Mech060826.qxd The Journal of Engineering Research Vol. 5, No.1 (2008) 30-36 Notation 1. Introduction In industrial operations, various augmentation tech- niques are in practice, which reduce equipment sizes and increase out-puts without impairing the efficiency of pro ___________________________________________ *Corresponding author’s e-mail: feroz@caledonian.edu.om duction. Impinging jet systems have been well estab- lished as an effective technique for heating, cooling, or drying a target surface and have a variety of industrial applications, including the tampering of glass, the drying of film and textile, and the cooling of hot steel plate and gas turbine cascade. Recently, the jet impingement has also been used for the cooling of microelectronic compo- nents. Literature reveals several investigations (Gardon and Cobonpue, 1962; Gardon and Akfirat, 1965; Korger and Krizek, 1966; Subba Rao et al. 1973; Coeuret, 1975; Chin and Tsang, 1978; Chang et al. 1995, Kendoush, 1998; Lee et al. 2002; Travnicek and Tesar, 2003) on heat/mass transfer using jets in open and closed contain- ers. However, studies on the effect of submerged imping- ing jets in closed cylindrical cell in the presence of solids are meager. The presence of solids provides a large sur- face of contact and causes rapid removal of heat in elec- tro-chemical processes such as electrochemical surface treatment of selective area, annealing of metals and plas- tic sheets. The present study is directed towards the inves- tigations on mass transfer with impinging submerged jets in closed cylindrical cell in the presence of solids and cor- relation of data in impingement and decreasing coefficient regions. Mass Transfer Studies with Submerged Impinging Jets in Closed Cylindrical Electrolytic Cell in the Presence of Solids S. Feroz *1, B.V.L. Rao2, C. Bhaskarasarma3 and V.S.R.K. Prasad3 1 Department of M&IE, Caledonian College of Engineering, Sultanate of Oman 2 Department of Chemical Engineering, Govt. Polytechnic College, Visakhapatanam 530 012, India 3 Department of Chemical Engineering, Andhra University, Visakhapatanam 530 003, India Received 26 August 2006; accepted 11 February 2007 Abstract: An experimental study of mass transfer in forced convective flow of fluid electrolyte through submerged jets impinging normal to the target surface in a closed cylindrical cell in the presence of solids (Porcelain beads) is reported. The pertinent dynamic and geometric variables of this study are flow rate, diameter of the nozzle, height of the nozzle from the target surface and solids fraction. The mass transfer measurements, made by the electrochemical method propose empirical correlations in the impingement and decreasing coefficient regions. Keywords: Impingement region, Decreasing coefficient region, Mass transfer coefficient, Target surface Cf Correlation factor Dc Diameter of the cell, m dp Diameter of solids, m dj Diameter of the nozzle, m E1 toE7 Electrodes on target surface h Height of the nozzle from target surface, m JD Mass transfer factor kL Mass transfer coefficient, m/s NRe Reynolds number vd jρ µ χ Solids fraction x Average radial distance of electrode from the target surface, m v Velocity of fluid electrolyte from the nozzle, m/s .áÑ∏°üdG OGƒŸG OƒLh ‘ á≤∏¨ŸG á«fGƒ£°SE’G á«∏ÿG ‘ IQƒª¨ŸG ΩG~£°U’G ÒaGƒf ™e IOÉŸG ∫É≤àfEG äÉ°SGQO RhÒa.¢S1*hGQ É檰ûµd .± .Ü ,2QÉ°SôH .∑ .Q .¢S .± h 3Éeô°UGôµ°ùH .¢S,3 áá°°UUÓÓÿÿGGOƒLh ‘ á≤∏¨e á«fGƒ£°SG á«∏ÿ iôFGO í£°ùH áe~£°üŸG IQƒª¨ŸG IQƒaÉædG ∫ÓN øe …ô°ù≤dG »∏ª◊G πFÉ°ùdG ≥a~J â«dhεdG ‘ IOÉŸG ∫É≤àfG ∫hÉæàJ ájÈàfl á°SGQO : áÑ°ùfh ,±~¡dG í£°S øe ܃ÑfE’G ágƒa ´ÉØJQG ,܃ÑfC’G ágƒa ô£b ,≥a~àdG ∫~©e πª°ûJ á°SGQ~dG √ò¡H á∏°üdG äGP á«°S~æ¡dGh ádÉ©ØdG äGÒ¨àŸG ¿G .(RôÿG ±õN) áÑ∏°üdG OGƒŸG .πeÉ©ŸG ¢ü≤fh ΩG~£°UG ≥WÉæà á«ÑjôŒ äÉbÓY ìÎ≤J ,á«FÉ«ª«chô¡µdG á≤jô£dÉH âjôLCG »àdG ,IOÉŸG ∫É≤àfG äÉ°SÉ«b ¿G .áÑ∏°üdG OGƒŸG ääGGOOôôØØŸŸGGáá««MMÉÉààØØŸŸGG.±~¡à°ùŸG ≈≤∏◊G í£°ùdG ,IOÉŸG ∫É≤àfG πeÉ©e ,πeÉ©ŸG ¢übÉæJ á≤£æe ,ΩG~£°U’G á≤£æe : 31 The Journal of Engineering Research Vol. 5, No.1 (2008) 30-36 2. Experimental Set-up and Procedure An electrolytic cell of 0.1475m diameter and 0.173m height fabricated with PVC has been employed for study- ing the effects of various parameters. A schematic diagram of the apparatus is shown in Fig. 1. The set-up mainly consists of a storage tank (T), a pump (P), a rotameter R , an electrolytic cell (C), and a selector switch (S). The storage tank is made of a copper sheet with a capacity of about 0.150 m3 and is provided with a drain valve (V1) at the bottom. The outlet line at the bottom of the tank is connected to the suction side of the pump and in between a valve (V2) was provided to isolate the pump from the storage tank whenever necessary. A 0.746 kW motor is used to drive the pump, which circu- lates the electrolyte solution. The outlet from the pump is divided into two lines - one for direct entry of electrolyte into the cell through a rotameter and the other to serve as a by-pass which returned to the storage tank, the by-pass being controlled by valve (V3). Three convergent nozzles of diameters 0.002112 m, 0.005623 m and 0.007831 m were fabricated using copper tube. Nozzle is provided with a screw threading to fit with the inlet tube. A rotame- ter of Fischer and Porter make, with a metering range of 0 - 3.33x10-4 m3/s has been used to measure the electrolyte flow rate. The cell is fixed between two circular hylam plates of the same dimensions. The top plate is provided with two holes, one for the insertion of nozzle and the other for exiting the electrolyte and returning to the electrolyte stor- age tank. Copper tubes are provided as inlet and outlet for the electrolyte. The bottom plate served as "Target sur- face". Concentric ring electrodes (E2 to E7) are cut out of copper sheet of 0.00125 m thick and fixed at equidistant ie. 0.01 m apart, flush with the inside surface. The central electrode (E1) is a small disc cut out of a copper rod of 0.02 m diameter, fixed at center point of the target plate where the jet impinges. All electrodes are provided with copper terminals for making electrical connections. The inlet copper tube served as the counter electrode. The electrolytic cell is charged with varying quantities of solids (SF) (0.5 % to 3% of total volume), which are a mixture of spherical and cylindrical shaped porcelain beads of 0.005 m diameter and 0.008 m length with a bulk density of 1200 kg/m3 and true density of 1267 kg/m3. Nozzles of three different sizes 0.007831 m, 0.005623 m and 0.002112 m are used to generate the impinging jet flow. Limiting currents are measured for reduction of ferri-cyanide ion, once the flow rate and temperature was stabilized. The electrochemical procedure adopted for the measurement is same as described in the earlier studies (Lin et al. 1951; Sakakihara et al. 1994; Subba Rao et al. 1973). An electric potential was applied in steps across the test and counter electrode and the corresponding cur- rents were noted. The attainment of limiting current was observed by a sharp rise in potential for a small increase in current. The electrolyte consisted of equi-molal solu- tions of (0.01M) of potassium ferricyanide and potassium ferrocyanide with an excess indifferent electrolyte 0.5N sodium hydroxide. Measurements of limiting currents are obtained at different flow rates. For any individual run the V3 E T V1 P V2 R C SF S Figure 1. Schematic diagram of experimental set-up 32 The Journal of Engineering Research Vol. 5, No.1 (2008) 30-36 3. Results and Discussion The hydrodynamics of the impinging jet flow may be visualized (Gardon and Cobonpue, 1962; Gardon and Akfirat, 1965; Korger and Krizek, 1966; Subba et al. 1973; Coeuret, 1975; Chin and Tsang, 1978) as the jet issuing from the nozzle onto the target surface, gradually flows away radially towards the outer edge of the target surface or the confining wall of the electrolyte cell depending upon whether it is free jet or submerged jet. In the presence of solids in closed cells with submerged impinging jets, the flow patterns are complex for any mathematical treatment and hence an experimental mod- eling has been attempted by the empirical method. The plot of mass transfer coefficients (kL) (calculated from the limiting current density, similar to earlier studies) (Lin et al. 1951; Sakakihara et al. 1994; Subba et al. 1973) for the reduction of ferricyanide ion is drawn against the dimensionless distance 'x/Dc', where 'x' is the average radial distance of the ring electrodes from the center of the cell and is shown in Fig. 2. A close inspection of the trends in these plots shows the existence of the following regions due to different flow patterns: (1) Impingement region - wherein the limiting current densities are com- paratively very high which can also be referred to as stag- nation zone. (2) Transition region - wherein the limiting current densities decrease with the radial distance of the electrode. (3) Wall jet region - wherein the limiting current densities decrease relatively very slowly as compared to those in the transition region. The change from region 2 to region 3 is not abrupt but smooth, hence these two region ie. transition and wall jet region are therefore combined to form a "decreasing coefficient region". It is also observed that only central electrode (x/Dc = 0) falls into the impingement region. 3.1 Effect of Flow Rate on Mass Transfer Coefficients The mass transfer coefficient data of E1, E3 and E7 are plotted against the flow rates as shown in the Fig. 3. It has been found that the values of kL increase gradually with an increase in the flow rate, due to high turbulence on the target surface both in the case of impingement (E1) and decreasing coefficient regions (E3 and E7). temperature remained constant within ± 0.1 0C and if the temperature varied more than ± 0.1 0C, the run was discarded. The electrolyte was analyzed for each and every run and the reproducibility of data was tested from time to time by repeating one of the previous runs under identical conditions. The diameters of nozzles, height of the nozzle from the target surface and solid fraction ar e varied and experiments are repeated for each case. The ranges of varia bles cove red are given in Table 1. S.No Variable Minimum Maximum 1. Flow rate (Qx10 5), m3/s 3.4 14.0 2. Velocity (v), m/s 0.705 40.0 3. Height of the nozzle from the target surface, (hx102), m 1.0 8.0 4. Diameter of the nozzle, (dj x102), m 0.2112 0.7831 5. Average radial distance of the electrode on the target surface from the center, (x*102), m 0.0 6.328 8. Reynolds number, NRe 6707 102405 9. Schmidt number, Sc 698.6 931.9 Table 1. Ranges of variables covered in the present day 0 0.5 1 1.5 0 0.1 0.2 0.3 0.4 0.5 x/Dc k L x1 05 m /s Qx105, m3/s 3.4 14.0 Figure 2. Variation of mass transfer coefficients with x/Dc (dp: 5x10-3 m, λ: 0.01, dj: 0.7831x10-2 m, h: 2.5x10-2 m) 0.01 0.51 1.01 1.51 2.01 1 6 11 16 Qx105, m3/s k L x1 05 , m /s Figure 3. Variation of mass transfer coefficients with flow rates (dp: 5x10-3 m, λ: 0.01, dj: 0.5623 x 10-2 m, h: 2.5x10-2 m) 33 The Journal of Engineering Research Vol. 5, No.1 (2008) 30-36 3.2 The Effect of Height of Nozzle from Target Surface on Mass Transfer Coefficients The effect of nozzle height from the target surface on the mass transfer coefficients is shown in Figs. 4a and 4b for three different heights covered in the present study ie. 0.01 m, 0.04 m and 0.08 m. It has been found that with an increase in the height of the nozzle from the target surface, the values of the mass transfer coefficient decreases. The variation of mass transfer coefficients with nozzle height is found to be marginal in both impingement (E1) and decreasing coefficient regions (E7), as shown in Figs. 4a and 4b, respectively. As the height increases the liquid jets that emerge out from the nozzle are loosing the impact of their momentum and not fully reach the target surface where the mass transfer occurs. Similar observations were reported in earlier studies on single, open free and sub- merged jets (Gardon and Cobonpue, 1962; Gardon and Akfirat, 1965; Korger and Krizek, 1966; Subba et al. 1973; Coeuret, 1975). 3.3 The Effect of Nozzle Diameter on Mass Transfer Coefficients The values of mass transfer coefficients of E1 and E7 are plotted against velocity for three different nozzle diameters keeping other parameters constant as shown in Figs. 5a and 5b. It has been found that in both regions kL increases with an increase in the nozzle diameter for a given velocity due to large scale eddy turbulence on the target surface. 3.4 The Effect of Solids on Mass Transfer Coefficients The variation in the mass transfer coefficients of E1 and E7 with the percentage of solids presence is shown in Figs. 6a and 6b. The percentage of solids varied from 0.5% to 3% of volume of the cell. Mass transfer coeffi- cients are found to increase marginally with the increase in solids up to 1% and then decreases in both impingement 0.01 1.01 2.01 0.1 0.6 1.1 1.6 2.1 2.6 3.1 v, m/s k L x1 05 , m /s Figure 4a. Variation of mass transfer coefficients with velocity for different heights of nozzle from target surface in impingement region ( E1, dp: 5x10 -3 m, χχ :0.01, dj: 0.21120x10 -2 m) 0.00 0.05 0.10 0.01 1.01 2.01 3.01 4.01 5.01 v, m/s k L x1 05 , m /s Figure 4b. Variation of mass transfer coefficients with velocity for different heights of nozzle from target surface in decreasing coefficient region ( E7, dp: 5x10 -3 m, χχ: 0.01, dj: 0.21120x10 -2 m) 0.50 1.00 1.50 2.00 0.1 10.1 20.1 30.1 40.1 v, m/s k L x1 05 , m /s Figure 5a. Variation of mass transfer coefficients with velocity for different nozzle diameters in impingement region ( E1, dp: 5x10 -3 m, χχ: 0.01, h: 2.5x10-2 m). 0.01 0.06 0.1 10.1 20.1 30.1 v, m/s k L x1 05 , m /s Figure 5b. Variation of mass transfer coefficient s with velocity for different nozzle diameters in decreasing coefficient region (E7, dp: 5x10 -3 m, χχ: 0.01, h: 2.5x10-2 m) 34 The Journal of Engineering Research Vol. 5, No.1 (2008) 30-36 and decreasing coefficient regions. It has been observed that at a lower solid fraction the scouring action together with the hydrodynamic turbulence increased the mass transfer coefficients marginally. At a higher solid fraction, the effect of solids is found to decrease gradually resulting in lower coefficients. 4. Comparison Figure 7 shows the variation of mass transfer coeffi- cients with and without the presence of solids. Plot 'A' showed the data of the present study (density: 1200 kg/m3). Plot 'B' gave the data on mass transfer coefficients in the absence of solids while plot 'C' gave the data in the presence of high-density solids (density: 2320 kg/m3). The plots showed that low-density solids (density: 1200 kg/m3) as in the case of the present study gave improvements up to 3 folds over the coefficients obtained in the absence of solids. The plots also revealed that the coefficients with low density solids (density: 1200 kg/m3) were found to be 200 times greater than those obtained with high density solids (density: 2320 kg/m3). Solid-Fluid mixing appears to be more significant and severe with low-density solids, favoring a scouring action as well as hydrodynamic turbu- lence on the transfer surface resulting in higher coeffi- cients. Figure 8 shows the comparison of the coefficients obtained from the study on heat transfer data on turbulent submerged liquid jets of (Chang et al. 1995), single sub- merged jets in open containers of (Rao et al. 1971), free multi-jets data of Venkateswarlu and (Raju, 1979) with those obtained from the present data in the impingement region. The coefficient data of the present study (Coeuret, 1975) however are found to be higher than those obtained with turbulent submerged liquid jets (heat transfer), single submerged jets and free multi-jets. 0.50 1.00 1.50 0.1 0.6 1.1 1.6 2.1 2.6 3.1 3.6 4.1 4.6 5.1 v, m/s k L x1 0 5 , m /s Figure 6a. Variation of mass transfer coefficients with velocity for different solids fraction in impingement region ( E1 dp: 5x10 -3 m, dj: 0.7831x10 -2 m, h: 2.5x10 -2 m) 0.01 0.06 0.11 0.16 0.1 0.6 1.1 1.6 2.1 2.6 3.1 3.6 4.1 4.6 5.1 v, m/s k L x 1 0 5 , m /s Figure 6b. Variation of mass transfer coefficients with velocity for different solids in decreasing coefficient region ( E7 dp: 5x10-3 m, dj: 0.7831x10 -2 m, h: 2.5x10 -2 m) 0.00 0.10 0.20 0.30 0.40 0 0.1 0.2 0.3 0.4 0.5 x/Dc k L x 1 05 ,m /s k Lx 1 0 5 , m / s x/Dc Figure 7. Comparison plots of present study (Pl ot ‘A’ density: 12 00 kg/m3) with the higher density solids (Plot ‘C’ density: 232 0 kg/m3) and without the presence of solids (Plot ‘B’) (dp: 5x10 -3 m, χχ: 0.01, h: 2.5x10 - 2 m, dj: 0.7831x10 -2 m, Q: 8.75x10 -5, m) 0.00 0.50 1.00 1.50 1.00 11.00 Qx105, m3/s k L x 1 0 5 , m /s Figure 8. Comparison plots of present study with Chang et al. 1995 [turbulent subme rged single jets, heat transfer] , Subba Rao et al. (1973) [Single S ubmerged jets in open container] , Venkateswarlu and Raju (1979) [Free Multi-jets in open container] 35 The Journal of Engineering Research Vol. 5, No.1 (2008) 30-36 5. Correlations Developed Earlier investigations (Gardon and Cobonpue, 1962; Gardon and Akfirat, 1965; Korger and Krizek, 1966; Subba Rao, et al. 1973; Chang et al. 1995) on heat and mass transfer in free and submerged impinging jets used conventional NU/JH-Re or Sh/JD-Re type of correlations. Similarly, the correlations are developed for mass transfer coefficient data, taking into account the effect of design and operating variables in the present study. The follow- ing format of the equations has been used to develop gen- eralized correlations by the Regression analysis. (1) The data on reduction of ferri-cyanide ion in the impingement region yielded the following equations of correlation: For the impingement region: (2) (3) For the decreasing coefficient region: (4) (5) The above equations are valid for the ranges of vari- ables covered in the present study. The plots of the data correlated in accordance with the Eqs. 2 to 5 (Correlation factor versus flow Reynolds number) are shown in Figs. 9-12. (6) JD = C (NRe) n1 (φ1)n2 (φ2)n3 (χ)n4 JD = 203.43 ( NRe) –0.92 (h/Dc) -0.038 (dj/Dc) 1.06 (χ) 0.151 for χ ≤ 0.01 JD = 28.41 ( NRe) –0.93 (h/Dc) -0.051 (dj/Dc) 1.07 (χ)-0.312 for χ ≥ 0.01 JD =7.10 (NRe) –0.96 (h/Dc) -0.042 (dj/Dc) 1.08 (χ)0.057 (x/Dc) -0.804 for χ ≤ 0.01 JD =2.43 (NRe) –0.95 (h/Dc) -0.018 (dj/Dc) 1.08 (χ)-0.18 (x/Dc) -0.814 for χ ≥ 0.01 Correlation factor (cf)=J D / ( (φ1 ) n 1 (φ2 ) n 2 (φ2 ) n 3 (χ) n4) 0.00 0.01 0.10 1.00 1000 10000 100000 1000000 NRe cf Figure 9. Correlation plot for impingement region for χ ≤ 0.01 0.00 0.00 0.00 0.01 0.10 1.00 1000 10000 100000 1000000 NRe cf Figure 10. Correlation plot for impingement region for χ ≥ 0.01 0.00 0.00 0.00 0.01 0.10 1000 10000 100000 1000000 NRe cf Figure 11. Correlation plot for decreasing coefficient region for χ ≤ 0.01 36 The Journal of Engineering Research Vol. 5, No.1 (2008) 30-36 6. Conclusions The correlations developed can be employed in the design of the electrochemical cell with submerged impinging jets in the presence of solids. The following inferences have been drawn from the results of the study: (i) The mass transfer coefficients is found to increase with an increase in the flow rate both in impingement and decreasing coefficient regions as a result of an increase in turbulence at the target surface (ii) The mass transfer coefficients is found to increase with an increase in the nozzle diameter at a given velocity in both the regions, due to large scale turbu- lence at a given velocity (iii) The mass transfer coefficients showed a decrease in their value with the increase in the height of the noz- zle from the target surface as the impact of jets did not fully reach the target surface due to local flow inter- actions. The decrease is found to be marginal in both the regions, and (iv) The mass transfer coefficients initially showed an increase in value with the increase in solids fraction up to 1.0% and then decreased in both regions. It has been observed that at a lower solid fraction the scour- ing action together with the hydrodynamic turbulence increased the mass transfer coefficients marginally. At higher solid fraction, the effect of solids is found to decrease gradually resulting in lower coefficients. References Coeuret, F., 1975, "Transfert De Matierelors Del'impact Normal De Jets Liquides Circulaires Immerges," J. of Chemical Engineering Science, Vol. 30, pp. 1257- 1263. Chin, D.T. and Tsang, C.H., 1978, "Mass Transfer to an Impinging Jet Electrode," J. of Electrochemical Society, Vol. 125(9), pp. 1461-1469. Chang, C.T., Kojasoy, G. and Landis, F., 1995, "Confined Single- and Multiple-Jet Impingement Heat Transfer- I. Turbulent Submerged Liquid Jets," Int. J. of Heat and Mass Transfer, Vol. 38(5), pp. 833-842. Gardon, R. and Cobonpue, J., 1962, "Heat Transfer Between a Flat Plate and Jets of Air Impinging on it," Int. Developments in Heat transfer, ASME, New York, pp. 454-460. Gardon, R. and Akfirat, J.C., 1965, "The Role of Turbulence in Determining the Heat Transfer Characteristics of Impinging Jets," Int. J. of Heat and Mass Transfer, Vol. 8, pp. 1261-1272. Korger, M. and Krizek, F., 1966, "Mass Transfer Coefficient in Impingement Flow from Slotted Nozzles," Int. J. of Heat and Mass Transfer, Vol. 91, pp. 337-344. Kendoush, A.A., 1998, "Theory of Stagnation Region Heat and Mass Transfer to Fluid Jets Impinging Normally on Solid Surfaces," Chemical Engineering and Processing, Vol. 37, pp. 223-238. Lin, C.S., Denton, E.B., Gaskill, H.S. and Putnam, G.L., 1951, "Diffusion Controlled Electrode Reactions," Industrial and Engineering Chemistry, Vol. 43(9), pp. 2136-2143. Lee, D.H., Lee, Y.M., Kim, Y.T., Won, S.Y. and Chung, Y.S., 2002, "Heat Transfer Enhancement by Perforated Plate Installed Between an Impinging Jet and the Target Plate," Int. J. of Heat and Mass Transfer, Vol. 45, pp. 213-217. Sakakihara, Y., Flora, J.R.V., Suidan, M.T. and Kuroda, M., 1994, "Measurement of Mass Transfer Coefficients with an Electrochemical Method using Dilute Electrolyte Solutions," J. of Water Research, Vol. 28(1), pp. 9-16. Subba Rao, B., Krishna, M.S. and Jagannadha Raju, G.J.V., 1973, "Ionic Mass Transfer with Submerged Jets-I," Periodica Polytecnica Chemical Engineering, Vol. 17(2), pp. 185-195. Travnicek, Z. and Tesar, V., 2003, "Annular Synthetic Jet used for Impinging Flow Mass Transfer," Int. J. of Heat and Mass Transfer, Vol. 46, pp.3291-3297. Venkateshwarlu, P. and Jagannadha Raju, G.J.V., 1979, “Ionic Mass Transfer with Imping Multi-Jets, Indian J. Technol, Vol. 17(1), pp. 1-4. 0.00 0.00 0.00 0.00 0.01 0.10 1000 10000 100000 1000000 NRe cf Figure 12. Correlation plot for decreasing coefficient region for χ ≥ 0.01