Showing papers by "Christos N. Markides published in 2009"

Experimental investigation of the effects of turbulence and mixing on autoignition chemistry

Abstract: The autoignition of acetylene, released from a finite-sized circular nozzle into a turbulent coflow of hot air confined in a pipe, has been the subject of a recent experimental study to supplement previous work for hydrogen and n-heptane. As with hydrogen and n-heptane, autoignition appears in the form of well-defined localized spots. Quantitative information is presented concerning the effects of turbulence intensity, turbulent lengthscale and injector diameter on the location of autoignition. The effects of these parameters on inhomogeneous autoignition have not been investigated experimentally before. The present study establishes that increasing the bulk velocity increases the autoignition length, as was reported for hydrogen and n-heptane. For the same turbulence intensity, the autoignition length increases as the injector diameter increases and as the turbulent lengthscale decreases. A simultaneous decrease in turbulence intensity and increase in lengthscale causes a reduction in autoignition length. Further, the frequency of appearance of the autoignition spots has also been measured. It is found to increase when autoignition occurs closer to the injector, and also at higher velocities. The observed trends are consistent with expectations arising from the dependence of the mixture fraction and the scalar dissipation rate on the geometrical and flow parameters. The data can be used for the validation of turbulent combustion models.

Ventilation characteristics of the built environment and their effects on the urban microclimate

Abstract: The thermal conditions within cities, as well as the energy efficiency of buildings, are determined by the urban climate, which is in turn influenced by the thermal-fluid characteristics of the built environment and particularly the transfer effects to the atmosphere. A good example of this interaction is the urban heat island phenomenon. In this case heat generated from urban activities has a direct impact on the urban microclimate, resulting in increased night time temperatures and a reduction in the observed temperature range. In order to understand the interaction between the built environment and the urban microclimate, one needs to consider: (1) the energy efficiency of the built environment and mainly the phenomena of heat transfer through radiation; and (2) the ventilation capability of the urban environment and the effects of different building geometries. The study presented in this paper is concerned with the ventilation of the built environment and involves Particle Image Velocimetry (PIV) measurements under street canyon flow conditions in a flow channel.