Influence the Nozzle Shape on Local Heat Transfer in Impinging Jet

Transcription

1 Issue 6, Volume 6, 12 Influence the Nozzle Shape on Local Heat Transfer in Impinging Jet M. Attalla an M. S. Ahme 1 Abstract The local Nusselt number istributions of circular nozzle on a heate flat plate were experimentally investigate for various nozzle geometries. Experiments have been conucte with variation of exit Reynol's number, Re, is varie from to an plate surface spacing to nozzle iameter, H/, in the range of 1 H/ 6 for single nozzle with square ege (non-chamfere) an chamfere nozzles of the same iameter, 5 mm. The chamfere length, Lc is varie from 1 mm to 3.65 mm with constant chamfere angle, θ = for each nozzle configuration. The temperature istributions on the heate flat plate were measure using a thermograph camera, IR with a igital image processing system. The results inicate that the stagnation Nusselt numbers have the highest value for square ege inlet nozzle when compare with other nozzle configurations. The Stagnation Nusselt number was correlate for the nozzle Reynol's number as Keywors Impingement Jet; Chamfere Ege; Local Heat Transfer; Turbulence Intensity. Nomenclature k Thermal conuctivity of air, W/m.K Nu Nusselt number (-) q α Convective heat flux, W/m2 Re Jet Reynol's number, (-) R Raial istance from stagnation point of jet, m X Stream-wise istance, from stagnation point of jet measure along the plate (horizontal irection), m Greek symbols α Convective heat transfer coefficient, W/m2K Subscripts lo Local value f Force Convective heat transfer coefficient I I. INTRODUCTION mpinging jet has been wiely use for applications where high heat an mass transfer rates are require. A variety of nozzle geometries ranging from slots to square ege (from profite plate) an roun jet [1]. Previous stuies have emonstrate the epenence of heat transfer on a number of parameters, incluing the Reynol's number of jet, M. Attalla, Mechanical Power an Energy Department, faculty of Engineering, South Valley University, Qena, Egypt, phone: ; fax: ; moha_attalla@yahoo.com. M. S. Ahme, Mechanical Engineering Department, Faculty of Inustrial Eucation, Sohag University, Egypt. nozzle to plate spacing, the presence of a confining wall, the ambient air temperature relative to the jet temperature, an jet iameter [2], [3]. The geometry of the nozzle has consierable effect on the heat transfer between the impinging jet an the plate. The nozzle geometry parameters such as orifice shape an iameters, as well as the orifice inlet an outlet shapes have been foun to play a etermine role in heat transfer rate. The heat transfer rate epens also on the thickness of the nozzle orifice [1]-[4]. The orifice geometry can be esigne to improve the heat transfer between the impinging jet an plate because it affects the evelopment of the jet before it impinges on the plate, principally in the case of confine jets [5]. The orifice esign is also important because it affects the pressure rop across the nozzle an the velocity profile along the target surface [1]. A few prior stuies have investigate the effects of nozzle2 configuration on impinging jet heat transfer. In[1] it is starte experimentally the effect of change the nozzle geometry from square ege to chamfere on pressure rop an heat transfer rate. They conclue that chamfering the nozzle inlets reuce pressure rop without affecting much the heat transfer characteristics. In [6] it is investigate the effect of nozzle shape on local heat transfer istribution for three-ifferent nozzle cross sections, circular, square an rectangular. In [7] prove a better unerstaning of the mean flow characteristics an heat transfer of jet array with cross-flow, using ifferent jet orifice geometries, circular an cuspe ellipse. In [8] stuie the effect of nozzle geometry on pressure rop an heat transfer on the free surface an submerge liqui jet arrays. They foun that the heat transfer coefficient in confine submerge test was greater than that for their free jet counterparts. In aition, the inlet/outlet chamfere an contoure inlet nozzles were best in terms of the obtaine heat transfer rate for a given pumping power. The objective of the present paper is to investigate experimentally the effect of the nozzle geometry of square ege (non-chamfere) an with chamfere ege on heat transfer characteristics in point of view of local heat transfer istribution in free single circular air jet. The experimental parameters inclue nozzle-chamfere length (Lc = 1, 2.3 an 3.65 mm), Reynols number, Re, varie from to, an nozzle to plate separation istance, H/, varie from 1 to 6. The specific goal is to ientify conition uner which nozzle shape may be reuce or enhance heat transfer rate by 376

2 Issue 6, Volume 6, 12 using the free single nozzle. The local heat transfer is presente as a function of separation istance, H/, Reynols number, Re, an chamfere length, Lc. II. EXPERIMENTAL SET-UP The air jet impingement system is shown in Fig. 1. Airflow rate is measure by calibrate orifice flow meter. Air filter an pressure regulator are installe upstream of the nozzle flow meter to filter the air an maintain the ownstream pressure at 5 bar. The air is elivere to a flow conitioning cylinrical plenum in the test section. The separation istance between nozzle an target plate is set by using screw fixe on frame to move up an own irection [9]. The heat source consists of a metal sheet (.1 mm thick) with a heate area of ( mmx17 mm). Because of the very small thickness of the metal sheet, the conuction resistance between the two sies is negligible. Therefore, the temperature of both sies coul be assume equal as reporte by [1] an [11]. The temperature was recore from the plate epening on the emissivity value of surface, which was calibrate by Attalla [11]. Power loss from the expose surface of the impinging plate ue to natural convection an raiation was estimate experimentally [1] [12]. The temperature istribution on the impinging plate is obtaine from IR camera image for each nozzle configuration. The local Nusselt number for the surface is etermine by Air Compressor Filter Gate Valve Rotameters Pressure Regulator DC Power Source Nozzle Fig. 1 Layout of Experimental Set-Up H PLenum Chamber Impinging Plate IR Camera Figure 2 shows the free nozzle geometry use in this stuy. The nozzle iameter was hel at = 5 mm for all the experiments. One set of chamfere nozzle involves three ifferent chamfere length values (Lc = 1 mm, 2.3 mm an 3.65mm), the chamfere angle is θ= for all nozzles, Fig. 2- a. The secon one involves a free nozzle with square ege an was selecte to etermine the local an average heat transfer impinging into flat plate, Fig. 2-b. The nozzle length, L = 5, was also consiere for all the nozzles use. θ L c Fig. 2 Section View of Nozzle Geometric Teste a- Chamfere Ege Nozzles, b- Nozzle L α (1) Nu k q f (2) αf To Tj Where α f is the force convective heat transfer coefficient an To an Tj are the heate wall an exit jet temperatures respectively. The total heat losses amount up to 2 % of the electrical heat flux for turbulent flow [1,]. The uncertainty of experimental results has been estimate following the recommenation an metho suggeste an escribe by [13]. The estimate total uncertainty in the local Nusselt number was range from 3.25 to 5.23 %. The uncertainty in the Reynols number was estimate to be within 2.4 %. Another important source of uncertainty was the emissivity measure by the thermo camera. The uncertainty of emissivity was range from.5 to 1.23 %. III. RESULTS AND DISCUSSION Experiments were conucte to stuy the influence of chamfering nozzle at the inlet on istribution of local Nusselt number at three ifferent mean jets Reynol's number of,, an. The mean jet Reynol's number is base on jet iameter (5 mm) an jet average outlet velocity. The separation istance between jet an impinging plate is, 2, 4 an 6 for all the configurations investigate. The separation istance between jet an impinging plate is, 2, 4 an 6 for all the configurations investigate. Figure 3 shows examples of the thermal footprints of four nozzles configurations (square ege, an chamfere ege, Lc = 1 mm, 2.3 mm an 3.65 mm) obtaine from the infrare measurements. Those examples of the thermal footprints have been obtaine at separation istances H/ = 1 an at high Reynol's number, Re =. It is observe that, the temperature istributions ue to impinging jets are perfectly symmetric aroun the stagnation point except for the far fiel of the region of impingement. In aition, the colors of the thermal footprints are whitene with increase of chamfere length. This is ue to ecrease of cooling egree. Figures 4 to 7 show the local istribution of Nusslet number -base on 377

3 Issue 6, Volume 6, 12 temperature istribution- along the horizontal line through the stagnation point for four ifferent nozzle outlet shapes, three with ifferent chamfere length an the fourth with nonchamfere (square ege). Stagnation Point X C C Also, the stagnation Nusslet number values of square ege an chamfere ege with Lc1 nozzles are nearly equal for all Reynols number. However, the stagnation Nusselt number values ecrease by about 8-13 % for chamfere ege Lc2 an Lc3 for all Reynols numbers. The experimental result of [1], showe that the pressure rop increases by about 15.7 % with chamfere hole (Lc2 = 2.31 mm) when compare with the square ege hole. This increase in pressure rop is accompanie by a ecrease in local Nusselt number [], [21]. a- 4.7 C C b- Chamfere Ege, Lc1= 1 mm 4.7 C c- Chamfere Ege, Lc1= 2.3 mm 4.7 C - Chamfere Ege, Lc1= 3.65 mm Fig. 3 An Examples of infrare measurement for all nozzles configurations (H/ = 1, an Re = ) Local Nusselt number istributions at H/ = 1, are shown in Fig. 4a-c. It is observe that with increases in the Reynols number, the local Nusselt number increases at all horizontal axes for the four nozzles configurations. This may be ue to increase of the turbulence intensity of jet Reynols number [14], [15]. The value of stagnation Nusselt number for nozzle with chamfere length Lc2 an Lc3 are nearly equal at Reynols number,. However, the Nusselt number at stagnation point are higher for the square ege an nozzle with chamfere length, Lc1 by about 7% as compare with chamfere length Lc2 an Lc3. For other Reynols number, an, the stagnation Nusslet number is highest for a square ege nozzle when compare with the other nozzles configurations. For the chamfere length, Lc2 is higher by about 3 to 5 % from the chamfere length Lc3. This is ue to; the pressure rop at high Reynols number has week effect than the other Renol's number [1], [7], [16], an [17]. In the stagnation region, ( R/ 1), The Nusslet number nearly remains constant for all nozzle configurations up to an R/ =.5 an rop sharply until R/ = 1.6, an then, the change in slope of Nusselt number istribution is clearly observe for all Reynols number. The seconary peak is observe aroun R/ = 2, in transition turbulence region [18], [19], for high Reynols number (, an ). However, seconary peaks are not istinctly seen for lower Reynols number, Re =. Generally, the local Nusslet number istribution with square ege nozzle shows higher values at raial location of (1,6 R/ 2,5). The local Nusselt number istribution for all nozzles configurations separation at istance/ iameter ratio H/ = 2 is shown in Fig. 5a-c. All local Nusselt number curves for four nozzles configurations (square ege, chamfere ege Lc1, Lc2, an Lc3), have similar trens as those with H/ = C (a) Re =, H/ = Fig. 4a Influence of Nozzle Geometry on Local Nusselt Number Distribution along Horizontal Axis for Re =, an H/ = (b) Re =, H/ = Fig. 4b Influence of Nozzle Geometry on Local Nusselt Number Distribution along Horizontal Axis for Re =, an H/ = 1. The istributions of local Nusselt number for various values of Reynol's number at separation istance ratio, H/ = 4, are shown in Fig. 6a-c. The stagnation Nusselt number values are nearly the same for all nozzle configurations for a particular Reynols number. The seconary peak nearly occurs at R/ = 2.2 for square ege an Chamfere ege with Lc1 nozzles at high Reynols number. However, the seconary peak is not clearly visible for nozzle configurations at all other Reynols number. 378

4 Issue 6, Volume 6, 12 Nulo [-] (c) Re =, H/ = Fig. 4c Influence of Nozzle Geometry on Local Nusselt Number Distribution along Horizontal Axisfor Re =, an H/ = (a) Re =, H/ = Fig. 5a Influence of Nozzle Geometry on Local Nusselt Number Distribution along Horizontal Axis for Re =, an H/ = (c) Re =, H/ = Fig. 5c Influence of Nozzle Geometry on Local Nusselt Number Distribution along Horizontal Axis for Re =, an H/ = (a) Re =, H/ = Fig. 6a Influence of Nozzle Geometry on Local Nusselt Number Distribution along Horizontal Axis for Re =, an H/ = (b) Re =, H/ = Fig. 5bInfluence of Nozzle Geometry on Local Nusselt Number Distribution along Horizontal Axis for Re =, an H/ = 2. For Chamfere ege, Lc3, nozzle, the stagnation region is move outwar in horizontal axis, at a value of R/ of about.65. This inicates that the pressure rop increases with increase of chamfere length, Lc3, [1], [22]. The local Nusselt number istributions for all nozzles are nearly constant at the transition region (1.5 R/ 2.35) at values of Reynols number, Re = an. This inicates that the separation istance ratio H/ = 4 can be a threshol for the seconary maximum to occur [18], [19], an [23]. On the other han, the local Nusselt number ecreases monotonically after transient region for all nozzles. (b) Re =, H/ = Fig. 6b Influence of Nozzle Geometry on Local Nusselt Number Distribution along Horizontal Axis for Re =, an H/ = 4. Figure 7a-c shows the istribution of local Nusselt number for large separation istance ratio H/ = 6, an ifferent values Reynol's number. Trens observe in the local Nusselt number istribution are similar to those of Figs. 4a-c to 6a-c. At the largest separation istance H/ = 6, the maximum Nusselt number occurre at the stagnation point an then ecrease monotonically along the stream-wise irections. For a particular Reynols number Nasselt number values for square ege nozzle up to R/ = 3, are higher than those for all 379

5 Issue 6, Volume 6, 12 three chamfere ege nozzles [17]. (c) Re =, H/ = Fig. 6c Influence of Nozzle Geometry on Local Nusselt Number Distribution along Horizontal Axis for Re =, an H/ = (b) Re =, H/ = Fig. 7b Influence of Nozzle Geometry on Local Nusselt Number Distribution along Horizontal Axis for Re =, an H/ = 6. 8 (a) Re =, H/ = Fig. 7a Influence of Nozzle Geometry on Local Nusselt Number Distribution along Horizontal Axis for Re =, an H/ = 6. (c) Re =, H/ = Fig. 7c Influence of Nozzle Geometry on Local Nusselt Number Distribution along Horizontal Axis for Re =, an H/ = 6. The effect of nozzle geometry on the local Nusslet number is examine in Fig. 8a-b, at two Reynols number 3 an an separation istance H/ = 2, for four nozzle geometry (un-chamfere ege, an nozzle with chamfere ege Lc = 1 mm, 2.3, an 2.65mm). The curves in Fig. 8 a an b are bell shape an the local Nusselt number is largest values at stagnation point an ecreases with increase of raial istance R/D for all nozzles configurations an both Reynols number. Moreover, the local Nusselt numbers values for square ege are highest than other nozzle geometry at two Reynols numbers. The effect of nozzles configuration on stagnation Nusselt number are escription in next section. The influence of separation istance H/ on Nuesslt number at stagnation point for ifferent values of Reynol's number an ifferent nozzle configurations (square ege, chamfere ege Lc1, chamfere ege Lc2, an chamfere ege Lc3) is shown in Fig. 9a-e. This figure shows that the stagnation Nusselt number is nearly constant from H/ = 1 up to aroun H/ = 4 an then slightly ecreases for all nozzles. This result may be ue to the istance/iameter in within the optional core jet length [21], [23]. Nulo [-] (a) Re = 3, H/ = Fig. 8a Variation in local Nussewlt number istribution with nozzle configurations for nozzle iameter of 5 mm an H/ = 2 for Reynols number = 3. The ifferences between maximum an minimum values of stagnation Nusselt number is nearly constant for all separation istance at high Reynols number,. However, this value change with ecrease in value Reynol's number an the values of stagnation point Nusselt number are close at high separation istance H/ = 6. This result is explaine by the turbulence has less effect with ecrees of Reynols number an increase of separation istance. 38

6 Issue 6, Volume 6, c- Re = Nu [-] 7 (b) Re =, H/ = Fig. 8b Variation in local Nussewlt number istribution with nozzle configurations for nozzle iameter of 5 mm an H/ = 2 for Reynols number =. 1 a- Re = 5 H/ Fig. 9c Variation of Stagnation Nusselt Number with Separation Distance, for all Nozzle configurations at Re = Re = H/ Fig. 9a Variation of Stagnation Nusselt Number with Separation Distance, for all Nozzle configurations at Re =. 5 3 H/ Fig. 9 Variation of Stagnation Nusselt Number with Separation Distance, for all Nozzle configurations at Re = 1. 1 e- Re = b- Re = 3 H/ Fig. 9b Variation of Stagnation Nusselt Number with Separation Distance, for all Nozzle configurations at Re = Squere Ege H/ Fig. 9e Variation of Stagnation Nusselt Number with Separation Distance, for all Nozzle configurations at Re =. The stagnation Nusselt numbers for square ege nozzle are correlate with jet Reynols number an nozzle to-plate separation istance as shown in Fig. 1. The values of stagnation Nusselt numbers measure by, [24] for straight pipe nozzles are also inclue for comparison of separation istance, H/, from 1 to 4. It observe that, the figure show that correlation of the present stuy for square ege are in goo agreement with the measure values of stagnation 381

7 Issue 6, Volume 6, 12 NuSt [-] Nusselt number for 1 H/ 4 of, [24]. A least-square metho is yiele to obtain following correlation with a 6.3% stanar eviation: Nu st.69 Re H Nu st =.69Re.54 (H/) Re Present Stuy,2 H/ 8 Katti an Prabhu, 1 H/ 4 [24] Fig. 1 Stagnation Nusselt number variation with jet Reynols number an comparison with the measure ata of Katti an Prabhu, [24] IV. CONCLUSIONS An experimental investigation was performe to stuy the effect of nozzle inlet chamfering on heat transfer istribution over flat plate. In this stuy, the Reynols number base on jet inner iameter is varie from to an the separation istance /iameter ratio H/ is also varie from 1 to 6. Four ifferent nozzles inlet configurations are teste in this stuy: square ege nozzle, an chamfere ege nozzle having Lc1 = 1 mm, Lc2 = 2.3 mm an Lc3 = 3.65 mm. The conclusions of the present stuy are as follows: The chamfere ege has no effect of local Nusselt number istribution along the stream-wise irection. The Stagnation point Nusselt number is highest value at separation istance H/ = 1 for all nozzles configurations. For a range H/ from 1 to 4, the stagnation Nusselt number values are nearly constant, an it is higher than at separation istance H/ =6 for all nozzle configurations teste. It is observe that, the local Nusselt number istribution aroun R/ =2, has a secon peak for all nozzles inlet was forme particularly at higher Reynol's number values. The stagnation Nusselt number was correlate for the jet Reynols number an nozzle-to-plate separation istance as, Nu Re H st. ACKNOWLEDGMENT This stuy was supporte by thermal lab - Faculty of Engineering South Valley University Qena Egypt. REFERENCES [1] L. A. Brignoni, an S. V. Garimella, Effect of Nozzle Inlet Chamfering on Pressure an Heat Transfer in Confine Air Jet Impingement, Int. Journal of Heat an Mass Transfer, vol. 43,, pp [2] R. 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8 Issue 6, Volume 6, 12 [23] K. S. Choo, an S. J. Kim, Heat Transfer Characteristics of Impinging Air Jet Uner a Fixe Pumping Power Conition, Int. Journal of Heat an Mass Transfer vol. 53, 1, pp [24] Katti V, an Prabhu SV. Experimental Stuy an Theoretical Analysis of Local Heat Transfer Distribution Between Smooth Flat Surface an Impinging Air Jet from a Circular Straight Pipe Nozzle, Int. J of Heat an Mass Transfer, vol. 51, 8, pp