Experimental Investigations Of Flow And Combustioin Characteristics For A Low NO_x Dual-swirl Gas Burner

As high-quality,high-efficiency and clean energy,the proportion of natural gas in primary energy increases year by year with the completion of "West-East natural gas transmission" project and the promotion of "coal-to-gas switch" policy.Except for its high combustion efficiency,low SO2 and dust emissions characteristics,the higher temperature generated from burning is suited for NOx formation.Therefore,optimal design of natural gas burners has important effects on the cleaning and efficient use of natural gas.This study has designed a new dual-stage counter-swirling nozzle that is used on new-type low-NOx gas burners,which combined air staged combustion and swirling jetting technologies.In this paper,the flow and combustion characteristics are studied experimentally with the new nozzle,and the work will provide reference to the optimal design of this type nozzles.For the reason that the measurement of the flow fields occupies an important place in the paper,the measuring error of the particle image velocimetry(PIV)system is analyzed first.This paper is organized as follows:the analysis of influence factors of PIV measuring accuracy;experimental study of the reacting and non-reacting flow characteristic of the nozzle under confined and unconfined conditions;investigation of the effects of airflow split between the two swirlers on the NOx emissions,thermal flow fields and the lean blowout limits.The main contents and conclusions of this paper are as follows:(1)PIV plays a very important role in the modern display and measurement of flow fields.In this paper,the influences of the interpretation area size and the inter-frame time interval At on the PIV measurement accuracy are conducted in an annular jet flow field test rig.Consequently,a criterion has been presented.Then the numerical simulation of annular jet flow field with standard k-? turbulent model was conducted to examine the criterion and they matched quite well.The investigation indicated that PIV measurement is quite precise with velocity error less then two percent on the condition that the fastest particles' displacement in ?t is no more than 1/5 of the side length of the interpretation area.(2)Swirlers are commonly used in combustion chamber as they provide recirculation zones and enhance flame stability.In this paper,the influences of confinement and combustion on swirling flow were investigated with a original dual-axial counter-swirling gas nozzle.The reacting and non-reacting flowfields were examined under confined and unconfined conditions using PIV.Experimental results showed that for the unconfined cases,the non-reacting flowfield exhibited an integral central recirculation zone.Upon confinement,this zone split up into two sections and corner recirculation zones occurred.Compared to the non-reacting cases,the central recirculation zone of the reacting flowfield was significantly enlarged with increased velocities and fluctuating velocities of main flow.In addition,combustion was beneficial to increase viscidity and density uniformity of airflow in the central recirculation zone,reducing its fluctuating velocities and improving its flow stability.(3)The variation of air split of dual-stage counter rotating swirlers will have an effect on the comprehensive swirl strength and recirculation zone size.It will further change the mixing uniformity of fuel and air,and finally makes a difference to the NOx emissions and the lean blowout limits.In order to determine the best project of airflow distribution,the effects of airflow split between two swirlers on the NOx emissions,thermal flow field structure and the lean blowout limits were studied experimentally with a new dual-stage counter-swirling nozzle equipped with venturi pipe type premixing section.The experimental results showed that the optimal scheme should be that the ratio of the inner swirler's airflow to the outer swirler's is equal to 1/0.67.Compared to the baseline project of the ratio equalling 1/1.38,the optimized scheme will respectively diminish the NOx emissions by 9.4%and 18.5%,with the value being 39.5mg/m3 and 33.4mg/m3 under large and small power conditions.In addition,the lean blowout limits of the optimized project are broadest and its lean blowout(LBO)equivalence ratio could be as low as zero within the range of study.