Investigation into falling film outside horizontal tube based on diode laser absorption spectroscopy

The flow of liquid film is utilized in a wide variety of industrial processes, and an accurate investigation into film flow is essential to understand the mechanisms in the relevant processes. Here, a sensor based on diode laser absorption spectroscopy (DLAS) with an integrated optical-electrical system was developed to determine the liquid film thickness of the falling film outside a horizontal tube. The heat transfer coefficient of the liquid film was calculated using the theory of the convective heat transfer of a fluid flowing across a single tube. The imaging method was first employed to validate the measurement accuracy of the DLAS sensor; it was found that the liquid film thickness measured by the DLAS sensor and the imaging method were in good agreement, and the deviation between these two techniques was 4.57%. The relationship between liquid film thickness and heat transfer coefficient under different conditions, including spray density q (0.16/0.24/0.32 kg (m . s)(-1)), hot water temperature at the inlet of the tube T-h (40/50/60 degrees C) and height of the liquid distributor h(ld) (3/9/15 mm), were then discussed. When q increased, the heat transfer coefficient of the liquid film increased with increasing film thickness; when Th increased, the liquid film thickness remained constant while the heat transfer coefficient increased; when hld increased, the liquid film thickness decreased and the heat transfer coefficient increased.