As a non-renewable resource, mineral oil has been re-evaluated for its value and suitability as the coolant and insulator in transformers because of its environmental unfriendliness, fire hazards, ...and operating efficiency dissatisfaction. As a result, alternative dielectric liquids meeting the demands for more reliable, safer, and cleaner energy has been a critical need. Plant-based insulating fluids, non-toxic to the environment and ultimately biodegradable, have been flowing into the mainstream for achieving cost efficiencies, performance advantages, and enhancing safety. With more than 20 years of field experience, plant-based oils are now employed in more than 600,000 transformers worldwide, and have been established an enviable performance track record. Far from being just a mineral oil replacement, but more significantly, plant-based oils have filled a gap in some specific application scenarios where mineral oils fail to satisfy the fire-safety and environmental standards. This article reviews the research status of plant-based insulating fluids for transformer, including both challenges and outlook circa 2020. Plant-based oils have many inherent advantages over mineral oil, but also have numerous and unique challenges.
•Research for plant-based oils as alternative insulating fluids has been advancing during the past 30 years.•Current status and development prospects of plant-based insulating fluids for transformer are summarized.•Offer an insight into the key challenges and future perspectives about further promotion of plant-based insulating oils.
•Transformer dielectric fluids critical properties and condition monitoring methods are articulated.•Dissolved gas analysis, thermal capabilities, electrical properties of transformer dielectric ...fluids are illustrated with graphical abstracts.•Environment concerns and sustainability aspects of alternate dielectric fluids are demonstrated.•Future scopes and challenges to adopt alternate dielectric fluids for transformer applications are exemplified.
Transformers are very critical elements of modern power system network and have evolved in many ways over its long history. Transformers are most significant apparatus in interconnected power systems with consideration of several aspects like reliability, efficiency, economy, wide applications and operation. However mineral oil also called transformer oil is attracting more attention from researchers and transformer industry due to environmental concerns, limited stock, high price of petroleum resources and its disposal issues after getting contaminated. Above issues have motivated researchers to turn their attention to biodegradable and/or renewable insulating oil. Multiple facets, such as environmental requirements, safety and economic considerations, availability of technology and raw material governs the development of new insulating liquid materials. In addition, the increased equipment efficacy and the adoption of more environment friendly alternate dielectric fluids for power system solutions is gaining popularity towards steps to reduce the carbon footprint and achieving net zero cum decarbonisation global policy. This article provides the detailed review of transformer condition monitoring and the critical investigation about environment friendly dielectric fluids. They claim excellent lifecycle of the insulation, better working compatibility and improved performance compared to the traditional mineral oil base fluids in adherence of compliance, standards and graphical abstracts. These alternate dielectric fluids are getting proved as superior transformer fluids. The diverse investigational work carried out is incorporated, providing an overview of research. In addition, the future scopes, challenges and environment friendliness of various fluids are studied and deliberately articulated.
•Two-phase cooling method is reviewed for a solution for cooling high-heat-flux electronics.•The pool boiling, submerged jet impingement and confined jet impingement are discussed.•Experiments on ...different working fluids are summarized.•The effects of surface characteristics are discussed in detail.
The increasing cooling demands of electronics have attracted more attention recently for heat removal at the component with higher heat flux and heat density. Boiling, as a two-phase cooling method, is able to achieve a rapid heat removal capacity by utilizing the latent heat from liquid to vapor phase change. It is being expected to be a good solution for cooling electronics with high heat dissipation rate. The present review examines heat transfer performance of the submerged boiling configurations, namely pool boiling, submerged jet impingement and confined jet impingement. The heat transfer characteristics examined are onset of nucleate boiling, nucleate boiling heat transfer coefficient and critical heat flux (CHF). The investigations on working fluids (traditional coolant, dielectric fluids and nanofluids) are summarized and compared. The effects of surface parameters (surface roughness, contact angle, heater size, surface orientation, surface aging and surface structures) on boiling heat transfer are discussed. Contradictory results regarding the effects of jet parameters in submerged/confined jet impingement boiling are reported, suggesting that the heat transfer mechanism of submerged/confined jet impingement boiling should be further investigated. On the other hand, it is confirmed that the submerged jet is effective in delaying CHF by providing sufficient liquid replenishing to the heated surface. A novel kind of surface structure which introduces separate liquid-vapor flow pathways in pool boiling, is discussed in detail. These surfaces introduce submerged jets in pool boiling by the elaborate surface structures but not the traditional jet generation systems. It can provide us the possibility of designing a system with heat removal capacity in submerged jet impingement level, keeping the system away from pumping power and moving parts.
•Pool and flow boiling of dielectric fluids on enhanced surfaces are reviewed.•Boiling models and fundamental studies of bubble dynamics are analyzed.•Enhanced surfaces are collated and ...categorized.•h and CHF of these surfaces are compared and their transport mechanisms elucidated.•Research shortfalls are identified and future research directions are proposed.
Pool and flow boiling of dielectric fluids are efficient direct liquid cooling techniques with immense potential in the thermal management of electronic/electrical components. However, due to the increasing demand for higher rate of heat flux dissipation, many enhanced surfaces have been developed to further augment the boiling performance of dielectric fluids. This has resulted in large collections of experimental data and predictive models being reported in the recent years. The present review seeks to consolidate and highlight the recent developments in pool and flow boiling of dielectric fluids on enhanced surfaces. The various models developed to characterize the nucleate boiling curves and to predict critical heat fluxes of plain and enhanced surface are critically examined. The effects of enhanced surfaces on the bubble dynamics in dielectric fluids are also examined and the fundamental studies on boiling found in the recent literature are provided. In addition, attempts are made to categorize the various enhanced surfaces based on their fabrication techniques and their heat transfer performances and to elucidate the thermal transport mechanisms involved. Based on the literature surveyed, the various experimental results are compared, existing shortfalls are identified and areas which require further investigations are proposed.
This article describes the design and performance of a dielectric fluid cooling concept for automotive power electronics. The concept combines a low-thermal-resistance package (which eliminates ...metalized ceramic substrates) with a high-performance convective cooling strategy (slot jets impinging on finned surfaces). Modeling was first used to design the cooling system to maximize thermal performance and minimize pumping power. A prototype was then fabricated, and experiments were conducted to validate the model predictions using three fluids at various fluid flow rates (16.7 cm 3 /s 1 L/min to 68.3 cm 3 /s 4.1 L/min) and inlet temperatures (30 °C and 70 °C). The final design was compact (120-cm 3 total volume, including heat exchanger and conceptual power modules) and cooled 12 devices (e.g., silicon carbide SiC). The validated model was then used to predict the junction-to-fluid thermal resistance and pumping power for various conditions, including −40 °C fluid temperature. The results predict thermal resistance values as low as 19 mm 2 ·K/W is possible using the dielectric fluid cooling approach. The dielectric fluid cooling system is predicted to provide thermal resistance and pumping power values that are approximately 56% and 90% lower, respectively, compared to an automotive power electronics cooling system. Moreover, the dielectric fluids can be used for direct cooling of the bus bars, which can be an effective capacitor and gate driver cooling strategy and enable the use of new driveline fluids tailored for this application.
•Flow boiling performances of HFE-7100 are examined in this study.•The HTC and CHF are significantly enhanced on the present design.•The enhancements of HTC and CHF are without escalating pressure ...drop.•HTC and CHF can be substantially enhanced up to 208% and 37%.•A peak CHF value of 216 W/cm2 is achieved at mass flux of 2772 kg/m2 s.
Flow boiling of dielectric fluids in microchannels is one of the most desirable cooling solutions for high power electronics. However, the flow boiling of dielectric fluids is hindered by their unfavorable thermophysical properties. Specifically, without precooling dielectric fluids, it is challenging to promote critical heat flux (CHF) due to its high vapor density, low surface tension and the resulted superior wettability. In this study, each side wall of a five-parallel silicon microchannel array was structured with an array of microscale reentry cavities and four micronozzles bypassed by an auxiliary channel. The present microchannel configuration aims to significantly enhance CHF of HFE-7100 flow boiling by improving global liquid supply using auxiliary channels and micrononozzles as well as by sustaining liquid film using capillarity induced by reentry cavity array. Equally important, these structures can promote nucleate boiling at low heat flux, generate intense mixing, and promote thin film evaporation at high heat flux, resulting in high flow boiling heat transfer rate. Flow boiling of HFE-7100 in the present microchannel configuration is characterized with mass flux ranging from 231 kg/m2 s to 1155 kg/m2 s. The effective two-phase heat transfer coefficients (HTCs) are ranging from 6 kW/m2 K to 117 kW/m2 K. Compared to the four-nozzle plain-wall microchannels, for example, the effective HTC and CHF can be substantially enhanced up to 208% and 37%, respectively, without escalating pressure drop at a mass flux of 462 kg/m2 s. Compared to plain microchannels with inlet restrictors, CHF is considerably enhanced up to 70% with a reduction of pressure drop ∼82% at a mass flux of 1155 kg/m2 s. Significantly reduced pressure drop is achieved by integrating bypass and the enhanced confined bubble removal. A peak CHF value of 216 W/cm2 is achieved at mass flux of 2772 kg/m2 s in the present microchannel configuration with inlet temperature at room temperature.
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