Scaling Effects for Liquid Flows in Microchannels

Many experimental works on the forced convection through microchannels seem to show that when the hydraulic diameter is less than 1 mm, the conventional theory can no longer be considered suitable to predict the pressure drop and convective heat transfer coefficients. This conclusion seemed valid for both gas and liquid flows. Sometimes the authors justified this claim by invoking “new” micro-effects. In the last few years, this conclusion seems to be controverted by additional, more accurate experimental data. For this reason, the explanation of the experimental results obtained for microchannels in terms of friction factors and convective heat transfer in the laminar regime is sought for within the bonds of the conventional theory. In particular, this study focuses on the role of viscous heating in liquids flowing through microchannels, considering them as scaling effects. The role of the cross-sectional geometry on the viscous heating is highlighted for adiabatic and diabatic channels. Design correlations that are useful in defining the limit of significance of the most important scaling effects for microchannels, such as viscous heating and conjugate heat transfer, are also presented.     

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.     

Effect of developing flow and thermal regime on momentum and heat transfer in micro-scale heat sink

A developing micro-channel heat transfer and fluid flow has been investigated experimentally in rectangular micro-channels of D_h = 440 μm, having water as a working fluid. Infrared technique was used to design and built a micro-channel test section that incorporate internal fluid temperature measurements. The new method that provides information about the fluid temperature distribution inside the channel and provides validation for the methods used to determine the local and average Nusselt numbers. The experimental results have been compared with theoretical predictions from the literature and results obtained by numerical modeling of the present experiment. The experimental results of pressure drop and heat transfer confirm that including the entrance effects, the conventional theory is applicable for water flow through micro-channels.These results differ from the conclusions of several researches. It was shown that data presented by some researches can be due to entrance effects. The present results highlight the importance of accounting for common phenomena that are often negligible for standard flows such as accounting for profile of inlet velocity, axial heat conduction, effect of the design inlet and outlet manifolds.This paper, to the best of knowledge, is the first presentation on the method of the bulk fluid temperature measurements along micro-channel using IR technique, and calculation of the local heat transfer coefficient based on the local heat flux and the local temperature difference between the heated wall and the bulk fluid temperature.     

Flow instability of evaporative micro-channels

When boiling occurs in a micro-channel, the growing bubbles could be confined. They expand both upstream and downstream and cause flow instability in the form of flow fluctuations. The instability occurs frequently when the Bond number of the fluid in the micro-channel is less than unity. To reduce the flow instability, installation of an inlet orifice at the upstream or the micro-channel expanding at the downstream are found to be effective. A generalized instability model for micro-channels was established, which includes the effects of both inlet orifice and channel expansion. Experiments of evaporating water in 48 parallel micro-channels with 353 μm hydraulic diameter were conducted, and the generalized instability criterion was validated.     

Flow condensation in parallel micro-channels – Part 2: Heat transfer results and correlation technique

This second part of a two-part study concerns heat transfer characteristics for FC-72 condensing along parallel, square micro-channels with a hydraulic diameter of 1 mm, which were formed in the top surface of a solid copper plate. Heat from the condensing flow was rejected to a counter flow of water through channels brazed to the underside of the copper plate. The FC-72 condensation heat transfer coefficient was highest near the channel inlet, where the annual liquid film is thinnest. The heat transfer coefficient decreased along the micro-channel because of the film thickening and eventual collapse of the annular regime. Notable heat transfer enhancement was observed for annular flow regions of the micro-channel associated with interfacial waves. Comparing the present data to predictions of previous annular condensation heat transfer correlations shows correlations intended for macro-channels generally provide better predictions than correlations intended specifically for mini/micro-channels. A new condensation heat transfer coefficient correlation is proposed for annular condensation heat transfer in mini/micro-channels. The new correlation shows excellent predictive capability based on both the present FC-72 data and a large database for mini/micro-channel flows amassed from eight previous sources.     

Heat Transfer Characteristics of Gaseous Flows in Micro-Channel with Negative Heat Flux

Two-dimensional compressible momentum and energy equations are solved to obtain the heat transfer characteristics of gaseous flows in micro-channels with constant heat flux for which the value is negative for no-slip flow. The numerical methodology is based on the Arbitrary-Lagrangian-Eulerian method. The computations are performed for channels with constant heat flux ranging from ?10⁴ to ?10² W/m². The channel height ranges from 10 to 100 μ m and the aspect ratio of the channel height and length is 200. The stagnation pressure is chosen such that the exit Mach number ranges from 0.1 to 0.7. The outlet pressure is fixed at the atmosphere. The wall and bulk temperatures in micro-channels with negative heat flux are compared with those of positive heat flux cases obtained in our previous work and also those of the incompressible flow in a conventional sized channel. In the case of fast flow, temperatures normalized by heat flux have different trends whether heat flux value is positive or negative. A correlation for the prediction of the wall temperature of the gaseous flow in the micro-channel is proposed. The rarefaction effect is investigated for the cases of channel height of 10 μ m with slip boundary conditions. The magnitudes of viscous dissipation term and compressibility term are also investigated. The effect of each term on heat transfer characteristics is discussed.     

跨努森数区域的微通道内流动计算模拟研究

采用一阶和修正二阶的滑移连续介质模型,对跨努森数区域的低速微通道流进行二维和三维数值模拟。用实验结果和DSMC方法验证滑移连续介质模型在跨努森数区域中的适用性,并详细讨论了微通道流的可压缩效应、稀薄效应、低雷诺数效应和三维特性。研究表明,努森数是表征稀薄效应和模型适用性的特征参数,滑移连续介质模型适用于努森数小于0.150的氮气流动;马赫数不再是微通道流可压缩效应的唯一标识参数;雷诺数是表征低雷诺数效应和三维特性的关键参数,高宽比大于20的微通道流具备良好的二维特性。     

Numerical investigation of fluid flow and heat transfer under high heat flux using rectangular micro-channels

A 3D-conjugate numerical investigation was conducted to predict heat transfer characteristics in a rectangular cross-sectional micro-channel employing simultaneously developing single-phase flows. The numerical code was validated by comparison with previous experimental and numerical results for the same micro-channel dimensions and classical correlations based on conventional sized channels. High heat fluxes up to 130 W/cm² were applied to investigate micro-channel thermal characteristics. The entire computational domain was discretized using a 120 × 160 × 100 grid for the micro-channel with an aspect ratio of (α = 4.56) and examined for Reynolds numbers in the laminar range (Re 500–2000) using FLUENT. De-ionized water served as the cooling fluid while the micro-channel substrate used was made of copper. Validation results were found to be in good agreement with previous experimental and numerical data [1] with an average deviation of less than 4.2%. As the applied heat flux increased, an increase in heat transfer coefficient values was observed. Also, the Reynolds number required for transition from single-phase fluid to two-phase was found to increase. A correlation is proposed for the results of average Nusselt numbers for the heat transfer characteristics in micro-channels with simultaneously developing, single-phase flows.     

Compressibility and rarefaction effect on heat transfer in rough microchannels

High pressure drop and high length to hydraulic diameter ratios yield significant compressibility effects in microchannel flows, which compete with rarefaction phenomena at the smaller scale. In such regimes, flow field and temperature field are no longer decoupled. In presence of significant heat transfer, and combined with the effect of viscous dissipation, this yields to a quite complex thermo-fluid dynamic problem. A finite volume compressible solver, including generalized Maxwell slip flow and temperature jump boundary conditions suitable for arbitrary geometries, is adopted. Roughness geometry is modeled as a series of triangular shaped obstructions, and relative roughness from 0% to 2.65% were considered. The chosen geometry allows for direct comparison with pressure drop computations carried out, in a previous paper, under adiabatic conditions. A wide range of Mach number is considered, from nearly incompressible to chocked flow conditions. Flow conditions with Reynolds number up to around 300 were computed. The outlet Knudsen number corresponding to the chosen range of Mach and Reynolds number ranges from very low value to around 0.05, and the competing effects of rarefaction, compressibility and roughness are investigated in detail. Compressibility is found to be the most dominant effect at high Mach number, yielding even inversion of heat flux, while roughness has a strong effect in the case of rarefied flow. Furthermore, the mutual interaction between heat transfer and pressure drop is highlighted, comparing Poiseuille number values for both cooled and heated flows with previous adiabatic computations.     

Effects of choking on flow and heat transfer in micro-channels

The gas flows through micro-channels are encountered in many engineering applications such as the cooling devices of electronic chips, semiconductors, micro-electro-mechanical systems (MEMS), etc. Many works have been performed to investigate the flow and heat transfer characteristics generally occurring in the micro channels. According to these investigations, the majority of heat was transferred in the entrance region of the channel, due to high strain rate of the developing flow. These findings are valid only for unchoked micro channel flows. Once the gas flow is choked, the major flow features may be changed but no detailed works have been carried out to date. In these regards, the choked flow characteristics should be known to investigate the heat transfer phenomena in the micro channel flows. In the present study, numerical simulations have been used to provide detailed flow and heat transfer characteristics of micro-channel gas flows. The main objectives of the present effort are to understand the evolution of choking inside micro-channels with isothermally-heated-walls and to elucidate the regions of high heat transfer. The results obtained show that for choked flow conditions, high heat transfer is generated at both the entrance and the exit of the micro-channel. The exit effects like increased strain rate, high temperature gradient and the thinning of the boundary layer cause a rapid increase in heat transfer at the exit of the micro-channel. The location where the flow is choked is practically important in determining the heat transfer phenomena at the vicinity of the channel exit.     

Critical heat flux for subcooled flow boiling in micro-channel heat sinks

Critical heat flux (CHF) was measured and examined with high-speed video for subcooled flow boiling in micro-channel heat sinks using HFE 7100 as working fluid. High subcooling was achieved by pre-cooling the working fluid using a secondary low-temperature refrigeration system. The high subcooling greatly reduced both bubble departure diameter and void fraction, and precluded flow pattern transitions beyond the bubbly regime. CHF was triggered by vapor blanket formation along the micro-channel walls despite the presence of abundant core liquid, which is consistent with the mechanism of Departure from Nucleate Boiling (DNB). CHF increased with increasing mass velocity and/or subcooling and decreasing hydraulic diameter for a given total mass flow rate. A pre-mature type of CHF was caused by vapor backflow into the heat sink’s inlet plenum at low mass velocities and small inlet subcoolings, and was associated with significant fluctuations in inlet and outlet pressure, as well as wall temperature. A systematic technique is developed to modify existing CHF correlations to more accurately account for features unique to micro-channel heat sinks, including rectangular cross-section, three-sided heating, and flow interaction between micro-channels. This technique is shown to be successful at correlating micro-channel heat sink data corresponding to different hydraulic diameters, mass velocities and inlet temperatures.     

Fabrication of high aspect ratio ceramic micro-channel in diamond wire sawing as catalyst support used in micro-reactor for hydrogen production

Ceramic is an ideal material for preparing micro-channel catalyst supports with their characteristics of high temperature resistance, corrosion resistance and mechanical strength. High aspect ratio micro-channel structure has the advantages of large specific surface area, strong mass and heat transfer performance and high material utilization. However, ceramic materials are hard and brittle, and it is difficult to fabricate micro-channel structures with aspect ratio more than 1.5:1 by traditional processing methods. In this paper, a cutting method of large diameter diamond wire sawing was proposed. The micro-channels with width of 520 μm and aspect ratio of more than 4:1 was successfully fabricated by this method. Furthermore, the integrity of the micro-channel structure processed by diamond wire sawing was analyzed. And than the effect of surface morphology in different processing parameters on the catalyst loading performance were studied. The catalyst loading strength of ceramic slices with different surface morphology was tested. Finally, the ceramic micro-channel array was used as the catalyst support in micro-reactor for hydrogen production via methanol steam reforming (MSR). The methanol conversion rate and H₂ production rate could reach 87.8% and 74.6 mmol/h, respectively under GHSV 12600 ml/g·h at 300 °C. The experimental results show that the large-diameter diamond wire sawing technology can be used to process ceramic microchannels with high aspect ratio; using ceramic microchannel arrays as catalyst supports in hydrogen production can obtain better reaction performance; the feasibility of ceramic materials were broadened as microchannel catalyst supports.     

Fluid flow in micro-channels

We consider the problem of liquid and gas flow in micro-channels under conditions of a small Knudsen and Mach numbers, that correspond to continuum model. Data from the literature on pressure drop in circular, rectangle, triangular and trapezoidal micro-channels with hydrodynamic diameter ranging from 1.01 μm to 4010 μm are analyzed. The Reynolds number at transition from laminar to turbulent flow is considered. Attention is paid to comparison between predictions of the conventional theory and experimental data, obtained during the last decade, as well as to discussion of possible sources of unexpected effects which were revealed by a number of previous investigations.     

Thermophoresis of particles in aqueous solution in micro-channel

A theoretical analysis of micro motion of particles under the action of thermophoresis in aqueous electrolyte solution is presented in this paper. A two-dimensional, incompressible and laminar fluid flow model is proposed to simulate the fluid flow and particle motion trajectory in micro-channel. The effects of thermophoretic force and viscous force on the particle are calculated. The particle trajectories considering the effects of particle size, inlet flow velocity and temperature difference between bottom side and top side in micro-channel with a length of 2000 μm and a height of 500 μm are simulated. The effect of thermophoresis on separation motion of particles in micro-channel is analyzed. The results show that thermophoretic force increases with the increase of temperature gradient in micro-channel and particles with smaller diameter, smaller density and smaller gravity acceleration can pass the micro-channel more easily.     

Flow boiling heat transfer in two-phase micro-channel heat sinks--II. Annular two-phase flow model

This paper is Part II of a two-part study devoted to measurement and prediction of the saturated flow boiling heat transfer coefficient in water-cooled micro-channel heat sinks. Part I discussed the experimental findings from the study, and identified unique aspects of flow boiling in micro-channels such as abrupt transition to the annular flow regime near the point of zero thermodynamic equilibrium quality, and the decrease in heat transfer coefficient with increasing quality. The operating conditions of water-cooled micro-channels fell outside the recommended range for most prior empirical correlations. In this paper, an annular flow model is developed to predict the saturated flow boiling heat transfer coefficient. Features unique to two-phase micro-channel flow, such as laminar liquid and vapor flow, smooth interface, and strong droplet entrainment and deposition effects, are identified and incorporated into the model. The model correctly captures the unique overall trend of decreasing heat transfer coefficient with increasing vapor quality in the low vapor quality region of micro-channels. Good agreement is achieved between the model predictions and heat transfer coefficient data over broad ranges of flow rate and heat flux.     

Single-phase and two-phase heat transfer characteristics of low temperature hybrid micro-channel/micro-jet impingement cooling module

This study examines the single-phase and two-phase cooling performance of a hybrid micro-channel/micro-jet impingement cooling scheme using HFE 7100 as working fluid. This scheme consists of supplying coolant from a series of jets that deposit liquid into the micro-channels. A single-phase numerical scheme that utilizes the k–ε turbulent model and a method for determining the extent of the laminarized wall layer shows very good predictions of measured wall temperatures. It is shown jet velocity has a profound influence on single-phase cooling performance. High jet velocities enable jet fluid to penetrate the axial micro-channel flow and produce a strong impingement effect at the wall. On the other hand, the influence of jets at low jet velocities is greatly compromised compared to the micro-channel flow. During nucleate boiling, vapor layer development along the micro-channel in the hybrid module is fundamentally different from that encountered in conventional micro-channels. Here, subcooled jet fluid produces repeated regions of bubble growth followed by bubble collapse, rather than the continuous growth common to conventional micro-channel flow. By reducing void fraction along the micro-channel, the hybrid scheme contributes greater wall temperature uniformity. Increasing subcooling and/or flow rate delay the onset of boiling to higher heat fluxes and higher wall temperatures, and also increase critical heat flux considerably. A nucleate boiling heat transfer coefficient correlation is developed that fits the present data with a mean absolute error of 6.10%.     

The entrance effect on gases flow characteristics in micro-tube

Entrance region may have important effect on gases flow characteristics in micro-channels. It’s concluded in the available papers that the entrance effect causes significant difference. An experimental system of single-phase gas flow characteristics in microchannels was set up. Flow characteristics of nitrogen in PEEK polymer micro-tube (hydraulic diameter is 553μm) was studied experimentally. According to the data of nitrogen flow in the micro-tube with the length ranging from 0.1m to1.524m, it is shown that the friction constant becomes higher when the tube becomes shorter. By using pipe cutting methods, it’s confirmed that entrance effect is one of the key factors that cause friction constant higher than conventional theory. It’s found that friction constant of fully developed flow is lower than the value predicted by conventional theory in turbulent region. The result indicates that the flow transition occurs at Reynolds number ranging from 1600–2000. The phenomenon of obvious early transition is not found.     

Temperature-Dependent Viscosity and Viscous Dissipation Effects in Microchannel Flows With Uniform Wall Heat Flux

A parametric investigation is carried out on the effects of temperature-dependent viscosity and viscous dissipation in simultaneously developing laminar flows of liquids in straight microchannels of constant cross sections. Reference is made to fluid heating conditions with a uniform heat flux imposed on the walls of the microchannels. Six different cross sectional geometries are considered, chosen among those usually adopted for microchannels (circular, flat, square, rectangular, trapezoidal, and hexagonal). Viscosity is assumed to vary with temperature according to an exponential relation, while the other fluid properties are held constant. A finite-element procedure is employed for the solution of the parabolized momentum and energy equations. Due to the high value of the ratio between the length and the hydraulic diameter in microchannels, such an approach is very advantageous with respect to the one based on the steady-state solution of the elliptic form of the governing equations in a three-dimensional domain corresponding to the whole microchannel. Computed axial distributions of the local Nusselt number and of the apparent Fanning friction factor are presented. Numerical results confirm that, in the laminar forced convection in the entrance region of straight microchannels, the effects of temperature-dependent viscosity and viscous dissipation cannot be neglected in a wide range of operative conditions.