Greenhouse gas emission rate under discharge condition by numerical simulation

Abstract

当水流通过泄洪建筑物下泄时，水体中所溶解的温室气体（二氧化碳（CO₂）、甲烷（CH₄）等）会因为所受压力的瞬间改变而导致溶解度降低，从而造成气液之间传质的发生及水中温室气体的排放.然而，目前对于泄流条件下水中温室气体排放的研究还较为缺乏.鉴于原型观测与模型试验的局限性，本文建立了大坝泄流条件下温室气体排放速率的数学模型，模型基于VOF （volume of fluid）气液两相流模型，考虑了温室气体在过坝下泄过程中发生的气泡传质和自由液面传质.本文以温室气体CO₂和CH₄为研究对象，分别通过模拟溶解态CO₂和CH₄在过流坝面和空中挑射过程中及在坝下消力池或水垫塘内的输移扩散，计算得到CO₂和CH₄在水中的浓度分布及在不同上游来流的温室气体浓度工况下的坝下温室气体排放速率.模拟结果表明，坝下温室气体在水中的浓度分布主要受到上游来流浓度大小、气液传质的发生及溶解气体输移扩散的影响.其中，上游来流的温室气体浓度大小为影响坝下温室气体排放速率的主要因素.本研究为明确不同泄洪方式下的温室气体排放速率的大小和科学评估水电碳足迹提供了新的研究思路和技术基础.;The solubility of dissolved greenhouse gas (GHG), such as carbon dioxide (CO₂) and methane (CH₄) in water will be decreased under discharge conditions due to the sudden change of pressure, which will lead to gas-liquid mass transfer and GHG emission. However, the research about GHG emission under flood discharge conditions is still rare. Considering the limitation of prototype observation and model experiment, a mathematical model for GHG emission rate under flood discharge conditions was built in this paper. The model is based on the VOF (volume of fluid) method and takes into account the bubble mass transfer and free surface mass transfer. We simulated the transport and diffusion of dissolved CO₂ and CH₄ in the spillway and jet flow respectively, and calculated their concentration distribution and emission rate. The simulated results show that the concentration distribution of GHG downstream of the dam is mainly affected by the upstream inflow concentration, gas-liquid mass transfer, the transport and diffusion of dissolved GHG. Among them, the concentration of GHG in the upstream flow is the main factor affecting the emission rate downstream of the dam. This study provides a new research approach and technical base for clarifying the amount of GHG emission rate under different discharge types, and scientifically evaluating the hydroelectric carbon footprint.