High-temperature, high-pressure burning velocities of expanding turbulent premixed flames and their comparison with Bunsen-type flames

Abstract

This paper reports high-temperature/pressure turbulent burning velocities and their correlation of expanding unity-Lewis-number methane/air turbulent flames, propagating in near-isotropic turbulence in a large dual-chamber, constant-pressure/temperature, fan-stirred 3D cruciform bomb. A novel heating method is used to ensure that the temperature variation in the domain of experimentation is less than 1°C. Schlieren images of statistically spherical expanding turbulent flames are recorded to evaluate the mean flame radius 〈R(t)〉 and the observed flame speeds, d〈R〉/dt and SF (the slope of 〈R(t)〉), where SF is found to be equal to the average value of d〈R〉/dt within 25mm ≤ 〈R(t)〉 ≤ 45mm. Results show that the normalized turbulent flame speed scales as a turbulent flame Reynolds number ReT,flame=(u'/SL)(〈R〉/δL) roughly to the one-half power: (SLb)−1d〈R〉/dt ≈ (SLb)−1SF=0.116ReT,flame0.54 at 300K and 0.168ReT,flame0.46 at 423K, where u′ is the rms turbulent fluctuating velocity, SL and SLb are laminar flame speeds with respect to the unburned and burned gas, and δL is the laminar flame thickness. The former at 300K agrees well with Chaudhuri et al. (2012) [16] except that the present pre-factor of 0.116 and ReT,flame up to 10,000 are respectively 14% and four-fold higher. But the latter at 423K shows that values of (SLb)−1d〈R〉/dt bend down at larger ReT,flame. Using the density correction and Bradley's mean progress variable 〈c〉 converting factor for schlieren spherical flames, the turbulent burning velocity at 〈c〉=0.5, ST,c=0.5≈ (ρb/ρu)SF(〈R〉c=0.1/〈R〉c=0.5)2, can be obtained, where the subscripts b and u indicate the burned and unburned gas. All scattering data at different temperatures for spherical flames can be represented by ST,c=0.5/SL=2.9[(u′/SL)(p/p0)]0.38, first proposed by Kobayashi for Bunsen flames. Also, these scattering data can be better represented by (ST,c=0.5-SL)/u'=0.16Da0.39 with small variations, where the Damköhler number Da=(LI/u′)(SL/δL) and LI is the integral length scale.