An experimental method to quantify local air-side heat transfer coefficient through mass transfer measurements utilizing color change coatings
Introduction
Heat transfer between liquid-to-gas is widely used in aerospace, transportation, electronics cooling, industrial processes, refrigeration, and air-conditioning areas. Therefore, liquid-to-gas heat exchangers are intensively investigated for energy saving considerations. Jacobi and Shah [1] reviewed the air-side enhancement induced by vortex generators. It is shown that trade-offs between increasing HTC, air-side pressure drop penalty, and high manufacturing cost limit air-side performance improvements. Joardar and Jacobi [2] have claimed that air-side convective resistance to be the most significant contributor, which accounts for 75% or more of the total heat transfer resistance in refrigerant-to-air heat exchangers. Due to the complexity of air-side geometries, empirical correlations from experimental studies are usually used to predict heat transfer performance. Therefore, experimental techniques have been intensively studied in the past decades to quantify air-side HTC in order to seek opportunities to improve the efficiency of the fluid-to-gas heat exchangers. The Wilson Plot Method has been proposed by Wilson [3] a century ago and is still widely used today. However, it is only possible to obtain averaged HTC with this method. There are three most common technical approaches to measure local HTC experimentally: (i) via temperature measurements; (ii) through flow velocity measurements; (iii) by mass transfer measurements. Furthermore, there are several specific techniques available for each approach. However, it is still challenging to quantify local air-side HTC on complicated geometries and entire heat exchanger surfaces.
The purpose of this study is to propose a practical experimental method which has the potential to be applied on complicated surfaces to obtain local air-side HTC. This method employs the analogy between heat and mass transfer. A visualization procedure is developed to quantify local mass transfer by color change observation. Since the method employs a coating material in combination with a tracer gas to achieve a local color change, this method will be abbreviated as CTC in this paper.
Section snippets
Review of experimental methods for measuring air-side heat transfer
The main experimental methods to quantify HTC are classified and summarized in Fig. 1. There are three technical approaches to obtain local HTC. For each approach, the most commonly applied technologies are studied and compared. As the CTC method employs a similar color analysis as the Thermochromic Liquid Crystal (TLC) method, more details about the TLC method are reviewed. Furthermore, the CTC method employs the analogy between heat and mass transfer. Therefore, the mass transfer methods,
Principle of the proposed method
A new visualization method which employs the analogy of heat and mass transfer is developed. It applies a similar principle as the AAM. A color change coating and tracer gas combination is developed; the chemical interaction between the coating material and a tracer gas is the driving force of mass transfer. As shown in Fig. 2, a color change acidic coating material is attached to the heat transfer surface of interest. A dye is mixed with the coating material, which changes color at a certain
Description of wind tunnel
A suction type, open-loop wind tunnel is designed and built for the mass transfer experiments, as shown in Fig. 5. The design followed the principles described by Barlow et al. [24] and ASHRAE Standard 41.2-1987 (RA 92) [25]. The size of the test section is W120 mm × H100 mm × L400 mm, which is to test small samples such as flat plates, single fins, and fin arrays. The flow velocity across the test section can be adjusted from 0 to 3.5 m/s. The air flow, water vapor (1), and anhydrous ammonia
Coating formulation development
Different coating solutions are developed and evaluated on aluminum alloy Al 1100. The criteria include solution stability, metal corrosion, coating quality, and color change characteristics. The first step is to evaluate the uniformity and stability of the solution. Different formulations are prepared and diluted with ethanol to obtain 25% solution by mass. No chemical reaction is expected in ethanol for at least one week. The number of hydrogen protons () are measured during the course of
Conclusions
A new method to quantify local air-side HTCs has been developed, and this paper describes preliminary results for air flow across a flat plate at laminar flow conditions. The following conclusions are obtained:
- (1)
The method (CTC) is based on the analogy between heat and mass transfer, similar to the well-known naphthalene sublimation method. The analogy between mass transfer and heat transfer is clear in laminar boundary layer flow, and the impermeable-surface assumption is valid at low
Acknowledgements
- 1.
The authors would like to thank the member companies of the Air Conditioning and Refrigeration Center at the University of Illinois at Urbana-Champaign for their financial and technical support.
- 2.
The authors would like to thank Creative Thermal Solutions, Inc. (CTS) for providing technical support and equipment.
- 3.
The coating measurements were carried out in part at the Frederick Seitz Materials Research Laboratory Central Research Facilities located at the University of Illinois at Urbana-Champaign.
Funding
Air Conditioning and Refrigeration Center at the University of Illinois at Urbana-Champaign and its membership companies provide funding for this project.
Declaration of Competing Interest
The authors declare that there is no conflict of interest.
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