Abstract:

Aiming at the problem of heat transfer enhancement outside the tube of immersed coil heat exchangers, the ultrasound is imposed into the heat exchanger by means of vibration surface, and the influence of ultrasonic field on the flow pattern, cavitation phenomenon and heat transfer enhancement outside the tube of immersed coil heat exchanger is studied. The cavitation and ultrasound propagation phenomenon are generated by ultrasonic technology. The cavitation phenomenon induces liquid-vapor phase transition near the vibration surface, while the expansion of tiny bubbles occurs away from the vibration surface. The average vapor volume fraction of the heat exchanger without the ultrasound is 0.01302, which increases to 0.01359 with the ultrasound. The ultrasound propagation makes the flow velocity of the fluid outside the tube have the same pulsating change characteristics as that of the ultrasonic wave, by which the fluid flows towards both sides of the heat exchanger with high-low velocity interval distribution. The fluid has the lowest velocity near 0, and the highest velocity of 4.93 m·s^-1, the average velocity of the whole heat exchanger increases from 0.0248 to 0.102 m·s^-1 under the significant influence of the ultrasound. Under the dual effects of cavitation and acoustic flow, the average value of turbulent kinetic energy on the outer surface of the heat exchange tube increased from 2.090×10^-4 to 0.01847 m²·s^-2, indicating that the fluid on the outer surface of the heat exchange tube was disturbed and enhanced , the convective heat transfer coefficient of the outer surface of the heat exchange tube increased from 1634.533 to 2031.069 W·m^-2·K^-1, and the heat transfer enhancement ratio reached 24.26%. This research is of great significance to the application of ultrasonic technology in immersed coil heat exchanger.

Key words: heat transfer enhancement, immersed coil heat exchanger, ultrasound, cavitation phenomenon, gas-liquid flow, hydrodynamic characteristics

摘要：

针对沉浸式换热器管外强化传热的问题，采用振动壁面的方式向换热器内输入超声波，研究了超声外场对沉浸式换热器内的管外流动、空化现象以及传热强化的作用。超声作用在流体中能够产生空化现象和声流的传播。其空化作用使得邻近振动面的流体发生液气相变，在远离振子的区域发生微小气泡的膨胀，换热器管外流体区域的平均气体体积分数由未加载超声时的0.01302最大增至0.01359。声流现象使得换热器管外流体的流速具有和超声波相同的脉动变化特性，呈高低速相间分布流向换热器两侧，最低速度接近0，最高速度4.93 m·s^-1，平均流速由0.0248 m·s^-1增至0.102 m·s^-1，超声作用效果显著。在空化和声流的双重作用下，换热管外表面湍动能均值由2.090×10^-4 m²·s^-2增大至0.01847 m²·s^-2，表明换热管外表面流体受到扰动增强，换热管外表面对流传热系数由1634.533 W·m^-2·K^-1增大至2031.069 W·m^-2·K^-1，传热强化比率达24.26%。本研究对超声技术在沉浸式换热器内的应用具有重要意义。

关键词: 强化传热, 沉浸式换热器, 超声波, 空化现象, 气液两相流, 流动特性