Henk Huisseune

An experimentally validated and parameterized periodic unit-cell reconstruction of open-cell foams

The physical behavior of open-cell foams depends on their microscopic structure. An open-cell geometrical model is proposed, which can serve as the basis for a future macroscopic analysis. The strut geometry is of particular interest, as it is reported to have substantial influence on the occurring thermo-hydraulic and mechanical phenomena. Axial strut size variation, as well as the porosity dependence of shape is quantified and included in a geometrical model. The foam is generated by placing the struts on an elongated tetrakaidecahedron. The required input parameters for the model are two cell dimensions, corresponding to the mean transverse and conjugate diameters of the ellipse encompassing a cell, and the strut cross-sectional surface area at its midpoint between two nodes. The foam geometry is generated iteratively, as porosity is used as validation. A high resolution micro-computed tomography scan is performed to measure the three parameters, the resulting porosity and surface-to-volume ratio. This allows to validate the model. The predictions are found to be within measurement accuracy. A numerical implementation of the model in the preprocessor of a commercial CFD package is demonstrated.

Thermal hydraulic study of a single row heat exchanger with helically finned tubes

In this study, the heat transfer and friction correlation of a single row heat exchanger with helically finned tubes are experimentally determined. The transversal tube pitch was parametrically varied. A detailed description of the test rig and the data reduction procedure is given. A thorough uncertainty analysis was performed to validate the results. The proposed heat transfer correlation can describe 95% of the data within +/- 11% and shows a 4.49% mean deviation. The friction correlation predicts 95% of the data within +/- 19% with a mean deviation of 6.84%. The new correlations show the same trend as most correlations from open literature, but none of the literature correlations are able to accurately predict the results of this study.

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.

Combined Experimental and Numerical Flow Field Study of Inclined Louvered Fins

In this study the flow behavior within an interrupted fin design, the inclined louvered fin, is investigated experimentally through visualization and numerically through computational fluid dynamics (CFD) simulation. The inclined louvered fin is a hybrid of the offset strip fin and standard louvered fin, aimed at improved performance at low Reynolds numbers for compact heat exchangers. The flow behavior is studied in six geometrically different configurations over a range of Reynolds numbers and quantified using the concept of “fin angle alignment factor,” which is related to the flow efficiency in louvered fins. The experimental data resulted in a discrete data set of local fin angle alignment factor values, which were used to validate the simulations. Using these validated cases it is shown that the graphical measurement method can be distorted by recirculation zones, resulting in erroneous values. Care should thus be taken when performing graphical measurement of the mean flow angle based on dye injection images. The transition from steady laminar to unsteady flow in inclined louvered fins is geometrically triggered and occurs at lower Reynolds numbers compared to slit fins and standard louvered fins. This property can potentially be used to further improve on the performance of interrupted fin surfaces.

Numerical optimization of louvered fin heat exchanger with variable louver angles

Several studies of the louvered fin heat exchanger have already been done. Both experimental and numerical studies are available. Investigations to the optimal louver angle have been performed, many times in combination with other fin parameters such as louver pitch and fin thickness. Most studies assume a single louver angle for all the louvers in the heat exchanger. Hsieh and Jang [1] on the other hand studied the effect of a variable louver angle for 5 different cases with successively increasing or decreasing louver angles. Tube-fin interactions were not taken into account. In this study, a round tube and fin geometry with individually varying louver angles is analyzed. The thickness of the fin was neglected. Any interactions between the optimal louver angles and the fin thickness are hence not captured. A laminar and steady calculation was performed, with symmetric boundary conditions. For the Reynolds number on the hydraulic diameter (ReDh) of 535 that was studied, a Von Karman vortex street is present behind the last tube row of heat exchanger. The steady calculation is hence only an approximation of the reality, but is shown to give reasonable results. An ordinary kriging response surface model was used to explore the entire parameter space. Updates to the model were made on the basis of improving the Pareto front, visualizing the tradeoff between heat transfer and pressure drop. It is shown that the use of individually varying louver angles allows increasing the Colburn j factor by 1.3% for the same friction factor, with respect to the optimal uniform louvered fin configuration.

Numerical Study of Flow Deflection and Horseshoe Vortices in a Louvered Fin Round Tube Heat Exchanger

In louvered fin heat exchangers, the flow deflection influences the heat transfer rate and pressure drop and thus the heat exchangers performance. To date, studies of the flow deflection are two-dimensional, which is an acceptable approximation for flat tube heat exchangers (typical for automotive applications). However, in louvered fin heat exchangers with round tubes, which are commonly used in air-conditioning devices and heat pumps, the flow is three-dimensional throughout the whole heat exchanger. In this study, three-dimensional numerical simulations were performed to investigate the flow deflection and horseshoe vortex development in a louvered fin round tube heat exchanger with three tube rows in a staggered layout. The numerical simulations were validated against the experimental data. It was found that the flow deflection is affected by the tubes in the same tube row (intratube row effect) and by the tubes in the upstream tube rows (inter-tube row effect). Flow efficiency values obtained with two-dimensional studies are representative only for the flow behavior in the first tube row of a staggered louvered fin heat exchanger with round tubes. The flow behavior 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 of the tubes of the downstream tube rows and thus the horseshoe vortex development at these locations. The results of this study are important because the flow behavior is related to the thermal hydraulic performance of the heat exchanger.

Thermal Analysis of a Commercial Plate Fin Heat Exchanger With Nonuniform Inlet Flow Conditions

In studies using computational fluid dynamics software, very often a uniform air stream is applied as an inlet boundary condition of a heat exchanger. In actual applications, however, the inlet flow conditions are not uniform. Therefore, the effect of nonuniformities on the thermal performance is characterized in a wind tunnel for a commercially available plate water/air heat exchanger. Three nonuniform flow conditions are investigated. The heat exchanger is 275 mm wide and 295 mm high. Three nonuniformities are created by placing a plate 10 cm upstream of the heat exchanger: The first one covers the right-hand side of the heat exchanger, the second one covers the top half of the heat exchanger, and the last obstruction consists of a circular hole of 150 mm diameter in the middle of a plate. Only the circular obstruction has a significant influence on the heat transfer rate: The external convective resistance is up to 25% higher compared to the uniform case. The measurement results presented in this study can be used by other researchers to validate numerical simulations with nonuniform inlet conditions.

An experimental study of natural convection in open-cell aluminum foam

Natural convecton n air-saturated alumnum foam has been measured. A carefully designed experimental setup is built for his ask. The calibraton is done by comparing he results of a flat plate wh literature data, revealing excellent agreement. The nvestigated foams have a pore densiy of 10 and 20 PPI. The bondng of the foam is performed via brazing, or by applying a single epoxy which is enriched wh highly conductve alumna particles. The Rayleigh number is varied between 2500 and 6000, wh he rato of he surface area o he perimeter of he substrate as characteristc length. The foam height is varied between 12 and 25.4 mm. A major difference between both he bondng methods is observed. The brazed samples showed a beter heat ransfer n all cases. Furthermore, when ncreasing he foam height, a clear augmentaton of he heat ransfer is observed. Based on hese results, a correlaton is presented.

Evaluation of heat transfer models with measurements in a hydrogen-fuelled spark ignition engine

Hydrogen-fuelled internal combustion engines are still investigated as an alternative for current drive trains because they have a high efficiency, near-zero noxious and zero tailpipe greenhouse gas emissions. A thermodynamic model of the engine cycle enables a cheap and fast optimization of engine settings for operation on hydrogen. The accuracy of the heat transfer sub model within the thermodynamic model is important to simulate accurately the emissions of oxides of nitrogen which are influenced by the maximum gas temperature. These emissions can occur in hydrogen internal combustion engines at high loads and they are an important constraint for power and efficiency optimization. The most common models in engine research are those from Annand and Woschni, but they are developed for fossil fuels and the heat transfer of hydrogen differs a lot from the classic fuels. We have measured the heat flux and the wall temperature in an engine that can run on hydrogen and methane and we have investigated the accuracy of simulations of the heat transfer models. This paper describes an evaluation of the models of Annand and Woschni with our heat flux measurements. Both models can be calibrated to account for the influence of the specific engine geometry on the heat transfer. But if they are calibrated for methane, they fail to calculate the heat transfer for hydrogen combustion. This demonstrates the models lack some gas or combustion properties which influence the heat transfer process in the case of hydrogen combustion.