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

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 International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:17 No:04 61 173604-8585-IJMME-IJENS © August 2017 IJENS I J E N S 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 0.000125 to 0.0005 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 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 50–80 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. International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:17 No:04 62 173604-8585-IJMME-IJENS © August 2017 IJENS I J E N S 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