KEY CONCEPTS
• A heat transfer model revealed the potential for using pulse electrothermal defrosting to remove ice.
• Ice removal can be facilitated by spontaneous melting at the interface through interfacial lubrication and gravitational shedding.
• Pulse electrothermal defrosting is dependent upon gravity and does not work on horizontal surfaces.

Ice remains a major problem that can hinder machinery, and its removal can be cumbersome, time consuming and expensive. Adding to the problem is that in using current approaches for deicing surfaces, machinery must be shut down.

A better understanding of how ice forms on a surface might lead to better approaches for removal. In a previous TLT article (1), researchers determined that the hydrophobicity of a surface is a key parameter influencing ice growth. When the contact angle of the surface is greater than 40 degrees, ice forms an off-surface growth mode that the researchers found was easier to remove. 

Nenad Miljkovic, associate professor in the mechanical science and engineering department at the University of Illinois in Urbana-Champaign, Ill., says, “The limitation to current approaches for deicing surfaces are dependent upon the application. For example, defrosting systems need to be installed in refrigeration/air conditioning systems, forcing the user to pay a penalty for efficiency leading to added cost. In automotive applications, a very big penalty is incurred, particularly in electric vehicles where an air source heat pump, which extracts heat from the cold air outside during winter heating mode, tends to ice up rapidly. This limits the efficiency of the electric vehicle which also limits the automobile’s range.”

Miljkovic points out that in aerospace applications, ice is a major safety problem. He says, “Ice removal adds cost and complexity.”

Miljkovic says, “Many current ice-removal systems are thermal based. This involves adding energy to deice a specific surface that includes not just melting ice but also heating and evaporating the water formed. Melting represents a process that requires a higher level of heat (latent heat) than raising the temperature of water by one degree centigrade (specific heat). 

An alternative technique for ice removal is known as pulse electrothermal defrosting. Miljkovic says, “In thinking about deicing, melting occurs at the interface between ice and the surface on which it builds up. In conventional methods, the ice completely melts in order to be removed. But with pulse electrothermal defrosting, large amounts of heat are introduced in short periods of time through pulses. The result is that more localized heating can be achieved on a surface leading to the removal of ice even before it has completely melted.”

The potential of using pulse electrothermal defrosting led Miljkovic and his colleagues to develop a heat transfer model to more efficiently use this technique. 

Interfacial lubrication
The researchers used their model to solve the heat equation, which maintains conservation of energy with thermodynamics calculations to determine the intensity and time duration of an energy pulse needed in a specific application to remove ice. Miljkovic says, “We needed to do a bit of optimization to take in consideration the geometry of the surface on which the ice is attached.”

To validate the model, the researchers conducted a series of experiments by heating indium tin oxide-coated glass samples oriented in the vertical position in the experimental setup (see Figure 3). Miljkovic says, “Indium tin oxide is a common heating material used in aerospace in applications such as airplane cockpit windows; this material has good adherence to glass and a high resistance to electricity leading to the conversion of energy to heat.”


Figure 3. The experimental setup and the researchers involved in demonstrating that pulse electrothermal defrosting has the potential to effectively removal ice from vertical surfaces. (Figure courtesy of the University of Illinois.)

Ice removal was evaluated at -15 C and -71 C. Miljkovic says, “The higher temperature is common for refrigeration systems while the lower temperature is common in aerospace applications.”

The researchers determined that ice removal can be facilitated by spontaneous melting at the interface through interfacial lubrication and gravitational shedding. Miljkovic says, “When the energy pulse reaches the surface, heat is allowed to diffuse into the interface between the surface and the ice. This leads to the melting of a finite amount of ice at a specific thickness right at the interface with the surface. Through a process known as interfacial lubrication and influenced by gravity, the ice then becomes detached and starts to slide off the surface.”

Miljkovic indicated that this technique for ice removal depends on gravity and will not work on horizontal surfaces. He says, “We believe that pulse electrothermal heating is effective because systems such as heat exchangers usually have vertical geometries.”

The model is used to enable predictions to be made about thickness of the water layer needed to facilitate ice removal. This figure then determines the intensity and duration of the energy pulse. Miljkovic indicates that the model will then predict how long it will take for the ice to be removed.

Initial experimental results show that ice removal using pulse electrothermal defrosting can utilize less than 1% of the energy needed and take less than 0.01% of the defrosting time required for more conventional thermal-based defrosting techniques.

Miljkovic says, “We intend in the future to prepare a more detailed model that considers three-dimensional geometries and curved surfaces. We also are hoping to conduct experiments on a heat exchanger.

Additional information on this research can be found in a recent article (2) or by contacting Miljkovic at nmiljkov@illinois.edu. 

REFERENCES
1. Canter, N. (2018), “Relationship between wettability and ice removal,” TLT, 74 (2), pp. 12-13.
2. Chavan, S., Foulkes, T., Curumukhi, Y., Boyina, K., Rabbi, K. and Miljkovic, N. (2019), “Pulse interfacial defrosting,” Applied Physics Letters, 115 (7), 071601.