Oscillatory impingement of liquid inside reciprocating pipes

Engine pistons incorporating heat pipe cooling technology are under development to improve the thermal-tribological performance of the piston assembly in internal combustion engines. Reciprocating heat pipes are needed to enhance the heat transfer process in dynamic engine environments. These heat pipes utilize piston reciprocating motion as a means for liquid phase return. Therefore, oscillatory impingement of liquid inside heat pipes is critical for the new piston design. A simplified analysis is conducted to investigate the liquid oscillatory motion in circular pipes. The results indicate that, for a given fluid and pipe combination, the motion of the liquid is a function of the piston stroke, cranking speed and heat pipe length, as well as drag and viscous resistances. Based on this analysis, correlations between heat pipe design parameters and the critical impingement frequency of engine cranking are derived. Experimental observations for a liquid impingement inside heat pipes are performed on an engine/heat pipe apparatus constructed based on a single-stroke internal combustion engine. Both straight and curved transparent pipes, which simulate real heat pipes, are employed in observations. The experimental results show that the pipe curvature has little influence on critical impingement frequency if the curvature is small, and the critical impingement frequency is higher for pipes having a longer length and smaller diameter. Based on the experimental results, proper liquid charge levels are determined for reciprocating heat pipe design. In general, experimental results are in good agreement with the derived correlation. They also indicate that full scale liquid impingement on the entire inner wall of the pipe can be achieved at low cranking speeds.