Liquefied natural gas (LNG) is stable and safe, which is why the natural gas is usually liquefied before transported. The heat exchanger is widely used as the key component of vaporizing LNG, and it is composed of a large number of microchannels. This paper mainly analyzes the flow of supercritical LNG in a spiral microchannel, and compares the flow and heat transfer characteristic of spiral microchannel with different pitch. The result was indicative that with the lessen of pitch, the heat transfer is improved, but the flow characteristic is decreased. Compared with the straight channel, the spiral channel with appropriate pitch value can markedly improve the heat transfer properties, but has less effect on the flow characteristic. The discussion also includes the flow and heat transfer of microchannel with different mass flux and heat flux.
The flow fields, emission levels, and static stability characteristics were investigated experimentally under various air split ratios (ASR, the ratio of the pilot stage air mass flow rate to the total air mass flow rate) at a fixed equivalence ratio of 0.8 of both main and pilot stages in a premixed centrally-staged swirl flame. The flame structures were captured by a CH* chemiluminescence high-speed camera and the corresponding results were processed by Abel deconvolution. Besides, the flow fields obtained by using planar Particle Image Velocimetry (PIV) technique were combined with flame structures to make a better study on the aerodynamic structures of the centrally-staged swirl flames. The emission levels of NOx and CO were measured by a gas analyzer. The stability boundaries and flame structures at different equivalence ratios under three ASRs were also studied.
It is found that the size of the reacting primary recirculation zone (PRZ) becomes larger as more air is distributed to the pilot stage. This can be explained by the fact that the majority of the pilot fluid participates in the formation of the PRZ and also as a result of a stronger penetrability of the pilot jet. Moreover, the NOx emission levels increase while CO levels decrease, which is because of the longer residence time of the radicals within a larger PRZ and less impingement of the main flame on the combustor liner. Finally, the stability boundary is extended, and the total blowout equivalence ratio was decreased as the air split ratio increases, which demonstrates the flame stabilization effect of the pilot flame. In brief, the above findings can be a help to choose the appropriate air split ratio in the early design stage of the centrally-staged aero-engine combustors.
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