Auswahl der wissenschaftlichen Literatur zum Thema „Mini/microchannel absorber“

With the growing demand for photovoltaic (PV) systems as a source of energy generation that produces no greenhouse gas emissions, effective strategies are needed to address the inherent inefficiencies of PV systems. These systems typically absorb only approximately 15% of solar energy and experience performance degradation due to temperature increases during operation. To address these issues, PV–thermal (PVT) technology, which combines PV with a thermal absorber to dissipate excess heat and convert it into additional thermal energy, is being rapidly developed. This review presents an overview of various PVT technologies designed to prevent overheating in operational systems and to enhance heat transfer from the solar cells to the absorber. The methods explored include innovative absorber designs that focus on increasing the heat transfer contact surface, using mini/microchannels for improved heat transfer contiguity, and substituting traditional metal materials with polymers to reduce construction costs while utilizing polymer flexibility. The review also discusses incorporating phase change materials for latent heat absorption and using nanofluids as coolant mediums, which offer higher thermal conductivity than pure water. This review highlights significant observations and challenges associated with absorber design, mini/microchannels, polymer materials, phase change materials, and nanofluids in terms of PV waste heat dissipation. It includes a summary of relevant numerical and experimental studies to facilitate comparisons of each development approach.

Les dispositifs à micro/mini-canaux suscitent un vif intérêt pour l'absorption chimique efficace du CO₂ dans le cadre du captage de CO2. Cette thèse de doctorat vise à caractériser et à étudier le processus de transfert de masse à travers l’écoulement diphasique accompagné de réactions chimiques dans les mini-canaux, ainsi qu'à concevoir et développer de nouveaux absorbeurs de CO₂ miniaturisés avec des structures optimisées vis-à-vis de leurs performances d'absorption. Tout d'abord, la dynamique des bulles dans un mini-canal droit en T a été observée par moyen optique, montrant que la réaction chimique supprime la fragmentation des bulles mais favorise leur rétrécissement. Ensuite, le champ de vitesse et le champ de concentration de CO₂ dans le liquide ont été déterminés respectivement par la PTV et la colorimétrie sensible au pH, permettant le développement d'un modèle de transfert de masse de cellule unitaire modifié, intégrant les effets de la recirculation du flux et de la réaction chimique. En outre, une structure de chicane en spirale a été intégrée dans le mini-canal, d’où une amélioration significative du transfert de masse avec une faible augmentation des pertes de charge. Enfin, sur la base de cette stratégie d'intensification, une conception innovante d'absorbeur de CO₂ multicanaux intégrés, caractérisée par des unités parallèles de minicanaux croisés en double hélice conjuguée (Codohec), a été proposée. Un module à l'échelle du laboratoire de cette conception a été réalisé et ses performances d'absorption ont été évaluées de manière exhaustive, mettant en évidence divers avantages tels qu'un coefficient de transfert de masse élevé, une consommation d'énergie acceptable, un taux d'élimination élevé et une grande capacité de traitement du CO₂. Les résultats de cette thèse pourraient fournir de nouvelles perspectives sur les mécanismes de transport sousjacents du transfert de masse gaz-liquide accompagné de réactions chimiques et contribuer à la conception et à l'optimisation d'absorbeurs de CO₂ miniaturisés hautement efficaces pour des applications industrielles<br>Micro/minichannel devices show great interests for their potential in efficient CO2 chemical absorption in the context of the carbon capture. This PhD these aims to characterize and investigate the transport mechanisms involved in chemical reactionaccompanied two-phase mass transfer in minichannel, and to design and develop novel miniaturized CO2 absorbers featuring intensified structures and optimized absorption performances. Firstly, bubble dynamics within a T-junction straight minichannel were optically observed, showing that the chemical reaction tends to suppress bubble breakup while promoting its shrinkage. Then, the velocity field and CO2 concentration field in the liquid slug were determined using PTV and pH-sensitive colorimetry, respectively, permitting the development of a modified unit-cell mass transfer model that incorporates the effects of flow recirculation and chemical reaction. Further enhancement was achieved by embedding a spiral distributed baffle structure into the minichannel, leading to a significant increase in mass transfer coefficient with only a minor rise in pressure drop. Finally, building on this intensification measure, a novel design for an integrated multichannel CO2 absorber was proposed, featuring paralleling units of conjugated double-helix cross minichannels (Codohec). A lab-scale module of this design was realized, and its absorption performance was comprehensively evaluated, highlighting various advantages including a high mass transfer coefficient, acceptable energy consumption, high remove rate, and large CO2 treatment capacity. These findings may provide new insights into the underlying transport mechanisms of chemical reaction-accompanied gas-liquid mass transfer and contribute to the design and optimization of highly efficient miniaturized CO2 absorbers for industry applications

The effect of microchannel geometry variations on heat transfer and absorption rate of an ammonia-water constrained thin film bubble absorber are presented. Experiments are performed at an absolute pressure of four bar and at a fixed inlet mass concentration of ammonia of 15 percent. The mass flow rate of the inlet weak solution are varied from 10 g/min to 30 g/min, and that of ammonia gas are varied from 1 g/min to 3 g/min. Five geometries, including two smooth-bottom-walled channels of differing depths, and three channels with structured bottom walls are considered. For identical rates of vapor absorption, the overall heat transfer coefficient for the 400 μm smooth microchannel is significantly larger than of the 150 μm smooth channel as well as that of the cross-ribbed and angled-cross-ribbed structured channel. The streamwise-finned channel exhibit high overall heat transfer coefficients for vapor flow rates of up to 2 g/min.

An experimental study of absorption of ammonia into a constrained thin film of ammonia-water solution is presented. A large aspect ratio microchannel with one of its walls formed by a porous material is used to constrain the thickness of the liquid film. An exit visualization section was used to confirm absorption of ammonia gas within the microchannel. Experiments were performed at a pressure of 1 bar and a fixed inlet temperature of the weak solution, for weak solution flow rates from 10 to 30 g/min, inlet mass concentrations from 0 to 15 percent, and gas flow rates between 1 and 3 g/min. Results indicate that the overall heat transfer coefficient changes little for lower inlet weak solution concentrations and for lower gas flow rates, but increases noticeably for a higher solution and gas flow rate. The solution side log-mean temperature distribution increases with an increase in inlet solution concentration. Absorber exit visualization revealed the presence of periodic ammonia bubbles, occurring in varying sizes and periods, indicating that improvements to the current design are necessary to ensure complete absorption within the microchannel.

This work describes a microfluidic drug dissolution testing method that was developed using a commercial quartz crystal microbalance (QCM) resonator combined with an axial microfluidic flow cell. Dissolution testing is used to obtain temporal dissolution profiles of drugs, which provide information on the bioavailability or the drug’s ability to be completely dissolved and then absorbed and utilized by the body. Feasibility of the QCM dissolution testing method was demonstrated using a sample drug system of thin films of benzoic acid dissolved in water, capturing the drug dissolution profile under different microflow conditions. Our analysis method uses the responses of resonance frequency and resistance of the quartz crystal during dissolution testing to determine the characteristic profiles of benzoic acid dissolved over a range of microflows (10–1000 μL/min). The initial dissolution rates were obtained from the characteristic profiles and found to increase with higher flow rates. This aligns with the expected trend of increased dissolution with higher hydrodynamic forces. The QCM-based microfluidic drug dissolution testing method has advantages over conventional dissolution test methods, including reduced sample sizes, rapid test durations, low resource requirements, and flow conditions that more closely model in vivo conditions.

Due to having considerably small diameters compared to the macro channels; validation of conventional models and correlations and, examination of heat transfer and flow characteristics for mini/micro channels have been an attractive subject for last decades. In this study, classical turbulence models are compared and applicability of the conventional correlations is investigated for the flow through minichannels having diameter range between 1.2 and 0.25 mm. For the flow considered, fluid (R134a) enters the horizontal channel with a prescribed temperature and velocity, absorbs heat from the surrounding and then leaves the channel. Reynolds number is chosen in a range between 5000 and 20000 in order to cover the turbulent regime. For the first step of the study, in order to investigate the use of conventional turbulence models, Standard k-ε, RNG k-ε, Realizable k-ε, Standard k-w and Reynolds Stress models are employed to estimate friction factor and Nusselt number values for 0.5 mm diameter channel. These numerical results are compared with those calculated by conventional correlations existing in the literature. According to the comparison, none of the models create a dramatically deviation and Standard k-ε is determined as the model giving the closest results to the conventional values. As second step of the study, Standard k-ε model is applied for the flow through the minichannels having diameter of 1.2, 1, 0.8, 0.5 and 0.25 mm, respectively. Friction factor and Nusselt number values estimated numerically via Standard k-ε model are compared with those calculated by conventional correlations and existing relevant experimental data. According to the study, it is concluded that the numerical friction factor values are found to be close to the conventional values. The most discrepancy exists when diameter is less than and equal to 0.5 mm. Furthermore, numerical Nusselt number values are found to be close to conventional values estimated with the correlation proposed by Gnielinski (1976) while they are lower estimated for channels having diameter of 1.2, 1 and 0.8 mm and over estimated for 0.5 and 0.25 mm diameter channels. As a result, conventional correlations and turbulence models are found to be applicable for the diameter range and the flow investigated.