Bernd Ameel

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.

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.

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.

Design Sensitivity Analysis of a Plate-Finned Air-Cooled Condenser for Low-Temperature Organic Rankine Cycles

ABSTRACT Due to increasing world energy demand and environmental concerns, sustainable energy production has become crucial. Among sustainable energy sources such as solar, wind, and geothermal, industrial waste heat (heat normally released to the environment) has a big potential. Organic Rankine cycles (ORCs) are promising systems for utilizing low-temperature (100–250°C) waste heat. For an ORC system, the condenser is a key component. An accurate condenser design is important for cycle efficiency and system cost. In the literature, there are in-tube condensation correlations that are used to design condensers. However, they are not necessarily valid for low-temperature ORC conditions and working fluids, and that might lead to inaccurate end designs. This study comprises a summarized literature survey about in-tube condensation correlations. Then an iterative heat exchanger design methodology is proposed that allows performing a design sensitivity analysis on a V-shaped condenser within an input range of geometric parameters and boundary conditions. Nineteen correlations are implemented to calculate rating parameters like pressure drops, total transferred heat, overall heat transfer coefficient, size, cost and degree of subcooling. The deviations at these parameters are represented as the coefficient of variation that indicates the design condition regions where the prediction methods differ or not.

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.

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.

Influence of apex angle on thermal hydraulic behavior of a polymeric triangular cross corrugated plate

Flow and heat transfer in triangular cross corrugated plate heat exchangers are complicated and strongly influenced by geometrical parameters. Due to high geometric flexibility of polymeric materials, different apex angle are feasible for HVAC & R application. In this study the influence of the apex angle and Reynolds number on the thermal hydraulic performance of triangular cross corrugated channels are investigated while the corrugation angle is fixed at 90. In addition the base remained unvaried. With the aid of computational fluid dynamics, three-dimensional simulations are performed for 426 < Re < 2021 in a periodic unitary cell. The Reynolds Stress model is used as the turbulence model. The numerical results are in a very good agreement with experimental results correlation. They show deviations between 0.8 – 4.84 %. The highest thermal performances are achieved by the both apex angle of 120 and 90. The lowest thermal performance is observed by the apex angle of 55. The heat exchanger with the apex angle of 90 has the highest friction factor. Correlations for the friction factor and Nusselt number are proposed. For the Reynolds number lower than 1300, the apex angle of 120 shows the lowest friction factor. However for the Reynolds number higher than 1300, the apex angle of 55 shows the highest hydraulic performance.

Discussion on the Effect of a Fluid Domain around Fins and Grid Discretization in Buoyancy-Driven Convection

ABSTRACT In the numerical study of heat sinks, it is known that a sufficient amount of fluid domain should be added at each side of the heat sink. However, the question in this context is: what can be defined as sufficiently far away from the heat sink? Different authors use different sizes of the computational domain around the heat sink. In this work the impact of the size and location of the fluid domain on the calculated heat transfer coefficient is investigated. Three fin row types are studied: a rectangular, an interrupted rectangular, and an inverted triangular fin row. First, the influence of adding fluid domain to the sides of the heat sink is studied. A large decrease of the heat transfer coefficient on both sides and bottom is observed. Next, the influence of adding fluid domain on both the top and the sides is studied. For the rectangular fins, the impact on the lumped heat transfer coefficient is +12% compared to the case without any fluid domain added. For the inverted triangular fin shape, no net effect is observed on the lumped heat transfer coefficient. So the impact of adding fluid domain depends on fin shape that is investigated. For the sides only, a small amount of fluid needs to be added, while for the fluid domain on top of the heat sink, 130% of the equivalent fin height is found as a good option to simulate the fin in computational fluid dynamics.