Triangular Microchannels Research Articles

Measurement and modeling of condensation heat transfer in non-circular microchannels

Heat transfer coefficients in six non-circular horizontal microchannels (0.424 < D h < 0.839 mm) of different shapes during condensation of refrigerant R134a over the mass flux range 150 < G < 750 kg m −2 s −1 were measured in this study. The channels included barrel-shaped, N-shaped, rectangular, square, and triangular extruded tubes, and a channel with a W-shaped corrugated insert that yielded triangular microchannels. The thermal amplification technique developed and reported in earlier work by the authors is used to measure the heat transfer coefficients across the vapor-liquid dome in small increments of vapor quality. Results from previous work by the authors on condensation flow mechanisms in microchannel geometries were used to interpret the results based on the applicable flow regimes. The effect of tube shape was also considered in deciding the applicable flow regime. A modified version of the annular-flow-based heat transfer model proposed recently by the authors for circular microchannels, with the required shear stress being calculated from a non-circular microchannel pressure drop model also reported earlier was found to best correlate the present data for square, rectangular and barrel-shaped microchannels. For the other microchannel shapes with sharp acute-angle corners, a mist-flow-based model from the literature on larger tubes was found to suffice for the prediction of the heat transfer data. These models predict the data significantly better than the other available correlations in the literature.

Three-dimensional numerical simulation of the slip flow through triangular microchannels

Three-dimensional numerical analysis for fully developed incompressible fluid flow and heat transfer through triangular microchannels over the slip flow regime is simulated in this paper. In order to study the flow through the channel, the Navier–Stokes equations are solved in conjunction with slip/jump boundary conditions. The influences of Knudsen number (0.001 < Kn < 0.1), aspect ratio (0.2 < A < 4.5), and Reynolds number (1 < Re < 15) on the fluid flow and heat transfer characteristics are extensively investigated in the paper. The numerical results reveal that the rarefaction decreases the Poiseuille number, while its effect on the Nusselt number completely depends on the interaction between velocity slip and temperature jump. It is also found that the aspect ratio has an important role in the analysis, but the variation of Reynolds number is less remarkable.

One dimensional numerical simulation for steady annular condensation flow in rectangular microchannels

A one dimensional model for steady annular condensation flow in rectangular microchannels is developed and numerically solved under constant heat flux condition. The results indicate that the annular condensation length is determined by the contact angle, heat flux, vapor pressure, hydraulic diameter and aspect ratio of rectangular microchannels. A larger inlet vapor pressure and hydraulic diameter or a smaller heat flux and contact angle can all result in a longer annular condensation length. In addition, the simulation results of steady annular condensation flow in rectangular microchannels are compared with that in triangular microchannels. The differences in curvature radius, condensate pressure and velocity, vapor velocity distributions in rectangular and triangular microchannels under the same conditions verify the considerable influence of cross-section shape on micro flow condensation.

Three-dimensional numerical simulation of heat and fluid flow in noncircular microchannel heat sinks

A three-dimensional model of heat transfer and fluid flow in noncircular microchannel heat sinks is developed and analyzed numerically. It is found that Nusselt number has a much higher value at the inlet region, but quickly approaches the constant fully developed value. The temperature in both solid and fluid increases along the flow direction. In addition, the comparison of thermal efficiencies is conducted among triangular, rectangular and trapezoidal microchannels. The result indicates that the triangular microchannel has the highest thermal efficiency.

Seed bubbles trigger boiling heat transfer in silicon microchannels

The smooth channel surface of microsystems delays boiling incipience in heated microchannels. In this paper, we use seed bubbles to trigger boiling heat transfer and control thermal non-equilibrium of liquid and vapor phases in parallel microchannels. The test section consisted of a top glass cover and a silicon substrate. Microheater array was integrated at the top glass cover surface and driven by a pulse voltage signal to generate seed bubbles in time sequence. Each microheater corresponds to a specific microchannel and is located in the microchannel upstream. Five triangular microchannels with a hydraulic diameter of 100 μm and a length of 12.0 mm were etched in the silicon substrate. A thin platinum film was deposited at the back surface of silicon chip with an effective heating area of 4,500 × 1,366 μm, acting as the main heater for the heat transfer system. Acetone liquid was used. With the data range reported here, boiling incipience was not initiated if wall superheats are smaller than 15°C without seed bubbles assisted. Injection seed bubbles triggers boiling incipience and controls thermal non-equilibrium between liquid and vapor phases successfully. Four modes of flow and heat transfer are identified. Modes 1, 2, and 4 are the stable ones without apparent oscillations of pressure drops and heating surface temperatures, and mode 3 displays flow instabilities with apparent amplitudes and long periods of these parameters. The four modes are divided based on the four types of flow patterns observed in microchannels. Seed bubble frequency is a key factor to influence the heat transfer. The higher the seed bubble frequency, the more decreased non-equilibrium between two phases and heating surface temperatures are. The seed bubble frequency can reach a saturation value, at which heat transfer enhancement attains the maximum degree, inferring that a complete thermal equilibrium of two phases is approached. The saturation frequency is about a couple of thousand Hertz in this study.

Visualization study of steam condensation in triangular microchannels

A visualization experiment is conducted to investigate the condensation of steam in a series of triangular silicon microchannels. The results indicate that droplet, annular, injection and slug-bubbly flow are the dominant flow patterns in these triangular silicon microchannels. With increased mass flow rate, or an increase in the hydraulic diameter under the same Reynolds number, the location at which the injection occurred is observed to move towards the channel outlet. The frequency of the injection increases, i.e. the flow of condensation instability is higher with increased inlet vapor Reynolds number, condensate Weber number and the prolongation of the injection location, or with a decrease in the hydraulic diameter of the channel. In addition, the wall temperature of the channel decreases along the condensation stream. The total pressure drop, the average condensation heat transfer coefficient and the average Nusselt number are observed to be larger with increased inlet vapor Reynolds number. Moreover, it is found that the condensation heat transfer is enhanced by a reduction in the channel scale.

Numerical simulation for steady annular condensation flow in triangular microchannels

A steady one-dimensional model for annular condensation flow in triangular microchannels is developed. The curvature radius distribution of the condensate stream along the channel has been determined numerically. The results indicate that the curvature radius of the liquid phase would increase rapidly at the beginning, and then as the condensation process progresses along the length of the microchannels, the radius increase would proceed more slowly. At the end of the condensation flow, the radius increases rapidly again. A smaller contact angle and heat flux or a larger hydraulic diameter and steam pressure will all result in a longer condensation length.

Thermal effects on mixed electro-osmotic and pressure-driven flows in triangle microchannels

A numerical investigation on the mixed electro-osmotic and pressure-driven flows in triangle microchannels with constant wall temperature is reported in the present study. The Galerkin method is employed to solve the Poisson equation, energy equation and Navier–Stokes equations for the flow driven by electro-osmotic and pressure gradient synchronously under the conditions of favorable pressure gradients and backpressure gradients. The physical properties of the electrolyte solution are considered to be varying with the temperature, and the dimensionless velocity profile, dimensionless temperature profile as well as dimensionless mass flux of the electrolyte solution are obtained. Furthermore, the parameters studies including pressure gradient, length ratio and Joule heating on mass flux of the electro-osmotic flows are performed, respectively. The numerical results show that a large Joule number leads to large dimensionless mass flux and dimensionless temperature of the electrolytic solution in the triangular microchannels for both conditions. For the electro-osmotic flows under the favorable pressure gradient, the increase in dimensionless mass flux resulted from Joule heating is enlarged with increasing pressure gradient and length ratio. However, for the electro-osmotic flows under the backpressure gradient, Joule heating results in a reverse flow in the channel, and the dimensionless mass flux of the reverse flow increases with increasing backward pressure gradient and decreasing length ratio. It is further found that the Joule heating induces a more significant increase in the dimensionless mass flux under favorable pressure gradient compared with that under backpressure gradient.

Heat Transfer for Water Flow in Triangular Silicon Microchannels

This study investigated the behavior of pressure drops and heat transfer coefficients for four sets of triangular microchannels. Results of this study indicated that for Reynolds numbers less than 100, the Nusselt number tends to be nearly linearly increasing as the Reynolds number increases. The product of the friction factor and the Reynolds number (fRe) is slightly lower than those predicated by the conventional theory. Besides, high temperature gradient exists in the regions between the inlet and the exit of the flow channel. Furthermore, the difference between the values for Nu of those obtained from the empirical correlation for triangular microchannel and those from the experimental measurements was found to be within 15%.

Uniform Mems Chip Temperatures in the Nucleate Boiling Heat Transfer Region by Selecting Suitable, Medium Boiling Number Range

The not only lower but also uniform MEMS chip temperatures can be reached by selecting suitable boiling number range that ensures the nucleate boiling heat transfer. In this article, boiling heat transfer experiments in 10 silicon triangular microchannels with the hydraulic diameter of 155.4 μm were performed using acetone as the working fluid, having the inlet liquid temperatures of 24–40°C, mass fluxes of 96–360 kg/m2s, heat fluxes of 140–420 kW/m2 , and exit vapor mass qualities of 0.28–0.70. The above data range correspond to the boiling number from 1.574 × 10−3 to 3.219 × 10−3 and ensure the perfect nucleate boiling heat transfer region, providing a very uniform chip temperature distribution in both streamline and transverse directions. The boiling heat transfer coefficients determined by the infrared radiator image system were found to be dependent on the heat fluxes only, not dependent on the mass fluxes and the vapor mass qualities covering the above data range. The high-speed flow visualization shows that the periodic flow patterns take place inside the microchannel in the time scale of milliseconds, consisting of liquid refilling stage, bubble nucleation, growth and coalescence stage, and transient liquid film evaporation stage in a full cycle. The paired or triplet bubble nucleation sites can occur in the microchannel corners anywhere along the flow direction, accounting for the nucleate boiling heat transfer mode. The periodic boiling process is similar to a series of bubble nucleation, growth, and departure followed by the liquid refilling in a single cavity for the pool boiling situation. The chip temperature difference across the whole two-phase area is found to be small in a couple of degrees, providing a better thermal management scheme for the high heat flux electronic components. Chen's [1] widely accepted correlation for macrochannels and Bao et al.'s [2] correlation obtained in a copper capillary tube with the inside diameter of 1.95 mm using R11 and HCFC123 as working fluids can predict the present experimental data with accepted accuracy. Other correlations fail to predict the correct heat transfer coefficient trends. New heat transfer correlations are also recommended.

Are the available boiling heat transfer coefficients suitable for silicon microchannel heat sinks?

The typical MEMS fabrication of micro evaporators ensures the perfect smooth wall surface that is lack of nucleation sites, significantly decreasing the heat transfer coefficients compared with miniature evaporators fabricated using copper or stainless steel. In the present paper, we performed the boiling heat transfer experiment in silicon triangular microchannel heat sink over a wide parameter range for 102 runs. Acetone was used as the working fluid. The measured boiling heat transfer coefficients versus the local vapor mass qualities are compared with the classical Chen’s correlation and other correlations for macro and miniature capillary tubes. It is found that most of these correlations significantly over-predict the measured heat transfer coefficients. New correlations are given. There are many reasons for such deviations. The major reason is coming from the perfect smooth silicon surface that lowers the heat transfer performances. New theory is recommended for the silicon microchannel heat sink that should be different from metallic capillary tubes.

Passive water removal in fuel cells by capillary droplet actuation

In this paper, a microstructured flow field for passive water management in proton exchange membrane fuel cells (PEMFC) is presented. It is based on the transport of liquid water droplets by capillary forces. A triangular microchannel forces droplets to detach from a fuel cell's gas diffusion layer (GDL) or membrane electrode assembly (MEA). Thus, it ensures proper oxygen supply of the reactive area. Water droplets are lifted into a secondary channel, and removed from the fuel cell by capillary action. Water formation in the flow field channels has been studied by analytic models and experiments. Three droplet configurations were identified, which show different properties in terms of capillary transport. Preferred shapes cover the GDL only slightly and can be found for contact angles typical for fuel cell materials. The actuation mechanism was studied by simulations for different designs, with varying contact angles and geometries. The new channel design was compared in a test fuel cell to standard rectangular channels, in the initial phase of the cell, at low working temperatures (22–30 °C). The new flow field design stabilized the cell at 95% of its initial performance compared to 60% when using the standard design.

A novel solution for pressure drop in singly connected microchannels of arbitrary cross-section

This paper outlines a novel approximate solution for determining the pressure drop of fully developed, laminar, single-phase flow in singly connected microchannels of arbitrary cross-section. Using a “bottom-up” approach, it is shown that for constant fluid properties and flow rate in fixed cross-section channels, the Poiseuille number is only a function of geometrical parameters of the cross-section, i.e., perimeter, area, and polar moment of inertia. The proposed model is validated with experimental data for rectangular, trapezoidal, and triangular microchannels. The model is also compared against numerical results for a wide variety of channel cross-sections including: hyperellipse, trapezoid, sine, square duct with two adjacent round corners, rhombic, circular sector, circular segment, annular sector, rectangular with semi-circular ends, and moon-shaped channels. The model predicts the pressure drop for the cross-sections listed within 8% of the values published.

Periodic boiling in parallel micro-channels at low vapor quality

Based on experimental investigations the present study evaluates instability and heat transfer phenomenon under condition of periodic flow boiling of water and ethanol in parallel triangular micro-channels. Tests were performed in the range of hydraulic diameter 100–220 μm, mass flux 32–200 kg/m 2 s, heat flux 120–270 kW/m 2, vapor quality x = 0.01–0.08. The period between successive events depends on the boiling number and decreases with an increase in the boiling number. The initial film thickness decreases with increasing heat flux. When the liquid film reached the minimum initial film thickness CHF regime occurred. Temporal variations of pressure drop, fluid and heater temperatures were periodic. Oscillation frequency is the same for the pressure drop, for the fluid temperature at the outlet manifold, and for the mean and maximum heater temperature fluctuations. All these fluctuations are in phase. The CHF phenomenon is different from that observed in a single channel of conventional size. A key difference between micro-channel heat sink and single conventional channel is amplification of parallel-channel instability prior to CHF. The dimensionless experimental values of the heat transfer coefficient are presented as the Nusselt number dependence on the Eotvos number and the boiling number.

Flow instability and transient flow patterns inside intercrossed silicon microchannel array in a micro-timescale

The objective of this study is to visualize the transient flow patterns and heat transfer behaviors at low mass fluxes and high heat fluxes. The silicon chip consists of the intercrossed microchannel array with 10 triangular microchannels with the hydraulic diameter of 155.4 μm, and five transverse trapezoid microchannels, separating the triangular microchannels into six independent zones. The chip is horizontally positioned. Liquid acetone is used as the working fluid. Tests were performed in the range of mass flux 40–80 kg/m 2 s and heat flux 107–216 kW/m 2. It is found that all the microchannels repeat the flow patterns in the timescale of milliseconds, with three substages: liquid refilling stage, transient stratified flow stage and partial/fully dry-out stage. Stratified flow is the dominant flow due to the low liquid Froude numbers. The axial liquid film thickness for each separated microchannel zone is increased along the flow direction, caused by the large momentum force due to evaporation on the interface of the vapor and the settled liquid film, and the high vapor shear stress applied on the vapor/liquid interface. The non-uniform axial liquid film distributions cause the earlier dry-out of the triangular microchannel upstream. However, dry-out always takes place earlier in the downstream zone than that in the upstream zone. The vapor slug near the triangular microchannel exit may be entrained in the long liquid plug during the liquid refilling stage. Besides, the steep jumped liquid film thickness may take place near the channel exit for the stratified flow. Both indicate the strong geometry effects. Due to the compact size and high thermal conductivity of the silicon chip, the chip surface temperature variations versus time are identified by the infrared radiator image system, especially in the ending area of the thin film heater.

Heat transfer for laminar slip flow in a microchannel of arbitrary cross section with complex thermal boundary conditions

Laminar forced convective heat transfer for a gas flowing through a microchannel of arbitrary cross section with the axially-constant heat flux and circumferentially-varied wall temperature boundary condition in the slip-flow and temperature-jump regime is studied theoretically. The dimensionless temperature profile and the average Nusselt numbers are obtained by solving the fluid flow and energy equations for hydrodynamically and thermally fully developed incompressible slip flow by means of a computation-oriented method of the orthonormal function analysis. The work is then focused on studying the heat transfer characteristics of rectangular and triangular microchannels, which are more frequently encountered in real applications. The effects of Knudsen number, aspect ratio, and thermal boundary conditions on the heat transfer characteristics of these two types of microchannels are examined. The results show that the orthonormal function method is applicable to the fluid flow and heat transfer problems with the slip-velocity and temperature-jump boundary conditions for unsymmetrically heated microchannels with an arbitrary cross section. It is also shown that the increased thermal resistance caused by the temperature jump at the channel walls predominates in the overall heat transfer behavior in microchannels within a definite extension of Knudsen number, leading to a decrease in the heat transfer coefficient. Thus, the average Nusselt numbers in the slip flow are generally smaller than that in the no-slip-flow.

Influence of the Fluid on the Experimental Performances of Triangular Silicon Microheat Pipes

An experimental study was conducted to determine the performances of silicon microheat pipes (MHPs) filled with different fluids. The MHP arrays are made of 27 triangular microchannels chemically etched in a silicon wafer. This wafer is closed by molecular bonding of a bulk oxidized silicon wafer. The results show that the use of MHPs with low liquid charges can increase the effective thermal conductivity of the array. The best improvement of the effective thermal conductivity of the MHP array is equal to 41% for methanol and pentane, 12% for ethanol and nil for water and FC72.

An Experimental Study and Numerical Modeling of the Flow in a Network of Triangular Microchannels

The flow characteristics of a network of parallel microchannels (hydraulic diameter: 110 μm) are investigated both experimentally and numerically. The microchannel cross-section was triangular, as in the case of microheat pipes. The pressure drop across the microchannel network showed a dramatic increase with a departure from the law of fully developed flow in ducts as soon as the Reynolds number of the flow exceeded about 10. Numerical computation of the flow was carried out using the classical laws of hydrodynamics in an attempt to explain this surprising result. There was a good agreement with experimental results, which suggests that there are no size effects at the length scales used in the experiments. Moreover, the mechanisms responsible for the large pressure drop for higher Reynolds numbers were identified in the numerical analysis as being extra head losses due to separation in several parts of the test section.

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.

A Theory of Film Condensation in Horizontal Noncircular Section Microchannels

The paper presents a theoretical model to predict film condensation heat transfer from a vapor flowing in horizontal square and equilateral triangular section minichannels or microchannels. The model is based on fundamental analysis which assumes laminar condensate flow on the channel walls and takes account of surface tension, interfacial shear stress, and gravity. Results are given for channel sizes (side of square or triangle) in the range of 0.5–5 mm and for refrigerants R134a, R22, and R410A. The cases considered here are where the channel wall temperature is uniform and the vapor is saturated at the inlet. The general behavior of the condensate flow pattern (spanwise and streamwise profiles of the condensate film), as well as streamwise variation of local mean (over section perimeter) heat-transfer coefficient and vapor mass quality, are qualitatively in accord with expectations on physical grounds. The magnitudes of the calculated heat-transfer coefficients are in general agreement with experimental data for similar, but nonidentical, channel geometry and flow parameters.