A Study in Heat and Mass Transfer with Boiling in Porous Deposits
B. G. Jones* (Nucl., Plasma & Radiol. Engr.), C. Pan, B. Shi
University of Illinois

A numerical/analytical model of boiling heat transfer in heterogeneous porous layers with and without chimneys has been conducted. Experimental observations have provided qualitative modeling information and model refinements. 1-D and 2-D models have been evaluated numerically with nonlinear coupling between mass, momentum, energy, capillary pressure, and evaporation rate. Good agreement with published data has been obtained. Examination of artificially created layer performance suggests broad potential applications for controlled boiling heat transfer, such as computer chip cooling via freon or other CFCs, with heat fluxes in excess of 100 W/cm2, and in steam generator tube performance.

An Experimental Study of Condensate Retention and Its Effect on the Air-Side Performance of Slit-Fin-and-Tube Heat Exchangers
A. M. Jacobi,* G. Kim
Samsung

Heat exchangers in air conditioning applications often operate below the dew point, and water condensing from the air onto the surface affect performance. At high Reynolds numbers, retained condensate enhances heat transfer by acting as a surface protuberance, generating local secondary flows or an early transition to turbulence. At low Reynolds numbers, condensate can accumulate and act as an added resistance to air flow and heat transfer. The overall thermal impact is dependent on geometry and operating conditions. The purpose of this project is to measure of water retention and shedding effects and to develop and validate a model for predicting these effects for a slit-fin heat exchanger.

An Experimental and Computational Study of Two-Phase Flow in Distribution Headers
S. P. Vanka,* P. S Hrnjak,* J. E. Peters,* C. Winkle, P. Feng
NSF I/UCRC Air Conditioning and Refrigeration Center

Maldistribution of two-phase refrigerant flow in headers is influenced by the geometry and orientation of the header, inlet flow velocities, the inlet quality, and flow regime. An understanding of the effects of these quantities on the distribution of two-phase flow into the individual passes is being sought both by experiments and computations.

An Integrated Approach to Experimental Studies of Air-Side Heat Transfer Enhancements
A. M. Jacobi,* N. DeJong, A. El Sherbini, M. Gentry
NSF I/UCRC Air Conditioning and Refrigeration Center, National Science Foundation, EEC 96-12120; American Society of Heating, Refrigerating and Air-Conditioning Engineers

Basic flow and heat transfer mechanisms in louvered-fin and vortex-generator geometries will be studied to answer several unresolved questions. For louvered surfaces, research will address the effects of the approach turbulence intensity and velocity profile on vortex shedding, the ability of the flow to follow the louvers, and sources of acoustic noise in the flow. For vortex generators, the role of the approach turbulence and velocity distribution and the interaction of multiple generators will be studied. Full-scale heat exchanger studies will be conducted, and each surface will be examined using performance evaluation criteria for particular applications.

Combined Spanwise and Streamwise Vorticity to Enhance Air-Side Heat Transfer
A. M. Jacobi,* J. C. Dutton,* H. Ge, M. L. Smotrys
NSF I/UCRC Air Conditioning and Refrigeration Center

An experimental study is being conducted of the heat transfer enhancement of interrupted fins using streamwise vorticity. The general approach is to create a flow with spanwise vorticity using the offset-strip geometry and to introduce streamwise vorticity using a delta wing at the leading edge of a fin. The following experimental methods are being employed: flow visualizations using smoke and dye, full-field velocity data using particle image velocimetry (PIV), and local convective heat transfer measurements using naphthalene sublimation. From these measurements and overall pressure drop data, the heat transfer enhancement and pressure-drop penalty obtained with this system will be assessed.

Condensation of Ammonia in Microchannel Tubes of Aluminum Air-Cooled Condensers
P. S. Hrnjak,* A. Litch
Modine; Hydro Aluminum

Ammonia is the best refrigerant based on thermodynamical and thermophysical properties but charge reduction is essential to overcome its problems with odor and combustability. Microchannel aluminum condensers could be excellent heat exchangers if proven to work satisfactorily. This is the first time this technology is being tried to be used for ammonia.

Developing and Validating Computational Tools to Predict Louvered Fin Performance
A. M. Jacobi,* D. Tafti* (NCSA), S. Balachandar* (Theo. & Appl. Mech.), N. Wartick
LG Electronics

The multi louver heat exchanger surface is primarily used in automotive refrigerant-to-air applications, where air-side face velocities are above 8 m/s. However, recent demands for compactness and effectiveness in domestic air conditioning systems have motivated the adaptation of louvered fins to systems with face velocities from 0.5 to 2 m/s. The current generation of louvered fins, developed for high face velocities, is not optimal for residential air conditioning systems. Our objective is to develop and validate a numerical tool that can evaluate louvered-fin performance for residential air conditioning systems.

Effect of Boiling on Interfacial Behavior during Melt Quench Processes
B. G. Jones* (Nucl., Plasma & Radiol. Engr.), B. W. Spencer,* J. Schneider, M. Marciniak
U.S. Department of Energy, DE-FG07-89ER12900

The effect of interfacial mixing and contact area between two liquids of differing densities and temperatures that result from a high-density, high-temperature liquid passing through a lower density, low-temperature liquid has been studied. Heat transfer effects, including the effects of vapor generation, break-up, and solidification, were modeled. Analytical modeling was carried out at the University of Illinois, while experimental studies using simulant materials of both single and multiple injected columns were conducted at Argonne National Laboratory. Good agreement between model predictions and experimental data is found.

Frost Growth on Conventional, Coated, and Enhanced Heat Transfer Surfaces
J. G. Georgiadis,* A. M. Jacobi,* J. Hoke
NSF I/UCRC Air Conditioning and Refrigeration Center; American Society of Heating, Refrigerating and Air-Conditioning Engineers

This research is directed at developing a fundamental understanding of frost formation and growth in air conditioning and refrigeration applications. This deeper understanding will be used to develop models of frost deposition that account for the microstructure. Detailed experiments to study frost initiation on conventional surfaces and on surfaces coated with hydrophilic and hydrophobic materials are complemented by careful measurements of mature frost growth on flat-plates and on surfaces with louvers and vortex generators. The data and resulting models will help guide the development of frost-tolerant design methods, frost mitigation techniques, and defrost strategies.

Heat Transfer Enhancement in Copper Tubing
T. Newell,* J. C. Chato,* M. Wilson
Copper Development Assn.

This investigation is examining the effects of microfinned copper tubing on heat transfer, pressure drop, and void fraction. Common refrigerants (R134a and R410A) under condensation conditions are being tested.

Heat Transfer of Viscous Single-Phase Secondary Refrigerants When Flowing in Pipes at Very Low Re Numbers
P. S. Hrnjak,* S. H. Hong, Y. Mao
Heatcraft; Kemira

It is reported that new single-phase secondary refrigerants perform better heat transfer than existing correlations predict at very low Re number (100 to 800). Our goal is to experimentally check those correlations for heat transfer inside tubes.

Investigation of Refrigerant/Oil Mixtures in Horizontal Tubes and Flat Plate Evaporators
T. Newell,* J. C. Chato,* C. Tran, S. Gupta
NSF I/UCRC Air Conditioning and Refrigeration Center

Void fraction, pressure drop, heat transfer, and oil concentration will be investigated in a variety of refrigerant tubes and passageways.

Radiative Interactions with Microstructures
L. M. Phinney,* J. W. Rogers
University of Illinois

Mechanical structures with dimensions as small as a few microns are being used in conjunction with electronic circuits to create microelectromechanical systems. These devices offer low weight and batch production methods, which are advantageous for many applications. The small size and mass of these devices is particularly suited for space applications. The reliability of these devices in space depends on their ability to remain functional while exposed to radiation. Additionally, controlling and optimizing laser processing of microdevices during fabrication requires a thorough understanding of radiative interactions with microstructures. This project examines the effect of radiation on microstructures.

Radiative Transfer in Absorbing and Scattering Media
R. O. Buckius,* W. L. Cheng
American Air Liquide

Radiation heat transfer in absorbing and scattering media including general multidimensional gaseous absorption is under consideration. The correlated-k approach is being developed and validated for thermal radiative transport in highly nonhomogeneous media containing water vapor and carbon dioxide. Simplified approaches are being used to model the entire infrared spectrum of water vapor and carbon dioxide, including band overlap regions, for temperatures up to 2500 K.

Refrigerant-side Heat Transfer in Microchannel Tubes
W. E. Dunn,* W. Mamani
NSF I/UCRC Air Conditioning and Refrigeration Center

This project deals with refrigerant-side heat transfer in microchannel tubes used in advanced-design condensers for mobile and stationary air conditioning systems. Microchannel tubes consist of 10 to 30 parallel ports that are typically circular, square, or triangular in cross section and between 0.3 and 1 mm in size. The ports may have internal heat transfer enhancements. Heat transfer is studied for liquid, vapor, and two-phase flow. The purpose of this study is to quantify the heat transfer characteristics of microchannel tubes so that more energy efficient refrigeration systems can be designed.

The Effect of Hydrodynamic, Substrate Energy, and Structure on Frost Growth
J. G. Georgiadis,* P. S. Hrnjak,* A. M. Jacobi,* J. Hoke, D. Carlson, C. Robinson
NSF I/UCRC Air Conditioning and Refrigeration Center

This research is directed at developing a fundamental understanding of frost formation and growth in air conditioning and refrigeration applications. Experiments to explore frost inception and growth on interrupted surfaces and on full-scale heat exchangers will be used to develop a model of frost initiation and growth. The data and resulting model will help guide the development of frost-tolerant design methods, frost mitigation techniques, and defrost strategies.

The Use of Unsteady Flow Effects to Enhance Heat Transfer
A. M. Jacobi,* R. P. Lucht,* S. Kearney
NSF I/UCRC Air Conditioning and Refrigeration Center, National Science Foundation, EEC 96-12120

This project addresses heat transfer in an unsteady, developing, channel flow. Controlling the unsteadiness of the flow may provide significant heat transfer enhancements. This research focuses on the application of this idea to the low Reynolds number flows associated with refrigeration and air conditioning applications. Experiments in rectangular and triangular channels will quantify the heat transfer enhancement as a function of the acoustic or mechanically induced pulse frequency and amplitude, and the boundary layer structure will be investigated in detail using laser diagnostics. The physical mechanisms for the enhancement will be determined with a focus on exploiting these effects.

Thermal Radiation Scattering from Very Rough Surfaces
R. O. Buckius,* P. A. Kawka
National Science Foundation, CTS 95-31772; National Center for Supercomputing Applications, CT 950044N

This research program consists of a combined analytical and experimental investigation of the scattering and emission from realistic interfaces, including those with surface length scales on the order of the wavelength. The objectives are to rigorously quantify the scattering of thermal radiation from electromagnetic theory, to develop approximate yet accurate models, and to experimentally determine reflection for such interfaces. Rigorous electromagnetic theory and approximate geometric optics and diffraction models have been developed and compared with experimental findings.

Transcritical CO2-Understanding In-Tube Heat Transfer
J. G. Georgiadis,* A. M. Jacobi,* J. Petterson* (SINTEF Refrigeration Engr., Norway), J. Aldana
NSF I/UCRC Air Conditioning and Refrigeration Center, National Science Foundation, EEC 96-12120

This project is directed at developing a fundamental understanding of heat transfer and flow phenomena in the transcritical section of CO2 air-conditioning and refrigeration systems. Microscopic temperature and heat flux sensors will be developed for high resolution mapping of the refrigerant side thermal field in metallic pipes. This, along with local visualization, will facilitate the design of the next generation of compact CO2 systems.

Water Retention and Shedding Effects on Air-Side Heat Transfer Behavior
A. M. Jacobi,* C. Korte, J. Yin
NSF I/UCRC Air Conditioning and Refrigeration Center, National Science Foundation, EEC 96-12120; American Society of Heating, Refrigerating and Air-Conditioning Engineers

In many applications, water retention and shedding affect heat exchanger performance. Unfortunately, there are no general models available for predicting condensate retention and its effects on heat transfer. The purpose of this project is to develop and validate a model that can predict condensate retention in these applications and to provide design correlations for the wet performance of heat exchangers. This research will provide the first geometrically generalized model of condensate retention. Special attention will be directed at condensate management for enhanced surfaces.