Here you will find all of my peer-reviewed conference and journal articles and related writings. I've organized them by subject matter as well as chronologically. This page covers phase change in multicomponent flows, and page two contains publications related to concentrating solar power.
Where the publishing agreement allows, I've included direct links to the articles here on this website or that of the other author(s).
Last updated May 2015.
Phase change in multicomponent flows
For my masters project I started looking at the problem of enhancing heat transfer using phase change in multi-component flows, i.e., a two-component mixture in which one fluid has a much lower boiling temperature than the other and undergoes boiling. In the context of cooling high-powered electronics, the idea is to take advantage of the qualities of water (a very good heat transfer fluid with high thermal conductivity, high heat capacity, reasonable viscosity, low cost, etc.), but get the boost to heat transfer coefficient associated with boiling within the operating temperature limits of electronics (usually below 100 °C, the saturation temperature of water at atmospheric pressure). It turns out, intimate contact between the two fluids is important to get any improvement in heat transfer, which led me in the direction of investigating emulsions.
Roesle, M. L. and F. A. Kulacki, 2008, "Characteristics of two-component two-phase flow and heat transfer in a flat microchannel", paper number HT2008-56084, in Proceedings of the 2008 ASME Summer Heat Transfer Conference, August 10-14, 2008, Jacksonville FL. doi: 10.1115/HT2008-56084.
This paper (my first) describes experiments performed with a flat microchannel with two fluids, water and FC-72, flowing through it simultaneously. FC-72 is a heat transfer fluid with boiling temperature around 56 °C. The microchannel had a heated lower surface and a clear lid, so that the fluids could be observed as they flowed through the channel. I didn't make any effort to mix the two fluids, instead just introducing them into an inlet plenum from opposite directions. The flow was surface tension-dominated, and we usually observed the water and FC-72 flowing through the microchannel parallel to each other in, essentially, separate flows. We found that boiling in the FC-72 had negligible effect on the water. Instead of enhancing heat transfer, the average heat transfer coefficient dropped according to the fraction of the channel occupied by the FC-72.
This finding, that the water and FC-72 needed to be mixed thoroughly, led me in the direction of looking at emulsions.
Roesle, M. L. and F. A. Kulacki, 2010, "Boiling of dilute emulsions - toward a new modeling framework", Industrial and Engineering Chemistry Research 49(11), pp. 5188-5196. doi: 10.1021/ie9013259.
As I started looking at emulsions, I found that one group at a Russian university had published a number of papers on the subject of heat transfer in boiling dilute emulsions already. In addition to a number of articles reporting experimental results, they also published a model of boiling in emulsions based on chain-boiling of the droplets. The idea is that as the emulsion flows through the boundary layer around a heated surface, the droplets that enter the boundary layer become superheated and eventually boil. The droplet doesn't boil until it becomes highly-superheated, and it does so so rapidly that it forms a shockwave. That shockwave sets off boiling in any nearby droplets, causing a chain reaction. After calming down, I decided that while I accepted their basic premise that boiling occurs in highly-superheated droplets moving through the boundary layer (without necessarily even touching the heated surface!), I disagreed with their mechanisms for boiling and chain-boiling.
This paper reviews the highlights in the literature on the subject of boiling heat transfer in emulsions. (And there isn't much!) It describes and critiques the existing model of heat transfer in boiling emulsions. I then lay out my own model of boiling emulsions. It is based on the idea of superheated droplets boiling suddenly in the thermal boundary layer, just like the previous model, but it proposes a different set of mechanisms for boiling and chain-boiling. The model of boiling is incorporated into an overall multiphase Eulerian model that rigorously accounts for the the flow and heat transfer in the emulsion as a whole. (In restrospect, it was rather presumptuous to criticize the former model before I had actually implemented my model in a numerical solver or run my own experiments!)
To get an idea of what happens to a small, highly-superheated droplet when it boils (and also to do a final project for a class on numerical methods for moving boundary problems), I performed a series of simulations of a small boiling droplet suspended in a second imiscible liquid. I assumed a spherical droplet at rest in the surrounding liquid that begins boiling from a nucleation site at the center of the droplet, so the problem remains spherically symmetrical and I could perform one-dimensional simulations. The results were that the droplets boiled quite rapidly (as expected), but not so fast that any shockwave would form. The droplets boil rapidly enough that the resulting bubble oscillates after boiling finishes. The thermal boundary layer around the bubble can get interesting.
These papers describe the experimental campaign I performed as part of my Ph.D. I measured heat transfer during pool boiling of emulsions of FC-72 in water and pentane in water on an electrically heated wire. For the most dilute emulsions (0.1 and 0.2 % emulsified component by volume) I was also able to record video of the boiling process. The electrically heated wire is the same sort of apparatus used in most of the previous studies in this field, and one notable outcome of my study is that my results are very similar to earlier studies of water-based emulsions, despite differences in the emulsion composition; the wire diameter, material, and orientation; and droplet size. The agreement is seen when comparing the increase in the heat transfer coefficient (or heat flux) above the single phase heat transfer coefficient (or heat flux) as a function of the wire superheat relative to the saturation temperature of the emulsified component.
The video recorded of the boiling emulsions revealed a very surprising behavior: at the moment that the onset of boiling is observed in the heat transfer data, large bubbles appear attached to the heated wire. The bubbles are much larger than could be formed by the boiling of a single droplet (or a small number of droplets together). They remain attached to the wire and, in fact, appear to be motionless. As the heat flux is increased during the experiment, the bubbles grow larger and more numerous until eventually they start to depart from the wire. At higher heat fluxes I do start to see very small bubbles that seem to form in the vicinity of the wire (but not on its surface), as one might expect from individual droplets boiling in the thermal boundary layer.
These large attached bubbles are not accounted for in my (or anyone else's) model of boiling emulsions. I believe they form from droplets that accumulate on the heated wire early in the experiment, before the temperature is high enough to cause boiling. They certainly deserve further study. My guess (and I haven't put numbers to it yet) is that they act like little heat pipes, with the droplet liquid evaporating at the bottom adjacent to the wire and the vapor condensing around the top of the bubble where it is surrounded by cooler emulsion and then running down the inside of the bubble.
Roesle, M. L., 2010, Boiling of Dilute Emulsions, Ph.D. thesis, University of Minnesota. Click here for the pdf.
My thesis encompasses the same material as in the emulsion papers listed above, and includes more detail about the experimental apparatus, data collection, and uncertainty analysis. The model is somewhat updated compared to the version given in the IECRes paper. The thesis also describes the implementation of the model in a numerical solver based on OpenFOAM and shows some results of 2D simulations of emulsion boiling around a heated wire. The results are only qualitatively in agreement with the experiments, and there are some empirical parameters in the model that need to be tuned to achieve that. But the model predicts that chain boiling of the droplets increases very rapidly with increasing droplet volume fraction, and I was only able to observe very dilute emulsions in my experiments, and the large attached bubbles seen in those experiments are not part of the model. So the model is definitely incomplete, but whether it accurately portrays the behavior of not-so-dilute emulsions (say, 1 or 2 % droplets by volume) is not certain.
Roesle, M. L. and F. A. Kulacki, 2013, Boiling Heat Transfer in Dilute Emulsions, Springer, New York. doi: 10.1007/978-1-4614-4621-7.
This mini-monograph is in Springer's SpringerBriefs in Applied Sciences and Technology - Thermal Engineering and Applied Science series, edited by F. A. Kulacki. It is similar in content to my thesis, but is updated, condensed, and cleaned up.
Roesle, M. L., D. L. Lunde, F. A. Kulacki, 2013, "Boiling of dilute emulsions on a heated strip", paper number HT2013-17102, in Proceedings of the ASME 2013 Summer Heat Transfer Conference, July 14-19, Minneapolis MN.
Roesle, M. L., D. L. Lunde, F. A. Kulacki, 2015, "Boiling Heat Transfer to Dilute Emulsions From a Vertical Heated Strip", Journal of Heat Transfer 137(4), p. 041503. doi: 10.1115/1.4029456.
This paper describes experiments performed by David Lunde while a Masters student at the U of Minnesota. (Both conference and journal paper cover the same experiments; the presentation and analysis of data in the journal paper is better.) It was a follow-on of my experiments and used the same apparatus to examine pool boiling, except he used a thin strip as the heated surface, thus providing a much taller surface, ~1.3 mm vs. 50 or 100 micron diameter wires. The resulting boiling curves are also in line with my own results, but boiling on the strip generally takes place at higher temperatures than on the wires. The same large attached bubbles were seen on the heated strip, but the camera was not available so video of them was not recorded.