Huizhu Yang

Improvements on Flow Distribution and Heat Transfer Performance of Plate-fin Heat Exchangers by Qusai-S Type Header Configuration

ABSTRACT In order to reduce flow maldistribution and enhance the heat transfer performance, an improved quasi-S-type header configuration of plate-fin heat exchangers is proposed. Based on the analysis of the fluid flow distribution, the results indicate that the outlet velocity of the conventional header is uneven. However, the qusai-S-type header not only effectively reduces the geometric mutation, but also extends the hydraulic path, which guides fluid to the two sides and thereby reduces the maldistribution. The qusai-S-type header was designed on the basis of the cubic curve (denoted as configuration B), Bézier curve (configuration C), or two semi-circular segments uniting with one-line segment (configuration D). Compared with the conventional header (configuration A), the maldistribution parameters for configuration B, C, and D decrease by 75.2–93.9%, 80–94.8%, and 78.4–94.3%, respectively. Yet, the power consumptions of them increase by 26.3%, 22.3%, and 42.3%, respectively. Besides, the effectiveness of the conventional plate-fin heat exchanger declines about 15.1% due to improper header configuration, while the decrease of effectiveness can be controlled within 2.0% using the improved header configurations. Therefore, the improved header configurations can effectively enhance the flow uniformity and the heat exchanger effectiveness, but with a low power consumption penalty.

Numerical prediction for subcooled boiling flow of liquid nitrogen in a vertical tube with MUSIG model

Multiple size group (MUSIG) model combined with a three-dimensional two-fluid model were employed to predict subcooled boiling flow of liquid nitrogen in a vertical upward tube. Based on the mechanism of boiling heat transfer, some important bubble model parameters were amended to be applicable to the modeling of liquid nitrogen. The distribution of different discrete bubble classes was demonstrated numerically and the distribution patterns of void fraction in the wall-heated tube were analyzed. It was found that the average void fraction increases nonlinearly along the axial direction with wall heat flux and it decreases with inlet mass flow rate and subcooled temperature. The local void fraction exhibited a U-shape distribution in the radial direction. The partition of the wall heat flux along the tube was obtained. The results showed that heat flux consumed on evaporation is the leading part of surface heat transfer at the rear region of subcooled boiling. The turning point in the pressure drop curve reflects the instability of bubbly flow. Good agreement was achieved on the local heat transfer coefficient against experimental measurements, which demonstrated the accuracy of the numerical model.