Spray evaporative cooling of vibrating surfaces with application to automotive combustion engines

This thesis examines spray evaporative cooling of vibrating surfaces with application to automotive engines. Two-phase evaporative cooling is advantageous because it uses the latent heat of vaporisation which has a much higher transfer rate than single-phase forced convection as employed in conventional engine cooling systems. There is no existing literature on the effect of boundary motion in spray boiling heat transfer. A theoretical and experimental study of spray boiling heat transfer conducted for engine vibration conditions are presented. Numerical simulations using Volume of Fluid method are presented to investigate droplet impingent and onset of evaporation on vibrating surfaces. Experimental investigation of spray boiling heat transfer on vibrating surfaces is presented. Control of engine evaporative cooling using a transient 1D conduction model that represent the engine cylinder head wall is investigated using a spray boiling heat transfer correlation. A Proportional Integral (PI) control model for evaporative cooling, created using Matlab Simulink, that solves 1D transient conduction through a cylinder head, is presented. Simulations were undertaken for control of the gas-side metal temperature of the cylinder head for step-by-step change in engine load from full-load to half-load. Results showed that control is achieved under one second during the engine load changes with total gas-side metal temperature fluctuations being less than 5 OC. Numerical simulations were undertaken for a droplet with a diameter of 49 micrometer impinging on a vibrating surface for a range of vibration amplitudes and frequencies of 0.02 mm to 10 mm, and 1000 Hz to 10 Hz, respectively. An enhancement of heat transfer is seen at high frequencies, whereas heat transfer deteriorated at high amplitudes. To verify theoretical findings three experimental rigs have been designed and built as part of a research team effort. Experiments of spray boiling heat transfer at two different sub-cooling levels (5 oC and 15oC) are presented for wall vibration amplitudes and frequencies of 0.02 mm to 7 mm, and 400 Hz to 10 Hz, respectively. An enhancement of heat transfer was seen at high frequencies for 15 oC sub-cooling, whereas heat transfer deteriorated at high amplitudes for both degrees of sub-cooling. Numerical simulation results are qualitatively compared to experimental results, to check whether any correlation in heat transfer exists between a spray and a droplet. The heat transfer from droplet evaporation was found to be only half that of the spray evaporation experiments and it was concluded that, no obvious correlation exists. Experimental results are analysed against vibrational Reynolds number and it was found that heat transfer starts to deteriorate after a vibrational Reynolds number of 1000. A dynamic correlation was created by adding a term containing vibrational Reynolds number to an existing correlation. This was to calculate critical heat flux for vibrating surfaces.