Pressure behavior and heat transfer behavior of air natural convection in an asymmetrically heated vertical channel with a damper

Abstract

The natural convection of air between vertical parallel plates under asymmetrical heating was investigated focusing on the pressure behavior and the heat transfer performance as varying two experimentally controlled parameters, namely, the damper closing angle (0 ∼ 45 °) and the thermal operating condition (1.7 ∼ 6.4 kW). By varying the damper closing angle, the form loss coefficient of the damper was estimated from the measured pressure and the calculated pressure difference relating to the thermal gravity effect. It was verified that the form loss coefficient estimated for natural convection was close to the reported form loss coefficient estimated for forced flow. The thermal stratification of the ambient air was additionally considered in the pressure analysis. In addition, the heat transfer was analyzed in terms of the heat transfer correlation. As the damper angle increased, the heat transfer performance decreased owing the reduced flow rate. However, as the thermal operating condition intensified, the heat transfer performance was increased by the improved flow rate. Interestingly, the trend of the Rayleigh number was the same regardless of the two control parameters, even though an intensifying thermal boundary condition improved the heat transfer performance and an increasing damper closing angle weakened the heat transfer performance, conflictingly. Therefore, the correlation conventionally used for natural convection cannot be applied under the conditions of a high wall temperature and control using a damper because it does not consider the flow rate effect. To consider the flow rate effect additionally in the heat transfer correlation, a correlation based on Jackson's mixed convection correlation is suggested. The suggested correlation predicts the heat transfer performance on each wall by considering the effects of the damper angle and operating condition, simultaneously, within the error of less than 30%.