Experimental investigation of the turbulence structure of medium-scale methanol pool fires

Abstract

An experimental investigation of the turbulence structure of a medium-scale methanol pool fire has been undertaken to provide further insight into the complex physical phenomena which drive mixing and entrainment and thereby control development of the fire flow field. Laser Doppler anemometry was used to measure radial and axial components of velocity in the flaming zones of a 31-cm-diameter methanol pool fire. Temperature was measured simultaneously using fine wire thermocouples. Mean and rms values, correlation coefficients, turbulence scales, and autospectra of both components of velocity and compensated temperature are presented. Turbulent energy production, turbulent Reynolds and Prandtl numbers and other quantities of interest in the description of the nature of the turbulent fire flow field are given. Through the results, a consistent quantitative description of the fire flow field is developed, with much new information contained in the correlations and turbulence quantities. The temporal coupling between the formation and shedding of the characteristic large-scale structures in the fire, the air entrainment and the flapping flame fronts at the base of the fire is described through interpretation of the distributions of mean velocity, turbulence correlations and autospectra of velocity which are based on time averaged statistics. The flow is found to develop in a manner similar to a buoyant shear flow, but approaches classical shear flow behavior only in strongly entraining regions and in the thermal plume far above the fuel surface. High negative values of flux Richardson number suggest similarity to some aspects of large-scale environmental flows, while low values of turbulent Reynolds and Prandtl numbers are consistent with the notion of a rapidly developing, buoyancy-driven turbulent flow field. The results provide a comprehensive set of data useful in the assessment of the attributes and limitations of assumptions in existing flow models and in the validation of numerical predictions of the pool fire flow field.