Conference Agenda

In this work, slippery liquid infused porous surfaces (SLIPS) are used in combination with a linear wettability gradient to influence the mobility of water droplets on surfaces ranging in size from 5-50 μL. The overall aim is to study the effects of surface wettability and micro-structural roughness on the water drainage characteristics of functionalized aluminum surfaces.

The primary focus is on comparing the critical air velocities of water droplets on aluminum surfaces containing laser-etched microchannels coated with various Krytox™ GPL series oils. The microchannel design is used to establish a linear wettability gradient on the surface through roughness. Four different Krytox™ oils were studied of varying viscosity and fluorination level (i.e. GPL 102, GPL 103, GPL 104, and GPL 106). The surfaces were then placed in a subsonic wind tunnel and tested to measure the critical air speed needed for the onset of droplet motion. The critical air speed was fully characterized for the GPL 103, 104, and 106 coated surfaces. On the GPL 103 treated surface, the average critical velocity for the movement of 5 μL droplets was the lowest (only 7.3 m/s), while the critical air speed on the GPL 104 and 106 treated surfaces averaged 8.2 m/s and 9.3 m/s, respectively. Wind tunnel tests for the SLIPS sample with the GPL 102 coating were inconclusive due to oil displacement caused by the air shear force generated during testing. Although the critical air speed was not determined, spontaneous droplet motion on the GPL 102 coated surface was still observed in the absence of air flow. Spontaneous translational droplet motion was also observed on the surface coated with the GPL 103 Krytox™ oil too. Analysis of the spontaneous droplet motion was then performed to determine the maximum travel distance and instantaneous speed of the moving droplets. In addition to wind tunnel testing, each surface was also characterized using a goniometer to determine its static contact angle and contact angle (CA) hysteresis. The CA hysteresis which is typically a good predictor of droplet mobility on a surface was found by measuring the advancing and receding contacts angles on a surface. To date, our data have shown that a carefully selected Krytox™ oil (such as GPL 103 or 104) has the potential to help with droplet mobility and drainage; however, additional testing is warranted to explore other gradient surface designs and to measure the long-term durability of these surfaces.

During the last decade the progressive phase-out of high GWP refrigerants has shifted the refrigerant options mainly towards natural or HFOs [1]. However, recent information [2] about the generation of forever chemicals (PFA/TFA) related to carbon-fluorine chemical bonds, is pointing towards a more restrictive scenario for HFOs.
Under this situation, is of interest to investigate the most convenient secondary thermal fluid for cooling and freezing in indirect systems, aligned with use of hydrocarbons on the primary side.
The paper will devote the first section to present a study focused on the selection of the best fluid considering temperature limits, thermal properties, corrosion features and range of application.
After that and taking the selected fluids, the article will present the suggested design methodology for the fin-and-tube heat exchanger considered for the interaction with the conditioned environment. A combination of options changing number of rows, columns and circuits will allow to select the best candidates, also incorporating cost-based goodness factors. The final designs and their comparison to the baseline heat exchanger used in direct expansion units will also shed some light on the impact of the refrigerant transition in this part of the system.
REFERENCES
[1] Sovacool, B.K., Griffiths, S., Kim, J., Bazilian, M., Climate change and industrial F-gases: A critical and systematic review of developments, sociotechnical systems and policy options for reducing synthetic greenhouse gas emissions, Renewable and Sustainable Energy Reviews 141 (2021) 110759.
[2] PFAS ban affects most refrigerant blends, Cooling Post, 12th February 2023 (https://www.coolingpost.com/world-news/pfas-ban-affects-most-refrigerant-blends/)

Isobutane flows through brazed-type capillary tube suction line exchangers are investigated both experimentally and numerically. The experimental work was conducted by means of a purpose-built testing facility, where 16 samples comprising of four heat exchanger arrangements considering different heat transfer lengths and positions, and capillary tube and suction line inner diameters ranging from 0.66 to 0.77 mm and 6.6 to 7.9 mm, respectively, were evaluated for two different condensing pressures (3.5 and 7.5 bar). In the numerical front, the mathematical formulation for modeling of non-adiabatic capillary tube flows introduced in a prior study was revisited, and the assumptions adopted for effectiveness calculation were assessed. Comparisons of the model predictions for the refrigerant mass flow rate and the suction line outlet temperature with their experimental counterparts showed errors within ±10% and ±3°C bands, respectively. It was found that the liquid-to-vapor counterflow heat exchanger approach for effectiveness calculation led to satisfactory results even in the cases where the heat exchanger is fulfilled with flashing refrigerant.

Due to efforts aimed at decarbonizing industries, the use of refrigerants with low global warming potential (GWP) is highly recommended in the air-conditioning and refrigeration sectors. Despite possessing low-GWP values of less than 150, hydrofluoroolefins (HFO) exhibit relatively lower heat transfer performance compared to conventional hydrofluorocarbons (HFC) under certain operating conditions. In contrast to HFCs, there is a high demand for enhanced surfaces to meet the needs of heat transfer systems utilizing low GWP refrigerants. Accordingly, this study analyzes the pool boiling performance of low-GWP refrigerants in microchannel geometries. The experiments were carried out at various heat flux levels on both smooth and enhanced surfaces. The reliability of the experimental facility was validated against standard pool boiling correlations, and the pool boiling behavior of low-GWP refrigerant was compared to that of R-134a refrigerant in terms of heat transfer coefficient and wall superheat. The results indicate that the heat transfer coefficients of the enhanced surface are significantly higher than those of the smooth surface. Furthermore, the microchannel geometry demonstrated a lower wall superheat compared to the smooth surface. Additionally, a visualization study was performed using a high-speed camera to understand the pool boiling mechanism of low-GWP refrigerants on both smooth and enhanced surfaces.

Developing low-cost and durable heat exchangers is critical for next-generation heating, ventilation, air conditioning, and refrigeration systems. The emerging requirement to minimize the total refrigerant charge inventory while ensuring a uniform distribution of refrigerant has led to the deployment of advanced manufacturing strategies. At the same time, corrosion is a longstanding issue for metal components, especially those used in heat exchanger (HX) applications. In this study, Al-Ce-Mg alloy-based HXs have been investigated. The corrosion resistance of HXs prototype manufactured by the casting process has been examined and the potential of a new manufacturing approach has been explored to meet the requirements of emerging needs.

The intersection between advanced manufacturing and thermal management is an emerging area of interest. Research efforts into pumped two-phase cooling have demonstrated the potential for significant data center and power electronics heat rejection efficiency improvements. Despite these benefits, pumped two-phase loops present challenges associated with flow instabilities in heat exchangers due to the multiple parallel flow path, presence of two-phase flow in headers, and possible transient behaviors. In-situ real-time sensing could be used to understand the flow distribution and enable active control of heat exchanger geometries and features. This paper aims to develop embedded sensing within heat exchangers. In the exploratory phase of this research, multiple designs for embedded sensors made of conductive filament are printed and compared. The sensors have been tested in a controlled chamber and their resistance as a function of a known temperature is calibrated. In addition, the time history of signals has been used to develop a memory-based algorithm to enable self-awareness within the heat exchanger. The next steps will entail integrating the sensors into a pumped two-phase loop and investigate passive control actions based on maldistributions within the flow.