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In rarefied gas flows, discontinuity phenomena such as velocity slip and temperature jump commonly appear in the gas layer adjacent to a solid boundary. Due to the physical complexity of the interactions at the gas-solid interface, particularly in the case of systems with local nonequilibrium state, boundary models with limited number of parameters cannot completely describe the reflection of gas molecules at the boundary. In this work, the Gaussian mixture (GM) model, which is an unsupervised machine learning technique, is employed to construct a statistical gas-solid surface scattering model based on the collisional data obtained from molecular dynamics (MD) simulations. The GM model is applied to study Couette flow for different inert gases (Ar and He) confined between two parallel infinite gold walls at different temperatures. A direct comparison between the results obtained from the GM model and the Cercignani-Lampis-Lord (CLL) scattering kernel against the MD collisional data in terms of the distribution of the predicted postcollisional velocities, and accommodation coefficients has shown that the results from the GM model are an excellent match with the MD results outperforming the CLL scattering kernel. As an example, for He gas, while the predicted energy accommodation coefficient by the CLL model is more than two times higher than the MD predictions, the value computed by the GM model is in excellent agreement with the MD results. This superior performance of the GM model confirms its high potential to derive a generalized boundary condition in systems encountered with highly nonequilibrium and complex gas flow conditions.

Thermochemical energy storage using salt hydrates is a promising concept to bridge the gap between supply and demand for solar thermal energy in residential buildings. Using a suitable thermochemical material such as a salt hydrate, a thermal energy storage device, also known as a heat battery, can be created to supply low-temperature thermal energy during colder periods. To generate adequate power from a heat battery for the production of domestic hot tap water or space heating, the hydration rate of the salt hydrate needs to be sufficiently fast. It is hypothesized that the hydration rate of the material increases over multiple charge and discharge cycles due to crack formation and volume increase of the salt hydrate particles. This hypothesis is tested by performing two kinds of experiments: optical microscopy experiments using a micro-climate chamber to evaluate the particle size, and Thermo Gravimetric Analysis (TGA) experiments to determine the hydration rate of the particles. The hydration rate and particle size are input for a nucleation and growth model that takes into account crack formation and particle growth. Optical microscopy experiments show a particle expansion of approximately 30 % over 12 cycles. Typical hydration rates are increased by a factor 15 comparing the first and the 12th TGA cycle. It is shown that particle growth and crack formation significantly contribute to the improvement of the hydration rate. Finally, taking into account crack formation and particle growth in the numerical model results in a good agreement between model and experiments. Such a numerical model can be used for heat battery design.

People's subjective response to any thermal environment is commonly investigated by using rating scales describing the degree of thermal sensation, comfort, and acceptability. Subsequent analyses of results collected in this way rely on the assumption that specific distances between verbal anchors placed on the scale exist and that relationships between verbal anchors from different dimensions that are assessed (e.g. thermal sensation and comfort) do not change. Another inherent assumption is that such scales are independent of the context in which they are used (climate zone, season, etc.). Despite their use worldwide, there is indication that contextual differences influence the way the scales are perceived and therefore question the reliability of the scales’ interpretation. To address this issue, a large international collaborative questionnaire study was conducted in 26 countries, using 21 different languages, which led to a dataset of 8225 questionnaires. Results, analysed by means of robust statistical techniques, revealed that only a subset of the responses are in accordance with the mentioned assumptions. Significant differences appeared between groups of participants in their perception of the scales, both in relation to distances of the anchors and relationships between scales. It was also found that respondents’ interpretations of scales changed with contextual factors, such as climate, season, and language. These findings highlight the need to carefully consider context-dependent factors in interpreting and reporting results from thermal comfort studies or post-occupancy evaluations, as well as to revisit the use of rating scales and the analysis methods used in thermal comfort studies to improve their reliability.My individual contribution: Runa T. Hellwig: Conceptualization, Methodology, Visualization, Writing - Original Draft \ my contribution like all authors Investigation, Writing - Review & Editing, ResourcesInvestigation, Writing - Review & Editing, ResourcesRuna T. Hellwig: Conceptualization,Methodology, Visualization, Writing - Original DraftRuna T. Hellwig: Conceptualization,Methodology, Visualization, Writing - Original DraftRuna T. Hellwig: Conceptualization,Methodology, Visualization, Writing - Original DraftAuthor statementAll authors: Investigation, Writing - Review & Editing, Resources Marcel Schweiker: Conceptualization,Methodology, Visualization, Formal analysis, Writing - Original Draft Rucha Amin: Conceptualization,Methodology Susanne Becker: Conceptualization, Methodology, Writing - Original Draft Joon-Ho Choi:Conceptualization, Methodology Chungyoon Chun: Conceptualization, Methodology Lyrian Daniel:Writing - Original Draft Renata De Vecchi: Writing - Original Draft Stephanie Gauthier:Conceptualization, Methodology, Writing - Original Draft Runa T. Hellwig: Conceptualization,Methodology, Visualization, Writing - Original Draft Gesche M Huebner: Conceptualization,Methodology, Writing - Original Draft Jungsoo Kim: Conceptualization, Methodology Jakub Kolarik:Writing - Original Draft M.C. Jeffrey Lee: Conceptualization, Methodology Karin Schakib-Ekbatan:Conceptualization, Methodology Despoina Teli: Conceptualization, Methodology Conrad Voelker:Writing - Original Draft

Thermal discomfort is one of the main triggers for occupants’ interactions with components of the built environment such as adjustments of thermostats and/or opening windows and strongly related to the energy use in buildings. Understanding causes for thermal (dis-)comfort is crucial for design and operation of any type of building. The assessment of human thermal perception through rating scales, for example in post-occupancy studies, has been applied for several decades; however, long-existing assumptions related to these rating scales had been questioned by several researchers. The aim of this study was to gain deeper knowledge on contextual influences on the interpretation of thermal perception scales and their verbal anchors by survey participants. A questionnaire was designed and consequently applied in 21 language versions. These surveys were conducted in 57 cities in 30 countries resulting in a dataset containing responses from 8225 participants. The database offers potential for further analysis in the areas of building design and operation, psycho-physical relationships between human perception and the built environment, and linguistic analyses., Measurement(s)Natural Language • Demographics • humidity • response to temperature stimulus • temperature of air • geographic location • room temperature ambient airTechnology Type(s)Survey • sensorSample Characteristic - OrganismHomo sapiensSample Characteristic - EnvironmentbuildingSample Characteristic - LocationOman • Kingdom of Spain • Ecuador • Chile • Malaysia • Iraq • Poland • Nigeria • Iran • Italy • United States of America • Germany • Brazil • Jordan • Kingdom of the Netherlands • Greece • Sweden • Kingdom of Denmark • India • Sri Lanka • Uganda • South Korea • Republic of China • United Arab Emirates • Australia • French Republic • United Kingdom • Japan • Indonesia • China Machine-accessible metadata file describing the reported data: 10.6084/m9.figshare.9805289

In-line electrolyte monitoring during hemodialysis treatment is an important step towards personalized treatment, for optimization of clinical blood pressure management and minimization of cardiovascular complications and bone-mineral disease. But how to achieve such in-line measurement? We propose a real-time electrolyte measurement system based on a molecular fluorescence “on-off” switching mechanism. We have fabricated a disposable polymeric micro-optofluidic device in polydimethylsiloxane (PDMS) with integrated optical fibers to enable in-line electrolyte monitoring. The sensing principle is based on photoinduced electron transfer (PET) that ion-selectively quenches the fluorescence of a molecular optical probe. The PET molecules are covalently immobilized onto the PDMS microchannel walls. As a proof-of-concept, we determine sodium concentration in a flowing medium. A clear relation between fluorescence intensity and sodium concentration is observed. The PET principle can also be applied to many other ion types.

The quality of local skin temperature prediction by thermophysiological models depends on the local skin blood flow (SBF) control functions. These equations were derived for low activity levels (0.8 - 1 met) and mostly in sitting or supine position. This study validates and discusses the prediction of foot SBF during activities of 1 - 3 met in male and females, and the effect on the foot skin temperature prediction (Delta T-skin,T- foot) using the thermophysiological simulation model ThermoSEM. The SBF at the foot was measured for ten male and ten female human subjects at baseline and during three activities (sitting, walking at 1 km/h, preferred walking around 3 km/h). Additional measurements included the energy expenditure, local skin temperatures (T-skin,T- loc), environmental conditions and body composition. Measured, normalized foot SBF is 2-8 times higher than the simulated SBF during walking sessions. Also, SBF increases are significantly higher in females vs. males (preferred walking: 4.8 +/- 1.5 versus 2.7 +/- 1.4, P

Molecular dynamics (MD) simulations are conducted to determine energy and momentum accommodation coefficients at the interface between rarefied gas and solid walls. The MD simulation setup consists of two parallel walls, and of inert gas confined between them. Different mixing rules, as well as existing ab-initio computations combined with interatomic Lennard-Jones potentials were employed in MD simulations to investigate the corresponding effects of gas-surface interaction strength on accommodation coefficients for Argon and Helium gases on a gold surface. Comparing the obtained MD results for accommodation coefficients with empirical and numerical values in the literature revealed that the interaction potential based on ab-initio calculations is the most reliable one for computing accommodation coefficients. Finally, it is shown that gas–gas interactions in the two parallel walls approach led to an enhancement in computed accommodation coefficients compared to the molecular beam approach. The values for the two parallel walls approach are also closer to the experimental values.

Wearable sensors are positioned close to, on, or even inside the human body and measure vital functions such as heart rate, temperature, or even biochemical parameters. [...]

An accurate local thermal sensation model is indispensable for the effective development of personalized conditioning systems in office environments. The output of such a model relies on the accurate prediction of local skin temperatures, which in turn depend on reliable input data of the local clothing thermal resistance and clothing area factor. However, for typical office clothing ensembles, only few local datasets are available in the literature. In this study, the dry thermal resistance was measured for 23 typical office clothing ensembles according to EN-ISO 15831 on a sweating agile manikin. For 6 ensembles, the effects of different air speeds and body movement were also included. Local clothing area factors were estimated based on 3D scans. Local differences can be found between the measured local insulation values and local area factors of this study and the data of other studies. These differences are likely due to the garment fit on the manikin and reveal the necessity of reporting clothing fit parameters (e.g., ease allowance) in the publications. The increased air speed and added body movement mostly decreased the local clothing insulation. However, the reduction is different for all body parts, and therefore cannot be generalized. This study also provides a correlation between the local clothing insulation and the ease allowance for body parts covered with a single layer of clothing. In conclusion, the need for well-documented measurements is emphasized to get reproducible results and to choose accurate clothing parameters for thermo-physiological and thermal sensation modeling. Electronic supplementary material The online version of this article (10.1007/s00484-018-1625-0) contains supplementary material, which is available to authorized users.

Introduction: End-stage renal disease (ESRD) patients depend on dialysis for removal of toxic waste products, fluid overload relief and maintenance of electrolyte balance. Dialysis prolongs millions of lives. To some extent, ESRD has become a manageable disease with a steadily growing dialysis population of increasing average age and associated comorbidity. During 7 decades many technical refinements have been developed e.g. sodium profiling, blood volume, ultrafiltration variation based on blood pressure measurement, urea kinetics etc. Despite its large potentials, in-line electrolyte monitoring lags behind in dialysis treatment.Areas covered: In this paper, we review the state of technologies available for in-line monitoring of the electrolytes sodium, potassium and calcium during hemodialysis.Expert commentary: We concluded that individual optimization of dialysate composition should be able to improve hard medical outcomes, but practical clinical implementation stands/falls with reliable and affordable in-line ion-selective sensing technology. Optical ion-selective microsensors and microsystems form a promising pathway for individualizing the dialysis treatment.

We present a spectroscopy technique to measure temperature locally in a polydimethylsiloxane (PDMS) micro-optofluidic chip with integrated optical fibers and minimal optical components. The device was fabricated in one step with fiber coupler grooves followed by manual integration of the optical fibers. The experimental setup consists of a micro-optofluidic chip with a pair of optical fibers for excitation and fluorescence collection, a laser module and a spectrometer. The laser module is coupled to one of the optical fibers to guide the light into the microchannel. The fluorescence signal is collected by a second integrated optical fiber placed orthogonally. A spectroscopy technique is used to measure the local temperature in a microchannel (500 μm wide and 125 μm in height) using Rhodamine B as a temperature indicator. It is shown that for a flow rate between 200 and 400 μL/min, the local temperature can be determined., We present a spectroscopy technique to measure temperature locally in a polydimethylsiloxane micro-optofluidic chip with integrated optical fibers and minimal optical components. The device was fabricated in one step with fiber coupler grooves followed by the manual integration of the optical fibers. The experimental setup consists of a micro-optofluidic chip with a pair of optical fibers for excitation and fluorescence collection, a laser module, and a spectrometer. The laser module is coupled to one of the optical fibers to guide the light into the microchannel. The fluorescence signal is collected by a second integrated optical fiber placed orthogonally. A spectroscopy technique is used to measure the local temperature in a microchannel (500 μm wide and 125 μm in height) using Rhodamine B as a temperature indicator. It is shown that for a flow rate between 200 and 400 μL/min, the local temperature can be determined.

Objectives: After cardiopulmonary bypass, patients often show redistribution hypothermia, also called afterdrop. Forced-air blankets help to reduce afterdrop. This study explores the effect of forced-airblankets on temperature distribution and peripheral perfusion. The bloodperfusion data is used to explain the observed temperature effects and the reduction of the afterdrop. Methods: Fifteen patients were enrolled in a randomised study. In the test group (n = 8), forced-air warmers were used. In the control group (n = 7), only passive insulation was used. Core and skin temperatures and thigh temperatures at 0, 8, 18 and 38 mm depth were measured. Laser Doppler flowmetry (LDF) was used to record skin perfusion from the big toe. Blood flow through the femoral artery was determined with ultrasound. Results: Afterdrop in the test group was smaller than in the control group (1.2 � 0.2 8C vs 1.8 � 0.7 8C: P = 0.04) whilst no significant difference in mean tissue thigh temperature was found between the groups. Local skin temperature was 2.5—3.0 8C higher when using forced-air heaters. However, skin perfusionwas unaffected. Ultrasound measurements revealed that leg bloodflow during the first hours after surgery was reduced to � 70% of pre- and peri-operative values. Conclusions: Forced-air blankets reduce afterdrop. However, they do not lead to clinical relevant changes in deep thigh temperature. LDF measurements show that forced-air heating does not improve toe perfusion. The extra heat especially favours core temperature. This is underlined by the decrease in postoperative leg blood flow, suggesting that the majority of the warmed blood leaving the heart flows to core organs and not to the periphery. # 2007 European Association for Cardio-Thoracic Surgery. Published by Elsevier B.V. All rights reserved.

We study the deviations for the results of the properties of a hard-sphere gas near the walls of a micro/nano channel using the hybrid MD-MC simulation method compared to the pure MD and MC results. Our model for the micro channel consists of two parallel infinite plates situated at distance L apart from each other, and of gas molecules confined between these two walls. We study the dependence of the deviations for higher densities, considering different lengths of the different simulation domains in the hybrid MD-MC method. We find that when density is increased, the deviations in the pure MC results are increasing compared to pure MD results. The deviations in the hybrid simulation results are decreasing and are very small when increasing the width of the solid-gas interface. The deviations of the pure MC simulation results from the pure MD simulation results for the number density are found to be around 0.9%, when the reduced density η = 0.1 and the width of the channel L = 50λ, where λ is the mean free path. When the hybrid method is used, the deviations are decreasing with a factor from two to three, and are between 0.32%–0.42%. For more dense gas (η = 0.2), the deviations of the MC simulation results for the number density are found to be 1.71%, and the deviations of the hybrid MD-MC simulation results between 0.246% and 0.6977%. We discuss how these deviations in case of a dense gas (η = 0.2) depend on the width of the interface, and we study it for the case when the MD domain is 10% and MC domain is 90% from the simulation domain, and also for the case when the MD domain is 50% and MC domain 50% from the whole simulation domain. For more dilute gas, the MC, MD and hybrid MC-MD simulations are in very good agreement and the deviations are negligible.Copyright © 2005 by ASME

A three dimensional Molecular Dynamics simulation method was used to study the effect of different geometries for rarefied gas flows in nanochannels. Argon molecules have been used. The velocity profiles in the channel were obtained and analyzed with three different channel geometries: a circular, a rectangular (square), and a slit channel. We found that when using the same driving force, the maximum velocity of the flow increases when the geometry changes in the order from circular geometry to rectangular geometry to slit geometry, where the latter becomes 2∼2.5 times as large compared with either the rectangular or circular channel. Rectangular channels showed a similar maximum and slip velocity as the circular channel while the velocity profile was qualitatively similar to the slit channel for Kn higher than 1.0. We also investigated the effect of different Knudsen numbers on the velocity profiles. A channel width of 50nm is used for the simulation. We found that for Kn higher than 2∼3, the Knudsen number has a comparably small influence on the slip velocity for circular channel and rectangular channel.

Intradialytic hypotension (IDH) is one of the most common complications of hemodialysis (HD) treatment. The initiating factor of IDH is a decrease in blood volume, which is related to an imbalance between ultrafiltration (UF) and refilling rate. Impaired reactivity of resistance and capacitance vessels in reaction to hypovolemia plays possibly a major role in the occurrence of IDH. These vessels also fulfill an important function in body temperature regulation. UF-induced cutaneous vasoconstriction would result in a reduced surface heat loss and an increase in core temperature. To release body heat, skin blood flow is increased at a later stage of the HD treatment, whereby possibly IDH can occur. The aim of the study is to develop a mathematical model that can provide insight into the impact of thermoregulatory processes on the cardiovascular (CV) system during HD treatment. The mathematical procedure has been created by coupling a thermo-physiological model with a CV model to study regulation mechanisms in the human body during HD + UF. Model simulations for isothermal versus thermoneutral HD + UF were compared with measurement data of patients on chronic intermittent HD (n = 13). Core temperature during simulated HD + UF sessions increased within the range of measurement data (0.23°C vs. 0.32 ± 0.41°C). The model showed a decline in mean arterial pressure of -7% for thermoneutral HD + UF versus -4% for isothermal HD + UF after 200 min during which relative blood volume changed by -13%. In conclusion, simulation results of the combined model show possibilities for predicting circulatory and thermal responses during HD + UF.

Skin blood flow is of major importance in human thermoregulation. Classic thermoregulation models require an explicit set point to control temperature. Normally such a set point is defined in the unit of the controlled variable (i.e. Celsius). However, the human body does not sense temperature directly, instead temperature information is coded into neuron fire rates. Here we explored the neurophysiology of thermoregulation to develop a mathematical model of skin blood flow that does not require a set point. The model was developed on measurement data of skin temperature, core temperature and skin blood flow and was validated using k-fold cross validation. The model explained over 90% of the variance in the measurements (r2=0.91). Hence, the results are promising and indicate that emulation of thermoregulatory neurophysiology is able to capture the dynamics of skin blood flow control.

In this work we use a three dimensional Molecular Dynamics simulation method to study the effect of different geometries and Knudsen number regimes on the gas flow in micro-nanochannels. Argon molecules have been used for the simulations. Thermal wall and diffusive-specular wall types were used for the boundaries of the channels. The velocity profiles in the channel were obtained and analyzed with three different channel geometries that are commonly used in the industry: circular, rectangular (square), and slit channel. We found that when using the same driving force, the maximum velocity of the flow increases when the geometry changes in the order from circular geometry to rectangular geometry to slit geometry, where the latter becomes 1.2∼1.5 times as large compared with either the rectangular or circular channel. While the absolute values of the velocity profiles show a distinct difference according to the different geometries, geometry effect on the shape of the velocity profile also shows interesting features. Rectangular tube shows much flatter profile compared with the other two channels. Also the effect of the size of the channels and different Knudsen numbers on the velocity profiles is investigated. Two different sizes were used here: 100nm and 10nm corresponding to typical sizes of a nano channel and carbon nanotubes. We found that the Knudsen number has an effect on the slip and maximum flow velocity for the slit geometry even for higher Knudsen number. For the Kn higher than approximately 3, it was found that the Knudsen number has a small influence on the slip flow velocity for the circular channel and rectangular channel than for lower Knudsen number.Copyright © 2011 by ASME

This paper describes a particle-separation device combining AC electroosmosis and dielectrophoresis under pressure-driven flow. The whole device comprises an initial hydrodynamic-focusing compartment with Y junction and an electrohydrodynamic compartment with interdigitated coplanar ITO electrode arrays. In the electrohydrodynamic compartment, the electrode arrays on the bottom of the microchannel are inclined at a 10 degree angle with regard to the direction of channel. A lateral flow is generated by AC electro-osmosis flow triggered by a low-voltage AC electric field on the surface of the electrode. Superimposed upon the axial pressure-driven flow applied by the external syringe pump, AC electro-osmosis flow induces a depressed vortical flow above the electrodes. We find that when homogeneously suspended micro polystyrene particles with different sizes (0.86 μm and 5 μm) in the KCl solution (0.1 mM) are transported through the vortical flow region, the small particles, 0.86 μm, successfully become trapped in the lateral flow above the electrode arrays under the combination of AC electroosmosis and positive DEP, whereas the large particles, 5 μm, completely pass through the vortices. The effectiveness of this separation is investigated for different axial flow rates and amplitudes of the applied voltage. It is shown that with increasing flow rate, it becomes hard for the small particle to get trapped. The possibility of trapping, however, is enhanced by increasing the amplitude of the applied voltage. In addition, we found that the effectiveness of particle separation is frequency dependent, tending to zero at both low and high frequencies. The peak of the effectiveness happens at a so-called characteristic frequency which depends on the conductivity and geometry of the electrodes. We expect that this electrohydrodynamic method can be used to separate the particles with high effectivity for various applications in microsystems.Copyright © 2011 by ASME

In simulations of micro channel cooling, the heat exchange from fluid to channel wall is an important aspect. Hence the heat exchange should be included in the model. Although numerically very expensive, it can be done by using a molecular wall. Numerically cheap implementations of a wall are the reflective wall and the thermal wall, and the combination of both, the diffusive-specular wall. In this paper we introduce the concept of a vibrating reflective wall as a boundary condition for molecular dynamics simulations. It is shown that the heat transfer with the vibrating reflective wall is the same as with a molecular wall, and that computation time is reduced greatly. As a competitive model, the diffusive-specular boundary condition is analyzed; it is shown that a good choice of parameters can give similar results in the same computation time, but the choice of parameters is not known a priori, therefore the vibrating reflective wall boundary condition is preferable.Copyright © 2008 by ASME

Most computer models of human thermoregulation are population based. Here, we individualised the Fiala model [Fiala et al. (2001) Int J Biometeorol 45:143-159] with respect to anthropometrics, body fat, and metabolic rate. The predictions of the adapted multisegmental thermoregulatory model were compared with measured skin temperatures of individuals. Data from two experiments, in which reclining subjects were suddenly exposed to mild to moderate cold environmental conditions, were used to study the effect on dynamic skin temperature responses. Body fat was measured by the three-compartment method combining underwater weighing and deuterium dilution. Metabolic rate was determined by indirect calorimetry. In experiment 1, the bias (mean difference) between predicted and measured mean skin temperature decreased from 1.8 degrees C to -0.15 degrees C during cold exposure. The standard deviation of the mean difference remained of the same magnitude (from 0.7 degrees C to 0.9 degrees C). In experiment 2 the bias of the skin temperature changed from 2.0+/-1.09 degrees C using the standard model to 1.3+/-0.93 degrees C using individual characteristics in the model. The inclusion of individual characteristics thus improved the predictions for an individual and led to a significantly smaller systematic error. However, a large part of the discrepancies in individual response to cold remained unexplained. Possible further improvements to the model accomplished by inclusion of more subject characteristics (i.e. body fat distribution, body shape) and model refinements on the level of (skin) blood perfusion, and control functions, are discussed.

We study the properties of a hard-sphere dense gas near the hard walls of micro and nano-channels. Analytical techniques, Monte Carlo (MC) methods and molecular dynamics (MD) simulation methods have been used to characterize the influence of the characteristic parameters such as number density, reduced density, width of the system and molecular diameter, on the equilibrium properties of the gas near the hard walls of micro and nano-channels. A mathematical model has been developed to characterize the density oscillations as the result of packing of molecules in case of a dense gas near the micro and nano-channels walls. The height and the position of the density oscillation peaks near the wall are characterized. These results are also confirmed by the MD and MC simulation results. Comparisons between MD and MC simulation results for particles having different diameter are also presented. For the same size of the particles and moderately dense gas, MC and MD results are similar, differences in the density profiles being limited only to the oscillatory region. For different particle sizes, MD and MC results are limited to a short distance near the wall for long size systems and moderately dense fluids. The effect of the boundary (particle size) on the simulation results is found to increase with ¿ (reduced density) and it is very small in case of a dilute gas. For small ¿ and small particle size (R) relative to the width of the system L, the height of the oscillation peaks is slowly increasing with R/L, and for high densities is always decreasing with R/L. The position of these peaks depends only on the size of the particles and when R is much smaller than L, it shows a small dependence on L. The deviations in the oscillatory region for the pure MC simulation results compared to pure MD simulation results are quantified, and more efficient hybrid MC–MD simulations are performed to reduce these deviations.

We present a hybrid method to study the properties of hard‐sphere gas molecules confined between two hard walls of a microchannel. The coupling between Molecular Dynamics(MD) and Monte Carlo(MC) simulations is introduced in order to combine the advantages of the MD and MC simulations, by performing MD near the boundaries for the accuracy of the interactions with the wall, and MC in the bulk because of the low computational cost. The effect of different gas densities, starting from a rarefied gas (reduced density η=πna3/6=0.001, where n=number density, a=molecular diameter) to a dense hard‐sphere gas (η=0.25), is investigated. We characterize the influence of different η’s and size of molecules on the equilibrium properties of the gas in a microchannel. The effect of the particle size on the simulation results, which is very small in case of a dilute gas, is increasing with η. Comparisons between MD, MC and hybrid MD‐MC simulation results are done, and comparisons between MD, MC, and hybrid MD‐MC computati...

We combine molecular dynamics (MD) and Monte Carlo (MC) simulations to study the properties of gas molecules confined between two hard walls of a microchannel or nanochannel. The coupling between MD and MC simulations is introduced by performing MD near the boundaries for accuracy and MC in the bulk because of the low computational cost. We characterize the influence of different densities and molecule sizes on the equilibrium properties of the gas in the microchannel. The effect of the particle size on the simulation results is very small in the case of a dilute gas and increases with the density. The hybrid MD-MC simulation method is validated by comparing the results for density and temperature profiles with those of pure MD and pure MC simulations. These results compare well for pure MD and pure MC, as well as hybrid MD-MC, both in the bulk and near the boundaries, when hard-sphere interactions are used. When Lennard-Jones potentials are used to accurately model the interactions between the gas and wall molecules instead, the results of pure MD simulations differ significantly from the pure MC simulations near the boundaries, but the results of the hybrid method compare well with the pure MD results near the wall, and with the pure MC and pure MD results in the middle of the channel. The hybrid method also very accurately simulates the interface between the MD and MC simulation domains. Comparisons between MD, MC, and hybrid MD-MC computational costs are outlined. The speedup when using 50% of the domain for MD simulations and 50% for MC simulations is very small compared to pure MD simulations times, but this speedup increases drastically for more realistic situations where the region near the wall is small compared to the bulk region.

A mathematical model has been developed to characterize the effect of packing of molecules of a hard-sphere dense gas near the hard walls of a microchannel. Analytical techniques, Monte Carlo (MC) methods and Molecular Dynamics (MD) simulation methods have been used to characterize the influence of the characteristic parameters such as number density, reduced density, length of the system and molecular diameter on the equilibrium properties of the gas near the hard walls of the microchannel. The height and the position of the density oscillation peaks near the wall are characterized. Comparisons between MD and MC results for particles having different diameter are presented. For the same size of the particles and moderately dense gas, MC and MD results are similar, differences in the density profiles being limited only to the oscillatory region. For different particle sizes, MD and MC results are limited to a short distance near the wall for long size systems and moderately dense fluids. The effect of the boundary (particle size) on the simulation results is increasing with η (reduced density) and it is very small in case of a dilute gas. For small η and small particle size (R) relative to length of the system L, the height of the oscillations peaks is slowly increasing with R/L, and for high densities is always decreasing with R/L. The position of these peaks depends only on the size of the particles and when R is much smaller than L, it shows a small dependence on L.Copyright © 2004 by ASME

To the Editor: During surgery under general anesthesia, thermal imbalances within the patient’s body are common. These imbalances are caused by a variety of factors, including exposure to a cold operating environment and heat loss from surgical incisions. Perhaps the most important thermal imbalance is hypothermia, resulting initially from redistribution of body heat from the core to the periphery caused by impaired thermoregulatory responses due to anesthetics. There is lack of clarity regarding the contribution made by heat loss from the exposed wound to the total heat balance in humans because of technical difficulties.1,2 The study by Lamke et al. elaborates on human wound heat loss.3 They assessed the evaporative water loss from wounds and exteriorized bowels by recording the rate of increase of vapour concentration in a closed measuring chamber placed over the exposed abdominal cavity. Information on wound temperature and wound heat loss are important because low wound temperatures are associated with a higher risk for wound infections and disturbed coagulation function, while evaporative heat losses may lead to hypovolemia. We recently developed a thermophysiological computer model, ThermoSEM (Eindhoven University of Technology and Maastricht University, Eindhoven and Masstricht, The Netherlands), that can be used to predict temperature responses and surgical wound heat of patients during surgery.4,1 The passive heat transfer processes in the body and between the body and the environment were modelled using fundamental heat transfer knowledge. A thermoregulatory model was formulated that describes the vasoconstriction response during surgery under general anesthesia and accounts for the decrease in metabolic rate during anesthesia. The model also contains flexible submodels that account for other thermal influences during surgery, e.g., surgical wound heat loss, use of heating mattresses and forced-air warmers, and ambient temperature. When running the model, all internal heat fluxes in the body are calculated as well as heat interactions between the body and the environment, leading to a transient whole body temperature prediction. In this study, this thermophysiological model was used to simulate three orthopedic surgeries and to compare the model’s output (temperature data of core, skin, and wound) to the data of three actual surgeries. Also, with help of the model, we intended to evaluate heat flows, including wound heat loss during orthopedic back surgery. Three patients were followed during orthopedic back surgery. The study protocol was approved by the Maastricht University Medical Ethical Committee, and all patients signed an informed consent. The weight and height of the patients were noted, and measurements of the patients’ skinfolds were taken prior to surgery to determine their body fat percentages. Skin temperatures were collected using wireless temperature sensors (maximum error 0.4°C), and an ambulant infrared camera was used to map the surgical wound temperatures (accuracy 0.3°C). For running the simulations, the following information was entered into the model: patient characteristics (height, weight, body fat percentage, and body shape); ambient temperature; use of forced-air heaters; insulation of blankets/socks/mattress; and start times of surgery, incision, and closure. For the general model characteristics, refer to the references.4,A Some assumptions were made for this particular type of surgery: The lengths of the abdomen segment and the actual incision were equal. Heat losses in circumferential directions were neglected. The air speed in the room was 0.05 m·sec−1. The administration of warmed fluids and the secretion of urine were in balance. The posterior abdomen segment was opened/closed stepwise from 0 to 75° in five minutes, representing the incision. The volumetric perfusion value of the wound was 2.25 L·m−3·sec−1. The blood layer was 0.5 mm thick and could evaporate. The blood properties were equal to water. The temperature of the surgery light was 50°C (measured with the infrared camera). Results from the comparison of the model with the actual measurements: The core temperature bias of measurements and simulations was −0.28 ± 0.75°C. Skin temperature bias was −0.11 ± 0.58°C. Wound temperature bias was −1.87 ± 1.53°C (model prediction 28.3 ± 0.8°C vs measured temperature 30.1 ± 1.5°C). Simulations showed that heat loss through the surgical wound was approximately 4.3 watts (W), which is about 6% of the total heat loss during orthopedic surgery. A heat balance for one patient during surgery is visualized in Figure 1. It was seen from simulations that the average evaporative heat loss was 9.8 W, the convective heat loss was 0.5 W, and the radiative heat gain (due to surgery light) was 6.0 W. The calculated evaporative water loss was 15.7 g·h−1 on average. Although the surgical interventions described in this paper are not fully comparable with the experiments performed by Lamke3 (back surgery vs abdominal surgery), the evaporative heat losses observed in this study are in the same range as Lamke’s values (range 2.1-32.2 g·h−1). Fig. 1 Heat balance of a patient following the simulation. White parts represent the heat fluxes through the open back segment. Black parts represent the heat flows through the other body parts. Visible from left to right: evaporative heat loss (Qe), radiative ... Despite there being a limited number of cases studied, the model seems capable of predicting patient temperatures and surgical wound heat loss during (orthopedic back) surgery.

Various and disparate technical disciplines have identified a growing need for tools to predict human thermal and thermoregulatory responses to environmental heating and cooling and other thermal challenges such as anesthesia and non-ionizing radiation. In this contribution, a dynamic simulation model is presented and used to predict human thermophysiological and perceptual responses for different applications and situations. The multi-segmental, multi-layered mathematical model predicts body temperatures, thermoregulatory responses, and components of the environmental heat exchange in cold, moderate, as well as hot stress conditions. The incorporated comfort model uses physiological states of the human body to predict thermal sensation responses to steady state and transient conditions. Different validation studies involving climate-chamber physiological and thermal comfort experiments, exposures to uncontrolled outdoor weather conditions, extreme climatic and radiation asymmetry scenarios revealed the model to predict physiological and perceptual responses typically within the standard deviation of the experimental observations. Applications of the model in biometeorology, clothing research, the car industry, clinical and safety applications are presented and discussed.