Microchannel Coolers Research Articles

Regulation of throttling distribution and characteristic analysis for a microchannel Joule-Thomson (J-T) cooler

Miniature J-T coolers are applied extensively in electronic devices, cryosurgery and infrared detectors. The utilization of microchannels in miniature J-T coolers can induce a noticeable pressure drop through the recuperative structure, leading to modifications in the refrigeration mechanism. This study delves into the performance and coupling mechanism of throttling and heat exchange. Herein, a microchannel cooler with a noticeable throttling effect throughout the refrigeration process serves as a prototype. The mathematical model is proved with the experiments, thereby exposing limitations in the improvement of the throttling mechanism and the thermodynamic process. The analysis reveals that the backflow exhibits a significant pressure reduction (ΔPl); thus, the utilization ratio of the throttling effect is low. Additionally, further optimization of the thermodynamic process is possible by regulating throttling distribution. The performance and coupling mechanism of throttling and heat exchange with constant working conditions are investigated. The findings reveal that an optimal ratio of ΔPl (14.3 %) yields the lowest cold terminal temperature, which is reduced by up to 38.9 K. In this case, the J-T efficiency and the effectiveness of the recuperative process are observed to peak at 56.7 % and 83.8 %, respectively, indicating the optimal use of the throttling effect and interaction effects. The analysis also suggests that the thermodynamic process has been improved.

Heat transfer enhancement of a microchannel cooler with V-shaped partitions

The ongoing evolution of electronic systems that operate under extreme conditions has led to persistent concern about potential failures caused by escalating temperature levels which provokes the decline the operating quality. In response to this challenge, cooling solutions based on microchannels have emerged as promising prospects for improving thermal management in such scenarios. This paper explores the importance of these innovative cooling techniques and their potential for mitigating the risks of overheating in electronic systems. Furthermore, the introduction of microfluidic techniques and microchannels, specifically constricted microchannels, offers promising approaches to improve cooling efficiency. These cooling systems enable efficient heat dissipation and thermal regulation, mitigating the risk of overheating and enhancing system performance. Constricted microchannels facilitate compact and efficient heat transfer by leveraging increased surface area-to-volume ratios and improved convective cooling. Nowadays, microchannel-based heat sinks, heat exchangers, and cooling systems have been developed, showcasing improved heat dissipation, reduced temperature gradients, and enhanced energy efficiency. This research focuses on a parametrical study that examines the fluid nature, Reynolds number analysis, and system design. Numerical results demonstrate successful thermal management of high-temperature electronic systems using constricted microchannel cooling. These results mitigate temperature-related failures and support the development of robust systems for harsh operating conditions.

Mikro-Frezeleme Yöntemi ile İşlenen İnce Duvarlı Yapılarda Deformasyonun İncelenmesi

Micro-thin-walled structures are frequently encountered in micro-pumps, micro-channel cooler plates, and micro-molds in the defense, aerospace and biomedical sectors. One of the micro-machining methods frequently used to obtain micro-thin-walled structures is micro-milling. The micro-milling method makes machining possible for micro-components with high accuracy and a good surface finish. However, there are many issues to consider when micro-milling thin-walled geometries. The fact that the wall deformation is directly related to the rate of progression necessitates knowing what effects the rate of progression cause on the wall deformation. Because the micro milling technique is generally used as the final cutting process in the creation of thin wall geometries in the industry. In this study, thin-walled structures were obtained by micro-milling the Al6061-T6 material. The influence of feed rate on wall deformation was investigated by applying different feed rates in micro milling experiments. Wall deformation measurements were performed using a motorized optical profilometer device. It has been observed that increasing feed value causes an enhance in cutting forces, it also causes an increase in the deformation of the micro-thin wall. The deformation distance between the end points of the micro-milled wall geometry using a feed rate of 1 µm/tooth is three times greater than the thin-wall geometry created using a feed rate of 0.2 µm/tooth. It has been determined that the deformation is much higher in the entrance and exit areas of the micro-thin wall. The wall deformation also decreases from the upper point to the lower points of the micro-thin wall. While the deviation distance in the measurement taken from the upper point of the wall geometry obtained by using the 1 µm/tooth feed rate, where the deformation is more, can reach 100 µm, the deviation at the lower point of the wall is negligible.

Embedded Microfluidic Cooling in Aluminum Nitride HTCC Substrate for High-Power Radio Frequency Chip Array

Abstract Radio frequency (RF) electronics are developing toward high power, high integration, and high-power density, resulting in a continuous increase in heat flux. The traditional high-power RF package, which is usually composed of aluminum nitride (AlN) substrate, aluminum silicon housing shell, and aluminum alloy cold plate, exhibits poor heat dissipation ability and high thickness due to excessive interfaces and a long thermal conduction path. In this paper, aimed at improving heat dissipation ability and reducing the thickness of RF electronics, the microchannel was transferred from the cold plate to the AlN high-temperature co-fired ceramic (HTCC) substrate which plays the role of electrical connection, structural support, and liquid cooling cold plate. The embedded AlN microchannel cooler was firstly designed. Then, a prototype of the AlN substrate with 64 simulated chip arrays and microchannels was fabricated and the thermal performance was evaluated using an experimental setup. Finally, the thermal performances of the proposed and traditional cooler were compared using a CFD simulation. The results indicated that the proposed embedded cooling structure could enhance the heat-flux dissipation ability by 61% and reduce the packaging thickness by 40% compared with the traditional cooling structure.

Fabrication and Performance of 300-mm Wafer-Scale Silicon Microchannel Cooler

This paper describes the design, fabrication, and thermal performance of 300 mm diameter wafer-scale Si microchannel coolers that demonstrated a junction temperature rise of less than 18°C when dissipating about 14 kW of power. A thermal test wafer, which included hot-spot regions to mimic high-power compute cores, was attached to the wafer-scale microchannel cooler with Pb-Sn solder. Glass manifold layers were attached to the microchannel wafer using a frit material. The average apparent heat transfer coefficient, h <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">app</sub> , was about 104,000 W/(K-m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ).

Hotspot thermal management of silicon-based high power hetero-integration

Extremely uneven thermal distribution due to the hotspots generating at the transistor channel regions on GaN power chip is one of the major obstacles to high power hetero-integration on silicon. In this work, a 12 mm × 18 mm × 0.7 mm embedded converging-diverging microchannel (CDMC) cooler silicon interposer (SI) is put forward to address the hotspot cooling. A 5.3 mm × 3.5 mm × 0.08 mm GaN monolithic microwave integrated circuit (MMIC) 3-stage power amplifier (PA) chip with DC heating power of ∼ 101 W was integrated on the SI to evaluate the on-chip hotspot cooling performance. The average hotspot heat flux was up to 306.8 ∼ 307.1 W/mm2, and the average chip heat flux achieved 545.4 ∼ 546.3 W/cm2. The maximum junction temperature of the GaN chip on CDMC SI was 137.2 ∼ 147.4 °C with the pressure drop of 122.8 ∼ 394.3 kPa and pumping power of 0.41 ∼ 3.29 W at 70 °C ambient temperature. The thermal and hydraulic characteristics of the CDMC SI are analyzed based on field synergy principle and thermodynamic properties using finite element method (FEM). The heat transfer coefficient (HTC) of the CDMC SI has 22.4 ∼ 35.4% increase compared with that of a typical microjet array (MJA) SI. The cooling performance and efficiency of the CDMC SI have been validated theoretically and practically to be prior to those of the MJA SI, and it is a promising solution for high power hetero-integration on silicon with brilliant hotspot thermal management capability.

Study on two microchannel coolers with different coupling mechanisms of distributed J-T effect and heat transfer

As a typical characteristic of the microchannel Joule-Thomson (J-T) coolers, the distributed J-T effect significantly impacts the cooling effect. To investigate the coupling mechanism of distributed J-T effect and heat transfer, two micro-coolers are fabricated by Printed Circuit Board (PCB) technology in this study. The throttling device is removed from the micro-cooler with staggered etched pillars (Cooler-Pillars), resulting in an entire coupling of heat recovery and throttling processes. The heat recovery and throttling structures are preserved in a micro-cooler etched with rectangular microchannels (Cooler-REC), and the two processes couple partly. The performance of the two micro-coolers is investigated experimentally and numerically at the inlet pressure of 2.02∼5.12 MPa. The experimental results show that the mass flow rate and total pressure drop of Cooler-Pillars are higher than Cooler-REC at the same inlet pressure. Compared with Cooler-REC, Cooler-Pillars can reach a lower cold end temperature and has a cold-end temperature of 190.6 K in steady-state operation at 5.03 MPa. A steady-state mathematical model is developed and compared favorably with the experimental results. The simulation result shows that at the same inlet pressure, the overall distribution of J-T coefficients and the total temperature drop caused by distributed J-T effect of Cooler-Pillars are higher. It is also found that the overall Nu distribution of Cooler-Pillars is higher than Cooler-REC, and heat transfer is enhanced, leading to an increase of 64.5% in average unit heat transfer at 5 MPa.

Parametric design analysis of a multi-level 3D manifolded microchannel cooler via reduced order numerical modeling

Waste heat flux from power dense electronics is expected to reach > 1 kW/cm2 in the next few decades, and they will require novel cooler designs with low thermal resistance, that can simultaneously dissipate large levels of heat and have high coefficient of performance (COP). 2D straight microchannel cold plates (CP) are an industrial go-to solution for active heat dissipation needs, but they suffer from a major drawback – very high pump pressure is required to force large quantities of fluid through miniscule channels in the CP and thus these coolers are very inefficient, achieving low COP. Recently, manifolded micro-coolers (MMC) have become popular which use a second manifold layer to distribute the fluid in 3D within the CP, thus shortening fluid travel length within the miniscule CP channels and significantly reducing the total device pressure drop. In this study, we first introduce a novel two-level manifold design which boasts a potential of > 2x improvement in COP compared to conventional single-level manifold concept without affecting the thermal performance. Recognizing the difficulty in simulating large area full MMCs, we then aim to simplify the 2-level MMC geometry into reduced order models to bring down simulation cost at an expense of accuracy. Two models were considered, the widely popular and convenient to use Single Cold Plate U-bend Channel (SCPUC) model which only simulates the CP channels, and the slightly more complicated Single Manifold Channel (SMC) model which also considers the effect of the manifold. The SMC model simulations were first validated against full device simulations for different heater footprint sizes (25, 100, 400 mm2) to establish accuracy and it was found that the SMC model could predict thermal performances of all device sizes with a nominal inaccuracy of 5%. In contrast, the widely accepted SCPUC model produced highly inaccurate (as high as 25–45%) predictions for thermal performance of the MMCs. The two models were then used under an extreme heat flux load of 800 W/cm2 and 0.2 liter per min (lpm) device flow rate, to simulate 54 different 2-level MMCs obtained by varying important geometric parameters on the manifold and cold plate side. Detailed analysis was performed to explain the trends in thermal performance and pressure drop with different geometric parameters. Finally, two pareto curves were reported, one between thermal resistance and pressure drop, and the other between COP and device size. It was seen that the proposed 2-level MMC showed record high COP as compared to state-of-the-art single-level MMCs. We hope that this study will act as a design guide for MMCs as well as act as a performance repository for a wide range and combinations of geometries of 2-level manifold structures.

Microchannel cooling for the LHCb VELO Upgrade I

The LHCb VELO Upgrade I, currently being installed for the 2022 start of LHC Run 3, uses silicon microchannel coolers with internally circulating bi-phase CO2 for thermal control of hybrid pixel modules operating in vacuum. This is the largest scale application of this technology to date. Production of the microchannel coolers was completed in July 2019 and the assembly into cooling structures was completed in September 2021. This article describes the R&D path supporting the microchannel production and assembly and the motivation for the design choices, together with the achieved fluidic and thermal performance. The Thermal Figure of Merit of the microchannel coolers is measured on the final modules to be between 1.5 and 3.5 K cm2 W−1, depending on glue thickness. The microchannel coolers constitute 18% of the total radiation length of the VELO and less than 2% of the material seen before the second measured point on the tracks. Microchannel cooling is well suited to the VELO implementation due to the uniform mass distribution, close thermal expansion match with the module components and resistance to radiation.

A Highly Efficient Integrated Silicon-Microchannel Cooler for Multi-Module Electronic Microsystems: Model Design, Optimization, and Performance Validation

Abstract Recently, the development trend of multi-module and multi-function in electronic microsystems makes the ever-increasing heat flux problem more serious. In this study, a highly efficient integrated single-phase microchannel cooler with four heat sources is presented for handling the challenges from both workings independently of all electronic modules and the high heat flux. Both numerical and experimental studies are conducted. By optimizing the structural design and the fabricated process, the presented microchannel cooler has outstanding cooling performance, which contains desired fluid flow distribution, pressure drop, heat transfer, and combination thereof. Results reveal uniform coolant flow dissipates four individual heaters independently, and their maximal temperature difference below 4 °C. Beyond this, high heat flux removal (707.6 W/cm2) is realized with an extremely low coolant flowrate (45 ml/min), and the maximum temperature rise is less than 60 °C. This study provides a referable solution for the thermal management of multi-module heat sources and high heat flux in compact electronic microsystems.

Fin Efficiency Design of Micro-Channel for Nd: YAG Slab Laser

The heat generated by a Slab Lasers can exceed 1,000 watts but the area available for cooling is too small. This results in localization of heat flux which makes heat dissipation a challenge in slab lasers. The Heat transfer coefficient can increase up to a very high range, which can’t be efficiently achieved by the conventional water cooling. Micro-channel coolers address this problem competently. These channels contain liquid, which transfers heat to the sink with high efficiency.The objective of this paper is to design the microchannel coolers that will be efficiently capable of removing the heat from the slab laser without causing any thermal distortion in the laser. The dimension of the slab is given as follows:-Length(l) =50mm, Width(w) =8mm and thickness(t)=2mm.

Thermal investigation of power diodes in novel integrated disc type packages

Paper focusses on electro-thermal numerical investigation of a novel disc type package for power diodes integrated with microchannel cooling system. Although the new solution ensures much lower average temperature of an electronic element, it results in much higher temperature non-uniformity obtained within the silicon structure. The distribution can be improved with the aid of redesigned microchannel cooler. The maximum junction temperature reduction of more than 25 °C is possible.

Novel High-Power, High Repetition Rate Laser Diode Pump Modules Suitable for High-Energy Class Laser Facilities

The latest generation of high-energy-class pulsed laser facilities, under construction or planned, such as EuPRAXIA, require reliable pump sources with high power (many kW), brightness (&gt;1 MW/cm2/sr) and electro-optical conversion efficiency (&gt;50%). These new facilities will be operated at high repetition rates (around 100 Hz) and only diode lasers are capable of delivering the necessary performance. Commercial (quasi-continuous wave, QCW) diode laser pulse-pump sources are, however, constructed as low-cost passively cooled stacked arrays that are limited either in brightness, efficiency or repetition rate. Commercial continuous wave diode laser pumps constructed using microchannel coolers (as used in high-value industrial machine tools) can fulfil all requirements, but are typically not preferred, due to their cost and complexity and the challenges of preventing cooler degradation. A custom solution is shown here to fill this gap, using advanced diode lasers in a novel passive side-cooling geometry to realize 100 … 200 Hz pump modules (10%–20% duty cycle) that emit peak power of 6 kW at wavelength = 940 nm. The latest performance of these modules is summarized and compared to literature. We show that a brightness &gt;1 MW/cm2/sr can be efficiently delivered across a wide range of laser pulse conditions with 10% duty cycle (pulse width: 100 µs … 100 ms … cw, repetition rate up to 1 kHz). Furthermore, we describe how these pumps have been used to construct and reliably operate (&gt;109 pulses without degradation) in high-energy-class regenerative and ring amplifiers at the Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie (MBI). We also show first results on 100 Hz pumping of cryogenically cooled solid-state Yb:YAG slab amplifiers, as anticipated for use in the EuPRAXIA laser, and note that peak temperature is disproportionately increased, indicating that improved cooling and more detailed studies are needed.

60 mm-aperture high average output power Nd:YAG composite ceramic disk laser

A laser with 60 mm-aperture composite ceramic disk pumped by laser diode arrays was developed. By optimizing waveguide and imaging lens, a pump power peak power of 75 kW, density of 318 W/cm2 is achieved with a pump uniformity of 96% by using a hexagonal waveguide. The temperature distribution and axial deformation of the cooler were optimized, and the axisymmetric micro-channel cooler was designed. Up to 2.3 kW average output power with an optical-optical efficiency of 37% is obtained in the stable resonator when pumping with a long-pulse.

Method to improve near-field nonlinearity of a high-power diode laser array on a microchannel cooler

Due to thermal stress, each emitter in a semiconductor laser bar or array is vertically displaced along the p-n junction; the result is that each emitter is not in a line, called near-field nonlinearity. Near-field nonlinearity along a laser bar (also known as “SMILE” effect) degrades the laser beam brightness, which causes an adverse effect on optical coupling and beam shaping. A large SMILE value causes a large divergence angle after collimation and a wider line after collimation and focusing. We simulate the factors affecting the SMILE value of a high-power diode laser array on a microchannel cooler (MCC). According to the simulation results, we have fabricated a series of laser bars bonded on MCCs with lower SMILE value. After simulation and experiment analysis, we found the key factor to affect SMILE is the deformation of the thin MCC because of the distribution of strain and stress in it. We also decreased the SMILE value of 1-cm-wide full bar AuSn bonded on MCCs from 12 to 1 μm by balancing force on MCC to minimize the deformation.

Design and Modeling of Membrane-Based Evaporative Cooling Devices for Thermal Management of High Heat Fluxes

We present a high-heat-flux cooling device for advanced thermal management of electronics. The device incorporates nanoporous membranes supported on microchannels to enable thin-film evaporation. The underlying concept takes advantage of the capillary pressure generated by small pores in the membrane, and minimizes the viscous loss by reducing the membrane thickness. The heat transfer and fluid flow in the device were modeled to determine the effect of different geometric parameters. With the optimization of various parameters, the device can achieve a heat transfer coefficient in excess of 0.05 kW/cm2-K, while dissipating a heat flux of 1 kW/cm2. When applied to power electronics, such as GaN high-electron-mobility transistors, this membrane-based evaporative cooling device can lower the near-junction temperature by more than 40 K compared with contemporary single-phase microchannel coolers.

Thermal Modeling of Extreme Heat Flux Microchannel Coolers for GaN-on-SiC Semiconductor Devices

Gallium nitride (GaN) high-electron-mobility transistors (HEMTs) dissipate high power densities which generate hotspots and cause thermomechanical problems. Here, we propose and simulate GaN-based HEMT technologies that can remove power densities exceeding 30 kW/cm2 at relatively low mass flow rate and pressure drop. Thermal performance of the microcooler module is investigated by modeling both single- and two-phase flow conditions. A reduced-order modeling approach, based on an extensive literature review, is used to predict the appropriate range of heat transfer coefficients associated with the flow regimes for the flow conditions. Finite element simulations are performed to investigate the temperature distribution from GaN to parallel microchannels of the microcooler. Single- and two-phase conjugate computational fluid dynamics (CFD) simulations provide a lower bound of the total flow resistance in the microcooler as well as overall thermal resistance from GaN HEMT to working fluid. A parametric study is performed to optimize the thermal performance of the microcooler. The modeling results provide detailed flow conditions for the microcooler in order to investigate the required range of heat transfer coefficients for removal of heat fluxes up to 30 kW/cm2 and a junction temperature maintained below 250 °C. The detailed modeling results include local temperature and velocity fields in the microcooler module, which can help in identifying the approximate locations of the maximum velocity and recirculation regions that are susceptible to dryout conditions.

Experimental study and numerical modeling of micro-channel cooler with micro-pipes for high-power diode laser arrays

A miniaturized micro-channel cooler with micro-pipes that transport cooling liquid was numerically modeled and experimentally studied. A prototype copper cooler with glass micro-pipes was manufactured and tested. An experimental arrangement for investigations of cooling performance of the proposed device was built. An infrared imaging method was employed for temperature profiling of high-power laser bar mounted on the cooler. Concurrently, mathematical and fully three-dimensional numerical models of the proposed new design of a cooler were developed and a series of thermo-fluid numerical simulations were performed for various mass flow rates of the cooling medium and different powers of laser diode. Obtained results were compared with experimental measurements and showed generally a good agreement. The possible origin of some discrepancies at boundaries was discussed. Both experimental and numerical findings indicate that the proposed cooler construction is an interesting solution for high-power laser diode arrays.

Modeling and simulation of power electronic modules with microchannel coolers for thermo-mechanical performance

This paper investigated the thermo-mechanical performance of microchannel-based technology to actively cool high power electronic modules, of which the power loss of the module has reached as high as 550watts. Finite element analysis (FEA) method was used to model and simulate the performance of insulated gate bipolar transistor (IGBT) modules. Thermal and mechanical performances of power electronic modules with and without microchannel were analyzed and compared. Residual stress caused by reflow soldering process and operating stress considering the residual stress in previous process were studied for the first time, in which plastic behavior of soft solder and copper were taken into account. Anand viscoplasticity constitutive model was used to describe the soft solder behavior in the assembly process. The Chaboche nonlinear kinematic hardening model for the copper was considered as well. Three dimensional temperature and stress distributions were presented, based on the optimization design by adjusting the thickness of each layer to reduce the operating stress and therefore extend the lifetime of the module.

Energy Efficient Two-Phase Microcooler Design for a Concentrated Photovoltaic Triple Junction Cell

The potential application of an R134a-cooled two-phase microcooler for thermal management of a triple junction solar cell (CPV), under concentration of 2000 suns, is presented. An analytical model for the triple-junction solar cell temperature based on prediction of two-phase flow boiling in microchannel coolers is developed and exercised with empirical correlations from the open literature for the heat transfer coefficient, pressure drop, and critical heat flux. The thermofluid analysis is augmented by detailed energy modeling relating the solar energy harvest to the “parasitic” work expended to provide the requisite cooling, including pumping power and the energy consumed in the formation and fabrication of the microcooler itself. Three fin thicknesses, between 100 μm and 500 μm, a variable number of fins, between 0 and 9, and 5 channel heights between 0.25 mm and 3 mm, are examined for a R134a flow rate of 0.85 g/s to determine the energy efficient microcooler design for a 10 mm × 10 mm triple junction CPV cell.