Effects Of Airflow On The Capture Characteristics For Cooking Contaminant

Cooking contaminant emissions have multiple adverse impact on indoor air quality and human health,so it is important to effectively discharge pollutants out of the room.Efficient and applicable kitchen exhaust technology not only maintains good indoor air quality,but also reduces exhaust system energy consumption and improves the efficiency of fume filtration equipment.Investigate the airflow and pollutant diffusion characteristics in the cooking area is not only the key to optimal air distribution and capture performance of the kitchen local exhaust system,but also a scientific problem to be solved urgently.This study first investigated the time series and size resolved distribution of particles of 0.01-10 ?m emitted from six typical Chinese dishes in an environmental controlled kitchen.The decayrate of super-micrometer was found increase with particle size.Compared to Western cooking in literature,it was found that Chinese cooking produces more super-micrometer.Moreover,this study proposed a modified indirect approach included the particles decay to obtain the capture efficiency.The unsteady flow characteristics of the cooking heat plume were studied using the pre-fed tracer particle PIV measurement method.The turbulence characteristics analysis results show that the region near the heat source and the impinging zone have higher turbulence intensity and intermittent.The Integral,Taylor and Kolmogorov spatial and temporal scales are obtained by turbulent structure analysis,which provides suitable grid size and time step for numerical simulation calculation.POD analysis revealed that the radial diffusion and contraction motion of the cooking plume to the surroundings is dominated by lateral velocity fluctuations.A PIV measurement method for the confluence flow field and particle concentration field in cooking area was established.The quantitative relationship between particle mass concentration and PIV scattering gray value was verified by experiments.Both measurement error is less than 20%,the global velocity and concentration fields of the cooking zone were measued.It is found that increasing the exhaust volume and reducing the installation height can reduce the turbulence intensity of the flow field.Also suppress the entrainment of the plume and reduce the plume perturbation escape.Based on the concentration field,the transmission rules of particles is determined.The inverse concentration decay coefficient and concentration half-width expansion coefficient both increase with particle size and concentration,and decrease with the increase of exhaust flowrate.Net escape/capture velocity(NEV)of particles was applied to discuss the relation between the contaminants transported and convection velocity.It was found that the convection velocity of the lower edge of the hood and the area near the pot should be increased to improve the system capture efficiency.Finally,the capture characteristics of the two new exhaust systems are studied.The influences of baffle ratio,different push-pull rate and operational disturbance on the capture characteristics of the exhaust system are further analyzed by the previously established measurement methods.It was found that the capture speed increase coefficient is proposed,and the optimal baffle ratio is about 2/3.The vorticity identification was applied to characterize the vorticity structure of thermal plume and capture jet.The effects of the push-pull rate and operational disturbance on the thermal plume and the capture jet entrainment are analyzed.Based on the principle of similarity criterion,an empirical mathematical model of the relationship between exhaust air volume,thermal buoyancy and capture jet characteristics is established.It is explained that the forced convection of the capture jet can effectively inhibit the development of cooking heat plume and the mechanism of improve pollutant capture efficiency.