A new correlation model is examined for capturing the combined influences of surface-to-nozzle distance and coolant flow rate on critical heat flux associated with spray evaporative cooling of vibrating surfaces. The correlation model is constructed using dimensional analysis by applying the Generalized Buckingham Π-Theorem. The model is calibrated using experimentally-measured spray evaporative cooling data, taken from an electrically-heated horizontal flat circular test-piece excited by a shaker through a range of low and high frequencies of vibration, from small to large amplitude. To understand the combined effect of frequency, amplitude, and surface-to-nozzle distance, at critical heat flux, Vibrational Reynolds Number, Acceleration Number, and Dimensionless Surface-to-Nozzle Distance are used. The results show that surface-to-nozzle distance, in the presence of dynamic effects, significantly influences the critical heat flux, whereas vibration amplitudes and frequencies have differing effects in response to variations in both surface-to-nozzle distance and flow rate. Surface-to-nozzle distance can either increase or decrease the heat transfer, depending on the vibration range. The calibrated correlation model is capable of predicting the effect of surface-to-nozzle distance on the critical heat flux with errors in the range −4.8% and + 10.5%.

A temperature control approach using evaporative spray cooling of vibrating surfaces in the nucleate boiling region is proposed and verified experimentally. This is relevant to temperature control of heat-generating automotive vehicle components. By exploiting an experimentally calibrated dynamic correlation model to represent evaporative spray cooling of a flat test-piece, a PID controller has been adopted with emphasis focused on the choice of gain parameters to ensure both stability of temperature control, and favourable responses in terms of relevant performance measures. Optimum linearisation of the correlation model has been achieved by solving an appropriate Wiener-Hopf equation, mainly to undertake a practical stability assessment of the closed-loop temperature control system. To verify the predicted control system performance, experimental measurements have been obtained from an instrumented, and spray-evaporatively-cooled, flat test-piece exposed to displacement vibration from a shaker. Experimental testing, appropriate to automotive vehicle component applications, includes large-amplitude, low frequency vibration at 12 mm and 1.9 Hz, and low amplitude, high-frequency vibration at 0.02 mm and 400 Hz. To assess the effects of different PID controller gains on the thermal performance of the thermal management system, a coefficient of performance (COP) is used, defined as the ratio of heat power removal to the required pumping power. To achieve a reduction in the settling time, and an increase in the rise time of stable control, a PID controller with a negative proportional gain showed most promising results. A 10.5% increase in COP was achieved in comparison to a PID controller with positive gains. This information is useful for the design and optimization of thermal management systems using evaporative spray cooling.

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.

An experimental investigation of the effect of surface vibration on spray evaporative cooling has been undertaken using a dynamic test rig. The horizontal circular test section involved a spray nozzle on top of a shaker being shaking at different frequencies and amplitudes to examine the effect of vibration on the nucleate boiling regime. The combination of the two-phase spray cooling and dynamic surface conditions has not previously been studied. The results clearly show that dynamic surface conditions influence nucleate boiling. In general, the evidence shows that vibration impedes heat transfer. The influence of amplitude and frequency are shown however not to have the same trend for all the excess temperatures. Depending on the mechanism, combinations of amplitude and frequency can either increase or decrease the heat transfer coefficient compared with the static cooling surface.

A new simulation capability is presented to enable the performance of a hardware-based temperature control system to be assessed in thermally-managing heat-generating automotive vehicle powertrain parts. Temperature control is assumed to involve spray evaporative cooling of powertrain parts exposed to vibration. Two hypotheses are proposed to enable construction of a practical simulation that is both accurate and computationally efficient. The first is that a dynamic correlation model for single-nozzle spray evaporative cooling of a flat test-piece exposed to vibration, can be used as a reasonable model for multiple-nozzle spray evaporative cooling of component parts with curved cooling surfaces of non-horizontal orientation. The second is that the transient heat diffusion properties of a particular 3-dimensional component can be replaced by a 1-dimensional (1D) equivalence. To test this hypothesis, Finite Element models for two representative parts have been constructed and used to demonstrate the quality of the 1D heat diffusion equivalence, for which a fast Finite Difference solution can be exploited. To test the accuracy of test-piece surface temperature control simulation, an experimental test facility has been built in hardware, in which the temperature of two instrumented test-pieces exposed to vibration (from a shaker) are controlled by spray evaporative cooling. Each test piece is electrically-heated and the hardware control system is configured using PID control, for which appropriate gains are selected. Detailed comparisons of temperature control by hardware and simulation are given for the two test-pieces under static and dynamic conditions. Good agreement is generally obtained between simulated surface temperatures compared with measurements taken from both test-pieces. The paper shows that temperature control of a hardware-based control system using spray evaporative cooling of powertrain parts can be confidently simulated.

A numerical study is presented to examine the behavior of a single liquid droplet initially passing through air or steam, followed by impingement onto a static or vibrating surface. The fluid dynamic equations are solved using the Volume of Fluid method, which includes both viscous and surface tension effects, and the possibility of droplet evaporation when the impact surface is hot. Initially, dynamic behavior is examined for isothermal impingement of a droplet moving through air, first without and then with boundary vibration. Isothermal simulations are used to establish how droplet rebound conditions and the time interval between initial contact to detachment vary with droplet diameter for droplet impingement onto a stationary boundary. Heat transfer is then assessed for a liquid droplet initially at saturation temperature passing through steam, followed by contact with a hot vibrating boundary, in which droplet evaporation commences. The paper shows that, for droplet impingement onto a static boundary, the minimum impact velocity for rebound reduces linearly with droplet diameter, whereas the time interval between initial contact and detachment appears to increase linearly with droplet diameter. With the introduction of a vibrating surface, the minimum relative impact velocity for isothermal rebound is found to be higher than the minimum impact velocity for static boundary droplet rebound. For impingement onto a hot surface, in which droplet evaporation commences, it is shown that large-amplitude surface vibration reduces heat transfer, whereas low-amplitude high-frequency vibration appears to increase heat transfer.

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.

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.

The accuracy of computational fluid dynamic (CFD)-based heat transfer predictions have been examined of relevance to liquid cooling of IC engines at high engine loads where some nucleate boiling occurs. Predictions based on (i) the Reynolds Averaged Navier-Stokes (RANS) solution and (ii) large eddy simulation (LES) have been generated. The purpose of these simulations is to establish the role of turbulence modeling on the accuracy and efficiency of heat transfer predictions for engine-like thermal conditions where published experimental data are available. A multiphase mixture modeling approach, with a volume-of-fluid interface-capturing method, has been employed. To predict heat transfer in the boiling regime, the empirical boiling correlation of Rohsenow is used for both RANS and LES. The rate of vapor-mass generation at the wall surface is determined from the heat flux associated with the evaporation phase change. Predictions via CFD are compared with published experimental data showing that LES gives only slightly more accurate temperature predictions compared to RANS but at substantially higher computational cost.

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.

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.

Evaporative cooling system concepts proposed over the past century for engine thermal management in automotive applications are examined and critically reviewed. The purpose of the review is to establish evident system shortcomings and to identify remaining research questions that need to be addressed to enable this important technology to be adopted by vehicle manufacturers. Initially, the benefits of evaporative cooling systems are restated in terms of improved engine efficiency, reduced CO2 emissions, and improved fuel economy. An historical coverage follows of the proposed concepts dating back to 1918. Possible evaporative cooling concepts are then classified into four distinct classes and critically reviewed. This culminates in an assessment of the available evidence to establish the reasons why no system has yet made it to serial production. Then, by systematic examination of the critical areas in evaporative cooling systems for application to automotive engine cooling, remaining research challenges are identified.