Triangular Microchannels Research Articles

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.

Explosive boiling of water in parallel micro-channels

The objective of this study is to visualize the flow pattern and to measure heat transfer coefficient during explosive boiling of water in parallel triangular micro-channels. Tests were performed in the range of inlet Reynolds number 25–60, mass flux 95–340 kg/m 2s, and heat flux 80–330 kW/m 2. The flow visualization showed that the behavior of long vapor bubbles, occurring in a micro-channel at low Reynolds numbers, was not similar to annular flow with interposed intermitted slugs of liquid between two long vapor trains. This process may be regarded as explosive boiling with periodic wetting and dryout. In the presence of two-phase liquid–vapor flow in the micro-channel, there are pressure drop oscillations, which increase with increasing vapor quality. This study shows strong dependence of the heat transfer coefficient on the vapor quality. The time when liquid wets the heated surface decreases with increasing heat flux. Dryout occurs immediately after venting of the elongated bubble.

Effect of axial conduction on the heat transfer in micro-channels

Experimental and numerical analysis were performed to evaluate heat transfer characteristics of water flowing through triangular silicon micro-channels with hydraulic diameter of 160 μm in the range of Reynolds number Re=3.2–64. It was shown that as the bulk water temperature, as well as the temperature of the heated wall, do not change linearly along the channel. The experimental results of temperature distribution on the heated wall agree with the numerical predictions. The behaviour of the Nusselt number along the channel has a singular point. At this point, the difference between the temperatures on the wall and the bulk water becomes negative and the flux changes the sign and is directed from the fluid to the wall. The singular point shifts closer to the channel outlet with an increase of the Reynolds number. It was shown that under conditions of the present study the heat transfer may be described by conventional Navier–Stokes and energy equations as a common basis.

Analysis of the heat transfer in unsymmetrically heated triangular microchannels in slip flow regime

A theoretical analysis of heat transfer characteristics is presented for the fully developed laminar flow of the incompressible gas in the triangular microchannels heated unsymmetrically with constant axial heat flux. Through solving the energy equation with temperature jump boundary conditions in slip flow regime by virtue of a computation- oriented method of the orthonormal function analysis, the dimensionless temperature profiles and the average Nusselt number for various thermal boundary conditions are obtained. The effects of Knudsen number, aspect ratio, and thermal boundary conditions on the heat transfer are discussed. The calculated results show that the orthonormal function method can be used to study the heat transfer characteristics of the unsymmetrically heated triangular microchannels. The average Nusselt number in triangular microchannels is lower for slip flow than for no-slip flow, and decreases with increasing Knudsen number. The aspect ratios and thermal boundary conditions of triangular microchannels have significant influences on the change of average Nusselt numbers with the increase in the Knudsen number. For the equilateral triangular microchannels, the decrease of the Nusselt number ratio due to temperature jump is smaller at large Knudsen number and larger at small Knudsen number on the boundary condition of bottom wall heated alone as compared with the one on the boundary condition of two heated hypotenuse walls. The correlations of the average Nusselt number with the Knudsen number for equilateral triangular microchannels are obtained.

CONVECTIVE BOILING IN PARALLEL MICROCHANNELS

Experiments were performed with clear water and with surfactant flowing in parallel triangular microchannels. The study is based on systematic measurements of temperature and flow pattern by infrared radiometry and high-speed digital video imaging. Different flow patterns were observed simultaneously in various microchannels at a fixed value of water or surfactant flow rates. Depending on flow and heat flux, pressure and temperature instabilities in the heated microchannels were studied. This work develops a practical modeling approach for two-phase microchannel heat sinks and also considers effect of surfactant on convective boiling in microchannels.

Two-phase flow patterns in parallel micro-channels

Micro-channel heat sinks with two-phase flow can satisfy the increasing heat removal requirements of modern micro-electronic devices. Some of the important aspects associated with two-phase flows in micro-channels, is to study the bubble behavior and flow regimes in diabatic, parallel micro-channels. Most of the reports in the literature present data of only a single channel and mostly adiabatic. This does not account for flow mixing and hydrodynamic instability that occurs in parallel micro-channels, connected by common inlet and outlet collectors. In the present study, experiments were performed for air–water and steam–water flow in parallel triangular micro-channels. The experimental study is based on systematic measurements of temperature and flow pattern by infrared radiometry and high-speed digital video imaging. In air–water flow different flow patterns were observed simultaneously in the various micro-channels at a fixed values of water and gas flow rates. In steam–water flow, instability in uniformly heated micro-channels was observed. This work develops a practical modeling approach for two-phase micro-channel heat sinks and considers the discrepancy between flow patterns of air–water and steam–water flow in parallel micro-channels.

Size and shape effects on two-phase flow patterns in microchannel forced convection boiling

An integrated microchannel heat sink consisting of shallow, nearly rectangular microchannels has been fabricated using standard micromachining techniques to highlight the effects of the micrometer sized channel shape on the evolving flow patterns and, consequently, on the thermal performance of the microsystem. An integrated heater serves as a local heat source, while an array of micro thermistors is used for temperature distribution measurements. The working fluid, DI water, is pressurized through the microchannels for forced convection heat transfer studies. Boiling curves for different flow rates have been recorded and analyzed based on the visualized flow patterns. Local nucleation, including bubble formation and bubble dynamics, is documented and found to be negligible. Although detected, in contrast with triangular microchannels, annular flow is observed to be unstable. Instead, the dominant flow pattern is an unsteady transition region connecting an upstream vapor zone to a downstream liquid zone with an average location depending on the input power. A physical mechanism based on the force balance across the vapor–liquid interface, and the development of a restoring force, is proposed to explain the flow visualization results.

Two- phase flow patterns and heat transfer in parallel microchannels

Microchannel heat sinks with two-phase flow can satisfy the increasing heat removal requirements of modern micro electronic devices. One of the important aspects associated with two- phase flows in microchannels is to study the bubble behavior. However, in the literature most of the reports present data of only a single channel. This does not account for flow mixing and hydrodynamic instability that occurs in parallel microchannels, connected by common inlet and outlet collectors. In the present study, experiments were performed for air- water and steam- water flow in parallel triangular microchannels with a base of 200–300 µ m. The experimental study is based on systematic measurements of temperature and flow pattern by infrared radiometry and high-speed digital video imaging. In air-water flow, different flow patterns were observed simultaneously in the various microchannels at a fixed values of water and gas flow rates. In steam-water flow, instability in uniformly heated microchannels was observed. This work develops a practical modeling approach for two-phase microchannel heat sinks and considers discrepancy between flow patterns of air- water and steam- water flow in microchannels.

A uniform temperature heat sink for cooling of electronic devices

Experimental investigation of a heat sink for cooling of electronic devices is performed. The objective is to keep the operating temperature at a relatively low level of about 323–333 K, using a dielectric liquid that boils at a lower temperature, while reducing the undesired temperature variation in the both streamwise and transverse directions. The experimental study is based on systematic measurements of temperature, flow and pressure, infrared radiometry and high-speed digital video imaging. The heat sink has parallel triangular microchannels with a base of 250 μm . Experiments on flow boiling of Vertrel XF in the microchannel heat sink are performed to study the effect of mass velocity and vapor quality on the heat transfer, as well as to compare the two-phase results to a heat sink cooled by single-phase water flow.

Co-current air–water two-phase flow patterns in vertical triangular microchannels

Characteristics of co-current upward air–water two-phase flow patterns in vertical equilateral triangular channels with hydraulic diameters of 2.886, 1.443 and 0.866 mm were investigated experimentally. Flow regimes were identified by both visual observations using a high-speed motion analyzer and dynamic pressure-drop measurements. The experimental results show that the typical flow patterns encountered in the conventional, large-sized vertical circular tubes, such as dispersed bubbly flow, slug flow, churn flow and annular flow, were also observed in the channels having larger hydraulic diameters ( d h=2.886 and 1.443 mm). However, for the smallest channel ( d h=0.866 mm), dispersed bubbly flow pattern, characterized by randomly dispersed bubbles in continuous liquid phase, was not found, although the other typical flow patterns remained in the channel. Moreover, the experiments reveal that, for the channel of d h=0.866 mm, a so-called capillary bubbly flow pattern, characterized by a single train of bubbles, essentially ellipsoidal in shape and spanning almost the entire cross-section of the channel, existed at low gas flow rates. It is further found that in the slug flow regime, slug-bubbles were substantially elongated. Finally, flow regime transition boundaries for the triangular microchannels were compared with relevant flow regime transition models and correlations as well as the existing experimental data for small round tubes and square channels.

A steady state, one dimensional, model for boiling two phase flow in triangular micro-channel

The potential advantages of triangular micro-channels incorporated into heat generating small devices are discussed. A simple one dimensional model of boiling two-phase flow and heat transfer in a single triangular micro-channel is investigated. The flow of the liquid phase inside the micro-channels is driven by surface tension and friction forces that exist at the interface between the fast moving vapor and liquid. The flow of the vapor phase is controlled by the heat flux generated and removed from the device. As the liquid flows through the channel it evaporates, its cross-section diminishes and the radius of curvature at the liquid vapor interface decreases. Thus, according to Young–Laplace equation, the liquid–vapor pressure difference increases along the channel. Consequently, a large decrease in the liquid pressure along the channel is obtained if the vapor pressure remains almost uniform. That pressure drop in the liquid phase is responsible for the onset of liquid flow. Along the micro-channel the increasing amount of generated vapor causes vapor velocity to increase and friction forces exerted on the liquid phase become significant until dry-out occurs. Since in the dry-out zone the heat transfer is drastically diminished, dry-out length estimates are of major concern in micro-channel design. A solution of a first order non-linear differentiated equation is required to predict dry-out lengths and their dependence on the dimensionless parameters governing the flow. A numerical simulation was carried out for the case of water flowing in a vertical channel of equilateral triangular cross-section. Hydraulic diameters from 0.1 to 1 mm, heat fluxes from 10 to 600 W/cm 2 and contact angles of 5° to 40° were assumed. The results validate the basic assumption that vapor pressure along the micro-channel is almost uniform. In many practical applications the differential equation can be simplified and solved analytically and the dry-out length are determined via a solution of an algebraic equation. Finally, it was demonstrated that the dry-out lengths seem to fit the dimensions of microelectronic devices.

NUCLEATION AND THERMODYNAMIC NONEQUILIBRIUM FOR BOILING IN MICROCHANNELS AND MICROSTRUCTURES

We review recent experimental and theoretical investigations that have contributed to developments and advances in boiling phase-change and transport phenomena in microchannels and microstructures, including bubble formation during boiling, phase transition, and the fluid flow and heat transfer characteristics. These investigations showed that bubble formation and phase transition in open and confined microstructures and the flow boiling heat transfer of pure water, pure methanol, and water-methanol binary mixtures through rectangular and triangular microchannels display some unusual phenomena and characteristics. The experimental results showed that the microchannel size, the geometric configuration including the aspect ratio (H/W) of the rectangular microchannels and the groove angle (6) of triangular microchannels, and the mole fraction of water-methanol binary mixtures have a significant impact on the flow boiling, especially for nucleate boiling, and on bubble formation and growth in microchannels. For boiling and nucleation in microchannels, two new concepts, evaporating space and fictitious boiling, were proposed to explain the unusual phase-change transport phenomena. We use classical thermodynamics to analyze the phase transition in microchannels and the thermodynamic aspects of boiling. For determining the phase transition in microchannels. we derive a dimensionless number and related criterion. The nonequilibrium effects during bubble formation and phase-change transitions are discussed theoretically, and the dynamics of microchannel flow boiling are analyzed to understand the phenomena. We propose a cluster model to describe the bubble formation and dynamic flow boiling process. The model shows that the phase-change transition and bubble formation are the consequence of cluster growth and interaction among clusters. The properties of nonequilibrium liquids with clusters are expected to be quite different from normal liquids, since they result in unconventional heat transport phenomena.

5560423 Flexible heat pipe for integrated circuit cooling apparatus

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.