A multi-segmented human bioheat model for transient and asymmetric radiative environments

This paper aims to improve the Salloum et al. multi-node multi-segmented model [M. Salloum, N. Ghaddar, K. Ghali, A new transient bio-heat model of the human body and its integration to clothing models, Int. J. Therm. Sci. 46 (4) (2007) 371–384] to accurately predict the circumferential skin temperature variation of nude and clothed human body segments when subject to complex transient and spatially non-uniform radiative environments. The passive bioheat model segments the body into the 15 cylindrical segments. Each body segment is divided into one core node, six angular skin nodes, one artery blood node, and one vein blood node. The model calculates the blood circulation using the Avolio model [A.P. Avolio, Multi-branched model of the human arterial system, Med. Biol. Eng. Comp. 18 (1980) 709–718] for arteries and arterioles up to 2 mm in diameter and the Olufsen et al., semi-analytical model [M.S. Olufsen, C.S. Peskin, W.Y. Kim, E.M. Pedersen, A. Nadim, J. Larsen, Numerical simulation and experimental validation of blood flow in arteries with structured tree outflow conditions, Ann. Biomed. Eng. 28 (11) (2000) 1281–1299] for small arteries and arterioles up to a minimum diameter of 0.3 mm; thus improving prediction of blood perfusion rates in the skin. Unsteady bioheat equations are simultaneously solved for the nodes of each body segment to predict the skin, tympanic, and core temperatures, sweat rates, and the dry and latent heat losses. The nude body thermal model is integrated to a clothing model that takes into consideration the moisture adsorption by the fibers to predict heat and mass diffusion through the clothing layers. The clothing layer is divided into six parts that are aligned to the skin sub-nodes for each clothed segment. The local and mean skin temperature can then be estimated in response to non-uniform environments.The nude body and the clothed model predictions were compared with published experimental data at a variety of ambient conditions, non-uniform conditions and activity levels. The current model agreed well with experimental data during transitions from hot to cold, dry to humid environments, and in asymmetric radiative environments. Both the nude and clothed human models have an accuracy of less than 6% for the whole-body heat gains or losses; the nude human model has an accuracy of ±0.35 °C for skin temperature values.     

The computer-based human thermal model

A computer model was developed that estimates the resistance to dry and evaporative heat transfer from fabric resistance data and fabric thickness data for five different clothing ensembles with the different total thermal insulation. Five different clothing ensembles with the different total thermal insulation were studied. In this study, a two-dimensional computer model was developed that estimates the resistance to of the insulation of the body were simulated with 16 sedentary subjects. In this model, the human geometry is described by 16 cylindrical elements representing the head, hands, arms, thigh and etc. Thermal and evaporative resistance of each sixteen body segments are calculated. Heat generated in the body by metabolism can be lost to the environment by conduction, convection, radiation, evaporation of the moisture from skin and through respiration. The preceding relationship can be used to determine the total thermal resistance and the total evaporative resistance for each segment. Evaporative heat loss from skin is a combination of a evaporation of sweat secreted due to thermoregulatory control mechanisms and the natural diffusion of water through the skin. The heat flows from body to environment through alternating clothing and air.The purpose of this paper is to develop a model for estimating total thermal insulation of clothing ensembles. Also latent and sensible heat transfer values for each body segment and whole body are found. Possible reasons for discrepancies between the observed data and predictions of the model are discussed. It seems that they are in agreement.     

Modeling additional solar constraints on a human being inside a room

Sun fluxes induce additional heterogeneous thermal constraints in buildings and may also lead to discomfort for the inhabitant. To calculate the local thermal sensation of a human being totally or partially situated in the sunlight, the solar radiation inside a room and its detailed distribution on parts of the human body are modeled. The present study focuses on the solar gains part of a complete modeling tool simulating an occupied building.The irradiated areas are calculated with a ray tracing method taking shadow into account. Solar fluxes are computed. Fluxes can be absorbed by each surface or reflected. The reflected fluxes are then absorbed at the next impact. A multi-node thermoregulation model (MARCL) represents the thermal behavior of the human body and all its heat exchanges with the environment. The thermal transient simulation of the whole occupied building is performed in TRNSYS simulation software. In the case presented here, the results show that, when a person is inside the building, the skin and clothing temperatures of the irradiated segments increase more or less depending on the segments but the global thermal equilibrium of the body is maintained thanks to strong physiological reactions.     

Modeling of heat and moisture transport by periodic ventilation of thin cotton fibrous media

In walking conditions, the air spacing between the fabric layer of a porous clothing system and the human skin changes with the walking frequency. This change will cause air penetration in and out of the clothing system depending on the fabric air permeability. The air passing through the fabric can considerably reduce the heat and moisture transfer resistance of the clothing system and its suitability for a given thermal environment. In this work, the coupled convection heat and moisture exchange within the clothing system subject to sinusoidal air layer thickness variation about a fixed mean is experimentally investigated and theoretically modeled to predict the periodic fabric regain, the fabric temperature and the transient conditions of the air layer located between the fabric and the skin.Experiments were conducted in environmental chambers under controlled conditions using a sweating hot plate at 35 °C that represents the human skin and a gear motor to generate the oscillating fabric motion. The first set of experiments was done using a dry isothermal hot plate to measure the sensible heat transfer. The second set of experiments was conducted with an isothermal sweating hot plate and the total heat (sensible and latent) transport from the plate was recorded.A mathematical model was developed for the heat and mass transport through the air spacing layer and the fiber clothing system. In the fabric, a three-node adsorption model was used to describe the effect of fabric motion (ventilation) on the sensible and latent heat flows from the human skin under different environmental conditions. The fiber model was linked to the transport model of the oscillating air spacing layer that falls between the fiber and the fixed boundary (human skin). The transport equations were solved numerically. The sensible and latent heat transport quantities at the moist solid boundary were calculated. A reasonable agreement was observed between the model predictions of heat loss or gain from the hot plate and the experimentally measured results.     

Numerical Simulation of Transient Heat Transfer in a Protective Clothing System during a Flash Fire Exposure

A finite volume model was developed to simulate the transient heat transfer in a protective clothing system. The model domain consists of a fire-resistant fabric, the human skin, and the air gap between the fabric and the skin. The model uses a more sophisticated treatment of the air gap compared to previous models: it accounts for transient combined conduction-radiation heat transfer within the air gap and includes the variation in the air gap properties with temperature. Predictions were obtained for the temperature and heat flux distributions in the fabric, skin, and air gap as a function of time, as well as the time to receive skin burn injuries. The numerical model was used to explore the physics of heat transfer in protective clothing, which could potentially be used to improve the performance of this clothing. This study illustrates the dependence of the temporal behavior of the heat fluxes on the specific model assumptions, as well as the associated sensitivity of skin burn predictions to these assumptions.     

Synergistic numerical and experimental simulations to evaluate a surface probe to determine body core temperatures

ABSTRACT

A highly accurate noninvasive means of determining body core temperature is proposed and characterized by synergistic use of numerical and experimental simulations. It was demonstrated that the new surface probe yields skin surface temperature measurements that are within a few tenths of a degree of the body core temperature. Advanced simulation techniques such as the Monte Carlo method for the determination of radiant heat losses were used to ensure high accuracy. Convective heat losses were also accounted. Full account was taken of the multilayer nature of the tissue bed beneath the skin surface, each layer with its specific thermophysical properties, blood perfusion, and metabolic heating. For the validation of the numerical simulation model, an experimental apparatus was fabricated and operated. The experimental data supported the numerical predictions. The capability of the probe to accurately follow thermal transients was the focus of a redesign, yielding small-fraction-of-a-minute following capability.     

Computational fluid dynamics (CFD) investigation of air flow and temperature distribution in a small scale bread-baking oven

Experimental and computational fluid dynamics (CFD) analyses of the thermal air flow distribution in a 3-zone small scale forced convection bread-baking oven are undertaken. Following industrial bread-making practise, the oven is controlled at different (constant) temperatures within each zone and a CFD model is developed and validated against experimental data collected within the oven. The CFD results demonstrate that careful selection of the flow model, together with implementation of realistic boundary conditions, give accurate temperature predictions throughout the oven. The CFD model is used to predict the flow and thermal fields within the oven and to show how key features, such as regions of recirculating flow, depend on the speeds of the impinging jets.     

Numerical Simulation of Heat Transfer in Firefighters' Protective Clothing with Multiple Air Gaps during Flash Fire Exposure

A finite volume model was developed to simulate transient heat transfer in firefighters' protective clothing during flash fire exposure. The model domain consists of three layers of fire-resistant fabrics (outer shell, moisture barrier, and thermal liner) with two air gaps between the clothing layers, the human skin, and the air gap between the clothing and the skin. The model accounts for the combined conduction-radiation heat transfer in the air gaps entrapped between the clothing layers, and between the clothing and the skin. The variation in the air gap properties and energy content during both the exposure and the cool down periods was accounted for. Predictions were obtained for the temperature and heat flux distributions in the fabric layers, skin, and air gaps as a function of time. The influence of each air gap on the clothing performance was investigated as well. This article demonstrates the importance of accurately modeling the contributions of the air gaps in order to predict the protective clothing performance.     

A Clothing Ventilation and Heat Loss Electric Circuit Model with Natural Convection for a Clothed Swinging Arm of a Walking Human

This work aims to develop a computationally effective electric circuit model to estimate the ventilation and heat transfer for walking human in the presence of natural convection. The ventilation circuit includes flow resistance, inductance, and electromotive force elements. It is coupled to an electric resistance circuit of heat flows to adjust the temperature difference inducing natural convection flow. The coupled ventilation and heat circuit models predicted both the segmental ventilation rate and heat loss from the arm at different walking and wind speeds. The developed model of the segmental ventilation and heat transfer from the clothed human segment was validated by performing experiments on a walking thermal manikin using tracer gas method. Good agreement was observed between the model predictions and the experiment at a maximum relative error of 10% lying within the standard deviation range. Results showed that the simplified ventilation-heat circuit models succeeded in estimating the natural convection effect at low computational cost. Moreover, it was shown that the effect of natural convection is more significant in walking at no wind than in windy condition. Accounting for natural convection effect increases the segmental ventilation and heat loss at low air permeability (0.02 m/s) by 68% and 20%, respectively.     

Numerical Heat Transfer Coupled with Multidimensional Liquid Moisture Diffusion in Porous Textiles with a Measurable-Parameterized Model

This article reports on the numerical simulation of the transient heat transfer coupled with multidimensional liquid diffusion in porous textiles with a measurable-parameterized model. This model is developed with the incorporation of measured multidimensional liquid diffusion properties into the parameterization of the liquid transfer model by the investigation of physical mechanisms. An improved two-node model of human body is employed to simulate the thermoregulatory behaviors and the thermal responses between the textiles and body skin are considered through the boundary conditions for accuracy simulation of realistic wearing situations. The predicted results of this model are compared with the experimental data for validating the model accuracy. The influence of the multidimensional liquid diffusion property of porous textiles on the moisture performance of clothing during the wearing period is investigated through a series of computational experiments. This model offers the ability to predict the multidimensional liquid transfer capacity and is more effective in realistic application due to the measurable properties.     

The passive solar heated school in Wallasey. VI. Thermal sensation and comfort in the classroom: A year-long study of the subjective and adaptive responses of children

In order to examine relationships between thermal parameters and subjective response, a class of 33 13-year-old children was studied over a period of 63 weeks. Children completed 7 point rating scales of thermal sensation, air movement and dryness and 5 point scales of perceived comfort and wakefulness. Information was collected about clothes worn, windows open and lights in use, as well as data on concurrent thermal variables. In general the classroom was perceived as warm, dry and airless. Children were found to maintain thermal neutrality down to globe temperatures of 17°C by adjusting clothing, windows and lighting. The adaptive responses served to moderate the effect of physical fluctuations on thermal sensation but appeared to be ineffective at globe temperatures over 24°C. A theoretical model of adaptive control of the environment would suggest low correlations between thermal sensation and variables which the children could adjust (clothing, ventilation, heating), and high correlations between these moderator variables and objective room temperatures. The observed set of correlations supports this model. Children seem to be sensitive to differences between mean surface and air temperatures but not to ceiling-floor temperature differences within the ranges studied. There are low correlations between thermal sensation and ‘comfort’. This has implications for energy conservation.     

Comparison of a small-scale 3D PCM thermal control model with a validated 2D PCM thermal control model

A three-dimensional (3D) numerical model was developed to simulate the use of a phase change material linked to a photovoltaic (PV) system to control the temperature rise of the PV cells. The model can be used to predict temperatures, velocity fields and vortex formation within the system. The 3D predictions have been compared with those from a previously developed experimental validated two-dimensional (2D) finite-volume heat transfer model conjugated hydro-dynamically to solve the Navier–Stokes and energy equations. It was found that for the systems simulated with appropriate boundary conditions, the 2D model predictions compare well with those of the 3D model. The 3D model was used to predict the temperature distributions when the heat transfer to the phase change material was enhanced by high thermal conductivity pin fins.     

Modelling airflow,heat transfer and moisture transport around a standing human body by computational fluid dynamics

In this study a combined computational model of a room with virtual thermal manikin with real dimensions and physiological shape was used to determine heat and mass transfer between human body and environment. Three dimensional fluid flow, temperature and moisture distribution, heat transfer (sensible and latent) between human body and ambient, radiation and convection heat transfer rates on human body surfaces, local and average convection coefficients and skin temperatures were calculated. The radiative heat transfer coefficient predicted for the whole-body was 4.6 W m^− 2 K^− 1, closely matching the generally accepted whole-body value of 4.7 W m^− 2 K^− 1. Similarly, the whole-body natural convection coefficient for the manikin fell within the mid-range of previously published values at 3.8 W m^− 2 K^− 1. Results of calculations were in agreement with available experimental and theoretical data in literature.     

Transient temperature distributions in canned products during heat sterilization and cooling

This reseach was performed using experimental temperature data and a simple analytical model to estimate the temperature distributions at the centres of cylindrically canned products (diameter/thickness ratio = 4/1) during heat sterilization and cooling under continuous flow conditions for two retort temperatures (115 and 121°C). The recorded experimental temperature data for an individual can for two different retort temperatures were then used to construct the dimensionless experimental centre temperature curves. In order to predict dimensionless centre temperature profiles with a mathematical model, the convection boundary condition (i.e. 0·1 < Bi < 100) in transient heat transfer was considered. The predicted temperature profiles were compared with the experimental centre temperature measurements for two cases—heat sterilization and cooling. The experimental temperature values were in very good agreement with the theoretical predictions. The results of this study show that the present technique is a reasonable tool for estimating in a simple and accurate form the temperature distributions of a cylindrically canned product subject to both heat sterilization and cooling.     

The Thermal Protective Performance of Firefighters' Clothing: The Air Gap Between the Clothing and the Body

The air gaps entrapped in firefighters' clothing play a crucial role in determining the protective performance of the clothing. Nevertheless, most of the models in the literature ignored the air gaps between the clothing layers and dealt with the air gap between the clothing and the body in an approximate way. In addition, none of these studies addressed the influence of these air gaps on the protective performance of the clothing. This paper introduces a finite-volume model that employs a more realistic analysis for the air gaps entrapped in firefighters' clothing as compared to the typical model in the literature. The paper further investigates the influence of different clothing parameters on the heat release from the clothing to the skin and their corresponding influence on the clothing protective performance. Skin burns predictions made by the present model were compared to those made by the typical model in the literature. The paper demonstrates the influence of the air gap between firefighters' protective clothing and the body on the protection provided by the clothing. The paper also illustrates that inappropriate modeling of the air gaps entrapped in firefighters' clothing would underestimate the protective performance of the clothing.     

Characterizing heat transfer within a commercial-grade tubular solid oxide fuel cell for enhanced thermal management

A thermal transport model has been developed for analyzing heat transfer and improving thermal management within tubular solid oxide fuel cells (TSOFCs). The model was constructed via a proven electrochemical model and well-established heat transfer correlations. Its predictions compare favorably with other published data. Air temperatures consistently approach that of the fuel cell. This is primarily due to the high operating temperature of the cell (1000°C), the moderate magnitudes of radiation and airflow, and cell geometry. The required inlet air temperature (for thermally steady-state operation) has linear dependence on operating voltage and fuel utilization. Inlet air temperature has an inverse proportionality with respect to air stoichiometric number (i.e., inverse equivalence ratio). The current standard for airflow within TSOFCs was found to be excessive in consideration of the regenerative preheat effect within the supply pipes that feed air to the cell. Thermal management of simple TSOFC systems could be enhanced if commonly used air stoichiometric numbers were decreased.     

Influence of moving heat source on skin tissue in the context of two-temperature memory-dependent heat transport law

Abstract

Modeling and understanding heat transport and temperature variations within biological tissues and body organs are key issues in medical thermal therapeutic applications, such as hyperthermia cancer treatment. In the present analysis, the bioheat equation is studied in the context of memory responses. The heat transport equation for this problem involving the memory-dependent derivative (MDD) on a slipping interval in the context of three-phase (3P) lag model under two-temperature theory is formulated and is then used to study the thermal damage within the skin tissue during the thermal therapy. Laplace transform technique is implemented to solve the governing equations. The influences of the MDD and moving heat source velocity on the temperature of skin tissues are precisely investigated. The numerical inversion of the Laplace transform is carried out using Zakian method. The numerical outcomes of temperatures are represented graphically. Excellent predictive capability is demonstrated for identification of an appropriate procedure to select different kernel functions to reach effective heating in hyperthermia treatment. Significant effect of thermal therapy is reported due to the effect of delay time and the velocity of moving heat source as well.     

Neural computing thermal comfort index for HVAC systems

The primary purpose of a heating, ventilating and air conditioning (HVAC) system within a building is to make occupants comfortable. Without real time determination of human thermal comfort, it is not feasible for the HVAC system to yield controlled conditions of the air for human comfort all the time. This paper presents a practical approach to determine human thermal comfort quantitatively via neural computing. The neural network model allows real time determination of the thermal comfort index, where it is not practical to compute the conventional predicted mean vote (PMV) index itself in real time. The feed forward neural network model is proposed as an explicit function of the relation of the PMV index to accessible variables, i.e. the air temperature, wet bulb temperature, globe temperature, air velocity, clothing insulation and human activity. An experiment in an air conditioned office room was done to demonstrate the effectiveness of the proposed methodology. The results show good agreement between the thermal comfort index calculated from the neural network model in real time and those calculated from the conventional PMV model.     

Thermal Model in ANSYS for a Comparison Between two Configurations of a Concentrating Photovoltaic System

The theoretical analysis of different concentrating photovoltaic and thermal systems (CPV/T) is presented in this paper; it allows to evaluate the cooling fluid temperature in different working conditions. In particular, two CPV/T systems with different optics, line-focus and point-focus, are studied and compared theoretically by means of a model developed in ANSYS-CFX able to evaluate the theoretical temperature values of the fluid that flows in the cooling circuit, designed with the Solidworks software. The comparison in terms of the thermal performances between the point-focus and line-focus configurations of a CPV/T system has been realized considering different working conditions. As model input the uniform cell temperature has been set to different values, included between 323 K and 363 K, or determined experimentally referring to the point-focus configuration. In particular, the outlet temperatures of the cooling fluid, the fluid and cells temperatures difference at the outlet section, the time necessary to reach a steady-state condition, the fluid temperature trend in different parts of the tube and along the circuit, have been evaluated. Moreover, the fluid temperature trend has been studied varying the fluid velocity, its mass flow rate, and the insulation thickness. Finally, the point-focus CPV/T system thermal performances have been also evaluated adopting some experimental measurements as input data; the fluid temperature trend has been tested under a variable cell temperature during different hours of a sunny and a cloudy day.