Abstract:

There are many strong endothermic chemical reactions achieved by super-high temperature, they need quickly quenching to avoid reverse reaction for obtaining substantial yield. Based on our experiments of CO₂ pyrolysis by thermal plasma, where a non-conventional quenching was achieved with setting a converging nozzle at the exit of reactor to lead the pyrolysis gas at high speed into cooling tube, the reverse reaction CO+O=CO₂ was unusually suppressed and very high CO₂ conversion was achieved. To understand the mechanism of the novel quench phenomena, a CFD Simulation was carried out. It verified that a quenching rate of 10⁷ K·s^-1 could be expected, but the quench phenomena cannot be understood only by gas dynamics principle. A deep analysis on simulation revealed that converging nozzle resulted in viscous fluid strong rotating eddy in the cooling tube, it is the strong rotating eddy that enhance greatly both of the fluid entrainment into body jet and the forced heat transfer of the fluid at the cooling tube before entrainment.

Key words: gas dynamics, converging nozzle, quenching, numerical simulation, CFD

摘要：

在许多强吸热化学反应的化工过程中，常常需要对反应流体流出反应器时进行快速急冷来避免副反应或逆反应发生，以期最终获得可观的目标产物。在本实验室前期开展的热等离子体裂解二氧化碳实验研究中，采取在高温反应器出口加装收缩喷管将裂解气高速导入夹套水冷管的方法，实现了对高温裂解气的快速急冷，显著地避免了裂解气中CO与O的逆反应，获得了意想不到的CO₂高转化率。本文利用计算流体力学软件模拟这一过程，以期揭示这种新的冷却方法导致极快速冷却的机制。模拟结果表明，加装收缩喷嘴确实可以期待对高温射流产生10⁷ K·s^-1量级的温降速率。深入分析表明，仅仅靠气体动力学效应不能完全解释如此快速的冷却速率。从喷管高速喷出的黏性流体在夹套水冷管内形成高速涡流，这种涡流一方面增强了主流体对周围气体的卷吸，另一方面加强了被卷吸流体在被卷入之前与夹套水冷管壁面的强制换热过程，是导致快速急冷的主要机制。

关键词: 气体动力学, 收缩型喷嘴, 急冷机制, 数值模拟, 计算流体力学