Fundamental understanding of condensation frosting and precise detection with capacitive sensors

Shen, Yuchen

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https://hdl.handle.net/2142/117621 Copy

Description

Title

Fundamental understanding of condensation frosting and precise detection with capacitive sensors

Author(s)

Shen, Yuchen

Issue Date

2022-08-10

Director of Research (if dissertation) or Advisor (if thesis)

Wang, Sophie

Doctoral Committee Chair(s)

Wang, Sophie

Committee Member(s)

• Wang, Xinlei
• Brewster, M. Quinn
• Elbel, Stefan

Department of Study

Mechanical Sci & Engineering

Discipline

Mechanical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Condensation
• Frosting
• Ice propagation
• Frost detection
• Frost porosity
• Microchannel heat exchanger

Abstract

Frost accumulation on surfaces can deleteriously impact thermal performance in many applications, such as in aerospace and automotive systems, particularly in heating, ventilating, air-conditioning, and refrigeration (HVAC&R) systems. In refrigeration and heat pumping systems, frost typically grows on the finned, air-side surface of the heat exchanger depending on environmental and operating conditions. It can increase air-side pressure drop and block air flow while simultaneously increasing the thermal resistance, reducing thermal performance. As a result, frost accumulation can significantly reduce the system coefficient of performance (COP). A better understanding of the frosting mechanisms is necessary to mitigate these impacts, and effective strategies for defrosting control must be proposed. Micro-/nano-scale surface structures have been widely explored as a promising tool for anti-freezing or frost avoidance on heat transfer surfaces. Despite studies of many surface feature designs, the mechanisms associated with condensation freezing and ice propagation on micro-structured surfaces have yet to be thoroughly elucidated, especially when it comes to a quantitative understanding. Micro-/nano-textured superhydrophobic surfaces with different features have been developed to investigate the condensation freezing droplet sizes and freezing propagation process. Bare hydrophobic and hydrophilic surfaces were also studied to provide baselines for comparison. Droplets growing near the sample edge are the initial self-frozen droplets due to the local diffusive flux, and their size can be determined based on heterogeneous nucleation theory. For the other droplets located away from the edges, freezing was triggered by a neighboring droplet due to the local vapor concentration difference, and the freezing size can be estimated by the sample size, nucleation site density, and droplet growth rate. A micro-pillar layout is found to have significant effects on both nucleation and neighboring droplet interactions, as reflected by the condensation droplet distribution prior to solidification and eventually the freezing front propagation velocity. This work provides a more complete picture of the frozen droplet profile on superhydrophobic surfaces, with insights into the freezing mechanisms that can predict frost accumulation on different surfaces. In order to optimize defrost control, a capacitance sensing approach is proposed for frost detection and growth measurement. A capacitive sensor with interdigital electrodes is designed and fabricated based on the fringing effect for the frosting stage and frost density measurement. Frost growth experiments under different surface temperatures, air temperatures, and air velocities are conducted with simultaneous frost imaging. The results show that the capacitance sensing approach can accurately capture the characteristics of different frost growth stages and the differences in frost growth under different conditions. The frost porosity measured by the capacitance sensing approach agrees well with the averaged measurement approach, with a maximum difference of 3.38% for data obtained after very early frost growth (after 5 min). The temporal variation of frost porosity can significantly reflect the frost growth period. For frost detection on a microchannel heat exchanger, a capacitive sensor with electrodes parallel to the fin has been developed to detect the real-time frost growth in the direction along and perpendicular to the airflow. Frosting-defrosting experiments were conducted under different air and coolant temperatures on both rectangular offset-strip fins and louvered fins. Results show that the capacitive sensor can measure the frost thickness variation during the frosting and defrosting processes. The sensor captures a delay of frost growth on the trailing edge, showing high sensitivity under low air and coolant temperatures. The sensor also detects fin shape effects on the frost distribution profile. Based on capacitance readings in five locations, a frost distribution map is constructed and agrees well with imaging results. The newly developed sensor can serve as a powerful tool for defrost control.

Graduation Semester

2022-12

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Yuchen Shen

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