Linan Jiang

Forced convection boiling in a microchannel heat sink

Micromachining technology was utilized to fabricate a transparent microchannel heat-sink system by bonding glass to a silicon wafer. The micro heat sink consisted of a microchannel array, a heater, and a temperature sensor array. This integrated microsystem allowed simultaneous qualitative visualizations of the flow pattern within the microchannels and quantitative measurements of temperature distributions, flow rates, and input power levels. Boiling curves of temperature as a function of the input power were established. No boiling plateau was observed in the boiling curves, consistent with our previously reported data but different from results reported for macrochannel heat sinks. Three stable boiling modes, depending on the input power level, have been distinguished from the flow patterns. Local nucleation boiling was observed in microchannels with a hydraulic diameter as small as 26 /spl mu/m at the lower input power range. At the higher input power range, a stable annular flow was the dominant boiling mode. Bubbly flow, commonly observed in macrochannels, could not be developed in the present microchannels. Consequently, no boiling plateau was detected in the boiling curves.

Phase change in microchannel heat sinks with integrated temperature sensors

A unique technique of mask-less and self-aligned silicon etch between bonded wafers was developed and applied to fabricate a microchannel heat sink integrated with a heater and an array of temperature sensors. The technique allowed the formation of self-aligned and self-stopped etching of grooves between the bonded wafers. The device, consisting of distributed temperature microsensors, allowed direct temperature measurements for different levels of power dissipation under forced convection using either nitrogen or water as working fluids. The measured temperature distributions are used to characterize the micro heat sink performance under forced convection boiling conditions. The onset of critical heat flux (CHF) condition was investigated for different channel sizes and liquid flow-rates. The results suggest that the bubble dynamic mechanism in the microchannel might be different compared with conventional channels.

Subsonic gas flow in a straight and uniform microchannel

A nonlinear equation based on the hydrodynamic equations is solved analytically using perturbation expansions to calculate the flow field of a steady isothermal, compressible and laminar gas flow in either a circular or a planar microchannel. The solution takes into account slip-flow effects explicitly by utilizing the classical velocity-slip boundary condition, assuming the gas properties are known. Consistent expansions provide not only the cross-stream but also the streamwise evolution of the various flow parameters of interest, such as pressure, density and Mach number. The slip-flow effect enters the solution explicitly as a zero-order correction comparable to, though smaller than, the compressible effect. The theoretical calculations are verified in an experimental study of pressure-driven gas flow in a long microchannel of sub-micron height. Standard micromachining techniques were utilized to fabricate the microchannel, with integral pressure microsensors based on the piezoresistivity principle of operation. The integrated microsystem allows accurate measurements of mass flow rates and pressure distributions along the microchannel. Nitrogen, helium and argon were used as the working fluids forced through the microchannel. The experimental results support the theoretical calculations in finding that acceleration and non-parabolic velocity profile effects were found to be negligible. A detailed error analysis is also carried out in an attempt to expose the challenges in conducting accurate measurements in microsystems.

Fabrication and characterization of a microsystem for a micro-scale heat transfer study

A micro-system consisting of micro-channels with integrated temperature sensors was successfully designed and fabricated for the study of the heat-transfer properties of fluid flow in micro-domains. Surface micro-machining technology was used to construct the micro-channels with dimensions of about 40 µm × 1.4 µm × 4000 µm. Polysilicon thermistors, 4 µm × 4 µm × 0.4 µm in size were suspended across the channels and directly exposed to the fluid for local temperature measurements. The micro-channels and the micro-sensors were calibrated and the micro-system was characterized. The integrated micro-system performance was theoretically analysed to provide a framework for the interpretation of the experimental data, and the various heat-transfer mechanisms are subsequently discussed.

A micro-channel heat sink with integrated temperature sensors for phase transition study

A unique technique of maskless and self-aligned Si etch between bonded wafers was developed and applied to fabricate a micro-channel heat sink integrated with a heater and an array of temperature sensors. The technique allowed the formation of self-aligned and self-stopped etching of grooves between the bonded wafers. The device, consisting of distributed sensors, allowed direct temperature measurements to evaluate local heat transfer rates under forced convection boiling conditions. Temperature distributions along the heat sink were measured for different levels of power dissipation. The onset of critical heat flux condition was investigated as a function of channel size and liquid flow rate. The results suggest that the bubble dynamic mechanism in micro-channels maybe different from that with conventional channels.

Germanium as a versatile material for low-temperature micromachining

Though germanium (Ge) shares many similar physical properties with silicon (Si), it also possesses unique characteristics that are complementary to those of Si. The advantages of Ge include its compatibility with Si microfabrication, its excellent gas and liquid phase etch selectivity to other materials commonly used in Si micromachining, and its low deposition temperature (<350/spl deg/C) that potentially allows Ge to be used after the completion of a standard CMOS run. Wider applications of Ge as a structural, sacrificial, and sensor material require a more systematic investigation of its processing and properties. The results of such an undertaking are presently reported. The topics covered are the formation of Ge thin films and novel application of the selective deposition of Ge to etch hole filling, characterization of the effects of thermal treatment on the evolution of the residual stress in Ge thin films, etch selectivity for etch mask and sacrificial layer applications, and gas phase release technique for stiction elimination.

Micromachined polycrystalline thin film temperature sensors

Polycrystalline thin films based on two elemental semiconducting materials, Si and Ge, have been utilized to fabricate microthermistors. The thermistors are designed in a heavy-light-heavy doping concentration arrangement. The design, fabrication, analysis and characterization of a variety of thermistors under different doping schemes is described. Finally, the operation of the thermistors in self-heating operation is discussed. The results provide a systematic framework for the application of semiconducting microthermoresistors.

Study of Boiling Regimes and Transient Signal Measurements in Microchannels

Two-phase microchannel heat exchangers can achieve very large heat removal rate due to the phase change of the coolant. However, the design of two-phase microchannel heat exchangers has not yet considered the change of boiling regimes in microchannels under 150 µm, while the boiling regime has significant impact on convective heat transfer coefficient. This paper presents our study of boiling regimes in these microchannels as well as the transient pressure fluctuation caused by nucleation. These results will provide quantitative information to others designing small-diameter parallel channel heat exchangers.

Phase change in microchannel heat sink under forced convection boiling

A microchannel heat sink system, consisting of parallel microchannels, distributed temperature micro-sensors and a local heater, has been fabricated and characterized. V-grooves with hydraulic diameter of either 40 /spl mu/m or 80 /spl mu/m were formed by bulk silicon etching. The heater and temperature microsensor array were fabricated using surface micromachining. Microchannels were realized by bonding a glass wafer to the silicon substrate, resulting in a transparent cover for flow visualization. Phase change during the boiling process was studied under forced convection conditions, where DI water was used as the working fluid. No boiling plateau, associated with latent heat, has been observed in the boiling curves of microchannel heat sinks. Flow visualization was carried out to understand the boiling mechanism in such a system. Three phase-change modes were observed depending on the input power level. Local nucleation boiling within the microchannels occurred at low power level. At moderate levels, large bubbles developed at the inlet/outlet regions, and the upstream bubbles were forced through the channels and out of the system. At higher input power levels, a stable annular flow mode was observed, where a thin liquid film coated each channel wall until critical heat flux conditions developed with a dryout of the system.

Unsteady characteristics of a thermal microsystem

A novel integrated microsystem consisting of a heater, microchannels and distributed temperature sensors was successfully fabricated. The transient temperature behavior of the device was experimentally studied for a variety of power dissipation levels and forced convection flow rates, where DI water was used as the working fluid. Both heating-up and cooling-down time constants were determined for a pulsed current input. The device frequency response to a sinusoidal input voltage, with the device operating either in a single or in a two-phase mode, was characterized. The dependence of the resulting temperature average and amplitude on the input signal was studied for a variety of cycle conditions. It was found that, contrary to expectations, the heating-up time constant was larger than the cooling-down time constant. Furthermore, the periodic temperature field stabilized the system to avoid the occurrence of the dryout phenomenon.