Man Lee

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.

Height effect on nucleation-site activity and size-dependent bubble dynamics in microchannel convective boiling

Forced convection boiling in microchannels is studied experimentally under the uniform heat flux boundary condition. Several microchannel heat sinks with integrated temperature sensors, spanning two orders of magnitude in height 5–500 µm, have been fabricated with designed nucleation sites on the bottom surfaces. The microchannels are capped by a glass wafer to monitor bubble activity using video microscopy. Distributed micro heater elements on the device backside are used as the heat source, while the working liquid flow rate is adjusted using a syringe pump. The boiling curves of the device temperature as a function of the input power have been measured for various flow rates. The curves for increasing and decreasing heat flux exhibit a hysteresis loop, while the conditions corresponding to the onset of nucleate boiling and critical heat flux (CHF) are clearly distinguishable. The activity of nucleation sites as well as the ensuing bubble dynamics, from incipience to departure, is found to depend on the channel height. The critical size above which a nucleation site is active, along with three aspects of bubble dynamics, namely growth rate, departure size and release frequency, have been characterized experimentally and proper control parameters have been identified. (Some figures in this article are in colour only in the electronic version)

Characterization of an integrated micro heat pipe

The characterization of a micro heat pipe system, integrated with a local heater, temperature and capacitive microsensors is presented. Two liquid charging schemes based on a single hole, requiring vacuum environment, and a pair of holes, utilizing capillary forces are compared. Taking advantage of the great disparity between the dielectric constants of liquids and gases, capacitance sensors are used for void fraction measurements. Since it is difficult to control the phase content of a liquid–gas mixture in a micro heat pipe, a calibration technique based on a traveling water–air interface due to evaporation is introduced. The integrated sensor capacitance for pure water is found to depend on measurement frequency, temperature and ion concentration, exhibiting trends that are different from previous reports. The measured temperature and void fraction distribution along the heat pipes are consistent with the two-phase flow patterns recorded during the microsystem operation.

Integrated micro-heat-pipe fabrication technology

This paper presents the design and fabrication of an integrated micro-heat-pipe system consisting of a heater, an array of heat pipes, temperature and capacitive sensors. Taking advantage of the large difference between the dielectric constants of liquid and vapor, the integrated capacitor can be used for void-fraction measurements in two-phase flows. Both CMOS-compatible and glass-based fabrication technologies are reported. In the CMOS-compatible technology, the heat pipes are capped by a thin nitride layer utilizing wafer bonding and etch back technique. In the glass-based technology, the heat pipes are covered by a glass substrate using die-by-die anodic bonding to allow visualization of the two-phase flow patterns. This approach also results in a significant reduction of the parasitic capacitance, thus enhancing the sensitivity of the capacitance sensor. A few particular problems related to this technology are discussed and proper solutions are proposed.

Design, fabrication and characterization of an integrated micro heat pipe

The design, fabrication and characterization of an integrated microsystem consisting of micro heat pipes, a micro heater, temperature and capacitive microsensors are presented. CMOS-compatible micromachining techniques are utilized to fabricate the micro heat pipe device capped by a nitride layer. In order to allow clear visualization of flow patterns during operation, the process has been modified using a glass wafer to cap the heat pipes. Temperature distributions along the micro heat pipes have been measured using the microsensors located next to the heat pipes. The capacitive microsensors have been used to measure the void-fraction, taking advantage of the large difference between the dielectric constants of the liquid and vapor phases.

Single-phase liquid flow forced convection under a nearly uniform heat flux boundary condition in microchannels

A microchannel heat sink, integrated with pressure and temperature microsensors, is utilized to study single-phase liquid flow forced convection under a uniform heat flux boundary condition. Utilizing a wafer–bond-and-etch-back technology, the heat source, temperature and pressure sensors are encapsulated in a thin composite membrane capping the microchannels, thus allowing experimentally good control of the thermal boundary conditions. A three-dimensional physical model has been constructed to facilitate numerical simulations of the heat flux distribution. The results indicate that upstream the cold working fluid absorbs heat, while, within the current operating conditions, downstream the warmer working fluid releases heat. The Nusselt number is computed numerically and compared with experimental and analytical results. The wall Nusselt number in a microchannel can be estimated using classical analytical solutions only over a limited range of the Reynolds number, Re: both the top and bottom Nusselt numbers approach 4 for Re 100. The experimentally estimated Nusselt number for forced convection is highly sensitive to the location of the temperature measurements used in calculating the Nusselt number.

Design, fabrication and characterization of a thermal microsystem integrated with heaters, pressure and temperature microsensors

The design, fabrication and characterization of an intergrated thermal microsystem are presented. The system, consisting of thin-film heater elements, an array of microchannels, pressure and temperature microsensor arrays, is designed for studying forced convection heat transfer under well-controlled thermal boundary conditions. Utilizing a wafer bond and etch back technology, the heat source, pressure and temperature sensors are separated from the fluid flow by a membrane only 1.5 µm in thickness, thus allowing experimentally improved approximation of classical boundary conditions, especially the uniform heat flux at the solid/fluid interface. A three-dimensional simulation model is constructed for numerical analysis to complement the experimental characterization of the liquid single-phase flow in this microsystem. Pressure and temperature distributions, for various operating conditions, have been measured and compared with computed profiles. The agreement between the experimental and numerical results confirms that, though not ideal, the heat flux boundary condition is nearly uniform.

Two-Phase Flow in Microchannel Heat Sink with Nearly Uniform Heat Flux Boundary Condition

A thermal microsystem, integrated with pressure and temperature microsensors, is fabricated to study convective boiling under nearly uniform heat flux boundary condition. The temperature and pressure distribution along the microchannel is measured correspondingly. The pressure increases with input power when two phase flow develops. A pressure peak appears at the location of liquid-vapor interface region. The transient temperature and pressure fluctuation is also measured. The dominant frequencies of the temperature and pressure fluctuation are the same values at the liquid-vapor interface region and this dominant frequencies increase with input power. Simultaneously, the qualitative visualizations of the evolving flow patterns have been correlated with the quantitative temperature and pressure measurements.

Size and shape effects on two-phase flow instabilities in microchannels

An integrated microchannel heat sink consisting of shallow, trapezoidal 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 response of the system. An integrated heater provides the local heat source, while an array of temperature microsensors is used for temperature distribution measurements. DI water, serving as the working fluid, is pressurized through the microchannels for forced heat convection. Temperature plateaus are observed in the boiling curves, corresponding to the latent heat of phase change of the working fluid from liquid to vapor phase. The evolving two-phase flow patterns have been recorded and analyzed using high-speed camera. Bubble formation, growth and detachment at specific nucleation sites have been observed. Annular flow mode has been found to be unstable in trapezoidal channels. Instead, a highly unsteady transition region from upstream vapor phase to downstream liquid phase flow is developed, and the average location of this region depends on the input power.

The Nusselt Number in Single-Phase Liquid Flow Forced Convection in Microchannels

A thermal microsystem, integrated with pressure and temperature microsensors, is fabricated to study single-phase liquid flow forced convection under uniform heat flux boundary condition. Standard micromachining techniques were utilized in the fabrication of the integrated microsystem. Utilizing a wafer-bond-and-etch-back technology, the heat source, temperature and pressure sensors are separated from the fluid flow by a 1.5 mum thick composite membrane; thus, allowing experimentally good control of the thermal boundary conditions. A three-dimensional numerical simulation model has been constructed to investigate the heat flux distribution. The results show that upstream the cold working fluid absorbs heat, while downstream the warmer working fluid releases heat. The Nusselt number is calculated based on the computations, which are compared with analytical and experimental results. The wall Nusselt number in a microchannel can only be estimated by conventional analytical solution in a limited Reynolds number range. The estimated Nusselt number for forced convection is found to be highly dependent on the location of the temperature measurements.