Unsteady heat transfer properties of spray falling over a horizontal tube in an oily sewage source heat pump

doi:10.1016/j.applthermaleng.2020.115675

Applied Thermal Engineering

Volume 179, October 2020, 115675

https://doi.org/10.1016/j.applthermaleng.2020.115675 Get rights and content

Highlights

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Spray falling film of oily wastewater over a horizontal tube was investigated.
•
VOF model was used to simulate the gas-liquid falling film flow.
•
The unsteady film and tube temperature distributions were analyzed.
•
The unsteady heat transfer properties were investigated.
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The effects of oil content, spray density and tube diameter were studied.

Abstract

The oil exploit in late-period oilfield usually produce large amount of oily wastewater at moderate and high temperature, the thermal energy of which can be recuperated through a sewage source heat pump (SSHP). Among different kinds of heat recovery facility in SSHP, the spray heat exchanger is a better choice to recover energy from the oily wastewater due to its advantages of simple structure, high heat transfer efficiency and anti-blocking ability. However, the spray falling heat transfer was an unsteady procedure, and the complex properties of oil-water mixer will lead to different heat transfer and flow characteristics in the heat exchanger, which also affect the unsteady heat transfer. Hence, in this paper, a numerical model with VOF method was established to study the unsteady heat transfer properties of oily wastewater spray falling film on a horizontal tube, and the effects of different oil contents, spray densities and tube diameters on the unsteady properties of heat transfer coefficient. The results show that the film temperature and tube temperature were continuously varying with time, and the slight variation in upper part and greater variation in lower part of the tube were observed. A further understanding on the flow field of liquid on the tube revealed the unsteady heat transfer was due to the falling flow instability. Thereafter, the influences of oil content, tube diameter and spray density on the unsteady heat transfer were analyzed. It was found that the decrease in oil content, increase in tube diameter will reduce the fluctuation intensity of heat transfer properties.

Introduction

In the late period of oilfield exploit, the recovering of oil from the oilfield production water will produce a large amount of wastewater with temperature of 30–70 °C, which results to huge pollution and waste of thermal energy [1]. Therefore, in recent years, the sewage source heat pump system (SSHP) has been used to recover the heat energy in this oily wastewater, which can improve the energy utilization efficiency and have good environmental protection benefits.

In recent years, a large number of studies have been carried out on the SSHP system [2], [3], [4], [5], and the advantages of spray heat exchanger with good performance in better anti-blocking and anti-fouling properties were reported previously. In the spray heat exchanger, the liquid spraying from the nozzle falls to the horizontal tube and then spreads around the tube and falls to the following tubes. Nusselt [6] firstly investigated this spray falling phenomenon and built theoretical foundation for the falling film flow and heat transfer analysis. Thereafter, the heat transfer characteristics of falling film over tube were widely investigated. Hu and Jacobi [7] conducted experiments on horizontal tube, and investigated the effect of spray density or liquid flowrate on heat transfer. It was found that the increase in spray density will enhance the heat transfer coefficient, however, this enhancement will be weakened after the spray density reaching to a certain value. Parken [8], Ganic [9], Mitrovic [10] and Zhao [11] also found the positive effect of spray density on heat transfer. However, Fujita and Tsutsui [12] pointed out a U-shape variation of heat transfer with the increase in spray density, which was caused by the transition from laminar flow to turbulent flow. Besides, Hu and Jacobi [13], Jafar [14] investigated the influence of tube diameter on heat transfer. They pointed out that the smaller tube diameter meant the relatively large impingement region and resulted to higher heat transfer performance. Ganic and Roppo [9] reported that the effect of tube spacing on the heat transfer differs based on different flow patterns. In addition, the physical properties also had obvious effects on the spray falling heat transfer. Gong et al. [15] found that the heat transfer coefficient of falling film around the tube decreased with the increase of salinity of feed seawater. Hao et al. [16] carried out a numerical study on oily wastewater spray falling film over a horizontal tube of heat exchanger, and investigated the influence of oil content on heat transfer coefficient. It was found that the local heat transfer coefficient decreased with the increase in oil content in most part of the tube.

In these studies, the unsteady heat transfer characteristics were also investigated. Qiu et al. [17] reported the fluctuation of film thickness of the spray falling over horizontal tubes at different circumferential angles at staggered flow. This was due to the interaction between the columns. Jayakumar et al. [18] used air-coupled ultrasonics to measure the falling liquid film thickness, and found the fluctuations in the thickness of the water film. The fluctuation in film thickness was found to increase with the flow rate. Yang and Shen [19] studied falling film outside horizontal tubes, pointed out the increase in falling velocity increased the fluctuation of film and then enhanced heat transfer. Hence, in some cases there exists unsteady flow and heat transfer properties of falling film over horizontal tube, and these properties have critical effects on heat transfer enhancements.

The comprehensive review on the previous studies on falling film heat transfer indicates that the spray falling heat transfer over horizontal tubes were influenced by the spraying parameters and tube properties, and the physical properties of the falling fluid such as component also influenced the characteristics of flow and heat transfer, and the heat transfer properties and falling film thickness varied with time. This variation was reported having influence on the heat transfer performance. Moreover, in previous studies [16], [20], it was reported that the oil content in the wastewater had a critical effect on fluid physical properties and the flow and heat transfer characteristics of spray falling. However, there were limited studies on the quantitative analysis on the unsteady properties of oily wastewater flow and heat transfer of spray falling film on horizontal tubes. Hence, to promote the enhancement of heat transfer, it was necessary to investigate the unsteady heat transfer properties of oil wastewater falling film over a horizontal tube and to understand the heat transfer and gas-liquid flow mechanism. Therefore, in this paper, a three-dimensional numerical model was established for the oily wastewater falling flow and heat transfer over a horizontal tube, and VOF model was used to track the gas-liquid interface. The fluctuations of film temperature, tube temperature, film thickness with time were analyzed. After that, the influence of oil content, tube diameter and spray density on fluctuation intensity of the heat transfer coefficient were investigated.

Section snippets

Methodology

The spray heat exchanger consists of spray nozzles and horizontal tube bundles, where the liquid flows downstream along the tube surface, as shown in Fig. 1(a). The present study focuses on one horizontal tube in a square pitch tube bundle, and the flow domain around the tube, as shown in Fig. 1(a) and (b).

Film temperature distribution

The conditions used for analysis are as follows: 10% of oil content, D = 25.4 mm, Γ = 0.168, q = 47.3 kW/m² and T_i = 46 °C. As shown in Fig. 1(d), the oily wastewater was sprayed from the inlet and flowed around the tube forming a thin liquid film on the tube. Fig. 4 shows the temperature distribution of falling film surface from 0.26 s to 0.54 s. As seen, temperature distribution varied during this period. At 0.34 s, there are three regions with much lower temperature, as seen in Fig. 4. The

Conclusions

In this paper, the unsteady heat transfer properties of oily wastewater falling flow over a horizontal tube for an oily sewage source heat pump were investigated using numerical method. The falling film temperature, tube surface temperature and falling film thickness distributions around the horizontal tube at different times were analyzed. It was found that film temperature and tube temperature were continuously varying with time. The variations in lower part of the tube were much greater than

Declaration of Competing Interest

We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work. The content of this manuscript have not been copyrighted or published previously. The contents of this manuscript are not now under consideration for publication elsewhere.

Acknowledgement

The study was supported by the Fundamental Research Funds for the Central Universities (No.: 18CX02077A), Shandong Provincial Natural Science Foundation (No.: ZR2016EEQ29) and National Natural Science Foundation of China (No.: 51606044).

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Citation Excerpt :
Besides, computational fluid dynamics (CFD) has received much attention as a powerful approach in reproduction of hydrodynamics in falling film since the rapid achievement in computer hardware in recent years. Among various modeling techniques for falling film, the interface acquisition methods (e.g. the interface capturing method and the interface tracking method (Zhao et al., 2022b)) were prevalently employed to solve this problem and some achievements were reported, among which the volume of fluid (VOF) method is the most widely adopted due to its reliability and stability (Cao et al., 2022; Lin et al., 2019; Mao et al., 2020; Wang et al., 2019; Yang et al., 2016; Zhao et al., 2018; Zhao et al., 2021; Zhao et al., 2022c). In the VOF method, surface tension is typically applied as a body forcing source term by a specific surface tension model in the momentum equations.
Computational fluid dynamics (CFD) modelling is extensively and successfully used for the reproduction of liquid film hydrodynamics. Surface tension significantly influences the numerical reproduction of falling liquid film flow. In this study, the hydrodynamics of the liquid film flow over a horizontal tube and a vertical plate is explored with different surface tension models. The results reveal that the neglect of surface tension leads to quantitative and qualitative distortions in the numerical simulation of the thin liquid film. Both continuum surface force (CSF) and continuum surface stress (CSS) models can produce the hydrodynamic behaviors of the falling film on the horizontal tube and the vertical plate, including pendant droplets, column necking or necking break, thickness of the film, vortex, liquid film detachment, as well as the values and trends of the velocity components. However, the CSF and CSS models produce different transient phase contours and streamlines, as well as different profiles of the time-averaged velocity components of the falling films on the horizontal tube and the vertical plate. Besides, there is no obvious difference between CSF and CSS in the perspective of time-averaged film thickness on a horizontal tube.

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Unsteady heat transfer properties of spray falling over a horizontal tube in an oily sewage source heat pump

Highlights

Abstract

Introduction

Section snippets

Methodology

Film temperature distribution

Conclusions

Declaration of Competing Interest

Acknowledgement

Energy

Exp. Therm Fluid Sci.

Int. J. Heat Mass Transf.

Int. J. Heat Mass Trans.

Inter. J. Heat Mass Trans.

Desalination

J. Comput. Phys.

Inter J Heat Mass Trans

Renewable Energy

Application of heat pump technology in waste heat recovery of oilfield sewage