Numerical study on pressure drop and heat transfer characteristics of gas-liquid Taylor flow in a microchannel based on FFR method

Highlights

• The whole heat transfer process is simulated through FFR method.

• Existence of internal circulation is demonstrated by wall shear stress.

• Thermal development process in liquid slugs was investigated in detail.

• Mixture velocity can greatly affect the temperature field distribution.

Abstract

Two-phase Taylor flow in microchannels as a critical way to enhance heat dissipation has attracted extensive attention due to the high integration and miniaturization development of electronic devices. To further elucidate the pressure drop and development process of heat transfer characteristics in Taylor flow, a 2-D planar T-junction microchannel with a width of 1 mm was investigated numerically by using the fixed frame computational domain method. The local Nusselt number distribution is divided into four parts for analysis, and the effects of mixture velocity and void fraction on the heat transfer coefficient are also discussed. The results indicate that the injection of gas-phase leads to an increase in pressure gradient, and the flow-pattern related model has a better predictive result. Strong evidence of internal circulation was found when the shear stress in Y direction is not equal to zero in Taylor flow. The investigation of Nusselt number has shown that the mixture velocity and the void fraction affect the temperature field distribution and heat absorption capacity in the liquid slug, respectively. Together these simulation data, nearly 1.8 times higher of the Nusselt number was observed compared to pure water flow.

Introduction

With the rising demand for high performance and integration in an electronic system, heat dissipation has become a critical problem limiting both the safe and reliable operation of microelectronic equipment and the rapid development of this filed. To disperse high heat flux in the electronic system, some technologies including heat pipes [1,2], jet impingement [3], and spray cooling [4,5] have been proposed and applied by numerous researchers, and a good cooling effect has been achieved. However, these techniques are not available for chip cooling applications [6]. The microchannel device as a high-performance cooling technique offers a larger contact area and shorter transport path than the macroscale and thus achieving the chip cooling. Meanwhile, the use of numerous parallel microchannels offers a potential approach to disperse large heat flux over small areas. However, flow in microchannels is usually laminar with low Reynolds numbers [7], which limits the thermal diffusion process of the fluid in the microchannel.

A few studies on improving the heat transfer capacity in the microchannels have been reported by researchers. They focus mainly on three approaches, namely the employing of high thermal conductivity fluid (e.g., nanofluids), the enhancing of turbulence in the channel and utilizing simultaneously. Many studies have shown that the thermal conductivities of nanofluids are relatively higher than those of conventional fluids [8,9], even nanoparticles with low volume fractions [10]. For example, Zirakzadeh et al. [11] experimentally studied the effects of Al₂O₃ water nanofluids on cooling heat sinks. The results showed that the heat transfer coefficient was 20% higher than the conventional heat sink. To determine the effects of nanofluids, Sohel et al. [12] compared the thermal performance of three nanofluids in a circular- shaped heat sink. Compared to pure water, 4% CuO-water nanofluid has the highest improvement (13.15%) in heat flux. Another option to promote heat exchange efficiency is to increase turbulence in the channel, which can be achieved by changing curvatures of the microchannel [13,14] or built-in barriers in the flow paths [15,16]. In addition, the above two methods also can be combined. For example, Zhou et al. [17] experimentally studied silver nanofluids through the micro-pin fin heat sink, and Mohammed et al. [18] investigated the comprehensive properties of nanofluid flow and heat transfer in micro-ribbed channels. However, much pumping power is consumed whether using nanofluids (higher viscosity than that of their base fluids) or setting vortex promoter in the flow paths (flow resistance increase), and the increase in heat transfer coefficient is limited.

Two-phase Taylor flow is a simple and effective method to produce vortices by introducing gas or immiscible liquid and has received more and more attention in recent years. This flow regime is characterized by the occurrence of regular bubbles/droplets in a continuous stream of liquid [19]. The presence of bubbles/plugs modifies the flow field in the continuous phase, and then internal circulation is formed. These internal circulations promote the radial mixing of the fluid, which provides a new mechanism for enhancing heat transfers. In recent years, some works have been devoted to study heat transfer characteristics of Taylor flow in the microchannel using experimental or simulation methods. Many investigations have shown that the Taylor flow increases the heat transfer efficiency 2–4 times for that of pure water flow [20,21]. Various parameters affect the Taylor flow heat transfer efficiency, including slug length, channel size, void fraction and flow patterns. An experimental study of Taylor flow heat transfer was first carried out in a 6.4 mm tube by Oliver and Wright [20], the results indicated that the strengthening effect of the short slug is more significant, and the overall heat transfer coefficient is up to 2.5 times than that for water-only flow. Similarly, Leung et al. [21] experimentally studied the flow and heat behaviors of Taylor flow in a vertical channel with a diameter of 2 mm. The effects of bubble length and homogeneous void fraction on Nusselt number were investigated, and the enhancement had been raised 3.2 times as against liquid-only flow. There is a relatively small body of literature that concerned with rectangular channels. Majumder et al. [22] studied the effects of air-water isolated single Taylor bubble and a train of Taylor bubble on heat transfer enhancement in a square channel. They reported that the injection of the Taylor bubble could effectively increase the average heat transfer coefficient by 1.2–1.6 times compared with pure water flow. Mehta and Khandekar [23] also confirmed the high heat transfer efficiency of this flow regime in a square cross-section channel by employing high-resolution InfraRed Thermography, and the enhancement is between 1.2 and 2 times.

Numerical simulation methods offer an effective complementary way to observe many local details difficult to capture in experiments with less expense. According to the simulation of Ua-arayaporn et al. [24], a higher Nusselt number exists in the liquid film region. They argued this could be for the small temperature difference between the local wall and bulk fluid. This finding is consistent with that of Mehdizadeh et al. [25] who showed that the heat transfer in the liquid film region accounts for about one-fourth of the overall heat transfer. More recently, Zhang et al. [26] carried out several investigations into the flow and thermal characteristics of two-phase Taylor flow in vertical channels. They found that as the Capillary number increased, the circulation volume decreased, and the dimensionless circulation time increased. These are all important parameters that affect heat exchange efficiency. Che et al. [27] studied the droplet heat transfer in rectangular microchannels by three-dimensional numerical simulation. The effects of droplets length and the aspect ratio of the channel cross-section on heat transfer process are discussed.

Although some numerical simulation research has been carried out on Taylor flow and heat transfer characteristics, the research to date has tended to focus on the moving frame of reference (MFR) rather than the fixed frame of reference (FFR) systems. Previous studies of Taylor flow heat transfer have not dealt with its complete development process, whereas the often overlooked FFR method is particularly useful in studying this issue. Hence, this paper has been divided into four sections and attempts to show the special feasibility of the FFR method in numerical simulation of Taylor flow and heat transfer. The specific numerical methods are described in section 2, and the characteristics of pressure drop and heat transfer are analyzed in section 3. This will be of practical significance to the application of Taylor flow in microchannels in the heat dissipation of electronic devices.