Hugo Canière

Horizontal two-phase flow characterization for small diameter tubes with a capacitance sensor

Two-phase flow modelling is strongly dependent on flow patterns. For the purpose of objective flow pattern identification, a capacitance sensor was developed for horizontal two-phase flow in small diameter tubes. Finite element simulations were made during design to study the effect of vapour distribution, wall thickness and electrode angle. A test rig was constructed and a series of experiments was done with horizontal air–water flow in a 9 mm tube. The sensor test results are presented in time, amplitude and frequency domain. Flow regime characterization with the capacitance measurements is clearly possible.

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.

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.

Refrigerant flow regime detection with a capacitance void fraction sensor

Flow regime prediction in air-conditioning units is of great importance for designing the inlet distributor and the pass layout of the evaporators/condensers. An important parameter of the two-phase flow inside the tubes is the void fraction. Void fractions and flow regimes are often related in flow pattern maps. In order to refine and expand these maps, more experiments have to be performed. At the Ghent University’s Applied Thermodynamics and Heat Transfer research group a precise void fraction sensor was developed based on the dielectric constant (capacitance) dierence between the gas and fluid phase. Measures were taken to improve the accuracy and reliability of the measurements. An electronic component was developed with a fast response that measures picofarad dierences. The sensor is, due to its inherently simple operating principle, both robust and accurate. The sensor was first tested for air-water flow. An analyses of all dierent flow regimes of the Baker map was made. At the University of Pretoria’s Thermoflow Research Group the sensor was mounted in a testrig for refrigerant two-phase flow (R22) to further analyze the dierent flow regimes. The void fraction signals analysis is used as a way to characterize the flow regimes. In addition, flow patterns were verified using high-speed digital videography.

Experimental study of inclined louvered fins: thermo- hydraulic characteristics

An experimental study of inclined louvered fins was performed. The study is aimed at determining the thermo-hydraulic properties of this novel fin type. To this end scaled models were tested in an open wind tunnel. The test rig was benchmarked to ensure the measurement data is accurate. To reduce the number of experiments required the ‘design of experiments’ methodology was used. First a numerical screening experiment was performed aimed at determining the parameters that had the strongest impact. These parameters were then studied in more detail. The impact of the fin pitch, the fin angle and the Reynolds number were studied. The data was compared to CFD results to further enhance understanding of the flow phenomena.

Experimental Study of Fin Effectiveness and Local Convection Coefficients for Different Fin Forms

*† ‡ § ** In this paper an experimental study of different types of longitudinal fins is presented. Unlike the current tendency to use thermal fin efficiency, fin effectiveness is used in this study. Fin effectiveness has the advantage of measuring the heat transfer gain by placement of the fin. With this gain, the performance and usefulness of a fin can be indicated. This is less obvious with the concept fin efficiency. Thus fin effectiveness can be used as a parameter to compare different types of fins. Also the local convection coefficient along the fin has to be determined for different Re-numbers. In this paper, the experimental setup, measurement techniques and methodology are described. Infrared thermography is used to measure fin surface temperatures and the temperature distribution on the fin.