Numerical study of vapor bubble effect on flow and heat transfer in microchannel

Abstract

Flow boiling in a microchannel is characterized by nucleation and dynamic behavior of vapor bubbles in the channel. In the present study, the effect of vapor bubble on fluid flow and heat transfer in a microchannel is investigated via lattice Boltzmann (LB) modeling. With respect to boiling flow in a single microchannel, the bubble nucleation, growth, and departure are simulated by using an improved hybrid LB model. Relating bubble behavior with fluid flow and boiling heat transfer provides some insight into the relevant fundamental physics on flow boiling in the microchannel. It is found that the bubble growth before its departure from the wall induces an obvious resistance to the fluid flow. The processes of nucleation and motion of different bubbles interact, leading to an alternate, either enhanced or weakened, effect of bubble behavior on the flow boiling.

Introduction

Flow boiling in a confined space involves many complex phenomena such as bubble nucleation, growth, departure, and coalescence. In flow boiling systems, different from single phase liquid flow, phase change dominated by heat transfer causes the fluid volume expansion due to density change, yielding the two-phase flow instability. Though many studies on flow instability in microchannels have been reported in recent years, the relevant mechanism is not fully understood yet, especially the relationship of flow boiling instability and bubble dynamical behaviors. Flow instability in microchannels not only causes an uneven thermal stress on the heated surface, but also leads to an early onset of critical heat flux (CHF). Recent studies worldwide on this topic focused on either the description of the flow instability phenomena, or measures to migrate or suppress the flow instability.

The two-phase flow instability was proposed firstly by Ledinggs in 1938 [1]. In 1966, Boure et al. [2] took the instability into categories and analyzed its trigger mechanism. Kandlikar et al. [3] reported the reversed flow induced by the expanding growth of vapor bubble in parallel microchannels of 1 mm hydraulic diameter using high speed video camera in 2001. Qu and Mudawar (2003) [4] have found the interaction of flow instability induced in a flow system of 21 parallel microchannels on the cuprum heat sink. Hetsroni et al. [5] investigated boiling heat transfer in parallel microchannels using water and ethanol as the working fluids. Their experiments covered data ranges: hydraulic diameter of 100–220 μm, mass flux of 32–200 kg/m²s, heat flux of 120–270 kW/m² and vapor mass quality of x = 0.01–0.08. The cycle period was dependent on the boiling number and decreased with the increase of boiling number. Dynamic changes of pressure drop, fluid across and temperature at the heated surface were both periodic at a same oscillation frequency. Chang and Pan [6] reported experimental results in a heat sink of 15 parallel microchannels. Flow patterns were found to be significantly different under stable and unstable flow conditions. Bubble nucleation, slug flow and slug or annular flow appeared sequentially along the flow direction for the stable flow, whereas forward or reversed slug/annular flows appeared alternatively in each channel for the unsteady flow. Huh and Kim (2006) [7] found the flow instability still occurred in a single microchannel. Huh et al. [8] studied the flow instability induced by the flow pattern transition in a single microchannel, which was made of polydimethylsiloxane (PDMS) and rectangular with 103.5 μm hydraulic diameter and 40 mm length. Fluid pressures, inlet and outlet fluid temperatures and heated surface temperatures were found to oscillate, matching the alternating flow pattern transitions with time in the microchannel. Qu and Mudawar [9] studied transport phenomena in two-phase microchannel heat sinks. Periodic pressure drop, large amplitude oscillations of inlet and outlet pressures and heat sink temperatures were observed. It is speculated that such kind of flow instability can be mitigated by setting a throttle valve at the microchannel upstream.

Kandilikar et al. (2005) [10] applied the throttle measure at the entrance of microchannel, which enforced water to pass through a smaller aperture into the microchannel, effectively preventing the reverse flow and suppressing the flow oscillation. Wang et al. [11] studied throttles of different inlet and outlet configurations to suppress the flow instability in microchannels. With different idea about throttle measures, Kuo and Peles [12] mitigated the flow boiling instabilities in microchannels by artificial reentrant cavities. Xu et al. [13], [14] proposed the idea using seed bubbles to eradicate the flow boiling instabilities in microchannels. The reentrant cavities increased the flow resistance. Moreover, the flow instability was sensitive to the number of seed bubbles especially in the situation of smaller heat flux. To unravel the relevant mechanisms, the effects of bubble growth on microchannel flow need to be studied.

Many ideas about how to stabilize flow in microchannels were proposed based on the sense of taking vapor bubble growth as the direct reason inducing flow instabilities. Although the corresponding experiments can explain the physical mechanism of bubble growth and bubble behavior qualitatively, it is generally hard to conduct quantitative research work. Fortunately, the development of numerical methods and computer technology provide a powerful tool to predict vapor bubble behavior in microchannel flow boiling.

Mukherjee et al. [15] applied the level-set method to investigate the bubble growth effect on the heat transfer and reverse flow in microchannels. Taha et al. [16] took the VOF method to study the slug bubble in microchannel flow boiling. These two, as representative works, proposed instructive reference for exploring the mechanisms of bubble behavior. Most of these numerical works focused on the exploding growth of bubble in microchannel due to large phase-change heat flux. But in the microchannel bubble flows, bubble behaviors like bubble nucleation, growth, and coalescence with small growth rate still have important influence on flow boiling. These influences should be taken into account for designing measures to suppress the flow instability like Kuo’s reentrant cavity [12] and Xu’s seed bubble [13], [14].

The objective of this paper is to numerically study the bubble dynamical behavior with smaller growth rate and its influence on the fluid flow and heat transfer in microchannel, using an improved hybrid lattice Boltzman model [17], [18].