Young Gil Park

A Review of Metal Foam and Metal Matrix Composites for Heat Exchangers and Heat Sinks

Recent advances in manufacturing methods open the possibility for broader use of metal foams and metal matrix composites (MMCs) for heat exchangers, and these materials can have tailored material properties. Metal foams in particular combine a number of interesting properties from a heat exchangers point of view. In this paper, the material properties of metal foams and MMCs are surveyed, and the current state of the art is reviewed for heat exchanger applications. Four different applications are considered: liquid–liquid, liquid–gas, and gas–gas heat exchangers and heat sinks. Manufacturing and implementation issues are identified and discussed, and it is concluded that these materials hold promise both for heat exchangers and heat sinks, but that some key issues still need to be solved before broad-scale application is possible.

Air-Side Heat Transfer and Friction Correlations for Flat-Tube Louver-Fin Heat Exchangers

The objective of this study is to develop an accurate, reliable, and updated predictive model for the air-side performance of flat-tube louver-fin heat exchangers. Using the most comprehensive experimental database to date-consisting of 1030 heat-transfer and 1270 pressure-drop measurements, from nine independent laboratories for 126 sample heat exchangers-j- and f-factor correlations are developed to predict the air-side performance of heat exchangers. The database is analyzed, the form of the curve fits is explored, and the predictive performance of the correlations is evaluated. The j- and f-factor correlations predict the experimental data with rms errors of 11.5% and 16.1%, respectively. Multiple regressions for a locally linearized data model were used to estimate the confidence intervals and covariances of the regression constants. A comparison to prior correlations shows the proposed correlations to provide more accurate predictions and to span a much broader parameter space than prior work. Practical utility in design and optimization, and unavoidable limitations in developing such correlations are discussed.

A Comparison of Metal-Foam Heat Exchangers to Compact Multilouver Designs for Air-Side Heat Transfer Applications

High-porosity metal foams, with novel thermal, mechanical, electrical, and acoustic properties, are being more widely used in various industrial applications. In this paper, open-cell aluminum foam is considered as a highly compact replacement for conventional louver fins in brazed aluminum heat exchangers. A model based on the ϵ-NTU method is developed to compare the flat-tube, serpentine louver-fin heat exchanger to the flat-tube metal-foam heat exchanger. The two heat exchangers are subjected to identical thermal-hydraulic requirements, and volume, mass, and cost of the metal-foam and louver-fin designs are compared. The results show that the same performance is achieved using the metal-foam heat exchanger but a lighter and smaller heat exchanger is required. However, the cost of the metal-foam heat exchanger is currently much higher than that of the louver-fin heat exchanger, because of the high price of metal foams. If the price of metal foam falls to equal that of louver-fin stock (per unit mass), then the metal-foam heat exchanger will be less expensive, smaller, and lighter than the louver-fin heat exchanger, with identical thermal performance.

The Air-Side Thermal-Hydraulic Performance of Flat-Tube Heat Exchangers With Louvered, Wavy, and Plain Fins Under Dry and Wet Conditions

The air-side thermal-hydraulic performance of flat-tube aluminum heat exchangers is studied experimentally for conditions typical to air-conditioning applications, for heat exchangers constructed with serpentine louvered, wavy, and plain fins. Using a closed-loop calorimetric wind tunnel, heat transfer and pressure drop are measured at air face velocities from 0.5 m/s to 2.8 m/s for dry- and wet-surface conditions. Parametric effects related to geometry and operating conditions on heat transfer and friction performance of the heat exchangers are explored. Significant differences in the effect of geometrical parameters are found for dry and wet conditions. For the louver-fin geometry, using a combined database from the present and the previous studies, empirical curve-fits for the Colburn j- and f-factors are developed in terms of a wet-surface multiplier. The wet-surface multiplier correlations fit the present database with rms relative residuals of 21.1% and 24.4% for j and f multipliers, respectively. Alternatively, stand-alone Colburn j and f correlations give rms relative residuals of 22.7% and 29.1%, respectively.

A simple air-side data analysis method for partially wet flat-tube heat exchangers

An air-side data analysis method is developed for flat-tube heat exchangers under partially wet conditions. In order to simplify the combined sensible and latent heat transfer, it is assumed that condensate drainage paths develop such that, at steady state, water does not spread to noncondensing surfaces, which therefore remain dry. The air dew point is compared to local fin-tip and fin-base temperatures, and a partially wet flat-tube heat exchanger is partitioned into fully wet, partially wet, and dry-fin regions, which are subsequently analyzed as separate heat exchangers. Using an enthalpy-based effectiveness–NTU (number of transfer units) method, average fin efficiency is calculated for each region, and the locations of region boundaries are determined iteratively. The proposed data analysis method is demonstrated with experimental data for a flat-tube louver-fin heat exchanger under various latent loads. The general approach presented can be extended to other heat exchanger geometries.

The Potential Impact of Nanofluid Enhancements on the Performance of Heat Exchangers

The potential benefit of using nanofluids in heat-exchanger applications is explored using an ϵ-NTU analysis. As expected, the enhancement in number of transfer units (NTU) is largest when the convection resistance associated with the flow to be enhanced is a large contributor to the overall thermal resistance of the baseline heat exchanger. Likewise, as expected, nanofluid enhancement has a larger impact for poorly performing baseline heat exchangers. For a single-pass cross-flow configuration, with both fluids unmixed, the ratio of heat capacity rates is unimportant in nanofluid enhancement. Among all flow arrangements investigated, the concentric-tube, counterflow arrangement shows highest improvement in heat duty for a prescribed convective heat-transfer enhancement. The enhancement in convection also suggests a significant saving of heat-transfer area/material, and the saving also increases with the resistance ratio of the enhanced stream to the overall thermal resistance. The impact of specific heat reduction due to the existence of nanoparticles in the stream is also investigated.

Numerical Modeling of Liquid Drop Spreading Behavior on Inclined Surfaces

A numerical study was conducted on the spreading behavior of liquid drops on flat solid surfaces. The model predicts the shape of liquid-vapor interface under static equilibrium using an unstructured surface grid composed of triangular elements. Incremental movement of base contour, i.e. solid-liquid-vapor contact line, is also captured such that the constrained boundary conditions, i.e. advancing and receding contact angles, can be satisfied. The numerical model is applied to a common experiment that studies the behavior of liquid drops on inclined surfaces, where the shape of the drops change in response to an alteration of total volume or gravitational direction. On a heterogeneous surface that has contact angle hysteresis, the shape of the base contour on the solid surface is not determined uniquely but rather dependent upon history. This study demonstrates such dependence by comparing the spreading of a liquid drop on a solid surface with different quasi-equilibrium paths.Copyright