An experimental study was conducted to determine the heat transfer for single and multiple confined and submerged impingement jets in single- and multi-phase flows. This type of heat transfer is of commercial interest in high performance finned coolers used in avionic equipment. Part I deals with single-phase liquid flows of Freon R-113 with Part II covering two-phase flows. Stagnation point and local average Nusselt numbers, suitable for design purposes, were determined as functions of the Reynolds number, the dimensionless spacing between the confining plates, and, for multiple jets, as a function of pitch spacing. The resulting correlation equations match the experimental data from +/-10% to +/-20%.