CHARACTERISTICS OF BOILING BUBBLE DEPARTURE DIAMETER AND FREQUENCY FROM POROUS FOAM STRUCTURES

An experimental study on the boiling phenomena and bubble dynamics from the porous graphite foams of different thermophysical properties with FC-72 dielectric coolant under saturated pool boiling condition is presented in this paper.  The pool boiling process from different graphite foams viz. “Pocofoam” of 61%, “Pocofoam” of 75%, “Kfoam” of 78% and “Kfoam” of 72% porosities were captured by using a high speed camera at 2005 frames per second and analyzed by using “Image Pro” image processing software. The bubble departure diameter and frequency from the porous foams are presented and discussed in this study. The results show that “Pocofoam” of 61% porosity has produced the smallest bubble departure diameter and highest bubble departure frequency as compared to other porous foams. It was found that the average bubble departure diameter and bubble frequency of “Pocofoam” of 61% porosity is 480.8 µm and 173 Hz, respectively.  The experimental results have also shown that the thermal properties of the porous graphite foam affected the bubble departure diameter and frequency

Global burden of 288 causes of death and life expectancy decomposition in 204 countries and territories and 811 subnational locations, 1990–2021: a systematic analysis for the Global Burden of Disease Study 2021

BACKGROUND Regular, detailed reporting on population health by underlying cause of death is fundamental for public health decision making. Cause-specific estimates of mortality and the subsequent effects on life expectancy worldwide are valuable metrics to gauge progress in reducing mortality rates. These estimates are particularly important following large-scale mortality spikes, such as the COVID-19 pandemic. When systematically analysed, mortality rates and life expectancy allow comparisons of the consequences of causes of death globally and over time, providing a nuanced understanding of the effect of these causes on global populations. METHODS The Global Burden of Diseases, Injuries, and Risk Factors Study (GBD) 2021 cause-of-death analysis estimated mortality and years of life lost (YLLs) from 288 causes of death by age-sex-location-year in 204 countries and territories and 811 subnational locations for each year from 1990 until 2021. The analysis used 56 604 data sources, including data from vital registration and verbal autopsy as well as surveys, censuses, surveillance systems, and cancer registries, among others. As with previous GBD rounds, cause-specific death rates for most causes were estimated using the Cause of Death Ensemble model-a modelling tool developed for GBD to assess the out-of-sample predictive validity of different statistical models and covariate permutations and combine those results to produce cause-specific mortality estimates-with alternative strategies adapted to model causes with insufficient data, substantial changes in reporting over the study period, or unusual epidemiology. YLLs were computed as the product of the number of deaths for each cause-age-sex-location-year and the standard life expectancy at each age. As part of the modelling process, uncertainty intervals (UIs) were generated using the 2·5th and 97·5th percentiles from a 1000-draw distribution for each metric. We decomposed life expectancy by cause of death, location, and year to show cause-specific effects on life expectancy from 1990 to 2021. We also used the coefficient of variation and the fraction of population affected by 90% of deaths to highlight concentrations of mortality. Findings are reported in counts and age-standardised rates. Methodological improvements for cause-of-death estimates in GBD 2021 include the expansion of under-5-years age group to include four new age groups, enhanced methods to account for stochastic variation of sparse data, and the inclusion of COVID-19 and other pandemic-related mortality-which includes excess mortality associated with the pandemic, excluding COVID-19, lower respiratory infections, measles, malaria, and pertussis. For this analysis, 199 new country-years of vital registration cause-of-death data, 5 country-years of surveillance data, 21 country-years of verbal autopsy data, and 94 country-years of other data types were added to those used in previous GBD rounds. FINDINGS The leading causes of age-standardised deaths globally were the same in 2019 as they were in 1990; in descending order, these were, ischaemic heart disease, stroke, chronic obstructive pulmonary disease, and lower respiratory infections. In 2021, however, COVID-19 replaced stroke as the second-leading age-standardised cause of death, with 94·0 deaths (95% UI 89·2-100·0) per 100 000 population. The COVID-19 pandemic shifted the rankings of the leading five causes, lowering stroke to the third-leading and chronic obstructive pulmonary disease to the fourth-leading position. In 2021, the highest age-standardised death rates from COVID-19 occurred in sub-Saharan Africa (271·0 deaths [250·1-290·7] per 100 000 population) and Latin America and the Caribbean (195·4 deaths [182·1-211·4] per 100 000 population). The lowest age-standardised death rates from COVID-19 were in the high-income super-region (48·1 deaths [47·4-48·8] per 100 000 population) and southeast Asia, east Asia, and Oceania (23·2 deaths [16·3-37·2] per 100 000 population). Globally, life expectancy steadily improved between 1990 and 2019 for 18 of the 22 investigated causes. Decomposition of global and regional life expectancy showed the positive effect that reductions in deaths from enteric infections, lower respiratory infections, stroke, and neonatal deaths, among others have contributed to improved survival over the study period. However, a net reduction of 1·6 years occurred in global life expectancy between 2019 and 2021, primarily due to increased death rates from COVID-19 and other pandemic-related mortality. Life expectancy was highly variable between super-regions over the study period, with southeast Asia, east Asia, and Oceania gaining 8·3 years (6·7-9·9) overall, while having the smallest reduction in life expectancy due to COVID-19 (0·4 years). The largest reduction in life expectancy due to COVID-19 occurred in Latin America and the Caribbean (3·6 years). Additionally, 53 of the 288 causes of death were highly concentrated in locations with less than 50% of the global population as of 2021, and these causes of death became progressively more concentrated since 1990, when only 44 causes showed this pattern. The concentration phenomenon is discussed heuristically with respect to enteric and lower respiratory infections, malaria, HIV/AIDS, neonatal disorders, tuberculosis, and measles. INTERPRETATION Long-standing gains in life expectancy and reductions in many of the leading causes of death have been disrupted by the COVID-19 pandemic, the adverse effects of which were spread unevenly among populations. Despite the pandemic, there has been continued progress in combatting several notable causes of death, leading to improved global life expectancy over the study period. Each of the seven GBD super-regions showed an overall improvement from 1990 and 2021, obscuring the negative effect in the years of the pandemic. Additionally, our findings regarding regional variation in causes of death driving increases in life expectancy hold clear policy utility. Analyses of shifting mortality trends reveal that several causes, once widespread globally, are now increasingly concentrated geographically. These changes in mortality concentration, alongside further investigation of changing risks, interventions, and relevant policy, present an important opportunity to deepen our understanding of mortality-reduction strategies. Examining patterns in mortality concentration might reveal areas where successful public health interventions have been implemented. Translating these successes to locations where certain causes of death remain entrenched can inform policies that work to improve life expectancy for people everywhere. FUNDING Bill & Melinda Gates Foundation

A critical review of filmwise natural and forced convection condensation on enhanced surfaces

The use of enhanced surfaces is an efficient method to improve filmwise condensation. Due to their immense potential, many enhanced structures were developed and investigated with the aim of improving natural and forced convection condensation heat transfer coefficients. This has resulted in large collections of predictive models and experimental data being reported. In this review, the developments in this field of research in the past few decades and the recent advances are collated and examined. This paper focuses on the review of natural convection condensation on the external surfaces of enhanced flat plates and tubes and forced convection condensation in enhanced tubes. The various models predicting the heat transfer coefficients on plain and enhanced surfaces are evaluated. For natural convection condensation, the liquid film-based models, semi-empirical models and numerical modes are reviewed whereas, for forced convection condensation, the gravity-dominated and vapor shear-dominated models are discussed. The effects of these enhanced structures on the liquid film and two-phase flow characteristics are analyzed and the various types of enhanced tubes and flat plates investigated are categorized. The manufacturing techniques employed to fabricate these surfaces are identified. A detailed evaluation of the heat transfer and pressure drop performances of the various enhanced surfaces is performed. In addition, their thermal performances are summarized and compared, and their associated heat transfer mechanisms are elucidated. For external condensation on a single tube row, three-dimensional fin structures were found to provide better thermal performance than two-dimensional structures, with some three-dimensional fin structures exhibiting more than 6 times the heat transfer coefficients of a plain surface. However, in a tube bundle, the heat transfer coefficient of three-dimensional fin tubes decreases more significantly with increasing tube row as compared to two-dimensional fin tubes. For convective condensation in circular tubes, the herringbone and pin fin tubes demonstrated better thermal and pressure drop performances than other internally enhanced tubes. Their efficiency indices were between 1.25 and 1.28. Based on the literature surveyed, the various experimental results are compared, existing research gaps are identified and frameworks for future research work are provided.Ministry of Education (MOE)Nanyang Technological UniversityThe first author would like to acknowledge the financial support for his research appointment at the University of Illinois at Urbana Champaign, USA under the College of Engineering (CoE) International Postdoctoral Fellowship Scholarship (IPS) provided jointly by the Ministry of Education, Singapore and Nanyang Technological University, Singapore

Experimental study of flow boiling of FC-72 in fractal-like flow channels

In this study, symmetrical and dichotomous fractal flow channel designs with various configurations in terms of the number of channels being split into smaller channels, were investigated experimentally for their flow boiling heat transfer performance with FC-72 as the working fluid. A multi-channel parallel channel design depicted as Parallel was used as a benchmark. The channels were fabricated by Selective Laser Melting (SLM). Three mass fluxes were studied, viz., 600, 900 and 1200 kg/m2⋅s. The peak heat transfer coefficient and pressure drop were investigated. High-speed visualization studies were also employed to examine the two-phase behaviors in the channels. The fractal flow channels were found to perform comparably with the Parallel channel at low mass fluxes but achieved higher heat transfer coefficients at higher mass fluxes, with the design n = 2, i.e., channel splitting into smaller channels twice, being the highest. The pressure drops of the fractal flow channel designs are higher than the Parallel channel due to their longer channel lengths, and the flow impacting multiple bifurcations at an angle, with n = 4, i.e., channel splitting into smaller channels four times, experiencing the highest pressure drop for all mass fluxes. Visualization studies show that fractal flow channels cause an early annular flow, enhancing heat transfer. Flow reversals in the Parallel channel were observed especially at higher mass fluxes, which suggest no enhancement in the heat transfer coefficient. The fractal flow channels, however, do not experience any flow reversal which could be attributed to the acceleration of the two-phase flow after each bifurcation, which in turn increases the inertia force to overcome the force from vapor expansion. The visualization studies also suggest that increasing the complexity does not result in better flow boiling heat transfer performance. This is due to the multiple flow separations which resulted in no two-phase heat transfer interaction and thus, under-utilization of the entire channel surface area for heat transfer. A comparison with a general correlation for flow boiling gives mean absolute errors of 31.6% and 35.3% for the fractal flow channels and Parallel, respectively.National Research Foundation (NRF)Accepted versionThe SLM 250 HL equipment used in this research is supported by the National Research Foundation, Prime Minister’s Office, Singapore under its Medium-Sized Centre funding scheme

Fabrication of minichannel fractal flow channels by selective laser melting for two-phase flow cooling applications

Fractal design flow channels provide better flow stability and heat dissipation capacity than conventional parallel channel flow channels under flow boiling conditions although they are difficult to fabricate. The Selective Laser Melting (SLM) technique is selected to explore the feasibility of fabricating minichannel fractal flow channels with four various branch levels, denoted as c = 1, 2, 3, 4, using AlSi10Mg, an aluminium alloy metallic powder. The largest inaccuracy of the flow channel diameters was found to be 1.6%. The flow boiling heat transfer performance was also investigated at a mass flux of 900 kg/m²∙s and was found that c = 2 has the highest heat transfer coefficient, being 3.5%, 0.9% and 5.8% higher than c = 1, c = 3 and c = 4. Preliminary studies shown that the higher flow channel designs experienced dryout at lower heat supplied, which hinders heat transfer performance. This trend may show that higher branch levels may not necessarily lead to better thermal performance.NRF (Natl Research Foundation, S’pore)Published versio

Study of heat transfer and coating properties in fluidized bed powder coating of metal surfaces

In this project, a comprehensive study of the fluidized bed coating process was carried out using a two-pronged experimental and numerical approach.RG 30/9

Thermal characteristics of non-conventional refrigerants and coolants

The objective of this present project is to focus on pool film boiling; which is also one of the least understood aspects of the boiling process. As for the refrigerants, various types have been tried in the experiment, and only the results for most complete sets for R-22 and R-134a are reported here.RP 42/9

Entropy generation for flow boiling on a single semi-circular minichannel

The paper investigates the effects of parameters such as mass flux, heat flux, channel diameter and inlet pressure on the entropy generation in flow boiling inside a semi-circular minichannel. A general entropy generation equation is derived for a single minichannel for flow boiling with developing flow, while also relaxing some heat transfer assumptions such as [Formula presented]≪1. The Romberg integration technique is used to solve for the entropy generation. Our results show that an increase in the mass flux causes an abrupt change in the entropy generation when the flow changes from laminar to transitional flow at small diameters although the effect is less significant at larger diameters. The heat transfer contribution in the entropy generation is also higher than the pressure drop contribution for the larger diameters. The larger channel diameters produce higher entropy generation compared to the smaller diameters for every heat flux investigated due to the increase in heat transfer contribution. An increase in the inlet pressure also decreases the entropy generation for every mass flux and heat flux. The aims of this study are to enhance the knowledge of the effects of heat transfer, pressure drop and flow behavior on the entropy generation, and to encourage researchers and designers to explore more novel features that can take advantage on the minimization of entropy generation

Experimental and numerical investigation of binary alloy solidification in a die-casting process

This report presents the findings of research work on the experimental and numerical investigations of pure material and binary alloy solidification under the AcRF research grant, RG 67/96. The pure material and binary alloy used in the solidification experiments are n-hexadecane and ammonium chloride-water solution, respectively. Several solidification experiments were performed under either constant wall temperature or constant heat flux (constant heat rate) condition. Numerical simulations were also performed using PHOENICS software to solve for the phase front and temperatures

Development of experimental rig for the simulation of solidification inside a die-casting cavity

56 p.This report presents the development of an experimental rig to carry out extensive investigations of the solidification behaviour inside a die-casting cavity. A rectangular enclosure was chosen to simulate the die-casting cavity. The results of the extensive solidification experiments inside a rectangular test cell with varying aspect ratio from 1 to 3, using nparaffin phase change materials are discussed in this report.RP 39/9