Single-phase heat transfer regimes in forced flow conditions under exponential heat inputs

We present an experimental and theoretical investigation of single-phase heat transfer under exponential power inputs. We conduct forced flow experiments with water, covering a broad range of mass fluxes (from 0 to 19,300 kg/m(2)/s), bulk temperatures (from 25 to 100 degrees C), pressures (from 0.1 to 1.2 MPa), exponential power escalation periods (from 2.5 to 200 ms), and considering two different heater and channel geometries. We use a high-speed infrared thermometry technique to measure the space- and time-dependent heat transfer coefficient between the heated surface and the coolant, building a database covering 73 different experimental conditions. We consider turbulence as a diffusive process and develop an analytic model that can predict 80% of our database within a +/- 10% error, and the entire database within a +/- 20% error. We discuss the presence of three heat transfer regimes, i.e., transient conduction, transient turbulent diffusion, and quasi-steady turbulent heat transfer, and derive analytically the two associated transition criteria. These transitions depend on the power escalation period, fluid properties, and are connected to the profile of the turbulent diffusion properties across the boundary layer. (C) 2021 Elsevier Ltd. All rights reserved.