Studying of Convective Heat Transfer Over an Aluminum Flat Plate Based on Twin Jets Impingement Mechanism for Different Reynolds Number

The impact of Reynolds number, jet-to-plate spacing, and jet-to-jet distance on the distribution of local heat transfer to impinging circular air jet in an aluminium flat surface is studied through an experimental investigation. For several decades, this has been an active research topic, Based on nozzle exit condition, the Reynolds number is 17,000, 13,000 and 10,000, nozzle-tonozzle spacing 1-3 cm and jet-to-plate distance is 1, 6 and 11 cm. Graphtec GL820 multichannel data logger was used to collect thermal data, while an infrared thermal imaging technique employing Fluke Ti25 was used to obtain thermal images for capturing the temperature distribution in front of the aluminium foil. The analysis of distributions of local heat transfer is based on theoretical foretelling and experimental results of the fluid flow characteristics in different places of jet impingement. The heat flux of the jet impinging on a flat plate surface was measured through an experimental setup by employing a heat flux microsensor placed away at radial positions of 0–14 cm from the stagnation point. The heat flux measurement was considered for the calculation of local Nusselt (Nu) number for air jet impingement as well as the local heat-transfer coefficient. Based on the results, calculation of the local Nusselt number was done at all mensuration points. Moreover, in a steady jet, with increase in the Re number, the Nu number also increases. On analysing the connection between the outcomes, it was observed that higher Reynolds number led to higher localised heat flux in the air jet that was steadily heated, which was impinged on the flat surface of the plate. Also, the best heat-transfer coefficient was found at the region near to the aluminium plate and nozzles as well on the distance between the nozzles when be close, particularly at the plate’s first points, which decreased gradually with increase in the distance from the centre of the aluminium plate for all Reynolds numbers employed. Index Term-Local Nusselt number; enhancement heat transfer; twin jets impingement; Reynolds number, heat flux; INTRODUCTION An important factor that has resulted in high usage of impingement jets is the high convective heat transfer coefficient, which also enhances the overall efficiency of many applications [1-4]. Many industrial applications employ jet impingement as it allows producing high heat transfer rates. It is employed in film cooling, turbine blade cooling, bearing cooling, automobile windshield de-icing/defogging, electronics cooling, glass tempering and drying of paper [5-8]. Many studies have been published on enhancement heat transfer by impingement jet, both in terms of experimental and numerical aspects [9-11]. Most of the information that is available focuses on impinging jets’ heat transfer characteristics for normal jet impingement on a flat surface. The impact of different velocities of twin jets on the heat transfer rate number is studied through an experimental investigation by employing the twin jet impingement technique. Impinging jets have a broad range of industrial applications and are crucial in the industry for cooling and heating applications. In several applications, to achieve efficient heat transfer rates, the fluids’ thermal conductivity needs to be enhanced [12, 13]. The wide popularity of jet impingement heat transfer technique in research studies is attributed to its ability to produce high heat transfer coefficients via forced convection action. In many industrial applications, impinging jets are employed in a broad range of configurations and disciplines, such as in the food industry, textile drying, electronic chip cooling, turbine blade cooling, metal annealing and glass tempering. Comprehensive research has been done to study how applying twin impinging steady jets can affect the heat transfer and flow characteristics. Most studies are based on enhancing heat transfer by employing single and twin impingement jets [14-16]. Zulkifli and Sopian [17] conducted many experimental studies on mechanism of jet impingement heat transfer and presented the results. Three Re numbers were used for measurements, namely 16,000, 23,300 and 32,000. The recorded value of the heat flux was referred to calculate the local Nusselt number of the twin air jet impinging on a plate. A heat flux sensor was International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:17 No:06 2 170206-3434-IJMME-IJENS © December 2017 IJENS I J E N S employed to measure the heat flux. Their results showed higher values of the calculated local Nu numbers for all radial locations, even those away from the stagnation point. A high Nu number at localised radial positions can be achieved through high instantaneous velocity, as demonstrated by the velocity profile plotted in the experiment’s first part. The local Nu numbers of the pulsating and steady jets were compared by Zulkifli et al. [18] with different jet velocities, frequencies, and radial locations away from the stagnation point. The steady-state heating of a surface plate measured patterned was evaluated by Dobbertean and Rahman [19]under free liquid jet impingement. The cooled plate was applied with a constant heat flux. Re numbers were calculated, which ranged from 500 to 1,000 with different depths. The local heat-transfer coefficient decreased with increasing Re number. The heat-transfer characteristics were studied by Wang et al. [20] via impingement jet at high-temperature surface plate measured to determine the effect of surface plate temperature, jet velocity and water temperature on heat transfer characteristics for different engineering applications. Water temperature, Jet velocity and surface temperature were found to influence heat flux maximum. High-velocity small-slot jet impingement was considered for experimentally studying heat-transfer characteristics [21], which was boiled on Nano scale modification flat in an effort to increase the heat flux of critical. This also allowed investigating the quantitative effects as well as surfacedistinguishing parameters’ effect mechanism. Moreover, the jet impingement heat transfer have been investigated by authors in [22] at a concave surface of the wing leading edge (correlation development and experimental study). At the stagnation point, heat-transfer increases with the Reynolds and numbers, and at the stagnation point, the preferable heattransfer efficiency that conforms to specific operating parameters is achieved by maintaining an optimal nozzle–plate distance (H/D). The authors in [9] studied the characteristics of transient heattransfer on a surface plate by employing air-jet impingement circular. While the air jet started its impingement, the local Nusselt number increased rapidly. However, the increase in Nu speed gradually decelerated with cooling down of the impinging jet at the fifty to eighty s region. Moreover, the heat transfer and fluid flow were studied by the authors in [23] for a slot impingement jet with a little nozzle-to-plate surface spacing, which showed a secondary peak in the Nusselt number. Based on the findings, it was observed that in the stagnation point, the mean velocity profile deviated from the standard law of the wall. The Nusselt number was better when compared to the condition where there were no perturbations, and near to the location of the secondary Nu number peak, large-scale vertical structures were spotted. The impact of nozzle-to-plate surface distance on the fluid flow and heat transfer of the submerged jet impingement was studied by [24]. Based on the results, the Nu number and pressure were found to be divided into three zones. In zone I, there was a drastic increase in the pressure and Nusselt number with decrease in nozzle-to-plate distance. In zone II, it was observed that there was negligible impact of the nozzle-to-plate distance on the pressure and Nusselt number, in zone III, the pressure and Nusselt number were observed to monotonically decrease with rise in nozzle-to-plate surface spacing. The impact of frequency, amplitude and pulse shape on time-averaged and instantaneous convective heat transfer was theoretically examined by Mladin and Zumbrunnen [25] in a planar stagnation area by employing a detailed boundary layer style. In their study, they recorded the presence of a threshold Strouhal number, St>0.26, below which there would be no enhancement of heat transfer effects significantly. Moreover, comprehensive data still remain limited regarding the impact of jet impingement on local heat-transfer profiles, which are placed at radial positions at plate from the stagnation point towards the plate surface end, and additional investigation is required. The main objective of the current study is to examine the twin circular jet heat transfer characteristics for various Reynolds number as well as put emphasis on the Nu number and local heat transfer coefficient. Moreover, the Nusselt numbers on the radial distance are compared for the aluminium flat plate impingement jet heat transfer. On the stagnation point, the local Nusselt number is considered to be radially symmetrical. EXPERIMENTAL SETUP To guarantee a flat impingement surface, the aluminium foil was firmly held. A heat flux-temperature sensor and a square aluminium foil of 30×30×0.4 cm dimension were settled on the surface measured of the aluminium foil by employing a highconductivity heat sink complex and the impact of air gaps, between the sensors and aluminium surface, was minimised by applying Kapton tape. International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:17 No:06 3 170206-3434-IJMME-IJENS © December 2017 IJENS I J E N S Fig. 1. The positions of thermocouples and thermal flux sensor on an aluminium measured surface Figure 1 displays the positions of thermocouples and the heat flux (thermal flux) sensor on an aluminium plate surface. For all models, the nozzles’ arrangement is pres