Editor (John Thome)

From Thermal-FluidsPedia

As a full-time consulting engineer for 15 years from 1984 through 1998 with my own firm, as an Assistant/Associate Professor in the USA for 5 years before that at Michigan State University, a Ph.D. at Oxford University in 1978, and now as Professor of Heat and Mass Transfer at the Swiss Federal Institute of Technology of Lausanne (EPFL) since 1998, I have developed and experimentally validated numerous engineering methods actually used for the thermal design of evaporators and condensers for the petrochemical industry, the refrigeration, air conditioning and heat pump industries, and more recently the micro-electronics and power-electronics industries. These methods are a new generation of design methods, based all on flow pattern maps and the characteristic flow structure on the individual regime. For the new flow pattern map and flow boiling model for pure fluids and mixtures inside tubes, I received the ASME Heat Transfer Division's Best Paper Award in 1998 for my 3-part paper published in the ASME Journal of Heat Transfer. Those methods have since been extended to flow pattern/flow structure based design methods for condensation of pure vapors and mixtures inside tubes and the prediction of two-phase pressure drops. The method has also been extended to predict boiling of very wide-boiling range mixtures (including refrigerant-oil mixtures) and very high pressure fluids (CO2). One of my first widely used methods predicts the mixture boiling effect for binary and multi-component mixtures. This method is used as the basis within HTRI thermal design programs to design most of the thermosyphon reboilers used throughout the world. I have been in fact a consultant to HTRI since the age of 28 on multi-component boiling and in 2006 did a complete thermal audit of all HTRI thermal design methods used in HTRI software and provided a "roadmap" for future developments. In this sense, I have in many ways followed the footsteps of D.Q. Kern both in two-phase thermal design methods and by working as a full-time independent consulting engineer for 15 years of my career. HTRI has also acted as the exclusive distributor of my engineering software Enhanced Heat Transfer (known as EHT) since 1998 that is now completely integrated into HTRI software programs for use by all HTRI members for the design of process heat exchangers using single-phase and two-phase enhanced tubes, inserts, etc. I recently received the J&E Hall Gold Medal given by the UK Institute of Refrigeration (Feb. 13, 2008) for my contributions to refrigeration heat transfer. A summary of thermal design methods I have proposed and developed for evaporators and condensers that have become widely used in industry include methods for predicting: (i) flow boiling heat transfer, flow pattern maps and two-phase pressure drops in horizontal tubes with plain and enhanced tubes for pure refrigerants and hydrocarbons (including ammonia), hydrocarbon and refrigerant mixtures and oil effects; (ii) condensation heat transfer in horizontal tubes with plain and enhanced tubes for pure fluids and mixtures, (iii) bundle boiling heat transfer and two-phase pressure drops on plain, low finned and enhanced tube bundles for pure hydrocarbons and refrigerants, hydrocarbon and refrigerant mixtures and also oil effects; (iv) falling film evaporation heat transfer on plain, low finned and enhanced tube bundles; (v) falling film condensation heat transfer on plain, low finned and enhanced tube bundles; (vi) flow boiling heat transfer and two-phase pressure drops for CO2 in microchannels, macrochannel tubes and multiport tubes and for its use as a secondary refrigerant; (vii) flow boiling heat transfer, flow pattern maps, critical heat flux and hot spot cooling in multi-microchannels for computer and power electronic cooling applications; (viii) numerical modeling of laminar condensation in circular and non-circular microchannels, and (ix) numerous methods for describing two-phase flows, bubble dynamics and flow pattern transitions to introduce a more theoretical background into engineering practice. This work is backed up by extensive experimental work including flow visualization for investigation of fundamental phenomena. This research work has been in part sponsored by the Swiss National Science Foundation, European projects such as HMTMIC and EFROST, the European Space Agency (ESA), industrial sponsorships by ABB, HTRI, Valeo, etc., my lab's Falling Film Research Club consortium (with Wolverine Tube, Wieland-Werke, The Trane Company, Johnson Controls, UOP, etc.), ASHRAE and ARI, etc. The emphasis of this work has always been to develop new thermal design methods that will actually be used in engineering practice (based on my 15-year consulting experience) and not only reside in research journals. As for my direct contributions to engineering practice and education, I have authored four books: Enhanced Boiling Heat Transfer (1990), Convective Boiling and Condensation, 3rd Edition (1994) with J.G. Collier, and the Wolverine Engineering Databook III (2004). My e-book Databook III is available for free on the website of Wolverine Tube Inc. and is the most widely used reference book by practicing heat transfer engineers around the world (and now includes 150 additional videos of two-phase phenomena and an EXCEL calculator for methods within the book). My 4th book entitled Nucleate Boiling on Microstructured Surfaces (with M.E. Poniewski) is ready for publication in early 2008 and will also be available free to download from the website of Heat Transfer Research Inc. I have authored (or have in press or in review) nearly 40 journal papers in the last two years (2006-2007). A list of publications is attached at the end of this summary. As a further general contribution to the heat transfer community, I have organized nearly twenty international 2- to 5-day long engineering short courses on the topics of boiling and condensation (my last two in Lausanne were attended by 74 participants from 17 different countries). My LTCM laboratory currently has 11 Ph.D. students (plus another starting March 1), 5 post-docs (2 more in arrival), 2 secretaries and currently two visiting professors. Besides directing the LTCM laboratory, I am also Director of the Doctoral School in Energy, which has about 55 Ph.D. students representing 4 different faculties at the EPFL. I am also the new Director of the European ERCOFTAC Coordination Centre (European Research Community On Flow, Turbulence And Combustion), which has about 120 member institutions (universities, national laboratories and companies) throughout Europe. One of my lab's current main areas of research is two-phase flow and heat transfer in microchannels. This work is presently supported by 6 different contracts with total funding of nearly $1 million. The emphasis is on fundamental research while the principal applications are cooling computer chips and power-electronics by boiling (other applications are cooling of micro-reactors and two-phase separators). While these applications are not original, what is original here are the following: (i) the first viable solution has been obtained to achieve very large cooling rates using a low pressure refrigerant as the working fluid rather than water (which is not a viable candidate for CPU's) for boiling in multi-microchannels made in either copper or silicon (we have attained a heat removal rate of 3.4 MW/m2 so far with a chip interface temperature below 60°C and our theory shows that we can extrapolate this to over 9 MW/m2 with new test geometries now being prepared (ii) the development of the first theoretically based thermal design tools to predict the critical heat flux, the local flow boiling heat transfer coefficients, void fraction, two-phase flow pattern transitions, the two-phase pressure drops, hot spot heat transfer, etc. for microchannels required for optimization of such systems and (iii) I have proposed and validated methods to overcome the most important operating problems with boiling in microchannel cooling elements with multiple parallel channels. Introducing an orifice at the inlet of each of these channels (via a perforated plate insert or formed naturally by the junction between the microchannel element and the inlet header itself), this solved four no-go problems simultaneously: (i) it yields a uniform flow distribution to all channels (so far we tested up to 134 parallel channels), (ii) it prevents back flow, (iii) it imposes flow stability even at low flow rates (others have reported very significant temperature and flow fluctuations while we have steady flow up to nearly the critical heat flux!) and (iv) it partially flashes the incoming liquid at the entrance of the microchannels (which eliminates the temperature overshoot associated with boiling nucleation) and kick starts the boiling process without a temperature overshoot, all while keeping the pumping power consumption quite low. This solution has a small footprint and height that allows numerous chips to be effectively cooled when set side by side in "blades" or to cool stacks of chips (which is the next step in server design to minimize the performance lost by inter-chip communication). This work has been done in collaboration with IBM and a new multi-year project has just got underway (and also a somewhat similar project with ABB for cooling of power electronics using micro-channel evaporators). Besides cooling of the new generation of high performance computer chips with extremely high heat flux dissipation rates, I have also been highly involved in the development of the thermal design tools for falling film evaporators for the replacement of flooded evaporators now for over 15 years, first as a consultant and now as a university professor. We have (and continue to) develop experimentally backed prediction methods for falling film evaporators and falling film condensers with enhanced tube bundles. Our work on falling film evaporators has resulted in local thermal methods for predicting local heat transfer coefficients and the onset of dryout in these systems for the new generation of enhanced tubes, while currently we are running prototype bundle tests ourselves to include tube row, vapor shear, distribution, etc. aspects into our methods. We have developed similar local prediction methods (and continue our work on this topic) for enhanced falling film condensers. I expect that this work will eventually be applied also in the petrochemical industry (for example, through my software EHT embedded within HTRI programs), potentially for changing intube vertical falling film evaporators into shell-side falling film evaporators with much higher performance and more energy effective operation. In other current work, I have a bundle boiling research program in which we have recently obtained two-phase flow videos with a high speed camera for flow in the center of a 20-tube bundle (using quartz tubular sections and mirrors), high frequency laser/diode light intensity and piezo-electric pressure signals for quantitative flow pattern identification, local flow boiling heat transfer data, and adiabatic and diabatic pressure drop data for a plain tube bundle with two test fluids. This work will be the basis, hopefully, for a new set of physically based (flow pattern/flow structure) thermal design methods for boiling on tube bundles. Currently preparations are nearly ready to continue this work with enhanced tube bundles.