Seed bubbles stabilize flow and heat transfer in parallel microchannels

Seed bubbles are generated on microheaters located at the microchannel upstream and driven by a pulse voltage signal, to improve flow and heat transfer performance in microchannels. The present study investigates how seed bubbles stabilize flow and heat transfer in micro-boiling systems. For the forced convection flow, when heat flux at the wall surface is continuously increased, flow instability is self-sustained in microchannels with large oscillation amplitudes and long periods. Introduction of seed bubbles in time sequence improves flow and heat transfer performance significantly. Low frequency (similar to 10 Hz) seed bubbles not only decrease oscillation amplitudes of pressure drops, fluid inlet and outlet temperatures and heating surface temperatures, but also shorten oscillation cycle periods. High frequency (similar to 100 Hz or high) seed bubbles completely suppress the flow instability and the heat transfer system displays stable parameters of pressure drops, fluid inlet and outlet temperatures and heating surface temperatures. Flow visualizations show that a quasi-stable boundary interface from spheric bubble to elongated bubble is maintained in a very narrow distance range at any time. The seed bubble technique almost does not increase the pressure drop across microsystems, which is thoroughly different from those reported in the literature. The higher the seed bubble frequency, the more decreased heating surface temperatures are. A saturation seed bubble frequency of 1000-2000 Hz can be reached, at which heat transfer enhancement attains the maximum degree, inferring a complete thermal equilibrium of vapor and liquid phases in microchannels. Benefits of the seed bubble technique are the stabilization of flow and heat transfer, decreasing heating surface temperatures and improving temperature uniformity of the heating surface.

Chaotic behavior of pulsating heat pipes

Chaotic behavior of closed loop pulsating heat pipes (PHPs) was studied. The PHPs were fabricated by capillary tubes with outer and inner diameters of 2.0 and 1.20 mm. FC-72 and deionized water were used as the working fluids. Experiments cover the following data ranges: number of turns of 4, 6, and 9, inclination angles from 5 degrees (near horizontal) to 90, (vertical), charge ratios from 50% to 80%, heating powers from 7.5 to 60.0 W. The nonlinear analysis is based on the recorded time series of temperatures on the evaporation, adiabatic, and condensation sections. The present study confirms that PHPs are deterministic chaotic systems. Autocorrelation functions (ACF) are decreased versus time, indicating prediction ability of the system is finite. Three typical attractor patterns are identified. Hurst exponents are very high, i.e., from 0.85 to 0.95, indicating very strong persistent properties of PHPs. Curves of correlation integral versus radius of hypersphere indicate two linear sections for water PHPs, corresponding to both high frequency, low amplitude, and low frequency, large amplitude oscillations. At small inclination angles near horizontal, correlation dimensions are not uniform at different turns of PHPs. The non-uniformity of correlation dimensions is significantly improved with increases in inclination angles. Effect of inclination angles on the chaotic parameters is complex for FC-72 PHPs, but it is certain that correlation dimensions and Kolmogorov entropies are increased with increases in inclination angles. The optimal charge ratios are about 60-70%, at which correlation dimensions and Kolmogorov entropies are high. The higher the heating power, the larger the correlation dimensions and Kolmogorov entropies are. For most runs, large correlation dimensions and Kolmogorov entropies correspond to small thermal resistances, i.e., better thermal performance, except for FC-72 PHPs at small inclination angles of theta < 15 degrees.

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 he 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 55.4 mu m were performed using acetone as the working fluid, having the inlet liquid temperatures of 24-40 degrees C, mass fluxes of 96-360 kg/m(2)s, heat fluxes of 140-420 kW/m(2), and exit vapor mass qualities of 0.28-0.70. The above data range correspond to the boiling number from 1.574 x 10(-3) to 3.219 x 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 he dependent on the heat Axes only, not dependent on the mass Axes 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 he small in a couple of degrees, providing a better thermal management scheme for the high heat flux electronic components. Chen's [11 widely accepted correlation for macrochannels and Bao et al.'s [21 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.

Numerical simulations of interrupted and conventional microchannel heat sinks

We provide three-dimensional numerical simulations of conjugate heat transfer in conventional and the newly proposed interrupted microchannel heat sinks. The new microchannel heat sink consists of a set of separated zones adjoining shortened parallel microchannels and transverse microchambers. Multi-channel effect, physical property variations, and axial thermal conduction are considered. It is found that flow rate variations in different channels can be neglected, while heat received by different channels accounts for 2% deviations from the averaged value when the heat flux at the back surface of the silicon chip reaches 100 W/cm(2). The computed hydraulic and thermal boundary layers are redeveloping in each separated zone due to shortened flow length for the interrupted microchannel heat sink. The periodic thermal developing flow is responsible for the significant heat transfer enhancement. Two effects influence pressure drops across the newly proposed microchannel heat sink. The first one is the pressure recovery effect in the microchamber, while the second one is the head loss when liquid leaves the microchamber and enters the next zone. The first effect compensates or suppresses the second one, leading to similar or decreased pressure drop than that for the conventional microchannel heat sink, with the fluid Prandtl number larger than unity.

Pool boiling heat transfer of ultra-light copper foam with open cells

High speed visualizations and thermal performance studies of pool boiling heat transfer on copper foam covers were performed at atmospheric pressure, with the heating surface area of 12.0 mm by 12.0 mm, using acetone as the working fluid. The foam covers have ppi (pores per inch) from 30 to 90, cover thickness from 2.0 to 5.0 mm, and porosity of 0.88 and 0.95. The surface superheats are from -20 to 190 K, and the heat fluxes reach 140 W/cm(2). The 30 and 60 ppi foam covers show the periodic single bubble generation and departure pattern at low surface superheats. With continuous increases in surface superheats, they show the periodic bubble coalescence and/or re-coalescence pattern. Cage bubbles were observed to be those with liquid filled inside and vented to the pool liquid. For the 90 ppi foam covers, the bubble coalescence takes place at low surface superheats. At moderate or large surface superheats, vapor fragments continuously escape to the pool liquid. Boiling curves of copper foams show three distinct regions. Region I and II are those of natural convection heat transfer, and nucleate boiling heat transfer for all the foam covers. Region III is that of either a resistance to vapor release for the 30 and 60 ppi foam covers, or a capillary-assist liquid flow towards foam cells for the 90 ppi foam covers. The value of ppi has an important effect on the thermal performance. Boiling curves are crossed between the high and low ppi foam covers. Low ppi foams have better thermal performance at low surface superheats, but high ppi foams have better one at moderate or large surface superheats and extend the operation range of surface superheats. The effects of other factors such as pool liquid temperature, foam cover thickness on the thermal performance are also discussed.

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.

Optimization and comparison of single- and dual-pump fiber-optical parametric amplifiers with dispersion fluctuations

Solutions for fiber-optical parametric amplifiers (FOPAs) with dispersion fluctuations are derived using matrix operators. On the basis of the propagation matrix product and the hybrid genetic algorithm, we have optimized and compared single- and dual-pump FOPAs with zero-dispersion-wavelength variations. The simulations prove that the design of FOPAs involves multimodal function optimization problems. The numerical results show that dual-pump FOPAs are highly sensitive to dispersion fluctuations whereas dispersion variations have less impact on the gain of single-pump FOPAs. To increase signal gain and reduce ripple, dual-pump FOPAs, instead of single-pump FOPAs, have to be carefully optimized with a suitable multisegment fiber structure rather than a one-segment fiber structure. The different combinations of multisegment fibers can provide highly different gain properties. The increase in gain is at the cost of the ripple.

Reducing group-velocity-dispersion-dependent broadening of stimulated Brillouin scattering slow light in an optical fiber by use of a single pump laser

We propose a method of effectively extending the stimulated Brillouin scattering (SBS) gain bandwidth in a single-mode optical fiber to reduce group-velocity-dispersion (GVD)-dependent pulse spread of SBS slow light. This can be done by overlapping doublet SBS gain spectra synthesized from a single pump laser. Numerical calculations are performed to verify our proposed method. We find that there exists the optimum spectral separation between two center frequencies of the doublet SBS gain spectrum with respect to the inherent spectral width of the pump laser, which makes it possible to effectively reduce the signal pulse broadening due to GVD. We show that the maximum time delay of the amplified signal pulse can be approximately two times longer than that by a previously reported method using a single broadband pump laser. (c) 2008 Optical Society of America.

Zero-broadening SBS slow light propagation in an optical fiber using two broadband pump beams

A new method of tailoring stimulated Brillouin scattering (SBS) gain spectrum for slow light propagation is proposed by use of two Gaussian-shaped broadband pump beams with different powers and spectral widths. The central frequency interval between the two pump beams are carefully set to be two inherent Brillouin frequency shift, ensuring that the gain spectrum of one pump has the same central frequency with the loss spectrum of the other one. Different gain profiles are obtained and analyzed. Among them a special gain profile is found that ensures a zero-broadening of the signal pulse independent of the Brillouin gain. This is owing to the compensation between the positive gain-dependent broadening and the negative GVD (group velocity dispersion) dependent broadening. The relationship of two pump beams is also found for constructing such a gain profile. It provides us a new idea of managing the broadening of SBS-based slow pulse by artificially constructing and optimizing the profile of gain spectrum. (c) 2008 Optical Society of America.