Corrections for the hydrodynamic instability based critical heat flux models in pool boiling – effects of viscosity and heating surface size

This paper presents corrections for existing hydrodynamic instability based Critical Heat Flux (CHF) models in pool boiling by taking into account the effect of the viscosity, geometry and size of the liquid-vapour interface. Based on the existing literature, the Kelvin – Helmholtz theory, used by the most commonly adopted CHF models, can lead to noticeable errors when predicting the instability conditions. The errors are mainly due to the inaccuracy of the inviscid flow assumptions and the oversimplification of the interface geometry. In addition, the literature suggests the most unstable condition predicted by the Viscous Correction for Viscous Potential Flow (VCVPF) theory for the cylindrical interfaces best match the observed air column breakup conditions in water. In this paper, the most unstable instability conditions predicted by the VCVPF theory are used to correct the existing CHF models. The comparison between the existing and corrected CHF models suggests that the corrected models always predict a higher CHF value. In addition, the corrected Zuber model predicts similar CHF value to the Lienhard and Dhir model. The comparison with experimental data suggests that the correction to the Zuber model can increase its prediction accuracy in most cases, but not necessary for the Lienhard and Dhir model. When compared to experimental CHF data for boiling cryogens at different pressures, the corrected CHF models are consistently more accurate than the original CHF models

Experimental investigation of the Kelvin-Helmholtz instabilities of cylindrical gas columns in viscous fluids

This paper derives analytical solutions for the critical Kelvin–Helmholtz (KH) instability conditions at the interface between a cylindrical gas column and a pool of viscous immiscible fluid confined in a chamber of finite size. The analysis focuses on conditions of negligible heat and mass transfer. The derivations are based on the established approaches reported in the literature with different boundary conditions. The most unstable instability conditions have also been calculated numerically. Experiments designed to measure the actual air column break-up conditions in water have been carried out to validate the analytical models. Comparisons show that the most unstable conditions predicted by the Viscous Corrections of the Viscous Potential Flow KH model are the best match to the experimentally measured break-up conditions. Parametric investigation of the instability theories shows that the vapour column size has a noticeable effect on the critical conditions, but has a negligible effect on the most unstable conditions when the column radius is greater than 1.2 mm. Furthermore, the critical instability conditions are sensitive to the chamber size and the perturbation symmetry, while the most unstable conditions are insensitive to these parameters

Modeling of a liquid nitrogen droplet evaporating inside an immiscible liquid pool

Evaporation of liquid nitrogen in another immiscible liquid occurs in many industrial applications. Existing oversimplified one-dimensional (1D) quasi-steady models, although can quantitatively predict the evaporation rate by introducing an empirical fitting parameter, rely on configurations inconsistent with experimental observation so more rigorous models are required to get in-depth physical insights and improve modeling capability. This study proposes a 2D quasi-steady-state theoretical model, free of fitting parameters, that predicts the bubble growth rate and estimates the heat transfer rate for a liquid nitrogen droplet evaporating inside a liquid pool within the spherical bubble regime. The droplet's shape and position within a spherical bubble are determined by the equilibrium between the gravitational force and the upward pressure force resulting from the vapor flow between the droplet and the pool. The vapor layer thickness is calculated to be on the order of 10 microns. Notably, the primary contribution to heat transfer arises from the lower portion of the droplet, leading to local heat flux values up to approximately six times higher at the bottom compared to the top. The predicted bubble growth is quantitatively consistent with experimental data within the capillary spherical bubble regime. Furthermore, the overall heat transfer rate Q exhibits a distinct scaling relationship with the volume ratio between the bubble and droplet, yielding

Investigation of the soot formation in ethylene laminar diffusion flames when diluted with helium or supplemented by hydrogen

A new optical diagnostic technique has been used to measure the spatially distributed temperatures, soot diameters, and soot volume fractions in several different ethylene laminar diffusion flames to investigate the effect of adding hydrogen and helium on the soot formation. The test results show that adding hydrogen increases the flame temperature in all regions, while adding helium does not significantly affect the flame temperature in the reaction region but does increase the flame temperature elsewhere. The flame heights when adding helium and hydrogen can be calculated using the correlation introduced by Roper if the ethylene diffusion coefficient is used. This indicates that the flame height is determined by the diffusion of ethylene molecules when the hydrogen fraction is below 20%. It was also found that either adding helium or hydrogen does not significantly affect the soot diameter but does reduce the soot volume fraction. A total of 20% of helium addition by volume was measured to reduce the total soot number by 19%, while a total of 20% of hydrogen addition reduced the total soot number by 23%. In comparison, replacing the hydrocarbon with hydrogen is much more effective in reducing soot formation. Replacement of 25% ethylene by hydrogen was measured to reduce the total soot number by 66%. Apart from demonstrating the influence of hydrogen and helium on ethylene diffusion flames, these measurements provide additional data for modelers of diffusion flames, especially those with an interest in the formation of particulate matter. © 2014 American Chemical Society

Measurement of the spatially distributed temperature and soot loadings in a laminar diffusion flame using a Cone-Beam Tomography technique

A new low-cost optical diagnostic technique, called Cone Beam Tomographic Three Colour Spectrometry (CBT-TCS), has been developed to measure the planar distributions of temperature, soot particle size, and soot volume fraction in a co-flow axi-symmetric laminar diffusion flame. The image of a flame is recorded by a colour camera, and then by using colour interpolation and applying a cone beam tomography algorithm, a colour map can be reconstructed that corresponds to a diametral plane. Look-up tables calculated using Planck's law and different scattering models are then employed to deduce the temperature, approximate average soot particle size and soot volume fraction in each voxel (volumetric pixel). A sensitivity analysis of the look-up tables shows that the results have a high temperature resolution but a relatively low soot particle size resolution. The assumptions underlying the technique are discussed in detail. Sample data from an ethylene laminar diffusion flame are compared with data in the literature for similar flames. The comparison shows very consistent temperature and soot volume fraction profiles. Further analysis indicates that the difference seen in comparison with published results are within the measurement uncertainties. This methodology is ready to be applied to measure 3D data by capturing multiple flame images from different angles for non-axisymmetric flame. © 2013 Elsevier Ltd

Analysis of the particulate emissions and combustion performance of a direct injection spark ignition engine using hydrogen and gasoline mixtures

Three different fractions (2%, 5%, and 10% of stoichiometric, or 2.38%, 5.92%, and 11.73% by energy fraction) of hydrogen were aspirated into a gasoline direct injection engine under two different load conditions. The base fuel was 65% iso-octane, and 35% toluene by volume fraction. Ignition sweeps were conducted for each operation point. The pressure traces were recorded for further analysis, and the particulate emission size distributions were measured using a Cambustion DMS500. The results indicated a more stable and faster combustion as more hydrogen was blended. Meanwhile, a substantial reduction in particulate emissions was found at the low load condition (more than 95% reduction either in terms of number concentration or mass concentration when blending 10% hydrogen). Some variation in the results occurred at the high load condition, but the particulate emissions were reduced in most cases, especially for nucleation mode particulate matter. Retarding the ignition timing generally reduced the particulate emissions. An engine model was constructed using the Ricardo WAVE package to assist in understanding the data. The simulation reported a higher residual gas fraction at low load, which explained the higher level of cycle-by-cycle variation at the low load

Predicting the critical heat flux in pool boiling based on hydrodynamic instability induced irreversible hot spots

A new model, based on the experimental observation reported in the literature that CHF is triggered by the Irreversible Hot Spots (IHS), has been developed to predict the Critical Heat Flux (CHF) in pool boiling. The developed Irreversible Hot Spot (IHS) model can predict the CHF when boiling methanol on small flat surfaces and long horizontal cylinders of different sizes to within 5% uncertainty. It can also predict the effect of changing wettability (i.e. contact angle) on CHF to within 10% uncertainty for both hydrophilic and hydrophobic surfaces. In addition, a linear empirical correlation has been developed to model the bubble growth rate as a function of the system pressure. The IHS model with this linear bubble growth coefficient correlation can predict the CHF when boiling water on both flat surfaces and long horizontal cylinders to within 5% uncertainty up to 10 bar system pressure, and the CHF when boiling methanol on a flat surface to within 10% uncertainty up to 5 bar. The predicted detailed bubble grow and merge process from various sub-models are also in good agreement with the experimental results reported in the literature

Association between cultural capital and health literacy during the COVID-19 pandemic among community residents in China: the mediating effect of social capital

BackgroundHealth literacy is crucial for managing pandemics such as COVID-19 and maintaining the health of the population; our goal was to investigate the impact of cultural capital on health literacy during the COVID-19 pandemic among community residents and to further examine the mediating role of social capital in the relationship between cultural capital and health literacy.MethodsA cross-sectional study was conducted among 1,600 community residents selected in Chongqing, China using a stratified random sampling method. Data were gathered through a questionnaire survey, including sociodemographic characteristics, cultural capital, social capital, and health literacy. Chi-square analysis, one-way ANOVA, t-test, and hierarchical linear regression were used to analyze the level of health literacy among community residents and the related elements; the structural equation model (SEM) was used to explore the influential mechanisms of health literacy and explore whether social capital acted as a mediator in the relationship between cultural capital and health literacy.ResultsCultural capital, community participation, community trust, reciprocity, and cognitive social capital had a significant positive effect on health literacy. In addition, the results of SEM indicated that cultural capital not only directly influences health literacy (β = 0.383, 95% CI = 0.265–0.648), but also indirectly influences health literacy through three types of social capital (β = 0.175, 95% CI = 0.117–0.465; β = 0.191, 95% CI = 0.111–0.406; β = 0.028, 95% CI = 0.031–0.174); its mediating effect accounting for 50.7% of the overall effect.ConclusionsOur results highlight the empirical link between cultural capital and health literacy, and suggest that social capital mediates this connection. These findings suggest that governments and communities should focus on the construction of community cultural capital and provide residents with better social capital to improve their health literacy to prepare for future pandemics

The dynamics of droplet impact on a heated porous surface

In this paper, droplet impact on a porous surface is experimentally investigated over a wide range of Weber numbers and surface temperatures. Regime transition criteria have been deduced to determine droplet post-impingement behaviour as a function of the Weber number and surface temperature for which a droplet impacting on a porous surface. Based on the energy balance, an analytical model with improved boundary layer description is proposed to predict maximum spreading of droplet following impact on porous surfaces when the effect of heat transfer is negligible. The results of the model indicate that the spreading process after droplet impact on porous surfaces is governed by the viscous dissipation and matric potential. The maximum-spread model predictions agreed well with experimental measurements reported in this paper and the literature over a large range of Weber numbers and different porous surfaces