Analysis of convective condensation heat transfer for moist air on a three-dimensional finned tube

https://doi.org/10.1016/j.applthermaleng.2021.117211Get rights and content

Highlights

Abstract

The condensation of moist air plays an important role in daily life and industry. During the condensation process, single-phase and phase-change heat transfers occur simultaneously. A three-dimensional finned tube is an effective element for enhancing heat transfer. However, the latent and sensible heat transfer during moist air condensation outside a three-dimensional finned tube has not been experimentally studied. In this study, experiments were performed using a three-dimensional finned tube to enhance the moist air condensation process. For a better understanding of the enhancement mechanism, latent and sensible heat transfer variations with relative humidity ranging from 0.75 to 0.95 and Reynolds numbers ranging from 838 to 3444 were obtained and analysed. A smooth tube was used for the comparison. The experimental results indicated that the three-dimensional finned tube was favourable for the enhancement of moist air condensation, especially for latent heat transfer. When the Reynolds number was 2504, the latent and sensible heat transfers of the three-dimensional finned tube were 43.6%–81.8% and 15.6%–22.9% higher than those of the smooth tube, respectively. In addition, the latent and sensible heat transfers of the three-dimensional finned tube and the smooth tube were obtained under different velocities of moist air, and empirical correlations were developed to predict the sensible and latent heat transfers with deviations of less than ± 20%. The results obtained can provide a reference for the design of three-dimensional finned tubes used in moist air condensation.

Introduction

The condensation heat transfer of moist air is a common process and can be observed in passive containment cooling systems [1], air conditioners [2], seawater desalination [3], natural gas exhaust heat recovery [4], CO2 capture and storage [5], power plant cooling [6], and so on. During the condensation process, single-phase and phase-change convection occur simultaneously on the condensing wall [7]. Meanwhile, noncondensable gases (NCG) accumulate near the condensing interface, and this significantly decreases the heat transfer coefficient.

Othmer [8] experimentally studied the influence of NCG on water steam condensation and found that a 0.5% volume fraction of NCG decreased the heat transfer coefficient by more than 50%. Sparrow and Eckert [9] developed a boundary layer model to predict the condensation heat transfer of moist air outside a horizontal plate, which considered the interfacial resistance, superheating, and forced flow. They made the conclusion that interfacial resistance has a negligible influence on the condensation process. Siddique [10] conducted experimental and theoretical investigations to determine the condensation of moist air under forced conditions, focussing on the condensation inside a vertical tube under different operation parameters and studying the influences of different types of NCG. This provided a reference for the design of nuclear safety systems. Tong [11] studied the condensation of moist air in the background of a natural gas turbine and found that the latent heat transfer coefficient decreased with an increase in the cooling water inlet temperature. Xu et al. [12] studied the condensation of steam in the presence of multiple NCGs. Zschaeck et al. [13] used computational fluid dynamics to model the condensation of moist air by defining a mass sink to simulate the removal of condensable components near the condensing wall, and they analysed the mass flow rate of the condensate. Tang et al. [14] used a double boundary layer to study the distribution of the velocity and temperature in the boundary layers. Alshehri et al. [15] studied the condensation of moist air over a wide range of NCG concentrations and found that the thermal resistance of the condensate liquid was at least one order of magnitude lower than that of the diffusion layer. Lowrey and Sun [16] studied the condensation of moist air in a plate heat exchanger. The progress of experimental and theoretical studies on the condensation of moist air has been reviewed in detail by Huang et al. [17].

Meanwhile, different methods have been developed to enhance the condensation of moist air. For example, Baghel et al. [18] studied the condensation of moist air outside a hydrophobic metallic substrate. Hu et al. [19] used metal foam to improve the heat transfer of moist air used in air conditioners. Pan et al. [20], [21] experimentally studied the condensation of moist air in gravity-driven flow outside a vertically finned tube and found that it can enhance heat transfer under high-NCG conditions. Hu et al. [22] carried out an experimental study on moist air condensation outside integral finned tubes with different wettabilities. Ge et al. [23] experimentally studied the condensation heat transfer of steam in the presence of a large amount of CO2 outside V-shaped plates and found that an increase in the fin height and fin spacing can enhance the condensation heat transfer. Ji et al. [24] studied the reduction effect of NCG outside superhydrophobic surfaces and used a steam jet to enhance this process.

The three-dimensional (3-D) finned tube is an effective heat transfer enhancement element because 3-D fins can increase the heat transfer area and help break the flow of the boundary layer, and studies have been conducted by many researchers. For example, Zhang et al. [25] experimentally studied the influence of 3-D fin parameters on single-phase convective heat transfer and found that fin height had the most significant influence on heat transfer and flow resistance. Chen et al. [26], [27] experimentally studied the convective heat transfer of air using a heat exchanger made up of 3-D finned tubes, and they found that it could achieve a 35% higher Nu number compared with that made of a smooth tube. At the same time, 3-D finned tubes have also been used to enhance the condensation heat transfer process. Chen and Wu [28] studied the condensation of hydro-fluoro-olefin refrigerant outside a 3-D finned tube and found a 10.8-times condensation heat transfer enhancement compared with a smooth tube. Wang et al. [29] numerically studied sulfuric acid vapour condensation on a 3-D finned tube. Gu et al. [30] experimentally studied the condensation heat transfer of moist air outside 3-D finned tubes with different wettabilities and found that the hydrophilic finned tube had the highest heat transfer coefficient. Chen et al. [31] used a stepped lattice-finned tube to enhance the condensation process.

The literature reviewed above shows that using a 3-D finned tube is an effective heat transfer enhancement method. Although considerable research has been conducted, there is still a lack of relevant studies that focus on the separate performance of single-phase and phase-change convective heat transfer of moist air outside a 3-D finned tube. To further analyse the performance of 3-D finned tubes during the condensation of moist air, in this study, latent heat and sensible heat transfers were obtained and analysed through an experiment, and a smooth tube with the same base tube diameter was also tested for comparison. In addition, empirical correlations were developed to predict sensible and latent heat transfers. The results obtained in this study is helpful to understand the mechanism of 3-D finned tube strengthening moist air condensation.

Section snippets

System introduction

The experimental system consisted of a test section, cooling water system, steam generator system, data acquisition system, and circulation pipeline, as shown in Fig. 1. At the beginning of the experiment, a circulating fan was turned on to circulate the air inside the pipeline. Meanwhile, heating tapes were turned on to heat the air inside the pipeline, and a temperature controller was used to control the power of the heating tapes and adjust the air temperature. When the temperature of the

Heat transfer rate changing with relative humidity

The variation in the heat transfer rate with relative humidity (RH) ranging from 75% to 95% was investigated, and the velocity of moist air remained constant at 2.4 m/s. Photographs of the condensation process are shown in Fig. 3.

The temperature of moist air was lower than its dew point. Near the condensing interface, steam was assumed in saturate state under local temperature. The density of steam and temperature of moist air were lower than that in the main stream. The condensation of water

Conclusions

The forced condensation of moist air outside a 3-D finned tube and a smooth tube was studied experimentally. The variation of latent and sensible heat flux and the heat transfer coefficient with relative humidity was obtained and analysed, and the following conclusions were drawn.

  • (1)

    The three-dimensional finned tube had a higher heat transfer ability than the smooth tube under all tested conditions. The total heat transfer rate of the 3-D finned tube was 38.7% to 57.1% higher when the moist air

CRediT authorship contribution statement

Yu-heng Gu: Data curation, Formal analysis, Methodology, Writing-original draft, Writing-review & editing. Yu-dong Ding: Data curation, Funding acquisition , Project administration, Supervision, Writing-review & editing. Qiang Liao: Data curation, Funding acquisition, Project administration, Supervision, Writing-review & editing. Qian Fu: Methodology, Supervision. Xun Zhu: Methodology, Supervision. Hong Wang: Methodology, Supervision.

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.

Acknowledgements

The authors would like to thank the National Key Research and Development Program of China (No. 2016YFB0601102), and the Fundamental Research Funds for the Central Universities (No. 2018CDXYDL0001).

References (36)

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