Flow Boiling and Heat Transfer of N-heptane Flow in a Microtube Heated by Concurrent Microflame

Micro-diffusion flames have received much research attention due to their use in micro-energy systems. Experiments are performed with liquid heptane fuel at room temperature to assess the effects of fuel flow rates on micro-diffusion flame characteristics, wall temperature, and evaporation phenomena. At low fuel flow rate (≤10 µl/min), a steady, laminar flame with a spherical shape is formed, the maximum wall temperature recorded was 1050 K, a stable flow regime was observed with a dynamic meniscus, and the liquid fuel evaporated completely at the interface. At a medium fuel flow rate (30 µl/min), an unsteady flame with periodic patterns was formed, the maximum wall temperature dropped to 970 K, and the flow regime became unstable with liquid droplets in vapor regime. At a high fuel flow rate (≥50 µl/min), an explosive flame with an erratic pattern was obtained, the maximum wall temperature substantially decreases to 890 K, and an explosive boiling regime prevails with nucleate bubbles in the downstream region. The flame instability relies on meniscus oscillation and liquid droplets variation at low and high fuel flow rate, respectively.