Optimization of waste heat recovery using an ORC in a large retail company

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.Combined heat and power (CHP) systems are able to increase the total energy use of primary energy sources. In the CHP system studied in this paper internal combustion engines produce electricity and the hot engine cooling water is used for the heating of buildings. However, there is still waste heat left which can be fed to an organic Rankine cycle (ORC) to produce electricity. The objective of this study is to design an economic optimal ORC system taking into account the variable load for heating and the change in ambient temperature during a year. Also the auxiliary equipment such as pumps and fans are considered. A thermodynamic steady-state model is developed to simulate the changing behavior hour-by-hour of the complete system in different operating conditions. The ORC efficiency strongly varies over a year. The model allows selecting the optimal size of the heat exchangers (condenser and evaporator), the optimal mass flow rates and the maximal power of fans and pumps needed for the considered application.dc201

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

Influence of porosity and pore density on the thermal and hydraulic performance of metal foam heat exchangers

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.Metal foam shows a great potential for heat transfer applications. In this work the influence of the volumetric porosity, the pore density and the foam material (aluminum or copper) on the heat transfer and pressure drop characteristics are investigated. Two-dimensional simulations are performed using a porous medium approach: the Darcy-Forchheimer-Brinkman flow model is combined with the two-equation energy model. Round tube heat exchanger with a staggered tube layout are considered. Simulations are performed for inlet velocities between 1.2 m/s and 3.2 m/s. The validation experiment shows a good match between the simulations and the measurements, proving the quality of the simulations. It is found that the friction factor is mainly determined by the porosity, while the Colburn j-factor is mainly determined by the pore density. For a given pumping power the heat exchanger volume increases with decreasing PPI value to perform the same heat duty. For the same PPI value and a fixed pumping power, the heat exchanger volume increases with increasing porosity. For the same fan power, the heat transfer rate for copper foam is up to 20% higher compared to foam made of the aluminum alloy AlSi7Mg0.3. Comparison to a bare tube bundle shows that the heat transfer rate of a foamed heat exchanger is up to 6 times larger for the same fan power. This article illustrates that optimization is required to design heat exchangers which fully benefit from the unique advantages of open-cell metal foam.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

Experimental investigation of the effects of foam heoght, emissivity and orientation on Bouyancy-driven convection in open-cell aluminium foam

Papers presented to the 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 20-23 July 2015.In this paper air-saturated buoyancy-driven convection in open-cell aluminium foam is studied. The effects of foam height, radiative heat transfer and orientation are experimentally investigated. Two aluminium foam heat sinks with the same baseplate dimensions (6” by 4”) are tested. Their respective foam height is 22 mm and 40 mm. The aluminium foam has a porosity of 0.946 and a pore density of 10 pores per linear inch. The heat sinks are tested in a vertical and a horizontal orientation. The effect of radiation is studied by comparing untreated heat sinks with painted versions. During the experiments the power dissipated by the heat sinks is measured as function of the temperature difference between the baseplate of the heat sink and the ambient. The temperature difference is varied from 10 to 70°C.am201

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

Thermo-hydraulic comparison of 10 PPi metal foam and louvered fins for low velocity applications

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.To maximize the effectiveness and thus minimize the airside resistance, a relatively new heat enhancing material, aluminium foam, is compared with the current state-of-the-art, louvered fins, in a wind tunnel experiment. The comparison between both heat exchangers is done based on a well-defined Performance Evaluation Criterion (PEC), taking heat transfer and pressure drop in account. Furthermore, as the studied heat exchangers are so-called ‘low-capacity’ units, a non-uniform temperature field downstream the heat exchanger is induced. Therefore an area-mean temperature reading has been developed, with the aid of an infra red camera. Finally, the contribution of the contact resistance to the overall thermal resistance of these pressed-fit heat exchangers is investigated.dc201

Feasibility study of a plastic helical coil heat exchanger for a domestic water storage tank

The main goal of this study is to investigate whether it is possible to use a polymeric helical coil heat exchanger as an alternative to conventional metallic helical coil. More specifically this work focuses on a helical coil design for a domestic water storage tank application. Corrosion and fouling resistance, scarcity of the materials, low weight and cost are the driving forces to consider designing polymeric heat exchangers rather than metallic heat exchangers. However, simply replacing the metallic material by the polymer material and applying the traditional design methods used for metallic heat exchangers do not lead to an acceptable design. If one wants to design a good polymer heat exchanger, heat transfer and structural problems have to be solved first. In addition, the pressure drop limitation should not be neglected. In order to reach this goal, a model is developed to predict the optimal design of a helical coil heat exchanger immersed in the water storage tank for a certain water mass flow rate and temperature. This design compensates for the low thermal conductivity and strength of the polymer.Papers presented to the 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Costa de Sol, Spain on 11-13 July 2016

Influence of the geometry on the thermohydraulics of a compound heat exchanger consisting of louvered fins and delta winglets

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.Louvered fin heat exchangers with round tubes are frequently used in heating, ventilation, air conditioning and refrigeration applications. In this paper delta winglet vortex generators are punched out of the louver surface, resulting in a so called compound design. Three-dimensional numerical simulations were performed. The delta winglets serve to reduce the size of the tube wakes, which are zones of poor heat transfer. A screening analysis of the most important geometrical parameters showed that the delta winglet geometry highly contributes to the thermal and hydraulic performance at low Reynolds numbers, while at higher Reynolds numbers the performance is mainly determined by the louver geometry. The compound heat exchanger has a better thermal hydraulic performance than when the louvers or the delta winglets are applied separately. The performance of the compound design is also compared to louvered, slit and plain fin heat exchangers. This clearly shows its potential. Especially for low Reynolds applications, the compound heat exchanger can be made smaller in size and thus more economical in cost