A Study on Comparison of Lewis-number-induced with Buoyancy-driven Self-excitation in Laminar Lifted Coflow-Jet Flames

A study on laminar jet flames in coflow air diluted with helium has been conducted to investigate self-excitations for various propane mole fractions and nozzle exit velocities. The stability map was represented as a function of nozzle exit velocity and fuel mole fraction for propane. The results show that two types of self-excitation were observed : (1) buoyancy-dreiven self-excitation and (2) Lewis-number-induced self-excitation coupled with (1) near extinction limit for 9.4mm nozzle diameter. It was shown that with 0.95mm nozzle diameter, Lewis-number-induced and buoyancy-driven self-excitations could be sperated. The differences between the two self-excitations were addressed and discussed. Introduction Since the first spot of laminar tribrachial flame in lifted jet configuration [1, 2], the behaviors of freeand coflow-jet flames have been successfully described by the cold jet similarity concepts [2]. Such a laminar lifted flame in freeand coflow-jet configurations propagates along a stoichiometric contour due to the intrinsic nature of tribrachial structure such that the flame speed is balanced to the local flow one. The tribrachial flame speed depends upon the mixture strength, Lewis number, heat loss, fuel concentration gradient, and buoyancy. Because of such a stabilization mechanism of tribrachial flame, the lifted flame could be self-excited by some factors of them that modify the edge flame speed, i.e. buoyancy due to a flame flicker, the repetitive interaction of burning rate and buoyancy-driven convection and conductive heat loss from premixed wings to trailing diffusion flame. Won et al [3] and Füri et al [4] investigated selfexcitation with the similar frequencies of O(1) Hz in laminar lifted coflow-jet flames, and identified as buoyancy-induced and Lewis-number-induced selfexcitations (hereafter called BDSE and Le-ISE), respectively. The extended work through the comparison of normaland micro-gravity experiments as well as numerical simulation verified that the self-excitations were caused by the repetitive interaction of burning rate and buoyancy-induced convection [5]. It has been believed to preclude all doubts since that. Nonetheless, it was noted that the self-excitation of tribrachial flame due to Lewis number in 2D mixing layer configuration had been described well numerically in zero gravity [6]. It was also recognized that heat-lossinduced selfexcitation (hereafter called HLISE) (f < 0.1 Hz) suppressed the BDSE in nitrogen-diluted non-premixed free-jet propane and butane flames [7]. With those backgrounds, Lee et al tried to distinguish the LeISE  Corresponding author: jeongpark@pknu.ac. kr Proceedings of the European Combustion Meeting 2015 from the BDSE in nitrogen-diluted laminar nonpremixed free-jet flames with an applied DC electric field of VDC = -10 kV [8]. Applying the DC electric field increased tribrachial flame speeds significantly and thereby forced the flame to be attached to the nozzle, such that HLISE could not appear. They also found that the Le-ISE can be suppressed evidently by the BDSE in the existence of buoyancy. By applying a horizontal injection method (in order to eliminate the accumulated partially premixed mixture in front of tribrachial flame and thereby buoyancy effect), it was shown that the Le-ISE could be separated from the BDSE, and also found that characteristics of the Le-ISE were very similar to those observed numerically by Kurdyumov et al [9, 10] in 2D mixing layer configuration. However, disadvantage of the experimental study was to apply the DC electric field and also to observe the Le-ISE not in lifted flame but in attached flame. Then, further experimental efforts may be required to find the existence of Le-ISE without applying electric fields and to further embody the characteristics in laminar lifted jet flame. Experimental facility The experimental apparatus schematically shown in Fig. 1 consisted of a coflow-jet burner, mass flow controller, a digital camera system and a visualization system. Two types of fuel tube nozzle (9.4 and 0.95 mm in diameter) in coflow-air jet configuration with 60.0 mm Figure 1. Schematic experimental setup and flow system about coflow-jet burner configuration. in outer diameter were used. Propane with a purity of 99.99 %, and helium and nitrogen with purities of 99.99 and 99.95 % were used, respectively. The fuel tube 100 cm in length such that the internal flow can be fully developed. The flow rates were controlled precisely by using mass flow controllers with maximum flow capacities of 50, 100, 500, 20000, 50000 ml/min, and a Flow Manager software (version 3.2). Their nominal accuracies for the full-scale flow rate were within of 1.0 %. A series of glass beads were installed in the lower half of the compartment to suppress external disturbances and to obtain uniform outer jet flows. A cylindrical acrylic compartment with 10 cm diameter and 40 cm length was used to reduce external derangement as well. Experiments were conducted by adding helium with a high thermal conductivity to the outer coflow air to control heat losses from the flames to ambience. Compressed air whose humidity (41 ~ 43 %) and temperature (23 oC) were controlled in a constant temperature, humidity room was also used to eliminate the uncertainty. The lift-off height was measured with a digital camera (SONY,HDR-CX560) attached to a 2-D transfer device. A Matlab-based code was used to analyze flame images. Flame length and lift-off height were defined as the brightest point in converted graylevel images of flame edge. Results and discussion Experiments in propane jet flames diluted with nitrogen were performed for various nozzle exit velocity UO, fuel mole fraction XF,O and coflow VCO was fixed at 9.4 cm/s. Figure 2 shows flame stability map as functions of UO and XF,O with D = 9.4 mm. Two types of selfexcited flames were observed: (I) BDSE existed at XF,O > 0.1 and (II) Le-ISE coupled with BDSE (hereafter called LCB, regime II) appeared at 0.1 < XF,O < 0.135, similarly to those in the previous studies. Stoichiometric laminar flame speeds were in range of 20.6 – 26.0 cm/s at 0.1 < XF,O < 0.135, and the nozzle exit velocity was also smaller than the stoichiometric laminar flame speeds. In this situation, the BDSE could appear due to the repetitive interaction of burning rate and buoyancy-induced convection [4]. Meanwhile the LCB was observed at very low XF,O (highly diluted with nitrogen), such that selfexcitation appeared just prior to flame extinction when fuel Lewis number was much larger than unity and radiation of heat loss was excessive near the extinction Damköhler number [4, 9]. Based on Figure 2. Flame stability map as a function of exit velocity and fuel mole fraction for D = 9.4 mm, and VCO = 9.4 cm/s. Figure 3. Various flame dimensions of BDSE (a) at XF,O = 0.13, UO = 7.25 cm/s, and VCO = 9.4 cm/s, and LCB (b) at XF,O = 0.10, UO = 5 cm/s, and VCO = 9.4 cm/s. aforementioned study, the Damköhler numbers of LCB and BDSE may be required to be compared as well in the current study. Here, the Damköhler number was defined as: