Two-Phase Cooling Research Articles

Effect of Groove Size on Heat Transfer Performance of Two-phase Immersion Cooling with Lotus Copper Jointed onto a Grooved Heat Transfer Surface

Effect of Groove Size on Heat Transfer Performance of Two-phase Immersion Cooling with Lotus Copper Jointed onto a Grooved Heat Transfer Surface

Single-Sided Digital Microfluidic (SDMF) Devices for Effective Coolant Delivery and Enhanced Two-Phase Cooling

Digital microfluidics (DMF) driven by electrowetting-on-dielectric (EWOD) has recently been attracting great attention as an effective liquid-handling platform for on-chip cooling. It enables rapid transportation of coolant liquid sandwiched between two parallel plates and drop-wise thermal rejection from a target heating source without additional mechanical components such as pumps, microchannels, and capillary wicks. However, a typical sandwiched configuration in DMF devices only allows sensible heat transfer, which seriously limits heat rejection capability, particularly for high-heat-flux thermal dissipation. In this paper, we present a single-sided digital microfluidic (SDMF) device that enables not only effective liquid handling on a single-sided surface, but also two-phase heat transfer to enhance thermal rejection performance. Several droplet manipulation functions required for two-phase cooling were demonstrated, including continuous droplet injection, rapid transportation as fast as 7.5 cm/s, and immobilization on the target hot spot where heat flux is locally concentrated. Using the SDMF platform, we experimentally demonstrated high-heat-flux cooling on the hydrophilic-coated hot spot. Coolant droplets were continuously transported to the target hot spot which was mitigated below 40 K of the superheat. The effective heat transfer coefficient was stably maintained even at a high heat flux regime over ~130 W/cm2, which will allow us to develop a reliable thermal management module. Our SDMF technology offers an effective on-chip cooling approach, particularly for high-heat-flux thermal management based on two-phase heat transfer.

Embedded Two-Phase Cooling of Large Three-Dimensional Compatible Chips With Radial Channels

Thermal performance for embedded two-phase cooling using dielectric coolant (R1234ze) is evaluated on a ∼20 mm × 20 mm large die. The test vehicles incorporate radial expanding channels with embedded pin fields suitable for through-silicon-via (TSV) interconnects of multidie stacks. Power generating features mimicking those anticipated in future generations of processor chips with eight cores are included. Initial results show that for the types of power maps anticipated, critical heat fluxes (CHFs) in “core” areas of at least 350 W/cm2 with at least 20 W/cm2 “background” heating in rest of the chip area can be achieved with less than 30 °C temperature rise over the inlet coolant temperature. These heat fluxes are significantly higher than those seen for relatively long parallel channel devices of similar base channel dimensions. Experimental results of flow rate, pressure drop, “device,” and coolant temperature are also provided for these test vehicles along with details of the test facility developed to properly characterize the test vehicles.

Assessment of Two-Phase Cooling of Power Electronics Using Roll-Bonded Condensers

Two-phase thermosyphons with condensers from roll-bonded panels, short “roll-bond thermosiphons,” are attractive for power-electronics cooling. Using simulations, the performance of roll-bond thermosyphons and classical heat sinks is compared. The roll-bond thermosyphons are advantageous in terms of trade-off between thermal resistance, cooler volume or mass, and sound-power level. Under forced convection, where air-side heat-transfer coefficients are comparatively high, the classical heat sink suffers from low fin efficiency and limited heat spreading. By increasing the number of panels, the roll-bond thermosyphon enables low thermal resistances that cannot be practically reached with classical heat sinks. For free air convection, the roll-bond thermosyphon allows a significant reduction of thermal resistance and cooler mass.

Numerical Simulation of the Mechanically Pumped Two-Phase Cooling Loop

Two-phase mechanically pumped cooling loop (MPCL) has emerged as a highly effective means for dissipating large amounts of heat from a small heat transfer area and provided a robust solution for significant design with flexibility, precise temperature control, and is othermalization. In this paper, the attempt of design optimization of the work fluid is introduced, by employing thermal simulation analysis with SINDA/FLUINT.

Flow Stability Analysis of Two-Phase Cooling Systems

AbstractEffective and efficient removal of heat is of critical importance in a wide range of applications, from human comfort to high power electronic devices and high efficiency photovoltaic arrays. This paper considers the stability and control of a cooling system operating in the two-phase regime. Two-phase cooling uses boiling to transfer heat from the source to the working fluid. Compared with the usual single-phase operation which relies on the temperature differential for heat transfer, boiling has superior efficiency by taking advantage of the latent heat of vaporization. However, it is known that two-phase operation is susceptible to various types of instability. We consider a cooling cycle consisting of two heat exchangers, an evaporator to extract heat from the source and a condenser to dissipate the heat to the ambient environment, and a pump and valves to regulate the flow. The heat exchangers are modeled by one-dimensional partial differential equations based on mass balance, momentum balance and energy balance. By using a novel Lyapunov function and impose the condition that the thermal subsystem time constant (in terms of enthalpy) is much slower than the fluid subsystem (in terms of pressure and mass flow rate), we obtain explicit stability condition in terms of the pressure demand curves (pressure drop as a function of the mass flow rate) of the heat exchangers. Using this Lypunov funciton, we derive passivity conditions for the fluid system which may in turn be used for flow stabilization. The general Lyapunov framework presented here may be extended to the stability analysis and control design of more complex thermodynamic systems.

High Heat Flux Two-Phase Cooling in Silicon Multimicrochannels

This paper presents performances of two-phase cooling of a chip at very high heat flux with refrigerant R236fa in a silicon multimicrochannel heat sink. This heat sink was composed of 134 parallel channels, 67 mum wide, 680 mum high, and 20 mm long, with 92- mum -thick fins separating the channels. The base heat flux was varied from 3 to 255 W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , the volume flow rate from 0.18 to 0.67 I/min, and the exit vapor quality from 0 to 80%. The working pressure and saturation temperature were set at 273 kPa and 25 degC, respectively. The present database includes 1040 local heat transfer coefficients. The base temperature of the chip could be maintained below 52 degC while dissipating 255 W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> with 10 degC of inlet subcooling and 90 kPa of pressure drop. A comparison of the respective performances with an extrapolation of the present results shows that two-phase cooling should be able to cool the chip 13 K lower than liquid cooling for the same pumping power at a base heat flux of 350 W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

Investigation of Microporous Coatings and Mesoscale Evaporator Enhancements for Two-Phase Cooling of Electronic Components

An empirical study of pool nucleate boiling enhanced with a microporous coating was conducted within the context of microprocessor cooling. A thermal test vehicle (TTV) emulated the heat load from a general purpose microprocessor and delivered a moderately nonuniform heat flux density distribution at the boiling surface that would be typical of mainstream microprocessors. The TTV was affixed to a copper test coupon that formed the bottom surface of a sealed boiling chamber containing FC-72 at atmospheric pressure. A design of experiments was conducted on multiple test coupons to identify how the microporous coating, the base thickness of the coupon, and the height of small pin fins affect the combined conduction and boiling heat transfer from the test coupon. The data revealed that the presence of the microporous coating was the most significant factor of the three tested and indicates that the presence of fins as short as 0.5mm may play a role in reducing hysteresis in the boiling curve.

Two-Phase Cooling Method Using the R134a Refrigerant to Cool Power Electronic Devices

This paper presents a two-phase cooling method using the R134a refrigerant to dissipate the heat energy (loss) generated by power electronics (PEs), such as those associated with rectifiers, converters, and inverters for a specific application in hybrid-electric vehicles. The cooling method involves submerging PE devices in an R134a bath, which limits the junction temperature of PE devices while conserving weight and volume of the heat sink without sacrificing equipment reliability. First, experimental tests that included an extended soak for more than 850 days were performed on a submerged insulated gate bipolar transistor (IGBT) and gate-controller card to study dielectric characteristics, deterioration effects, and heat-flux capabilities of R134a. Results from these tests illustrate that R134a has high dielectric characteristics and no deterioration of electrical components. Second, experimental tests that included a simultaneous operation with a mock automotive air-conditioner (A/C) system were performed on the same IGBT and gate-controller card. Data extrapolation from these tests determined that a typical automotive A/C system has more than sufficient cooling capacity to cool a typical 30-kW traction inverter. Last, a discussion and simulation of active cooling of the IGBT junction layer with the R134a refrigerant is given. This technique will drastically increase the forward current ratings and reliability of the PE device

Two-Phase Cooling Flows with Magnetic Reconnection

Motivated by the observations of high Faraday rotation measures measured in cooling flow clusters, we propose a model relevant to plasmas with comparable thermal and magnetic pressures. Magnetic field reconnection may play a major role in changing the topology of the magnetic field in the central cooling flow regions. The effect of the topology change is that cool flux loops can reconnect to hot flux loops that are connected to the overall thermal reservoir of the cluster. There can be a rapid recycling of mass between hot and cold phases on a time scale of ~3 x 10^8^-10^9^ yr which may reduce the inferred inflow and mass condensation rates by at least an order of magnitude. A central multiphase medium is a direct consequence of such a model. Throughout the cooling flow the filling factor of the hot loops (T &gt; 2 x 10^7^ K) is of order unity. The filling factor of the cool loops (&lt;2 x 10^7^ K) is 0.1%- 1% with a corresponding mass fraction of cold phase of 1%-10%. A crucial parameter is the coherence length of the field relative to the cooling radius and the distribution of field energy with scale. When the cooling radius is greater than the field coherence length, then cooling flows proceed as usual. When the coherence length is greater than the central cooling radius, however, the thermal energy of the reservoir can be tapped and the mass condensation rates may be very significantly reduced. Three additional conditions must be satisfied: (1) cold loops must be able to fall at least as far as the mean distance between hot loops in a cooling time; (2) loops must enter an evaporative phase on reconnecting; and (3) a sufficient number of hot loops penetrate the cold-phase region to power the radiative losses.

Design Parameters and Practical Considerations in the Two-Phase Forced-Convection Cooling of Multi-Chip Modules

Forced-convection boiling was investigated with a dielectric coolant (FC-72) in order to address some of the practical issues related to the two-phase cooling of multi-chip modules. The module used in the present study featured a linear array of nine, 10 × 10 mm2, simulated microelectronic chips which were flush-mounted along a 20-mm wide side of a rectangular channel. Experiments were performed with a 5-mm channel gap (distance between the chip surface and the opposing channel wall) at eight orientations spaced 45 degrees apart. Two other channel gaps, 2 and 10 mm, were tested in the vertical up flow configuration. For all these configurations, the velocity and subcooling of the liquid were varied from 13 to 400 cm/s and 3 to 36°C, respectively. Changes in orientation did not affect single-phase or nucleate boiling characteristics, but did have a major impact on CHF. Upflow conditions were found to be the best configuration for the design of two-phase cooling modules because of its inherently stable flow and relatively high CHF values. The CHF value for the most upstream chip in vertical upflow agreed well with a previous correlation for an isolated chip. Combined with the relatively small spread in CHF values for all chips in the array, this correlation was found to be attractive for design purposes in predicting CHF for a multi-chip array. To achieve a given CHF value, it is shown how the strong CHF dependence on velocity rather than flow area allows for a reduction in the required flow rate with the 2-mm, as compared to the 5-mm gap, which also required a smaller flow rate than the 10-mm gap. This reduction inflow rate was significant only with subcooled conditions corresponding to high CHF values.