Georges El Achkar

In this paper, an experimental thermohydraulic characterisation of flow boiling and condensation in additive manufactured plate-fin heat exchanger was conducted. An original experimental setup was developed allowing the operation of the heat exchanger in both boiling and condensation modes. This heat exchanger mainly consists of two identical rectangular cross-section channels, each of hydraulic diameter 3 mm, with n-pentane and water used as refrigerant and heat transfer fluid, respectively. An experimental metrology, based on the infrared thermography, was implemented in order to determine the temperature distribution of the heat exchanger external walls. Based on this temperature distribution, the mean temperature profiles of the n-pentane and water were determined, allowing the identification of three main flow zones: the subcooled liquid zone, the two-phase (boiling/condensation) zone and the superheated vapour zone. These mean temperature profiles were then coupled to a developed thermal model in order to determine the local thermal parameters, such as heat flux, vapour quality and heat transfer coefficient. The total pressure drop in the two-phase zone was finally determined. All the obtained results were analysed and compared with data available in the literature.

In this paper, the effects of operating conditions on the energy efficiency of a reciprocating room temperature magnetic refrigerator were experimentally investigated in order to optimise its refrigeration performance. An experimental setup of a magnetic refrigerator allowing the thermal characterisation of an active magnetic regenerator, with gadolinium particles (diameter 0.3 mm) used as a magnetocaloric material and water used as a working fluid, was developed. The effects of the magnet velocity (0.04-0.16 m.s −1 ), the water filling ratio (2.985.96) and the room temperature (14.5-18.1 ◦ C) on the cold end temperature of the active magnetic regenerator were determined and analysed. An optimal operating point of the magnetic refrigerator was found for a magnet velocity of 0.16 m.s −1 , a water filling ratio of 5.21 and a room temperature of 18.1 ◦ C, for which a characteristic time of 70 s, a maximum cold end temperature variation of 12.4 ◦ C, a cold power of 1450 W and a coefficient of performance of 6.7 were obtained. A synthesis on the suitable choice of the magnetic refrigerator operating conditions is given in order to optimise its refrigeration performance.

In this paper, the thermohydraulic performance of a reciprocating room temperature active magnetic regenerator (AMR), with gadolinium (Gd) particles used as a magnetocaloric material (MCM) and water used as a working fluid, was numerically investigated. A two-dimensional transient flow model was developed using COMSOL Multiphysics, in order to determine the water flow distribution in two AMRs of cross and parallel Gd particles distributions for different water inlet velocities of 0.06 m.s^-1, 0.08 m.s^-1 , 0.1 m.s^-1 and 0.12 m.s^-1. The Gd particles have a radius of 1.5 mm and a distance from one another of 0.9 mm. Based on the simulations results of the first model, a two-dimensional transient coupled flow and heat transfer model was then developed using COMSOL Multiphysics, in order to characterise the convective heat transfer in the AMR of cross Gd particles distribution for the same water inlet velocities.

Heat pumps are widely investigated for their versatile use in a wide range of applications. In this study, three types of heat pumps were experimentally compared. These heat pumps include an one-stage without injection vapor heat pump, an one-stage injection vapor heat hump and a two-stage injection vapor heat pump with an economizer. The results showed that the heating capacity of all three variants of heat pumps decreases with the decrease of the evaporation temperature. However, the attenuation ratio of the heating capacity is found to be different from one pump to another. On the one hand, the largest attenuation rate is found to be 68.84% for evaporating temperatures ranging from −1 °C to −23 °C and for the case of one-stage heat pump. On the other hand, the smallest attenuation rate is found to be 31% for the two-stage injection vapor heat pump in the same temperature range. It is worth noting that the heating efficiency is improved by the amount of vapor injection, nevertheless, there is a maximum value due to the limitations of the economizer. The two-stage injection vapor heat pump can exhibit an enhanced heating efficiency of 127% compared to the one-stage without injection vapor heat pump and 13% compared to the one-stage injection vapor heat pump for an evaporating temperature of −23 °C.

In this paper, a modelling investigation of water droplet evaporative freezing was conducted in order to better understand the snowmaking process and hence to optimise the design of the artificial snowmaking device. To this end, mass and heat transfer theoretical models of a single water droplet cooling in an air space were established and implemented in a numerical model developed using the software COMSOL Multiphysics. The effects of the air temperature, relative humidity and velocity and the water droplet initial diameter and temperature on this process were identified and analysed, and their appropriate ranges for the snowmaking were determined.