) 2.49* *10-12 LCV (Mj/kg) Octae Number Carbo (%, w/w) Hydroge (%, w/w) Oxyge (%, w/w) Sulphur (%, w/w) 0.25 ppm < 5 Electrical coductivity (ps/m) Relative permittivity Several studies have bee carried out as show i the literature review regardig effects of uiform magetic fields o electric coductive vertical twophase flows or o-uiform magetic fields o twophase flows. Though a complete study has ot bee foud about the effects of variatios i the itesity of a uiform magetic field o the bubbly flow iside the horizotal pipes. Therefore, this study has bee doe o the effect caused due to the variatio i the itesity of a uiform magetic field o the bubbly flow of Diesel ad CNG iside a horizotal pipe. CNG s bubbly flow behavior iside diesel exists i order to have a extesive variety of characteristics of two-phase flow (size ad shape of bubbles, gas velocity, ad gas area fractio). For this purpose, a aalysis was executed over the bubbles of CNG iside lamiar Diesel flow. Furthermore, this paper shows the outcomes of differet magetic field itesities o the CNG bubbly flow. All the experimetal work was doe by makig the use of digital image processig techique. Properties of diesel fuel as well as CNG are show i Table EXPERIMENTAL APPARATUS A schematic diagram of the experimetal facility for the cocurret two-phase (CNG-diesel) flow i a horizotal pipe is show i Fig. 1(a). For the purpose of observatio, the test portio of the facility was made up of trasparet polycarboate. The pipe was of legth measurig 1.2 m ad ier diameter (Di) of 0.02 m. The CNG bubbles geerator had a iductio system fitted o it, whose task was to produce liquid ad gas mixture iside the test portio that was situated at the startig sectio of the test. The Diesel supply lie ad the compressed CNG lie were liked to the bubble geerator. Diesel flows by the bypass valve that is supplied ad filtered by the fuel pump (90 watts, 5 l/mi, 1.5 m) for the flow meter, pump alog with the pressure gauge. After that, it goes ito the side hole of the bubble geerator ad gets combied alog with the CNG i order to a make two-phase flow. The the compressed CNG goes ito the mixer situated at the first edge of the bubble geerator, via the ceter cross-sectioal area. Use of a pressure regulator was made to cotrol the pressure of CNG ad flow rate of CNG was calculated by gas flow meter prior to its etrace i the bubble geerator. Oe way valves were used for the purpose of cotrollig the CNG flow rate precisely. Oe eedle of m (ier diameter) was passed through the ceter of the cross-sectioal area of pipe iside the bubble geerator. Calibratio of flow meter was doe to record the flow rate of Diesel by makig the use of a stopwatch. Moreover, the volume of the calibrated tak was pre-calibrated by height ad its volume. The time cosumed to fill 5 liters of diesel iside the tak was calculated. The time was approximately aroud 15 l/mi; alog with this, the accuracy was aroud ± 95%. The electromaget charger (excitatio coils) was supplied by the DC power (Fig. 1b), it was made up of two coils havig 250 wraps of copper wire that was covered by eamel havig a diameter measurig m ad a U-type yoke. The yoke was made with steel, the cross-sectioal area for both of the yoke poles was m that was perpedicular to the directio of the flow (X-axis) ad 0.0 m that was parallel to the directio of flow (Z-axis) to embrace the test pipe i order to decrease the loss caused due to eddy curret i the 150

3 core. A pair of the Tesla meter sesors was fixed betwee the yoke poles, ad a test pipe was used to measure the magetic itesity sigig. The magetic field betwee the excitatio coils value desiged i this work was measured havig a variable magetic field flux from 0 to 2000 Gauss. Frame grabber board, high-speed camera, PC computer ad a lightig system are the compoets of a image acquisitio system. A 120 Watt lamp ad a source of light, was kept at the back of the viewig pipe. The pictures were clicked usig a Phatom Miro digital camera M 310 (maximum resolutio: 1280*800). The frame rate of the camera was 320 frames per secod at 1280*800 ad miimum exposure was 1.0 µsec. Use of the techique kow as backlightig photography was utilized to record the shadow of CNG bubbles i bubbly flow as both diesel fuel ad CNG bubbles were trasparet. For the purpose of acquirig a smooth illumiatio of the sectio of the test, oe layer of drawig sulfate paper was fixed at the back of the test sectio that resulted i a icrease i the itesity of the cotrast of the image ad the silhouette of CNG bubbles. measuremet ca be obtaied by combiig the sesor accuracy ad measurig istrumet accuracy. A more realistic method is to use the rootsum-of-squares method, that is, by takig the square root of the sum of the idividual errors Bevigto ad Robiso (2003). total 2 sesor 2 istrumet (1) Where, δtotal is the total ucertaity. The ucertaity percetage to the maximum ad miimum values of recorded data was calculated from: Diesel flow meter, digital gas meter, ad magetic meter (Teslameter). Table 2 shows the ucertaity percetage error of the experimetal yield values measured. Table 2 the ucertaity percetage error of the experimetal yield values measured parameter Mi. for recorded value Max. for recorded value Mi. ad Max. ucertaity (%) error Diesel flow rate cm 3 /s cm 3 /s Gas flow rate 9.714*10-4 cm 3 /s Magetic field itesity 1.119*10-2 cm 3 /s gauss 2000 gauss DIGITAL IMAGE PROCESSING (a) The experimetal rig (b) Electromagetic iductio ad Camera systems arragemet. Fig. 1. The experimetal rig ad measuremet system. 2.1 Ucertaity Aalysis The error aalysis performed o the experimetal data i order to evaluate the ucertaities i the measuremets. The total ucertaity i every The acquired images displayed that both the shape alog with the size of the movig CNG bubbles are varyig all the time uder the ifluece of the magetic field. The desity variatios, shape, gray level distributio, the size of the bubbles also had ueve variatios from frame to frame. The edges of the CNG bubbles were ot clearly observable particularly, which idicates that it is tough to get cotour precisely through a simple detectio method. Hece it is more appropriate for measurig ad idetifyig the CNG-Diesel bubbly flow by makig use of digital image processig. Image processig algorithm was developed to extract the iformatio from the high-speed video data. The program was divided ito three stages: bubble idetificatio, verificatio of bubble parameters ad bubble trackig. Fig. 2 shows a schematic of image processig used i the preset work. Each image was processed as a graphical object where each of the matrix cotaied three color itesities, the RGB values. The RGB image was coverted ito a grayscale image with 8 bit depth usig rgb2gray fuctio i Matlab. For gray level, all pixels cosisted of a umerical value from 0 (black) to 255 (white). Images were cropped i order to retai oly the part of the image immediately at the dowstream of the ozzle, as show i Fig. 3. After this croppig, the program recogizeed all frames whe CNG bubbles started to release from the ozzle, thus providig the set of frames to be processed. 151

4 i the bubble ad scalig them to physical dimesios, the area of bubble was measured. By calibratig with the square mesh sectio (0.004*0.004 m) fixed i calibratio pipe as show i Fig. 5 (f), the area of the bubble could be determied. CNG bubbles i Diesel, the bubble shapes with chagig magetic itesity were estimated to spheres ad oblate spheroids. The diameter of the bubble ca be calculated as a equivalet diameter by assumig a roud bubble shape: Fig. 2. Schematic of Image Processig Algorithm. a) b) Fig. 3. The cropped area for Preprocessig of the images. A umber of preprocessig processes that should be performed i order to ehace the quality of the image as well as to decrease the oises are show i Fig. 4. Previously image subtractio was utilized to lesse the backgroud oise. Hece a calibratio image was obtaied with o bubbles. This image cotaied light distributio, iformatio regardig the camera oise ad backgroud. The calibratio image was subtracted from each image to provide a image of the bubble with reduced backgroud oise. Moreover, each image was coverted ito a biary image by usig a threshold of aroud All pixels i the iput image were replaced by output image havig a larger lumiace as compared to the threshold havig a value 1 (white) as well as replacig all other pixels havig value 0 (black), where it was possible to suppress light structures coected to the image. Thirdly, some morphological fuctios were used to improve the quality of the image ad the bubble shape. Fially, images with the bubble were prepared for the ext step that is the quatitative aalysis which icluded evaluatio of the ceter ad size of the bubble. By coutig the umber of pixels Fig. 4. Image processig procedures. Ab d b 2 (2) The gas area fractio ca be calculated by calculatig the total area of all CNG bubbles i the image as show below: 1 A f A bi i1 (3) 152

5 Fig. 5. Images ad shape variatios of CNG bubbles i Diesel flow with magetic field itesity (0, 750, 1550, 2000 Gauss). Where Ab is the bubble area, Af stads for the area of the image frame, ad deotes the total umber of bubbles i the image frame. I Fig.4 (d), the top left coordiates are (0, 0), ad the right bottom coordiates are (58, 239). The bubbles were umbered from top to bottom as well as from left to right. By usig scree pixels ad covertig them ito millimeter scale, sizes of CNG bubbles were measured. By coutig the umber of square pixels i each bubble, the area of bubble Ab, ca be calculated. The ext step was to track the CNG bubble from frame to frame i order, its trajectory ad bubble velocity, this required bubbles ceter. The geometric ceter of the bubble ca be foud as the ceter of mass of the regio belogig to the bubble: G G y z A G i 1 i 1 i 1 i1 A A y A G z (4) (5) Where Gy is the y coordiate of the th shape with area A, ad Gz is the z coordiate of the th shape with the area A. Oce cetroids for the first cropped image ear the ozzle edge have bee foud, the algorithm moved the bubble border by a shift determied usig the measured bubble velocity. 4. RESULTS AND DISCUSSION 4.1 The Effect of the Magetic Field o CNG Bubble Sizes ad Gas area Fractio The experimet was carried out to ivestigate the effect o the growth of CNG bubbles of a magetic field orieted ormal to the flow. The results showed that CNG bubbles o the vertical magetic field grow larger i the Diesel flow, icreasig i the diameter at (750 Gauss, 1550 Gauss, ad 2000 Gauss). Hece, the magetic field caused by the Loretz force to the adverse effect of the buoyace force devote to elogatio i the bubbles ad the mechaism to break off rather tha pump drive. Fig. 5 shows images ad shape variatios of CNG bubbles i Diesel flow ad the time icremet is 0.1 sec, with magetic flux of 0, 750, 1550 ad 2000 Gauss. I this case, the CNG bubbles are chagig from sphericity at ijectio time (t = 0) ad early a oblate sphericity at the upper wall of test pipe. Moreover, this behavior is similar to 153

6 a icrease i the magetic field itesity, icrease i bubble s size ad gas area fractio. The experimetal procedure was validated by comparig the curret results with umerical results reported by Abdul Wahhab et al. (201) ad a good agreemet was achieved. The bubbles size ad gas area fractio with variatio i magetic field itesity are show i Table 3. Table 3 Measuremets of CNG bubbles sizes ad gas area fractio B = 0 Bubbles Area 10 - (m 2 ) Diameter 10-3 (m) Ceter of bubbles (z, y) (17.42, ) (58.7, 94.3) (23.44,.98) (24.71, 73.52) (398.44, 9.2) (422.73, 5.11) (448.93, 78.7) Gas area fractio 1.05 % B = 750 Gauss Bubbles Area 10 - (m 2 ) Diameter 10-3 (m) Ceter of bubbles (z, y) (38.14, 98.41) (78.74, 71.41) (211., 9.48) (239.4, 33.42) (24.74, 23.7) (453.7, ) (472.12, 2.12) Gas area fractio 1.23 % B = 1550 Gauss Bubbles Area 10 - Diameter Ceter of bubbles (m 2 ) 10-3 (m) (z, y) (9.71, ) (3.48, ) (232.44, 8.43) (353.52, 28.43) (434.22, 1.7) (48.74, ) (521.42, 5.34) Gas area fractio % B = 2000 Gauss Bubbles Area 10 - Diameter Ceter of bubbles (m 2 ) 10-3 (m) (z, y) (7.32, ) (40.33, ) (209.41, ) (321., 51.84) (429.31,.39) (41.44, ) (531.42, 5.94) Gas area fractio % 4.2 The Effect of Magetic Field o the CNG Bubble Velocity Figure shows the traslatioal average velocity of the bubbles (Ub) durig 1.0 sec time, at various magetic field itesities. The geeral tred of the velocity of CNG bubble decreased as the magetic field stregth icreased. The figure presets the time developmet of the CNG bubble velocity for chagig magetic field itesity (0, 750, 1550, ad 2000 gauss). It ca be observed i Fig. that icreasig magetic field itesity decreases the average CNG bubble velocity. The effect of the magetic field ormal to the flow directio results i a drag-like or resistive force bearig the tedecy to suppress the movemet of the liquid i the pipe to make it slow, which, i tur, decreases the motio of the gas bubble. This is traslated ito the reductios i the average velocity of bubbles (CNG) phase ad their flow rate. Moreover, the reduced motio of the CNG bubbles i the pipe that occurs by icreasig the stregth of the magetic field results i lower velocity gradiets at the wall. This has a direct effect i icreasig the period of trasfer of mometum of both of phases. Fig.. The average bubble velocity at various magetic field itesity(0, 750, 1550, ad 2000 Gauss). 5. CONCLUSION This work coducts a visualizatio of the CNG bubbles iside the lamiar diesel flow i a horizotal pipe uder the ifluece of differet magetic field stregths. I order to carry out the aalysis of the experimetal data, digital image processig techique was implemeted. The followig coclusios could be draw from the aalysis of the results: CNG bubbles chage from sphericity at ijectio time (t = 0) ad early a oblate sphericity at the upper wall of test pipe. Furthermore, this behavior is similar whe magetic field itesity ad the bubble size is icreased. CNG bubbles ted to move towards the upper wall uder buoyacy effect ad these bubbles develop ito a bigger size ad expad i the diesel flow field. By icreasig magetic field itesity, gas area fractio value icreases. 154

7 By icreasig the magetic field stregth, the average bubble velocity decreases. It is highly recommeded to visualize the CNG bubbles i the lamiar Diesel flow i a horizotal pipe uder the effect of various magetic field stregths of higher rages that were utilized i this work. ACKNOWLEDGEMENTS The authors are grateful to the Uiversity Techology PETRONAS for providig the ceter for automotive research ad eergy maagemet (CAREM). REFERENCES Abdul Wahhab, H. A., A. R. A. Aziz, H. H. Al Kayiem ad M. S. Nasif (201). Modelig of mixig Diesel-CNG i a horizotal pipe uder the ifluece of a magetic field, Asia Joural of Applied Scieces. 4(4), Abdul Wahhab, H. A., A. R. A. Aziz, H. H. Al Kayiem ad M. S. Nasif (2015). Modelig of diesel/cng mixig i a pre-ijectio chamber. 3rd Iteratioal Coferece Mechaic Egieerig Research, IOP Coferece Series. 100, Bevigto, P. R. ad D. Keith Robiso (2003). Data reductio ad error aalysis. McGraw- Hill. Bhaga, D. ad M. E. Weber (1981). Bubbles i viscous liquids: shapes, wakes ad velocities, Joural of fluid Mechaics 105, Bruer, K. ad J. S. Chag (1980). Flow regime trasitio uder electric fields i horizotal two-phase flow. i proceedigs, 15th IEEE Idustry Applicatios Society Coferece Bruer, K., P. T. Wa ad J. S. Chag (1983). Flow patter maps for horizotal gas liquid two-phase flow uder d.c. electric field. I Electrostatics, Istitute of Physics Coferece Series, Dih, T. B. ad T. S. Choi (1999). Applicatio of image processig techiques i air/ water two phase flow, Mechaics Research Commuicatios 2(4), Ekambara, K., R. S. Saders, K. Nadakumar ad J. H. Masliyah (2008). CFD simulatio of bubbly two-phase flow i horizotal pipes, Chemical Egieerig Joural. 144, Fossa, M. (1998). Desig ad performace of a coductace probe for measurig the liquid fractio i two-phase gas-liquid flows. Flow Measuremet ad Istrumetatio 9, Hat, J. G. ad J. D. Buckmaster (197). Spherical cap bubbles ad skirt formatio. The Physics of Fluids 19, Ishimoto, J., M. Okubo, S. Kamiyama ad M. Higashitai (1995). Bubble behavior i magetic fluid uder a o-uiform magetic field, Iteratioal Joural of JSME 38(3), Jarrahi, M., C. Castelai ad H. Peerhossaii (2011). Lamiar siusoidal ad pulsatile flows i a curved pipe, Joural of Applied Fluid Mechaics 4(2), Ki, H. (2010). Level set method for two-phase flows uder magetic fields, Computer Physics Commuicatios Lugga, R. D. (1999). Eergy dispersive X-ray scatter for measuremet of oil/water ratios, Uclear Istrumets ad Methods i Physical Research 422, Scheer, A. M. (199). Multiphase Flow Measuremet Usig Multiply Eergy Gamma Ray Absorptio (MFGRA) Compositio Measuremet. SPE 3593 Preseted at 199 Aual-Techical Coferece ad Exhibitio, Dever Colorado 10, -9. Sussma, M. ad J. A. Smereka (1997). Axisymmetric free boudary problems. Joural of Fluid Mechaics 341,