Mechanical Engineering

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Numerical and Experimental Study of a Novel Additively Manufactured Metal-Polymer Composite Heat-Exchanger for Liquid Cooling Electronics
(MDPI, 2022-01-14) Kalikhura, Gargi; Mandel, Raphael Kahat; Shooshtari, Amir; Ohadi, Michael
In order to meet increasing power-dissipation requirements of the electronics industry, compact, low-cost, and lightweight heat exchangers (HXs) are desired. With proper design, materials, and manufacture, polymer composite heat exchangers could meet these requirements. This paper presents a novel crossflow air-to-water, low-cost, and lightweight metal-polymer composite HX. This HX, which is entirely additively manufactured, utilizes a novel cross-media approach that provides direct heat exchange between air and liquid sides by using connecting fins. A robust numerical model was developed, which includes the dimensional effects of additive manufacturing. The study consists of a simplified 3D CFD model based on ellipsoidal-shaped staggered tube banks for the laminar range. It then uses an analytical approach to compute entire HX performance. The model is validated experimentally within 8% for thermal performance, 12% for air-side impedance, and 18% for water-side impedance. Finally, HX is compared with a conventional CPU radiator and performs within 10% of the conventional unit for reasonable flow rates and pressure-drop ranges. Moreover, HX also provides added design and cost advantages over the conventional unit, which makes the HX a potential candidate for electronic cooling applications.
A 1D Reduced-Order Model (ROM) for a Novel Latent Thermal Energy Storage System
(MDPI, 2022-07-14) Kailkhura, Gargi; Mandel, Raphael Kahat; Shooshtari, Amir; Ohadi, Michael
Phase change material (PCM)-based thermal energy storage (TES) systems are widely used for repeated intermittent heating and cooling applications. However, such systems typically face some challenges due to the low thermal conductivity and expensive encapsulation process of PCMs. The present study overcomes these challenges by proposing a lightweight, low-cost, and low thermal resistance TES system that realizes a fluid-to-PCM additively manufactured metal-polymer composite heat exchanger (HX), based on our previously developed cross-media approach. A robust and simplified, analytical-based, 1D reduced-order model (ROM) was developed to compute the TES system performance, saving computational time compared to modeling the entire TES system using PCM-related transient CFD modeling. The TES model was reduced to a segment-level model comprising a single PCM-wire cylindrical domain based on the tube-bank geometry formed by the metal fin-wires. A detailed study on the geometric behavior of the cylindrical domain and the effect of overlapped areas, where the overlapped areas represent a deviation from 1D assumption on the TES performance, was conducted. An optimum geometric range of wire-spacings and size was identified. The 1D ROM assumes 1D radial conduction inside the PCM and analytically computes latent energy stored in the single PCM-wire cylindrical domain using thermal resistance and energy conservation principles. The latent energy is then time-integrated for the entire TES, making the 1D ROM computationally efficient. The 1D ROM neglects sensible thermal capacity and is thus applicable for the low Stefan number applications in the present study. The performance parameters of the 1D ROM were then validated with a 2D axisymmetric model, typically used in the literature, using commercially available CFD tools. For validation, a parametric study of a wide range of non-dimensionalized parameters, depending on applications ranging from pulsed-power cooling to peak-load shifting for building cooling application, is included in this paper. The 1D ROM appears to correlate well with the 2D axisymmetric model to within 10%, except at some extreme ranges of a few of the non-dimensional parameters, which lead to the condition of axial conduction inside the PCM, deviating from the 1D ROM.
EMBEDDED TWO-PHASE COOLING OF HIGH FLUX ELECTRONICS VIA MICRO-ENABLED SURFACES AND FLUID DELIVERY SYSTEM (FEEDS)
(2016) Mandel, Raphael Kahat; Ohadi, Michael M; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
A novel cooler utilizing thin Film Evaporation on micro-Enabled surfaces and fluid Delivery System (FEEDS) is embedded into silicon die with the goal of achieving the metrics proposed by the Defense Advanced Research Project Agency’s (DARPA) ICECool fundamentals program: a heat flux of 1 kW/cm2 at superheats below 30 K, vapor qualities above 90%, pressure drops below 10% of absolute pressure, and heat densities of 1 kW/cm3. Preliminary models were used to investigate the various physical phenomena affecting two-phase flow in manifold-microchannels, including nucleate boiling, flow regime, annular film evaporation, void fraction, single-phase fully developed and developing forced convection, intra-microchannel flow distribution, and fin conduction. The various physical phenomena were then combined into a novel “2.5-D” microchannel model, which uses boundary layer assumptions and simplifications to model the 3-D domain with a 2-D mesh. The custom-coded microchannel model was first validated by comparing single-phase thermal and hydrodynamic performance to a 3-D laminar flow simulation performed in ANSYS Fluent with errors of less than 5% as long as the flow remains two-dimensional. Two-phase validation was conducted by comparing past experimental data to model predictions, and found to provide heat transfer predictions that were qualitatively accurate and correct in order of magnitude, and pressure drop predictions accurate to within 30%. A parametric study was then performed in order to arrive at a baseline geometry for meeting the ICECool metrics. A system level model was created to select the working fluid, and a manifold model was created to evaluate manifold flow configuration. A novel flow configuration capable of providing an even inter-microchannel flow distribution in two-phase mode was proposed, and a working manifold designed for the baseline geometry. Experiments with a press-fit FEEDS chip were then conducted, obtaining heat fluxes in excess of 1 kW/cm2 at 45% vapor quality. The volume of the FEEDS assembly was then reduced by bonding a chip directly to a FEEDS manifold. A bonding apparatus capable of providing a uniform and conformal clamping force was designed, fabricated, and used to hermetically bond the manifold to the chip. Three bonded chips were then tested, obtaining a maximum heat flux of 700 W/cm2 at vapor qualities approaching 30% and a heat density of 220 W/cm3.

Mechanical Engineering

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