Search for: All records

Jet impingement can be particularly effective for removing high heat fluxes from local hotspots. Two-phase jet impingement cooling combines the advantage of both the nucleate boiling heat transfer with the single-phase sensible cooling. This study investigates two-phase submerged jet impingement cooling of local hotspots generated by a diode laser in a 100 nm thick Hafnium (Hf) thin-film on glass. The jet/nozzle diameter is ∼1.2 mm and the normal distance between the nozzle outlet and the heated surface is ∼3.2 mm. Novec 7100 is used as the coolant and the Reynolds numbers at the jet nozzle outlet range from 250 to 5000. The hotspot area is ∼ 0.06 mm2 and the applied hotspot-to-jet heat flux ranges from 20 W/cm2 to 220 W/cm2. This heat flux range facilitates studies of both the single-phase and two-phase heat transport mechanisms for heat fluxes up to critical heat flux (CHF). The temporal evolution of the temperature distribution of the laser heated surface is measured using infrared (IR) thermometry. This study also investigates the nucleate boiling regime as a function of the distance between the hotspot center and the jet stagnation point. For example, when the hotspot center and the jet are co-aligned (x/D = 0), the CHF is found to be ∼ 177 W/cm2 at Re ∼ 5000 with a corresponding heat transfer coefficient of ∼58 kW/m2.K. While the CHF is ∼ 130 W/cm2 at Re ∼ 5000 with a jet-to-hotspot offset of x/D ≈ 4.2.

 

Hemiwicking is the phenomena where a liquid wets a textured surface beyond its intrinsic wetting length due to capillary action and imbibition. In this work, we derive a simple analytical model for hemiwicking in micropillar arrays. The model is based on the combined effects of capillary action dictated by interfacial and intermolecular pressures gradients within the curved liquid meniscus and fluid drag from the pillars at ultra-low Reynolds numbers$${\boldsymbol{(}}{{\bf{10}}}^{{\boldsymbol{-}}{\bf{7}}}{\boldsymbol{\lesssim }}{\bf{Re}}{\boldsymbol{\lesssim }}{{\bf{10}}}^{{\boldsymbol{-}}{\bf{3}}}{\boldsymbol{)}}$$$\left({10}^{-7}\lesssim \mathrm{Re}\lesssim {10}^{-3}\right)$. Fluid drag is conceptualized via a critical Reynolds number:$${\bf{Re}}{\boldsymbol{=}}\frac{{{\bf{v}}}_{{\bf{0}}}{{\bf{x}}}_{{\bf{0}}}}{{\boldsymbol{\nu }}}$$$\mathrm{Re}=\frac{v_0{x}_0}{\nu }$, wherev₀corresponds to the maximum wetting speed on a flat, dry surface andx₀is the extension length of the liquid meniscus that drives the bulk fluid toward the adsorbed thin-film region. The model is validated with wicking experiments on different hemiwicking surfaces in conjunction withv₀andx₀measurements using Water$${\boldsymbol{(}}{{\bf{v}}}_{{\bf{0}}}{\boldsymbol{\approx }}{\bf{2}}\,{\bf{m}}{\boldsymbol{/}}{\bf{s}}{\boldsymbol{,}}\,{\bf{25}}\,{\boldsymbol{\mu }}{\bf{m}}{\boldsymbol{\lesssim }}{{\bf{x}}}_{{\bf{0}}}{\boldsymbol{\lesssim }}{\bf{28}}\,{\boldsymbol{\mu }}{\bf{m}}{\boldsymbol{)}}$$$\left(v_0\approx 2m/s,25\mu m\lesssim x_0\lesssim 28\mu m\right)$, viscous FC-70$${\boldsymbol{(}}{{\boldsymbol{v}}}_{{\bf{0}}}{\boldsymbol{\approx }}{\bf{0.3}}\,{\bf{m}}{\boldsymbol{/}}{\bf{s}}{\boldsymbol{,}}\,{\bf{18.6}}\,{\boldsymbol{\mu }}{\bf{m}}{\boldsymbol{\lesssim }}{{\boldsymbol{x}}}_{{\bf{0}}}{\boldsymbol{\lesssim }}{\bf{38.6}}\,{\boldsymbol{\mu }}{\bf{m}}{\boldsymbol{)}}$$$\left(v_0\approx 0.3m/s,18.6\mu m\lesssim x_0\lesssim 38.6\mu m\right)$and lower viscosity Ethanol$${\boldsymbol{(}}{{\boldsymbol{v}}}_{{\bf{0}}}{\boldsymbol{\approx }}{\bf{1.2}}\,{\bf{m}}{\boldsymbol{/}}{\bf{s}}{\boldsymbol{,}}\,{\bf{11.8}}\,{\boldsymbol{\mu }}{\bf{m}}{\boldsymbol{\lesssim }}{{\bf{x}}}_{{\bf{0}}}{\boldsymbol{\lesssim }}{\bf{33.3}}\,{\boldsymbol{\mu }}{\bf{m}}{\boldsymbol{)}}$$$\left(v_0\approx 1.2m/s,11.8\mu m\lesssim x_0\lesssim 33.3\mu m\right)$.

 

The demand for flexible microelectronics has increased significantly within the last decade. This study investigates the cooling performance of flexible pulsating heat pipes (PHPs) made from acrylic with a bend radius of ≈300 mm. The fabricated devices support two-phase, pulsating fluid flow inside the rectangular microchannels. Both water and ethanol are used as coolants, where local hot spots are generated by cobalt-alloy foil heaters inside the flexible PHPs. The PHP's dissipate the heat generated to the environment via copper condensers with controlled setpoint temperatures. Based on a heater surface area of ≈1.5 cm 2 and a condenser setpoint temperature of 25°C, the maximum heat flux observed for sustained and repeatable cooling with water and ethanol was 8 W /cm 2 . These heat fluxes correlate well with other PHP studies with similar heater power loads, channel geometries, and coolants. 

Jet impingement can be particularly effective for removing high heat fluxes from local hotspots. Two-phase jet impingement cooling combines the advantages of both the nucleate boiling heat transfer with the single-phase sensible cooling. This study investigates two-phase confined jet impingement cooling of local, laser-generated hotspots in a 100 nm thick Hafnium (Hf) thin film on glass. The jet/nozzle diameter is ∼1.2 mm and the normal distance between the nozzle outlet and the heated surface is ∼3.2 mm. The jet coolants studied are FC 72, Novec 7200, and Ethanol with jet nozzle outlet Reynolds numbers ranging from 250 to 5000. The hotspot area is ∼0.06 mm2 and the applied hotspot-to-jet heat fluxes range from 20 W/cm2 to 350 W/cm2. This heat flux range facilitates studies of both the single-phase and two-phase heat transport mechanisms for heat fluxes up to critical heat flux (CHF). The temporal evolution of the temperature distribution of the laser-heated surface is measured using infrared (IR) thermometry. This study focuses on the stagnation point heat transfer - i.e., the jet potential core is co-aligned with the hotspot center. For ethanol, the CHF is ∼315 W/cm2 at Re ∼ 1338 with a corresponding heat transfer coefficient of h ∼ 102 kW/m2·K. For FC 72, the CHF is ∼94 W/cm2 at Re ∼ 5000 with a corresponding h ∼ 56 kW/m2·K. And for Novec 7200, the CHF is ∼108 W/cm2 at Re ∼ 4600 with a corresponding h ∼ 50 kW/m2·K. 

Asymmetric microstructures are of particular interest to many technical fields. Such structures can produce anisotropic flow-fields, which, for example, can be used to control heat and mass transport processes. Anisotropic wicking structures can now be systematically engineered with unique micro-pillar geometries and spatial pillar-placement distributions. Such asymmetric wicking structure designs are of particular interest to the thermal management community due to need to cool heterogeneous materials with specific heat load configurations. In this study, asymmetric half-conical micropillars have been fabricated utilizing two-photon polymerization. Macroscopic characterization of anisotropic flow-field velocities is performed via high-speed videography. High-speed thin-film interferometry and microscopic side-angle videography are also used to characterize the microscale evolution of meniscus curvature during inter-pillar wicking. The wicking velocity is observed to be directly proportional to both the meniscus curvature and the cross-sectional area of the micro-pillars (normal to the flow). An anisotropic hemiwicking model is also described with comparisons to experimental data. The hemiwicking model predicts the macroscopic wicking behavior (within 20% or less) for the relatively broad range of pillar geometries and pillar spacing configurations. These anisotropic flow-field predictions can help engineers design the next-generation of micro-structured heat sinks, fluid-based sensors and chemical harvesting systems. 

Fabricating micro and nanosized structures to induce hemiwicking on a heated surface has risen in popularity due to the higher heat flux the surface can experience. Recent studies have focused on the effects on the pillar geometry and spacing on the wicking velocity and the critical heat flux. As a result, a majority of the models that have been derived focus on the fluid properties and the wicking structure geometry and spacing. This study presents changes to the wicking performance when the stiffness of a soft material is taken into effect. Multiple similar wicking structures were fabricated using a negative mold method utilizing an in-house stamping apparatus. Using the mold, multiple polydimethylsiloxane (PDMS) samples were created, where the stiffness of the samples was varied by altering the mixing ratio and the curing time. The wicking velocity of ethanol, isopropyl alcohol, and isooctane did not vary for the samples that had a Young's Modulus greater than 1 MPa, but a notable decrease in the wicking velocity for all three fluids were observed for samples with a Young's Modulus less than 1 MPa. This study provides insight to the importance of the stiffness of the material is for hemiwicking on soft materials and that deformation effects have to be taken into account for Young's Moduli less than 1 MPa. 

During air or liquid cooling, thermal resistance of the devices is measured precisely from the thermal information at the junction. But existing thermal measurement technologies fall short because of highly transient events such as unstable vortex formation (air cooling) and bubble growth (two-phase liquid cooling). In solving this problem, this paper reports a novel and low-cost thermal mapping technique that can capture highly spatio-temporal temperature evolution at the solid-liquid interface. Essentially, a robust interface is fabricated with CuInS 2 /ZnS Quantum dots (λ peak = 550 nm and 750 nm) trapped inside nanopores (20 nm-30 nm) of a ceramic membrane (50 μm) and/or everyday use paper. It is observed that such nanoconfinement assisted Quantum dots provided sustained thermal photoluminescence coefficient (-0.1 nm/°C) at high number of heating-cooling cycle. This unique yet low cost thermal mapping technique is applied to capture thermal evolution during micro-droplet impingement cooling and hemiwicking flows through anisotropic wicks which showcase commendable spatio-temporal benefits.