One-dimensional Transient Model Research Articles

A detailed pyrolysis model for a thermally large biomass particle

Pyrolysis of a single biomass pellet involves multiple processes, such as evaporation, decomposition, flow field changes, shrinking or swelling. These processes can be simulated by consolidating pyrolysis theories and computational fluid dynamics. In this paper, a one-dimensional transient model is formulated in a C++ code to simulate the process of pyrolysis of a single biomass pellet. Experimental data from the literature is used to validate this model. This paper presents detailed modelling results for a single biomass pellet under pyrolysis in nitrogen. The model can determine the temperature distribution at different locations at any certain time, and the temperature distribution, at the same location, over time. In this paper, the focus is on how different parameters develop with time within the biomass pellet.

Transient simultaneous heat and mass transfer model to estimate drying time in a wetted fur of a cow

To reduce heat stress that contributes to complications in milk production and breeding, dairy cows are typically cooled by intermittent water sprays coupled with fan-induced air flow. Industry currently utilises fixed duration on and off-cycles; however, water efficiency may be improved by matching the sprinkler off-time to the cow drying time that corresponds to current outdoor environmental conditions. Electricity use can also be reduced by varying fan speeds to achieve the required heat rejection rate for a given set of conditions. To achieve savings in water and electricity, a transient, one-dimensional simultaneous heat and mass transfer model of evaporation within the wetted fur layer of a dairy cows was developed to estimate drying time and heat rejection rate based on ambient conditions. Water vapour concentration, liquid water content, and temperature within the fur layer were used to estimate a global drying time for fur. Model results was compared against a steady-state model in literature and the predicted net heat load is within 15% with a mean average error of 6.0%. Parametric analyses were performed to estimate drying time as a function of outdoor air temperature, air speed, humidity, and mean radiant temperature. Simulation results were used to develop a correlation for use in a control algorithm to predict the fan speed and sprinkler operation frequency needed to meet specified cooling load thresholds given outdoor conditions. Estimated savings demonstrated that the model-based controller could reduce annual electricity and water consumption by 20% and 40%, respectively. • Transient one-dimensional model for prediction of drying time from wetted cow fur. • Drying variation with outdoor temperature, humidity, air speed and roof temperature. • Predictive controller correlation based on regression of model simulation runs. • Controller shown to lower annual electricity by 20% and water consumption by 40%.

Modeling and assessment of two-phase transonic steam flow with condensation through the convergent-divergent nozzle

In the Fukushima accident, it was found that the Reactor Core Isolation Cooling (RCIC) system played a crucial role in delaying core meltdown by almost three days in Fukushima Daiichi Unit 2, because of self-regulated operation of the steam driven RCIC turbine-pump injection system. Steam flow in the convergent-divergent nozzles of the RCIC Terry turbine is two-phase non-equilibrium transonic flow with homogenous nucleation condensation. To more accurately predict the dynamic process and behavior of the transonic compressible steam flow, a one-dimensional transient two-phase analytical model is presented. A simplified four-fluid model was employed in the present work with the consideration of four separate fluid fields: vapor, liquid film, entrained droplets and condensed droplets. The mass, momentum and energy interactions between the fluids were considered and modeled. An extended seven-equation non-equilibrium critical flow model was developed to obtain the critical pressure and velocities of each phase at the nozzle throat. To predict the wetness in the divergent section, a mechanistic nucleation condensation model was integrated in the nozzle analysis model, considering the generation and consequent growth of droplets. The governing differential equations on a staggered grid were discretized using the second-order Lax-Wendroff scheme with a flux limiter, and the Semi-Implicit Method for Pressure-Linked Equation (SIMPLE) algorithm was employed to solve the discrete linear system. To demonstrate the predictability and reliability of the physical models and the numerical method proposed in the present work, three representative nozzles were modeled and simulated. The results show good agreement with the available experimental data, even for condensation shock.

Hygrothermal characterization and modeling of cross-laminated timber in the building envelope

Cross-laminated timber (CLT) is a type of mass timber panel used in floor, wall, and roof assemblies. An important consideration in design and construction of timber buildings is moisture durability. This study characterized the hygrothermal performance of CLT panels with laboratory measurements at multiple scales, field measurements, and modeling. The CLT panels consisted of five layers, four with spruce-pine-fir lumber and one with Douglas-fir lumber. Laboratory characterization involved measurements on small specimens that included material from only one or two layers and large specimens that included all five layers of the CLT panel. Water absorption was measured with panel specimens partially immersed in water, and a new method was developed where panels were exposed to ponded water on the top surface. This configuration gave a higher rate of water uptake than the partial immersion test. The rate of drying was much slower when the wetted surface was covered with an impermeable membrane. Measured hygrothermal properties were implemented in a one-dimensional transient hygrothermal model. Simulation of water uptake indicated that vapor diffusion had a significant contribution in parallel with liquid transport. A simple approximation for liquid transport coefficients, with identical coefficients for suction and redistribution, was adequate for simulating panel-scale wetting and drying. Finally, hygrothermal simulation of a CLT roof assembly that had been monitored in a companion field study showed agreement in most cases within the sensor uncertainty. Although the hygrothermal properties are particular to the wood species and CLT panels investigated here, the modeling approach is broadly applicable.

Cold-crystallizing erythritol-polyelectrolyte: Scaling up reliable long-term heat storage material

Renewable energy usage would benefit from efficient and high-capacity long-term heat storage material. However, these types of material solutions still lack reliable and durable operation on bulk level. Previously, we showed that cold-crystallizing material (CCM), which consists of erythritol in cross-linked polymer matrix, stored heat for a long-term period in a milligram scale by supercooling stably and preventing undesired crystallization during storage. Crystallization of CCM can be triggered efficiently by re-heating the material (i.e. cold-crystallization). Supercooling and cold-crystallization are stochastic phenomena which manifest in a way that the properties in bulk scale often deviate from the microscale. In this work, we scale up CCM to a bulk size of 160 g, and analyze its supercooling and crystallization characteristics for long-term heat storage. In order to identify the impact of the scale-up on the tested compositions and to discover optimal storage conditions, CCM samples are maintained in storage mode at constant temperature between 0 and 10 °C and up to 97 days. To this end, the thermal chamber measurement procedure estimates the heat release of CCM samples based on the measured temperature data and the one-dimensional transient heat conduction model. Results indicate that the heat release in cold-crystallization is over 70% of the melting heat. This heat can be stored without reduction for at least 97 days, demonstrating the reliable performance of long-term heat storage. Analysing the thermal properties of CCM compositions indicates a maximum volumetric storage capacity of 250 MJ/m3 and excellent properties for further heat storage applications.

Investigations on thermal energy transfer between two contrasting media and its application to protect skin burn

Thermal energy transfer between two materials with different thermal properties is due to not only the temperature difference of the two objects but also the discrepancy in thermal inertia between the two materials. In this study, composite materials, called syntactic foams, with low thermal inertia feature were introduced as a new type of thermal shield preventing skin burn. Some types of syntactic foams are designed and manufactured by varying the mixing ratio of hollow glass microsphere and epoxy resin. An experimental study on thermal properties ranging from density to specific heat is conducted to deduce the thermal inertia of the composite materials as well. In order to confirm the effect of the composite materials intended to provide prevention to skin burns, a one-dimensional transient heat transfer model with heat generation and absorption is developed to scrutinize the situation when human skin is abruptly in contact with a heat source with or without the composite materials. The numerical results show that all the syntactic foams combined with hollow glass microsphere (HGM) and epoxy resin (ER) have a considerable delay effect on thermal energy transfer to skin tissue, and the HGM80/ER20 was superior to others. Based on the results, even with the equivalent temperature for the heat source, as the thermal inertia of the syntactic foams is lowered, the heat transfer impediment effect becomes higher. It is expected that the skin damage factor (Ω) can be reduced by 34.8% within the designed syntactic foams.

Warm mix asphalt (WMA) for rapid construction in airfield pavement

Warm-mix asphalt (WMA) is commonly mixed and compacted 15°C to 30°C below that of conventional hot mix asphalt (HMA). The lower production temperature of WMA is expected to give an advantage of a lower cooling time of newly laid asphalt overlay before it can be opened to traffic during the nighttime airfield pavement construction. The reduced cooling time of WMA would allow shorter airport closure time window and/or extend the time for the contractor for paving, and thus shorten the overall construction period due to more volume done each night. This is a beneficial practical advantage to hectic airports where the typical off-peak period is as short as 6–8 h. In this research, two different WMA technologies, Sasobit® and Rediset® (organic and chemical) were investigated through laboratory tests to see if there were notable differences in WMA rutting performance that could have considerable practical effects on the permissible temperature at the opening to traffic as compared to the HMA. Furthermore, a validated finite element (FE) solution for one-dimensional transient heat-transfer model is used to simulate the effect of the use of warm mix on shortening the cooling time and the overall project time. The laboratory test shows that the WMA with Sasobit has a significantly better rutting performance at the elevated temperature, enabling the WMA to be opened to traffic at a higher temperature, compared to HMA. The cooling analysis showed that, in comparison to HMA, the use of WMA could shorten the closure time of airport during the night time construction by 8–67 min (5–73% reductions), depending on the asphalt overlay thickness, specified traffic opening temperature and WMA production temperature. The use of WMA could also reduce the overall construction period by 2–16 nights for a single lift overlay, for the cases studied – which would represent a shortening of the overlay period by 4–24%.

The model development and case study for transient thermal response of the human body interacting with the coupled fabric-environment system

A one-dimensional transient thermal response model for the human body subjected to the influence of coupled fabric-environment system was developed in this study. To account for the human body's thermophysiological response, the bio-heat equation was used to predict the temperature distribution within each body tissue. The fabric composed of multiple pores full of wet air was treated as the porous media. The movement, heat transfer, fiber absorption, and evaporation of sweat within the porous media were considered in the developed model and it was solved by the CFD package software-ANSYS FLUENT to simulate the transient behavior of heat and moisture transfer in the coupled human body and fabric-environment system. Two typical settings of human activities, including human acclimatization without sweating and human activity with sweating, were considered in the numerical study. To investigate the effect of ambient temperature on the transient response of the body's skin temperature, we explored three different ambient temperatures. The numerical results showed that sweating caused skin temperature to drop gradually and that the magnitude to which the skin temperature changed was nearly insensitive to the change of environmental temperatures. Moreover, the sweating rate increased with the increase of ambient temperature.

Dynamics of gas flow between interconnected vessels: Experiments and simulations

The emergency relief systems of pressure vessels often direct their discharge to containment vessels known as catch tanks or dump tanks, which are part of effluent handling systems. Although many models exists for vent sizing and critical flow calculation, there is less information about the impact of depressurization on the catch tank. This work addresses this topic by presenting experimental and simulation studies. We report experimental temperatures and pressures in the pressurized vessel and in the catch tank during the depressurization of air or helium from several initial conditions. A reproducibility study demonstrates the excellent agreement among the results of different experimental runs. On simulation, we present results of the depressurization of a vessel connected to a catch tank. To accomplish this, an existing dynamic simulator is extended by including a one-dimensional transient heat transfer model and by enabling the simulation of interconnected vessels. The experimental and calculated pressure profiles are generally in very good agreement. The agreement of the experimental and calculated temperature profiles is good but the calculated profiles tend to overestimate the temperature variations in each vessel during the depressurization process compared to the experimental data.

Mathematical modeling of the electrochemical behavior of a polyaniline film for the fast electron transfer kinetic

The electrochemical behavior of a polyaniline (PANI) film as a conductive polymer has been studied using a new proposed mathematical model (PMM). A one-dimensional transient model, considering the doping-dedoping and redox processes for a thin PANI film, was developed based on the cyclic voltammetry method. It was assumed that two reactions occur at the interface of film/electrolyte during the potential sweep: an electrochemical reaction and an adsorption-desorption reaction. The diffusion process of dopant ions was governed by Fick's second law equation. The modeling results were evaluated using the experimental data reported in the literature. The effect of different parameters including bulk concentration of dopants (CA⁎), potential scan rate (v), adsorption rate constant (kad) and desorption rate constant (kde), on the electrochemical behavior of a PANI film were parametrically studied using the PMM. The increase of kad and CA⁎ causes the shift of redox couple to the lower potentials while the increment of kde moves the redox couple to the higher potentials. The increase of parameter v was provides the enhancement of both peak couple currents and the higher peak potential separation.

A comprehensive proton exchange membrane fuel cell system model integrating various auxiliary subsystems

A comprehensive proton exchange membrane fuel cell (PEMFC) system model is developed, including a pseudo two-dimensional transient multiphase stack model, a one-dimensional transient multiphase membrane humidifier model, a one-dimensional electrochemical hydrogen pump model, an air compressor model with proportion-integral-derivative control and a ribbon-tubular fin radiator model. All sub-models have been rigorously validated against experimental data to guarantee the system model accuracy. The effects of stack operating temperature, gas flow pattern and humidifier structural design are investigated to cast insights into the interaction among stack and auxiliary subsystems. The results indicate that the stack is successfully maintained at required operating temperatures (60 °C, 70 °C, 80 °C) with help of the radiator when the whole system starts from ambient temperature (25 °C). However, the stack is likely to suffer from membrane dehydration when operated at 70 °C, and the problem becomes more severe at 80 °C, causing significant performance deterioration. The water and temperature distribution inside the system are further demonstrated. The co-current flow pattern contributes to better water utilization of the whole system which may lead to higher output performances. But the counter-current flow pattern has positive effects on parameter distribution uniformity inside fuel cell, which is beneficial for the stack durability. As regards the membrane dehydration, it is found that optimizing membrane humidifier area does not fundamentally solve the problem. Increasing humidifier area contributes to higher water vapor transfer rate, however, it results in much slower humidification responses.

A novel transient thermohydraulic model of micro-channel heat sink

The purpose of this work is to develop an analytical model for prediction of the thermal fluid performance of a micro-channel heat sink by applying liquid thin-film theory. For this purpose, a one-dimensional transient model has been developed for a single channel of micro-channel heat sink and a series of numerical simulation was performed for various operating conditions. Mass, momentum and energy equations were applied separately for the vapor and liquid phases to predict more effective physical performance of the transient behavior. As a result, the axial distributions of mass flow, pressure, and temperature for the working fluid were obtained for vapor and liquid in the micro-channel. In particular, the phase change interface profile with the liquid thin-film was obtained using an augmented Young Laplace equation. Consequently, the interface areas of heat and mass transfer occupied by the liquid and vapor were predicted and incorporated into the governing equations of each phase. The analytical model was validated by comparison of its numerical solutions with experimental results. The discrepancy between the experimental and numerical results for the axial wall temperature was less than 4 °C, and that for the transient time to reach a steady-state wall temperature was within 1 sec. Through a series of performance simulations for a micro-channel model with pure water as the working fluid, transient response of the wall temperature, liquid film thickness, mass flow rate, liquid pressure, and vapor and liquid temperatures were investigated. The model developed in this study might be utilized for more refined performance prediction of a micro-channel heat sink.

A lab-scale rotary kiln for thermal treatment of particulate materials under high concentrated solar radiation: Experimental assessment and transient numerical modeling

Rotary kilns are worldwide used for industrial processes that involve thermal treatments of particulate materials. However, a great amount of fossil fuels is employed in such processes. As alternative, solar rotary kilns are considered for this application due to their versatility and potential to substitute traditional fossil-fuel driven devices. In order to boost the development of this technology, efforts have to be focused on the control of the particle temperature during the treatment. In this context, a lab-scale rotary kiln was built and tested using a 7-kWe high-flux solar simulator at University of Antofagasta. It was conceived to treat particulate materials of different nature and it is able to reach temperatures higher than 800 °C under different operation strategies. Silicon carbide was selected for initial tests because it is inert, endures high temperatures (up to 1600 °C) and it has been proposed as thermal storage vector in several researches on concentrated solar power. In a first stage, the empty kiln was preheated up to about 800 °C, reaching a steady state in less than three hours and with a power of approximately 370 W entering the kiln cavity. Afterwards, 43 g of silicon carbide were introduced in the furnace and the system was heated again up to a second steady state above 800 °C. In this stage, particles showed a fast increment of their temperature and exceeded 700 °C in less than three minutes after loading. A one-dimensional transient numerical model was also developed to perform the thermal analysis of the kiln and the estimation of both the particle temperature and the system efficiency. Numerical results showed good agreement with experimental data and thermal losses could be quantified in detail. Therefore, the model was also used to predict the thermal behavior of a solar rotary kiln working in batch mode.

Waste Gasification in a Melting Updraft Moving Bed Reactor: Preliminary Analysis on Oxidation and Melting Zone

Waste conversion into valuable chemical products seems to be a good overcoming strategy both for the fossil fuel resources employment and for the current inefficient waste management. The main step related to the production of chemical from waste is its thermochemical conversion via gasification process. Taking into account the variable composition of waste and its high ash amount, among others, the high temperature melting gasifier is the most suitable system for ensuring the required flexibility. Indeed, besides syngas production, this system allows the melting of waste ash fraction in order to produce a vitrified and inert granular material. A simplified transient one-dimensional model of the oxidation zone including melting phenomenon could help in the understanding of process behavior.

Effect of gas on the polymer temperature in external gas-assisted injection molding

Abstract The introduction of gas is the principal difference between external gas-assisted injection molding (EGAIM) and conventional injection molding. In this study, the effects of gas thickness and gas delay time on polymer temperature were discussed. A modified one-dimensional transient heat conduction model of polymer was established to reveal the relationships between polymer temperature and gas thickness and gas delay time in EGAIM. The temperature histories of polymer were obtained by the simulation methods, including Moldflow and ANSYS, and were verified by comparing the experimental data to numerical simulation results. The effects of gas thickness and gas delay time on the temperature histories of polymer will be discussed in detail. The results showed that the polymer temperature is strongly affected by the heat preservation of gas, which in turn, increases with the increase of gas thickness and delay time. This paper provides quantitative methods and theoretical guidance for the study of the effects of gas on the polymer temperature in EGAIM.

Thermal and Electromagnetic Transients in Liquid Metal Surfaces of the FNSF

The Fusion Nuclear Science Facility (FNSF) is an intermediate step in the path to commercial fusion energy that will accommodate the extreme fusion nuclear environment and the complex integration of components and their environment as well as the relevant nuclear science and plasma physics. The transient thermal and electromagnetic loads on plasma-facing components in FNSF have been shown to offer significant design challenges that are difficult to meet with solid walls. Hence, the project team is investigating the feasibility of using liquid walls to ameliorate some of the risk associated with solid wall designs.In this paper, we examine the effects these transient loads will have on a liquid wall. Mass loss is considered using standard evaporation models accounting for transient surface temperatures. The heat transfer is modeled with a one-dimensional transient conduction model that accounts for evaporative losses. No liquid motion is considered. Loss rates of tens of microns per edge-localized mode (ELM) are predicted. Peak heat fluxes are treated parametrically to help address the substantial uncertainty inherent in models for the timing and spatial distribution of the heat deposited during the ELM. Boiling is considered but is found to not be of consequence, as the temperatures required for homogeneous nucleation of bubbles are substantially higher than a conventional boiling point. It should be noted that all evaporation calculations are for evaporation into a vacuum. In the future, we intend to incorporate these evaporation rates into an edge physics code to self-consistently model the net mass flows at the liquid surface in a tokamak.Electromagnetic effects due to ELMs and disruptions are accounted for by assuming a stationary plasma quench. ELMs are addressed assuming a small fluctuation in the plasma current during an event, while disruptions are addressed assuming a full quench of the current. The variation in the plasma current induces currents in the conducting fluid, leading to forces on the liquid (and subsequent motion). A commercial finite element code is used to calculate the induced currents and forces associated with a static liquid divertor. Liquid motion is not considered in this calculation, so no magnetohydrodynamic (MHD) currents are addressed, but a simplified model is presented to estimate the impact of these currents on the liquid motion. Based on these calculations, the acceleration of the liquid is expected to be quite high, and containment of the liquid is likely not possible. The MHD effects appear to be relatively minor.

Mathematical simulation of heating and melting of solid ice in a carbon steel pipe coated with a resistive heating system

The heating and melting processes of solid ice inside a closed pipe were investigated by performing experiments and developing mathematical models. Several experiments were conducted in a cold room to measure the transient temperature and the times that were required to heat and melt ice inside a nominal 51-mm (2-inch) diameter Schedule 40 carbon steel pipe. The resistive heating system that was developed for this study consisted of several coating layers that were fabricated by using flame and cold spraying processes. By passing current through the coating heating system, sufficient heat was generated by way of Joule heating to heat and melt the ice in the closed pipe. One-dimensional transient heat conduction models were developed in cylindrical coordinates to predict the transient temperature of the ice during the heating process and the transient location of the moving solid-liquid interface during the melting process. The models were developed based on the separation of variables and the superposition methods. The data obtained from the models were compared with the results that were from the experiments. It was found that the models predicted the solid ice heating times to within 16%, 12%, and 15% and the melting times to within 6%, 8%, and 9% of those that were measured in experiments with supplied powers of 10 W, 20 W, and 40 W, respectively. The results suggest that the mathematical models that were developed based on one-dimensional transient heat conduction in cylindrical coordinates can be employed to provide reasonable predictions of the total time that is required for a thermal-sprayed coating heating system to heat and melt the ice within carbon steel pipes.

Examining the effects of microencapsulated phase change materials on early-age temperature evolutions in realistic pavement geometries

This study examines experimentally and numerically the effects of microencapsulated phase change material (PCM) additions on temperature rise and cool-down rate, and the associated impact on reducing the risk of thermal cracking in concrete pavements. Specimens representative of a realistic pavement geometry were exposed to diurnal temperature cycling at early ages while internal temperature evolutions were monitored. In spite of the fact that the lower thermal conductivity of the PCM inhibited heat dissipation, the results showed the presence of PCMs can reduce considerably the temperature rise and cool-down rate in the first 24 h following placement provided the PCM melting temperature is chosen suitably. A transient one-dimensional thermal model of a pavement section suitably captured experimental temperature evolutions and thereby offers the ability to inform the design of concrete pavements containing PCMs that feature early-age thermal cracking resistance.

Numerical investigation and thermodynamic analysis of syngas production through chemical looping gasification using biomass as fuel

Chemical looping gasification (CLG) is an innovative technology for the thermochemical conversion of biomass to produce high quality synthesis gas. In this study, a one-dimensional transient model considering partial oxidation of fuels and iron-steam reaction for biomass CLG fuel reactor was developed and validated against the experimental study utilizing Fe2O3 as oxygen carrier (OC) and microalgae as fuel. The simulation results and the experimental data are in good agreement in terms of the detailed time-varying outlet gas species concentrations, gas yield, carbon conversion efficiency, and gasification efficiency. The effects of different operating temperature and steam flow on the gasification performance are also well predicted by the adopted reaction mechanisms. Energy and exergy analyses are applied to evaluate the energy sources based on the validated model, which demonstrates that the presence of OC can enhance both the quantity and quality of the valuable syngas.

Development and validation of a one-dimensional transient rotodynamic pump model at component scale

For reactor design and safety purposes, the French Alternative Energies and Atomic Energy Commission (CEA) is currently working on the implementation of a predictive transient two-phase flow 4-quadrant rotodynamic pump model in the CATHARE-3 code (Code for Analysis of THermalhydraulics during an Accident of Reactor and safety Evaluation). This paper presents the pump model and its validation in single-phase first quadrant conditions at component scale. Explanations are first given on code architecture, meshing, equations to solve and how to switch from fixed frame to rotating frame and vice versa at impeller endpoints. Then, verification results in an ideal case are compared to Euler equations. Finally, validation results on real cases including the prediction of single-phase first quadrant steady performance curves and the simulation of a fast startup transient are presented.