## 2 Materials and Methods
### Experimental Parameters and Planning
Experiments were conducted using three different nozzle diameters (\(d\) = 25, 32, and 40 mm) with nine nozzles each, three different h/d ratios (h/d = 6, 7, and 8), and five velocity values (\(V\) = 5, 6, 7, 8, and 9 m/s). Figure 1 presents a schematic representation of the experimental setup. The system is mostly composed of aluminum and reinforced with a steel pipe structure. It consists of an impingement jet system, a data collection thermography system, heat sinks, and a thermally insulated portion fitted with heating components that provide continuous heat flux. Data are sent to a computer for storage and further analysis. The impingement jet system comprises an axial-flow fan unit, a butterfly valve, reducers, and a multi-nozzle configuration, including three distinct nozzle sizes. Air cooling is accomplished using a blower, with a butterfly valve at the fan output regulating air velocity. Air is designated as the operating fluid in the system. \begin{table}
\begin{tabular}{l  
The experimental configuration for measurements comprises a jack, a flat surface with an adjustable heat sink angle, a heating unit, a base plate, and rectangular finned heat sinks. The dimensionless h/d distances are modified using a jack positioned underneath the heating unit. The heating unit, including square base plates and finned heat sinks, measures 300 mm \(-\) 300 mm \(\times\) 170 mm. It mainly comprises an electric heater, a 50 mm-thick fireproof brick, and thermal insulation. The electric heater, situated on the fireproof brick, has dimensions of 250 mm \(\times\) 250 mm and functions at 200 W with a voltage of 50 V and a current of 4 A. The electrical power delivered to the heater is regulated by a variac transformer to maintain a consistent heat flux over the test plate. The base plate, composed of aluminum (Al 1050), is 300 mm \(\times\) 300 mm \(\times\) 5 mm. The rectangular fins affixed to the base plate measure 2 mm in thickness, 15 mm in width, and 100 mm in height, fabricated from the same aluminum alloy (see Figure 2). A thermal compound was used between the fins and the base plate, as well as underneath the fins, where they interfaced with the base plate to reduce contact resistance to HT. Fifteen thermocouples were positioned on the plate to assess the average surface temperature, two were designated for ambient temperature monitoring, and one was used to evaluate air temperature. Thermocouple measurements were obtained using a data collecting system Ordel UDL100 Data Logger (Ordel Electronic, Ankara, Turkey) and SBA200 signal converter (Ordel Electronic, Ankara, Turkey) linked to a computer, with the average values representing the steady-state surface temperature of the test surface. Copper-constantan thermocouples were calibrated in a thermostat with an accuracy of \(\pm\)0.1 \({}^{\circ}\)C prior to their use in the tests. The system was run for 190 to 200 min to achieve steady-state conditions. Average velocities were measured at the center of the jet nozzle exit using a Testo 400 anemometer (Testo SE & Co. KGa  
Figure 1: Test setup. (1) Velocity regulator, (2) fan, (3) valve, (4) regulator, (5) variac, (6) mechanism for the lifting jack and heat sink angle, (7) impingement multi-jet nozzles, (8) angularly adjustable finned heat sink, (9) thermocouples, (10) datalogger, and (11) computer. Figure 2: The general characteristics of the heat sink. The controllable parameters selected based on their potential impact on HT characteristics and the values used in the experiments are presented in Table 3.