Conference Agenda

This paper advances a two-dimensional model for predicting local pressure drop, and sensible and latent heat transfer rates in the so-called ‘no-frost’ evaporators used in frost-free refrigerators. The model was devised based upon the first principles of mass, momentum and energy conservation for both the air stream and frosted medium, being able to predict the evaporator blocking over time. The coupling between the heat exchanger air-side pressure drop and the fan characteristic curve has also been considered by means of a fluid-dynamic model able of predicting the two-dimensional airflow redistribution. Frost growth experiments were carried out using a purpose-built test facility comprised of a bottom-mount refrigerated cabinet placed with controlled humidity and temperature in both fresh-food and freezer compartments. A camera was also employed to capture the time evolution of the frost thickness on the evaporator. The model validation exercise showed that the predicted results follow the experimental trends inside the experimental band of errors. The model allowed the identification of the critical region of the evaporator in terms of airflow blockage, which leads to an uneven airflow pattern and underutilization of the air-side heat transfer surface area. Finally, the model was used to propose a new fin distribution aimed at a better utilization of the evaporator. The new geometry reduced the local air-side pressure drop by 65% and homogenized the airflow, resulting in a more uniform utilization of the evaporator area for heat transfer.

One of the primary concerns during the heating operation of air source heat pumps (ASHP) is the frosting phenomena on the evaporator's surface, which serves as the outdoor heat exchanger. In outdoor conditions characterized by low temperatures and high humidity, the water vapor in the air condenses and freezes on the external surface of the evaporator. Various factors, including air temperature, humidity, and surface temperature, influence frost formation on heat transfer surfaces. This frost formation increases heat resistance and air pressure drop, leading to the deterioration of the evaporator's performance and a reduction in the evaporation temperature of the refrigerant. Consequently, the specific volume increases, the mass flow rate decreases, and the overall heat capacity of the heat pump is diminished. Once the system performance reaches its minimum acceptable level, a defrost process is initiated to eliminate the frost layer and attempt to restore the performance at the start of the cycle. During the defrosting process, ice can be melted by reversing the cycle, using resistive heating methods, or through natural defrosting processes.

The current study offers an experimental comparison of frosting heat transfer rates, frosting cycle durations, and frost patterns on a microchannel evaporator within a vapor compression system using R134a refrigerant under different ambient conditions. Three distinct defrosting techniques are developed and implemented following the determination of full frosting on the evaporator, which was based on air side pressure drop data and microscopy analysis, aiming to evaluate the most effective defrosting technique.

Frost formation, a detrimental phenomenon that occurs in an air-source heat pump (ASHP) in low temperature and high humidity conditions impedes the performance of the ASHP. Prevalent thermal defrosting approaches, such as reverse cycle defrosting, and hot gas bypass, are energy-intensive and may cause occupant discomfort. In our prior work, we designed and demonstrated a purely mechanical defrosting approach that can overcome these challenges. We designed and manufactured a shape-morphing fin with a negative stiffness structure that displays snap-through instability. This instability entails the sudden release of strain energy resulting in large deflections and vibrations that facilitates frost fracture and shedding from the morphing fin. We conducted frosting [1] and defrosting experiments [2] to measure the properties of frost and compared the energy consumption between mechanical and thermal defrosting. Subsequently, we designed an optimal shape morphing fin to maximize the defrosting performance [3]. The mechanical defrosting strategy doesn’t cause occupant discomfort and is orders of magnitude more energetically efficient than the thermal strategy, demonstrating high effectiveness for glaze-like or high-density frost. However, the efficacy of the purely mechanical defrosting strategy diminishes for low-density frost.

In this study, we propose a thermo-mechanical strategy involving heating and refreezing stages to convert the low-density frost into high-density frost and hypothesize this hybrid approach to be a highly effective defrosting mechanism even in low-density conditions. We investigate the effectiveness of different multistable heat exchanger (HX) configurations and actuation combinations. We design and construct an experimental setup, capable of conducting frosting and defrosting studies, thereby enabling a robust evaluation of the technological feasibility of the proposed approach. This setup utilizes a thermal bath with a water/glycol mixture to form frost on the scaled-down multistable HX fin pack, and embedded linear actuators to actuate the multistable fins. This setup allows rapid testing of different HX designs and thermo-mechanical strategies, as well as a comparison of energy consumption to the prevalent thermal defrosting strategy. The paper concludes with recommendations for the most energy-efficient multistable heat exchanger configuration, actuation method, and thermo-mechanical strategy, paving the way for more effective and less energy-intensive defrosting strategies.

References:

[1] Aman Thakkar, Jiacheng Ma, James E. Braun, W. Travis Horton, Andres F. Arrieta. (2021). De-Icing in heat pump fins using shape morphing. In 2021 ASHRAE Annual Conference. ASHRAE.

[2] Aman Thakkar, Jiacheng Ma, James E. Braun, W. Travis Horton, Andres F. Arrieta. (2021). Frost Formation in Evaporator Fins with Embedded Negative Stiffness Structures. In 18th International Refrigeration and Air Conditioning Conference

[3] Aman Thakkar, Jiacheng Ma, James E. Braun, W. Travis Horton, Andres F. Arrieta. (2023). Energy-efficient defrosting of heat exchanger fins with embedded negative stiffness structures. Applied Thermal Engineering, 222, 119850.

Frost formation and growth on the evaporator surface is a common process that deteriorates the air-refrigerant heat transfer and restricts airflow. This degrades the performance of the vapor compression system by increasing temperature lift and air-side pressure drop. To accurately predict these effects during coil frosting, as well as the energy use and duration of the defrost process, there is a need to estimate the heat and mass transfer, momentum transport, and solid-liquid and liquid-vapor phase change. Therefore, in the past few decades, continuous effort has been made to model frosting and defrosting processes using approaches ranging from empirical correlations to computational fluid dynamic models. To provide a clearer overview for researchers, engineers, and manufacturers in this field, this paper provides a comprehensive literature review for frosting and defrosting models. The paper begins with theoretical background of frost formation and defrost processes, and then reviews the common modeling approaches in literature and their underlying assumptions when trying to account for various physical phenomenon. Based on the literature review, the most critical modeling effort for frost formation is the determination of frost densification rate and frost growth rate. Various methods to predict these two parameters are reviewed. Empirical correlations commonly used for frost density and thermal conductivity are presented and compared. For the defrost process, various multi-stage models have been proposed with different assumptions. Some assume the presence of air gap between the tube wall and the frost, while others consider the melted frost flow due to gravity. We also review physics-based and empirical approaches to integrate defrost models into heat pump models. We conclude by identifying research gaps and providing recommendations.

Heat pump systems are attractive for mildly cold climates because they can be more efficient than conventional heating sources and are able to reuse pre-existing air conditioning layouts (if present), but they are also quite susceptible to frost formation. It is of utmost importance to understand the frost growth in these systems and to use that understanding to develop an appropriate defrosting strategy in order to prevent system inefficiencies or failures from occurring. A predictive model for frosting and defrosting is indispensable to gain this understanding. To that end, we have developed and implemented in GT-SUITE a unified nonequilibrium model for condensation, frosting, melting and sublimation to account for these important processes in an accurate, fast and robust manner. The ’nonequilibrium’ approach employs a driving force between the bulk and the saturation mass fractions of the water vapor to calculate rates of these processes. Special care has been taken to handle the transitions between these phenomena in a smooth manner in order to keep numerical instabilities at bay. For defrosting, we model both sublimation and melting phenomena, which can occur simultaneously. We demonstrate the usefulness of our novel mathematical model for simulating frosting and defrosting phenomena for a detailed heat pump system in this work. This simulation helps us study the build-up and abatement of frost, and the effects the defrost strategy have on the indoor (cabin) temperature and the power consumed, clearly showing the usefulness of a defrost strategy for heat pump operation.