Two-Phase Flow Development of R134a in a Horizontal Pipe: Computational Investigation Raid A. Mahmood 1,2* , Khalid Saleh 1 , Veyan A. Musa 2 , Enass Massoud 3 , Ahmad Sharifian-Barforoush 1 , Lokman A. Abdulkareem 4,5 1 School of Mechanical and Electrical Engineering, University of Southern Queensland, Toowoomba 4350, Australia 2 Department of Mechanical Engineering, University of Zakho, Zakho City, P O Box 12, Kurdistan Region of Iraq, Iraq 3 Mechanical Engineering Department, Arab Academy for Science, Technology and Maritime Transport, Egypt 4 Department of Petroleum Engineering, University of Zakho, Zakho City, P O Box 12, Kurdistan Region of Iraq, Iraq 5 Institute of Fluid Dynamics, Helmholtz-Zentrum Dresden-Rossendorf, Dresden 01328, Germany Corresponding Author Email: Raid.Mahmood@usq.edu.au https://doi.org/10.18280/ijht.390515 ABSTRACT Received: 9 June 2021 Accepted: 26 September 2021 To improve the performance of vapor compression refrigeration systems that use vertical gravitational flash tank separators, the liquid separation efficiency of the vertical gravitational flash tank separator requires to be approved. To approach this improvement, the two-phase flow development and its behavior after the expansion device need to be investigated and predicted. For thus, this paper presents a three-dimensional computational investigation of the two-phase flow development of R134a after the expansion device in a horizontal pipe. Computational Fluid Dynamic (CFD) was used to predict the two-phase development and its behavior in the horizontal pipe. ANSYS 16.2 program was used to generates the geometry of the three-dimensional horizontal pipe of 2 meters long and 25 mm inner diameter. The hexahedral mesh was generated and it is assessed to obtain the optimum mesh size and number. Eulerian-Eulerian two-phase model was used with k-ɛ turbulence model. R134a was used as a working fluid in the horizontal pipe utilizing four different inlet diameters: 12, 12.5, 25, and 50.0 mm. Mass flux and vapor quality have been changed from 288 to 447 kg/m 2 .s and from 10 to 20% respectively. Results were validated against experimental results from the literature and revealed that the separation region length is affected by the initial phase velocities, inlet vapor quality, and inlet tube diameter. An empirical correlation to predict the expansion region length is proposed as a function of Froude, Webber, and Lockhart-Martinelli numbers. Keywords: two-phase flow, R134a, computational fluid dynamic, expansion length, two-phase flow development, flow in a horizontal pipe 1. INTRODUCTION Two-phase flow has been investigated in many studies due to the importance of such flow that represented in many applications [1-3]. Two-phase liquid-gas flow is involved in air conditioning and refrigeration systems [4, 5]. Two-phase flow pattern plays a significant role to distribute the refrigerant in headers where effective refrigerant distribution improves the heat transfer characteristic of the evaporators and condensers [6, 7]. The two-phase flow is generated after the expansion device, and investigated by many studies dealing with a two-phase flow characteristic in a horizontal tube; Awwad [8], Canière et al. [9], Bhramara, et al. [10], Dalkilic et al. [11], Ekambara et al. [12], Kondou et al. [13], Dasari et al. [14], Chen et al. [15], Becker et al. [16], Bottin at al. [17], Rana et al. [18], Duan et al. [19] Mahmood et al. [20]. However, few studies were reported for the adiabatic R134a two-phase flow after the expansion device. Duan et al. [19] reported that in two-phase flows there are various flow patterns such as stratified, slug, annular, and dispersed flows, for different fluid properties and flow conditions. Therefore, two-phase flow patterns after the expansion valve, which is the inlet to the flash tank and/or evaporator, have a significant effect on the separation performance of the flash tank and refrigerant distribution in the evaporator [21]. Mahmood et al. [22] considered the two-phase flow behavior in the gravitational flash tank separator using water-vapor two-phase flow. The results demonstrated that the liquid separation efficiency can be affected by the inlet two-phase flow behavior. Mahmood et al. [23] also reported that the inlet two-phase flow direction can influence the two-phase flow behavior and separation efficiency of a vertical separator. Tong [24] used a numerical approach to predict the two-phase flow behavior of water-oil flow in gravity separators. Saidj et al. [25] conducted an experimental study to investigate the behavior of air-water mixtures flowing through 90-degree bends. The results revealed that the void fraction increased with the gas superficial velocity and for the experimental condition, plug, slug, and stratified flow patterns occurred in the horizontal pipe while slug and churn flow patterns were presented in the vertical part. In the present study, CFD simulations were conducted to simulate the adiabatic R134a two-phase flow after the expansion valve using four internal diameters for horizontal tube (12.0, 12.5, 25.0, and 50.0 mm) and investigate the effects of the liquid droplets size, mass flux and vapor quality on the expansion length. Bottin et al. [17] conducted an experimental study of adiabatic two-phase flow in a horizontal tube. Mixture of water and air have been used as working fluid. Flow pattern maps were recorded using high speed video. The results revealed that at certain distance which was defined as 20D from the inlet of the tube, the flow was separated into two International Journal of Heat and Technology Vol. 39, No. 5, October, 2021, pp. 1532-1540 Journal homepage: http://iieta.org/journals/ijht 1532