Christophe T’Joen

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.

Capacitance sensor design for refrigerant two-phase flow characterization

A capacitance sensor configuration has been designed to characterize horizontal two‐phase flow of refrigerants, based on the difference in electrical permittivity of the vapour and liquid phase. A concave electrode design with active guarding was chosen. Finite element simulations were made for optimizing the design. The thickness of the dielectric layer separating the two‐phase flow from the electrodes is an important design parameter and was investigated in detail. The effect of the different flow regimes on the electrical field distribution and total capacitance was simulated for several inscribed electrode angles. An electronic circuit was built to accurately measure the capacitance. Experimental test results with air‐water flow in transparent 9mm tubes are presented. Experimental results with refrigerants are necessary for further development of objective and accurate flow regime transition and two‐phase flow pattern maps during evaporation or condensation of refrigerants.

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.

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.

Earth-Air and Earth-Water Heat Exchanger Design for Ventilation Systems in Buildings

Earth-air heat exchangers are often used for (pre)heating or (pre)cooling of ventilation air in low energy or passive house standard buildings. Several studies have been published in the passed about the performance of these earth-air heat exchangers [1–8]. Often this is done in relation to the building energy use. Several software codes are available with which the behaviour of the earth-air heat exchanger can be simulated. De Paepe and Janssens published a simplified design methodology for earth-air heat exchangers, based on thermal to hydraulic performance optimisation [7]. Through dynamic simulations and measurements it was shown that the methodology is quite conservative [9–10]. Hollmuller added an earth-air heat exchanger model to TRNSYS [11]. In stead of using earth-air heat exchangers, earth-water heat exchangers are now getting more attention. In this system the ventilation air is indirectly cooled/heated with the water flow in a fin-tube heat exchanger in the inlet of the ventilation channel. The water-glycol mixture transfers heat with the earth by flowing through e.g. a polyethylene tube. In the second part of this paper a design methodology is first derived and then applied to this type of system.Copyright

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