This article describes the results of an experimental study aimed at revealing the heat transfer physics of a twisted-tape insert in a uniformly heated tube for transitional and turbulent flow situations when the test tube remains stationary and reciprocates. Dry air is used as the test fluid, and this study is motivated by the need to understand the effectiveness of twisted-tape inserts on the heat transfer augmentation when applied to the piston cooling of a compression-ignition engine. Prior to the experimental tests, a parametric analysis was performed for which the dimensionless groups that characterized heat convection were identified and adopted to formulate the experimental program and procedures of data reduction. The parametric test matrix involves Reynolds, pulsating, and buoyancy numbers in the ranges 3, 000-12, 000, 0-0.9, and 0.003-0.009, respectively, with five different reciprocating frequencies tested, namely, 0, 0.83, 1.25, 1.67, and 2 Hz. The nonreciprocating experimental data confirmed considerable heat transfer enhancement due to the twisted-tape insert. When the test tube was reciprocated orthogonally to the main flow direction, both pulsating and reciprocating buoyancy forces arose, which modified the heat transfer from nonreciprocating situations. Initially, a range of heat transfer impediments relative to the nonreciprocating results that could reduce local Nusselt numbers to levels about 75% of nonreciprocating values took place when these reciprocating forces were relatively weak. Further increases of relative strengths of reciprocating forces resulted in subsequent heat transfer recovery and could lead to about 60% heat transfer enhancement relative to nonreciprocating values.
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