Study of spray cooling by advanced optical diagnostics

The use of a liquid spray that subsequently undergoes a liquid to vapor phase change can efficiently remove heat at low degrees of superheat from electronic circuits, lasers and turbine blades, etc. In the process of spray cooling, the impingement of high velocity droplets on the hot surface elicits complex droplet impact dynamics, phase change and two-phase flow, which cause difficulties in the understanding of the mechanism of spray cooling. Current research focuses on the fundamental phenomena affecting heat flux in spray cooling, including the evaporation process of individual droplets impinging on a flat surface and the physical mechanism that triggers critical heat flux. It is crucial to develop advanced optical diagnostic tools for analyzing these phenomena quantitatively. A novel laser interferometric method was developed to measure the contact diameter, profile, and volume of a liquid droplet evaporating on a hot quartz substrate. Validation experiments were conducted with different initial volumes of droplets. It is shown that an accuracy of ±l percent for the contact diameter measurement and ±5 percent for the volume measurement can be achieved. By using this newly developed optical diagnostic method, the dynamic impingement, evaporation and boiling of a liquid droplet were investigated. Ethanol, pure water and a water solution of a surfactant (sodium dodecyl sulfate) were used. The impact velocity of the droplets was 7.5m/s and the diameters ranged from 0.19mm to 0.46mm. The evolutions of the contact diameter, profile and volume of the evaporating droplets were measured quantitatively. According to the measurement results of single-droplet evaporation, a new 2-D model that takes the moving liquid-vapor interphase into consideration was proposed in toroidal coordinates. In order to extend the single-droplet model to the multi-droplet system, droplets expelled from the hot surface are taken into consideration. Expulsion rate, defined as the ratio of the outgoing liquid mass flux to the incoming liquid mass flux, was first introduced in this research. A concise analysis of the relation between heat flux and expulsion rate was derived, which indicates accurate measurement of spray characterization is helpful to reveal the mechanism of spray cooling. To improve the accuracy in spray characterization, a high accuracy laser phase-Doppler anemometry (PDA) system was developed. This system, based on a dual-mechanism scattering model, can greatly eliminate the measurement uncertainty caused by the measurement volume effect (MVE) in a conventional PDA system. Therefore, this PDA system provides an advanced tool for quantitatively analyzing the impacting and expelling droplet dynamics in spray cooling. By using the advanced PDA system, spray cooling with both pure water and water solutions containing 100 ppm by weight of sodium dodecyl sulfate was investigated. The liquid mass fluxes were ranged from 0.156Kg/m2s to 1.20Kg/m2s. According to the measurement results, it was found that the expulsion rate, defined as the ratio of the outgoing to the incoming liquid mass fluxes, might be used as an indication for describing different boiling regimes in spray cooling. The expulsion rate is also surmised to connect with droplet dynamics, surface conditions and bubble nucleation