An infrared thermographic technique is developed to obtain the transient solid surface temperatures surrounding the droplet during vaporization. This technique is appealing because it is non-intrusive, detailing the surface response to the droplet without affecting the evaporation process. Surface recovery can also be monitored using this thermographic method. The transient temperature distribution of a Macor solid is detailed. It is found that contact temperature is held in the vicinity of the droplet during the majority of the droplet's evaporation until the droplet thickness diminishes greatly, where upon the temperature of the solid surrounding the droplet begins to rise. The non-dimensional radius of influence of droplet cooling is also detailed. The data obtained on the cooling effect induced on aluminum and on Macor from previous studies is used in concert with new data obtained on a quartz surface to characterize the induced cooling of a hot surface by an evaporating droplet. The role of the droplet size and shape is investigated for various high and low thermal conductivity surfaces. Droplet evaporation time, surface heat transfer coefficient and droplet shape parameter are also examined.

This report describes the research performed during the period March 1987 - July 1988 under a joint research program between the Mechanical Engineering Department of the University of Maryland and the Center for Fire Research of the National Bureau of Standards. The research is conducted in the laboratories of the CFR by a Graduate Research Assistant of the ME Department under the joint supervision of Dr. Marino di Marzo (ME Dept.- UMCP) and of Dr. David D. Evans (CFR - NBS). The formulation of a model for the prediction of the cooling induced by an evaporating droplet impinging a semi-infinite solid is the subject of this report. The thermal interactions during the evaporation of a liquid droplet deposited on a low conductivity semi-infinite solid are complex because the evaporative process is coupled to the solid intense local cooling. Numerical techniques based on finite difference methods have failed to provide meaningful results. This is due to the sharp temperature gradients in the proximity of the droplet edge which cause instabilities in the solution for reasonable time steps due to the explicit coupling of the liquid-vapor regions. An integral method was proposed by Dr. Baum (CFR - NBS) in order to overcome these difficulties. The methodology and its application to this specific problem is described in detail. Preliminary result will also illustrate its validity. A brief note on the convective heat transfer coefficient measured in the previous experiments on aluminum and Macor is also included in this report.

This report describes the research performed during the period July 1990-July 1991 under a joint research program between the Mechanical Engineering Department of the University of Maryland and the Building and Fire Research Laboratory of the National Institute of Standards and Technology. The research is conducted by Graduate Research Assistants of the ME Department under the joint supervision of Dr. di Marzo (UMCP) and Dr. Evans (CFR-NIST). This joint research program was initiated in January 1985. The long term objective of the study of droplet-solid interaction is to obtain information applicable to the extinguishment of fire through a droplet array (e.g. spray). The solids of concern include low thermal conductivity materials, typical of fire applications.

This report describes the research performed during the period July 1990 - July 1991 under a joint research program between the Mechanical Engineering Department of the University of Maryland and the Building and Fire Research Laboratory of the National Institute of Standards and Technology. The research is conducted by Graduate Research Assistants of the ~E Department under the joint supervision of Dr. di Marzo (UMCP) and Dr. Evans (CFR - NIST). This joint research program was initiated in January 1985. The long term objective of the study of droplet-solid interaction is to obtain information applicable to the extinguishment of fire through a droplet array (e.g. spray). The solids of concern include low thermal conductivity materials, typical of fire applications. Several important results were obtained in the first years of research. In particular, the modelling of the boundary condition at the liquid-vapor interface (at the droplet exposed surface) was validated with the data collected for water droplets evaporating on an aluminum block (diMarzo 1986a, 1986b, 1988).

A computer code is developed and tested which simulates the transient evaporation of a single liquid droplet from the surface of a semi-infinite solid subject to radiant heat input from above. For relatively low temperature incident radiation, it is shown that the direct absorption of radiant energy by the droplet can be treated as purely boundary conditions, while a model for higher temperature incident radiation would require the addition of constant heat source terms. The heat equation is numerically coupled between the liquid and solid domains by using a predictor-corrector scheme. Three one-dimensional solution schemes are used within the droplet: a start-up semi-infinite medium solution, a tridiagonal Crank-Nicholson transient solution, and a steady-state solution. The solid surface temperatures at each time step are calculated through careful numerical integration of an axisymmetric Green's functions solution equation with the forcing function given by the past lower droplet surface and solid-vapor boundary heat fluxes. The time step is increased after a sensitive initial period to allow for reasonable run times. Two geometry models are included which give the droplet height as a function of current droplet volume and initial wetted radius; the second allows inclusion of the effects of initial contact angle and receding angle. Using water as the liquid and Macor, a low-thermal conductivity material, as the solid, the program output was compared to the experimental results in this line of research. They correlate well to the experiments in which the critical geometric shape factor and evaporation time were most easily measured.