Experimental Investigation on Comparison of Local Nusselt Number Using Twin Jet Impingement Mechanism

Transcription

1 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:17 No:04 60 Experimental Investigation on Comparison of Local Nusselt Number Using Twin Jet Impingement Mechanism Mahir Faris Abdullah*, Rozli Zulkifli*, Zambri Harun*, Shahrir Abdullah, Wan Aizon W.Ghopa, Ashraf Amer Abbas Department of Mechanical Engineering and Materials, Universiti Kebangsaan Malaysia, Bangi, Selangor *Corresponding Author Abstract-- Jet impingement is one of the best methods for achieving high heat-transfer coefficient over a flat plate surface. It has been an active research topic for several decades [1]. This study performed experiments on various parameters, such as nozzle-to-nozzle spacing (S/d = 1, 2, and 3 cm) and the distance between the nozzle and the aluminum plate (H/d = 1, 6, and 11 cm), to determine the effect of different Reynolds (Re) numbers using the twin jet impingement mechanism on the local heat transfer of an impinged flat aluminum plate. The same setup was used to measure the heat flux of the jet impinging on a flat aluminum plate surface. The heat flux of the heated air jet impinging on the plate was measured using a heat flux micro-sensor at radial positions 0 14 cm away from the stagnation point. The measurement of the heat flux was used to calculate the local heat-transfer coefficient and local Nusselt (Nu) number for steady air jet and air jet impingement. The Re used were 17,000, 13,000. Results show that the local Nu number was calculated at all measurement points. Furthermore, the Nu number increases with the Re number in the steady jet. The relationship between the results shows that higher flow velocity results in the higher localized heat flux of the steadily heated air jet impinging on the aluminum plate. In addition, the best heat-transfer coefficient in the area near the nozzles and aluminum plate and the nearest distance between the nozzles, especially in the first five points at the plate, decrease away from the center of the aluminum plate for all Re numbers used. Thermal data were collected by Graphtec GL820 multichannel data logger and Fluke Ti25 to capture the temperature distribution in front of the aluminum foil. Index Term-- Twin jet impingement; enhancement heat transfer; Reynolds number, heat flux; Nusselt number; stagnation point INTRODUCTION Heat transfer is in the key to increasing the efficiency of engineering applications. Jet impingement is utilized in many industrial applications for its ability to produce high heat transfer rates. It is used in inclined turbine blade, film cooling, bearing cooling, electronics cooling, automobile windshield deicing/defogging, drying of paper, and glass tempering [2-5]. Numerous studies on impingement heat transfer, both in numerical and experimental aspects, have been published [6-8]. Most of the available information on the heat transfer characteristics of impinging jets focus on normal jet impingement on a flat surface. An experimental investigation was conducted to study the effect of different Reynolds (Re) numbers of air jets on the heattransfer rate number using the twin jet impingement mechanism (TJIM). Impinging jets have wide industrial applications. They are very important in the industry for heating and cooling. In various applications, the thermal conductivity of fluids should be enhanced for efficient heat transfer [9, 10]. The jet impingement heat transfer technique has attracted considerable research interest because of the high heat-transfer coefficients produced by the forced convection action. Impinging jets are increasingly used in industrial applications over a wide range of disciplines and configurations, such as in textile drying, food industry, turbine blade cooling, electronic chip cooling, metal annealing, aircraft engine nacelle and blade, and glass tempering. Extensive research has been conducted to study the effects of applying multiple impinging steady jets on flow and heat-transfer characteristics. Many studies discussed how to enhance heat transfer using single and twin impingement jets [11-13]. Unlike the review articles by Jambunathan [14], which discussed steady impingement in remarkable detail, more studies have begun to investigate the effect of flow pulsations on heat-transfer enhancement experimentally and numerically. Sheriff and Zumbrunnen [15] experimentally discussed the effect of flow on cooling performance using jet arrays. The presence of coherent structures was observed, but no significant enhancement with respect to the heat-transfer characteristics was recorded. Zulkifli and Sopian [16] presented the results of two experimental studies on jet impingement heat transfer. Measurements were carried out with three Re numbers, namely, 16,000, 23,300, and 32,000. The local Nusselt (Nu) number of an air jet impinging on a plate was calculated from the recorded value of the heat flux. The heat flux was measured using a heat flux sensor. Their results revealed that the calculated local Nu numbers were higher in all radial positions away from the

2 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:17 No:04 61 stagnation point. High instantaneous velocity can result in a high Nu number at localized radial positions, as shown by the velocity profile plotted in the first part of the experiment. Zulkifli et al. [17] compared the local Nu numbers of the steady and pulsating jets at various frequencies, different jet Re numbers, and different radial positions away from the stagnation point. Dobbertean and Rahman [18] analyzed the steady-state heating of a patterned surface plate under free liquid jet impingement. A constant heat flux was applied to the cooled plate. Calculations were done for Re numbers ranging from 500 to 1000 and depths from to m. increasing the Renumber decreases the local heat-transfer coefficient. Wang et al. [19] studied the heat-transfer characteristics through jet impingement at a high-temperature plate surface to investigate the impact of initial surface temperature, water temperature, and jet velocity on heat transfer characteristics for various industrial applications. Heat flux maximum is influenced by water temperature, jet velocity, and surface temperature. The heat-transfer characteristics were studied experimentally [20] through high-velocity small-slot jet impingement boiling on nanoscale modification surfaces to increase the critical heat flux and to investigate the quantitative effects and the impact mechanism of surface-distinguishing parameters. Furthermore, [21] studied the jet impingement heat transfer at a concave surface in a wing leading edge (experimental study and correlation development). Heat-transfer increases at the stagnation point with the Re and <alpha> numbers, and an optimal H/d nozzle plate distance exists to achieve the preferable heat-transfer efficiency at the stagnation point that conforms to specific operating parameters. The transient heat-transfer characteristics on a flat plate using circular air-jet impingement were studied by [6]. The local Nu number rapidly increases when the air jet begins its impingement. The increase in Nu speed slows down as the impinging jet continues to cool down at the s region. Furthermore, [22] studied the heat transfer and fluid flow of a slot jet impingement with a small nozzle-to-plate spacing in which a secondary peak in the Nu number was observed. The results showed that the mean velocity profile in the stagnation point swerved from the standard law of the wall. The Nu number was better than in the case with no perturbations, and large-scale vertical structures were spotted near the location of the secondary Nu number peak. [23] studied the influence of nozzle-to-plate spacing on the fluid flow and heat transfer of submerged jet impingement. The results revealed that the Nu number and pressure are divided into three zones. In zone I, the Nu number and pressure drastically increase with the decreasing nozzle-to-plate spacing. In zone II, the effect of the nozzle-to-plate spacing is negligible on the Nu number and pressure. In zone III, the Nu number and pressure monotonically decrease with the increasing nozzle-to-plate spacing. Mladin and Zumbrunnen [24] theoretically investigated the influence of pulse shape, frequency, and amplitude on instantaneous and time-averaged convective heat transfer in a planar stagnation region using a detailed boundary layer model. They reported the existence of a threshold Strouhal number, St > 0.26, below which no significant heat transfer enhancement can be obtained. However, comprehensive data on the effect of impingement jets on local and average heat-transfer profiles at radial positions from the stagnation point to the end of the plate surface are still limited and require further investigation. The present study aims to investigate the steady twin circular jet heat transfer characteristics at different Re numbers and focuses on the local heat transfer coefficient and Nu number. Furthermore, The Nu numbers on the radial distance at the aluminum plate impingement jet heat transfer were compared. The local Nu number was assumed radially symmetrical on the stagnation point. The total heat flux was proportional to the average Nu number. EXPERIMENTAL SETUP Figure 1 shows a schematic of the experimental instruments. Compressed air was supplied from the main compressor of 4 psi (0.275 bar). The compressed air was stored in air reservoir and controlled by a ball valve. We used a refrigerated air dryer to remove moisture from the compressed air. A pressure gauge and a regulator were respectively installed to control air pressure and avoid fluctuation from the cyclic on/off of the main compressor. On the contrary, the air flow rate was measured using a digital air flow meter (VA 420, CS Instruments). The air entered the twin jet impingement mechanism (TJIM) through two identical pipe lines. Each line was controlled by a ball valve to ensure identical flow characteristics for the twin jets.

3 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:17 No:04 62 Fig. 1. Schematic of heat transfer tests and thermal imaging setup The aluminum foil was held tightly to ensure a flat impingement surface. A square aluminum foil of 30 cm 30 cm 0.4 cm and a heat flux-temperature foil sensor were fixed on the front surface of the aluminum foil using a highconductivity heat sink compound and Kapton tape to reduce the effect of air gaps between the sensor and the aluminum surface (Figure 2). Fig. 2. The locations of the thermocouples and Heat flux sensor on the flat impingement surface. Figure 3 shows the arrangement of nozzles for all models. A square aluminum foil of surface dimensions and thickness (L) was used as a jet impingement target. The locations of the heat flux sensor and thermocouples on the aluminum plate surface are shown in Figure 2.

4 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:17 No:04 63 Fig. 3. Arrange of nozzles for the 9 models In this study, we used two K-type thermocouples that were 120 mm apart and attached to an aluminum plate to monitor the plate temperature. Data from all sensors, such as temperature, static pressure (pitot tube), room humidity, atmospheric pressure, and due point, were collected by the comet model H7331 [25]. The high-thermal conductivity (k) and small thickness of the aluminum foil ensured uniform temperature distribution through the foil thickness for obtaining accurate temperature measurement at the surface [18]. Thermal data were collected by the Graphtec GL820 multichannel data logger. The Fluke Ti25 Infrared thermal imager was used to capture the temperature distribution at the front of the aluminum foil. It is suitable for different kinds of thermocouples (i.e., J, K, T, E, R, S, and B types) [17]. First, the air flow was set to achieve a Re number of 17,000 and 13,000 for each jet in the steady jet case by measuring the velocity of the twin jet center point at the nozzle exit using a pitot tube. Second, the digital airflow meter was installed in the TJIM to measure the flow rate and velocity of the steady jet flow in constant temperature mode at 100 C. The flow meter anemometer used in this experimental setup was purchased from Dantec Dynamics. This flow meter was placed between the refrigerated air dryer and the PJIM pipes passing the twin jets. Meanwhile, the run in the twin impingement jets was executed by obtaining the velocity obtained from the pitot tube, and this velocity was verified by the flow meter. Then, the highest Re number=17000 was obtained and also to capture the heat transfer per unit time (q) from the data logger and to calculate the convective heat transfer coefficient (h) by units (W/(m²K)). EXPERIMENTAL PROCEDURES AND METHODOLOGY Figure 4 shows the schematic diagram of the experimental setup. The experimental procedures were performed as follows.

5 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:17 No:04 64 Fig. 4. Schematic of twin impingement jets tests setup. Subsequently, the localized Nu number was calculated for the 15 points at a radial distance from the stagnation point on the surface measured. Third, the differential pressure provided the pressure difference as an analog used as input to data acquisition Ni 6008, converted to a signal, then convert to a value using the Scilab code developed to carry out the results. This differential pressure was set up between the pitot tube and the Ni 6008 data acquisition. Fourth, the aluminum foil was installed at 1, 6, and 11 cm away from the nozzle exit to the surface measured, and the space between the twin nozzles was 1, 2, and 3 cm, which means that nine models were built for the experimental test. This preparation was carried out to start measuring the heat flux and the surface temperature on the impingement surface. Fifth, the Fluke Ti25 Infrared thermal imager was used to capture the thermal images and temperature distribution at the surface simultaneously until the heat transfer reached the steady state. The steady heat transfer was achieved when the heat inlet to the aluminum foil by the jets equaled the heat lost by natural convection. A total of 540 samples were recorded to reduce the experimental error in heat flux temperature sensor measurements, and the average value was considered (Figure 5). Fig. 5. Twin impingement jets effect and original image of the setup Prior to the experiments, several parameters related to the TJIM and thermal imager were kept constant as listed in Table 1.

6 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:17 No:04 65 Table I Fixed value parameters Constant parameter Value Nozzle to target distance 1 to 11 cm Nozzle to nozzle distance 1 to 3 cm Reynolds number 17000,13000 Ambient temperature 24 o C Aluminium plate temperature 100 o C Emissivity of foil aluminium 0.97 Background temperature 25 o C Transmission 100% Jet impingement heat-transfer problems require fluid mechanics and heat-transfer considerations. Consequently, related dimensionless numbers should be determined. The Re number of the air jet, which relates the inertial forces due to the viscous forces of the fluid, is computed as follows [19]: Re vd (1) where μ is the dynamic viscosity of the fluid (Pa s or N s/m 2 or kg/(s)), ν is the velocity of the fluid (m/s), and ρ is the density of the fluid (kg/m 3 ). In jet impingement heat transfer, forced convection is dominated. The heat-transfer coefficient (h) could be obtained from Newton s law equation, [26] Q h( T T ), which results in s j h T s q T j where Ts is the surface temperature, T j (2) is the jet temperature, and q is the amount of heat transferred (heat flux), W/m 2. The ratio of convective to conductive heat transfer can be calculated by the Nu number equation as follows [27]: hd Nu (3) k Where h is the convective heat transfer coefficient, d is the pipe diameter, and k is the thermal conductivity of the fluid. RESULT AND DISCUSION Heat-transfer enhancement tests were carried out at nozzle-tonozzle spacing (S/d=1, 2, and 3 cm) and nozzle to plate distance (H/d=1 to 11 cm) to obtain the Nu numbers shown in Figures These results confirm that Nu number improved using the TJIM. Moreover, steady jets produce different Nu numbers using the nine models with different velocities.

7 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:17 No:04 66 Fig. 6. Nusselt number values at Model (1) Figure 6 presents the first model with the spacing between nozzles of S/d=1 cm and the distance between the nozzles and the aluminum plate surface of H/d=1 cm. The heat-transfer enhancement in the first points decreased gradually when the heat flux sensor moved away to the end of the aluminum plate until it reached around 44 at Re = 17,000 and around 41 at Re = 13,000, and the maximum Nu numbers were approximately 136 at Re = 17,000 and 122 at Re = 13,000. Fig. 7. Nusselt number values at Model (2)

8 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:17 No:04 67 Figure 7 presents the second model when the spacing between nozzles was S/d = 1 cm and the distance between the nozzles and the aluminum plate surface was H/d = 6 cm. The heat-transfer enhancement in the first four points decreased gradually when the movement of the twin jets was in the opposite horizontal direction to the end of the plate surface until it reached around 48 when Re = 17,000 and 44 when Re = 13000, and the maximum Nu numbers were approximately 136 at Re = 17,000 and 127 at Re = 13,000. Fig. 8. Nusselt number values at Model (3) The figure above displays the Nu number values when the spacing between nozzles was S/d = 1 cm and the distance between the nozzles and the aluminum plate surface was H/d = 11 cm. The heat-transfer enhancement in the first points decreased gradually when the heat flux sensor moved away to end of the aluminum plate until it reached 48 at Re = 17,000 and 41 at Re = 13,000, and the maximum Nu numbers were approximately 149 at Re = 17,000 and 136 at Re = 13,000.

9 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:17 No:04 68 Fig. 9. Nusselt number values at Model (4) Figure 9 presents the fourth model when the spacing between nozzles was S/d = 2 cm and the distance between nozzles and the aluminum plate surface was H/d = 1 cm. The heat-transfer enhancement in the first 4 points decreased gradually when the twin jets moved in the opposite horizontal direction to the end of the plate surface until it reached around 49 when Re = 17,000 and 42 when Re = 13,000 and the maximum Nu numbers were approximately 156 at Re = 17,000 and 135 at Re = 13,000. Fig. 10. Nusselt number values at Model (5)

10 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:17 No:04 69 Figure 10 presents the fifth model when the spacing between nozzles was S/d=2 cm and the distance between the nozzles and the aluminum plate surface was H/d = 6 cm. The heat-transfer enhancement in the first 4 points decreased gradually when the twin jets moved in the opposite horizontal direction to the end of the plate surface until it reached 52 when Re = 17,000 and 46 when Re = 13,000 and the maximum Nu numbers were approximately 143 at Re = 17,000 and 131 at Re = 13,000. Furthermore, the maximum Nu numbers of approximately 152 when Re = 17,000 and 136 when Re = 13,000 were obtained by the TJIM in the first 4 points at a radial distance from the stagnation point and decreased gradually to the end of the aluminum plate surface until it reached less than 50 when Re = Fig. 11. Nusselt number values at Model (6) 17,000 and 42 when Re = 13,000 at the end of the aluminum plate, when the spacing between nozzles was S/d=2 cm and the distance between the nozzles and the aluminum plate surface was H/d = 11 cm (Figure 11).

11 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:17 No:04 70 Fig. 12. Nusselt number values at Model (7) Figure 12 presents the seventh model when the spacing between nozzles was S/d = 3 cm and the distance between the nozzles and the aluminum plate surface was H/d = 1 cm. The enhancement of heat transfer in the first 4 or 5 points decreased gradually when the twin jets moved in the opposite horizontal direction to the end of the plate surface until it reached 48 when Re = 17,000 and 42 when Re = 13,000, and the maximum Nu numbers were approximately 151 at Re = 17,000 and 133 at Re = 13,000.

12 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:17 No:04 71 Fig. 13. Nusselt number values at Model (8) Figure 13 presents the eighth model when the spacing between nozzles was S/d = 3 cm and the distance between the nozzles and the aluminum plate surface was H/d = 6 cm. The heattransfer enhancement in the first points increased gradually then decreased when the heat flux sensor moved away to the end of the aluminum plate until it reached around 52 at Re = 17,000 and 48 at Re = 13,000, and the maximum Nu numbers were approximately 144 at Re = 17,000 and 129 at Re = 13,000. Fig. 14. Nusselt number values at Model (9)

13 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:17 No:04 72 Finally, the maximum Nu numbers of the ninth model when the spacing between nozzles was S/d= 3 cm and the distance between the nozzles and the aluminum plate surface was H/d = 11 cm were approximately 150 when Re = 17,000 and 133 when Re = 13,000 and obtained by the TJIM in the first 5 points at an approximate radial distance from the stagnation point and decreased gradually to the end of the aluminum plate surface until it reached less than 50 when Re = 17,000 and 43 when Re = 13,000 at the end of the aluminum plate. Figure 14 shows the Nu numbers when the spacing between nozzles was S/d = 3 cm and the distance between the nozzles and the aluminum plate surface was H/d = 11 cm.. These results are more logically on comparing with other research work. likewise to confirm the accuracy of the present work, the value of the steady jet nusselt number versus Reynolds number was plotted and Compared with the results of other previous researchers as reported by [17, 29] In summary, we can understand that the Nusselt number value presented a sensible change with increase in values of Reynolds number [14, 28]. These results are more logically on comparing with other research work. likewise to confirm the accuracy of the present work, the value of the steady jet Nusselt number versus Reynolds number Compared with the results of other previous researchers as reported by [17, 28] In summary, these results reflect the behavior of the Nu number quantitatively and qualitatively when steady and when twin jets impinge on a hot flat plate at the center line of interference zone passing to all the holes of the twin jets to the end of the surface plate. Results from the experimental data are presented in this section. Figures 15 and 16 illustrate the TJIM effect on the surface temperature measured by the heat flux-temperature sensor on the front surface and the thermal image on the surface, respectively. The surface temperature increases at the first 4 or 5 points on the plate surface and then starts to decrease after the 5-point distance on the aluminum surface plate. Presenting how TJIM affects the Nu number in the midpoint or center between the twin jets that pass to end of the interference zone at the end of the aluminum plate surface is important. The figures present the influence on the Nu number based on micro foil sensor measurements. The Nu number was recorded and then decreased away from the center of the aluminum plate surface to the end of the surface (low rates at distant points from the center of the surface). This result is logical because in the current experiment the heat-transfer rate increases as long as the twin jets are close to the surface and when the heat flux sensor is under the direct impact of the twin jets air flow on the surface. It gradually decreases as we move away from the center of the interference zone. Figures 15 and 16 show the images captured by the thermal imager. These thermo-images represent the effect and distributions of TJIM on the surface of the impinged target. The center of the temperature values are labeled in the images. The steady jet cases are illustrated below for the nine models, respectively. Furthermore, we captured 18 pictures for all models, one picture for every model at Re= and Some observations can be recognized in these images. First, the hottest spots due to the effect of twin jets impingement can be clearly seen. Then, an elliptical temperature distribution after the midlevel point of temperature can be observed. Moreover, the steady jet case achieved higher temperature rates due to the high flow rate of the jets. In contrast, the lower flow rate was supplied in TJIM given its duty principles. Moreover, the higher center temperature of C in Figure 15 was produced by the fourth model when H/d = 1 and S/d = 2. Furthermore, twin jets exchange their superiority in having the highest temperature because of the high sensitivity of the thermal imager to the minimal changes in the temperature between both jets. See Figure 15 below.

14 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:17 No:04 73 Fig. 15. Thermographic temperature distribution on Aluminuim foils Model (1) to (9) at Re=17000 In Figure 16, the steady jet case shows the heat-transfer behavior and thermal distributions. We have captured nine pictures for all models. Some observations can be recognized in these images. First, the hottest spots due to the effect of twin jets impingement can be seen clearly. Then, an elliptical temperature distribution after the midlevel point of temperature can be observed. The steady jet case achieved higher temperature rates because of the high flow rate of the jets. In contrast, the lower flow rate was supplied in TJIM given its duty principles. Moreover, the higher center temperature of C in Figure 16 was produced by the fourth model when H/d = 1 and 6 and S/d = 2.

15 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:17 No:04 74 Fig. 16. Thermographic temperature distribution on Aluminuim foils Model (1) to (9) at Re=13000 CONCLUSION The present study extensively investigated the impact of twin jet impingement heat-transfer mechanism for heat-transfer enhancement. The present investigation included heat flux temperature micro foil sensor measurements and IR thermal imaging. The results revealed a significant enhancement in the localized Nu number of the steady flow at positions of radial distance on the aluminum measured surface 1 5 cm at Re numbers of 17,000 and 13,000 and gradually decrease as we move away from the center of the interference zone. Subsequently, the thermography capturing process was carried out on the surface of the aluminum foil flat target while the heat flux temperature data were collected for the nine models at different Re on the impinged surface of the target. The results revealed logical behavior for all parameters under consideration. The identical effect of twin jets verifies the performance of twin jets system which was designed to generate identical two jets. Moreover, the distance between nozzles and the spacing between jet and nozzles can be considered the optimum condition for achieving higher heat transfer rates for the present problem. In conclusion, the variously presented results could describe the effect of the nine models on heat-transfer characteristics of TJIM that may contribute to the performance improvement of various industrial and engineering applications. ACKNOWLEDGEMENTS We would like to thank the financial supports provided by FRGS/1/2013/TK01/UKM/02/3, FRGS/1/2016/TK03/UKM/02/1 and Prof. Dr. faris Abdullah Aljanaby. REFERENCES [1] Penumadu, P.S., and Rao, A.G.: Numerical investigations of heat transfer and pressure drop characteristics in multiple jet impingement system, Applied Thermal Engineering, 2017, 110, pp [2] Ibuki, K., Umeda, T., Fujimoto, H., and Takuda, H.: Heat transfer characteristics of a planar water jet impinging normally or obliquely on a flat surface at relatively low Reynolds numbers, Experimental thermal and fluid science, 2009, 33, (8), pp [3] Baffigi, F., and Bartoli, C.: Heat transfer enhancement in natural convection between vertical and downward inclined wall and air by pulsating jets, Experimental Thermal and Fluid Science, 2010, 34, (7), pp [4] Attalla, M., and Salem, M.: Experimental investigation of heat transfer for a jet impinging obliquely on a flat surface, Experimental Heat Transfer, 2015, 28, (4), pp [5] Harinaldia, D.R., and Defriadic, R.: Flow and Heat Transfer Characteristics of an Impinging Synthetic Air Jet under Sinusoidal and Triangular Wave Forcing [6] Guo, Q., Wen, Z., and Dou, R.: Experimental and numerical study on the transient heat-transfer characteristics of circular air-jet impingement on a flat plate, International Journal of Heat and Mass Transfer, 2017, 104, pp [7] Roy, S., and Patel, P.: Study of heat transfer for a pair of rectangular jets impinging on an inclined surface, International Journal of Heat and Mass Transfer, 2003, 46, (3), pp

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