Conference Agenda

Fuel cell systems play a crucial role in the development of new energy systems, striving to meet economic and ecological requirements for a secure and environmentally friendly energy supply. Particularly in mobile applications like automotive or aviation sectors, fuel cell systems, are gaining prominence due to their emission-free and efficient operation. In previous studies, it has been demonstrated that two-phase cooling systems offer advantages over liquid cooling systems, which represents the current state of the art technology. This study investigates the impacts of various heat transfer correlations on evaporation within the cooling channel of a two-phase cooling system, particularly in the context of fuel cell-powered aircraft applications utilizing methanol as a refrigerant.Formularbeginn The investigation is conducted under cruise phase conditions, considering an electrical power of the stack of 100 kW and a mean fuel cell membrane temperature of 90 °C. The results reveal significant variations in heat transfer coefficients along the cooling channel, ranging from 500 to 6000 W/m²·K, with evaporation temperatures between 79 and 88 °C. Higher average heat transfer coefficients lead to increased evaporation temperatures. Furthermore, employing the Kim&Mudawar correlation results in more than a 13% lower mass flow rate compared to the Lazareck&Black correlation. This study contributes to understanding the influence of the evaporation heat transfer coefficient within the fuel cell cooling channel at the system level, providing valuable insights into the optimization of two-phase cooling systems for fuel cell-powered aircraft. Further research is necessary to experimentally determine the heat transfer coefficient, thereby validating the findings presented in this paper.

Plate Heat exchangers (PHEs) are gaining popularity as condensers and evaporators for their compact design and high efficiency. The demand of PHEs for air-to-water heat pumps has increased significantly in recent years. Enhancing the efficiency of the PHEs is crucial to save energy usage and reduce carbon emissions. To achieve this improvement, gathering data on the local heat transfer characteristics is essential since uneven fluid distribution in channels poses a challenge to effective heat transfer. In addition, there has been a noticeable surge in the need for refrigerants with low Global Warming Potential (GWP). Due to the limited availability of pure low-GWP refrigerants, which are insufficient to meet all applications, various blends of refrigerants have been suggested such as R454B, R454C, R455A, and others. However, there is currently no available data on the local heat transfer characteristics of these refrigerant mixtures. Existing literature lacks experimental analysis of local heat transfer coefficients for pure and zeotropic mixture refrigerants in plate heat exchangers. A specially constructed test section was used in the inquiry to measure the local heat transfer coefficient of refrigerants. Eight stainless steel plates were brazed together to construct the test part. This section has three channels: two for water flow and one for refrigerant flow. Within the test section, there are eight locations specifically allocated for temperature measurements. These spots are arranged in a grid of 5 rows and 4 columns, totaling 160 measurement locations. The thermocouples are strategically placed at intervals of 4, 12, 20, 28, 36, 44, 52, and 60 mm from the right to the left, with a 22 mm gap between each row. This study experimentally analyzed the three-component zeotropic mixture R455A's local heat transfer at various conditions. The local distribution of heat transfer coefficients is essential for both condensation and evaporation. This information aids in establishing a connection between the heat transfer coefficient and the flow regime, allowing for a more accurate correlation for predicting heat transfer. This, in turn, facilitates enhancements in the design of heat exchangers, leading to improved heat transfer performance.

Except for uncommon surfaces manifesting superhydrophilicity, water forms droplets on heat transfer and air-handling surfaces in air conditioning, heat pumping, and refrigeration systems. Droplet evaporation is ubiquitous in these systems, and it is especially important when water is present on the air side of a condenser, during defrosting when liquid water is present on a refrigerator or heat-pump evaporator, and when water is present on air-handling surfaces of a ventilation system. Droplet evaporation is also important in related applications, such as desalination, power generation, etc. Often the evaporating droplet is impure, whether due to a contaminant—say acquired from the heated surface—or to a naturally occurring salt or mineral. The evaporation of a water droplet with a solute is complex, involving phase change, simultaneous heat and mass transfer, wetting behavior, and thermosolutal convection with Marangoni effect.

We have experimentally investigated the evaporation of ASTM D1141-98 artificial seawater droplets on a heated stainless surface. After the seawater droplet is completely evaporated, a ring-like deposition is formed on the surface. The flow pattern inside the evaporating seawater droplet is measured using particle image velocimetry (PIV) technique. To better understand the physics, a numerical simulation of droplet evaporation is performed using COMSOL and the numerical results are compared with PIV results. The surface-tension gradient (Marangoni stress) at the liquid-vapor interface changes the flow patterns inside the evaporating droplets. During evaporation, the contact angle decreases, and secondary eddies form near the contact line. The formation of these eddies increases the salt concentration near the pinned contact line, because salt is ‘trapped’ in the recirculating eddies. As the process continues, the higher evaporation rate near the contact line causes the salt concentration to increase inside the eddies, leading to supersaturation and the formation of ring-like deposition. When Marangoni stress is neglected, the formation of eddies near the contact line occurs earlier and the flow pattern is in completely opposite direction compared to the case where Marangoni stress is considered. The general flow pattern inside the evaporating droplet obtained from PIV is similar to the numerical results where Marangoni stress is not considered. The effects of initial salt concentration on flow pattern evolution and salt concentration distribution are also examined.

This research describes an experimental approach focusing on the two-phase boiling heat transfer and pressure drop characteristics of R410A refrigerant within microchannels. An experimental test facility designed to investigate the thermal-hydraulic performance of R410A at various operational scenarios. The experimental loop comprises three circuits: the refrigerant circuit, coolant circuit, and the chiller circuit. The refrigerant circuit is instrumental in regulating mass flow rate, whereas the heat exchange fluid circuit adjusts the heat transfer rate. The chiller loop, circulating an ethylene glycol/water mixture, functions to cool both the refrigerant and brine circuits. This test rig accurately measures heat transfer in channels and exchangers with capacities ranging from 0.05 to 15 kW. It features a two-system configuration, allowing for differentiated control over diverse heat exchange requirements. The refrigerant flow is adjusted using impeller pumps and expansion valves,. The vapor quality is controlled using a plate heat exchanger. This setup enables the adjustment of the vapor quality within the range of 0.02 to 0.98. Once the desired vapor quality is attained and the flow stabilizes in the development phase, it then proceeds to the experimental test segment. This experimental framework not only assures data precision and reliability but also paves the way for gaining insights into the performance characteristics of R410A in microchannel heat exchangers. These findings contribute to the understanding of R410A heat transfer and flow behavior in microchannel heat exchangers and may be useful for advancements in refrigeration technology.

In order to prevent global warming, there is a strong demand for greenhouse gas control and energy saving in air conditioning systems, particularly in the selection of refrigerants. The Kigali Amendment to the Montreal Protocol adopted in 2016 sets a target for developed countries to phase out their production and consumption of HFC refrigerants by 2036. To meet the requirement without reducing refrigerant capacity, non-azeotropic mixtures of HFC and HFO refrigerants is attracting attention as an alternative refrigerant to HFC refrigerants. For non-azeotropicrefrigerants, heat transfer performance with phase change would deteriorate due to changes in saturation temperature depending on the vapor quality and due to concentration gradients near the gas-liquid interface. Therefore, to cover the degradation of heat transfer performance, the use of compact heat exchangers using multi-port tubes with small diameter refrigerant channels are consideredbecause of its high heat transfer area density, light weight, and small refrigerant channel volume. The channel shape in flat multi-port tubes are often non-circular, such as square and triangular. Itschannel diameter is about 1 mm or less, so if the refrigerant flows as a gas-liquid two-phase flow, the gas-liquid interfacial structure is affected by surface tension. For annular flows which are main flow pattern formed over a wide range of vapor quality, the liquid film in the corners becomes thicker due to surface tension. In this case, it was found in our previous study that disturbance wave generation was suppressed despite of the low cross-sectional average void fraction. Such flow structure might lead to heat transfer deterioration. The target of our study is evaporating heat transfer and the final purpose is to improve the evaporating heat transfer coefficient by agitation of the liquid film flow through nucleate boiling. As the first step, the objective of this study is to clarify the wall superheat at the onset of nucleate boiling. Here, experimental results of saturated pool boiling to measure the wall superheat at the onset of nucleate boiling are reported. Three types of refrigerants were used in the experiments: R454C, a mixture of HFC refrigerant R32 and HFO refrigerant R1234yf in the ratio of 21.5%:78.5%; R134a, an HFC refrigerant; and R1234yf, an HFO refrigerant, as reference data. Experiments were conducted at three saturation temperatures of 5°C, 10°C, and 20°C. Two types of measurements with different heating processes were conducted. The one is to increase the heat flux stepwise to the maximum value and then decrease. The other is to increase the heat flux very slowly to obtain a stable wall temperature. The test section was made of a copper block with a smooth circular surface of 10 mmdiameter. K-type thermocouples were placed at 5 mm, 10 mm, and 15 mm from the heat transfer surface to measure heat flux and wall temperature. The copper block was heated by a cartridge heater. The effect of thermophysical properties on the onset of nucleate boiling, especially non-azeotropic mixture refrigerant, will be discussed.

The reduction of environmental load has recently become a serious issue, and it must be converted into refrigerants with lower global warming potential (GWP). Therefore, hydrofluoroolefin (HFO) refrigerants with a lower GWP have been proposed as alternative refrigerants. A falling film type evaporator using heat transfer through thin liquid film can reduce refrigerant charge amount compared with flooded type evaporators. However, a liquid film breakdown and dry patch became a major factor in heat transfer deterioration in falling film. Although the many studies for heat transfer outside horizontal tube have been reported, the knowledges of heat transfer and flow characteristics of falling film of HFO refrigerants on vertical plate are limited. This study experimentally investigated the boiling heat transfer and flow characteristics of falling film of HFO refrigerant on a vertical plate to clarify the effects of behaviors of boiling bubble, liquid film breakdown, and dry patch on the heat transfer and flow characteristics. The liquid film flowed down with ripples at the gas-liquid interface. The boiling bubble generation and collapse were observed in the liquid film under high heat flux conditions. The dry patches expanded with increasing heat flux and decreasing film Reynolds number. The heat transfer coefficients enhanced with increasing heat flux, however rapidly deteriorated under lower film Reynolds number owing to the dry patch expansion.