Prediction of the onset of nucleate boiling in microchannel flow

The onset of nucleate boiling in the flow of water through a microchannel heat sink was investigated. The microchannels considered were 275 μm wide by 636 μm deep. Onset of nucleate boiling was identified with a high-speed imaging system and the heat flux at incipience was measured under various flow conditions. An analytical model was developed to predict the incipient heat flux as well as the bubble size at the onset of boiling. The closed-form solution obtained sheds light on the impact of the important system parameters on the incipient heat flux. The model predictions yield good agreement with the experimental data.     

Effects of inlet/outlet configurations on flow boiling instability in parallel microchannels

A simultaneous visualization and measurement study has been carried out to investigate effects of inlet/outlet configurations on flow boiling instabilities in parallel microchannels, having a length of 30 mm and a hydraulic diameter of 186 μm. Three types of inlet/outlet configurations were investigated. Fluid flow entering to and exiting from the microchannels with the Type-A connection was restricted because the inlet and outlet conduits were perpendicular to the microchannels. The fluid flow had no restriction in entering to and existing from the microchannels with the Type-B connection. In the Type-C connection, fluid flow was restricted in entering each microchannel but was not restricted in exiting from the microchannels. It is found that amplitudes of temperature and pressure oscillations in the Type-B connection are much smaller than those in the Type-A connection under the same heat flux and mass flux conditions. On the other hand, nearly steady flow boiling exists in the parallel microchannels with the Type-C connection under the experimental conditions. Therefore, this configuration is recommended for high-heat-flux microchannel applications. As predicted, the stability threshold is determined by the minimum in the pressure-drop-versus-flow-rate curve. The pressure drop and heat transfer coefficient versus vapor quality for flow boiling in microchannels with the Type-C connection are presented. It is found that experimental data of pressure drop are higher and heat transfer coefficients are lower for boiling flow at high vapor quality in microchannels than those predicted from correlation equations for boiling flow in macrochannels, due to local dryout.     

An experimental study of flow friction and heat transfer in wavy microchannels with rectangular cross section

Experimental investigation has been conducted on the flow friction and heat transfer in sinusoidal microchannels with rectangular cross sections. The microchannels considered consist of ten identical wavy units with average width of about 205 μm, depth of 404 μm, wavelength of 2.5 mm and wavy amplitude of 0–259 μm. Each test piece is made of copper and contains 60–62 wavy microchannels in parallel. Deionized water is employed as the working fluid and the Reynolds numbers considered range from about 300 to 800. The experimental results, mainly the overall Nusselt number and friction factor, for wavy microchannels are compared with those of straight baseline channels with the same cross section and footprint length. It is found that the heat transfer performance of the present wavy microchannels is much better than that of straight baseline microchannels; at the same time the pressure drop penalty of the present wavy microchannels can be much smaller than the heat transfer enhancement. Conjugate simulation based on the classical continuum approach is also carried out for similar experimental conditions, the numerical results agree reasonably well with experimental data.     

Effects of compressibility and transition to turbulence on flow through microchannels

A detailed experimental study of flow through long microchannels of hydraulic diameter ranging from 60.5 to 211 μm has been carried out. The internal pressure distribution along the length of the channel has been measured to analyze the local flow behaviour. The effects of compressibility and transition to turbulence occurring in the microchannel flow were investigated in detail. In addition, the resulting flow has been analyzed numerically using a commercially available CFD code, FLUENT. It has been shown that there are no special micro-scale effects, including early transition to turbulence at least in the present range of hydraulic diameters after the significant effects of compressibility are accounted appropriately.     

Condensation flow patterns in silicon microchannels

A simultaneous visualization and measurement experiment was carried out to investigate condensation flow patterns of steam flowing through an array of trapezoidal silicon microchannels, having a hydraulic diameter of 82.8 μm and a length of 30 mm. The degassed and deionized water steam flowing in the microchannels was cooled by flowing water of 8 °C from the bottom. The silicon microchannels were covered with a thin transparent pyrex glass from the top which enabled the visualization of flow patterns. Experiments were performed at different inlet pressures ranging from 4.15 × 10⁵ Pa to 1.25 × 10⁵ Pa (with corresponding mass fluxes decreasing from 47.5 g/cm² s to 19.3 g/cm² s) while the outlet pressure was maintained at a value of 10⁵ Pa. Different condensation flow patterns such as fully droplet flow, droplet/annular/injection/slug-bubbly flow, annular/injection/slug-bubbly flow, and fully slug-bubbly flow were observed in the microchannels. At a given inlet pressure and mass flux, the flow pattern depended on both the location and time. Of particular interest is that the vapor injection flow, consisting of a series of bubble growth and detachment activities, appeared and disappeared periodically. During the disappearance period of injection flow, the slug-bubbly flow at downstream changed to the single-phase liquid flow due to the reversed flow of outlet condensate, while the annular flow at upstream changed to the vapor flow due to the effect of incoming vapor. Therefore, two-phase flow and single-phase flow appeared alternatively in the microchannels, causing large fluctuations of wall temperatures as well as other measurements. It was also found that the occurrence of vapor injection flow moved from the outlet toward the inlet as the mass flux was decreased. The vapor injection flow and its induced condensation instabilities in microchannels are reported here for the first time.     

Microchannel size effects on local flow boiling heat transfer to a dielectric fluid

Heat transfer with liquid–vapor phase change in microchannels can support very high heat fluxes for use in applications such as the thermal management of high-performance electronics. However, the effects of channel cross-sectional dimensions on the two-phase heat transfer coefficient and pressure drop have not been investigated extensively. In the present work, experiments are conducted to investigate the local flow boiling heat transfer of a dielectric fluid, Fluorinert FC-77, in microchannel heat sinks. Experiments are performed for mass fluxes ranging from 250 to 1600 kg/m² s. Seven different test pieces made from silicon and consisting of parallel microchannels with nominal widths ranging from 100 to 5850 μm, all with a nominal depth of 400 μm, are considered. An array of temperature sensors on the substrate allows for resolution of local temperatures and heat transfer coefficients. The results of this study show that for microchannels of width 400 μm and greater, the heat transfer coefficients corresponding to a fixed wall heat flux as well as the boiling curves are independent of channel size. Also, heat transfer coefficients and boiling curves are independent of mass flux in the nucleate boiling region for a fixed channel size, but are affected by mass flux as convective boiling dominates. A strong dependence of pressure drop on both channel size and mass flux is observed. The experimental results are compared to predictions from a number of existing correlations for both pool boiling and flow boiling heat transfer.     

Heat transfer in micro-channels: Comparison of experiments with theory and numerical results

This paper considers experimental and theoretical investigations on single-phase heat transfer in micro-channels. It is the second part of general exploration “Flow and heat transfer in micro-channels”. The first part discussed several aspects of flow in micro-channels, as pressure drop, transition from laminar to turbulent flow, etc. [G. Hetsroni, A. Mosyak, E. Pogrebnyak, L.P. Yarin, Fluid flow in micro-channels, Int. J. Heat Mass Transfer 48 (2005) 1982–1998]. In this paper, the problem of heat transfer is considered in the frame of a continuum model, corresponding to small Knudsen number. The data of heat transfer in circular, triangular, rectangular, and trapezoidal micro-channels with hydraulic diameters ranging from 60 μm to 2000 μm are analyzed. The effects of geometry, axial heat flux due to thermal conduction through the working fluid and channel walls, as well as the energy dissipation are discussed. We focus on comparing experimental data, obtained by number of investigators, to conventional theory on heat transfer. The analysis was performed on possible sources of unexpected effects reported in some experimental investigations.     

Numerical simulation of thermofluid characteristics of two-phase slug flow in microchannels

A fundamental study of heat transfer characteristics of two-phase slug flow in microchannels is carried out employing the Volume-of-Fluid (VOF) method. Despite of the fact that numerical simulations of two-phase flows in microchannels have been attempted by many investigators, most efforts seem to have failed in correctly capturing the flow physics, especially those pertaining to the slug flow regime characteristics. The presence of a thin liquid film in the order of 10 μm around the bubble is a contributing factor to the above difficulty. Typically, liquid films have a significant effect on the flow field and heat transfer characteristics. In the simulations reported in this paper, the film is successfully captured and a very high local convective heat transfer coefficient is observed in the film region. A strong coupling between the conductive heat transfer in the solid wall and the convective heat transfer in the flow field is observed and characterized. Results showed that unsteady heat transfer through the solid wall in the axial direction is comparable to that in the radial direction. Results also showed that a fully developed condition could be achieved fairly quickly compared to single-phase flows. The fully developed condition is defined based on the Peclet number (Pe) and a dimensionless length of the liquid slug. Local and time-averaged Nusselt numbers for slug flows are reported for the first time. It was found that significant improvements in the heat transfer coefficient could be achieved by short slugs where the Nusselt number was found to be 610% higher than in single-phase flows. The study revealed new findings related to slug flow heat transfer in microchannels with constant wall heat flux.     

Unstable and stable flow boiling in parallel microchannels and in a single microchannel

A simultaneous visualization and measurement study have been carried out to investigate flow boiling instabilities of water in microchannels at various heat fluxes and mass fluxes. Two separate flow boiling experiments were conducted in eight parallel silicon microchannels (with flow interaction from neighboring channels at headers) and in a single microchannel (without flow interaction), respectively. These microchannels, at a length of 30 mm, had an identical trapezoidal cross-section with a hydraulic diameter of 186 μm. At a given heat flux and inlet water temperature, it was found that stable and unstable flow boiling regimes existed, depending on the mass flux. A flow boiling map, in terms of heat flux vs mass flux, showing stable flow boiling regime and unstable flow boiling regime is presented for parallel microchannels as well as for a single microchannel, respectively, at an inlet water temperature of 35 °C. In the stable flow boiling regime, isolated bubbles were generated and were pushed away by the incoming subcooled liquid. Two unstable flow boiling regimes, with long-period oscillation (more than 1 s) and short-period oscillation (less than 0.1 s) in temperature and pressure, were identified. The former was due to the expansion of vapor bubble from downstream while the latter was owing to the flow pattern transition from annular to mist flow. A comparison of results of flow boiling in parallel microchannels and in a single microchannel shows that flow interaction effects from neighboring channels at the headers are significant.     

Experimental study of compressibility,roughness and rarefaction influences on microchannel flow

The existing experimental data in the literature on friction factor in microchannels are analyzed. Flow characteristics for nitrogen and helium in stainless steel microtubes, fused silica microtubes and fused silica square microchannels are studied experimentally. The data in fused silica microtubes with diameters ranging from 50 to 201 μm and the data in fused silica square channels with hydraulic diameter ranging from 52 to 100 μm show that the friction factors are in good agreement with the theoretical predictions for conventional-size channels. The friction factors in stainless steel tubes (D = 119–300 μm) are much higher than the theoretical predictions for tubes of conventional size. This discrepancy is resulted from the large relative surface roughness in the stainless steel tubes. From the literature review and the present test data it is suggested that for gaseous flow in microchannels with a relative surface roughness less than 1% the conventional laminar prediction should still be applied. A positive deviation of the friction factor from the conventional theory is observed due to the compressibility effect. In addition, smaller friction factors in fused silica microtubes with inner diameters ranging from 10 to 20 μm are obtained and the decrease in friction factor from the rarefaction effect is observed.     

Non-linear analyses of flow boiling in microchannels

Recently, four unstable boiling cases with different fluctuating amplitudes were observed in parallel silicon microchannels having a hydraulic diameter of 186 μm. These were: the liquid/two-phase alternating flow (LTAF) at two different heat fluxes, the continuous two-phase flow (CTF) at medium heat flux and medium mass flux, and the liquid/two-phase/vapor alternating flow (LTVAF) at high heat flux and low mass flux. In this paper, data of these unstable boiling cases are analyzed using the following methods: correlation coefficient, attractor reconstruction, correlation dimension and largest Lyapunov exponent. The processes responsible for appearance of chaotic oscillations in microchannels, such as nucleation, stability of bubbly flow, vapour core stability and vapour-phase flow stability, are discussed. It is shown that under certain conditions, the microchannels system works as a thermal oscillator. It was found that heat supplied to the microchannels increases the heating surface temperature while the appearance of the two-phase flow inside the channels decreases the heating surface temperature. The mechanism involving an increase in heating surface temperature is supported by phenomena of blocking the liquid flow in microchannels by the two-phase flow.     

Measurement and interpretation of terrestrial heat flow in Israel

Sixty-eight new determinations of terrestrial heat flow in Israel have a range of 0.17-11.07 μcal/cm²s. The average value of deep conductive heat flow in the undisturbed complex of the Arabo-Nubian Massif is 0·94 μcal/cm²s; it is least affected by circulation of groundwater. This value is only slightly higher than the heat flow of 0·88 μcal/cm²s in the Levantine Basin of the Mediterranean Sea. Several values that exceed 2·0 μcal/cm²s are due either to (probable) deep hydrothermal activity or to small domal structures of the basement.Within the sedimentary sequence which blankets the crystalline massif, terrestrial heat flow is often redistributed by circulating groundwater. Recharge regions, particularly Judean-Samarian Galilee, where cool meteoric waters percolate into the subsurface have anomalously low heat flow, ranging from 0·17 to about 1·0 μcal/cm²s. Part of the original deep thermal flux in those regions is intercepted at moderate depths by the recharge flow, and is carried into deeper aquifers of the Foothills, Coastal Plain, or the Jordan-Dead Sea Rift. Movement of groundwater occurs mainly along faults.Deep faults associated with the Jordan-Dead Sea Rift system act as conduits for hot waters ascending from deep confined aquifers. The most tangible surface expression of the convective hydrothermal system are the numerous warm to hot springs, emerging along the margins of the Rift. However, the waters emerging on the surface as the warm and hot springs are a minor fraction of the convective system. Most of the ascending thermal waters are absorbed by shallow aquifers with lower hydraulic potential. Such regions are characterized by anomalously high heat flow; several values exceed 2 and one value is 11 μcal/cm²s.     

Predictions of turbulent flow and heat transfer in gas–droplets flow downstream of a sudden pipe expansion

Droplets-laden turbulent flow downstream of a sudden pipe expansion has been investigated by using Euler/Euler two-fluid model for the gaseous and dispersed phases. Significant increase of heat transfer in separated flow at the adding of evaporating droplets has been demonstrated (more than 2 times compare with one-phase flow at the value of mass concentration of droplets M_L1  0.05). Addition of dispersed phase to the turbulent gas flow results in insignificant increase of the reattachment length. Low-inertia droplets (d₁  50 μm) are well entrained into the circulation flow and present over the whole pipe section. Large particles (d₁ ≈ 100 μm) go through the shear layer not getting into the detached area. Comparison with experimental data on separated gas–droplets flows behind the plane backward-facing step has been carried out.     

Experimental and numerical study on the flow characteristics in curved rectangular microchannels

In this paper, the flow characteristics in curved rectangular microchannels with different aspect ratios and curvature ratios for Re numbers ranging from 80 to 876 are investigated. The obtained experimental results are compared with simulated values based on classical Navier–Stokes equations and available correlation in the literature. An empirical equation based on experimental data is proposed to provide a better prediction of the frictional pressure drop in the curved rectangular microchannels.     

Flow and Heat Transfer Characteristics of Liquid Nitrogen in Mini-/Microchannels

Flow and heat transfer of liquid nitrogen in mini-/microchannels have many particular characteristics and are very important for many cooling applications. In this study, the investigation of flow and heat transfer characteristics of liquid nitrogen in mini-/microchannels is presented by summarizing the experimental studies carried out in the author's group. In addition, some recent results about flow and heat transfer of liquid nitrogen in microchannel heat sink are also presented. It is found that small viscosity of liquid nitrogen enables the single-phase liquid flow in mini-/microchannels to be turbulent state, which proves that the classical theory for pressure drop is still valid if the surface roughness of the passage is properly taken into consideration. Experiments of flow boiling of liquid nitrogen are conducted under both adiabatic and diabatic conditions. It is shown that confinement number can be applicable in classifying the heat transfer characteristics of liquid nitrogen in macro- and microchannels. Flow visualization in microchannels at low temperatures poses big challenges on experimental aspects, which have been subtly overcome and clear images have been obtained. The flow patterns and flow regimes of two-phase flow of liquid nitrogen exhibit different features from the room-temperature fluids. Furthermore, three-dimensional (3D) flow visualization by only one high-speed camera is conducted to obtain more detailed information of flow patterns. Finally, the experiments of flow boiling of liquid nitrogen in microchannel heat sink are also presented and discussed.     

Friction factor and heat transfer in multiple microchannels with uniform flow distribution

Predictions of flow and heat transfer in microchannels are ongoing issues in microfludics. This work focused on laminar flow (69 < Re < 800) within rectangular microchannel with hydraulic diameter from 106 μm to 307 μm for single-phase liquid flow. The friction factors obtained by experiments on the microchannels showed that conventional theory for fully-developed flow is applicable within the range of our experiments. A manifold configuration which ensured uniform flow through the microchannel array is thought to contribute to the improvement of accuracy. The average Nusselt number for the microchannel array was also evaluated experimentally in the condition of constant heat transfer rate. We found that there were deviations between the experimental and theoretical values of heat transfer rate in the microchannels. In order to predict heat transfer rate accurately, we proposed an empirical correlation in terms of Nu/(Re^0.62 Pr^0.33) and Brinkman number confined to the experimental range. The correlation is expected to be useful to design the microchannel devices related to heat transfer.     

Pressure effects on flow boiling instabilities in parallel microchannels

The effects of pressure on flow boiling instabilities in microchannels were experimentally studied. Experiments were conducted using water in 223 μm hydraulic diameter microchannels with mass fluxes ranging from 86 to 520 kg/m² s and pressures ranging from 50 to 205 kPa. Onset of flow oscillation, critical heat flux (CHF) conditions, local transient temperature measurements along with flow boiling visualization were obtained and studied. System pressure was found to significantly affect flow instabilities. For high pressure, it was observed that boiling instabilities were significantly delayed and CHF was extended to high mass qualities. Local temperature measurements also revealed lower magnitudes and higher frequencies of oscillations at high system pressures.     

Adiabatic two-phase flow in rectangular microchannels with different aspect ratios: Part I – Flow pattern,pressure drop and void fraction

An aspect ratio is an important parameter for two-phase flow in a rectangular microchannel. To study the aspect ratio effect on the flow pattern, pressure drop and void fraction, experiments of adiabatic liquid water and nitrogen gas two-phase flow in rectangular microchannels were conducted. The widths and heights of rectangular microchannels are 510 μm × 470 μm, 608 μm × 410 μm, 501 μm × 237 μm and 503 μm × 85 μm. Therefore, the aspect ratios of the rectangular microchannels are 0.92, 0.67, 0.47 and 0.16; and the hydraulic diameters of the rectangular microchannels were 490, 490, 322 and 143 μm, respectively. Experimental ranges were liquid superficial velocities of 0.06–1.0 m/s and gas superficial velocities of 0.06–71 m/s. Visible rectangular microchannels were fabricated using a photosensitive glass. And pressure drop in microchannels was directly measured through embedded ports. The visualization of the flow pattern was carried out with a high-speed camera and a long distance microscope. Typical flow patterns in the rectangular microchannels observed in this study were bubble flow, transitional flow (multiple flow) and liquid ring flow. As the aspect ratio decreased, the bubble flow regime became dominant due to the confinement effect and the thickness of liquid film in corner was decreased. A void fraction in the rectangular microchannels has a linear relation with the volumetric quality. And the two-phase flow becomes homogeneous with decreasing aspect ratio owing to the reduction of the liquid film thickness. Like Zhang et al.’s [19] correlation, as the confinement number increased, the C-value in Lockhart and Martinelli correlation decreased. And a frictional pressure drop in the rectangular microchannels was highly related with the flow pattern.     

A novel time strip flow visualisation technique for investigation of intermittent dewetting and dryout in elongated bubble flow in a microchannel evaporator

A flow visualisation study of flow boiling of R245fa in silicon multi-microchannels at low mass flux and moderate heat flux has been carried out with a high speed digital camera. The micro-evaporator had 67 channels of length 20 mm, width 223 μm, and height 680 μm while the fin width between adjacent channels was 80 μm. The base heat flux ranged from 2 to 26 W cm^?2 for a mass velocity of 100 kg s^?1 m^?2, resulting in exit vapour qualities ranging from 10% to 70%. In particular, a novel time strip technique was developed to analyse the recorded image sequences and significantly highlight the various phenomena occurring along given channels. Notably, this technique was able to reveal profound details regarding the intermittent dryout mechanism of liquid films trapped between the elongated bubbles and the heated channel walls. The results show that the intermittent dryout of the evaporating liquid film is comprised of four stages with distinct time scales and dynamics: (i) the growth of liquid film thinning perturbations to a critical amplitude causing the rupture of the metastable liquid film, (ii) a dewetting stage involving expanding dry spots leading to a rivulet flow regime, (iii) evaporation of the rivulets leading to full dryout, and (iv) a rewetting stage. This intermittent dryout mechanism appears to explain the many seemingly contradictory heat transfer coefficient trends observed with changes in vapour quality in microchannels, thus resolving an important heat transfer dilemma. Furthermore, since dryout is an undesirable event during the practical application of a microchannel evaporator, it is important to delay or even suppress the initial rupture of the liquid film that leads to dryout. This can be achieved by manufacturing or treating the channel surfaces to be highly wettable with the chosen refrigerant.     

Heat, mass and momentum transfer in coating formation by plasma spraying

The plasma jets produced by d.c. spray torches exhibit unusual properties: high flow velocities (up to 2 500 m · s ⁻¹ ), high temperatures (up to 14 000 K), steep temperature and velocity radial gradients (up to 10 ⁷ K · m ⁻¹ and 5.10 ⁵ s ⁻¹ ) and low gas density ( 1/30 to 1/50 that of the cold gas). They are laminar in their core and turbulent in their fringes. When they exit the torch nozzle, the resulting vortices coalesce inducing an engulfment process of the ambient gas with large scale eddies entraining bubbles of cold gas. The latter do not mix instantaneously with the plasma due to the high density difference. Mixing occurs after the heating of the cold inclusions. In addition, the plasma jets are continuously fluctuating in length and position because of the continuous movement of the arc root on the anode wall at frequencies ranging between 3 and 20 kHz. This results in a sort of piston flow. In plasma spraying, the solid particles are injected in the plasma jet through an injector set downstream or upstream of the nozzle exit. In this injector, particles collide between themselves and the injector wall. Therefore, they have trajectory and velocity distributions at the injector exit. It results in a dispersion of their trajectories within the jet. The flow rate of the powder carrier gas has to be adjusted to give the particles about the same momentum as that of the plasma jet at the injection point. The large difference between particle and flow velocity can induce convective movements within the molten droplets resulting in a continuous renewing of the liquid material at the particle surface. For metal or alloy particles sprayed in air this internal movement brings about a high oxidation rate enhanced by the presence of atomic oxygen in the jet. Particles impact on the part to be covered at velocities between 150 and 300 m · s ⁻¹ . The liquid material spreads out from the point of impact and forms a lamella called “splat”. The flattening time is below a few μ s and splat solidification generally starts before the flattening process is completed. The next particle that impacts a few tens of μ s later, flattens on already solidified particles. The piling up of a few splats forms a pass in less than one millisecond, then, the next pass is deposited a few seconds later. The thickness of a pass varies between 3 and 60 μ m. The flow and heat phenomena during the impact and solidification processes control the microstructure and thermo-mechanical properties of coatings. The build-up of a coating in plasma spraying is a multiscale problem with time scales ranging between microseconds and seconds and length scales ranging between a few micrometers and a few hundred micrometers or more. Therefore, models and experiments deal with either the formation of splats or the piling of layers. This paper will review what is our present knowledge of the modeling and measurement of the transient phenomena involved in the various subsystems of the plasma spray process: jet formation, particle injection, particle heating and acceleration and coating formation.