Visualization of bubble flow in the channel of a dimple-type embossing plate heat exchanger under different fluid inlet/outlet ports

Abstract

The plate heat exchanger, which uses air and water mixture as the medium of cold flow, is efficient, compact and widely used heat exchange equipment. Its heat transfer efficiency is directly related to the air bubble flow in the cold flow. Considering different positions of fluid inlet and outlet ports, such as the upper or lower inlet combined with the upper or lower outlet, a high-speed camera was employed to visualize and analyze the air bubble movement in a flow passage machined between two transparent dimple-type embossing plates, including the bubble trajectory, distribution, and size, etc. Results showed that regardless of the inlet and outlet ports for the fluid, a faster flow velocity of air and water mixture has a larger Reynolds number, Froude number, Weber number, and smaller bubble size as well as a higher sectional void fraction on the top section of the passage. However, the fluid inlet and outlet positions have an important effect on the bubble distribution, and this indicates that the bubble buoyancy has an obvious influence on the bubble distribution. As the liquid flows away from the lower outlet slowly, there appears a stagnant air area, meanwhile, a foam zone is formed below the air area only when the fluid gets into it from the lower inlet. Nevertheless, as the fluid flows away from the upper outlet of the passage, there is neither a stagnant air area nor a foam domain irrespective of where the fluid enters from. It is worth pointing out that all the bubble velocities fluctuated, and small bubbles have a larger velocity fluctuation range and frequency than middle and large bubbles.