## 2 Experimental Setup and Methodology  
Experiments were conducted in a high-pressure constant volume combustion chamber (CVCC) at the Mechanical Engineering Department of the University of Babylon, Iraq. A controllable injection system was manually and automatically controlled based on pre-calculated partial pressure data. The experimental setup is depicted in Figure 1, where direct measurements of the laminar flame speed were obtained for different fuel blending ratios. The chamber had an inner diameter of 305 mm, a length of 400 mm, and a thickness of 10 mm. Its volume was approximately 29 L. The mixture was ignited centrally using two spark electrodes with a diameter of 1 mm and a spark gap of 1.5 mm. An electronic circuit was employed to control the timing and power delivery, ensuring a strong spark. Two optical quartz windows, 120 mm in diameter and 20 mm thick, were positioned opposite each other to allow for monitoring and recording of flame propagation. A schlieren technique was utilized, using two lenses of plano-convex type with a high-speed camera (AOS-QPRI-Switzerland) with \(800\times 800\) pixels. A controllable light source was used to visualize the flame. Methane and ammonia were used as the main blending fuels; hence, this mixture blends with hydrogen, and the air was used as the oxidizer. A four-way manifold containing four solenoid valves was used to control the process of the mixture preparation; gaseous fuels, air injection, and exhaust gas scavenging. An electrical control board was utilized to  
Figure 1: Schematic of the experimental setup. facilitate and regulate the experiments accurately by controlling the solenoid valves and regulating, controlling, and measuring the voltage entering the transformer. Furthermore, it was used to control and measure the initial pressure, temperature, and heating system. In this study, the fuels can be blended according to two fractions of the fuel. Ammonia is the main fuel blended with methane based on the ammonia mole fraction (\(X_{NH_{3}}\)) as shown in Equation (1). Moreover, hydrogen can be blended based on the hydrogen mole fraction (\(X_{H_{2}}\)), computed as shown in Equation (2). \[X_{NH_{3}}=\frac{V_{NH_{3}}}{(V_{CH_{4}}+V_{NH_{3}})} \tag{1}\]  
\[X_{H_{2}}=\frac{V_{H_{2}}}{(V_{CH_{4}}+V_{NH_{3}}+V_{H_{2}})} \tag{2}\]  
where \(V_{x}\) is the volume fraction of species \(x\) in the initial unburned fuel-air mixtures. The experimental conditions for the mixing are summarized in Table 2. All experiments were conducted at an unburned mixture temperature of 298 K and variations in the temperature were kept within \(\pm\)2 K, which was monitored by a thermocouple; the initial mixture pressure of 0.1 MPa was measured and monitored by a pressure transmitter fixed on the combustion chamber. The main reasons for performing the experiments at an initial pressure of 0.1 MPa and an initial temperature of 298 K was to inhibit the development of Darrieus-Landau (DL), and due to the hydrodynamic instability of the finite flame thickness in relation to the flame size. The methane and ammonia blending ratio in this study varied from 100% to 0% with a 20% increment. The hydrogen additions to the main fuel were 0%, 20%, 40%, and 100%. The equivalence ratio was from 0.8 to 1.2, which is calculated according to the following relation:  
\[\left[\gamma\mathrm{H}_{2}+\beta\mathrm{NH}_{3}+\sigma\mathrm{CH}_{4}+\frac{0.5 \gamma+0.75\beta+2\sigma}{\phi}(\mathrm{O}_{2}+3.76\mathrm{N}_{2})\longrightarrow Product\right] \tag{3}\]  
To start each experiment, the initial step involves a vacuum process to purge the chamber of any residual gases. Following that, the flushing process is carried out to introduce fresh air into the chamber.