Multiobjective Optimization of Staggered Elliptical Pin-Fin Arrays

This work presents a numerical procedure for multiple objectives to optimize staggered elliptic-shaped short pin-fin arrays. The multiobjective problem is to achieve an acceptable compromise between augmentation of turbulent heat transfer and reduction in friction loss. Four nondimensional variables, pin-fin height-to-channel height ratio, major axis length-to-channel height ratio, minor-axis length-to-channel height ratio, and pin-fin pitch-to-channel height ratio are chosen as design variables. The D-optimal method is used to determine the training points. The response surface method is used to approximate the Pareto optimal front with Reynolds-averaged Navier-Stokes analysis of the flow and heat transfer using the shear stress transport (SST) turbulence model. The Pareto-optimal solutions are obtained using a combination of an evolutionary algorithm and a local search.     

Inverse Design of Cooling Arrays of Micro Pin-Fins Subject to Specified Coolant Inlet Temperature and Hot Spot Temperature

Given a micro pin-fin array cooling scheme with these constraints: (a) given maximum allowable temperature of the material (the hot spot temperature), (b) given inlet cooling fluid temperature, (c) given total pressure loss (pumping power affordable), and (d) given overall thickness of the entire micro pin-fin cooling array, find the maximum possible average heat flux on the hot surface and find the maximum possible heat flux at the hot spot under the condition that the entire amount of the inputted heat is removed by the cooling fluid. The goal was to create an optimum performance map for a cooling micro array having specified inlet coolant temperature and maximum temperature. Fully 3D conjugate heat transfer analysis was performed on each of the randomly created candidate configurations. Response surfaces based on Radial Basis Functions were coupled with a genetic algorithm to arrive at a Pareto set of best trade-off solutions. These Pareto optimized configurations indicate the maximum physically possible heat fluxes for specified material and constraints. Detailed off-design performance maps of such Pareto-optimized cooling arrays of micro pin-fins were calculated that demonstrate superior on-design and off-design performance of pin-fins having symmetric convex cross sections as opposed to the commonly used circular cross sections.     

The Effect of Micro Pin-Fin Shape on Thermal and Hydraulic Performance of Micro Pin-Fin Heat Sinks

Single micro pin-fin configurations having the same chord thickness/diameter but different shapes are numerically modeled to assess their heat transfer and hydraulic performances for Reynolds number values changing between 20 and 120. The configurations are three-dimensionally modeled based, and their heat transfer performances are evaluated using commercially available software COMSOL Multiphysics 3.5a. Navier–Stokes equations and continuity and energy equations are solved under steady-state conditions for single-phase water flows. To increase the computational efficiency, half of the domain consisting of a micro pin-fin located inside a microchannel is modeled using a symmetry plane. To validate the model, experimental data available in the literature are compared to simulation results obtained from the model of the same geometrical configuration as the experimental one. Accordingly, the numerical and experimental results show good agreement. Furthermore, performance evaluation study is performed using three-dimensional (3D) numerical models in the light of flow morphologies around micro pin-fins of various shapes. According to the results obtained from this study, the rectangular-shaped micro pin-fin configuration has the highest Nusselt number and friction factor over the whole Reynolds number range. However, the cone-shaped micro pin-fin configuration has the best thermal performance index, indicating that it could be more preferable to use micro pin-fins of unconventional shapes in micro pin-fin heat sinks.     

State of the Art of High Heat Flux Cooling Technologies

The purpose of this literature review is to compare different cooling technologies currently in development in research laboratories that are competing to solve the challenge of cooling the next generation of high heat flux computer chips. Today, most development efforts are focused on three technologies: liquid cooling in copper or silicon micro-geometry heat dissipation elements, impingement of liquid jets directly on the silicon surface of the chip, and two-phase flow boiling in copper heat dissipation elements or plates with numerous microchannels. The principal challenge is to dissipate the high heat fluxes (current objective is 300 W/cm²) while maintaining the chip temperature below the targeted temperature of 85°C, while of second importance is how to predict the heat transfer coefficients and pressure drops of the cooling process. In this study, the state of the art of these three technologies from recent experimental articles (since 2003) is analyzed and a comparison of the respective merits and drawbacks of each technology is presented. The conclusion is that two-phase flow boiling in microchannels is the most promising approach; impingement cooling also has good prospects but single-phase liquid cooling is probably only a short-term solution. As an example of the state of the first technology, the Heat and Mass Transfer Laboratory at Ecole Polytechnique Fédérale de Lausanne has already achieved 200 W/cm² of cooling in a first prototype, with a low pumping power, good temperature uniformity, and at the required maximal operating temperature.     

Performance Analysis and Design Optimization of Gapped Pin-Fin in a Cooling Channel

Performance analysis and optimization of a circular pin-fin with inside gaps in a rectangular cooling channel were performed at Reynolds number, 10,000, using three-dimensional Reynolds-averaged Navier–Stokes equations and a multi-objective genetic algorithm. The low-Reynolds-number version of the shear stress transport model was used as turbulence closure. A parametric study was also performed to identify the geometrical effects of the pin-fin on heat transfer and pressure drop. The straight and reference gapped pin-fins yielded better performances than those of the circular pin-fin without the gap in terms of both heat transfer and pressure drop. The objective of the optimization was to maximize the heat transfer and minimize the pressure loss, simultaneously. The area-averaged Nusselt number and pressure loss coefficient were considered as objective functions, and three design variables related to the geometry of the gapped pin-fin were chosen for the optimization. Twenty-seven design points were generated using Latin hypercube sampling in the design space, and response surface approximation models were constructed for the objective functions. The optimization results were analyzed using five representative solutions on the Pareto-optimal front. The objective functions were found to be significantly affected by variation in the design variables, especially, the width of front gap and the rear gap angle.     

Monodisperse Spray Cooling of Small Surface Areas at High Heat Flux

Spray cooling is an effective method to remove high heat fluxes from electronic components. To understand the physical mechanisms, this work studies heat transfer rates from single and dual nozzle distilled water sprays on a small heated surface (1.3 mm × 2 mm). Thermal ink jet atomizers generate small droplets, 33 μm diameter, at known frequencies, leading to controlled spray conditions with a monodisperse stream of droplets interacting with the hot surface. Of particular interest in this work is the dissipated heat flux and its relation to the liquid film thickness, the surface superheat, and the cooling mass flow rate. Experimental results show the heat flux scales to the cooling mass flow rate. In comparison to published spreading–splashing correlations, these experiments indicate that the drops impinge on the liquid film and spread without generating splashing, leading to high-efficiency stable heat transfer. Surface temperatures range from 120 to 140°C. In addition, the liquid film thickness is investigated in relation to the heater superheat and a stable thin film is seen at superheats beyond 20°C. The efficiency of the spray system is inversely related to the film thickness and may be due to ejection of liquid from the surface due to bursting of vapor bubbles.     

Mini-Contact Enhanced Thermoelectric Coolers for On-Chip Hot Spot Cooling

Shrinking feature size and increasing transistor density, combined with the high performance demanded from next-generation microprocessors, have led to on-chip high heat flux “hot spots,” which have emerged as the primary driver for thermal management of today's integrated circuit (IC) technology. This article describes the use of a mini-contact to enhance the cooling flux of a miniaturized thermoelectric cooler (TEC) for on-chip hot-spot remediation. A package-level numerical simulation is developed to predict the on-chip hot spot cooling capability achievable with such a mini-contact enhanced TEC. Attention is focused on the hot-spot temperature reduction associated with variations in mini-contact size and thermoelectric element height, as well as the parasitic effect from the thermal contact resistance introduced by the mini-contact. A preliminary experiment has been conducted to verify the numeric model and to demonstrate the effects of the mini-contact on hot-spot cooling.     

双层壁冷却结构综合冷却效率和相对压力损失多目标优化

在典型燃烧室工作环境下,针对特定双层壁冷却结构,以综合冷却效率和相对压力损失为优化目标,采用径向基神经网络构建数学模型,通过遗传优化算法实现多目标优化,旨在提高其气动和传热性能。在给定的双层壁冷却结构参数范围内,优化后双层壁冷却结构的最大综合冷却效率为0.89,而相对压力损失可降至0.17%。     

Next Generation Spray Cooling: High Heat Flux Management in Compact Spaces

For many years, spray cooling has been known as a promising technology for the removal of high heat fluxes. However, that promise has yet to be fully realized. This work presents a current understanding of the mechanisms of spray cooling and its limitations. To address these limitations, a novel spray nozzle array is described that allows for scalable, high-performance cooling. However, fluid management remains a challenging concern that may limit the application of this technology.     

Multi-Objective Optimization Analysis of Structural Design for Large Cooling Towers

ABSTRACT

Equivalent static wind loads (ESWLs) on cooling towers derived even by the refined multiple-mode dynamic algorithm such as the complete quadratic combination theory usually shows quite different types for various design targets. So, it is evitable to bring complex and burdensome calculation for structural design. To this problem, the improved multi-objectives ESWLs method was proposed by combining the wind vibration characteristics of large cooling towers itself. Through several numerical examples, the multi-objective ESWLs distribution characteristics were discussed, and its rationality of application in the cooling towers was also validated.     

高实度菱形扰流柱冷却通道的传热及流动特性研究

采用数值模拟的方法对高实度(45%)菱形扰流柱在恒定通道和收敛通道内的传热及流动特性进行三维数值模拟研究,分析了不同雷诺数下两种通道内的传热及压力损失。研究结果表明:随着雷诺数的增加,恒定通道和收敛通道传热效果均增强,两种通道内的流动阻力损失也减小,但减小的趋势逐渐降低;在相同雷诺数下,收敛通道内扰流柱的表面尤其是背风面能够被有效冷却;恒定通道的压力损失变化平缓且呈线性分布,而收敛通道内的压力损失存在急剧增大区域。     

Evaluation of Jet Impingement,Spray and Microchannel Chip Cooling Options for High Heat Flux Removal

Thermal management for high heat flux removal from microelectronic chips is gaining critical importance in many earth-based and space-based systems. Heat fluxes greater than 1 MW/m² (100 W/cm²) have already been realized in high-end server applications, while cooling needs in next generation chips and advanced systems such as high-power electronics and electrical systems, pulsed power weapons systems, solid-state sensors, and phased-array radars are expected to reach 5–10 MW/m² (500–1000 W/cm²). After evaluating the contributions from different thermal resistances in the chip-to-ambient thermal path, this paper presents a critical review and research recommendations for three prominent contending technologies: jet impingement, spray cooling, and microchannel heat sinks.     

Heat Flux Correlation for Spray Cooling in the Nonboiling Regime

An experimental facility was developed to investigate the nonboiling heat transfer performance of water spray cooling. The effects of mass flux and wall temperature on heat transfer coefficient and heat flux were experimentally studied. It was found that heat transfer coefficient increased with the increasing of mass flux and wall temperature. Generalized correlations were developed for the Nusselt number related to wall temperature and the average Nusselt number as a function of the spray Reynolds number and the nondimensional temperature with an absolute error of 4% and 15% when the Reynolds number is more than 440. Compared with the data of Oliphant et al., it was observed that the usage field of the correlations could be extended to Reynolds number greater than 240.     

Advanced Micro-Heat Exchangers for High Heat Flux

Three micro-heat exchangers for use in a liquid cooling system with a long offset strip, short offset strip, and chevron flow path based on the traditional heat transfer enhancement concepts were designed and tested. A straight channel heat exchanger was also made for comparison. The liquid crystal thermography method described by Lin and Yang (2005) was used to observe the flow and temperature distributions in the micro-heat exchangers. The test results show that the chevron channel heat exchanger provides the lowest thermal resistance. However, its pressure drop is also the highest, approximately five times higher than that for other three heat exchangers. The offset strip heat exchangers provide better thermal performance than does the straight channel heat exchanger. The performance of the heat exchanger with the shorter strip is better than that of heat exchanger with longer strip. From the above results, all of the three micro-heat exchangers with conventional heat transfer enhancement showed less thermal resistance than the straight channel heat exchanger. The conventional heat transfer techniques may be effectively applied in the high-flux micro-heat exchanger design.     

Boiling Heat Transfer Enhancement Using Micro-Machined Porous Channels for Electronics Cooling

Boiling heat transfer enhancement for a passive electronics cooling design is presented in this paper. A novel pool boiling enhancement technique is developed and characterized. A combination of surface modification by metallic coating and micro-machined porous channels attached to the modified surface is tested and reported. An experimental rig is set up using a standard BGA package with 12 mm × 12 mm thermal die as a test surface. The limiting heat flux for a horizontally oriented silicon chip with fluorocarbon liquid FC-72 is typically around 15 W/cm². Boiling heat transfer with the designed enhancement techniques is investigated, and the factors influencing the enhancement are analyzed. The metallic coated surface at 10°C wall superheat has a heat flux six times larger than an untreated chip surface. Micro-machined porous channels with different pore sizes and pitches are tested in combination with the metallic coated surface. The boiling heat flux is seven times larger at lower wall superheat compared to the plain chip surface. Maximum critical heat flux (CHF) of 38 W/cm² is obtained with 0.3 mm pore diameter and 1 mm pore pitch. A ratio of pore diameter and pore pitch is found to correlate well with the heat transfer enhancement obtained by experiments. Structures with smaller pore diameter to pitch ratio and larger pore opening are found to have higher heat transfer enhancement in the tested combination.     

Numerical Investigation of Jet Impingement Cooling with Supercritical Pressure Carbon Dioxide in a Multi-Layer Cold Plate during High Heat Flux

Jet impingement cooling with supercritical pressure carbon dioxide in a multi-layer cold plate during the heat flux of 400 W/cm2 is investigated numerically. The generation and distribution of pseudocritical fluid with the high specific heat of supercritical pressure carbon dioxide and the mechanism of the heat transfer enhancement led by the high specific heat are analyzed. For a given nozzle diameter, the effects of the geometric parameters of a multi-layer cold plate such as the relative nozz...     

Critical Heat Flux (CHF) Correlations for Subcooled Water Flow Boiling at High Pressure and High Heat Flux

The subcooled water flow boiling is beneficial for removing the high heat flux from the divertor in the fusion reactor,for which an accurate critical heat flux(CHF)correlation is necessary.Up to now,there are many CHF correlations mentioned for subcooled water flow boiling in the open literatures.However,the CHF correlations’accuracies for the prediction of subcooled water flow boiling are not satisfactory at high heat flux and high pressure for reactor divertor.The present paper compiled 1356 CHF experimental data points from 15 independent open literatures and evaluated 10 existing CHF correlations in subcooled water flow boiling.From the evaluation,the W-2 CHF correlation performs best for the experimental CHF data in all existing critical heat flux correlations.However,the predicted mean absolute error(MAE)of the W-2 correlation is not very ideal for all database and the MAE of the W-2 correlation is from 30%to 50%for some database.In order to enhance the CHF prediction accuracy in subcooled water flow boiling at high heat flux and high pressure,the present paper developed a new CHF correlation.Compared with other existing CHF correlations,the new CHF correlation greatly enhances the prediction accuracy over a broad range of pressures and heat fluxes which are desired in the cooling of high heat flux devices,such as those in the fusion reactor divertor.The validation results show that the new correlation has a MAE of 10.05%and a root mean squared error(RMSE)of 16.61%,predicting 68.1%of the entire database within±10%and 81.5%within±15%.The MAE of the new CHF correlation is 7.4%less than that of the best existing one(W-2 correlation),further confirming its superior prediction accuracy and reliability.Besides,the new CHF correlation works well not only for a uniform power profile but also for a non-uniform power profile in subcooled water flow boiling at high pressure and high heat flux.     

Flow Boiling of Ammonia in a Diamond-Made Microchannel Heat Sink for High Heat Flux Hotspots

To solve the heat dissipation problem of electronic devices with high heat flux hotspots,a diamond microchannel heat sink consisting of 37 parallel triangular microchannels with channel lengths of 45 mm and hydraulic diameters of 280 μm was designed.The flow boiling heat transfer characteristics of ammonia in the microchannels were investigated under high heat fluxes of 473.9-1000.4 W/cm².Saturated flow boiling experiments with saturation temperatures of 25℃,30℃,and 35℃ and mass fluxe...     

Design Optimization of a New Hot Heat Sink with a Rectangular Fin Array for Thermoelectric Dehumidifiers

The objective of this research was aimed at conducting an experimental investigation to study the heat sink performance of a new rectangular fins array. Various operating conditions were considered, namely the distance between the fan and the fins, varied from 0 mm to 40 mm, heat supplied to the sink and the fan voltage. It was concluded that a fan installed at 30 mm height from the fins is recommended as the hot side temperature was the lowest. Next a pre-experimentation of small scale prototype of thermoelectric Dehumidifier (TED) was designed and constructed. It was composed of two thermoelectric (TE) cooling modules, MT2-1, 6-127, (two in serial) mounted between the rectangular fin heat exchangers with dimension 140 × 240 × 35 mm. The hot side was cooled by a ventilation fan whereas the air flow on the cold side was free convection. The effect of position of fan was investigation experimentally. Preliminary tests confirmed the good performance of the hot heat sink design for the intended thermoelectric application.