Effect of micro-fin geometry on flow and heat transfer during evaporation in round tubes

Yang, Cheng-Min

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https://hdl.handle.net/2142/105734 Copy

Description

Title

Effect of micro-fin geometry on flow and heat transfer during evaporation in round tubes

Author(s)

Yang, Cheng-Min

Issue Date

2019-05-17

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

Hrnjak, Predrag S.

Doctoral Committee Chair(s)

Hrnjak, Predrag S.

Committee Member(s)

• Jacobi, Anthony M.
• Dutton, J. Craig
• Wang, Xiaofei
• 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)

• micro-fin
• flow boiling
• heat transfer
• visualization

Abstract

In order to understand the mechanism of heat transfer enhancement due to micro-fin geometry, this dissertation develops a novel approach to visualize the two-phase flow pattern in transparent micro-finned tubes under both adiabatic and diabatic conditions. Rather than the conventional sight-glass method (without internal micro-fin geometry in the clear tube), clear resin tubes are made by a 3D printer to reproduce the real internal geometry in the commercial metal micro-finned tubes. The 3D printed tube is placed inside a glass tube and heated with the secondary fluid flowing between the two tubes for simulating the evaporation conditions. Thanks to the diabatic visualization, new insights for the flow behavior inside the smooth and micro-finned tubes during evaporation are brought. Additionally, quantification and analysis of the captured high speed videos help further examine the heat transfer mechanisms in different flow regimes. The first focus of this thesis is to experimentally investigate the effect of axial micro-fins on heat transfer and pressure drop during evaporation in commercial aluminum round tubes. The axial micro-finned tube used in this study is special compared to most of the work in the literature because the tube was expanded before testing to reflect the real situation. In the manufacturing process of fin-tube heat exchanger, the tube expansion technique is widely used to provide a better contact of the micro-finned tubes and external air-side fins. Experiments of R410A flow boiling inside axial micro-finned tubes and smooth tubes with the inner diameter of 6.3 mm were conducted. The results showed that the average heat transfer enhancement factor of the axial micro-finned tube is 1.34 and the average pressure drop penalty factor is around 1.23. In the second part of this thesis, two-phase flow behavior was visualized under near adiabatic conditions through a clear smooth tube and micro-finned tubes of 0°, 10° and 18° helix angles. The results showed that axial micro-fins (0° helix angle) do not provide additional force to pull up the liquid inside the round tube, so the annular flow does not occur at an earlier condition (lower vapor quality or mass flux). Therefore, the flow pattern in transparent axial micro-finned tubes is generally similar to that in transparent smooth tubes. For the helical micro-finned tube, the annular flow pattern occurs at a lower vapor quality than the smooth and axial micro-finned tubes. This is because the swirling flow triggered by the helical micro-groove geometry enables the liquid refrigerant to reach a higher location in the round tube, and the capillary force keeps the liquid flowing along the micro-grooves. Helix angle affects the transition boundary of stratified wavy flow to wavy annular flow, which shifts downward (to lower mass flux and vapor quality conditions) as the helix angle increases. Other transitions are not significantly affected by the helix angle. Based on the experimental data, a flow pattern map for flow boiling in horizontal micro-finned tubes considering the effect of helix angle was built. In the third part of this thesis, flow boiling visualization under diabatic conditions is carried out. Compared to the traditional adiabatic visualization technique, the more realistic evaporative condition is simulated and the developing evaporation process is captured. The diabatic method reveals new insights for the flow behavior at low vapor quality such as formation of a vapor plug and rewetting phenomenon in slug flow. Moreover, the effect of micro-fin geometry on evaporation in different flow patterns is further studied. Micro-fin geometry influences flow behavior in round tubes especially for low vapor quality conditions. Bubbles are mainly generated in the groove region due to a higher superheat, and the bubbles in the helical micro-finned tube are easier to merge and travel with a higher velocity. The annular flow pattern is almost the same as that visualized through the traditional adiabatic method since the nucleate boiling mechanism is suppressed when the fluid velocity is high. Heat flux and vapor quality conditions affect the heat transfer mechanisms during the flow boiling. The objective of the fourth part of this thesis is to quantify the bubble parameters such as departure diameter and frequency and analyze the heat transfer mechanisms through the captured videos from the diabatic visualization. The wall heat flux transferred to the refrigerant comes from nucleate boiling (bubble generation) and forced convection. Since the forced convection plays an important role under the current heat flux condition, the liquid becomes warmer. The heat flux for bubble growth (film boiling) can be also quantified through the video. The heat is transferred through forced convection mechanism thoroughly as the heat flux is low. As the heat flux increases, the proportion of evaporative mechanism becomes important. In the visualization section, the tube material is resin (polymer) instead of aluminum. The relevance of the data through the visualization approach to the real evaporator tube may be questionable due to the change of the material. In other words the effect of surface tension for these materials is challenged and this is the main deliverable of the last part of this thesis. The visualization technique as well as comparative measurements of contact angles for aluminum and polymer explored with water, R245a and R410A are presented. The results showed that the changes of the contact angles are small such that they do not affect the generalization of the conclusion obtained in the transparent polymer tube to actual metal tube used in the real heat exchangers.

Graduation Semester

2019-08

Type of Resource

text

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Copyright and License Information

Copyright 2019 Cheng-Min Yang

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