Experimental study on the propagation dynamics of ammonia/hydrogen/air premixed flames in a two-stage pipe with sudden contraction

• A two-stage pipe with sudden contraction. • The γ N H 3 is negatively correlated with the roughness of the cross-sectional flame. • The effect of the pipe on pressure has changed from promoting to inhibiting. • The proportion of time through the same pipe is the same. • The first peak pressure value of a wave is correlated with the shape of the flame. This study experimentally investigates the flame propagation characteristics of ammonia/hydrogen/air in a two-stage pipe with sudden contraction by varying the equivalence ratio ( Φ ) and ammonia volume fraction ( γ N H 3 ). The results reveal that the dynamic pressure history under this configuration exhibits a distinct “three-peak, two-valley” pattern. Notably, the pressure difference between the second and third peaks (ΔP) transitions from positive to negative and decreases continuously with increasing ammonia addition, indicating that the pipe’s influence on pressure development shifts from enhancement to suppression which intensifies with higher ammonia content. For a fixed ammonia proportion, ΔP also decreases as the equivalence ratio increases. Furthermore, the relative duration of flame propagation through the three pipe segments (head, middle, and tail) remains consistent across conditions. At Φ = 0.8, the time proportions are approximately 75%, 18%, and 7%, respectively. In addition, a notable spatial discrepancy is observed between the flame position corresponding to the pressure and the actual flame front, which is governed collectively by both ammonia fraction and equivalence ratio. This study provides key experimental insights and a theoretical foundation for safety assessment and efficient utilization of ammonia fuel in complex piping systems.