Measurement of heat flux in dense air-mist cooling: Part I - A novel steady-state technique

This work presents a technique for characterizing water air-mist cooling of surfaces at temperatures from 200-1200 °C while subjected to intense spraying conditions, so that the heat flux could reach values in excess of 10 MW/m2. In this new steady-state method, a local region of an air-mist/spray impinges upon the exposed surface of a hot Pt-disk (8 mm ∅ by 2.5 mm thickness) which is surrounded by an induction coil while both are embedded in a cast ceramic monolith. A digital controller adjusts the output power of a high-frequency generator to balance the induction heating of the Pt-specimen with the heat removed by the boiling of the spray droplets impinging on its active surface, in order to maintain the control temperature of the sample at the predetermined set-point. Measurement of the RMS current flowing through the coil to maintain this temperature, together with the solution of a two-dimensional axi-symmetric computational model of the electromagnetic field and heat conduction equations, enabled estimation of the heat extracted locally by the mist. A rigorous experimental procedure was performed to determine the boiling curves from 200 °C to 1200 °C and then back to 200 °C. The curves for both trajectories revealed that strong boiling hysteresis occurred in the nucleate and transition boiling regimes and that this was almost absent in the stable film boiling regime. On the other hand, hysteresis was not present in thermal loops involving surface temperatures only between 600 and 1200 °C. The existence of hysteresis, in a certain temperature interval, points out the importance of considering the thermal history of actual cooling processes when simulating them in the laboratory. The ability of the new technique to maintain the sample temperature for prolonged periods, e.g., hours, enables future studies to improve the understanding of boiling phenomena during intense heat-removal processes. In Part II of this article, the effects that the mist droplet size, droplet velocity and water impact density have on the heat extracted under steady-state conditions from a surface with large superheat are investigated for a wide variety of those characteristics.