This paper describes an experimental investigation of heat transfer for impinging jet and ribbed duct flows under system reciprocation which focuses on the variable reciprocating effects on these two flow systems for piston cooling application. The experimental data reconfirmed the unsteadiness of reciprocating flow which led to the local temporal heat transfer variations when both test sections reciprocated. For impinging jet flow, reciprocation did not modify the fundamental physics that facilitated heat transfer, but the local instantaneous heat transfer could be improved or impeded by reciprocation, depending on the Reynolds number and reciprocating frequency. For reciprocating ribbed duct flow, the fundamental heat transfer was considerably modified from the nonreciprocating situation so that the distribution of heat transfer along the ribbed surface evolved toward a wavy-like pattern from the nonreciprocating zigzag pattern when the reciprocating frequency or Reynolds number increased. Comparing these two reciprocating systems at high mass flow rate and high reciprocating frequency showed a higher spatial-time averaged heat transfer level for ribbed duct flow while the reciprocating impinging jet flow provided better cooling performance when the Reynolds number or reciprocating frequency was relatively low Therefore, the ribbed duct and impinging jet flows might be suitable for piston cooling of high-and low-speed engines respectively.

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