Experimental study of cyclic frosting and defrosting on microchannel heat exchangers with different coatings

doi:10.1016/j.enbuild.2020.110382

Energy and Buildings

Volume 226, 1 November 2020, 110382

https://doi.org/10.1016/j.enbuild.2020.110382 Get rights and content

Abstract

This paper aims to further explore the frosting and defrosting performance of coatings applied to microchannel heat exchangers. A visualization cycle experiment of hydrophilic, desiccant and hydrophobic coated microchannel heat exchangers was performed, and compared with the uncoated sample, under frosting condition. The results show that the delayed frost formation of the hydrophilic coating and the desiccant coating is not obvious. Even worse, the average heat transfer of the hydrophilic coating sample is 19% lower than that of the uncoated coating. By contrast, in the first round, the hydrophobic coating has a significant anti-frost effect, with a 24% reduction in pressure drop and a 40% increase in heat transfer. But it attenuated the most after four cycles. Taken together, the application potential of hydrophobic coatings is the greatest, but the horizontal fins and crest and trough structures, making drainage difficult, are key factors that limit its superior performance. Additionally, when evaluating the frosting performance of the coating of the microchannel heat exchanger, it is not recommended to use the frosting amount alone or the index of thickness and pressure drop, but to comprehensively consider the heat exchange effect.

Introduction

Heat exchanger is an important part of air conditioning system. Compared with the conventional fin and tube heat exchanger, the microchannel heat exchanger has the advantages of having lower weight, higher heat exchange efficiency, smaller refrigerant charge and compact structure, widely used in automotive air conditioning. In recent years, due to the prominent cost and performance advantages of microchannel heat exchangers, the application research of which in household air conditioners has drawn increasing attention. Many experimental researches [1], [2], [3], [4] have shown that, when air conditioning systems using micro-channel heat exchangers instead of finned tube heat exchangers as condensers, cooling capacity of the system is increased, COP is improved, and the pressure drop of the refrigerant is declined.

However, the market replacement rate in the field of commercial or domestic air-conditioning is still low, mainly because the microchannel heat exchanger has serious frosting problems when operating as an evaporator at low temperature conditions, leading to system performance dramatically drop [5], [6], [7]. Therefore, the performance of microchannel heat exchangers must be optimized when operate under high humidity and low temperature conditions.

At present, most research on frosting of microchannel heat exchangers is limited to uncoated. Y. Xia and Y. Zhong [8] compared the air-side thermal–hydraulic performance of louvered-fin and flat-tube heat exchangers during frosting. Kyoungmin Kim [9] investigated the thermal performance uniformity and frost growth of microchannel heat exchangers as a function of the depth of the heat exchanger, the pitch of the fins, and the pitch of the channels. Bo Xu et al. [10] investigated the cycle frosting and defrosting performance of horizontal-tube and vertical-tube microchannel heat exchangers, showed cycle operation increased the severity of the ice blockage of the horizontal sample and shorten the effective operating time by 40 min than vertical sample. Further, Bo Xu et al. [11] proposed a solution to improve the drainage problem. The new type sample exhibited stable performance in 5 test cycles under frost conditions. Lately, Y. Hu and A. Ebrahimifakhar [12] experimentally studied the effects of outdoor air-side fouling on frost growth and heat transfer characteristics of a microchannel heat exchanger,

Previous studies have shown that hydrophilic, desiccant, and hydrophobic materials can delay frosting. However, most studies are all carried out on flat samples [13], [14], [15], [16], [17]. And even for the application research of coatings, they mainly focused on the geometry of fins or tube heat exchangers [18], [19], and the impact of operating conditions or surface temperature. Due to the differences in the structure of the micro-channel heat exchanger, the characteristics of the fins, and the installation orientation of the flat tubes etc., these experimental results and disciplines cannot be generalized directly to the microchannel heat exchanger.

Thus, it is necessary to systematically study the frosting performance of surface-coated microchannel heat exchangers. Lorenzo Cremaschi [20] experimentally studied microchannel heat exchangers, with coatings of different contact angel, operated in different environments. He evaluated the degree of influence of frosting factors, and adopted a new method of frost layer thickness. But the performance of material cycling frosting is not included, and comprehensive evaluation of material properties is lacking. For microchannel heat exchangers with desiccant coating, Li Zhang [21] introduced a diffusion model to predict the distribution of moisture concentration and temperature in the desiccant layer of microchannel heat exchanger; X.Y. Sun et al. [22] tested and compared the heat transfer, mass transfer and pressure drop performances of desiccant coated microchannel heat exchanger and fin-and-tube heat exchanger. So far, no experimental study on the effect of desiccant coating on frosting and defrosting of microchannel heat exchangers.

To address the existing gap in the study about experimental data of frosting-defrosting performance on coated microchannel heat exchangers, in this paper, frosting-defrosting cycle tests were performed on uncoated and three other microchannel heat exchangers with desiccant, hydrophobic and hydrophilic coatings. Considering the insufficient evaluation of the performance of coating applications in the existing research, this experiment comprehensively analyzes the anti-icing performance of coating materials when applying to microchannels from the perspective of frosting process, clogging, defrosting process and interfacial durability. At the same time, the coatings were evaluated from the perspective of heat exchangers, such as heat transfer efficiency and pressure drop. This work could provide more reference data and more inspired testing and evaluation methods for the subsequent application research of the anti-frost coating of microchannels.

Section snippets

Test facilities

The entire experiment was performed in an enthalpy different laboratory, which could establish an artificial simulation environment that meets the test requirements and is relatively stable, without being disturbed by the surrounding space. The wet and dry bulb temperatures of the return air and supply air of the enthalpy different room are collected by wet and dry bulb thermometers, and the air condition is precisely adjusted by air-conditioning processing system, to achieve the required

Data sources and uncertainty analysis

According to the principle of mass conservation, the accumulated frost mass can be obtained from the dry air mass flow and the moisture content of the air before and after passing through the heat exchanger:

$M_{f} = \int {\dot{m}}_{a} (d_{in} - d_{out}) d t$ (1)

${\dot{m}}_{a} = ρ_{a} {\dot{v}}_{a}$ (2)

${\dot{v}}_{a} = S \cdot \bar{υ}$ (3)Where $\dot{m_{a}}$ is the mass flow rate of dry air, kg·s⁻¹, d_in and d_out are moisture content of the windward and leeward sides of the heat exchanger, kg·kg⁻¹, t is frosting time, s, ρ_a is the density of dry air, kg·m⁻³, and $\dot{v_{a}}$ is air volume flow rate, m

Frosting process

Fig. 5 shows the frosting process of the first cycle. Under the standard frosting condition, the uncoated and hydrophilic coated samples both started to appear white as soon as the fan was turned on. The high-humidity air flows through the heat exchanger, rather to condense, it directly condenses into frost on the surface of the low-temperature fins, forming a white layer. Although a study has shown that a water film can form on the vertical plane of a hydrophilic surface and flow down to

Conclusions

Through four frosting cycles of experiments on four microchannel heat exchangers, the following conclusions can be drawn:

(1)
When the hydrophilic coating is applied to a microchannel heat exchanger, it does not significantly delay the frosting. In contrast, the average heat transfer of the four cycles was 19% lower than that of the uncoated sample, mainly due to the higher density of frost layers. The heat exchange capacity decreases by 24% after four rounds.
(2)
Desiccant coating can slightly reduce

CRediT authorship contribution statement

Pu Liang: Conceptualization, Supervision. Liu Ran: Writing - original draft, Methodology. Huang Hai: Supervision, Writing - review & editing. Zhang Shengqi: Writing - review & editing. Qi Zhaogang: Writing - review & editing. Xu Weidong: Writing - review & editing. Zhou Jing: Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

The authors are very grateful to acknowledge the collaborator of this project experiment, Sanhua Holding Group. Thanks for the financial support and the provision of all heat exchanger samples in the experiment, including the manufacture of heat exchangers and the application of coatings.

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Citation Excerpt :
The contact angle represents the wetting characteristics, and the larger the contact angle, the weaker the surface wettability. Pu et al. [49] comparatively studied the effects of desiccant coating, hydrophilic coating, and hydrophobic coating on the frosting performance of MCHX. The experimental results showed that hydrophilic coating and desiccant coating had no clear effect on delaying frosting.
Currently, microchannel heat exchangers (MCHXs) are widely employed in refrigeration and heat pump systems due to their advantages of being more efficient, smaller in size, lower refrigerant charge, and cheaper price. However, when used as evaporators, MCHXs have the issues of a faster frosting rate and frost melt water, condensate that is more difficult to remove than fin-and-tube heat exchangers. This review paper focuses on the research progress on the thermal-hydraulic and drainage performance of MCHX under frosting, defrosting, and wet conditions from the year 2000–2022. First, experimental research on the frosting and drainage characteristics of MCHX is summarized and analyzed in periodic frosting and defrosting conditions. Then, three typical methods for improving the frosting and defrosting performance of MCHX are introduced. Moreover, empirical correlations and simulation models developed to predict the frosting and defrosting characteristics of MCHX are discussed. Uneven frost layer distribution should be considered in the frosting model and the effect of drainage characteristics on defrosting performance should be considered in the defrosting model. The thermal-hydraulic and drainage performance of MCHX under wet conditions is also reviewed. First, methods for testing real-time condensate retention are introduced. Then, the parameters affecting the thermal-hydraulic and drainage characteristics of the MCHX are analyzed. Based on the above research, two methods for improving the performance of MCHX under wet conditions are introduced. Empirical correlations and numerical models for predicting thermal-hydraulic performance under wet conditions are classified and compared. A novel one-dimensional numerical model based on the fin theory combined with the moving boundary technique can accurately capture the actual dehumidification state with a small computational cost. The main conclusions regarding the thermal-hydraulic and drainage performance of MCHX based on the open literature are presented. Finally, some research initiatives and vital improvement measures are suggested for future studies. This review aims to serve as a reference guide for the thermal-hydraulic and drainage characteristics studies of MCHX under frosting, defrosting, and wet conditions.

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Experimental study of cyclic frosting and defrosting on microchannel heat exchangers with different coatings

Abstract

Introduction

Section snippets

Test facilities

Data sources and uncertainty analysis

Frosting process

Conclusions

CRediT authorship contribution statement

Declaration of Competing Interest

Acknowledgment

Int. J. Refrig.

Int. J. Heat Mass Transf.

Appl. Energy

Appl. Therm. Eng.

Appl. Energy

Int. J. Heat Mass Transf.

Appl. Therm. Eng.

Exp. Therm. Fluid Sci.

Int. J. Refrig.

Appl. Therm. Eng.

Exp. Therm. Fluid Sci.

Performance test and analysis of micro-channel domestic air conditioner refrigeration [J]

J. Zhongyuan Inst. Technol.

Research status of microchannel heat exchangers in air conditioning applications [J]

Mech. Eng.

Comparative experimental study of the performance of the microchannel heat exchanger used in the home cabinet air conditioner [J]

J. Refrig.