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Visualization of flow and heat transfer characteristics for swirling impinging jet
International Communications in Heat and Mass Transfer 39 (2012) 640–648
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Visualization of flow and heat transfer characteristics for swirling impinging jet☆ C. Nuntadusit a,⁎, M. Wae-hayee a, A. Bunyajitradulya b, S. Eiamsa-ard c a b c
Energy Technology Research Center and Department of Mechanical Engineering, Faculty of Engineering, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand Department of Mechanical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok, 10330, Thailand Department of Mechanical Engineering, Faculty of Engineering, Mahanakorn University of Technology, Bangkok 10530, Thailand
a r t i c l e
i n f o
Available online 30 March 2012 Keywords: Heat transfer enhancement Impinging jet Swirling jet Flow visualization Twisted tape Thermochromic liquid crystal
a b s t r a c t Flow and heat transfer characteristics of swirling impinging jet (SIJ) were studied experimentally at constant nozzle-to-plate distance of L = 4D. The swirling jet is generated by inserting twisted tapes within a pipe nozzle. Effects of swirl on the impinged surface are investigated at twist ratios (y/W) of ∞ (straight tape), 3.64, 2.27, 1.82, and 1.52. The flow patterns of the free swirling jet and the swirling impinging jet were visualized by mixing dye with the jet flow. Distributions of temperature and convective heat transfer coefficient on the impinged surface were measured with thermochromic liquid crystal (TLC) sheet and image processing technique. Additionally, an oil film technique was performed as a complementary technique for flow visualization on the impinged surface. The experimental results reveal that there appear to be two peaks of heat transfer in the jet impingement region. The heat transfer enhancements in jet impingement region can be achieved at a low twist ratio of 3.64 which corresponds to the swirl number of 0.4. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Impinging jet is a jet whose axial direction is oriented toward and perpendicular to a target surface, resulting in an intensive heat transfer on the impinged area. Impinging jet has been widely used in many engineering applications such as cooling of turbine blades and electronic components, heating of steel plates, tempering of glass, drying of papers and food products [1–9]. Heat transfer enhancement of impinging jet can be made effective by proper controlling of various parameters such as nozzle-to-plate distance, jet velocity, jet array assembly (in multiple jet configurations), nozzle shape, external forcing, and imposed swirl, etc. [1-24]. In general, the flow structure of impinging jet can be classified into three regions: a free jet region, which forms around the jet exit with V(r) velocity distribution; an impingement flow region, which forms upon the jet impact and deflection; and a wall jet region, which forms due to re-acceleration of the flow along the impinged surface, as shown in Fig. 1. Large heat transfer coefficients are attainable in the stagnation region. In addition, the wall jet region, which possesses a larger transfer area, can potentially contribute to further heat transfer enhancement. To enhance heat transfer on an impinged surface, swirling jet has been adopted instead of non-swirling jet or conventional jet. Ward and Mahamood [3] showed that a swirling jet provided more uniform radial distribution of velocity than non-swirling one. They also observed that the Nusselt number decreased as nozzle-to-plate distance increased. ☆ Communicated by W.J. Minkowycz. ⁎ Corresponding author. E-mail address: [email protected] (C. Nuntadusit). 0735-1933/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.icheatmasstransfer.2012.03.002
Huang and El-Genk [4] performed heat transfer and flow visualization of conventional impinging jet (CIJ), swirling impinging jet (SIJ) with swirl angles (θ = 15, 30 and 45°) and multi-channel impinging jets (MCIJ), using smoke flow, smoke wires, and air bubble techniques in water jet visualization. Their results revealed that SIJ offered superior Nusselt number as well as radial heat transfer uniformity when compared to MCIJ and CIJ. Lee et al. [5] showed the influence of swirling jet generated through multi-narrow channels on the local and average heat transfer distribution. Their results demonstrated that swirling effect was intense at small nozzle-to-plate distance, L/D = 2, but the effect became rarely observed at L/D beyond 10. Shuja et al. [6] studied swirling confined jet impinging on an adiabatic wall. They found that the jet axis tilted toward radial direction as the swirl velocity increased and lowering of the velocity profile number enhanced the entropy generation due to heat transfer. Bakirci and Bilen [7] visualized temperature distribution and evaluated heat transfer rate of swirling impinging jet (SIJ), multi-channel impinging jet (MCIJ), and conventional impinging jet (CIJ) using thermochromic liquid crystal technique. They found that as the swirl angle increased, the radial Nusselt number distribution became more uniform and the optimum result was found at the swirl angle of 50° and jet-to-surface distance of L/D = 14. Alekseenko et al. [8] performed visualization of impinging jets using PIV and Stereo PIV techniques and found that the swirling impinging jet possessed a greater spread rate and a faster decay in absolute velocity compared to conventional jet. Wen and Jang [9] examined flat surface cooling by impinging jet issuing through two different swirling strips including a longitudinal swirling-strip and a crossed swirling-strip at various nozzle-to-plate distances. They used smoke flow visualization to
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free jet region
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Fig. 1. Flow structure of an impinging jet on a surface [24].
investigate swirling flow jet impinging at different jet to plate distances. Heat transfer from a flat surface to each impinging jet was investigated and correlation of the average Nusselt number has been derived as a function of nozzle exit-to-plate distance and Reynolds number for all nozzle types. Yang et al. [10] demonstrated detailed impinging annular jet behaviors induced by a swirling motion. They reported that at a short and intermediate separation distances (jetto-surface distance), swirling annular jet provided non-uniform wall pressure and Nusselt number distributions on the impinged surface.
Several swirl generators have been investigated for enhancing the heat transfer rate [11–23]. The features of some swirl generators are shown along with the flow pattern of a conventional impinging jet in Fig. 2. While other swirl generators have been commonly investigated, twisted tapes received less attention. This motivates the present work to investigate the use of twisted tapes as a swirl generator for heat transfer enhancement of impinging jets, especially on impinged surfaces. According to above literatures, only a few research works focused on the heat transfer enhancement mechanisms of swirling impinging jet.
Fig. 2. Geometries of various swirl generators for impinging jets: (a) SWJ [4], (b) SWJ [8], (c) impinging annular jet [10], (d) swirl jet with eight narrow channels [5], (e) multichannel conventional impinging jet [7] and (f) longitudinal swirling strips [9].
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To gain a better understanding on the behaviors of swirling impinging jet (SIJ), as well as parameters influencing the heat transfer enhancement, flow visualization of SIJ and effects of swirl number on heat transfer distributions on the impinging surface are examined. In the experiment, thermochromic liquid crystal (TLC) sheet was applied for visualization of temperature distribution on an impinged surface struck by the swirling impinging jet (SIJ). The parameters tested were of five different twist ratios (y/W): ∞ (straight tape), 3.64, 2.27, 1.82, and 1.52, corresponding to swirl numbers (Sw*) of 0.0 (straight tape), 0.4, 0.62, 0.78, and 0.94, respectively. Flow visualization of the swirling jet was conducted using the dye technique. The conventional impinging jet (CIJ) and free jet (without an impinged surface) were also investigated for comparison at similar test conditions.
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Fig. 4. Injection nozzle with twisted tape (left) and twisted tapes at various twist ratios (right).
2. Experimental details 2.2. Flow visualization with dye injection in water tank 2.1. Test rig The schematic of the experimental setup is shown in Fig. 3. In the experiments, air at room temperature of about 28 °C was drawn by a 7.5 kW blower and directed to the jet chamber which acts as a reservoir. Subsequently, the air was heated by an electrical heater which was regulated by a temperature controller. Volumetric flow rate of the air through the pipe nozzle was kept constant corresponding to a Reynolds number of 20,000. The pipe nozzle, into which a twisted tape was inserted, had an inner diameter of 16.5 mm and a length of 300 mm. The outer pipe nozzle surface and the jet chamber were well insulated to minimize heat losses to the surrounding. The air delivered from the nozzle pipe was then impinged perpendicularly to the impinged surface. Details of the heat transfer and flow visualizations of the impinging jets by various techniques are described in the following section. In the experiments, the swirls were generated using twisted tapes, made from stainless steel strips of 2.5 mm in thickness, 16.5 mm in width (W), and 300 mm (L) in length, which were inserted inside the pipe nozzle. These were fabricated by twisting a straight tape about its longitudinal axis with 180° for each twist length (y = L/n, where n is the number of twist), while being held under tension. Swirl intensity is varied by changing the twisted length or the twist ratio (y/W). The twist ratios investigated are y/W = ∞ (straight tape), 3.64 (5 twists), 2.27 (8 twists), 1.82 (10 twists), and 1.52 (12 twists), resulting in swirl numbers of Sw* = 0.0 (straight tape/no swirl), 0.4, 0.62, 0.78 and 0.94, respectively, as shown in Fig. 4. The figure also outlines the coordinate system being used in this study.
The schematic dye visualization setup is shown in Fig. 5. The water tank was made of acrylic transparent plates with crosssection of 600 × 600 mm 2 and 500 mm in height. In this experiment, two colors of dye were used as tracers. The blue dye was prepared by using thymolphthalein and sodium hydroxide dissolved in water. Similarly, to prepare the magenta dye, phenolphthalein and sodium hydroxide were dissolved in water. Firstly, the jet flow was mixed with the magenta dye and issued in clear water to reveal the effect of swirl on a free jet flow. Then, two colors of the dye were injected into the clear water jet flow through two needles mounted at the nozzle exit to acquire the flow path in jet structure. In each test, water at room temperature from the reservoir water tank was pumped from the reservoir through a main pipe and then through a rotameter, the setting chamber and then the circular pipe nozzle. At the pump exit, a bypass pipe and a valve were used to adjust the flow rate. The flow pattern of the dye was recorded by a digital video camera. For the case of impinging jet, flow patterns of the dyes were recorded through the acrylic transparent wall surface in front view and top view. The nozzle-to-plate distance was kept constant at L = 4D where D is the nozzle diameter. 2.3. Temperature distribution visualization with TLC Schematic of the heat transfer experimental setup is shown in Fig. 3. In this experiment, the electrically heated impinged surface was cooled by the jet issuing from a pipe nozzle. The surface was made from a stainless steel foil (SUS304) with length, width and
W y
Fig. 3. Schematic for experimental setup.
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Digital camera
input power to the stainless steel foil was evaluated from the power supply electric current and the resistance of the stainless foil. When the temperature distribution on the impinged surface, as indicated by the color pattern of the TLC, reached a steady state, the color pattern on the TLC sheet was recorded by a digital camera. Because the stainless steel foil used in this study was extremely thin, the temperature of the target could be assessed from the color pattern of the TLC sheet on the rear side. The recorded image was then converted from RGB (red, green, blue) to HSI (hue, saturation, intensity) color domain. Distribution of the hue was then converted to surface temperature via a calibration curve between the hue and the temperature. Correlation of the curve was found by calibration against thermocouples. Heat transfer results are presented in the form of Nusselt number. The local convective heat transfer coefficient and local Nusselt number can be determined from:
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h ¼ q=ðT w −T j Þ
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where q_ is the net heat flux supplied to the impinged wall. Tj is the jet temperature measured in the settling chamber, and Tw is the local temperature on the impinged wall measured by the TLC sheet.
Top view Chamber
2.4. Surface flow visualization with oil film technique An oil film visualization technique was applied to visualize the flow pattern on the impinged surface. In this experiment, a transparent acrylic plate was used as the impinged surface instead of a stainless foil. A mixture of liquid paraffin, powder of titanium dioxide, and oleic acid was prepared and painted uniformly on the impinged surface to form a white oil film. In an air jet impingement, the oil film was disturbed and flowed out from the jet impingement region. The oil film pattern during exposure to an impinging jet was recorded by a digital camera every 30 s.
Water pump
Fig. 5. Schematic for dye visualization of the impinging jet setup.
3.1. Free jet Fig. 6 shows the dye visualization of free jet for conventional jet and swirling jets with swirl numbers (Sw*) of 0.0 (straight tape),
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3. Results and discussion
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thickness of 250 mm, 250 mm and 0.03 mm, respectively. A TLC sheet was attached onto the rear of the stainless foil with a film binder. TLC sheet is temperature sensitive; its color changes from black, brown, red, yellow, green to blue from 29 °C to 36 °C. Direct current was supplied to the stainless steel foil from a power supply unit via copper bus bar electrodes, which were attached to the stainless steel foil and the acrylic plate. As a result, the boundary condition of uniform heat flux could be imposed on the impinged surface. The
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0.4, 0.62, 0.78, and 0.94 at Reynolds number of 760. For free jet from the conventional pipe nozzle, the jet flow is stable up to Y/D of 2. After that, it becomes unstable and consequently makes transition into a turbulent jet. Similar pattern is observed for the free jet with a straight tape inserted (Sw* = 0.0), but the ambient fluid appears as a colorless strip at the center of the jet due to partial flow blockage by the inserted tape. On the other hand, due to swirling effect, all swirling jets become unstable and make transition into turbulent flows sooner within one diameter downstream of the jet exit. As a result, they spread sooner than both free jets (without and with straight tape insertion). In addition, the spreading rate increases with increasing swirl number. It is obvious that the jet with the highest swirl number (Sw* = 0.94) spreads instantly at the nozzle exit (Y/D = 0.0). 3.2. Swirling impinging jet (SIJ) 3.2.1. Visualization with the dye technique Dye visualization of swirling impinging jets (SIJ) at different swirl numbers for the normalized nozzle-to-plate distance (L/D) of 4 for
both front and top views are shown in Fig. 7a and b, respectively. The result for conventional impinging jet (plain nozzle) is also given for comparison. The magenta and blue dye streaks show the streaklines from the right and left of the pipe nozzle, respectively. The results show that both impinging jets, in the case of plain nozzle and in the case with straight tape inserted in the pipe, emerge as laminar jet and, while some instability can be observed, practically remain so even right before the impingement. In addition, there is little indication of spreading of the jets before impingement normal to the wall. On the other hand, due to swirling effect, all swirling impinging jets transit to turbulent jets much earlier very close to the jet exit, and considerably upstream before impingement. This results in complicated characteristics of the jets and larger spread before the impingement. The degrees of jet turbulence and jet spread before impingement are observed to qualitatively increase with increasing swirl. This indicates better entrainment and mixing between the jet fluid and the ambient fluid before impingement as swirl number increases. The increase in the spreading rate of the jet with increase swirl number implies reduction in local axial velocity of the jet
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Fig. 7. Streakline visualization of conventional, splitted, and swirling impinging jets: (a) front view and (b) top view.
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Fig. 8. Surface flows on the impinged surfaces of conventional, splitted, and swirling impinging jets by using oil film technique.
For all cases, there are no swirl effects to flow pattern on the wall jet region. Since, streaklines on the impinged surface flow out in radial direction from the jet impingement region and the locations of streaklines do not change with time.
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before impingement. This further implies reduction in local axial momentum flux on impingement on the target surface as swirl number increases, with two of the streaklines impinge obliquely to the wall.
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Fig. 9. Temperature distributions on the impinged surfaces of conventional, splitted, and swirling jets at various swirl numbers by a thermochromic liquid crystal (TLC) technique.
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3.2.2. Surface flow with oil film technique Fig. 8 shows surface flows on the impinged surfaces by using oil film technique. For the jet without tape insert (conventional jet), the un-removed oil film area is located at the center of the pipe nozzle location (X/D = 0.0 and Y/D = 0.0 or r/D = 0.0) that corresponded to the stagnation point. And the region where the oil film is removed from the wall by strong shear flow corresponds to the jet impingement region. For the jet from pipe with straight tape insert (Sw* = 0.0), there are two stagnation points of un-removed oil film area and two jet impingement regions for removed oil film region. For swirling jets with low swirl number Sw* = 0.4, the un-removed oil film area is found at the stagnation points on both sides of the tape and also along the tape ridge plane. On the other hand, at higher swirl numbers (Sw* = 0.62, 0.78 and 0.94) the un-removed oil film areas are noticeably shifted from the center of the pipe nozzle location in a radial direction due to high tangential velocity of the swirling flows. For all cases of swirling impinging jet, there appear to be two jet impingement regions which extend in radial direction behind the stagnation points.
the stagnation point. Similarly, for the jet issued from nozzles equipped with straight tape (Sw* = 0.0) and twisted tape with low swirl number Sw* = 0.4, heat transfer is maximized around the stagnation point. On the other hand, when the swirl number increases, the maximum heat transfer location shifts away considerably from the stagnation point in the radial direction due to high tangential velocity components. For all swirling jets, the intensified heat transfer regions are separated due to blockage by the twisted tape ridge. The separation becomes notable as swirl number increases. It can be observed that the maximum heat transfer rates associated with the nozzles equipped with all tapes appear to be higher than that provided by nozzles without tape. This could be caused by an increase of the jet velocity and thus the flow momentum due to the decrease of nozzle cross-sectional area (blockage effect of the tape) and jet confinement by the tape ridge. However, as swirl number increases further, heat transfer rate decreases. This might be caused by subsidizing effect of the reduction of axial velocity by an increase in the spreading rate prior to impingement. 3.2.4. Nusselt number Fig. 11(a) shows the effect of swirl number (Sw*) on local Nusselt number along the X axis. For the conventional impinging jet (CIJ), the maximum Nusselt number takes place at the stagnation point at X/D = 0 as mentioned earlier. For the case of impinging jet from nozzle with straight tape (Sw* = 0.0), the maximum heat transfer rate is found on either side of the tape in the vicinity of the tape ridge and development of secondary maxima in heat transfer is also found in this case. For the swirling impinging jet (SIJ) at Sw* = 0.4, the maximum Nusselt number occurs at the stagnation point and then the number decreases monotonically along the radial direction. Heat transfer behaviors associated with swirling jets at higher swirl numbers (Sw* = 0.62, 0.78 and 0.94) are similar in patterns, as double peaks of Nusselt number are found in the impingement region. Fig. 11(b) shows the effect of swirl number (Sw*) on local Nusselt number along the Y axis. The patterns of Nusselt number contour of both jets are found to be similar to those along the X axis. On the other hand, the Nusselt number for swirling impinging jet at Sw* = 0.4 along the Y-direction is different from that along the X axis, as double peaks of Nusselt number appear. The double peaks of Nusselt number are also prevailed for jets at higher swirl numbers. The positions of the peaks seem to shift further in radial direction with increasing swirl number, which coincides with the Nusselt
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3.2.3. Local Nusselt number Heat transfer patterns on the impinged surfaces via thermochromic liquid crystal (TLC) sheet are demonstrated in Fig. 9. In the figure, the lower temperature region corresponds to the higher convective heat transfer. The results indicate that the maximum heat transfer rate is found at the center of the pipe nozzle location or the stagnation point (X/D = 0.0 and Y/D = 0.0) and diminishes along the radial direction. However, secondary peak heat transfer appears in a donutshape. This result is similar to that reported in Ref. [23], where an impingement plate was placed within the potential core of the jet. This is attributed to transition of low turbulence in the stagnation region to a turbulent wall jet. It is also observable that secondary peak heat transfer is less notable as heat flux increases. Fig. 10 shows Nusselt number distributions on the impinged surfaces obtained from thermochromic liquid crystal (TLC) sheet with an image processing technique. In the figure, the lower temperature region corresponds to the higher convective heat transfer which coincides well with the results of the Nusselt number contour on the impinged surfaces shown in Fig. 9. For the CIJ, the maximum heat transfer rate is found at the center of the pipe nozzle location, or
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Fig. 10. Nusselt number distributions on the impinged surfaces.
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a)
loss of the local momentum flux of the jets during jet dispersion prior to the impingement. The effect of SIJ on heat transfer found in the present work, partly agrees with that reported by Bakirli and Bilen [7]. The result in the previous work revealed that all SIJ provided lower heat transfer than CIJ because the experiments were performed with large normalized nozzle-to-plate distance (L/D) of 8, leading to significant local momentum loss during jet dispersion prior to an impingement, whereas the experiments in the present work were carried out at smaller normalized nozzle-to-plate distance of 4, thus the influence of jet dispersion is less significant, especially at low swirl number.
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The present paper reports visualization of SIJ along with that of CIJ. Effects of swirl number (Sw* = 0.0, 0.4, 0.62, 0.78 and 0.94) on flow and heat transfer behaviors of an impinging jet are examined. The main findings are:
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Y/D Fig. 11. Local Nusselt numbers on the impinged surfaces: (a) along the Y axis (X/ D = 0.0) and (b) along the X axis (Y/D = 0.0).
number contour. In addition, by comparing the Nusselt number distributions along the X and Y axes, it is found that the double peaks of Nusselt number along the Y axis is more symmetric than that along the X axis. Fig. 12 shows the average Nusselt numbers on the impinged surfaces in the range 0 b r b 2D, for the case of L/D = 4. Apparently, Nusselt numbers for the cases of Sw* = 0.0 (straight tape) and 0.4 are found to be enhanced over that of the CIJ. This is responsible of the increase of the jet velocity on the impingement surface due to the decrease of nozzle cross-sectional area and jet confinement by the tape ridge. However, as swirl number further increases, Nusselt number decreases. The average Nusselt number given by the SIJ at Sw* =0.62 is comparable to that provided by the CIJ while those of the SIJ at Sw* =0.78 and 0.94 are lower than that of the CIJ. Lowering of the Nusselt number is caused by
Average Nusselt number
160 140 120
L=4D 0
CIJ SIJ,Sw* - 0 SIJ,Sw* - 0.4 SIJ,Sw* - 0.62 SIJ,Sw* - 0.78 SIJ,Sw* - 0.94
100 80 60 40
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0.0
.2
.4
.6
.8
1.0
1.2
Sw* Fig. 12. Average Nusselt numbers on the impinged surfaces for jets with different swirl numbers.
1. From dye visualization, the jet delivered from the nozzle with tape insert is divided into two main streams due to the presence of the twisted tape (ridge). The degree of jet dispersion or spreading rate prior to impingement is gradually increased with increasing swirling number (Sw*). 2. Heat transfer rates of the jet with straight tape insert and the swirling jet with swirl number of 0.4 are found to be enhanced over that of the jet without tape insert (CIJ) while those of larger swirl numbers (Sw* = 0.78 and 0.94) are diminished compared to that of the CIJ. This is due to momentum distribution of the jets before an impingement. Acknowledgments The authors are grateful for the supports of this research by the Thailand Research Fund (TRF) and the Office of Higher Education Commission through grant no. MRG4980085. References [1] H. Martin, Heat and mass transfer between impinging gas jets and solid surfaces, Advances in Heat Transfer, Academic Press, New York, 1977, pp. 1–60. [2] R. Viskanta, Heat transfer to impinging isothermal gas and flame jets, Experimental Thermal and Fluid Science 6 (1993) 111–134. [3] J. Ward, H. Mahamood, Heat transfer from a turbulent swirling, impingement jets, Proceedings of the Seventh International Heat Transfer Conference, 1982, pp. 401–408. [4] L. Huang, M.S. El-Genk, Heat transfer and flow visualization experiments of swirling, multi-channel, and conventional impinging jets, International Journal of Heat and Mass Transfer 47 (1998) 583–600. [5] D.H. Lee, S.Y. Won, Y.T. Kim, Y.S. Chung, Turbulent heat transfer from a flat to a swirling round impinging jet, International Journal of Heat and Mass Transfer 45 (2002) 223–227. [6] S.Z. Shuja, B.S. Yilbas, M. Rashid, Confined swirling jet impingement onto an adiabatic wall, International Journal of Heat and Mass Transfer 46 (2003) 2947–2955. [7] K. Bakirci, K. Bilen, Visualization of heat transfer for impinging swirl flow, Experimental Thermal and Fluid Science 32 (2007) 182–191. [8] S.V. Alekseenko, A.V. Bilsky, V.M. Dulin, D.M. Markovich, Experimental study of an impinging jet with different swirl rates, International Journal of Heat and Fluid Flow 28 (2007) 1340–1359. [9] M.Y. Wen, K.J. Jang, An impingement cooling on a flat surface by using circular jet with longitudinal swirling strips, International Journal of Heat and Mass Transfer 46 (2003) 4657–4667. [10] H.Q. Yang, T. Kim, T.J. Lu, K. Ichimiya, Flow structure, wall pressure and heat transfer characteristics of impinging annular jet with/without steady swirling, International Journal of Heat and Mass Transfer 53 (2010) 4092–4100. [11] S. Eiamsa-ard, P. Seemawute, K. Wongcharee, Influences of peripherally-cut twisted tape insert on heat transfer and thermal performance characteristics in laminar and turbulent tube flows, Experimental Thermal and Fluid Science 34 (2010) 711–719. [12] P. Seemawute, S. Eiamsa-ard, Thermohydraulics of turbulent flow through a round tube by a peripherally-cut twisted tape with an alternate axis, International Communications in Heat and Mass Transfer 37 (2010) 652–659. [13] M. Rahimi, S.R. Shabanian, A.A. Alsairafi, Experimental and CFD studies on heat transfer and friction factor characteristics of a tube equipped with modified
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[19] K.V. Sharma, L.S. Sundar, P.K. Sarma, Estimation of heat transfer coefficient and friction factor in the transition flow with low volume concentration of Al2O3 nanofluid flowing in a circular tube and with twisted tape insert, International Communications in Heat and Mass Transfer 36 (2009) 503–507. [20] S. Eiamsa-ard, P. Promvonge, Thermal characteristics in round tube fitted with serrated twisted tape, Applied Thermal Engineering 30 (2010) 1673–1682. [21] S. Eiamsa-ard, P. Promvonge, Performance assessment in a heat exchanger tube with alternate clockwise and counter-clockwise twisted-tape inserts, International Journal of Heat and Mass Transfer 53 (2010) 1364–1372. [22] C. Thianpong, P. Eiamsa-ard, K. Wongcharee, S. Eiamsa-ard, Compound heat transfer enhancement of a dimpled tube with a twisted tape swirl generator, International Communications in Heat and Mass Transfer 36 (2009) 698–704. [23] S. Ashforth-Frost, K. Jambunathan, C.F. Whitney, Velocity and turbulence characteristics of a semiconfined orthogonally impinging slot jet, Experimental Thermal and Fluid Science 14 (1997) 60–67. [24] M. Angioletti, R.M. Di Tommaso, E. Nino, G. Ruocco, Simultaneous visualization of flow field and evaluation of local heat transfer by transitional impinging jets, International Journal of Heat and Mass Transfer 46 (2003) 1703–1713.
Contents lists available at SciVerse ScienceDirect
International Communications in Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ichmt
Visualization of flow and heat transfer characteristics for swirling impinging jet☆ C. Nuntadusit a,⁎, M. Wae-hayee a, A. Bunyajitradulya b, S. Eiamsa-ard c a b c
Energy Technology Research Center and Department of Mechanical Engineering, Faculty of Engineering, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand Department of Mechanical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok, 10330, Thailand Department of Mechanical Engineering, Faculty of Engineering, Mahanakorn University of Technology, Bangkok 10530, Thailand
a r t i c l e
i n f o
Available online 30 March 2012 Keywords: Heat transfer enhancement Impinging jet Swirling jet Flow visualization Twisted tape Thermochromic liquid crystal
a b s t r a c t Flow and heat transfer characteristics of swirling impinging jet (SIJ) were studied experimentally at constant nozzle-to-plate distance of L = 4D. The swirling jet is generated by inserting twisted tapes within a pipe nozzle. Effects of swirl on the impinged surface are investigated at twist ratios (y/W) of ∞ (straight tape), 3.64, 2.27, 1.82, and 1.52. The flow patterns of the free swirling jet and the swirling impinging jet were visualized by mixing dye with the jet flow. Distributions of temperature and convective heat transfer coefficient on the impinged surface were measured with thermochromic liquid crystal (TLC) sheet and image processing technique. Additionally, an oil film technique was performed as a complementary technique for flow visualization on the impinged surface. The experimental results reveal that there appear to be two peaks of heat transfer in the jet impingement region. The heat transfer enhancements in jet impingement region can be achieved at a low twist ratio of 3.64 which corresponds to the swirl number of 0.4. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Impinging jet is a jet whose axial direction is oriented toward and perpendicular to a target surface, resulting in an intensive heat transfer on the impinged area. Impinging jet has been widely used in many engineering applications such as cooling of turbine blades and electronic components, heating of steel plates, tempering of glass, drying of papers and food products [1–9]. Heat transfer enhancement of impinging jet can be made effective by proper controlling of various parameters such as nozzle-to-plate distance, jet velocity, jet array assembly (in multiple jet configurations), nozzle shape, external forcing, and imposed swirl, etc. [1-24]. In general, the flow structure of impinging jet can be classified into three regions: a free jet region, which forms around the jet exit with V(r) velocity distribution; an impingement flow region, which forms upon the jet impact and deflection; and a wall jet region, which forms due to re-acceleration of the flow along the impinged surface, as shown in Fig. 1. Large heat transfer coefficients are attainable in the stagnation region. In addition, the wall jet region, which possesses a larger transfer area, can potentially contribute to further heat transfer enhancement. To enhance heat transfer on an impinged surface, swirling jet has been adopted instead of non-swirling jet or conventional jet. Ward and Mahamood [3] showed that a swirling jet provided more uniform radial distribution of velocity than non-swirling one. They also observed that the Nusselt number decreased as nozzle-to-plate distance increased. ☆ Communicated by W.J. Minkowycz. ⁎ Corresponding author. E-mail address: [email protected] (C. Nuntadusit). 0735-1933/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.icheatmasstransfer.2012.03.002
Huang and El-Genk [4] performed heat transfer and flow visualization of conventional impinging jet (CIJ), swirling impinging jet (SIJ) with swirl angles (θ = 15, 30 and 45°) and multi-channel impinging jets (MCIJ), using smoke flow, smoke wires, and air bubble techniques in water jet visualization. Their results revealed that SIJ offered superior Nusselt number as well as radial heat transfer uniformity when compared to MCIJ and CIJ. Lee et al. [5] showed the influence of swirling jet generated through multi-narrow channels on the local and average heat transfer distribution. Their results demonstrated that swirling effect was intense at small nozzle-to-plate distance, L/D = 2, but the effect became rarely observed at L/D beyond 10. Shuja et al. [6] studied swirling confined jet impinging on an adiabatic wall. They found that the jet axis tilted toward radial direction as the swirl velocity increased and lowering of the velocity profile number enhanced the entropy generation due to heat transfer. Bakirci and Bilen [7] visualized temperature distribution and evaluated heat transfer rate of swirling impinging jet (SIJ), multi-channel impinging jet (MCIJ), and conventional impinging jet (CIJ) using thermochromic liquid crystal technique. They found that as the swirl angle increased, the radial Nusselt number distribution became more uniform and the optimum result was found at the swirl angle of 50° and jet-to-surface distance of L/D = 14. Alekseenko et al. [8] performed visualization of impinging jets using PIV and Stereo PIV techniques and found that the swirling impinging jet possessed a greater spread rate and a faster decay in absolute velocity compared to conventional jet. Wen and Jang [9] examined flat surface cooling by impinging jet issuing through two different swirling strips including a longitudinal swirling-strip and a crossed swirling-strip at various nozzle-to-plate distances. They used smoke flow visualization to
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d
free jet region
stagnation region
V(r) h
Boundary layer
Boundary layer
Z r Impingement surface Wall jet region
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Fig. 1. Flow structure of an impinging jet on a surface [24].
investigate swirling flow jet impinging at different jet to plate distances. Heat transfer from a flat surface to each impinging jet was investigated and correlation of the average Nusselt number has been derived as a function of nozzle exit-to-plate distance and Reynolds number for all nozzle types. Yang et al. [10] demonstrated detailed impinging annular jet behaviors induced by a swirling motion. They reported that at a short and intermediate separation distances (jetto-surface distance), swirling annular jet provided non-uniform wall pressure and Nusselt number distributions on the impinged surface.
Several swirl generators have been investigated for enhancing the heat transfer rate [11–23]. The features of some swirl generators are shown along with the flow pattern of a conventional impinging jet in Fig. 2. While other swirl generators have been commonly investigated, twisted tapes received less attention. This motivates the present work to investigate the use of twisted tapes as a swirl generator for heat transfer enhancement of impinging jets, especially on impinged surfaces. According to above literatures, only a few research works focused on the heat transfer enhancement mechanisms of swirling impinging jet.
Fig. 2. Geometries of various swirl generators for impinging jets: (a) SWJ [4], (b) SWJ [8], (c) impinging annular jet [10], (d) swirl jet with eight narrow channels [5], (e) multichannel conventional impinging jet [7] and (f) longitudinal swirling strips [9].
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To gain a better understanding on the behaviors of swirling impinging jet (SIJ), as well as parameters influencing the heat transfer enhancement, flow visualization of SIJ and effects of swirl number on heat transfer distributions on the impinging surface are examined. In the experiment, thermochromic liquid crystal (TLC) sheet was applied for visualization of temperature distribution on an impinged surface struck by the swirling impinging jet (SIJ). The parameters tested were of five different twist ratios (y/W): ∞ (straight tape), 3.64, 2.27, 1.82, and 1.52, corresponding to swirl numbers (Sw*) of 0.0 (straight tape), 0.4, 0.62, 0.78, and 0.94, respectively. Flow visualization of the swirling jet was conducted using the dye technique. The conventional impinging jet (CIJ) and free jet (without an impinged surface) were also investigated for comparison at similar test conditions.
Y t
y
y/W = 1.52 y/W = 1.82
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d y/W = 3.64
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Fig. 4. Injection nozzle with twisted tape (left) and twisted tapes at various twist ratios (right).
2. Experimental details 2.2. Flow visualization with dye injection in water tank 2.1. Test rig The schematic of the experimental setup is shown in Fig. 3. In the experiments, air at room temperature of about 28 °C was drawn by a 7.5 kW blower and directed to the jet chamber which acts as a reservoir. Subsequently, the air was heated by an electrical heater which was regulated by a temperature controller. Volumetric flow rate of the air through the pipe nozzle was kept constant corresponding to a Reynolds number of 20,000. The pipe nozzle, into which a twisted tape was inserted, had an inner diameter of 16.5 mm and a length of 300 mm. The outer pipe nozzle surface and the jet chamber were well insulated to minimize heat losses to the surrounding. The air delivered from the nozzle pipe was then impinged perpendicularly to the impinged surface. Details of the heat transfer and flow visualizations of the impinging jets by various techniques are described in the following section. In the experiments, the swirls were generated using twisted tapes, made from stainless steel strips of 2.5 mm in thickness, 16.5 mm in width (W), and 300 mm (L) in length, which were inserted inside the pipe nozzle. These were fabricated by twisting a straight tape about its longitudinal axis with 180° for each twist length (y = L/n, where n is the number of twist), while being held under tension. Swirl intensity is varied by changing the twisted length or the twist ratio (y/W). The twist ratios investigated are y/W = ∞ (straight tape), 3.64 (5 twists), 2.27 (8 twists), 1.82 (10 twists), and 1.52 (12 twists), resulting in swirl numbers of Sw* = 0.0 (straight tape/no swirl), 0.4, 0.62, 0.78 and 0.94, respectively, as shown in Fig. 4. The figure also outlines the coordinate system being used in this study.
The schematic dye visualization setup is shown in Fig. 5. The water tank was made of acrylic transparent plates with crosssection of 600 × 600 mm 2 and 500 mm in height. In this experiment, two colors of dye were used as tracers. The blue dye was prepared by using thymolphthalein and sodium hydroxide dissolved in water. Similarly, to prepare the magenta dye, phenolphthalein and sodium hydroxide were dissolved in water. Firstly, the jet flow was mixed with the magenta dye and issued in clear water to reveal the effect of swirl on a free jet flow. Then, two colors of the dye were injected into the clear water jet flow through two needles mounted at the nozzle exit to acquire the flow path in jet structure. In each test, water at room temperature from the reservoir water tank was pumped from the reservoir through a main pipe and then through a rotameter, the setting chamber and then the circular pipe nozzle. At the pump exit, a bypass pipe and a valve were used to adjust the flow rate. The flow pattern of the dye was recorded by a digital video camera. For the case of impinging jet, flow patterns of the dyes were recorded through the acrylic transparent wall surface in front view and top view. The nozzle-to-plate distance was kept constant at L = 4D where D is the nozzle diameter. 2.3. Temperature distribution visualization with TLC Schematic of the heat transfer experimental setup is shown in Fig. 3. In this experiment, the electrically heated impinged surface was cooled by the jet issuing from a pipe nozzle. The surface was made from a stainless steel foil (SUS304) with length, width and
W y
Fig. 3. Schematic for experimental setup.
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Digital camera
input power to the stainless steel foil was evaluated from the power supply electric current and the resistance of the stainless foil. When the temperature distribution on the impinged surface, as indicated by the color pattern of the TLC, reached a steady state, the color pattern on the TLC sheet was recorded by a digital camera. Because the stainless steel foil used in this study was extremely thin, the temperature of the target could be assessed from the color pattern of the TLC sheet on the rear side. The recorded image was then converted from RGB (red, green, blue) to HSI (hue, saturation, intensity) color domain. Distribution of the hue was then converted to surface temperature via a calibration curve between the hue and the temperature. Correlation of the curve was found by calibration against thermocouples. Heat transfer results are presented in the form of Nusselt number. The local convective heat transfer coefficient and local Nusselt number can be determined from:
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h ¼ q=ðT w −T j Þ
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Top view Chamber
2.4. Surface flow visualization with oil film technique An oil film visualization technique was applied to visualize the flow pattern on the impinged surface. In this experiment, a transparent acrylic plate was used as the impinged surface instead of a stainless foil. A mixture of liquid paraffin, powder of titanium dioxide, and oleic acid was prepared and painted uniformly on the impinged surface to form a white oil film. In an air jet impingement, the oil film was disturbed and flowed out from the jet impingement region. The oil film pattern during exposure to an impinging jet was recorded by a digital camera every 30 s.
Water pump
Fig. 5. Schematic for dye visualization of the impinging jet setup.
3.1. Free jet Fig. 6 shows the dye visualization of free jet for conventional jet and swirling jets with swirl numbers (Sw*) of 0.0 (straight tape),
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thickness of 250 mm, 250 mm and 0.03 mm, respectively. A TLC sheet was attached onto the rear of the stainless foil with a film binder. TLC sheet is temperature sensitive; its color changes from black, brown, red, yellow, green to blue from 29 °C to 36 °C. Direct current was supplied to the stainless steel foil from a power supply unit via copper bus bar electrodes, which were attached to the stainless steel foil and the acrylic plate. As a result, the boundary condition of uniform heat flux could be imposed on the impinged surface. The
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0.4, 0.62, 0.78, and 0.94 at Reynolds number of 760. For free jet from the conventional pipe nozzle, the jet flow is stable up to Y/D of 2. After that, it becomes unstable and consequently makes transition into a turbulent jet. Similar pattern is observed for the free jet with a straight tape inserted (Sw* = 0.0), but the ambient fluid appears as a colorless strip at the center of the jet due to partial flow blockage by the inserted tape. On the other hand, due to swirling effect, all swirling jets become unstable and make transition into turbulent flows sooner within one diameter downstream of the jet exit. As a result, they spread sooner than both free jets (without and with straight tape insertion). In addition, the spreading rate increases with increasing swirl number. It is obvious that the jet with the highest swirl number (Sw* = 0.94) spreads instantly at the nozzle exit (Y/D = 0.0). 3.2. Swirling impinging jet (SIJ) 3.2.1. Visualization with the dye technique Dye visualization of swirling impinging jets (SIJ) at different swirl numbers for the normalized nozzle-to-plate distance (L/D) of 4 for
both front and top views are shown in Fig. 7a and b, respectively. The result for conventional impinging jet (plain nozzle) is also given for comparison. The magenta and blue dye streaks show the streaklines from the right and left of the pipe nozzle, respectively. The results show that both impinging jets, in the case of plain nozzle and in the case with straight tape inserted in the pipe, emerge as laminar jet and, while some instability can be observed, practically remain so even right before the impingement. In addition, there is little indication of spreading of the jets before impingement normal to the wall. On the other hand, due to swirling effect, all swirling impinging jets transit to turbulent jets much earlier very close to the jet exit, and considerably upstream before impingement. This results in complicated characteristics of the jets and larger spread before the impingement. The degrees of jet turbulence and jet spread before impingement are observed to qualitatively increase with increasing swirl. This indicates better entrainment and mixing between the jet fluid and the ambient fluid before impingement as swirl number increases. The increase in the spreading rate of the jet with increase swirl number implies reduction in local axial velocity of the jet
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Fig. 7. Streakline visualization of conventional, splitted, and swirling impinging jets: (a) front view and (b) top view.
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For all cases, there are no swirl effects to flow pattern on the wall jet region. Since, streaklines on the impinged surface flow out in radial direction from the jet impingement region and the locations of streaklines do not change with time.
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before impingement. This further implies reduction in local axial momentum flux on impingement on the target surface as swirl number increases, with two of the streaklines impinge obliquely to the wall.
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3.2.2. Surface flow with oil film technique Fig. 8 shows surface flows on the impinged surfaces by using oil film technique. For the jet without tape insert (conventional jet), the un-removed oil film area is located at the center of the pipe nozzle location (X/D = 0.0 and Y/D = 0.0 or r/D = 0.0) that corresponded to the stagnation point. And the region where the oil film is removed from the wall by strong shear flow corresponds to the jet impingement region. For the jet from pipe with straight tape insert (Sw* = 0.0), there are two stagnation points of un-removed oil film area and two jet impingement regions for removed oil film region. For swirling jets with low swirl number Sw* = 0.4, the un-removed oil film area is found at the stagnation points on both sides of the tape and also along the tape ridge plane. On the other hand, at higher swirl numbers (Sw* = 0.62, 0.78 and 0.94) the un-removed oil film areas are noticeably shifted from the center of the pipe nozzle location in a radial direction due to high tangential velocity of the swirling flows. For all cases of swirling impinging jet, there appear to be two jet impingement regions which extend in radial direction behind the stagnation points.
the stagnation point. Similarly, for the jet issued from nozzles equipped with straight tape (Sw* = 0.0) and twisted tape with low swirl number Sw* = 0.4, heat transfer is maximized around the stagnation point. On the other hand, when the swirl number increases, the maximum heat transfer location shifts away considerably from the stagnation point in the radial direction due to high tangential velocity components. For all swirling jets, the intensified heat transfer regions are separated due to blockage by the twisted tape ridge. The separation becomes notable as swirl number increases. It can be observed that the maximum heat transfer rates associated with the nozzles equipped with all tapes appear to be higher than that provided by nozzles without tape. This could be caused by an increase of the jet velocity and thus the flow momentum due to the decrease of nozzle cross-sectional area (blockage effect of the tape) and jet confinement by the tape ridge. However, as swirl number increases further, heat transfer rate decreases. This might be caused by subsidizing effect of the reduction of axial velocity by an increase in the spreading rate prior to impingement. 3.2.4. Nusselt number Fig. 11(a) shows the effect of swirl number (Sw*) on local Nusselt number along the X axis. For the conventional impinging jet (CIJ), the maximum Nusselt number takes place at the stagnation point at X/D = 0 as mentioned earlier. For the case of impinging jet from nozzle with straight tape (Sw* = 0.0), the maximum heat transfer rate is found on either side of the tape in the vicinity of the tape ridge and development of secondary maxima in heat transfer is also found in this case. For the swirling impinging jet (SIJ) at Sw* = 0.4, the maximum Nusselt number occurs at the stagnation point and then the number decreases monotonically along the radial direction. Heat transfer behaviors associated with swirling jets at higher swirl numbers (Sw* = 0.62, 0.78 and 0.94) are similar in patterns, as double peaks of Nusselt number are found in the impingement region. Fig. 11(b) shows the effect of swirl number (Sw*) on local Nusselt number along the Y axis. The patterns of Nusselt number contour of both jets are found to be similar to those along the X axis. On the other hand, the Nusselt number for swirling impinging jet at Sw* = 0.4 along the Y-direction is different from that along the X axis, as double peaks of Nusselt number appear. The double peaks of Nusselt number are also prevailed for jets at higher swirl numbers. The positions of the peaks seem to shift further in radial direction with increasing swirl number, which coincides with the Nusselt
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3.2.3. Local Nusselt number Heat transfer patterns on the impinged surfaces via thermochromic liquid crystal (TLC) sheet are demonstrated in Fig. 9. In the figure, the lower temperature region corresponds to the higher convective heat transfer. The results indicate that the maximum heat transfer rate is found at the center of the pipe nozzle location or the stagnation point (X/D = 0.0 and Y/D = 0.0) and diminishes along the radial direction. However, secondary peak heat transfer appears in a donutshape. This result is similar to that reported in Ref. [23], where an impingement plate was placed within the potential core of the jet. This is attributed to transition of low turbulence in the stagnation region to a turbulent wall jet. It is also observable that secondary peak heat transfer is less notable as heat flux increases. Fig. 10 shows Nusselt number distributions on the impinged surfaces obtained from thermochromic liquid crystal (TLC) sheet with an image processing technique. In the figure, the lower temperature region corresponds to the higher convective heat transfer which coincides well with the results of the Nusselt number contour on the impinged surfaces shown in Fig. 9. For the CIJ, the maximum heat transfer rate is found at the center of the pipe nozzle location, or
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Fig. 10. Nusselt number distributions on the impinged surfaces.
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a)
loss of the local momentum flux of the jets during jet dispersion prior to the impingement. The effect of SIJ on heat transfer found in the present work, partly agrees with that reported by Bakirli and Bilen [7]. The result in the previous work revealed that all SIJ provided lower heat transfer than CIJ because the experiments were performed with large normalized nozzle-to-plate distance (L/D) of 8, leading to significant local momentum loss during jet dispersion prior to an impingement, whereas the experiments in the present work were carried out at smaller normalized nozzle-to-plate distance of 4, thus the influence of jet dispersion is less significant, especially at low swirl number.
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The present paper reports visualization of SIJ along with that of CIJ. Effects of swirl number (Sw* = 0.0, 0.4, 0.62, 0.78 and 0.94) on flow and heat transfer behaviors of an impinging jet are examined. The main findings are:
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number contour. In addition, by comparing the Nusselt number distributions along the X and Y axes, it is found that the double peaks of Nusselt number along the Y axis is more symmetric than that along the X axis. Fig. 12 shows the average Nusselt numbers on the impinged surfaces in the range 0 b r b 2D, for the case of L/D = 4. Apparently, Nusselt numbers for the cases of Sw* = 0.0 (straight tape) and 0.4 are found to be enhanced over that of the CIJ. This is responsible of the increase of the jet velocity on the impingement surface due to the decrease of nozzle cross-sectional area and jet confinement by the tape ridge. However, as swirl number further increases, Nusselt number decreases. The average Nusselt number given by the SIJ at Sw* =0.62 is comparable to that provided by the CIJ while those of the SIJ at Sw* =0.78 and 0.94 are lower than that of the CIJ. Lowering of the Nusselt number is caused by
Average Nusselt number
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1. From dye visualization, the jet delivered from the nozzle with tape insert is divided into two main streams due to the presence of the twisted tape (ridge). The degree of jet dispersion or spreading rate prior to impingement is gradually increased with increasing swirling number (Sw*). 2. Heat transfer rates of the jet with straight tape insert and the swirling jet with swirl number of 0.4 are found to be enhanced over that of the jet without tape insert (CIJ) while those of larger swirl numbers (Sw* = 0.78 and 0.94) are diminished compared to that of the CIJ. This is due to momentum distribution of the jets before an impingement. Acknowledgments The authors are grateful for the supports of this research by the Thailand Research Fund (TRF) and the Office of Higher Education Commission through grant no. MRG4980085. References [1] H. Martin, Heat and mass transfer between impinging gas jets and solid surfaces, Advances in Heat Transfer, Academic Press, New York, 1977, pp. 1–60. [2] R. Viskanta, Heat transfer to impinging isothermal gas and flame jets, Experimental Thermal and Fluid Science 6 (1993) 111–134. [3] J. Ward, H. Mahamood, Heat transfer from a turbulent swirling, impingement jets, Proceedings of the Seventh International Heat Transfer Conference, 1982, pp. 401–408. [4] L. Huang, M.S. El-Genk, Heat transfer and flow visualization experiments of swirling, multi-channel, and conventional impinging jets, International Journal of Heat and Mass Transfer 47 (1998) 583–600. [5] D.H. Lee, S.Y. Won, Y.T. Kim, Y.S. Chung, Turbulent heat transfer from a flat to a swirling round impinging jet, International Journal of Heat and Mass Transfer 45 (2002) 223–227. [6] S.Z. Shuja, B.S. Yilbas, M. Rashid, Confined swirling jet impingement onto an adiabatic wall, International Journal of Heat and Mass Transfer 46 (2003) 2947–2955. [7] K. Bakirci, K. Bilen, Visualization of heat transfer for impinging swirl flow, Experimental Thermal and Fluid Science 32 (2007) 182–191. [8] S.V. Alekseenko, A.V. Bilsky, V.M. Dulin, D.M. Markovich, Experimental study of an impinging jet with different swirl rates, International Journal of Heat and Fluid Flow 28 (2007) 1340–1359. [9] M.Y. Wen, K.J. Jang, An impingement cooling on a flat surface by using circular jet with longitudinal swirling strips, International Journal of Heat and Mass Transfer 46 (2003) 4657–4667. [10] H.Q. Yang, T. Kim, T.J. Lu, K. Ichimiya, Flow structure, wall pressure and heat transfer characteristics of impinging annular jet with/without steady swirling, International Journal of Heat and Mass Transfer 53 (2010) 4092–4100. [11] S. Eiamsa-ard, P. Seemawute, K. Wongcharee, Influences of peripherally-cut twisted tape insert on heat transfer and thermal performance characteristics in laminar and turbulent tube flows, Experimental Thermal and Fluid Science 34 (2010) 711–719. [12] P. Seemawute, S. Eiamsa-ard, Thermohydraulics of turbulent flow through a round tube by a peripherally-cut twisted tape with an alternate axis, International Communications in Heat and Mass Transfer 37 (2010) 652–659. [13] M. Rahimi, S.R. Shabanian, A.A. Alsairafi, Experimental and CFD studies on heat transfer and friction factor characteristics of a tube equipped with modified
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