Peter De Jaeger

How to Study Thermal Applications of Open-Cell Metal Foam: Experiments and Computational Fluid Dynamics

This paper reviews the available methods to study thermal applications with open-cell metal foam. Both experimental and numerical work are discussed. For experimental research, the focus of this review is on the repeatability of the results. This is a major concern, as most studies only report the dependence of thermal properties on porosity and a number of pores per linear inch (PPI-value). A different approach, which is studied in this paper, is to characterize the foam using micro tomography scans with small voxel sizes. The results of these scans are compared to correlations from the open literature. Large differences are observed. For the numerical work, the focus is on studies using computational fluid dynamics. A novel way of determining the closure terms is proposed in this work. This is done through a numerical foam model based on micro tomography scan data. With this foam model, the closure terms are determined numerically.

Particle deposition effects on heat transfer from a metal foam‐wrapped tube bundle

Purpose - The purpose of this paper is to clarify the relationship between dust deposition effects on the thermohydraulic performance of a metal foam heat exchanger. Design/methodology/approach - The paper uses finite volume approximation to solve the two-dimensional volume-averaged form of governing equations through and around a metal foam-covered tube bundle. Modified porosity, permeability, and form drag coefficient for a dusty foam layer are obtained through the application of a thermal resistance network model. Findings - The paper provides novel data to predict the fouling effects on the performance of foam-wrapped tube bundles as air-cooled heat exchangers. It is observed that depending on the deposited layer thickness, the increased pressure drop and heat transfer deterioration can be very significant. Originality/value - This paper fulfils an identified need to study fouling effects on thermohydraulic performance of a foam heat exchanger.

Influence of geometrical parameters of open-cell aluminum foam on thermohydraulic performance

The influence of the geometry of open-cell aluminum foam on the thermohydraulic behavior in channel flow is investigated. The mean cell diameter and the strut cross-sectional surface area are chosen as geometrical parameters, ranging respectively between 1.2 and 5.2 mm and between 0.0125 and 0.17 mm2. The flow arrangement and the operating conditions are fixed. A numerical model is implemented in a commercial solver, based on volume averaging theory. The model is validated against experimental data. The porous properties, which take the sub-REV scaled physics into account, are written as a function of both geometrical parameters. The thermohydraulic characteristics of 16 well-chosen foams are used to build a surrogate model. An ordinary Kriging model is used for this, indicating that the root mean square error of interpolated results is lower than 0.6 and 3% for, respectively, heat transfer and total pressure. The resulting heat transfer and total pressure difference are nondimensionalized by dividing them by the results obtained from an empty channel. The relative increment of the pressure drop is an order of magnitude higher than the increment observed for heat transfer. Consequently, the applied performance evaluation criterion (defined as the ratio of dimensionless heat transfer versus total pressure) is mainly influenced by the hydraulic performance. For the given application, a clear optimum is found. The proposed method allows performing the parameter study with acceptable computational cost with a sufficient level of detail from an engineering perspective.

Heat transfer through vertically downward blowing single-jet air curtains for cold rooms

One of the major sources of heat gain in refrigerated storage rooms is the infiltration of warm ambient air through doorways. Air curtains reduce this amount of heat transfer by blowing a plane air jet in the doorway while allowing an easy passage of the traffic. An air curtain device installed at the doorway of a cold room in a supermarket was studied in detail. Thermographic images were taken, recording the temperature field across the doorway. Tracer gas decay measurements were used to estimate the airflow rate through the door. These measurements were then used to validate a computational fluid dynamics (CFD) model of the air curtain. With this CFD model the impact of some important air curtain parameters, such as the jet velocity and the jet nozzle width, on the heat transfer rate through the opening is determined. Finally, an expression to estimate the heat transfer rate through the air curtain is proposed.

A Discussion on the Interpretation of the Darcy Equation in Case of Open-Cell Metal Foam Based on Numerical Simulations

It is long known that for high-velocity fluid flow in porous media, the relation between the pressure drop and the superficial velocity is not linear. Indeed, the classical Darcy law for shear stress dominated flow needs to be extended with a quadratic term, resulting in the empirical Darcy–Forchheimer model. Another approach is to simulate the foam numerically through the volume averaging technique. This leads to a natural separation of the total drag force into the contribution of the shear forces and the contribution of the pressure forces. Both representations of the total drag lead to the same result. The physical correspondence between both approaches is investigated in this work. The contribution of the viscous and pressure forces on the total drag is investigated using direct numerical simulations. Special attention is paid to the dependency on the velocity of these forces. The separation of the drag into its constituent terms on experimental grounds and for the volume average approach is unified. It is shown that the common approach to identify the linear term with the viscous forces and the quadratic term with the pressure forces is not correct.

Three-Dimensional Simulations of the Flow Deflection in a Louvered Fin Heat Exchanger With Round Tubes in a Staggered Arrangement

In this study three-dimensional numerical simulations were performed to investigate the flow deflection and its influence on the horseshoe vortex development in a louvered fin round tube heat exchanger with three tube rows in a staggered layout. The numerical results are validated against experimental data. It was found that the flow deflection is affected by the tubes in the same tube row and by the tubes in the upstream tube rows. The flow efficiency values obtained with two-dimensional studies are only representative for the flow behaviour in the first tube row of a staggered louvered fin heat exchanger with round tubes. The flow behaviour in the louvered elements of the subsequent tube rows differs strongly due to its three dimensional nature. Furthermore it was found that the flow deflection affects the local pressure distributions upstream the tubes of the downstream tube rows which explains the stronger horseshoe vortex system at these locations.Copyright

Flow Field Study in Louvered Fin and Round Tube Heat Exchangers

Three-dimensional flow structures influence the heat exchanger’s performance. In this study flow visualization experiments were performed in six scaled-up models of a louvered fin heat exchanger with round tubes. The models have a staggered tube layout and differ only in their fin spacing and louver angle. A water tunnel was designed and built and the flow visualizations were carried out using dye injection. For small Reynolds numbers no horseshoe vortices are developed in front of the tubes and the recirculation regions downstream the tubes are small. As the Reynolds number is increased, the horseshoe vortices become larger and stronger. The recirculation bubbles grow until they cover the entire back of the tube. When the Reynolds number is further increased, the recirculation region becomes unsteady. At the same Reynolds number the vortex strength and the number of vortices in the second tube row is larger than in the first tube row. Reducing the fin pitch suppresses the vortex and wake development. Further it was found that the first unsteady flow patterns appear in the wake of the heat exchanger and these instabilities move upstream with increasing Reynolds number. The onset of unsteadiness is postponed to higher Reynolds numbers when the fin pitch or louver angle is reduced.Copyright

A Transient Technique to Determine Thermal Conductivity and Thermal Contact Resistance of Porous Materials

The application of a transient technique for the measurement of effective thermal conductivity and thermal contact resistance of porous media is discussed. A sensitivity analysis has proven that direct measurement of thermal contact resistance from a single temperature recording is not feasible. It requires the measurement of at least one additional sample with different height. The estimation of effective thermal conductivity is done by solving the inverse heat conduction problem (IHCP). The direct problem is treated analytically by describing the system with a quadrupole formalism in Laplace domain. The inversion procedure was found to be computational expensive. For this reason, the analytical solution of a reference case was obtained and used to validate a finite difference scheme. The indirect problem of the IHCP is solved via the Levenberg-Marquardt algorithm. Preliminary results are shown to demonstrate the method. Future actions consist of calibrating the experimental setup, benchmark with known materials and report uncertainty.© 2010 ASME

Thermo-Hydraulic Performance of a Heat Exchanger Consisting of Metal Foam Covered Tubes

Open cell metal foam offers an interesting combination of materials properties from a heat exchanger point of view such as a high specific surface area, tortuous flow paths for flow mixing and low weight. A heat exchanger design with metal foams is studied in this work, aimed at low airside pressure drop. It consists of a single row of Al tubes covered with thin layers (4–8 mm) of metal foam. Through wind tunnel testing the impact of various parameters on the thermo-hydraulic performance was considered, including the Reynolds number, the tube spacing, the foam height and the type of foam. The results indicated that if a good metallic bonding between the foam and the tubes can be achieved, metal foam covered tubes with a small tube spacing, small foam heights and made of foam with a high specific surface area potentially offer strong benefits at higher air velocities (> 4 m/s) compared to helically finned tubes. The bonding was done by conductive epoxy glue and was found to have a strong impact on the final results, showing a strong need for a cost-effective and efficient brazing process to connect metal foams to the tube surfaces.Copyright