This paper reports two-phase cooling in compact cross-flow microchannel heat exchangers with high power density up to 180 W/cm(3). The performance is enabled by high-speed air flow through microchannels and two-phase condensation of refrigerant R245fa. The heat exchangers were realized in 1 cm(3) blocks of copper alloy, using micro-electrical-discharging machining. Two heat exchanger designs were analyzed, fabricated, and tested. The first device has 150 air-side channels of diameter 520 mu m, and the second device has 300 air-side channels of diameter 355 mu m. In both cases the refrigerant channels are 2.0 x 0.5 mm(2). The heat exchangers were operated with Reynolds number between 7500 and 20,500 for the air flow and with mass flux between 330 and 750 kg/m(2) s for the refrigerant flow. The refrigerant temperature at the channel entrance was 80 degrees C, which is near the maximum operating temperature for some electronic devices. For comparison purposes, the devices were also tested with single-phase refrigerant flows. This work demonstrates the potential of high power density heat exchangers that leverage advanced manufacturing technologies to fabricate miniature channels.

We present single-phase heat transfer in a compact cross-flow microchannel heat exchanger, with air flowing through the heat exchanger to remove heat from a closed-loop flow of refrigerant R245fa. The 1 cm(3) heat exchanger was monolithically fabricated from a block of copper alloy using micro electrical-discharge machining. Air carrying channels of diameter 520 gm were oriented in cross-flow to the refrigerant-carrying channels of size 2.0 x 0.5 mm(2). High-speed air flowed with Reynolds number between 1.2 x 10(4) and 2.05 x 10(4), which corresponded to air speeds between 20 and 100 m/s, while refrigerant flowed at Reynolds number between 1000 and 2300. Using an equivalent fin model and finite element simulations, we predicted the heat exchanger performance and used the simulations to interpret the measured behavior. Temperature, pressure, and flow rates were measured over a variety of operating conditions to determine heat transfer rate, j-factor, and friction factor. We observed a maximum power density of 60 W/cm(3) when the air inlet temperature was 27 degrees C and the refrigerant inlet temperature was 80 degrees C. The high speed of air flow caused large friction on the air side, resulting in goodness factor j/f near 0.5. This work demonstrates that high power density can be achieved in miniature heat exchangers, and that micromachined metal devices can enable this performance. The results could be broadly applied to other types of microchannel devices. (C) 2017 Elsevier Ltd. All rights reserved.

This work proposes a theoretical model for predicting the apparent equilibrium contact angle of a liquid on an ideal rough surface that is homogeneous and has a negligible body force, line tension, or contact angle hysteresis between solid and liquid. The model is derived from the conservation equations and the free-energy minimization theory for the changes of state of liquid droplets. The work of adhesion is expressed as the contact angles in the wetting process of the liquid droplets. Equilibrium contact angles of liquid droplets for rough surfaces are expressed as functions of the area ratios for the solid, liquid, and surrounding gas and the roughness ratio and wetting ratio of the liquid on the solid for the partially and fully wet states. It is found that the ideal critical angle for accentuating the contact angles by the surface roughness is 48 degrees. The present model is compared with existing experimental data and the classical Wenzel and Cassie-Baxter models and agrees with most of the experimental data for various surfaces and liquids better than does the Wenzel model and accounts for trends that the Wenzel model cannot explain.

An experimental and numerical study of a planar freezing front propagating in a water layer above microgrooved substrates is presented. Classical photolithographic technology is employed to fabricate the microgrooves, and the morphological effect on the front propagation speed is quantified and compared with that predicted by the numerical simulation. The simulation is performed using enthalpy method and the finite element analysis package FIDAP in order to understand the physical mechanisms. The experimental results show that the speed of a freezing front oscillates when the front moves across the adjacent crests and troughs of microgrooves. The propagation speed on crests is about two to eight times that in troughs. The simulation results agree well with experiments and demonstrate that the silicon crests change the heat transfer direction into vertical as the latent heat is released from the freezing front, leading to a fast propagation on crests. The shape of the freezing front, the impact of the sample geometry, and the cooling rate of the system are also reported and discussed. The findings provide insight into how the speed and shape of a freezing front can be manipulated and might find broad application in systems with solidification.

High porosity metal foams with novel thermal, mechanical, electrical, and acoustic properties are being more widely adopted for application. Due to their large surface-area-to-volume ratio and complex structure which induces better fluid mixing, boundary layer restarting and wake destruction, they hold promise for heat transfer applications. In this paper, the thermal-hydraulic performance of open-cell aluminum metal foam heat exchanger has been evaluated. The impact of flow conditions and metal foam geometry on the heat transfer coefficient and gradient have been investigated. Metal foam heat exchanger with same geometry (face area, flow depth and fin dimensions) consisting of four different type of metal foams have been built for the study. Experiments are conducted in a closed-loop wind tunnel at different flow rate under dry operating condition. Metal foams with a smaller pore size (40 PPI) have a larger heat transfer coefficient compared to foams with a larger pore size (5 PPI). However, foams with larger pores result in relatively smaller pressure gradients. Current thermal-hydraulic modeling practices have been reviewed and potential issues have been identified. Permeability and inertia coefficients are determined and compared to data reported in open literature. On the basis of the new experimental results, correlations are developed relating the foam characteristics and flow conditions through the friction factor f and the Colburn j factor. (C) 2017 Elsevier Ltd. All rights reserved.

Drainage of frost melt water from a number of microgrooved brass surfaces, fabricated by micro-end-milling process, is investigated experimentally and compared to that of a flat baseline surface. Frost is grown on sample surfaces (45 mm x 45 mm in dimension) inside a thermally controlled chamber, at a plate temperature of about -25 degrees C in the presence of cold air (-6 degrees C) and retention of water on these surfaces after defrosting is studied for three different defrosting energy inputs. Microgrooved surfaces drain up to 70% more condensate than does the flat baseline. The groove geometry is found to considerably affect the water retention. Drainage is promoted by an increase in the pillar width but is relatively insensitive to the changes in the groove depth. Effects of defrosting heating rate on the frost surface temperature and substrate temperature during defrosting process is also investigated. Frost mass per unit area is found to be lower for the grooved surfaces in the 1st frost and 2nd frost cycles. (C) 2011 Elsevier Ltd. All rights reserved.

This paper investigates heat transfer enhancement by means of additively manufactured static mixers during liquid water cooling of a horizontal, heated flat plate. The static mixers disrupt the thermal boundary layer and induce mixing, resulting in an increased heat transfer rate of about 2X larger than flows without mixers. Simulations of the flows provided insights into the flows near the mixers, and guided selection of specific mixer geometries. The mixers were fabricated directly into the flow channels using additive manufacturing and then assembled onto the heated plate. Two types of mixing structures were analyzed: twisted tape structures that are similar to conventional static mixers; and novel chevron shaped offset wing structures. Heat transfer performance was measured for liquid water (510 <=3D Re <=3D 1366) cooling the heated section with convective heat flux ranging between 0.1 and 0.8 W/cm(2). This work demonstrates the potential of additive manufacturing to enable novel flow geometries that can enhance convection heat transfer whilst minimizing pressure drop penalties and volumes. (C) 2019 Elsevier Ltd. All rights reserved.

The objective of this study is to develop an accurate, reliable, and updated predictive model for the air-side performance of flat-tube louver-fin heat exchangers. Using the most comprehensive experimental database to date-consisting of 1030 heat-transfer and 1270 pressure-drop measurements, from nine independent laboratories for 126 sample heat exchangers-j- and f-factor correlations are developed to predict the air-side performance of heat exchangers. The database is analyzed, the form of the curve fits is explored, and the predictive performance of the correlations is evaluated. The j- and f-factor correlations predict the experimental data with rms errors of 11.5% and 16.1%, respectively. Multiple regressions for a locally linearized data model were used to estimate the confidence intervals and covariances of the regression constants. A comparison to prior correlations shows the proposed correlations to provide more accurate predictions and to span a much broader parameter space than prior work. Practical utility in design and optimization, and unavoidable limitations in developing such correlations are discussed.

A technique for fabricating micropatterned aluminum surfaces with parallel grooves 30 mu m wide and tens of microns in depth is described. Standard photolithographic techniques are used to obtain this precise surface-feature patterning. Positive photoresists, S1813 and AZ4620, are selected to mask the surface, and a mixture of BCl3 and Cl-2 gases is used to perform the etching. Experimental data show that a droplet placed on the micro-grooved aluminum surface using a micro-syringe exhibits an increased apparent contact angle, and for droplets condensed on these etched surfaces, more than a 50% reduction in the volume needed for the onset of droplet sliding is manifest. No chemical surface treatment is necessary to achieve this water repellency; it is accomplished solely by an anisotropic surface morphology that manipulates droplet geometry and creates and exploits discontinuities in the three-phase contact line. These micro-structured surfaces are proposed for use in a broad range of air-cooling applications, where the management of condensate and defrost liquid on the heat transfer surface is essential to the energy-efficient operation of the machine.

Thermal analysis with comprehensive treatment of conjugate heat transfer is performed in this study for discrete flush-mounted heat sources in a horizontal channel cooled by air. The numerical model accounts for mixed convection, radiative exchange and two-dimensional conduction in the substrate. The model is first used to simulate available experimental work to demonstrate its accuracy and practical utility. A parametric study is then undertaken to assess the effects of Reynolds number, surface emissivity of walls and heat sources, as well as thickness and thermal conductivity of substrate, on flow field and heat transfer characteristics. It is shown that due to radiative heat transfer, the wall temperatures are brought closer, and the trend of temperature variation along the top wall is significantly altered. Such effects are more pronounced for higher surface emissivity and/or lower Reynolds numbers. The influence of substrate conductivity and thickness is related in that a large value of either substrate conductivity or thickness facilitates redistribution of heat and tends to yield a uniform temperature field in the substrate. For highly conductive or thick substrate, the "hot spot" cools down and may occur in upstream sources. Radiation loss to the ambient increases with substrate conductivity and thickness due to the elevated temperature near the openings, yet the total heat transfer over the bottom surface by convection and radiation remains essentially unaltered. [DOI:10.1115/1.4005299]

This paper presents the fabrication of metallic micro-mushroom re-entrant structures and the characterization of their hydrophobicity and oleophobicity. Five different microstructure geometries are introduced, with typical feature sizes in the range of 10-100 mu m. These microstructures are realized in steel, and are fabricated over the cm-scale using micro electrical discharge machining (mEDM). The liquid repellency of these surfaces is characterized using droplets of either water (surface energy gamma(lg) = 72.4 mN m(-1)), RL-68H oil (gamma(lg) = 28.6 mN m(-1)), or Isopropanol (IPA) (gamma(lg) = 21.7 mN m(-1)). The water droplets form nearly perfect spheres, with contact angles in the range 146-162 degrees, and contact angle hysteresis of 19-35 degrees. The oil droplet contact angles are in the range 106-152 degrees and the IPA contact angles are in the range 75-123 degrees. Strong re-entrant features and close spacing are necessary to support a fully non-wetting state for use with oil and IPA. Water forms the highest contact angles with narrow, post-like, and widely spaced micro-mushroom geometries.