Multi-phase CFD-conjugate heat transfer for spray cooling in the non-boiling regime

A numerical study is described to predict, in the non-boiling regime, the heat transfer from a circular flat surface cooled by a full-cone spray of water at atmospheric pressure. Simulations based on coupled Computational Fluid Dynamics and Conjugate Heat Transfer are used to predict the detailed features of the fluid flow and heat transfer for three different spray conditions involving three mass fluxes between 3.5 and 9.43 kg/m2s corresponding to spray Reynolds numbers between 82 and 220, based on a 20 mm diameter target surface. A two-phase Lagrange-Eulerian modelling approach is adopted to resolve the spray-film flow dynamics. Simultaneous evaporation and condensation within the fluid film is modelled by solving the mass conservation equation at the film-continuum interface. Predicted heat transfer coefficients on the cooled surface are compared with published experimental data showing good agreement. The spray mass flux is confirmed to be the dominant factor for heat transfer in spray cooling, where single-phase convection within the thin fluid film on the flat surface is identified as the primary heat transfer mechanism. This enhancement of heat transfer, via single-phase convection, is identified to be the result of the discrete random nature of the droplets disrupting the surface thin film

Correlation models of critical heat flux and associated temperature for spray evaporative cooling of vibrating surfaces

Prediction models have been constructed to investigate the effect of vibrating surfaces on the critical heat flux (CHF) and its associated temperature in spray evaporative cooling. Dimensional analysis has been used to construct the models to account for the influence of key dynamic parameters. Experimental measurements have been obtained from a flat, electrically-heated, copper test-piece, located inside a spray-chamber mounted on top of a shaker. A wide range of large-amplitude and high-frequency measurements have been obtained which correspond to test conditions for a piece of hardware mounted on board a light-duty automotive vehicle with vibration amplitudes ranging from 0 to 8 mm and frequencies from 0 to 200 Hz. Three nozzle types have been fed with distilled water at flow rates ranging from 55 to 100 ml/min being used to cool with subcooling degrees ranging from 10°C to 45°C. Measured data for both static and dynamic cases have been used to explore the influence on the CHF and the surface-to-fluid saturation temperature at which this occurs, of subcooling degrees, surface vibration amplitude and frequency, vibrational Reynolds Number and vibrational Acceleration Number. The measured data has also subsequently been used to calibrate the predictive models for use in thermal management systems. Static measurements (without vibration) show that the influence of flow rate, volumetric flux, and subcooling are largely in agreement with published literature. For dynamic cases, the influence of vibration is best explained in terms of the nondimensional parameters: Vibration Reynolds Number and Acceleration Number. The effect of vibration on CHF and associated temperature is assessed in detail for the three nozzle types at different flow rates and degrees of subcooling. Predictions of CHF and associated excess temperature, using the calibrated correlation models for the dynamic conditions, are very reasonable, and suitable for the intended purpose of ensuring safe operation of thermal management systems using spray evaporative cooling

Heat flux correlation models for spray evaporative cooling of vibrating surfaces in the nucleate boiling region

New empirical correlation models are constructed to characterise heat transfer associated with spray evaporative cooling of vibrating surfaces - a process involving complex two-phase physics well beyond current numerical simulation capabilities. The proposed correlation models, which account for dynamic, rather than just static surface conditions as in existing models, are constructed using dimensional analysis involving the Generalized Buckingham Π-Theorem. Experimentally-measured spray evaporative cooling data is used to fit the model using the Vibrational Reynolds number and a dimensionless acceleration number which better correlate the influence of surface frequency and amplitude in the nucleate boiling regime. Different coolant flow-rates through a full-cone spray nozzle are used to cool a flat circular test-piece acting as a horizontal surface. The test-piece surface is excited by a shaker through a range of low and high vibration frequencies and amplitudes. The results show that surface dynamic effects certainly influence nucleate boiling, but they also show that surface vibration does not have the same effect for all excess temperatures - dynamic effects can either increase or decrease heat transfer depending on the heat transfer mechanism. These new models are important for thermal management in several areas, particularly involving batteries, power electronics, and electrical machines in automotive and aerospace applications

Control of spray evaporative cooling in automotive IC engines

A novel approach is proposed for precise control of two-phase spray evaporative cooling for thermal management of road vehicle internal combustion engines. A reduced-order plant model is first constructed by combining published spray evaporative cooling correlations with approximate governing heat transfer equations appropriate for IC engine thermal management. Control requirements are specified to allow several objectives to be met simultaneously under different load conditions. A control system is proposed and modelled in abstract form to achieve spray evaporative cooling of a gasoline engine, with simplifying assumptions made about the characteristics of the coolant pump, spray nozzle, and condenser. The system effectiveness is tested by simulation to establish its ability to meet key requirements, particularly concerned with precision control during transients resulting from rapid engine load variation. The results confirm the robustness of the proposed control strategy in accurately tracking a specified temperature profile at various constant load conditions, and also in the presence of realistic transient load variation