The effect of ambient pressure on the heat transfer of a water spray

The present work is aimed at quantifying the effects of ambient pressure in the heat transfer at single injections of a full cone spray over a hot metal surface. The experimental configuration is that of a spray impinging down perpendicularly onto a flat surface located at 55 mm inside an injection chamber. The experiments were conducted for prescribed initial wall temperatures ranging from single phase to local nucleate boiling and transition regimes of heat transfer. Ambient pressures ranged from atmospheric to 30 bar. The analysis is based on spatial resolved measurements of the instantaneous surface temperature during the injection period. The measurements are then processed in order to obtain estimates of the time-averaged values of the local heat flux. The overall cooling rate is also obtained by integrating the local values within the total area of the spray impact Results show that the amount of heat extracted by the impinging spray increases 3.4 times when ambient pressure is increased from atmospheric to 20 bar at the same superheating degree at the wall of 45 degrees C. This corresponds to an increase from 13.3% to 47.7% in the ratio between the actual cooling and the theoretical maximum cooling, defined here as cooling efficiency. This is a result of a better spreading of the liquid film at the wall, covering a larger footprint upon impact. Instantaneous peak heat flux is also increased, as a clear indication of the improved heat transfer between the impinging droplets and the wall.The work presented herein derives from a broader research program devised to develop a system for in cylinder cooling of internal combustion engines using high pressure water sprays produced by gasoline direct injectors.The authors would like to acknowledge LiquidPiston INC. for providing all the laboratorial conditions to perform the experiments, MEtRICs - Mechanical Engineering and Resource Sustainability Centre (UID/EMS/04077/2019), and Diogo Ferreira for aiding in the highspeed visualization setup and experiments. T. Costa is supported by the Portuguese Foundation for Science and Technology (FCT) under the PhD grant PD/BD/105929/2014, MIT Portugal Program, and F.P. Brito is supported by FCT under the Post doctoral grant SFRH/BPD/89553/2012 and J. Martins is supported by the FCT grant SFRH/BSAB/142994/2018, financed by FEDER funds through Programa Operacional Fatores de Competitividade - COMPETE and National funds through PIDDAC and FCT