An experimental study was performed on combustion of lean-and stoichiometric-premixed spays in counterflow. N-decane was used as a liquid fuel with low volatility. Flame structure and stabilization are discussed based on the flame-spread mechanism of droplet array with a low-volatility fuel. Both blue flame and yellow flame were established near the stagnation region. The flame spread among droplets and group flame formation were observed on the premixed spray side. Envelope flames were also observed on the approach air flow side. The blue-flame region consisted of the propagating premixed flame, the envelope diffusion flame around each droplet, the group diffusion flame at the initial stage, and the envelope flame around droplets passing through the group flame. The flame was stabilized within a specific range of the mean droplet diameter through the balance between the droplet velocity and flame-spread rate of the premixed spray.
The flame structure of rich-premixed-spray jets in quiescent atmosphere was experimentally studied for different overall equivalence ratios of the fuel spray/air mixture. N-decane was used as a low-volatility fuel to elucidate the local heterogeneity effect. The overall equivalence ratio of the premixed-spray jet was varied with the fuel flow rate while the air flow rate in the premixed-spray jet was kept constant. As the overall equivalence ratio was decreased from a quite large value, the combustion mode transited from the combustion with the external flame alone to that with the external flame and the internal flame. Further decrease caused disintegration of the internal flame. In this case, the internal flame oscillated largely and a part of the internal flame was sometimes disintegrated into bubble-like internal flames, which disappeared inside the external flame. The oscillation and disintegration of the internal flame was possibly caused by non-uniformity in local flame-spread rate due to non-uniform droplet spatial distribution. Characteristics of the internal flame, including transition from combustion with the internal flame to that without the internal flame, were discussed based on the analogy to the spreading flame in the premixed spray.
Effects of radiation feedback on flame spread behavior along polymer-insulated wire in microgravity are investigated numerically. Polyethylene-insulated nickel-chrome (NiCr) wire is considered as the test sample since the experimental data of this sample are available. Time-dependent, 2-D heat and mass transport processes with one-step finite rate reaction model in gas phase as well as solid phase are numerically solved. Forced weak flow is given from one end of the domain and opposed flame spread mode is of interest in the present study. Radiation energy transport from the (sooty) flame to the unburned solid is modeled with the simplest way and parametric study are performed to elucidate the importance of radiative energy transfer on the precise prediction of the flame spread behavior. It turns out that flame spread behavior in near-quiescent regime has strong sensitivity on the imposed radiation properties, suggesting that proper radiation model must be included for the precise prediction of the flame spread behavior in microgravity. In addition, our model predicts that 10 s (corresponding to the maximum microgravity duration brought by JAMIC) might be insufficient to achieve steady state in an ideal quiescent environment. To make further concern on the feasibility study of the numerical model, long-term microgravity test would be necessary.
To apply DME fuel to existing heavy oil boilers, technology that attains the luminous combustion is vital for maintaining the radiant heat transfer. Through this research, a simple method with installation of circular conic shroud on the burner tip is proposed to realize the luminous DME flame. By attaching the optimum-designed shroud, DME spray flame becomes more radiative, and the intensity of radiant heat transfer is increased. Measurements of PM (Particulate Matter) size distribution prove the formation of soot from DME in the luminous flame. Attaching a shroud induces the inhibition of mixing DME spray and circulation flow beside the burner tip, and a fuel-rich area is formed near the burner. As a result of the flow field change, in comparison to a conventional burner, a wider area for thermal decomposition of DME and formation of soot is attained. The improvement of radiant heat transfer of this luminous combustion method is confirmed by application test to the actual boiler.