Void fraction measurements of refrigerant flow in small diameter tubes using a capacitive sensor

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.The void fraction is one of the key parameters in two phase flows. Quite a few methods have been proposed to measure this void fraction. Yet many of these methods are quite complex, intrusive and/or are not applicable to all possible void fraction ranges. Capacitance measurements have already been shown to characterize the two phase flow behaviour of refrigerants. Due to the curvature of the electrodes for circular tubes, the measured capacitance does not vary linearly with the void fraction. In this work a method is proposed to calibrate a capacitive sensor and thus determine the void fraction based on capacitive measurements. The proposed method was applied to 270 measurement points. The tube diameter D for all these points is 8mm, the mass flux ranges from 200 to 500 kg/m²s and the vapour fraction ranges between 2.5% and 97.5%. Refrigerants R134a and R410A were used. The results for these data points were compared to the Rouhani-Axelsson drift flux void fraction model. A very good agreement with this model was observed.dc201

Ice fraction estimation for ice slurries through impedance measurements

Papers presented to the 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 20-23 July 2015.A new approach to ice concentration measurement for ice slurries is proposed and investigated in this work. The technique is based on in-line measuring of the impedance of an ice slurry flow. An impedance sensor was designed and built to conduct experiments. The sensor measures the impedance of the ice slurry flow in a frequency domain ranging from 20 to 120 kHz. Using this broad range of measurements, multiple parameters are determined which are then linked to the ice concentration. Ethanol is utilized as freezing point depressant. The initial ethanol concentration was varied from 3 to 6 wt-% and ice fractions from 0 to 50 wt-% were used. The ice concentration was also determined using temperature, density and calorimetric measurement. These other techniques were used as a reference and compared to the proposed impedance measurement technique. The measurement accuracy was shown to be at odds with the accuracy of the other measuring techniques. Other advantages of the proposed measuring method are the possibility to perform the measurement in-line and the low cost of theam201

Small diameter fibres as new wick material for capillary-driven heat pipes

Papers presented to the 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 20-23 July 2015.Heat pipes with a wick material consisting of small diameter metal fibres of 12 μm are investigated. The container material is copper and the working fluid is water. The fibre mesh heat pipe is compared with two other wick structures: a screen mesh (145 meshes per inch) and a sintered powder wick. All three heat pipes have an outer diameter of 6 mm, a length of 200 mm. The heat pipes are tested in a vertical orientation, both gravity-opposed and gravity-assisted. In the gravity-opposed orientation the heat pipes are tested for a heat input up to 50 W and an operating temperature of 70°C. In the gravity-assisted orientation the heat pipes are tested up to 160 W and 120°C. The thermal resistance and the temperature difference between evaporator and condenser are used as performance indicators. For the gravity-assisted orientation, the screen mesh wick clearly outperforms the fibre and sintered powder wick, due to its higher permeability and better ability to distribute the working fluid over the circumference of the wick. For the gravity-opposed orientation, the fibre and screen mesh heat pipe perform equally well. Both have a lower thermal resistance than the sintered powder heat pipe, as the small diameter fibres and fine mesh create more and very small capillary channels in comparison with the sintered powder wick.am201

Prediction of mold risk in cavity walls combining a coupled CFD/HAM-model and a 2D hygrothermal model: the influence of the outer cavity layer on the inner cavity layer

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.Preventing mould risk in buildings is important to ensure a healthy environment for the people and to avoid material damage. A reliable prediction is especially important for ventilated cavity walls made of a moisture sensitive material such as wood. In this paper the influence of the outer cavity layer on the inner cavity layer has been analyzed. The cavity wall consists of a timber frame on the inside and a brick veneer on the outside separated by an air layer. For this hygrothermal evaluation of air cavities, coupled CFD/HAM-software and a commercial hygrothermal software package WUFI-2D® are used. First the coupled CFD/HAM-software is used to examine the heat and mass transfer coefficients at the surfaces between the air and the material layer and the applicability of the heat/mass-analogy. Afterwards, the effect of long-wave radiation in the cavity will be simulated with the coupled CFD/HAM-model. Finally the model developed in WUFI to simulate a ventilated cavity wall and the influence of different materials for the outer layer will be examined combining the coupled CFD/HAM-model and WUFI-2D.dc201

Two-phase flow behaviour in a smooth hairpin tube: analysis of the disturbance using capacitive measurements

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.Two-phase refrigerant flow up-and downstream of a sharp return bend is studied. The capacitance of the flow is logged at several locations up-and downstream of the return bend. Analysis of the capacitance time traces is performed to evaluate the presence of a flow disturbance due to the bend. A vertically oriented bend is studied with an inner diameter of 8 mm and a radius of 11 mm. Smooth straight tubes with an internal diameter of 8 mm are connected to the in- and outlet of the return bend. Upward as well as downward directed flows are studied. The refrigerant R134a is used and the mass flux G and vapour quality x are varied between 200 and 400 kg/m²s and 0-1, respectively. A downstream disturbance up to 21.5D is observed for both up-and downward flow.cf201

Thermal influence of non-uniform inlet flow conditions for a commercial plate fin heat exchanger

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.In this study, a commercially available plate heat exchanger with wavy fins is experimentally characterized under uniform and three non-uniform flow conditions. The heat exchanger is 275 mm wide and 295 mm high and has a distance in-between the plates of 3 mm and the wavy fins are 11 mm high. Internally, the heat exchanger is finned with offset strip fins. Hot water at 55°C is sent through the plates and the waterside mass flow rate is kept high in order to minimize the internal thermal resistance. For the internal convection coefficient, a correlation from literature is used. The airside mass flow rate is varied by adjusting the fan speed. In case of uniform flow conditions, the frontal air velocity varies between 1.7 and 7.6 m/s. The airside velocity is measured over a nozzle and the uniformity of the wind tunnel is verified by hot wire measurements. Three non-uniformities are placed 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 (and most severe) non-uniformity consists of a circular hole of 150 mm diameter in the middle of a plate. Only the third non-uniformity has a significant influence on the heat transfer rate: up to 25% higher for the external convective resistance in comparison with the uniform case.dc201

Applying the volume averaging theory to open-cell metal foam in natural convection/radiation

Papers presented to the 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 20-23 July 2015.Heat sinks made out of open-cell aluminium foam are investigated numerically in natural convection. Results derived from a 2D numerical model are compared to results for in-house experiments. Different foam heights are studied. The numerical model is based on the volume averaging theory. The aluminium foam that is used has 10 pores per linear inch and a porosity of 93%. The temperature of the substrate was varied between 55°C and 95°C. The geometry used in the numerical model replicates the experimental test rig as well as possible. A discussion of the determination of the closure terms is given. If only convective heat transfer is taken into account in the numerical model, the relative differences between the numerical and experimental results are smaller than 29% for all foam heights studied. However, when the influence of radiation is included in the numerical model, it is shown that the numerical results differ less than 9% with the experimental ones. This validates the choice of closure terms used in the model and this shows that it is necessary to properly model radiative heat transfer in numerical models of open-cell aluminium foam in natural convection.am201

Reorganization Energy for Internal Electron Transfer in Multicopper Oxidases.

We have calculated the reorganization energy for the intramolecular electron transfer between the reduced type 1 copper site and the peroxy intermediate of the trinuclear cluster in the multicopper oxidase CueO. The calculations are performed at the combined quantum mechanics and molecular mechanics (QM/MM) level, based on molecular dynamics simulations with tailored potentials for the two copper sites. We obtain a reorganization energy of 91-133 kJ/mol, depending on the theoretical treatment. The two Cu sites contribute by 12 and 22 kJ/mol to this energy, whereas the solvent contribution is 34 kJ/mol. The rest comes from the protein, involving small contributions from many residues. We have also estimated the energy difference between the two electron-transfer states and show that the reduction of the peroxy intermediate is exergonic by 43-87 kJ/mol, depending on the theoretical method. Both the solvent and the protein contribute to this energy difference, especially charged residues close to the two Cu sites. We compare these estimates with energies obtained from QM/MM optimizations and QM calculations in a vacuum and discuss differences between the results obtained at various levels of theory