Experimental study on the unsteady laminar heat transfer downstream of a backwards facing step

The objective of this work is to study experimentally the unsteady heat transfer downstream of a backward-facing step in the 2-D laminar regime when the inlet flow is pulsated. To this aim, an experimental set-up has been prepared with water as the working fluid. The Reynolds number based on the hydraulic diameter of the inlet channel and average inlet velocity is 300. Inlet flow temperature is 30 °C and a region downstream of the step is heated up to 74 °C. Pulsation is achieved using a piston pump and heat transfer is studied up to a maximum pulsation Strouhal number of 1.2. The results obtained confirm previous numerical simulation work in the sense that pulsation could be used to partially recover the heat transfer efficiency that is lost in steady flow conditions downstream of a backward-facing step. It has also been confirmed that the behaviour of the averaged Nusselt number versus pulsation Strouhal number is of the resonant type. That is: the Nusselt number increases from the steady situation up to a certain value of the Strouhal number (0.41 in our case) and, then, it degrades as the frequency of the pulsation is further increased.