Boiling Heat Transfer Characteristics of Water Jet Impingement on Hot Steel Surface : Experiment and Analysis

Abstract

Boiling heat transfer of subcooled water impingement on highly superheated platee is inverstigated by means of heat transfer analysis and high-definition flow visualization. The stainless-steel plate initially heated up to 900ºC by an induction heating method is cooled by water of 15ºC. The surface temperature and heat flux are estimated by solving 2-D inverse heat conduction problem. The temporal visualization during quench subcooled-jet impingement boiling is synchronized with the heat transfer measurement in the corresponding surface temperature and heat flux. Spread of the subcooled jet over the horizontal plate shows a quasi-steady regime where the wetting front spreads linearly with time. The time for onset of the quasi-steady regime can be explained by a quasi-steady time. The front separates the single-phase/collapsed-bubble region from the outside region which is dry if not for the impinging droplets ejected from the front. As the front expands, the surface experiences sequence of single-phase, collapsed-bubble, wetting front evaporation, ejected-droplet evaporation cooling. The fraction of water ejected from the front increases linearly with time (reaches over 10%) and is predicted. After that, enhancement resulting from interaction of two adjacent jets impinging on largely superheated stainless-steel plate is investigated. Subcooled water jets with varying jet separation distance (up to 30 nozzle diameter) show that following an elapsed time of isolated-jet behavior, the spreading liquid wetting fronts merge and in the merged region the heat transfer coefficient increases substantially. This enhancement is presented as the two-jet cooling efficiency and we show this efficiency initially increases with the increase in separation distance, reaching a peak and then vanishes for large separation. Finally, the comparative study of 4-type of quenching methods on hot steel block is experimentally performed with heat transfer analysis and microscopic examination. For the comparison, water/oil immersion, forced immersion and multiple jet quenching are selectively adopted. Two material types of steel such as stainless steel (SUS310S) and alloy steel (SNCM439) are used to compare boiling heat transfer characteristics and microscopic examination, separately. The surface temperature and heat flux are determined by solving inverse heat conduction and followed boiling phenomena are visualized by high-speed imaging. Furthermore, microscopic examination was performed with SEM to verify the metallographic structures after each quenching method, and the mechanical properties are also examined. All 4-type methods showed qualitative agreement in the variation of martensitic fraction depending on each cooling rate. As the surface is cooled more rapidly, the higher volume fraction of martensite are formed and shows enhanced mechanical properties.