Numerical Modeling of Heat and Moisture Through Wet Cotton Fabric Using the Method of Chemical Thermodynamic Law Under Simulated Fire

The paper deals with numerical modeling of heat and moisture transfer behavior of a fabric slab during combined drying and pyrolysis. The model incorporates the heat-induced changes in fabric thermo physical properties and the drying process is described by a one-step chemical reaction in the model. The new model has been validated by experimental data from modified Radiant Protective Performance (RPP) tests of fabrics. Comparisons with experimental data show that the predictions of mass loss rates, temperature profiles within the charring material and skin simulant, and the required time to 2nd skin burn are in reasonably good agreement with the experiments. It is concluded that moisture increases the time to 2nd degree skin burn for fabrics exposed to low intensity heat flux of 21 kW/m², but under high heat flux exposures, such as 42 kW/m², moisture tend to increase heat transfer through the thermal protective fabric system and the tolerance time of the same fabrics will reduce. The model can find applications not only in thermal protective clothing design, but also in other scientific and engineering fields involving heat transfer in porous media.     

Heat Transfer in Thin Fibrous Materials Under High Heat Flux

A heat-transfer model has been developed for two common, inherently flame-resistant fabrics, Nomex® IIIA and Kevlar®/PBI, when subjected to the high heat fluxes used in bench top tests, such as the thermal protective performance (TPP) test, ASTM D 4108. The apparent heat capacity method was used to model thermochemical reactions in these materials with information from thermal gravimetric analysis (TGA) and differential scanning calorimeter (DSC) tests. Also included were in-depth radiation absorption, variable thermal properties, and heat transfer across an air space from the fabric to a test sensor. The finite element method was used to solve the resulting equations. Absolute temperatures predicted by this relatively simple model fall within 4% of those measured by an infrared thermometer. Estimated times to the Stoll second-degree burn criterion are within 6% of those derived from actual tests.     

Skin Burn Translation Model for Evaluating Hand Protection in Flash Fire Exposures

A model is described for use in translating measured heat flux to predict second and third degree hand burn injury in fire exposures. The model adapts a burn translation algorithm for estimating burn injuries used in established instrumented fire test manikin technologies. It facilitates more accurate prediction of burns to human hands by accounting for the cylindrical geometry of the fingers, bone tissue beneath the skin, and different skin thickness data that represents the different areas of the hand. A numerical modeling approach is used to demonstrate the response of the skin burn model for predicting hand burn injury in heat exposures encountered in fire manikin testing.     

Spatial homogenization algorithm for bridging disparities in scale between the fire and solid domains

The analysis of structures exposed to non-uniform heating from localized fires is a challenging task due to the spatially varying boundary conditions and the differences in scale between the fire simulation and solid heat transfer model. This paper presents a spatial homogenization algorithm for capturing non-uniform boundary conditions from a high-resolution fire simulation in a low-resolution finite element heat transfer model of a structure. The homogenization algorithm uses numerical integration by the trapezoid rule to calculate the equivalent thermal flux vector in the finite element heat transfer model for a spatially varying surface flux. The proposed method is compared to other approximating techniques, including averaging, sampling, and least squares methods, for a 2D heat transfer problem. The results demonstrate that the proposed homogenization algorithm converges rapidly due to the energy-equivalent representation of the thermal boundary condition. The homogenization algorithm is then implemented in a 3D heat transfer model that uses macro-level plate elements. For an application involving a horizontal plate exposed to a localized fire, the model is shown to converge to the results obtained by a solid finite element model. The homogenization algorithm combined with the plate heat transfer element proves to be an accurate and highly efficient means for analyzing structures with spatially varying thermal boundary conditions calculated by computational fluid dynamics.     

Heat Transfer Model of Flame Resistant Fabrics During Cooling After Exposure to Fire

Protective clothing is used in many industries to protect firefighters and other workers from fire and other hazards. While skin burns can occur during a fire, protective fabric temperatures remain high for some time even after a fire ends. Therefore, skin burn injuries can occur during the time in which a fabric is cooling. A heat transfer model has been developed that can predict inherently flame resistant fabric temperatures and skin burn injuries during this cooling phase. This paper describes the heat transfer model, including methods used to calculate the apparent heat capacity and the convection heat transfer coefficient as the fabric cools. The new model has been validated using data from bench top tests of Kevlar®/PBI fabric specimens. Parametric studies using the model demonstrate the importance of selected thermal properties and boundary conditions on fabric temperatures and bench top test results.     

Experimental and numerical investigation on bare tube cross flow heat exchanger-using COMSOL

ABSTRACT

In this study, COMSOL multi-physics modelling software was used to make a computational model of a bare helical tube cross flow heat exchanger in order to simulate the temperature changes in the heat exchanger. The computational results of heat transfer are validated by using the analytical models. A conjugate convection/conduction heat transfer model was developed, which exhibited good agreement to the experiments. A different velocity of air taken into the consideration to find out the temperature distribution through the pipe and air temperature inside the duct. The temperature profile, and the overall heat transfer rate from the wall of the tube were calculated and plotted for theoretical, experimental and Numerical method using the k- conjugate heat transfer model. The model is validated through comparison with theoretical relations for single-pass cross-flow arrangements and with experimental results also. Simulation results showed good agreement with experimental values with respect to different mass flow rates.     

An integrated numerical simulator for thermal performance assessments of firefighters’ protective clothing

Research and development of firefighters’ protective clothing relies on a large number of fire disaster experiments in order to assess the thermal performance. It would be substantially advantageous to substitute a virtual numerical experiment for a real one in terms of time, cost and safety. The present article reports the development of an integrated numerical simulator that makes possible the estimation of burn injuries originating from fire disasters. In the simulator, a general-purpose computational fluid dynamics program computes the fluid flow and heat transfer in an in situ fire event, while a one-dimensional program calculates the radiative–conductive heat transfer through the clothing and human skin. A data interface combines the two simulations by loose coupling so as to give the real-time burn injury progress output. The predicted surface heat fluxes and burn degrees agree with experimental measurements reasonably well. Possible numerical error sources are discussed that call for potential improvements in the future.     

ICARUS: A code for evaluating burn injuries

A computer code, ICARUS (Injuries CAused by Radiation Upon the Skin), has been developed for evaluating time to second-degree burn injury caused by thermal radiation. This paper introduces the modeling methodology incorporated in ICARUS and illustrates the code's validity and application. ICARUS enables studies of the effects of thermal radiation on the skin and benefits assessments of the shielding effects of clothing layers. ICARUS uses a unique method of solving the complex heat transfer problem associated with simultaneous radiation, conduction, and convection in a multilayered diathermanous clothing/skin assembly, which is especially useful when coupled with the thermal responses (moisture loss, charring, shrinking, and so on) of the clothing fabrics themselves. The code is designed to run on an IBM-PC compatible.     

Radiative and convective heat transfer coefficients of the human body in natural convection

The purpose of this study was to investigate the convective and radiative heat transfer coefficients of the human body, while focusing on the convective heat transfer area of the human body. Thermal sensors directly measuring the total heat flux and radiative heat flux were employed. The mannequin was placed in seven postures as follows: standing (exposed to the atmosphere, floor contact); chair sitting (exposed to the atmosphere, contact with seat, chair back, and floor); cross-legged sitting (floor contact); legs-out sitting (floor contact); and supine (floor contact). The radiative heat transfer coefficient was determined for each posture, and empirical formulas were proposed for the convective heat transfer coefficient of the entire human body under natural convection, driven by the difference between the air temperature and mean skin temperature corrected using the convective heat transfer area.     

Full-field surface heat flux measurement using non-intrusive infrared thermography

A method was developed to measure full-field, transient heat flux from a fire onto a surface using infrared (IR) thermography. This research investigated metal plates that were directly exposed to fire while the unexposed side temperature of the plate was measured using IR thermography. These temperatures were then used in a two-dimensional finite difference inverse heat transfer analysis to quantify the heat flux. The method was demonstrated through a series of experiments with direct fire exposures onto vertical and horizontal plates. Fires were produced using a propane sand burner and ranged from 20 to 100 kW. Point heat flux measurements were also measured using a Schmidt–Boelter heat flux gauge. It was found that heat fluxes obtained via IR thermography were within one standard deviation of those from the Schmidt–Boelter gauge. The effect of plate material was studied both numerically and experimentally for stainless steel and aluminum plates. It was found that although precision is affected by material, appropriate resolutions can be selected to obtain similar precision for both materials. Spatial and temporal resolution effects were also investigated and it was found that the precision of the heat flux measurement is inversely proportional to both spatial and temporal resolutions.     

湿传递对建筑墙体传热性能的影响

建筑物的耗能与建筑围护结构的传热传湿密切相关,了解建筑墙体内部的热湿传递对建筑节能有重要影响。以相对湿度和温度梯度为驱动势建立墙体一维非稳态热、湿和空气耦合传递模型(HAM模型),并利用有限元法进行了数值求解,重点关注了湿传递对传热的影响。数值结果表明:考虑传湿时墙体内部温度波动小,墙体进行热湿传递会产生湿积累,降低墙体使用年限;考虑传湿时通过墙体总传热量比不考虑传湿时多7.5%;考虑传湿时内壁面最大平均数比不考虑传湿时大0.78。     

Study on transient heat transfer through multilayer thermal insulation: Numerical analysis and experimental investigation

This paper reports on a numerical and experimental study of heat transfer phenomena through two different multilayer fibrous insulations for building applications. The investigated samples were composed of different layers of fibrous materials and aluminium foils, placed between one or two air gaps in the vertical dimension. An experimental apparatus (a guarded hot box) has been used to measure heat transfer through the samples, while a finite volume numerical model combined radiation/conduction heat transfer was developed to predict the temperature distribution and heat transfer in such insulation systems comprised of the materials separated by multiple reflective foils. The model takes into account the coupling between the solid conduction of the fibrous system and the gaseous conduction and radiation. The radiation heat transfer through the insulation system has been modelled via the two flux approximation. The numerical results were compared with the experimental data from the guarded hot box for model validation, as well as to assess the effectiveness of the reflective foils in changing the resistance of the insulations. The comparative verification of the model showed that the numerical results were consistent with the experimental data through the environmental conditions under examination.     

Numerical study of a M-cycle cross-flow heat exchanger for indirect evaporative cooling

In this paper, numerical analyses of the thermal performance of an indirect evaporative air cooler incorporating a M-cycle cross-flow heat exchanger has been carried out. The numerical model was established from solving the coupled governing equations for heat and mass transfer between the product and working air, using the finite-element method. The model was developed using the EES (Engineering Equation Solver) environment and validated by published experimental data. Correlation between the cooling (wet-bulb) effectiveness, system COP and a number of air flow/exchanger parameters was developed. It is found that lower channel air velocity, lower inlet air relative humidity, and higher working-to-product air ratio yielded higher cooling effectiveness. The recommended average air velocities in dry and wet channels should not be greater than 1.77 m/s and 0.7 m/s, respectively. The optimum flow ratio of working-to-product air for this cooler is 50%. The channel geometric sizes, i.e. channel length and height, also impose significant impact to system performance. Longer channel length and smaller channel height contribute to increase of the system cooling effectiveness but lead to reduced system COP. The recommend channel height is 4 mm and the dimensionless channel length, i.e., ratio of the channel length to height, should be in the range 100 to 300. Numerical study results indicated that this new type of M-cycle heat and mass exchanger can achieve 16.7% higher cooling effectiveness compared with the conventional cross-flow heat and mass exchanger for the indirect evaporative cooler. The model of this kind is new and not yet reported in literatures. The results of the study help with design and performance analyses of such a new type of indirect evaporative air cooler, and in further, help increasing market rating of the technology within building air conditioning sector, which is currently dominated by the conventional compression refrigeration technology.     

A CFD study of convection in a double glazed window with an enclosed pleated blind

A numerical study has been conducted of steady free convection in a double glazed window with a between-panes pleated cloth blind. The model geometry is based on a commercial product that is available on the consumer market in North America. The study considers the effects of Rayleigh number, enclosure aspect ratio, and blind geometry on the convective heat transfer. A simplified model of the coupled convective and radiative heat transfer is also presented. Sample results show that the average Nusselt number data from the CFD study can be combined with a one-dimensional model to closely predict the glazing-to-glazing U-value.     

Numerical and experimental study of conjugate heat transfer in a horizontal air cavity

The demand for general reduction of the energy consumption in civil engineering leads to more frequent use of insulating materials with air gaps or cavities. Heat transfer through a constructional part can be decreased by adding an air gap and low emissivity reflective foils to the structure. In the first part of this paper, the impacts of cavity thickness and inner surface emissivity on combined conduction, convection and radiation heat transfer was experimentally explored in the case of constructional part with a horizontal cavity subjected to constant downward heat flux. The heat flow meter Netzsch HFM 436 Lambda was used for steady-state measurements. Results suggest that the studied parameters seriously affect the combined heat transfer in the composed structure. In the second part the paper reports the numerical study of two-dimensional conjugate heat transfer in closed horizontal cavity having air as the intervening medium. Numerical models validated by related experimental results were performed to further investigate the effect of radiation heat transfer. It was found that in general, the total heat flux through the composed structure decreases with increasing air cavity thickness, which is significant especially when low emissivity inner surfaces are taking into account. The direction of heat flow (downward or upward heat flow) has a significant impact on the convection heat transfer. An important contribution from the present work is the analysis of the optimal thickness of the cavity at different boundary conditions. The optimal thickness of the enclosure with low emissivity surfaces is 16 mm when subjected to upward heat flux.     

Semi-analytical solution of roof-on-grade attached underground engineering envelope

Using a mathematical model of heat transfer of attached underground Engineering Envelope, the calculation area is divided into 13 rectangular blocks according to the Interzone Temperature Profile Estimation (ITPE) technology, and the solutions are obtained for all the parts using the technique of variable separation. During the solution, the Fourier coefficients are determined based on the continuity of the heat flux and boundary conditions. The influences of the temperature of earth surface and the heat transfer coefficient between air and wall on heat flow through the envelope has been quantitatively analyzed. The results show that the heat flux through the envelope is no longer monotonously increase or decrease with increase of the influencing factors. By analyzing the curve of r-value (the ratio of the flux through the roof to the overall flux through the engineering section including the floor and walls), it can be concluded that the heat flow between the first floor of the building and the engineering is the major reason and the shape of the engineering is another one.     

Modelling of wall heat transfer using modified conduction transfer function, finite volume and complex Fourier analysis methods

The original conduction transfer function (CTF) method (which was derived from the EnergyPlus source codes), and the present modified CTF method (which uses a higher order discretisation scheme for the surface heat flux as well as finer grids at the layer boundaries for multi-layer constructions) were used to calculate wall surface heat fluxes based on monitored wall surface temperatures as the inputs. At the same time, the finite volume method and the matrix method (based on the complex Fourier analysis) were also used for the numerical predictions. The matrix transfer method was updated to treat the non-linear long wave length thermal radiation and proved to be consistent with the results from the finite volume method for all wall types ranging from single-layer wall, two-layer wall with air gap, cavity brick wall and brick veneer wall. Numerical predictions using the matrix transfer method, the conduction transfer function method and the finite volume method were compared with the long period measurements for single- or multi-layer materials with and without air gaps. At the same time, CTF coefficients for modified CTF methods were tabulated and analysed for all computational cases in this study.     

Systematic analysis on combined heat and water transfer through porous materials based on thermodynamic energy

A new thermodynamic energy ‘water potential’ based on the principles of chemical potential of an element of mixed gas is defined as the driving force of gaseous phase water flux. Adhesive power or ‘capillary action’ and a portion of the water potential, is confirmed as the driving force of liquid phase water flux. A numerical model of combined heat and water transfer using the water potential is introduced and influences of forces, such as gravity and pressure on water flux are incorporated from the viewpoint of thermodynamics. A way to estimate diffusivities of gaseous and liquid phase water through porous materials is also shown. Accuracy of the numerical model is demonstrated through a comparison between calculation and experiment for different temperature gradients and water content in a porous material.     

Determining the thermal capacitance,conductivity and the convective heat transfer coefficient of a brick wall by annually monitored temperatures and total heat fluxes

The finite volume scheme and complex Fourier analysis methods are proposed to determine the thermal capacitance (defined as the product of density and specific capacity) and thermal conductivity for a building construction layer using the monitored inner/outer surface temperatures and heat fluxes. The overall heat transfer coefficient for the air gap, and the convective heat transfer coefficient for air gap surfaces and room surfaces are determined by the linear relationship between the surface convective heat flux and the temperature difference. Convective heat flux is obtained by removing the thermal radiation flux from the total surface heat flux. Finally, the predicted surface heat fluxes using the calculated thermal properties and ASHRAE values were compared with the measurements.     

Modeling natural convection in a pitched thermosyphon system in building roofs and experimental validation using particle image velocimetry

Attics in Europe are more and more used as a living room. In tropical regions, summer comfort in attics becomes critical when the roof system is badly designed. The European standards advise to form an open thermosyphon system into the roof under the tiles for many purposes. In order to evaluate the air channel's efficiency, an experimental study using a 2D-PIV system was carried out. A numerical model representing the natural convection within the thermosyphon was also developed. The predicted velocity distributions and the induced mass flow rate were in good agreement with the experimental results. Correlations for the air flow rate and heat transfer coefficients were proposed. The impact of the inclination, height, opening sections and Rayleigh number on the channel's efficiency was also investigated.