Start-up heat transfer process in a parallel-plate channel has been studied in this article. Transient Graetz problem has been modeled to investigate the effects of geometry, flow regime, and fluid properties on the time of start-up process. An analytical solution based on the Graetz number has been presented as a solution to the partial differential equation of energy with associated initial and boundary conditions in details. Start-up process has been studied regarding Reynolds and Prandtl numbers. Obtained results showed that start-up process was shorter for flow regime with greater Graetz number, which corre- sponded to greater Reynolds and Prandtl numbers.
