查看更多>>摘要：Jet impingement are widely used in the piccolo hot air anti-icing chambers of aircraft. The high pressure at the nozzle and low pressure in the chamber lead to a relatively large nozzle pressure ratio, triggering an unexpanded jet state. The unexpanded jet impingement significantly differs from the subsonic jet impingement in flow and heat transfer. Therefore, this study conducts numerical investigations on the flow and heat transfer characteristics of unexpanded jet impingement. In addition, the effects of nozzle pressure ratio (NPR) and dimensionless impinging distance (L/d) are considered. According to the numerical results, the impinging air cannot be fully expanded in the free jet zone, and will continue to expand after entering the wall jet zone, triggering an abrupt drop in the hot air temperature, under a small dimensionless impinging distance (L/d = 2.5 and L/d = 5.0). In this state, the hot air will not heat the surface effectively, as the surface will heat the air reversely. Furthermore, it is also determined that the ambient temperature should be included as an influencing factor in the analysis of jet impinging heat transfer. For high-speed jet impingement, especially unexpanded jet state, the ambient air will influence the total temperature of jet boundary due to the large difference of static temperature between free jet zone and chamber environment.
查看更多>>摘要：It is significant to evaluate whether the classical correlations based on conventional tube size (>= 10 mm) is suitable for predicting the flow and heat transfers characteristics of the precooler composed of mini-tube bundles (around 1 mm). In this work, a full-scale heat transfer experiment of precooler is carried out, and the high temperature incoming air (686 K) in the experiment was produced by burning the mixture of air and alcohol. The numerical simulation of a unit precooler is also conducted with Computational Fluid Dynamics. The classical correlations for the design of heat exchanger are evaluated by both experiments and simulations. The results indicate that the prediction error of classical correlations of flow resistance is less than 10% for both the flow inside the mini-tube and across mini-tube bundles. The classical correlation of heat transfer inside tube can meet the engineering requirements. However, the maximum deviation of Nusselt number prediction for the outside flow across tube bundles reaches 32.1%. A new correlation for calculating heat transfer across a compact pre cooler composed of staggered mini-tube bundles with a prediction error below 5% is finally established based on the experimental and numerical results.
NSTL
Elsevier