A series of experimental investigations on the studies related to fluid flow and transient mixed convection from a horizontally unconfined stationary or rotating ceramic-based multichip module (MCM) disk with unconfined jet impingement have been successfully conducted. The fluid flow and heat transfer behavior explored includes the streamwise velocity and turbulence intensity distributions, transient dimensionless temperature distribution on the MCM disk surface, transient heat flux distribution of input power, and transient chip and average heat transfer characteristics on the MCM disk surface. Besides, two new correlations of transient stagnation and average Nusselt numbers in terms of jet Reynolds number, ratio of jet separation distance to nozzle diameter and time elapsed during the transient period, are presented for the cases of stationary MCM disk. For the cases of rotating MCM disk, a complete composite correlation of steady-state average Nusselt number for mixed convection due to jet impingement, disk rotation and buoyancy is proposed. As compared with the steady-state results, if the transient chip and average heat transfer behaviors may be considered as a superposition of a series of quasisteady states, the transient chip and average Nusselt numbers in all the present transient experiments can be properly predicted by the existing steady-state correlations when in the power-on transient period.