Flow boiling in microchannels: Fundamentals and applications

The rapid advances in performance and miniaturization of electronics and high power devices resulted in huge heat flux values that need to be dissipated effectively. The average heat flux in computer chips is expected to reach 2–4.5 MW/m2 with local hot spots 12–45 MW/m2 while in IGBT modules, the heat flux at the chip level can reach 6.5–50 MW/m2. Flow boiling in microchannels is one of the most promising cooling methods for these and similar devices due to the capability of achieving very high heat transfer rates with small variations in the surface temperature. However, several fundamental issues are still not understood and this hinders the transition from laboratory research to commercial applications. The present paper starts with a discussion of the possible applications of flow boiling in microchannels in order to highlight the challenges in the thermal management for each application. In this part, the different integrated systems using microchannels were also compared. The comparison demonstrated that miniature cooling systems with a liquid pump were found to be more efficient than miniature vapour compression refrigeration systems. The paper then presents experimental research on flow boiling in single tubes and rectangular multichannels to discuss the following fundamental issues: (1) the definition of microchannel, (2) flow patterns and heat transfer mechanisms, (3) flow instability and reversal and their effect on heat transfer rates, (4) effect of channel surface characteristics and (5) prediction of critical heat flux. Areas where more research is needed were clearly mentioned. In addition, correlations for the prediction of the flow pattern transition boundaries and heat transfer coefficients in small to mini/micro diameter tubes were developed recently by the authors and presented in this paper

Advancements and prospects of thermal management and waste heat recovery of PEMFC

Despite that the Proton Exchange Membrane Fuel Cell (PEMFC) is considered to be an efficient power device; around half of the energy produced from the electrochemical reaction is dissipated as heat due to irreversibility of the cathodic reaction, Ohmic resistance, and mass transport overpotentials. Effective heat removal from the PEMFC, via cooling, is very important to maintain the cell/stack at a uniform operating temperature ensuring the durability of the device as excessive operating temperature may dry out the membrane and reduces the surface area of the catalyst hence lowering the performance of the cell. In addition to cooling, capturing the produced heat and repurposing it using one of the Waste Heat Recovery (WHR) technologies is an effective approach to add a great economic value to the PEMFC power system. Global warming, climate change, and the high cost of energy production are the main drivers to improve the energy efficiency of PEMFC using WHR. This paper presents an overview of the recent progress concerning the cooling strategies and WHR opportunities for PEMFC. The main cooling techniques of PEMFCs are described and evaluated with respect to their advantages and disadvantages. Additionally, the potential pathways for PEMFC-WHR including heating, cooling, and power generation are explored and assessed. Furthermore, the main challenges and the research prospects for the cooling strategies and WHR of PEMFCs are discussed

Ecoulements diphasiques aux petites échelles - Application au refroidissement d'une pile à combustible de type PEMFC

This study is devoted to boiling flows in mini-channels of small dimensions. The objective was the enhancement of the thermal performance of PEM fuel cells. Confinement is nowadays one of the key parameters when considering a fuel cell. Indeed, the amount of heat flux to be removed from the fuel cell increases when its dimensions are decreased. As a consequence, thorough investigations of heat transfer in fuel cells are required. In our study, Forane 365 HX was selected to be used as the working fluid for the purpose of cooling. In comparison with single-phase flows, an important amount of energy can be extracted in forced boiling conditions while keeping a moderate wall temperature gradient. Thus, in order to properly design a heat exchanger in forced boiling conditions, it is important to understand the influence of the dimensions of one single mini-channel on the pressure drop across the channel and the heat transfer coefficients. Consequently, we investigated the flow and associated heat transfer in four rectangular mini-channels of hydraulic diameter ranging from 0.727 mm to 1.45 mm. We especially studied the influence of the aspect ratio by considering two mini-channels of same hydraulic diameter and different aspect ratios. We found that the heat flux necessary to the onset of nucleate boiling or the critical heat flux are dependent on the geometric dimensions of the mini-channel in our experiments. For the saturated boiling flow conditions, the pressure drop is also related to the channel cross-section area whereas the heat transfer coefficients seem to be nearly independent of the mini-channel under study. Simultaneously to these measurements, a high-speed video camera was used for visualizing the boiling flow. We observed that the mini-channel cross-section has a significant influence on the flow pattern transitions. Most of our experiments were performed for a working pressure of 70 kPa (relatively to atmospheric pressure) whereas the mass flow rate was in the range [100- 800 kg.m−2.s−1]. The heat flux removed by the boiling flow reached values as large as 250 kW.m−2 for the critical heat flux conditions.Cette thèse est consacrée à l’étude des écoulements internes en ébullition dans des canaux de petites dimensions. L’application visée est le refroidissement des piles à combustible de type PEMFC. Le confinement de plus en plus marqué des PEMFC et l’accroissement des densités de flux de chaleur produites au coeur de pile nécessitent une étude approfondie des transferts de chaleur dans les mini-canaux. Pour cette étude, le fluide utilisé a été le forane 365 HX, qui présente des propriétés intéressantes pour l’objectif visé. Comparativement aux écoulements monophasiques, une quantité élevée d’énergie peut être extraite des parois en limitant le gradient thermique pariétal en condition d’ébullition, lorsque le fluide se trouve à saturation. De manière à effectuer un dimensionnement adéquat d’un échangeur thermique, nous avons étudié l’influence de la dimension de la section droite de la mini-conduite sur les performances de l’appareil. Nous avons ainsi étudié quatre mini-canaux de section rectangulaire et de diamètre hydraulique compris entre 0, 727mm et 1, 45mm. Nous avons centré notre étude sur l’influence du rapport d’aspect en définissant deux mini-canaux de même diamètre hydraulique et de rapports d’aspect différents. Nous avons constaté que les valeurs de flux thermiques conduisant à l’apparition de l’ébullition (ONB) ou à l’assèchement du liquide en paroi (CHF) dépendent de la géométrie du mini-canal. Les évolutions des pertes de pression en régime diphasique sont aussi influencées par ces données géométriques tandis que les coefficients d’échange évoluent peu avec celles-ci. De plus, des visualisations de l’écoulement ont été effectuées simultanément aux mesures et nous avons montré que les paramètres géométriques avaient un impact direct sur les transitions entre les régimes d’écoulement. Pour la majeure partie des expériences réalisées, la pression de fonctionnement était de 70kPa (relativement à la pression atmosphérique), les vitesses massiquesétaient comprises entre 100kg.m−2.s−1 et 800kg.m−2.s−1 et les flux thermiques testés ont pu atteindre la valeur maximale de 250 kW.m−2, lors de la détermination du flux de chaleur critique (CHF)

Écoulements diphasiques aux petites échelles – Application au refroidissement d’une Pile à Combustible (PEMFC)

This study is devoted to boiling flows in mini-channels of small dimensions (hydraulic diameter between 0.727 mm and 1.45 mm). The objective was the enhancement of the thermal performance of PEM fuel cells. In our study, Forane 365 HX was selected to be used as the working fluid for the purpose of cooling. In comparison with single-phase flows, an important amount of energy can be extracted in forced boiling conditions while keeping a moderate wall temperature gradient. We especially studied the influence of the aspect ratio and hydraulic diameter We found that the heat flux necessary to the onset of nucleate boiling or the critical heat flux are dependent on the geometric dimensions of the mini-channel in our experiments. For the saturated boiling flow conditions, the pressure drop is also related to the channel cross-section area whereas the heat transfer coefficients seem to be nearly independent of the mini-channels.Cette thèse est consacrée à l’étude des écoulements internes en ébullition dans des canaux de petites dimensions (diamètre hydraulique compris entre 0,727 mm et 1,45 mm). L’application visée est le refroidissement des piles à combustible de type PEMFC. Pour cette étude, le fluide utilisé a été le forane 365 HX. Comparativement aux écoulements monophasiques, une quantité élevée d’énergie peut être extraite des parois en limitant le gradient thermique pariétal en condition d’ébullition, lorsque le fluide se trouve à saturation. Nous avons centré notre étude sur l’influence du rapport d’aspect et du diamètre hydraulique. Nous avons constaté que les valeurs de flux thermiques conduisant à l’apparition de l’ébullition (ONB) ou à l’assèchement du liquide en paroi (CHF) dépendent de la géométrie du mini-canal. Les évolutions des pertes de pression en régime diphasique sont aussi influencées par ces données géométriques tandis que les coefficients d’échange évoluent peu avec celles-ci

Solidification monitoring of supercooled phase change materials

ACTIInternational audienceSalt hydrates are often used as heat storage materials according to their interesting thermal properties (high latent and specific heat). Their main drawback is their large supercooling, where the material may remain liquid below its melting point. However, this property should be turned into a benefit if a reliable way of triggering crystallization is developed, as heat could be released on demand. Sodium Acetate Trihydrate (SAT, CH3COONa.3H2O), which can exhibit 90K of supercooling, was used for this study. For a better understanding of how heat is released after crystallization triggering, caused by seeding, a fully new method was developed to measure the samples’ opacification, using a HeNe laser (633 nm) coupled with a photodetector, measuring the solid fraction thanks to the difference between liquid and solid specular transmittance. This technique was compared with temperature measurement and image processing, showing consistency between the different methods

Nucleation triggering methods in supercooled phase change materials (PCM), a review

ACLSupercooling is an undesired property of phase change materials due to the poorly predictable occurrence of crystallization during cooling. For such situations, the stored latent heat cannot be recovered which can be an issue for temperature-controlled applications. This review illustrates the techniques used for triggering crystallization in phase change materials having a supercooling property. The development of triggering devices should constitute a breakthrough for heat on demand applications, as heat can be released even when the temperature drops far below the liquidus temperature. Several techniques appear to be promising for nucleation triggering. They have been classified into two categories: passive (reduction of supercooling) or active (triggering of crystallization on demand) devices. They were accurately investigated for water freezing for: meteorological comprehension, food preservation or the pharmaceutical industry. In this paper, several nucleating agents (passive) have been explored, and most of them, added by 1 wt%, can decrease the supercooling degree by more than 90%. In addition, the heat would be immediately released on demand from a supercooled material by the use of seeding or electrofreezing (active methods). Solidification can also be externally triggered by the application of high pressure or ultrasonic waves (active). In addition to the analysis of the efficiency of the different techniques in terms of supercooling reduction, this review also discusses the solidification process at a microscopic scale

Thermal management design of transformers for Dual Active Bridge power converters

International audienceA methodology for the design of compact transformers operating at high frequency is presented in this paper. A particular emphasis is paid to the thermal management of the magnetic core and of the winding components. For a 7kW Dual Active Bridge DC/DC converter, the objective is to reduce the core volume ( 99 %) and integrate the serial inductance (8.7 µH) as being the leakage inductance of the transformer. A parametric study shows that the heating in the copper winding is very sensitive to the anisotropic thermal conduction behaviour of the wire. Due to this characteristic, a pot-core configuration is prone to a higher warming compared to the E-E core geometry, as the efficiency of the winding cooling is lower. In order to take part of the self-shielding ability of pot-cores, we studied new configurations in which internal thermal drains are inserted into voids specially designed to shorten the distance between the external cooled walls and the hottest points of the winding. The heating of internal components of the transformer and resulting thermal stress peak is reduced by 40 % paving the way for robust transformers with a power density that could theoretically reach up to 200 kW/dm3

Solidification monitoring of supercooled phase change materials

ACTIInternational audienceSalt hydrates are often used as heat storage materials according to their interesting thermal properties (high latent and specific heat). Their main drawback is their large supercooling, where the material may remain liquid below its melting point. However, this property should be turned into a benefit if a reliable way of triggering crystallization is developed, as heat could be released on demand. Sodium Acetate Trihydrate (SAT, CH3COONa.3H2O), which can exhibit 90K of supercooling, was used for this study. For a better understanding of how heat is released after crystallization triggering, caused by seeding, a fully new method was developed to measure the samples’ opacification, using a HeNe laser (633 nm) coupled with a photodetector, measuring the solid fraction thanks to the difference between liquid and solid specular transmittance. This technique was compared with temperature measurement and image processing, showing consistency between the different methods