Showing papers by "Anthony M. Jacobi published in 2004"

Heat Transfer Model for Evaporation in Microchannels, Part I: Presentation of the Model

Abstract: Reference LTCM-ARTICLE-2005-021View record in Web of Science Record created on 2005-07-06, modified on 2017-05-10

Heat Transfer Model for Evaporation in Microchannels, Part II: Comparison with the Database

Abstract: Reference LTCM-ARTICLE-2005-020View record in Web of Science Record created on 2005-07-06, modified on 2017-05-10

Liquid drops on vertical and inclined surfaces: II. A method for approximating drop shapes

Abstract: In this article, a new method is proposed to approximate the shapes of liquid drops on vertical and inclined surfaces. Based on observations from Part I, the profile of a drop at a given azimuthal angle is approximated by two circles sharing a common tangent at the maximum height. The drop volume is obtained by integrating all profiles over the circumference of the base. The volume is thus described as a function of the contact angles and the three-phase contact line. The new method accurately predicts the volumes of drops tested in Part I and independent measurements from the literature. Simplifying the drop shape to a spherical cap can lead to a 75% error in drop-volume prediction. The proposed method is used to study the effect of drop parameters on volume prediction. The two-circle geometry can also be used to measure contact angles from profile images.

Effect of Substrate Wettability on Frost Properties

Abstract: Microscopic observations of frost deposition on clean glass (hydrophilic) and polytetrafluoroethylene (PTFE) (hydrophobic) substrates allow the quantification of frost growth and characterization of structure. In contrast to early growth behavior, the thickness of the frost layer during the mature growth increases faster on substrates that have lower contact angles, and the frost density is less than that measured for high-contact angle substrates. This behavior is explained in terms of the effects of substrate wettability and its impact on condensate distribution, the initial condition for frost growth. A higher conductivity layer is formed on the hydrophilic than on the hydrophobic substrate. Modeling that is based on a relation between thermal conductivity and frost structure is used to predict the growth rate and density on the two different substrates. These predictions agree with the experimental data and support the explanation that substrate wettability affects mature frost growth through its effect on condensate distribution at frosting incipience.

Modified wilson-plot technique for heat exchanger performance: strategies for minimizing uncertainty in data reduction

Abstract: The so-called ‘modified Wilson-Plot’ technique is commonly accepted as the preferred method for interpreting airside heat transfer performance data for liquid- and refrigerant-to-air heat exchangers. Indeed, there are good reasons to believe Wilson-Plot techniques are superior to alternative data interpretation schemes. Unfortunately, Wilson-Plot methods are conceptually and computationally more complex than the other methods and it is not possible to simply extend the single-sample uncertainty analysis of Kline and McClintock to the modified Wilson-Plot technique. A rational approach to uncertainty analysis for the modified Wilson-Plot is not available in the literature, but if this method—accepted as superior to others—is to be widely adopted, a well-defined approach to uncertainty analysis is needed. The purpose of this paper is to provide an uncertainty analysis appropriate for the modified Wilson-Plot technique.

Work in progress: a concept inventory for heat transfer

Abstract: A concept inventory has been developed for heat transfer, which is typically taken in the junior or senior year in mechanical engineering. The subject builds on material previously introduced in thermodynamics and fluid mechanics, and the concepts in these subjects are intertwined. The course has been given as a pretest in one course. An item analysis of the inventory has been made to assess the validity of the inventory, showing that further work needs to be done. The inventory will be given as a post test and then modifications to the inventory will be made. The current draft inventory is available for testing and evaluating.

Assessing the condensate-drainage behavior of dehumidifying heat exchangers

Abstract: Contemporary compact heat exchangers often employ very high fin densities. In many air-conditioning applications such as automotive cooling, the heat exchanger operates to dehumidify the conditioned air. It has been observed that the amount of condensate retained on the air-side surface significantly increases with increased fin density. Retained condensate not only affects the thermal-hydraulic performance, but it also has adverse implications on the quality of conditioned air. Conventional wind tunnel experiments to assess performance are expensive. A method for characterizing water drainage from heat exchanger surfaces, the so-called dynamic dip test, is described. The dynamic dip test is explored as a fast and efficient method for comparative and screening studies for applications where off-cycle drainage is important. Results from more than 30 automotive-style heat exchangers are presented, and condensate drainage characteristics are explored to assess surface tension effects in inter-fin and louver gaps of highfin-density, compact heat exchangers.

Critical contact angles for liquid drops on inclined surfaces

Abstract: The contact angles of liquid drops on inclined plane surfaces have been investigated at critical conditions of sliding. Drops of two liquids were tested on eight surfaces covering a wide range of advancing and receding contact angles. Contact angles and contact line dimensions were observed for various drop sizes and surface inclination angles. The contact-angle variation along the circumference of the drop was best fit by a third-degree polynomial of the azimuthal angle. The findings lead to the development of a general relation between the advancing contact angle, the receding contact angle, and the maximum Bond number for a liquid–surface combination. This relation is well supported by data from the literature. The new relation allows the characterization of a surface by its advancing contact angle for a given liquid, and prediction of the corresponding receding angle and maximum drop size.

Experimental Verification of Ultra-High-Performance, Advanced-Propulsion Heat Exchangers

Abstract: Hi gh-performance compact heat exchangers are a critical technology for many advanced air-breathing propulsion systems, including liquid air cycle engines (LACE). In order to make these systems practical, large performance gains over current state -of-practice ground-based heat exchangers must be realized to reduce heat exchanger mass. Theoretical investigations have predicted these gains to be achievable through advanced geometry and operation at very high Reynolds numbers. This paper presents the initial results of experimental studies at the University of Illinois, designed to verify these predicted gains. The experimental apparatus used to test heat transfer performance is based on the naphthalene sublimation mass transfer technique. Experimental results are presented for Reynolds numbers as high as 100,000 – more than ten times higher than any published study to date. Comparison of the results to previous predictions for high-Re heat exchanger performance shows order-of-magnitude increases in the heat transfer coefficient, suggesting that ultra-high performance compact heat exchangers are achievable for propulsion applications.

Air-side heat transfer augmentation of a refrigerator evaporator using vortex generation

Abstract: In air-cooling applications with frost production, heat exchanger performance is often air-side limited, because the two-phase flow of the refrigerant provides excellent heat transfer coefficients, and because designers typically use a large air-side fin spacing for frost tolerance. The large fin spacing prolongs operation of the heat exchanger by mitigating the effect of the accumulating frost, but it does not allow for a very high air-side heat transfer coefficient. Longitudinal vortex generation is a proven and effective technique for thinning the thermal boundary layer and enhancing heat transfer, but its efficacy in a frosting environment is essentially unknown. In this study, an array of delta-wing vortex generators is applied to a plain fin-and-tube heat exchanger with a fin spacing of 8.5 mm. Heat transfer and pressure drop performance are measured to determine the effectiveness of the vortex generator under frosting conditions. For Reynolds numbers between 500 and 1200, a reduction of 35.0% to 42.1% is observed in the air-side thermal resistance. Correspondingly, the heat transfer coefficient is observed to be between 33-53 W/m 2 -K for the enhanced heat exchanger and between 18-26 W/m 2 -K for the baseline heat exchanger. A modified volumegoodness parameter is also calculated and shows that the enhanced exchanger outperforms the baseline specimen for the range of Reynolds numbers examined.

Enhancement of air-side heat transfer in offset-strip fin arrays using unsteady forcing

Abstract: A series of thin, mechanically oscillated vanes is placed upstream of an offset-strip fin array in an attempt to enhance the thermal performance of a heat exchanger by exciting flow instabilities that have previously been linked to an enhancement in heat transfer. Dye-in-water flow visualization experiments are conducted to provide a qualitative proof-of-concept for this work-in-progress, by matching flow characteristics in a forced array to the welldocumented unforced case. Flow visualization experiments demonstrate that when the array is subjected to harmonic forcing at specific frequencies and amplitudes, flow instabilities appear within the array, and these flow instabilities are qualitatively similar to natural flow instabilities that have been previously shown to enhance heat transfer.

An Empirical Study of Frost Accumulation Effects on Louvered-Fin, Microchannel Heat Exchangers

Abstract: The use of folded-louvered-fin, microchannel heat exchangers in refrigeration and heat pump applications, where normal operating conditions give rise to frost accumulation on the airside heat transfer surface, is experimentally studied. Heat transfer and pressure drop data are presented for heat exchangers with different fin geometries. The visualization of frost growth via a 0.5mm fiberscope indicates that the bridging of louver gap by frost contributes to a decrease in airside heat transfer coefficient as the frosting process evolves. The bridging of the louver gap implies that the airside flow pattern changes from a louver-directed to a duct-directed flow, with a deleterious impact on heat transfer. The impacts of frost growth on heat transfer and thermal-hydraulic performance for different fin geometries are compared in terms of f and j factors.