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Elizabeth Favreau
Marketing Writing Team Lead
As someone who grew up in Northern Minnesota, where no one bats an eye at temperatures below 0°F (-18°C) in the winter, I’m acutely aware of how important it is to have a reliable and effective method of heating your home. At the same time, we’ve all become well aware of the need to reduce greenhouse gas (GHG) emissions, and heating and cooling buildings contributes to a significant portion of today’s GHG emissions. According to the U.S. Department of Energy (DOE), the building sector accounted for about 35% of total GHG emissions in 2021, and 8% of total GHG emissions came from on-site combustion.1 Transitioning from traditional furnaces to heat pumps is one way that we can reduce those on-site building emissions.
Heat pumps use electricity to transfer heat from outside to inside to heat your building, or vice versa to cool your building. Heat pumps are highly energy efficient because they don’t generate heat, as a furnace does; instead, they just move heat from one area to another. In addition, because they can both heat and cool a building, they can reduce the number of required HVAC systems. However, heat pumps can struggle in colder climates—like where I grew up—and they use liquid refrigerants, which can have high global warming potentials (GWPs). To make heat pumps a more widely viable and environmentally friendly solution, we need to develop heat pumps that are compact, have low power requirements, and can operate on low-GWP refrigerants in extreme climate conditions.
Current design methods for heat pumps tend to rely on simplified thermodynamic cycle analysis and 0D/1D simulations, which struggle to capture important physical phenomena such as turbulent flow through the expansion valves and phase change in the evaporators and condensers. In addition, these methods require experimental data for empirical models, which can be very expensive to generate.
Researchers in the Advanced Propulsion and Power Department at the DOE Argonne National Laboratory, together with Convergent Science, are using innovative simulation techniques to overcome these limitations. Three-dimensional computational fluid dynamics (CFD) simulations offer a predictive approach that can substantially reduce the time and costs associated with the heat pump design cycle. With accurate submodels, CFD can replicate the complex physics in heat pump components to provide deeper insight into the flow and heat transfer phenomena that cannot be captured with simplified approaches or easily studied with experimental methods. In particular, CONVERGE’s autonomous meshing and advanced physical models make it well suited to simulations of complex geometries with multi-phase flows.
In a project funded by the DOE, Argonne researchers Muhsin Ameen and Katherine Asztalos, along with Convergent Science engineers Ameya Waikar, Michael Xu, and David Rowinski, are employing multi-fidelity simulations coupled with high-performance computing (HPC) to model and optimize heat pump components, starting with microchannel condensers.
Compared to macrochannel condensers, microchannel condensers exhibit superior heat transfer due to their greater surface area-to-volume ratio, making them well suited for compact systems. They are also typically lighter weight and require a smaller refrigerant charge. Microchannel condensers are suitable for applications with very high heat flux (≥10,000 W/m2), finding uses in HVAC systems, heat pump water heaters, refrigeration systems, and electronics.
The physics of microchannel condensers differs significantly from their macrochannel counterparts; for example, condensation in microchannels is dominated by surface tension forces, as opposed to macrochannels where gravity is the dominant force. Various parameters affect the mechanism for condensation in microchannels, including heat flux, vapor quality, fluid properties, and channel geometry; CFD provides researchers with a valuable tool for examining how these parameters impact the performance of the condenser.
To investigate the performance of microchannel condensers, the team from Argonne and Convergent Science conducted multi-phase CFD simulations in CONVERGE, validating the model against experimental data available in the literature.2,3 The team used CONVERGE’s volume of fluid (VOF) modeling, with the High Resolution Interface Capturing (HRIC) scheme, in conjunction with the Lee condensation model to simulate the multi-phase flow.
For the initial validation, the team performed simulations of FC-72—the liquid coolant used in the experimental setup—flowing along parallel square microchannels. They investigated three different mass flow rates (ṁ) at the inlet and compared the predicted liquid mass fraction at the outlet with the experimental measurements. The results, shown in Figure 2, show good agreement between the simulations and experiments.
Having validated the CFD model, the research team next wanted to investigate the effects of changing various parameters on the performance of the microchannel condenser. They started by looking at two different low-GWP refrigerants, R-1234yf (GWP < 1) and R290 (GWP = 3), and compared them to the performance of FC-72. They found that similar performance could be achieved between the low-GWP refrigerants and FC-72 by modifying the inlet operating conditions and boundary conditions. Figure 3 shows an example, where similar performance was achieved with a high mass flow rate of FC-72 and a low mass flow rate of R-1234yf. The spatial distributions of the refrigerants in the microchannels also show similar patterns under these conditions.
The next parameter the team investigated was the effect of the cross-sectional geometry on the performance of the microchannel condenser. They tested a circular cross-section and a square cross-section, using FC-72 as the refrigerant and similar operating conditions for each case. They found improved performance with a circular cross-section, as shown in Figure 4.
Finally, the research team turned their attention to the effects of adding a turbulence model to their simulation setup, comparing their results to experimental data. The previous simulations described in this blog post have been laminar, and while laminar simulations are able to capture end-state conditions, they struggle to accurately capture other parameters such as pressure drop and phase change distribution. As shown in Figure 5, the addition of the k-ω SST turbulence model enables the simulations to accurately capture the pressure drop, and the phase change distribution better reflects a pressure-driven flow.
The team from Argonne and Convergent Science were able to develop and validate a multi-phase approach for modeling microchannel condensers with CONVERGE. With this model, they were able to gain a deeper understanding of the influence of low-GWP refrigerants and geometric parameters on the performance of microchannel condensers.
In the future, the team plans to incorporate conjugate heat transfer modeling into the CONVERGE setup to more accurately replicate the real-world devices. In addition, they are working on modeling other heat pump components, with the goal of simulating the complete heat pump system down the line. They have already started work on modeling a supersonic ejector, with preliminary results shown in the video in Figure 6. Two-equation k-ε large eddy simulation turbulence modeling and Adaptive Mesh Refinement are able to capture the shock trains and other complex flow features in the mixing chamber and diffuser.
Overall, this work is paving the way to developing more efficient, more effective, and more environmentally friendly heat pumps. Enabling a more widespread adoption of heat pumps could make a significant impact in reducing on-site building GHG emissions, while still keeping us Minnesotans warm in the winter. Learn more about this work in the team’s International Refrigeration and Air Conditioning Conference paper!
[1] Department of Energy. (2024). Decarbonizing the U.S. Economy by 2050 (No. DOE/EE-2830).
[2] Kim, S.-M., Kim, J., & Mudawar, I. (2012). Flow condensation in parallel micro-channels-part 1: Experimental results and assessment of pressure drop correlations. International Journal of Heat and Mass Transfer, 55(4), 971-983.
[3] Kim, S.-M., & Mudawar, I. (2012). Flow condensation in parallel micro-channels-part 2: Heat transfer results and correlation technique. International Journal of Heat and Mass Transfer, 55(4), 984-994.
